MEDICAL SCHOOL LHIBTBA1EY Gift of Dr. Chaunoey D. Leake HUMAN ANATOMY l.ippinrott Company. . l.\ J. I'.. I.ippino>tt Company. •iKlit, igoX. 1>\ J H. I.ippr ;>;uiy. l.y J. B. I.ippnu-.'-t ('..nipany. i.ippiiicott Ciiinpany. •ii;ht. i')i'). 1>\ J. H. I.inpiiu-ntt I'ompany. -. !•> J. I'., l.ippiiu-iitt ('ntnpaiiy. Him .11. H.trrved, T^^ CONTENTS. VOL. II. THE NERVOUS SYSTEM. General Considerations 996 The Nervous Tissues 997 The Nerve-Cells 997 The Nerve-Fibres 1000 Neuroglia io°3 The Nerve-Trunks 1006 The Ganglia 1007 Development of the Nervous Tissues . . . 1009 Nerve-Terminations 1014 Motor Endings 1014 Sensory Endings 1015 THE CENTRAL NERVOUS SYSTEM. THE SPINAL CORD 1021 Membranes 1022 Cord-Segments 1024 Form of the Spinal Cord 1026 Columns of the Cord 1027 Gray Matter 1028 Central Canal 103° Microscopical Structure 1030 White Matter 1036 Fibre Tracts 1039 Blood- Vessels of Spinal Cord 1047 Development of Spinal Cord 1049 Practical Considerations : Spinal Cord. . 1051 Malformations IQ51 Injuries I052 Localization of Lesions 1053 THE BRAIN I055 General Description 1056 General Development 1058 Derivatives from the Rhombencephalon 1063 The Medulla Oblongata 1063 Internal Structure 1068 The Pons Varolii i°77 Internal Structure 1078 The Cerebellum 1082 Lobes and Fissures 1084 Architecture IC)88 Internal Nuclei 1088 Cerebellar Cortex 1090 Cerebellar Peduncles 1093 The Fourth Ventricle 1096 Development of the Hind-Brain Derivatives "°o The Medulla "01 The Pons "°3 The Cerebellum "°3 The Mesencephalon "°5 The Corpora Quadrigemina nob The Cerebral Peduncles 1107 The Sylvian Aqueduct "<» Internal Structure of the Mid-Brain 1112 The Tegmentum 1112 TheCrusta i"5 The Median Fillet i"5 The Posterior Longitudinal Fas- ciculus The Mesencephalon — Continued Development of Mid-Brain 1117 The Fore-Brain i"9 The Diencephalon i J T9 TheThalamus i"9 Structure 1120 Connections 1121 The Epithalamus "23 The Trigonum Habenulae 1123 The Pineal Body 1124 The Posterior Commissure 1125 The Metathalamus "26 The Hypothalamus "27 The Subthalamic Region 1127 The Corpora Mammillaria 1128 The Pituitary Body 1129 The Third Ventricle 1131 The Telencephalon 1132 The Cerebral Hemispheres 1133 Cerebral Lobes and Interlobar Fissures i T35 Lobes of the Hemispheres 1139 Frontal Lobe "39 Parietal Lobe "43 Occipital Lobe 1 145 Temporal Lobe 114? Insula "49 Limbic Lobe "5° The Rhinencephalon "5* The Olfactory Lobe "5i Architecture of the Hemispheres "55 The Corpus Callosum "55 The Fornix "5$ The Septum Lucidum "59 The Lateral Ventricles "60 Internal Nuclei of the Hemisphere "69 The Caudate Nucleus "69 The Lenticular Nucleus "69 The Claustrum TI72 The Amygdaloid Nucleus "7 The Internal Capsule "73 Structure of the Cerebral Cortex i The Nerve-Cells of Cortex "7° The Nerve-Fibres of Cortex "79 Variations in Cerebral Cortex "»o White Centre of the Hemisphere i The Association Fibres i The Commissure) Fibres i " The Projection Fibres "°7 Development of the Derivatives of Fore- Brain T The Pallium * The Sulci and Gyri "9° Histogenesis of Cerebral Cortex. .. . i The Rhinencephalon "93 The Corpus Stratum "93 The Diencephalon "93 The Cerebral Commissures i Measurements of the Brain "95 v VI CONTENTS. The Membranes of the Brain 1 197 Tin- I Hir.i Mater 1 198 The Pia Mater The Arachnoid 1203 The Pacchionian B<>f the I '.rain 1206 Practical Considerations : The Brain and Its Membranes 1207 Congenital Krrors of Development. . 1207 Tin- Melinites 1208 Cerebral I lemorrhage 1209 Cerebral Localization 1210 Cranio-Cerebral Topography 1214 THE PERIPHERAL NKKVOIS SYS: THE CRANIAL NERVES 1220 The Olfactory Nerve 1 220 The Optic Nerve 1 223 The Oculomotor Nerve 1225 The Trochlear Nerve 1228 The Trigeminal Nerve 1230 The Gasserian Ganglion 1232 The ( Ophthalmic Nerve and Branches 1233 The Ciliary Ganglion 1236 The Maxillary Nerve and Branches 1237 The Spheno-Palatine Gang- lion 1240 The Mandibular Nerve and Branches 1242 The Otic ( .an-lion 1246 The Sobtnaxttlary Ganglion 1247 Practical Considerations : The Tri- geminal Nerve 1 248 The Abducent Nerve 1249 The Facial Nerve 1250 Practical Considerations 1254 The Auditory Nerve 1256 The Glosso-Pharyngeal Nerve 1260 The Vagus or Pneumogastric Nerve 1265 Practical Considerations 1272 The Spinal Accessory Nerve 1274 Practical Considerations 1275 The Hypoglossal Nerve 1275 Practical Considerations 1277 THE SPINAL NERVES 1278 The Posterior Primary Divisions 1279 The Cervical Nerves 1281 The Thoracic Nerves The Lumbar Nerves 1282 The Coccygeal Nerve The Anterior Primary Divisions 1284 The Cervical NYrves 1285 The Cervical Plexus and Branches 1286 The Phn-nic Nerve 1290 Practical Considerations 1292 The Brachial Plexus and Branches 1292 The Kxternal Anterior Thorn ic Nerve 1297 The Muscnlo Cutaneous Nerve 1298 The Median Ner\e 1298 The Brachial Plexus and Branches— Continued Practical < 'onsiderations 1301 The Internal Anterior Thoracic Nerve 1303 The Lesser Internal Cutaneous Nerve 1303 Tht- Internal Cutaneous N'-rve 1303 The I'lnar Net \ e 1303 Practical Considerations 1306 The Subscapular Nerves 1306 The Circumflex Nerves 1307 Practical Considerations 1308 The Musculo-Spiral Nerve 1308 Practical Considerations 1374 The Thoracic Nerves 1314 :c,d ( 'oiisiderations 1318 The Lumbar Plexus and Branches 1319 The Ilio-Hvpogastrfc * 1320 The Ilio-In^uinal Nerve 1321 The < ieniti >-Crural Nerve 1322 The Kxternal Cutaneous Nerve 1324 The < ibturator N 1324 The Accessory Obturator Nerve .. . 1324 The Anterior Crural Nerve 1327 Practical Considerations: Lumbar Mexus 1330 The Sacral Plexus and Branches 1331 The Great Sciatic Nerve 1335 TheLxtern.il Popliteal Nerve 1336 The Anterior Tibial Net \v 1336 The Musculo-Cntaneous 1338 The Internal Popliteal Nerve 1339 The Posterior Tibial ' 1342 The Pudenda! Plexus and Branches 1345 The Small Sciatic Nerve 1348 The Pudic Nerve 1349 The Coccypal I'l'-x'is 1352 Practical Considerations : Sacral Plexus 1352 THE SYMPATHETIC NKKVKS 1353 General Constitution and Arrange- ment 1355 The Gangiiated Cord 1356 Kami Communicantes 1356 Cervico-Cephalic Portion of Gangiiated Cord 1358 The Superior Cervical ( ianglion 1359 The Middle Cervical (ianglion 1362 The Inferior Cervical ( ianglion 1362 Thoracic Portion of < ian.uliated Cord. . . . 1364 The Splanchnii 1364 Lumbar Portion of C.anuhatcd Cord. .. . 1366 ' Portion of Gangiiated Cord 1367 The Plexuses of the Sympath. -tic Nerves 1367 The Cardiac Pi- 1367 The Solar Plexus 1368 Subsidiary Plexuses 1369 The Hvpogjastric Plexus Subsidiary !'!• ... 1374 Practical Consideratii S\ni|a- thet; 1375 Development of the Peripheral Nerves. . 1375 THE ORGANS OF SENSE. Tin SKIN. General Description . Structure The n di Struiture . 1381 1394 The Cutaneous ( ,l.ir 1397 « '.lands 1397 The Sueat ('.lands 1398 ..prneut of the Skin and its Append- 1400 CONTENTS. VI! THE NOSE. The Outer Nose 1404 Cartilages of the Nose 1404 Practical Considerations : The External Nose 1407 The Nasal Fossae 1409 The Vestibule 1409 The Septum 1410 The Lateral Wall 1410 The Nasal Mucous Membrane 1413 The Olfactory Region 1413 The Respiratory Region 1415 Jacobson's Organ 1417 Practical Considerations : The Nasal Cavities 1417 The Accessory Air-Spaces 1421 The Maxillary Sinus 1422 The Frontal Sinus 1423 The Ethmoidal Air-Cells 1424 The Sphenoidal Sinus 1425 Practical Considerations : The Accessory Air-Spaces 1426 Development of the Nose 1429 THE ORGAN OF TASTE The Taste-Buds 1433 Structure 1434 Development 1436 THE EYE. The Orbit and its Fasciae 1436 Practical Considerations . . 1438 The Eyelids and Conjunctiva 1441 Practical Considerations 1446 The Eyeball 1447 Practical Considerations 1448 The Fibrous Tunic 1449 The Sclera 1449 The Cornea 1450 Practical Considerations 1453 The Vascular Tunic 1454 The Choroid 1455 The Ciliary Body 1457 Practical Considerations 1459 The Iris 1459 Practical Considerations 1461 The Nervous Tunic 1462 The Nervous Tunic — Continued The Retina 1462 Practical Considerations 1468 The Optic Nerve 1469 Practical Considerations 1470 The Crystalline Lens 1471 Practical Considerations 1473 The Vitreous Body 1473 Practical Considerations 1474 The Suspensory Apparatus of the Lens. . 1475 The Aqueous Humor and its Chamber. . 1476 Practical Considerations 1476 The Lachrymal Apparatus 1477 The Lachrymal Gland 1477 The Lachrymal Passages 1478 Practical Considerations 1479 Development of the Eye 1480 THE EAR. The External Ear 1484 The Auricle 1484 The External Auditory Canal 1487 Practical Considerations 1490 The Middle Ear 1492 The Tympanic Cavity 1492 The Membrana Tympani 1494 The Auditory Ossicles 1496 The Mucous Membrane 1500 The Eustachian Tube 1501 The Mastoid Cells 1504 Pract. Consid.: The Middle Ear 1504 The Tympanic Cavity 1504 The Tympanic Membrane 1505 The Eustachian Tube 1507 The Mastoid Process and Cells 1508 The Internal Ear 1510 The Osseous Labyrinth 1511 The Vestibule 1511 The Semicircular Canals 1512 The Cochlea 1513 The Membranous Labyrinth 1514 The Utricle 1514 The Saccule 1515 The Semicircular Canals 1515 The Cochlear Duct 1517 The Nerves of the Cochlea 1521 Development of the Ear 1523 THE GASTRO-PULMONARY SYSTEM. General Considerations 1527 Mucous Membranes 1528 Structure 1528 Glands 1531 Types of Glands 1531 Simple Tubular Glands 1532 Compound Tubular Glands .... 1532 Tubo- Alveolar Glands 1532 Serous Glands 1534 Mucous Glands 1534 Simple Alveolar 1535 Compound Alveolar Glands. .. . 1535 Development of Glands 1537 THE ALIMENTARY CANAL. The Mouth 1538 The Lips, Cheeks and Vestibule 1538 The Teeth 1542 Description of Individual Forms. .. . 1543 Structure of the Teeth 1548 The Enamel 1548 The Dentine 155° The Teeth— Continued The Cementum 1552 The Alveolar Periosteum 1553 Implantation and Relations of the Teeth 1554 Development of the Teeth 1556 First and Second Dentition 1564 The Gums 1567 The Palate 1567 The Hard Palate 1567 The Soft Palate 1568 The Tongue 1573 General Description 1573 The Glands of the Tongue 1575 The Muscles of the Tongue 1577 The Sublingual Space 1581 The Salivary Glands 1582 The Parotid Gland 1582 The Submaxillary Gland 1583 The Sublingual Gland 1585 Structure of the Salivary Glands. . . . 1585 Development of the Oral Glands. ... 1589 vm CONTENTS. Practical Considerations : The Mouth . . . 1589 Malformations : Harelip and Cleft Palate ...................... 1589 The Lips .......................... 1590 The Gums ........................ 1590 The Teeth ........................ 1591 The Roof of the M< »uth ............ 1592 The Floor of the Mouth ............ 1593 The Cheeks ....................... 1594 Tin- Ton-ue ....................... 1594 The Pharynx .......................... 1596 The Naso-Pharynx ................ 1598 The Oro-Pharynx .................. 1598 The Laryngo-Pharynx .............. 1598 The Lymphoid Structures .......... 1599 The Faucial Tonsils ............ 1600 The Pharyngeal Tonsil ........ 1601 Relations of tin- Pharynx ........... 1601 Development and Growth of Pharynx 1603 Muscles of the Pharynx ............ 1604 Practical Considerations: The Pharynx. . 1606 The (Esophagus ....................... 1609 General Description ............... 1609 Course and Relations .............. 1609 Structure .......................... 1611 Practical Considerations : (Esophagus . . 1613 Congenital Malformations .......... 1613 Foreign Bodies .................... 1613 Strictures ......................... 1614 Carcinoma ........................ 1614 Extrinsic Disease .................. 1614 Diverticula ....................... 1614 The Abdominal Cavity ................ 1615 The Stomach .......................... 1617 General Description ............... 1617 Peritoneal Relations ............... 1619 Position and Relations ............. 1619 Structure .......................... 1621 Growth ........................... 1629 Variations ........................ 1629 Practical Considerations : The Stomach 1629 Congenital Malformations .......... 1629 Injuries of the Stomach ............ 1630 Ujcers and Cancer ................. 1631 Dilatation and Displacement ........ 1631 Operations on the Stomach ........ 1632 The Small Intestine .................... 1633 General Description ................ 1633 Structure .......................... 1634 The Duodenum .................... 1644 Duodrao-TeiunaJ Fossa- ............ 1647 Interior of the Duodenum .......... 1648 The Jejuno- Ik-urn .................. 1649 The Mesentery and Topography .... 1650 Mecke!' s 1 >iverticulum ........ ..... 1652 Practical Considerations: The Small In- testine ...................... 1652 The P< ritoiie il ("oat ................ 1652 The Muscular (.'oat ................ 1653 Tin- Mucous and Suhmucous ' Ulcers of the Duodenum, 1653 Infection ........... . 1654 T\phoid I'lcers Contusion and Kuptiin 1654 truction .......... 1655 1656 1657 1657 1657 1660 1664 1665 1666 Tin- I .aiv.e Intestine Structure . 'I li'- < The Vermitoiiii App< n •lle.il Kel.ition ,. . 1 I ossae. . . rge I ntestine — Continued Retro-Colic Fossa; The Colon General I >esrripti< >n Peritoneal Relations The Sigmoid Flexure 1 h-velopment and ( irowth The Rectum The Anal Canal The Anus Muscles and Fascia,- of Rectum and Anus The Ischio-Rectal Fossa Pract. Consid. : The Large Intestine . . . The Caecum The Vermiform Appendix Etiology of Appendicitis Anatomical Points relating to the Symptoms and to the Treat- ment of Appendicitis Operations for Appendicitis .... The Colon and Sigmoid Flexure. .. . I >istention and Rupture I >isplacet:it -nts Obstruction and Stricture Wounds Operations The Rectum and Anal Canal Development of the Alimentary Canal . . Formation of the Mouth Formation of the Anus Differentiation of the Body-Cavity . . 1 1' velopment of the Peritoneum. .. . The Liver General I K-s. ription Borders and Surfaces Blood- Vessels Structure The Hepatic Duct The Gall-Bladder The Common Bile- Duct Peritoneal Relations of the Liver . . . Position of the Liver Development and < irowth Practical Considerations : The Biliary Apparatus Anomalies in Form and Position of the Liver Hepatoptosis and I lepatopexy Obstruction of 1 1< p.itic Circulation. . Wounds ar.d I lepatic Abscess Malformations of Gall-Bladder Wounds and Rupture Distention and Cholecystitis The Cystic and Common Bile- Ducts. Operations on dall-lUadder and P.ili- ary Ducts The Pancreas General 1 Vscription Structure Pancre.it! Development Practical Considerations : The Pain Malformation • Injuries Pancreatitis The Peritoneum ( '.enel.ll Considerations The Anterior Parietal Peritoneum Tile Anterior Mesentery The Posterior Mesentery : Part I The Posterior Mesentery : Part II Tin- 1 Me* utery : Part III. . 1667 1668 1668 1670 1671 1671 1672 1673 1673 1675 1678 1680 1680 1681 1681 1683 1685 1685 1686 1686 1687 1688 1688 1689 1694 1694 1695 1700 1702 1705 1705 1707 1709 1712 1718 1719 1720 1721 1722 1723 1726 1726 1726 1727 1727 1729 1729 1729 1731 1732 1732 1732 1734 1736 1737 1738 1738 1739 1740 1740 1742 1744 1746 1751 1753 CONTENTS. IX Practical Considerations : The Perito- neum Anatomical Routes for Infections. . . Peritonitis anatomically considered. . Abdominal Hernia General Considerations Predisposing anatomical conditions. Inguinal Hernia Anatomy of Inguinal Canal Anatomy of Indirect Inguinal Hernia Varieties of Inguinal Hernia. .. . Anatomy of Direct Inguinal Her- Anatomical Considerations of Treatment Femoral Hernia Anatomy of Femoral Canal .... Anatomical Considerations of Treatment Umbilical Hernia Ventral Hernia Lumbar Hernia Obturator Hernia Sciatic Herniae Perineal Herniae Diaphragmatic Hernice Intraabdominal Herniae ACCESSORY ORGANS OF NUTRITION. 1754 i?54 1756 1759 1759 1759 1763 1763 1766 1767 1770 1770 1773 1773 1774 1775 1776 1777 1777 1778 1778 1778 1779 The Spleen 1781 General Description 1781 Structure 1783 Peritoneal Relations 1785 Development and Growth 1 787 Accessory Spleens 1787 Practical Considerations : The Spleen. . . 1787 The Thyroid Body 1789 General Description 1789 Structure 1791 Development 1793 Accessory Thyroids 1793 Practical Considerations : The Thyroid Body 1794 The Parathyroid Bodies 1795 General Description 1795 Structure 1 795 The Thymus 1796 General Description 1796 Structure 1798 The Thymus — Continued Development and Changes 1800 The Suprarenal Bodies 1801 General Description 1801 Structure 1802 Development and Growth 1804 Accessory Suprarenals 1805 Practical Considerations: The Suprarenal Bodies 1806 The Anterior Lobe of the Pituitary Body, 1807 Development 1808 The Carotid Body 1809 The Coccygeal Body 1811 The Aortic Bodies 1812 THE ORGANS OF RESPIRATION. The Larynx 1813 Cartilages, Joints and Ligaments. . . . 1813 Form of Larynx and Mucous Mem- brane : 1818 »MuscIes of the Larynx 1825 Changes with Age and Sex 1828 Practical Considerations : The Larynx. . 1828 The Mediastinal Space 1832 Practical Considerations 1833 The Trachea 1834 General Description 1835 Structure 1835 Relations 1836 Growth and Subsequent Changes 1837 Bifurcation of Trachea and Roots of Lungs 1837 The Bronchi 1838 Practical Considerations : The Air-Pas- sages 1840 The Lungs 1843 General Description 1843 Lobes and Fissures 1845 Physical Characteristics 1846 The Bronchial Tree 1847 The Lung Lobule. 1849 Structure 1851 Blood- Vessels 1853 Relations to Thoracic Walls. . . . 1855 The Pleurae 1858 General Description 1858 Relations to the Surface 1859 Structure 1860 Development of the Respiratory Tract . . 1861 Practical Considerations : The Lungs and Pleurae 1864 THE URO-GENITAL SYSTEM. THE URINARY ORGANS. The Kidneys 1869 General Description 1869 Position and Fixation 1870 Relations 1873 Architecture 1875 Structure 1877 Practical Considerations: The Kidneys . . 1887 Anomalies of Form, Size or Num- ber 1887 Anomalies of Position 1887 Renal Calculus 1890 Injuries and Tumors 1893 Operations 1893 The Renal Ducts 1894 Pelvis of the Kidney 1894 The Ureter 1895 Structure 1896 Practical Considerations: The Ureters . . 1898 Congenital Anomalies 1898 Ureteral Calculus 1899 Wounds 1900 Operations 1901 The Bladder 1901 General Description 1901 Peritoneal Relations 1904 Fixation and Relations 1905 Structure 1908 Practical Considerations: The Bladder. . 1910 Congenital Anomalies 1910 Effects of Distention 1911 Retention of Urine 1912 Rupture and Wounds 1913 Cystitis and Vesical Calculus 1914 The Male Perineum 1915 The Triangles 1916 CONTENTS. Pract. Consul.: The- Bladder— The Perincal Interspaces 1916 1 .audmarks 1918 Lateral Lithotomy 1919 Median Lithotomy. ..... 1921 Suprapubic Lithotomy 1921 The Female Illaddei 1922 The Urethra 1922 The Prostatic Portion 1922 The Membranous Portion 1923 The Spongy I'ortion 1923 The Female Urethra 1924 Structure 1924 Practical Considerations: Male Urethra. . 1927 Congenital Abnormalities 1927 • Clinic.il 1 >ivision of I 'rethra 1928 Rupture of Urethra 1930 Anatomical Consideration of Ure- thritis 1930 Stricture of Urethra 1931 I 'n-thral Instrumentation 1933 Development of the Urinary Organs. .. . 1934 The Pronephros 1934 The Mesonephros (\Volffian Body). . 1935 The Metanephros (Kidney) 1937 The Bladder and I "ret lira 1938 THE MALE REPRODUCTIVE ORGANS. The Testes 1941 General Description 1941 Architecture 1942 Structure 1942 Spermatogenesis 1944 The Spermatozoa 1946 The Epididymis 1947 General Description 1947 Structure 1947 The Appendages of the Testicle 1949 The Appendix Test is 1949 The Appendix Epididymidis 1949 The Paradidymis ....'. 1950 The Vasa Aberrantia 1950 Practical Considerations: '1 he Tes- ticles 1950 Congenital Anomalies 1950 Orchitis 1951 Epididymo-Orchitis 1952 Castration 1952 Hydrocele 1953 The Spermatic Ducts 1953 The Yas 1 H -ferens 1954 The Ejaculatory I >u<-t 1955 Structure <>f Spermatic Duct 1956 The Seminal Vesicles 1956 Gener.il I Ascription 1956 Structure . 1958 Practical Considerations: The Seminal les 1959 The Spermatic (Wil 1960 Practi< v -rations: The Spermatic 1961 The Scrotum . 1961 1961 of the Testicle . 1963 'Mill . 1964 The 1 1965 • '965 . 1966 Hw Corpus Spongionun . 1967 The ('dans Penis . 1968 Structure . 1968 1 he 1'elli Congenital Abnormalities . 1972 Pract. Consid. : The Penis— Continue J Circumcision J973 Contusions and Wounds 1974 Amputation J975 The Prostate < .land 1975 ( ieneral I )escriptit >n 1975 Position and Relations I976 Structure 1977 Development '979 Pract. Consid.: The Prostate Gland .. 1979 Relations to Generative System .... 1979 Injuries J979 Hypertrophy 1980 Operations 1982 The Glands of Couper 1984 General Description 1984 Structure 1984 Development 1984 THE FK.MAI i: REPRODUCTIVE ORGANS. The Ovaries 1985 General Description 1985 Position and Fixation 1986 Structure 1987 Follicles and Ova 1988 The Human Ovum 1990 Corpus Luteum 1990 Development 1993 Variations 1995 Practical Considerations : The Ovaries. . 1995 The Fallopian Tubes 1996 General Description 1996 Course and Relations 1997 Structure 1997 Development and Changes 1999 Variations 1999 Practical Considerations : The Fallopian Tubes 1999 Rudimentary Organs 2000 The Epoophoron 2000 Gartner's Duct 2001 The Paroophoron 2002 Vesicular Appendages 2002 The Uterus 2003 General Description 2003 Attachments and Peritoneal Rela- tions 2004 The Broad Ligament 2004 The Round Ligament 2005 Position and Relations 2007 Structure 2007 Development and Changes 2010 Menstruation and Pregnancy 2012 Practical Considerations: Uterus and Attachments 2012 Compartments of Pelvis 2013 1 Hspfacements of Uterus 2014 The Broad Ligament 2014 The Round Ligaments 2015 The Vagina 2016 General Description 2016 Relations 2016 Structure 2017 Development 2019 Variation- 2019 Practical Considerations: The Vagina. 2019 Relations to Uterine Cervix 2019 Fistula- 2020 The 1 .abia anil the Vestibule 2O2I The Labia Majora 2O2I The Mons Pubis 2021 1 abia Minora 2022 The Vestibule . 2O22 CONTENTS. XI The Clitoris 2024 The Bulbus Vestibuli 2025 The Glands of Bartholin 2026 Pract. Consid. : The External Genitals . . 2027 The Mammary Glands 2027 General Description 2027 Structure 2029 Milk and Colostrum 2030 Development 2032 Variations 2033 Practical Considerations : The Mammary Glands 2033 The Nipple 2033 Paths of Infection 2034 Carcinoma 2035 Practical Considerations : The Mammary Glands — Continued Removal of the Breast 2036 Development of Reproductive Organs. . . 2037 General Considerations 2037 The Indifferent Stage 2038 Differentiation of the Male Type. .. . 2038 Descent of the Testis 2040 Differentiation of the Female Type. . 2042 Descent of the Ovary 2043 The External Organs 2043 In the Female 2044 In the Male 2044 Summary of Development 2045 The Female Perineum 2046 VOLUME II. THE PERIPHERAL NERVOUS SYSTEM THE CRANIAL, SPINAL AND SYMPATHETIC NERVES THE ORGANS OF SENSE THE GASTRO-PULMONARY SYSTEM THE ALIMENTARY CANAL AND ITS GLANDS THE ACCESSORY ORGANS OF NUTRITION OR THE DUCTLESS GLANDS THE RESPIRATORY ORGANS THE URO-GENITAL SYSTEM THE NERVOUS SYSTEM. FIG. 834. THE nervous system — the complex apparatus by which the organism is brought into relation with its surroundings and by which its various parts are united into one coordinated whole — consists essentially of structural units, the neurones, held together by a special sustentacular tissue, the neuroglia, assisted by ingrowths of connective tissue from the investing membrane, the pia mater. The neurone, the morphological unit of the nervous system, includes a nucleated protoplasmic accumulation, the cell-body, and the processes. The former, usually spoken of as the nerve-cell, presides over the nutrition of the neurone and is the seat of the subtle changes giving rise to nervous impulse. The processes arise as outgrowths from the cell-body and provide the paths along which impulses are conveyed. They are very variable in length, some extending only a fraction of a millimeter beyond the cell-body, while others continue for many centimeters to distant parts of the body. The longer processes, which usually acquire protecting sheaths, are known as the nerve-fibres, and these, associated in bundles, constitute the nerve-trunks that pass to the muscles and various other organs. Reduced to its simplest terms, the nervous system consists of the two parts rep- resented in the accompanying diagram (Fig. 834). The one, the sensory neurone, {A} takes up the stimulus received upon the integument or other sensory surface and, by means of its process (nerve-fibre), conveys such impulse from the periphery towards the central aggregations of nerve-cells that commonly lie in the vicinity of the body-axis. Functionally, such a path constitutes a centripetal or afferent fibre (#). The impressions thus carried are transferred to the second element, the motor neurone (.#), which in response sends out the impulse originating within the cell-body (nerve-cell) along the process known as the centri- fugal or efferent fibre (e*), to the muscle-cell and causes contraction. The simple relations of the foregoing apparatus are, in fact, superceded by much greater complexity in consequence of the introduction of additional neurones by which the afferent impressions are distributed to nerve-cells situated not only in the immediate vicinity of the first neurone, but at different and often distant levels. Although very exceptionally the relation between the neurones may perhaps be that of actual continuity in consequence of a secondary union of their processes (Held), the view concerning the constitution of the nervous system most worthy of confidence, notwithstanding the bitter attacks by certain histologists, regards the neurones as separate and distinct units. While chained together to form the various paths of conduction, they are probably seldom, if ever, actually united to one another but only intimately related, since their processes, although in close contact, are not directly continuous, — contiguity but not continuity being the ordinary relation. During the evolution of the nervous system from the simpler type, the cell- bodies of the neurones forsake their primary superficial position and recede from the periphery. In vertebrates this recession is expressed in the axial accumulation of cell-bodies either within the wall or in the immediate vicinity of the neural tube (brain and spinal cord), from or to which the processes pass. The nervous system is often divided, therefore, into a cenfra/and a peripheral portion. The former, also known as the cerebro- spinal axis, includes the brain and spinal cord and contains the chief axial collections of nerve-cells ; the peripheral portion, on the contrary, 996 Diagram showing fundamental units of nervous system. A, sensory neurone, conducting afferent impulses by its pro- cess (a) from periphery (S) ; B, motor neurone sending efferent impulses by its process (e) to muscle. THE NERVOUS TISSUES. 997 contains the nerve-cells of the sensory ganglia and is principally composed of the nerve-fibres that pass to and from the end -organs. Intimately associated with and in fact a part of the peripheral nervous system, but at the same time possessing a certain degree of independence, stands the sympathetic system, which provides for the innervation of the involuntary muscle and glandular tissue throughout the body and the muscle of the heart. When sectioned, the fresh brain and spinal cord do not present a uniform appear- ance, but are seen to be made up of a darker and a lighter substance. The former, the gray matter, owes its reddish brown color not only to the numerous nerve-cells that it contains, but also to its greater vascularity ; the hue of the lighter substance, the white matter, is due to its chief constituents, the medullated nerve-fibres, in conjunction with its relatively meagre blood supply. THE NERVOUS TISSUES. The Neurones. — The neurones, the essential morphological units of the nervous system, consist of the cell-body and the processes. The latter, as seen in the case of a typical motor neurone (Fig. 835), are of two kinds : (a) the branched protoplasmic extensions, the dendrites, which may be multiple and form elaborate arborescent ramifications that establish relations with other neurones, and (^) the single unbranched axone (neuraxis, neurite) that ordinarily is prolonged to form the axis-cylinder of a nerve-fibre, and, hence, is often termed the axis-cylinder process. The dendrites are usually uneven in contour and relatively robust as they leave the cell-body, but rapidly become thinner, due to their repeated branching, until they are reduced to delicate threads that con- stitute the terminal arborizations, the telodendria, formed by the end-branches. The latter are beset with minute varicosities and finally end in terminal bead-like thickenings. The axones, slender and smooth and of uniform thickness, are much less conspicuous than the dendrites. They may be short and only extend to nearby cells ; or they may be of great length and con- nect distant parts that lie either wholly within the FIG. 836. Dendrites FIG. 835. Dendrites Aiborization of axone Telodendrion Diagram of typical neurone. / / Diagram of nerve-cell of type II, in which axone is not prolonged as nerve-fibre. cerebro-spinal axis (as from the brain-cortex to the lower part of the spinal cord) or extend beyond (as from the lower part of the cord to the plantar muscles of the foot). 99« HUMAN ANATOMY. I-u,. 837. Semidiagrammatic representation of strut/line iii neurone; a, axone. On reaching their destination the- axones terminate in end-arborizations ( telodendria) of various forms, in a manner similar to tin- dendrites. According to the distribution of their axones, the neurones are divided into two In those of the first, known as ee//s of type /, th<- axone is continued as a nerve-fibre and is, therefore, relatively 'oni;. Soon after leaving the cell-body such axones -iv ot'f delicate lateral processes, the collaterals, which, alter a longer or shorter course, break up into arborizations ending in relation with other and often remote neurones. Neurones of the second and much less frequent cla->s, cells of type If, possess short axones that are not continued as iierve-tibres, but almost immediately break up into complex end-arborizations or neuropodia ( Kollikcr >, limited to the gray matter. The processes of the sensory neurones, as in the case of those constituting the spinal and other ganglia connected with afferent Derves, are so modified during development (Fig. 839) that later both dendrites and axones arise in common from the Dingle robust stalk of an apparently unipolar cell. Branching T-like, one process (the dendrite) passes towards the periphery and the other (the axone) extends to and into the cerebro-spinal axis. The nerve-cells, as the bodies of the neurones are called, possess certain structural details in common, although in some instances they present characteristics that suffice to identify them as In-longing to particular localities. Nerve-cells are relatively large elements, those in the anterior horns of the spinal cord measuring from .070-. 150 mm. in diameter, and contain a large spherical nucleus, poor in chromatin but usually pro- vided \\ith a conspicuous FIG. 838. niuleolus. Their cytoplasm varies in appearance with the method of fixation and staining to such an extent that considerable uncertain- kfl to the relation of many described detaiU to the actual structure of the cells. It may be accepted as established, ho\\rvrr, that tile cell body of the IK uroneconsjv ,,mi substance, homogeneou finely uranular, in which delicate jibrill<,- and m.. of ckromaiopkilit granules an embedded ; in addition, a variable amount ot brown or blackish pigment is coin monly present in the vicin- itv of the nucleus. The • n.e , ,f the tilnill.e within the ner\e -cell, '. maintained 1,\ M,i\ c I,,,],,. I,,,, 1,, ,. i; , garded, has been plai ed •nd (pit -stion liy t! :! and ..th.Ts. The signifi- • ami relations of the tibrill.r to the nei \ e -< ell. however, ha- «• ^i\ en rise to warm ( human spinal cord stained to show Nl»«l bodlet ; ^. A, axones; < . tmpbnil 400. THE NERVOUS TISSUES. 999 discussion. The observations based upon the improved methods of silver-staining introduced by Cajal have contributed much towards the solution of these questions, and, at present, the most experienced histologists incline towards the view that the fibrillae demonstrable within the nerve-cell are limited to the body and processes of that particular neurone and do not unite with the nbrillae of other neurones. When adequately differentiated by successful staining, the nbrillae form an intracellular net-work within the cell-body, from which they are continued into the dendrites and axone and in all cases end free in the terminal arborizations (Retains;. After special staining with methylene blue, or other basic anilines, the chro- matophilic granules appear deeply colored and arranged in groups or masses of vary- ing form and size. Such aggregations, known as Nissl bodies, after the German histologist whose elaborate studies and theories concerning the structure of the nerve- cell have given prominence to these masses of " stainable substance," are usually most conspicuous in the vicinity of the nucleus. Collectively, they constitute the tigroid substance of Lenhossek and are least marked at the periphery of the nerve- cell. They are continued into the dendrites as elongated flakes or pointed rod-like tracts that finally are resolved into scattered granules along the processes. The axone, on the contrary, is not invaded by the Nissl bodies, and usually joins the nerve-cell at an area free from the stainable substance, the axis-cylinder process com- monly arising from a slight elevation known as the implantation cone. Exception- ally, the axone may arise from one of the dendrites, either at its base or at a point some distance from the cell-body. Notwithstanding the elaborate classification of nerve-cells and the theories based upon the Nissl bodies, their significance is still debatable, although in the light of the more recent studies by Carrier, Holmes and others it seems probable that they are normal constituents of the cell and are directly related to functional activity, undergoing increase under unusual stimulus. Critical study of the structure of ganglionic nerve-cells has established the presence of four fundamental components within their cytoplasm. These are, according to Cowdry, (1) the Nissl bodies, (2) the mitrochondria, deeply staining minute rod-like granules, (3) an intracellular system of clefts or canaliculi, and ( 4 ) the neuro-fibrils. That these canaliculi are not artefacts is probable from their demonstration after staining intra vitam with a solution of pyronin, when the clefts appear as a network of clear, continuous, but tortuous spaces within the red-tinted cytoplasm. FIG. 839. Every neurone possesses at least one process, which is then an axone, although usually provided with both dendrites and axone. Very rarely more than a single axone is present. Depend- ing upon the number of their processes, nerve-cells are described as unipolar, bipolar, or multipdar. The unipolar condition is often secondary, since two processes may be so blended for part of their course that they form a single process. Conspicuous examples of such relation are seen in the spherical nerve-cells composing the spinal and other ganglia connected with the sensory nerves. Primarily such neurones possess an axone and a dendrite that arise from opposite ends of what is for a time a spindle-shaped bipolar cell. During development, however, the unilateral growth of the cell-body towards the surface of the ganglion brings about the gradual approximation of the two processes until they fuse in the single extension into which the spherical or flask-like cell is prolonged. This process sooner or later undergoes a Y- or T- like division, one process, usually identified as the dendrite, passing to the periphery to end in the free terminal arborization, whilst the other, the axone, passes centrally to end in an arborization around the neurones lying within the cerebro-spinal axis. Examples of bipolar neurones, in which the dendrite and axone pass from opposite sides of the spherical cell-body, are found in the retina and the ganglia Diagram showing transformation of young bipolar sensory neurone into one of unipolar type. 1000 HTMAN ANATOMY. FIG. 840. connected with the acoustic nerve. An interesting modification of bipolar neurones is presented by the olfactory cells, whose denclrites are represented by the extremely short p: mbedded within the nasal mucous membrane, whilst tin- axones are prolonged as the fibres of the olfactory nerves into the cranial cavity to end in telodendria within the glomeruli of the olfactory bulb. The cell-bodies of the multipolar neurones, which possess one axone and several dendrites, vary in form ( Fig. ^41 ). Some, as those within the sym- pathetic ganglia, are approximately spherical and of moderate size, with short delicate dendrites; many are of large size and irregularly stellate form, the dendrites passing out in all directions, as seen in the conspicuous motor neurones within the gray matter of the spinal cord ; others possess a regular and characteristic form, as the flask-shaped cells of Purkinje within the cerebellum, or the pyramidal cells of the cerebral cortex. Certain multipolar neurones within the cerebral cortex, and especially those constituting the chief components of the granule layer of the cerebellum, are distinguished by the small size of their cell-bodies and the peculiar ramifications and claw-like telodendria of their dendrites (Fig. 945^). Within the cerebellar cortex are likewise found examples of Bipolar neurones; a.frpmolfactcnv mucous membrane — dendrite is above; b, from retina. (Modified from Co/at.) FIG. 841. Multipolar nerve-celli of various forms; .-(, from spinal »-nnl , /.', (I..MI unbial cortex; C. from cerebellar cortex (Purkinje cell) ; a, axone; c, implantation tin- multipolar neurones of ('lolgi's type II, whose uxoiu-s almost immediately undergo elaborate branching within the gray matter to which they are confined. The Nerve-Fibres. — From the foregoing considerations it is evident that the nerve -fibres are not independent elements, but that all are the processes of neurones — either the a. \ones , ,f those that are prolonged into fibres , type I >, or the dendrites of those situated within the spinal and other seiis(,r\ peripheral ganglia. Although nem ones exist which aienot continued as nerve-til >res, the latter are always connected THE NERVOUS TlSSU-iS. 1001 FIG. 842. Axis-cylinders Axolemma Medullary sheath Node of Ranvier Neurilemma Medullated nerve-fibres, as seen in longi- tudinal sections of spinal nerve. X 500. with neurones. Recognizing, therefore, that the nerve-fibres are only processes of neurones, their separate description is justified only as a matter of convenience. The fundamental part of every nerve-fibre is the central cord, commonly known as the axis-cylinder, which is composed of threads of great delicacy, the a.vis- fibrilla, prolonged from the nerve-cell and embedded within a semifluid interfibrillar substance, the neiiroplasm, the entire cord per- haps being enclosed by a delicate structureless sheath, the axolemma. The existence of the axolemma as a distinct sheath, however, is ques- tionable, the appearance of such investment not improbably being due to a local condensation of the framework of the medullary coat immediately around the axis-cylinder. In the case of the typical fibres, such as form the chief constituents of the peripheral nerves distributed to various parts of the body, the axis- cylinder is surrounded by a relatively thick coat, known as the medullary sheath, outside of which lies a thin structureless envelope, the neurilcinnia or sheath of Schwann, that invests the entire nerve-fibre. In the case of fibres proceeding from neurones composing the sensory ganglia, the neurilemma is continuous with the nucleated sheath enclosing the individual ganglion-cells. The medullary sheath consists of two parts, a delicate reticular framework and a fatty substance, the my din, that fills the meshes of the supporting reticulum. The latter, arranged for the most part as anastomosing membranous lamellae, that in transverse sections of the nerve-fibre appear as faint concentric lines, resists pancreatic digestion and fat-dissolving reagents, and was regarded by Ewald and Ktihne as possessing properties similar to the keratin of horny substances and, hence, was named by them ncnrokcratin. The blackening after treatment with osmic acid and other reactions exhibited by myelin indicate its fatty nature, and it is probable that this substance exists during life in the form of a fine emulsion supported by the framework. When fresh, myelin appears highly refracting and homogeneous, and confers upon the medullated nerve-fibres their characteristic whitish color. It is, however, prone to post-mortem changes, so that after death it loses its former uniformity and presents irregular contractions and collections, or at the broken end of the fibre extrudes in irregular globules, due probably to fusion of the normal individual minute droplets into larger masses. The medullary sheath is not uniformly continuous, but almost completely inter- rupted at regular, although in different fibres variable, intervals marked by annular constrictions. These constrictions, the nodes of Ranvier, correspond to narrow zones at which the medullary sheath is practically wanting and the neurilemma dips in and, some- what thickened, lies in close relation with the axis-cylinder. According to Hardesty1 the medullary sheath does not suffer complete suppression at the nodes, but is represented by part of its reduced framework which trans- verses the constriction, a conclusion which we can confirm. The nodes occur at regular intervals along the fibre, which they thus divide into a series of internodal segments. In general, the latter are longer in large fibres, where they have a length of about i mm. , and shorter in those of small diameter, in which they may measure . I mm. or less in length. The axis-cylinder passes uninter- ruptedly across the nodes, although it often presents a slight fusiform enlargement FIG. Axis-cylinder Neurilemma Medullary sheath Medullated nerve-fibres in transverse section. X 550. 1 Amer. Journal of Anatomy, vol. iv., 1905. 1002 HTMAN ANATOMY. opposite each constriction (Ranvier). The neurilemma also suffers no break at the nodes, but is continuous from one segment to the other. In addition to the partial interruptions at the nodes, the medullary sheath after treatment with osmic acid frequently appears broken by clear narrow clefts that extend obliquely from the neuri- lemma to the axolemma and thus subdivide each internodal Moment into a number of smaller known as the S, Iiinidt- I.aiitt -rniann segments ( l-'ig. >44 >. The oblique clefts do not all extend in the same direction, even wkhin the same inter- nodal segment, since they are usually directed from without inward and towards the nodal constrictions and, therefore, have an opposed disposition at the ends of the same a> well as of the adjoining seg- ments. The significance of this subdivision is un- certain ; many regarding the details as artefacts. •ding to Capparelli ', however, the apparent clefts are in reality unstained membraneous septa that pass obliquely from the axolemma to the inner suifaee of the neurilc inina and serve to hold the a\i.-.-cylinder in place and to enclose the myelin. The studies of Hatai - on the arrangement of the neurokeratin seem to support these conclusions. Within each internodal segment, beneath the sheath of Schwann, lies a single (sometimes more than one > small nfnrilennna-cell which consists of an elongated oval nucleus surrounded by a meagre amount of cytoplasm. These cells represent the remains of the formative elements (sheath-celt's) that during the growth of the nerve-fibre were active in providing its envelope (page 101 i FIG, Node of Ranvier Medullated nerve-fibres after treatment with usmif acid; A, fibre showing lutn within medullary coat; £. OM showing same coat divided into segments. X 500. .itcd nervr ' hi"K tin-it li-m;iii..Ui'ii. g upon the ])i-esence or absenc«- <-f the medullary sheath throughout the greater part <>i their course, ner\e tibres are distinguished as medullated or non- 1 . \rdiiv f. niikrns. Anat. u. I-'.ntwi.k., M. '>*), [9 *Joonml of Comparative Neurology, \"i. \iii., i9°3- THE NERVOUS TISSUES. 1003 medullated. The medullated fibres constitute the great majority of those making up the peripheral nerves and the tracts of the cerebro-spinal axis ; the component fibres of the latter, however, while medullated arc without the neurilemma. The nonmedullated fibres, on the other hand, are chiefly prolongations < a\om-s i from the ganglion cells of the sympathetic system, although in the case of the olfactory nerves the fibres are also without a myelin-coat. The dis- tinction between these two classes of fibres is relative rather than FIG. 846. absolute, since every medullated nerve-fibre becomes nonmed- ullated before reaching its termination, central or peripheral. Nonmedullated nerve- fibres in longitudinal section of splenic nerve. X 310. Medullated nerve-fibres vary greatly in thickness, the smallest having a diameter of only .001 mm., whilst the largest may measure as much as .020 mm. According to their diameter, as determined by T'611iker, the medullated fibres may be grouped as fine (.oo2-.oo4 mm.), medium (.oos-.oog mm.), and coarse (.oio-.o2o mm.). In general, the thicker fibres are the longer and are the processes of large nerve-cells ; conversely, the finer have shorter courses and belong to small cells. Although subject to many exceptions, ihe motor fibres are usually the thicker and the sensory the smaller. Since there are many more nerve-fibres than nerve-cells, it is evi- dent that the former must undergo division along their course. Such •doubling always occurs at a point corresponding to a node of Ranvier, never within the internodal segment, the sheaths being continued over the two resulting fibres. On approaching their peripheral termination the branching becomes more frequent and the medullary sheath thinner until it ends, after which the axis-cylinder continues invested with only the attenuated neurilemma. The latter, now reduced to an extremely delicate covering beset with occasional nuclei, sooner or later disappears, the naked axis-cylinder alone being prolonged to end finally in the varicose threads of the telodendrion. The nonmedullated nerve-fibres proper, also termed pale fibres or fibres cf Remak, include those that are without the myelin sheath throughout their course. They are chiefly the axones of sympathetic neurones. Devoid of medullary sheath, these fibres, often .002 mm. or less in diameter, consist of only the axis-cylinder and the neurilemma, the latter being thinner and more delicate than on the medullated fibres. Like the latter, the pale fibres end in telodendria composed of naked axis-cylinders, bearing irregular varicosities. Neuroglia. — The neurones (nerve-cells and fibres) within the cerebro-spinal axis are everywhere held together by a special supporting tissue known as neuroglia. The latter is primarily derived from the in vagi - nated ectoblast lining the neural tube, certain elements, the spongioblast$.< being devoted to the production of the neuroglia, while others, the neuroblasts, give rise to the neurones. At first the supporting tissue is represented by greatly elongated, radially disposed fibre-cells that often extend the entire thickness of the wall of the neural canal. Later, the neurogliar elements become differentiated into (a) those bordering the lumen of the canal, which are partly retained as the cpcndyntal cells, and (b~) those which have early migrated to more peripheral locations and given rise to stellate cells that are converted into spider-like elements, the aslron'tcs. Seen in chrome-silver preparations (Fig. 847) these appear as irregular triangular or quadrilateral cells from whose angles numerous delicate fibrillae extend between the surrounding nervous elements. According to Rubaschkin,1 the astro- cytes are transformations from larger branched gliogenetic cells, by the conversion of whose robust protoplasmic processes the delicate fibril It? that later form the chief 1 Archiv f. mikros. Anat. u. Entwick., Bd. 64, 1904. FIG. 847. Young neuroglia cells ; astrocytes, from brain of child. X 300. 1004 HIM AN ANATOMY. constituents of the neuroglia arise. So long as neuroglia is being produced, as in the nervous axis of young ;mim;ils, the large gliogenetic cells are present and directly concerned in the production of additional ribrillae, their cytoplasm becoming pro- Mvely less granular and reduced through the various transition phases until in tin- final condition, as the small x/ia : lie scattered at uncertain intervals. In all parts of the central nervous system the mature neuroglia consists of essentially the same tissue, the differences presented in certain localities depending largely upon variations in its compactness. Kverywhere the chief part of the sup- porting tissue consists of the intricate felt-work of fibrillae, glia-Jibres, as they are called, which are usually free but to some extent connected with the spider-cells or astrocytes. Where, however, the neuroglia borders the neural tube (the ventricles of the brain and the central canal of the spinal cord) as the ependymal layer, its Arrangement exhibits peculiarities that call for later special mention. In the immediate vicinity <>f the neurones the felt-work of the fibrilke is unusually close, so that the cell-bodies and the nx.ts of the processes are surrounded by a protecting sheath, the glia-caf>sule. This diminishes along the demlrites, and after these begin to branch the neuroglia no longer forms a complete special investment. The medullated nerve-fibres within the brain and spinal cord are also provided with delicate ncitrogliar sheaths which replace the neurilemma which on these fibres is wanting. These sheaths are prolonged for some distance on the fibres of the roots of the spinal nerves. The fibres of the optic ner\e and of the olfactory tract are accompanied through- out their length by neurogliar sheaths, those of the remaining cranial nerves losing these envelopes shortly after leaving the brain I Rubaschkin). I'.eiieath the pia mater the neuroglia is especially dense and forms the external subpial layer that every- where invests the nervous mass, following all the inequali- ties < if its surface. In this manner the pia mater is excluded and, except where its connective-tissue strands accompany the blood-vessels that enter the nervous mass, takes no part in the make-up of the supporting stroma. The subpial layer consists of a dense felt-work of glia-fibres, disposed in various planes, which are partly free and partly the processes of spider cells. Internally the layer fades into the adjoining diffuse neuroglia without demarcation. At the periphery the fibres often exhibit a radial disposi- tion, their outer ends usually being somewhat expanded. Within the white matter the neuroglia, both in its distri- bution and density, is fairly uniform, although special tracts often separate the larger bundles of nerve-fibres. Its arrangement within the gray matter presents less uniformity, since more or less marked condensations occur where the nerve-cells are collected into nuclei, as Conspicuously seen in the inferior olive. FIG. 848. t nriiro- K!I:I siirriiuniliiiK j.iii.il (Kubiifih- Where the neuroglia borders the neural tube i.illy the central canal of the spinal cord) it constitutes the ependymal layer, the peculiari- : \\hich call for special mention. The imme- diate lining of the tube consists of a single layer of pyramidal epithelial elements, the cpcndynml cells, whose free surfaces or bases look towards the lumen, and the apices towards the MM i ounding nervous tissue. At least during the earlier years in man, and throughout life in many lower mammals, the h cell is beset with a number of hair-like processes that in their relations with the cytoplasm correspond to ordinarv cilia. The pointed distal end of the epeiidvmal cell is prolonged into a conical process that is directly continii'-d into usually a single neurogliar fibre which, after a . our>«- of uncertain length becomes THE NERVOUS TISSUES. 1005 lost in the surrounding complex of glia-fibres. In young tissue the apical processes often exhibit evidences of breaking up into a number of fine fibrillae. Where the processes enter robust tracts of neuroglia, as in the posterior longitudinal septum of the spinal cord, they are of unusual length. In addition to the radially directed fibres connected with the ependymal cells, the fibre-complex of the ependymal zone includes many fibrillae that are circularly and longitudinally disposed. Scattered glia cells, some stellate but mostly small, are also present and represent the elements from which the neuroglia-fibrillae have been derived. In the preceding account of the elements composing the nervous tissues the neurones have been regarded as the morphological units, each retaining its individual anatomical indepen- dence, although functionally closely related with other similar units. This conception, com- monly referred to as the Neurone Doctrine and strikingly formulated by Waldeyer in 1891, stands in contrast to the prior views by which actual continuity was attributed to the nerve-cells by means of the union assumed to exist within the terminal net-works of their processes. The independence and true relation of the neurone was established largely through the convincing embryological investigations of His and the renewed study of the nerve-cells as demonstrated by the improved applications of the Golgi silver-impregnations, supplemented by the method of vital staining by methylene blue introduced by Ehrlich. The Neurone Doctrine has gained wide acceptance and the support of the most distinguished anatomists, among those who have materially strengthened its position being Kolliker, Ramdn y Cajal, Retzius, Lenhosse'k, Waldeyer, van Gehuchten, and Edinger. The neurone conception, securely founded as it is upon a vast mass of evidence collected from a wide field by the most painstaking and accurate observation, has not escaped challenge, and at present is assailed by a group of histologists headed by Apathy and Bethe, who not only bitterly oppose the integrity of the neurone as an independent unit, but also strive to depose the nerve-cell from its dignity as the fundamental physiological factor. In 1897 Apathy1 published his observations on the structure of the ganglia of certain invertebrates, as revealed by a new mercuric gold-chloride method, and thereby established the important fact that the cell-body and processes of the neurone are pervaded by fine neurofibrillae, thus confirming the fibrillar structure of the nerve-cell advanced by Max Schultze more than a quarter of a century before. Following Apdthy, Bethe2 investigated the tissues of the higher animals and succeeded in dem- onstrating the existence of the neurofibrillae within the neurones of man. According to these observers, the neurofibrillae, although interlaced without junction within the cell-bodies, are independent threads, that are not confined to the neurones but pass beyond and unite with fibres from other sources. The neurofibrillae, therefore, and not the nerve-cells, are the essen- tial elements of the nervous system, the cells being only interposed along the path of conduc- tion. Indeed, according to these views, the neurofibrillae are independent of and, in a sense, foreign to the nerve-cells, leaving or entering the latter at pleasure and constituting by their union a continuous path of conduction from the receptive element to the muscle-fibre. Apathy, moreover, assumes the existence throughout the central nervous system of a fibrillar net-work formed outside and between the nerve-cells by the neurofibrillae from which the axones may arise independently of the nerve-cells. It is evident that if such be the case the conception of the neurone as an individual unit falls. The criticism made by the newer school, that the supporters of the neurone theory relied upon methods which inadequately demonstrated the ultimate terminal relations (the assumed union in net-works) has been met by the introduction of the still newer methods of Beilschow- sky and especially of Cajal, which have yielded preparations that demonstrate that the neuro- fibrillae everywhere form net-works within the cell-bodies of the neurones, are confined to their processes, and even in their ultimate endings form ununited terminal arborizations. It seems, indeed, that, at present at least, the defenders of the neurone theory may with justice charge their opponents in turn with depending upon methods that only partially show the relations of the neurofibrillae within the neurones. Retzius, than whom no more experienced and competent authority in this difficult field of research can be consulted, has recently reviewed the entire question and presented 3 most convincingly the facts that enable him, as well as the most distinguished anatomists of to-day, still vigorously to champion the Neurone Doctrine. After a critical and scientific discussion of the arguments advanced by Apathy, Bethe and Nissl,4 Retzius rests his case with little concern as to the verdict of those to whom facts and not speculation most appeal. 1 Mitteilungen aus d. Zoolog. Station zu Neapel, Bd. xii., 1897. * Allgemeine Anat. u. Physiol. des Nervensystems, 1903. 3Biologische Untersuchungen, N. F., Bd. xii., 1905. 4 Die Neuronenlehre und ihre Anhanger, 1903. ioo6 HUMAN ANATOMY. The Nerve-Trunks. — The fibres composing the peripheral nervous system are grouped into the larger and smaller nerve-trunks which extend to various parts of the body. In the make-up of those that supply both muscles and sensory surfaces gument or mucous membranes , as, for example, the median or the third division of the trimeminal nerve, ti are included: (i) the efferent axones of motor neurone^ whose cell -bodies are situated within the spinal cord or brain ; (2) the afferent dend: :v neurones within the spinal and other sensory ganglia ; and .} > the efferent axones of neurones within the sympathetic ganglia that accompany the spinal fibres to the periphery and serve for the innervation of the involuntary muscle of the bloc. ! and of the skin and the glands. The in: the various kinds usually more or less intermingled, are ;>eifnrii<»i. When well re-presented, the sheath of the funiculus C' insists of concentric lamellae of fibrous tissue which enclose pcrhicurial lymph-spaces. 849. I ; • .- • Blood-vessels -Perineurium *> Funiculus of nerve- fibres Transverse section ..I sin. ill n ,,-d of loosely united funiculi. X 20. The latter, lined by flattened ,,,nn nw plates, are in relation with the clefts between the nerve-fibres, , ,M the ..IK- hand, and with the lymphatics within the inter- hmi< tilar tissue on the other. When-, as usual, the nerve IS composed of several funiculi. these are loosely bound together and the entire trunk so formed is invested • ral tibr. the the medullarv substance exhibits a dark • -"id th. :,der ajipr.irs surroun-led by a deeply tinted ring The neuri THE NERVOUS TISSUES. 1007 lemma nuclei are occasionally seen as deeply stained crescentic figures that partially embrace the nerve-fibre, lying beneath the neurilemma within depressions in the medullary substance. FIG. 850. Perineurium Endoneuriutn Nerve-fibre FIG. 85 r. Epineurium Blood-vessel Transverse section of funiculus composed of nerve-fibres held together by endoneurium and surrounded by perineurium. X 175. Viewed in cross-section, the nonmedullated fibres appear as small irregularly round figures arranged in groups that correspond to bundles (Fig. 851). When numerous, the latter are aggregated into secondary bundles between which extend delicate connective- tissue septa, continuous with the general envelope investing the nerve- trunk. The medullary substance being wanting, the pale fibres are of small size and often possess a diameter of less than .001 mm. The Ganglia.— The cell- bodies of the neurones that consti- tute the sensory pathways within the peripheral nerves and of the neu- rones of the sympathetic system are collected at various points into aggregations known as ganglia. Familiar examples of the latter are the spinal ganglia on the posterior roots of the spinal nerves, certain cranial ganglia (as the Gasserian connected with the fifth nerve, the acoustic with the eighth, and those on the trunks of the seventh, ninth and tenth cranial nerves), and the sympathetic ganglia along the gangliated cords and within various plexuses of the sympathetic. A longitudinal section of a spinal ganglion (Fig. 852), which may be taken as a type of such collections, shows the entire ovoid mass to be enclosed by a fibrous capsule continuous with that ensheathing the nerves. Immediately beneath the capsule the ganglion-cells are arranged in a fairly continuous layer of varying thick- ness, while the cells, more deeply placed, are broken up into groups by the tracts of iter-fascicular septum Transverse section of small splenic nerve consisting chiefly ol nonmedullated fibres. X 200. ioo8 HUMAN ANATOMY. intervening nerve-fibres, a small amount of connective tissue prolonged from the endoneurium of the nerve-bundles and accompanying the blood-vessels being also FIG. 852. Posterior root (sensory) Spinal cord Spinal ganglion FIG. 853. Nerve-fibres, cut transversely Nerve-cell Anterior division Section of spinal tierve, showiiiK its roots, ganglion, COmmon trunk ami primary divisions. X 10. present. The chief ganglion -cells are from .o6o-.o8o nun. in diameter, but some measure as much as .170 mm. and others as little as .025 mm. In sections (Fig. 853) they usually appear round or oval, since only exceptionally are their processes to be seen. Each cell is enclosed by a richly nucleated fn/>sn/<- which is continuous with the sheath of the nerve-tibiv>. M«.st of the many other oval nuclei that are conspicuous in sections of the ^aiiijia belong to tin- neurilemma of the fibres and. hence, are seen as chains ex« tending in different planes. Although many of the nerve-cells within the spinal ganglia are the cell bodies of the senary neurones, whose processes course as medullated fibres within the spinal nerves, many more are small cells, \\lu»e axoiu-s never acquire, a medullary coat and, dividing into peripheral and central branches, run within the trunks and posterior loots of the nerves as nonmedullated fibres. i largely on the behavior of their axones, Dnial^.m^li<-n, Ji-n.i, 1908. Dogiel describes eleven varieties of nerve-cells. mi. shf>\viiiK lid •. surrounded In nu> Icau-d capsules, x 300. DEVELOPMENT OF THE NERVOUS SYSTEM. 1009 FIG. 854. and has traced the nonmedullated fibres along the dorsal roots into the spinal cord. The presence of fibres probably derived from sympathetic neurones has been demonstrated. The sympathetic ganglia are represented by those of the great gangliated cords, certain cranial ganglia (ciliary, spheno-palatine, otic, and submaxillary), the ganglia within the three prevertebral plexuses, and the innumerable small and often micro- scopic ganglia associated with the muscular tissue of the digestive, respiratory and uro-genital tracts, in the heart and in the various glands. In their general structure the sympathetic ganglia are similar to those connected with the spinal nerves, forming definite masses enclosed by a fibrous capsule, from which connective-tissue processes pass into the interior of the ganglion for the support and separation of the nervous elements. The individual gangli- on-cells — unipolar, bipolar or multi- polar — are ensheathed by nucleated capsules continuous with the neuri- lemma of the nerve-fibres. The sympathetic ganglion-cells are vari- ously related to the terminal ramifi- cations of (#) other sympathetic neurones and of (£) the neurones of the central nervous system (by way of the white rami fibres or their equivalents). In both cases, the ramification of the nonmedullated and fine fibre in the one and of the medullated fibre in the other, a pericellular plexus, commonly en- closes the cell-body. In the lower vertebrates (amphibians and reptiles) , the spinal fibre frequently winds spirally around the single process of the ganglion-cell before breaking up into the pericellular plexus (Huber1). The broader relations of the component nervous elements of the spinal ganglia are considered in connection with the Sympathetic System (page 1354). DEVELOPMENT OF THE NERVOUS TISSUES. Reference to the account of the early development of the nervous system (page 26) will recall the fact that the neural groove, later the neural tube, is lined by invaginated and thickened ectoblast from which the essential nervous tissues are derived. For the fundamental facts concerning the histo- genesis of these tissues we are in large measure indebted to the labors of His, whose account, supplemented by the important contributions of Kolliker, Cajal, Lenhosse"k, Schaper and others, forms the basis of our knowledge concerning these processes. Although in its principal features the histogenesis is similar in all parts of the neural tube, in that portion which becomes the spinal cord the changes are most typical and will, therefore, be here described. During the approximation and closure of the neural tube the cells composing its wall undergo active prolife- ration, whereby the wall, at first composed of only one or two rows of definitely outlined cells, is converted into a multinucleated tract in which the cell boundaries dis- appear and the nuclei lie embedded within a general protoplasmic sheet or syncytium (Hardesty2). The large dividing elements within the latter, the germinal cells of His, are conspicuous on account of their mitotic Diagram of constituents of spinal ganglion ; blue lines repre- sent efferent fibres ; black, afferent ; red, sympathetic; a, sensory ganglion cells; c, cells of type II, whose axones end (d) around sensory cells ; d, sympathetic neurone ; AR, PR, anterior and posterior roots; AD, PD, anterior and posterior primary divi- sions of spinal nerve; RC, ramus communicans. FIG. 855. ihn Segment from lateral wall of neural tube of pig embryo of 5 mm. ; syncytium replacing distinctly outlined cells, a, inner zone; g, germinal cells; ilm, internal limiting mem- brane ; m, peripheral zone ; r, radial strands of cytoplasm. X 690. (Hardesty.) figures and are situated close to the lumen of the neural tube. His regarded them as special cells directly concerned in the production of the neurones, a conclusion, however, that has not 1 Journal of Morphology, 1899. * Amer. Journal of Anatomy, vol. iii. , 1904. 64 IOIO HI 'MAN ANATOMY. been sustained ( Kollike: and othersi since the primary germinal cells probably only represent proliferating elemen. in forming wliat for a time is an undinerentiated tissue. The cells composing the neural wall are at first in close contact, their blended cytoplasm ytiumj forming an almost unbroken sheet. Soon, however, this continuity is interrupted in ( onsequence of the longitudinal expansion of the tissue and the appearance of spaces, and the cell-substance is resolved into a delicate reticulum, the m\fh>sf>oHginm of His, which becomes condensed at the inner and outer margins of the wall of the neural tube into the internal and •ml limiting monln tint: The ni'-shes of the reticulum enlarge, the intervening nucleated tracts of cytoplasm elongate and the increasing nuclei become radially disposed. My reason uf these changes the elements next the lumen of the tutie assume a columnar form and radial arrangement and become the primary t-f>cndymul idls. The remaining elements, appropriately named the indifferent cells (Schaper }, increase in number in consequence of the continued division of the germinal cells and gradually - the nut 'It'ar /) the neuroblasts that are directly converted into the neu- rones. Within the resulting cell- complex that for a time occupies the greater part of the wall of the neural tube, it is difficult to distinguish with certainty between the neuroglia and neuron-producing elements, since both are often elongated in shape and prolonged into processes. Histogenesis of the Neuroglia. — In addition to the extension, condensation and moulding (by the developing nerve some not until after birth), a variation that is of much service in enabling the anatomist to trace the course of the individual paths of con- duction. The origin and method of formation of the medullary substance has been, and in fact still is, a subject of discussion. It is, however, certain that its production is not dependent upon the neurilemma, since the niedullated fibres within the •o-spinal axis are devoid of this sheath, and, further, that the myelin sometimes appears before the neurilemma ( Kol.ster, Hardeeii). \Vhileit is doubtful whether the myelin is directly formed from the outer part of the axis-cylinder, as suggested by Kolliker, it is probable that this structure exerts some influence resulting in the deposit of the myelin- droplets either from the blood iWIassak), or from the apparently fluid substance that after a time surrounds the axis-cylinder ( I'.ard.-. -n \. Regarding the formation of \\\^ framework supporting the droplets of myelin, Han!' inclines to the view that certain sheath cells, which appear during mednllation, are probably concerned. From the foregoing account it is evident that the axis-cylinder is derived from the ectoblast and the neurilemma from the ectoblast ; the origin of the medullary sheath is still unde- termined, but most probably is also ectoblastic. Development of the Ganglia. — The < irigin t >f the afferent (sensory) neurones, whose cell-bodies are situate. 1 within the spinal and other ganglia, is entirely dim-rent from that of the efferent ( motor) ones above described. In the • of the spinal nerves, the di-vlopment of the ganglia pro- •oup of ectoblastic cells that form a ridge, the ganglion-erest, <>n the margin of either lip of the still open neural ti sr»), just where the general ectoblast M into that lining \\ "i approximation of the lips of the latter, the cells of the ganglion-crests fuse into I Mpe.l mass that completes tin- closure of the 1 tub.- ami constitutes a centre of proliferation from which the cells migr.it.' outward over the dorso -lateral wall of the tul»-. 111. prohter.ition is not uniform but most marked at points that correspond to the mesoblastic somites, in consequence of which a series of segmentally ins appears on . •.». li side of the !-•( tions are the ai the spinal ganglia. Within them certain cells soon become fusiform and, assuming the role of TJeuroM.ists, send out ftp) .ill either end. One process the axone grows Centrally, while the other the <|.-n.!rite extend- peripherally and becomes the chief part of ft sensory nerve-fibre. The subsequent growth of the neurone is not symmetrical, but to one side, and so Transverse sections of dorsal r ^ion of human i-mlnwis, shnum- .-.ul\ iliffcr eiitiali.ni »t "-piiKil K.iiiKlion ; A. //, neural tut«- -til! opni; < '. /;. tub,- closed ; <7, ganglion-ridges; A, fused mini's; <-, out- Riouth to form '-Million ; rf, ectoblast. X 230. (Ltnkos*- 1 Amer. Journal of Anatomy, vol. jv., 1905. DEVELOPMENT OF THE NERVOUS TISSUES. 1013 Cross-section of part of dorsal region of human embryo, showing developing spinal ganglion; dz, vz, mz, dorsal, ventral and marginal zones of spinal cord ; dr, vr, dorsal and ventral root-fibres of spinal nerve (n) ; sg, spinal ganglion on dorsal root. X 85. ordered that the two processes are approximated and finally joined to the cell-body by a common stalk (Fig. 839), the neurone being thus converted into an unipolar ganglion-cell. The centrally directed processes, the later posterior root-fibres of a spinal nerve, grow into the develop- FIG. 861. ing cord and enter the peripheral zone (later the white matter) to end, when their development is completed, at various levels in relation with neu- rones formed within the neural axis. The peri- pherally directed processes of the spinal sensory neurones, on the other hand, mingle with the axones from the motor neurones to form the mixed nerves distributed to the various parts of the body. The essential parts of the sensory neurones, the cell-body and the processes, are derived from ectoblastic elements, as well as the sheaths of the fibres, while the sheath of the entire ganglion is contributed by the mesoblast. The development of the sympathetic ganglia, which include essentially three sets — those of the gangliated cords, those of the prevertebral plexuses (cardiac, solar and hypogastric ) , and the terminal — has given rise to much discussion. According to one view, the sympathetic neurones have an independent origin and only secondarily form con- nections with the cerebro-spinal nerves. The other view, on the contrary, regards the sympathetic neurones as the direct descendents of neurogenetic elements derived from the developing spinal nerves. The evidence in support of the last view is so convincing that there is little question as to the correctness of its principle, although many details of the process, as relating to man, are still to be studied. It is, however, equally true that the sympathetic ganglia are neither produced by constriction and isolation of parts of the spinal ganglia, as sometimes assumed, FIG. 862. nor by tne migration of fully differentiated ganglion - cells, but, as emphasized by Neu- mayer, from undifferentiated neuroblasts which undergo in loco their development. The earliest suggestions of definite sympathetic ganglia in the human embryo appear about the beginning of the second ^^t'5S^_ foetal month as aggregations of 5», k^5 ce^s at the distal ends of the \ \V £ visceral rami of the developing '"•"'*? ^v spinal nerves. From these cells ; '.;.' > are derived the definite sympa- ^RH (•: • •'•'* thetic neurones < >f the gangliated "J^''^ /)/ cord, as well as those which &£:3i £•'' follow the mesial ingrowth of the spinal fibres for the pro- duction of the prevertebral and terminal ganglia. The lateral ganglia thus formed constitute for a time a series of isolated nodes ; subsequently these are connected bv the differentiation of sympathetic axones which grow from one ganglion to the next and, in conjunction with the spinal fibres, establish the longitudinal commissural strands of the gangliated cord. Other sympathetic cells send axones centrally and give rise to the efferent splanchnic nerves, whilst the axones of still others pass to the growing spinal nerves, " f v , ..;---,„..,. • Spina ganglia Sagittal section of rabbit embryo showing several developing spinal ganglia and nerve-trunks ; A, aorta; S, intersegmental artery. X 52. HUMAN ANATOMY. NERVE-TERMINATIONS. The terminations of the fibres composing the peripheral nerves— the axones of certain mot. ,r neurones situated within the crrcbro-spmal axis and the sympatt •: the neurones of the sensory ganglia— supply the means bv which the various structures < ,i the body are brought into intimate relation with the nen -"in. Some of these terminations transfer impulses resulting in muscular contractions; others convey impressions that produce various sensations (pain, pressure, muscle-sense, Kic;. 863. . temperature). The nerve- Nerve A terminations, therefore, may be grouped according to func- tion into motor and sensory endings. MOTOR NERVE-ENDINGS. The motor endings in- clude (a) terminations of the axones of neurones situated within the motor nuclei of the spinal cord and brain- stem that pass to voluntary muscle ; (£) terminations of sympathetic neurones that end in involuntary muscle and in cardiac muscle. Endings in Voluntary Muscle. — On approaching their peripheral destination --l-^Eiid- the medullated nerve-fibres branch repeatedly, each fibre in this manner coming into relation with a number of mus- Motor nerve-endings in voluntary muscle; bundle of nerve-fibres is , H!, ,.,..- Wk,™ fl,^ A •en separating to supply th< cle-nbres. When the med- ullated nerve-fibre reaches the muscle-fibre which it supplies, its medullary sheath abruptly ends and the neurilemma becomes insepara! ly fused \\ ith the sarcoh inina, whilst the a xis-cy lin'der passes beneath this sheath to terminate in an cnd-platf. The latter appears as an oval area, from .O4o-.o6omm. in it ' diameter, which is applied to the inns. le-Mibstance ; in profile it shows a slight FIG. 864. projection beyond the contour of the muscle-fibre, although this i> often wanting. Embedded within ral mi. leated sheet of granular protoplasm, the sotc-f>/iift\ lie the brush-like terminal arbori/atioiis of the vlinder formed of irregular varirosites and club- shaped ends. I-'roin the details of the development of the motor end plates, as d. -scribed by I'.ardeen, it is £*-y ^ probable that the granular sole-plate and its nuclei are differentiated from the sarcoplasm and the nuclei of the ''\.*fr^ muse le hbi . iv^.rciively. The much discnsscd relation of tin- end plate to the sarn >lrmma — whether • mtsideor •h serins to be derided in favor of a subs., lemmal position, sinci- the musrlr-sheath appears sub- ,^ srqnentlv to the formation of the motor ending, a fact , ' . . Motor nerve-ending iti voluntary that explains the apparent piercing ot the san olemma muscle; a, axone u-rnimatmK in i-nd- bvthraxi, rylind.-r. Fsnallv earh musdr -fibre is pro- '.''^ "• n< ll<-'"": >'lale- videil u ith a single motor end- jilate, which may lie at an J or um-(|ual distance from the ends of the fibre. Exci-ptionally two end-plates louinl on one muscle-fibre. \\\ uhich case the endings lie near each other. NERVE-TERMINATIONS. 1015 Endings in Involuntary Muscle. — The terminations of the axones of the sympathetic neurones supplying the nonstriated muscle are comparatively simple. The neurones contributing the immediate fibres of distribution usually occupy the nodal points of plexuses from which bundles FIG. 865. of nonmedullated nerve-fibres extend to and enclose the muscle fasciculi. Entering the latter the nerve-fibres divide into delicate varicose threads that pass between the muscle-cells, parallel with their long axes. As they course within the intercellular substance, the varicose fibrils give off short lateral branches that end, as does also the parent fibre, in minute terminal knots on the surface of the muscle-cells, often in the vicinity of the nucleus. Probably by no means every muscle- cell individually receives a nerve-ending, a longitudinal group including three or four rows of muscle-cells lying between two adjoining terminal nerve-fibrils (Huber). Endings in Cardiac Muscle. — These, also the termi- nations of sympathetic neurones, have been studied by, among .1 /"• • i V. • TT i i r- . J , • Nerve-ending in involuntary others, Lajal, Ketzius, Huber and Smirnow. According to muscle. (Huber.) the last-named investigator, the varicose nerve-fibrils may be followed between the muscle-cells, during which course side branches arise that, as well as the main fibril, terminate on the muscle elements in endings of varying com- plexity. In some cases these are merely minute simple end-knots, resembling those found in involuntary muscle ; in other cases they are more elaborate and consist of a group of secondary fibrillae bearing nodular endings, the whole recalling somewhat the motor end-plates in striped muscle. It is probable that most of the cardiac muscle-cells are in direct relation with nerve-endings (Huber). SENSORY NERVE-ENDINGS. Since the sensory endings are the peripheral terminal arborizations of the neurones whose cell-bodies lie in the spinal and other sensory ganglia, such teloden- dria are functionally the beginnings of the paths conducting the sensory stimuli to the central nervous system. According to their relations to the surrounding tissue, the sensory endings are broadly grouped into free and encapsulated. Free Sensory Endings. — These endings include vast numbers of nerve- terminations found in the skin and the mucous membranes, chiefly within the epithelium but to some extent also within the connective tissue strata. As a rule the sensory (afferent) nerve-fibres do not branch to any extent until near their peripheral destination, where they undergo repeated divisions, always at a node of Ranvier and in various directions. The medullary sheath of the main fibre is retained until close to its termination, although some of its branches may course as nonmedullated fibres for a considerable distance before ending or entering the epithelium. In the skin — and the same general plan applies to the mucous mem- branes— the fibres destined for the epidermis lose their myelin coat beneath the basement membrane and enter the epithelium as vertically coursing nonmedullated fibrils. Within the epidermis they break up into numerous delicate fibrils which undergo further divi- sion into still finer varicose threads that ramify between the cells of the stratum germinativum and terminate in minute free end-knobs (Fig. 866). Although an intracellular position of these nerve- endings has been described by various writers, it is probable that the endings are extracellular and lie upon the surface of and not within the epithelial Free sensory ending within epidermis elements. Similar, but far less numerous, free end- of rabbit; in several places nerve-fibrillae Jnp-s variCOse and club-like in form, OCCUr within terminate in end-knobs. (Dogiel.) . . . c i i • j *.t_ the connective tissue layers of the skin and the .tunica propria of mucous membranes. Within the integument, conspicuous end- ramifications of sensory neurones surround the hair follicles, lying upon the outer surface of the glassy membrane. ioi6 HTM AN ANATOMY. Fie. Tactile cells of Merkel lyin« within inter- .ry epithelium; broken line (f) indicates junction ni i-|.iUiL-lium and connective tissue layer; (H I nerve passing into epithelium. X 160. (H'orthmann.) FIG. 868. The tactile cells of Merkel, found in the deeper layers of the epidermis, represent a somewhat mure differentiated form of intraepithelial terminations and suggest transitions to the more specialized end- organs. In these endings the nerve-fibrils terminate in cup-shaped expansions or menisci, against which rest the modified epithelial cells. The latter may be regarded as an imperfectly differentiated ne*roeptiheliumt examples of which are seen in the gustatory cells in the taste buds and in the highly specialized visual and auditory cells in the retina and in the organ of Corti respectively. Encapsulated Sensory Endings. — In their most highly developed forms these end- ings (corpuscula nervorum terminalia) are represented by relatively large special end- organs in which the terminations of the axis- cylinder are enclosed within an elaborate laminated capsule. The latter, however, is more often present as a much simpler and thinner envelope consisting of strands of fibrous tissue. Transition forms between the intraepithelial tactile cells above noted and the more specialized encapsulated end-organs, always within the connective tissue, are seen in the corpuscles of Grandry (not found in man but conspicuous in the skin covering the bill and in the tongue of many water-fowl), in which the nerve ends in a disc-like expansion enclosed between large modified epithelial cells and the neuromuscular and neurotendinous end-organs, presently to be described (page 1020). The group of simpler encapsulated endings includes three well-known examples : the end-bulbs and the genital corpuscles of Krausc and the cor- pmc/es of Meissner, all of which possess a common structural plan — interwoven telodendria embedded within a semifluid interfibrillar substance and surrounded by a thin fibrous envelope. The End-Bulbs of Krause.— These endings include a variety of irregularly spherical or ellipsoidal bodies found in the edge of the eyelid, the conjunctiva and corneal margin, the lips and the oral mucous membrane, the glans penis and clitoridis and probably other parts of the integument highly endowed with sensil>ilitv. Within the conjunctiva, as described by Pogiel1. they lie superficially placed within the con- nective tissue near the summit of the papillae and folds, when such elevations exist, but always close beneath the epithelium. They vary considerably in si/c, often being small (.002-. 004 mm. ), but some- times measuring from .o$-.io mm. in diameter. I ">uallv a single nerve-fibre, exceptionally two or even m..re. enters each bulb, losing its medullary sheath as it pierces the thin fibrous capsule. Within the latter tile nei vc, now represented by the naked axis-cylinder, divide-, int.. from two to four branches, which, after ril-ing several annular or spiral turns, give off fibrils that undergo further division, the terminal threads forming a more or • Intricate m.t/e within the semifluid substance enclosed by the fibrOUfl capsule. 1 An liiv f inik. An. it., Hd. xliv., 1895. TWO corpuM-u-s repeated divisions, tin- resulting fibrillae becoming varicose- and inu-rt wined and ending in free terminal knob-like enlargements. In contrast to the foregoing end-organs, in which the axis-cylinder subdivides into numerous terminal threads disposed as more or less elaborate intertwinings, :ul group is distinguished by the possession of a thick laminated capsule that encloses a cylindrical core or inner hulb containing the slightly branched axis-cylinder. < -Tidings, of which the Pacinian corpuscle is repre- sentative, an- relatively large and ellipsoidal. A transitional form, connecting them with the spherical end-bulbs, is presented by the cylindrical end-bulbs of Krause. These are found in various parts of the corium, the oral mucous membrane and between the bundles of striped muscle and of tendon. They are irregularly cylindrical in form, often more or less bent, and consist of a thin laminated capsule that encloses a core of semifluid substance in which lies the centrally placed axis-cylinder. The latter, after losing the nn-dullary sheath on entering at the proximal end of the capsule, traverses the core without branching until near the distal pole, where it ends in a single or slightly subdivided terminal enlargement The Vater-Pacinian Corpuscles. — These structures, the most highly special- ized sensory end-organs, are relatively large ellipsoidal bodies, from .5-1.5 mm. in length and about one-third as much in breadth, situated within the connective tissue in many parts of the body. In man they are found in FIG. 874. the deeper layer-, of the connective tissue layer of the skin, especially on the palmar and plantar aspects of the fingers and toes, in the connective tissue in the vicinity of the joints, in tendi.ns, in the sheath of muscles, in the periosteum and in the tunica propria of the serous membrane-,. the peritoneum, pleura and pericardium. They are particularly large in the mesentery of the cat, where they may be readily de- •1 with the unaided eye .il pearly bodies smne- times two millimeters or more in length. The most conspicuous part of the Pacinian body is the mlnist ctipsu/i- that litutes almost the en- tire bulk of the corpuscle ami < • insists of from one to three do/en thin con- centric lamell.i- of til'- tissue. The surfaces of tile i.unell.r are ic. \-ered with endothelial plates whose nuclei appear as fusiform thicken- ings, along t: :.tric stri.e of the corpuscle. The axis of the Pacinian body •\ verso section ; *, nerve eiiti-iiiiK i:i|>Milr to n-:u h inner b aneit ulb. NERVE-TERMINATIONS. 1019 FIG. 875. Corpuscles of Herbst from bill of duck ; a, longitudinal, b, transverse section ; n, nerve traversing lamellae of capsule ; axis-cylinder within core is surrounded by cells. X 360. is occupied by a core or inner bulb of semifluid substance in which the naked axis-cylinder is embedded. On joining the proximal pole of the corpuscle, the fibrous (Henle's) sheath of the nerve-fibre blends with the outer lamellae of the capsule, while the medullary coat is retained during the somewhat tortuous path of the fibre through the capsule as far as the core. Here the remaining envelope of the nerve-fibre disappears, the terminal part of its course, through the core, being as the naked axis-cylinder. At a variable distance but often just before gaining the distal pole of the core, the axis- cylinder divides into from two to four branches, each of which terminates in a slightly expanded end-knot. Some- times shortly after penetrat- ing the capsule, the nerve- fibre splits into two or more axis-cylinders which then share the common envelope of semifluid axial substance. Similar end-organs, the corpuscles of Herbst, occur in the velvety skin covering the bill and in the tongue of water-fowl. They closely resemble the Pacinian bodies of mammals, but differ in being generally smaller, relatively broader, and in exhibiting a row of cubical cells within the core and around the axis-cylinder. These cells are regarded as corres- ponding to the large cells enclosing the tactile discs in the Grandry's corpuscles. The Golgi-Mazzoni corpuscles, found in the subcutaneous tissue of the pulp of the fingers, are modifications of the ordinary Pacinian end-organs. They differ from the latter in possessing fewer lamellae, a relatively larger core and a more branched axis-cylinder. Neuromuscular Endings. — First described by Kolliker and by Kiihne, although previously seen by Weissmann, these end-organs, often termed muscle- spindles, are now regarded as sensory endings that are probably concerned in afford- ing impressions as to tension or "muscle-sense". They lie within the connective tissue separating the bundles of voluntary muscle-fibres and are long spindle-shaped structures, varying in length from 1-5 mm. or more and in width from .I-.T, mm. where broadest. They are widely distributed, being probably present in all the skeletal muscles, and are especially numerous in the small muscles of the hand and foot. They are uncertainly found, however, in the intrinsic muscles of the tongue and in the eye muscles, although within the tendons of the latter very similar (neuro- tendinous} end-organs have been demonstrated. Each spindle consists of a capsule, composed of a half-dozen concentric layers of fibrous tissue, which encloses a group of usually from three to ten, but sometimes as many as twenty, striped muscle-fibres, medullated nerves, blood-vessels and inter- spersed connective tissue. These intrafusal fibres, as. they are called, differ from those of the surrounding muscle in being much smaller in diameter and length, markedly tapering towards either end, more coarsely but less distinctly striated, and in possessing nuclei within the sarcous substance. The striations are not equally distinct in all parts of the fibres, being much less evident in the middle zone than towards the ends. The fibres are more numerous and of greater diameter in the equatorial region than near the poles of the spindle. The intrafusal fibres collectively are surrounded by a thin special connective tissue envelope, the axial sheath, between which and the capsule lies the periaxial 1020 HTM AN ANATOMY. Nerve-fibre • Capsule lymph-space. Each spindle receives usually several medullated nerve-fibres, which, after incorporation of their sheaths of Henle with the capsule, pierce the latter at various points and proceed to the individual muscle-fibres. The terminal relations of the nerves to the intrafusal fibres have been studied by means of the newer methods especially by Ruffini, Huber and DeWitt and Dogiel. After repeated division during their course through the cap- sule and periaxial space, the nerve-fibres pierce the axial sheath, lose their medullary coat and terminate either as one or more ribbon-like branches that encircle the mus- cle-fibres in annular or spiral windings, or, after further subdivision, as branched telo- dendria in which the ultimate fibrils end in irregular spherical or pyriform enlargements. Neurotendinous End- ings. — These end-organs, described by Golgi and sub- sequently more fully investi- gated by Kolliker, Ciaccio, and Huber and DeWitt, in their general architecture resemble closely the sensory endings in muscle. They lie embedded within the interfascicular con- nective tissue and are usually found in the vicinity of the junction of muscle and tendon. Like the neuromuscular end- ings, the tendon- spindles are long fusiform structures, from I. -1. 5 mm. in length, sur- rounded by a fibrous capsule. The latter encloses a group of from eight to twenty intrafusal tendon fasciculi, which are smaller and apparently less mature than those of the sur- rounding tendon-tissue. ' The intrafusal fasciculi are invested by a film .us axial sheath be- t\v.-rn which and the capsule lies a periaxial lymph-space. On reaching the spindle, after repeated branching, the medullated nerve-fibres pene- trate the capsule, with which their fibrous ( Henle's) sheaths Mend, and undergo further The medullary mat is lost alter they pierce the axial sheath, the naked axis- up mll> *i»-'iller fibrils that extend alon^ the intrafusal fasciculi. The Nerve-fibre riirolcMclin :\ loiiKitllfli- -•'«. (Drawn ln.in pit- pa rat ion • r llui division, cylinders terminal ramifications, applied to th<- surface of the fasciculi, vary in details (Huber). SOUK- arise as short lateral branches that partly encircle the fasciculi and end in ular plate-like expansions, while others terminate between the smaller fasciculi. THE CENTRAL NERVOUS SYSTEM. THE central nervous system includes the spinal cord and the brain. In principle these parts are to be regarded as the walls of the primary neural tube, modified by unequal growth and expansion, which even after acquiring their definite relations enclose the remains of the canal, as represented by the system of ventricular spaces. In contrast to the spinal segment of the neural tube, which always remains a rela- tively simple cylinder, the spinal cord, the cephalic segment early differentiates into three primary cerebral vesicles, the anterior and posterior of which subdivide, so that five secondary brain-vesicles are present. Coincidently marked flexure of the cephalic segment occurs at certain points and in consequence this part of the neural tube becomes bent upon itself to such a degree that the axis of the anterior vesicle lies almost parallel with that of the spinal segment (Fig. 912). From the five secondary divisions of the flexed and sinuously bent cephalic segment of the neural tube are developed the fundamental parts of the brain in the manner presently to be described (page 1060), whilst from the relatively straight spinal segment proceeds the development of the spinal cord, in which process growth and differentiation convert the originally thin-walled tube into an almost solid cylinder, the minute central canal alone remaining as the representative of the once conspicuous lumen. THE SPINAL CORD. The spinal cord (medulla spinalis) is that part of the central nervous system, or cerebro-spinal axis, which lies within the vertebral canal. Its upper limit, where it becomes continuous with the medulla oblongata, is in a measure conventional, since there is no demarcation on the cord itself to indicate exactly its junction with the brain. Accurately considered, the superior limit of the cord may be assumed to correspond with the emergence of the uppermost root-fibres of the first spinal nerve which pass out between the atlas and the skull ; this level also corresponds to the lowest strands of the pyramidal decussation of the medulla oblongata and to the upper border of the posterior arch of the atlas. For practical purposes, however, the lower margin of the foramen magnum defines with sufficient accuracy the upper limit of the spinal cord. Below, the spinal cord terminates somewhat abruptly in a pointed end, the conus medullaris, that usually ends opposite the disc between the first and second lumbar vertebrae. The level to which the cord extends inferiorly, however, is subject to considerable variation, very rarely being as high as the middle of the body of the last thoracic vertebra (Moorhead), or as low as the upper border of the body of the third lumbar vertebra (Waring). In the female subject the spinal cord, although absolutely shorter than in the male, extends to a relatively lower level in the vertebral canal. Marked bending of the spine produces slight alterations in the position of the cord, during strong flexion an appreciable ascent of the lower end taking place. The relation of the cord to the vertebral canal varies at different periods. Until the third month of foetal life the cord occupies the entire length of the canal, but subsequently, owing to the more rapid lengthening of the spine than of the spinal cord, the latter no longer reaches to the lower limit of the canal and, therefore, apparently rises, so that by the sixth foetal month the lower end of the cord lies opposite the first sacral vertebra, and at birth terminates usually on a level with the body of the third lumbar vertebra. Measured from its upper conventional limit to the lower end of the conus medullaris, the spinal cord in the adult male has an average length of 45 cm. (17^ in.), and in the female of 43.7 cm. (i7>4^ in.), in both sexes the proportion of the length of the cord to that of the pre- sacral spine being approximately as 64 : 100 (Ziehen). The cord-length bears no constant rela- tion to stature, although in a general way tall individuals may possess long cords. The weight of the spinal cord, stripped of its membranes and nerves, is something less than 30 grammes (i oz.), or about 1-2000 of the body-weight. Its proportion to the weight of the brain is i :43. When fresh the spinal cord possesses a soft cheesy consistence and a specific gravity of 1035. IO22 Ht'.MAN ANATOMY. Medulla Laminae, cut Transverse processes .. ,J»- Pedicla 'ural sheath XII 1 Pedicles Sheath <>( filum Ki dura) sheath 5SH tilum 1 1:1 it t>\ It .ral shrath lyini; within last The Membranes of the Cord.— The spinal cord, together with the roots of the thirty-one pairs of spinal nerves, lies within the vertebral canal enclosed by three pn Meeting membranes, ormeninges, which, from without inward, are (i) the dura tnattr, ( 2 ) the aniclinoidca, and (;, i the pia inattr, all of which are directly continuous through the foramen magnum with the corres- ponding coverings of the brain. The external sheath, or thcca, formed by the dura, is a robust fibre-elastic tubular -nvelope, much longer and considerably wider than the cord, that does not lie against the wall of the vertebral canal, but is separated by an interval containing thin-walled plextform veins and loose fatty con- nective tissues ( Fig. ^79). The dural sheath, about .5 mm. in thickness, extends to the level of the second sacral vertebra and is, therefore, considerably longer than the spinal cord. The part of the sac not occupied by the cord encloses the longitudinal bundles of root-fibres, that pass obliquely to the levels at which the correspond- ing nerves leave the vertebral canal, and a fibrous strand, the_/r7/^w tcr- >ni)uil(\ prolonged from the cord to the lower end of the spine. The pia constitutes the imme- diate investment of the cord and supports the blood-vessels destined for the nutrition of the enclosed nervous cylinder. The pial sheath is composed ,,f an outer fibrous and an inner vascular layer, the connective tissue of the latter ac- companying the blood-vessels into the -nbstance of the cord. The arachnoid, a delicate veil- like structure made up of interlacing bundles of libro-elastic tissue, lies between the other two membranes and imots loosely the inner surface of tin- dura and closely the outer surface of the pia. It effectually subdixides the con>idcrable space between the external and internal sheaths into two compartments, the one beneath the dura, the snbditral s/>aff, beini^ little more than a capil- lary cleft tilled with modified lymph, and the other, the subareuJmtrid space, between the arachnoid and THE CENTRAL NERVOUS SYSTEM. 1023 The spinal cord, therefore, hangs FIG. 878. • Pons •Arachnoid Medulla the pia, containing the ccrcbro-spinal Jluid. suspended within the tube of dura, surrounded by a cushion of fluid an arrangement well adapted to insure the nervous cylinder against the inju- rious effects of shocks and of undue pressure during changes in the position (- of the spine. Both spaces, but par- ticularly the subarachnoid, are crossed by fibrous trabeculae and thus imper- fectly subdivided into secondary com- partments, all of which are lined with endothelium. The spinal cord is fixed within the loose dural sheath not only by the root- fibres of the spinal nerves that pass between the cord and the outer envelope, but also by two lateral fibrous bands, the ligamenta denticulata, that are continu- ous with the pia along the cord, one on each side. Mesially they are attached between the anterior and posterior root- fibres and externally to the inner surface of the dura by the tips of pointed pro- cesses, about twenty-one in all, that stretch across the subarachnoid space, which they imperfectly divide into a general anterior and a posterior com- partment. The ligaments, covered by prolongations of the arachnoid, extend the entire length of the cord, the first pro- cess being attached to the margin of the foramen magnum, immediately above the vertebral artery as it pierces the dura. The succeeding ones meet the dura between the pairs of spinal nerves, the lowest process lying between the last thoracic and the first lumbar nerve. In the cervical and thoracic region, a median fibrous band, the septum posticum, connects the posterior surface of the cord FIG. 879. Anterior roots of spinal nerves ., Dura. reflected \ Spinal cord, covered with v '. ,' arachnoid . • * and pia Upper part of spinal cord within dural sheath, which has been opened and turned aside ; ligamenta denticulata and nerve-roots are shown as they pass outward to dura. Dural sheath Periosteum Spinal cord Posterior root Ligamentum denticulatum Extradural areolar tissue \nterior root Spinal ganglion Spinal nerve Vertebral artery . Body of fourth cervical vertebra - Transverse section of vertebral canal at level of fourth cervical vertebra, spinal cord in position. with the dura and partially subdivides the subarachnoid space. Lower, this partition, IO24 HUM AX ANATOMY. FIG. 880. Skull Vertebral artery Spinal accessory nerve Pedicles, cut Mes oval ; its width is not uniform on account of two conspicuous swellings that are associated with the origin and reception of the large nerves supplying the limbs. The upper or cervical enlargement ( intumescentia ccrvicalis • begins just below the upper end of the cord and ends opposite the second thoracic vertebra, having its greatest expansion at the level of the fifth and sixth . ieal vertebra-, where the sagittal diameter is about 9 mm. and the transverse from 13-14 mm. The lower or lumbar enlargement (intumescentia lumbalis) begins opposite the tenth thoracic vertebra, slightly above the origin of the first lumbar nerve, and fades away in the con us medullaris below. It appears very gradually and reaches its maximum opposite the twelfth thoracic vertebra, where the cord has a sagittal diameter of 8. 5 mm. and a transverse diameter of from 11-13 mm. (Ravenel). The lumbar enlargement is associated with the great nerve-trunks supplying the lower limbs. The inter- vening part of the thoracic region is the smallest and most uniform portion of the cord and is almost circular in out- line. Where least expanded, opposite the middle of the thoracic- spine, the cord measures 8 mm. in its sagittal and 10 mm. in its transverse diameter. These enlarge- ments appear coincidently with the formation of the limbs, an- relatively small during foetal life, and acquire their full dimensions only after the limbs have attained their definite growth. In a general way, a similar relation between the size of the enlargements and the degree of development of the limbs is observed in the lower animals. At the tip of the conus medullaris the spinal cord is prolonged into a delicate tapering strand, the filum terminale, that consists chiefly of fibrous tissue con- tinued from the pia mater and invested by arachnoid. It extends to the bottom of the pointed and closed end of the dura! sac, which it pierces at tin- level of the second sacral vertebra and, ensheathed by a prolongation of dura -inn li-nninn/is), as the fihtni terminate cxternnni, proceeds downward through the lower end of the sacral canal for a distance of about 8 cm. (3^ in.), finally to be attached to the periosteum covering the posterior surface of the coccyx. The part within the dural the fi/nm terminate internnm, is about 16 cm. (6# in.) in length and surrounded by the nerve-bundles of the cauda e-iuina .Fig. 8S2), from which it is readily dis- tinguished by its -listening silvery appearance. Thoracic 1 iu- upper IL ilf <>r less ,,f the internal filum contains the terminal part of the o-ntral r.m.il of the spinal cord walled by a thin and variable layer of nervous substance in which small ••IK are usualK pn-s.-nt. The minute 1. undies of nerve- fibres often found adhering to the filum, which sometimes may be followed to and even through the dural sheath, are re-arded by Kauber as representing one or two additional (second and third) s, homologous with the caudal nerves of the lower animals. ! ilcllinlcil .tli • rmtrTbuted • • THE CENTRAL NERVOUS SYSTEM. 1027 The Columns of the Cord. — Inspection of the surface and particularly of cross-sections of the spinal cord (Fig. 885) shows the latter to be partially divided into a symmetrical right and left half by a median cleft in front and a partition in the mid-line behind. The cleft, the anterior median fissure (flssura mediana anterior) extends the entire length of the cord, and is continued on the upper part of the filum terminale. It is narrow, from 2-3.5 mm- in depth, penetrating for less than one-third of the ventro-dorsal diameter of the cord, and occupied by a process of pia mater. Along its floor, which lies immediately in front of the white commissure, it is frequently deflected to one side of the mid-line and presents a slight expansion. The separation into halves is completed by the posterior median septum (septum medianum posterius), the so-called posterior median jftssure. With the ex- ception of a shallow groove in the upper cervical cord, the lumbar enlargement and the conus medullaris, no fissure exists, but in its place a dense partition extends from the posterior surface to the middle of -the interior of the cord, ending in close relation to the gray commissure. The character of the septum is a subject of dispute, according to some anatomists con- sisting exclusively of condensed neuroglia, while others regard it as composed of pial tissue blended with the neuroglia and, therefore, of both mesoblastic and ectoblastic origin. The latter view is substantiated by the mode of development of the posterior septum, the immature pial covering of the developing blood-vessels being imprisoned within and fused with the neu- rogliar partition derived from the expanding dorsal halves of the developing cord (page 1050). The application of differential stains also demonstrates the composite nature of the septum. Each half of the spinal cord is further subdivided by the lines along which the root-fibres of the spinal nerves are attached. The root-line of the dorsal (sensory) fibres is relatively straight and narrow, and marked by a slight furrow, the postero- lateral sulcus (sulcus lateralis posterior) that lies from 2.5-3.5 nim- lateral to the posterior septum and is evident even on the intersegmental intervals where the root- fibres are practically absent. The ventral root-line, marking the emergence of the anterior (motor) fibres, is much less certain, since the bundles of fibres of the indi- vidual nerves do not emerge in the same vertical plane, but overlie one another to some extent, so that each group occupies a crescentic area, whose greatest width cor- responds in a general way with that of the subjacent ventral horn of gray matter. The anterior root-line, which lies from 2-4 mm. lateral to the median fissure, is neither indicated by a distinct furrow nor con- FIG. 884. tinuous. ^fi^^^rttttfadRw^fe^P -^ In this manner two longitudinal tracts, the posterior columns (funiculi posteriores) are marked off between the posterior median septum and the sulci of the posterior root- lines. These columns include something less than one-third of the semi-circumference of the cord, and are -ibout 6 mm. in width in the thoracic cordand 8mm. and 7 mm. in the cervi- cal and lumbar enlarge- ments respectively. The tracts included -Medulla Cut pedicle of third cerv. vertebra Ganglion on 4 nerve i of 5 cerv. nerve Upper end of spinal cord, viewed from behind after partial removal of dural sheath ; cord-segments are indicated by groups of converging bundles of posterior root-fibres; spinal ganglia are seen lying within the mtervertebral foramina; spinal accessory nerve is seen ascending on each side. between the dorsal and ventral root-lines constitute the lateral columns (funiculi laterales) and those between the ventral root-lines and anterior median fissure are the anterior columns (funiculi anteriores). Such subdivision into anterior and lateral 1028 HUMAN ANATOMY. columns is, however, largely artificial, since neither superficially nor internally is there a definite demarcation between these tracts. They may be, therefore, conveniently regarded as forming a common antero-latcral column, that on each side embraces something more than two-thirds of the semicircumference of the cord. In the lower cervical and upper thoracic cord, each posterior column is subdivided by a shallow furrow that lies from 1.5-^ mm. lateral to the posterior medium septum. This, the paramedian sulcus i sulcus inter medius posterior;, corresponds in position with the peripheral attachment of a radial septum of neuroglia that penetrates the white matter for a variable distance, sometimes almost as far as the gray matter, and subdivides the posterior column into two unequal tracts, of which the inner and smaller is the pos- tero-median column ( fasciculus gracilis), or column of Goll, and the outer and larger is the postero-lateral column ( fasciculus cuneatus), or column of Burdach. The Gray Matter. — Inspection of the transversely sectioned spinal cord, even with the unaided eye, shows it to be composed of an irregular core of gray substance enclosed by a mantle of white matter. Within each half of the cord the gray FIG. 885. -Posterior median septum asterior column ./Posterior root-furrow Caput cornu- , iteral column Cervix cornul .Posterior root-fibres T_ EaSa Central canal Lateral cornu ~^B :4 — in gray commissure Basis cornu Caput comu Anterior median fissure Anterior column Anterior white commissure Transverse section of thoracic cord, showing disposition of gray and white matter and division of latter into :mti-ii.ii l.iti t.il and posterior columns. X 13. matter forms a comma-shaped area, the broader end of which lies in front and the narrower behind, with the concavity directed laterally. The convex surfaces of the tracts of the two sides, which look towards each other and the mid-line, are connected by a transverse band of gray matter, the gray commissure ( commissura »risea) that extends across the mid-line, usually somewhat in advance of the middle of the sagittal diameter, and encloses the minute central canal of the cord. By this canal the connecting band, or central gray matter, is divided into a dorsal and a ventral part, i\\t- posterior and the anterior ^ ray commissure, which lie behind and in front of the tube resprcti\ely. While the posterior median septum reaches the dorsal surface of the gray com- missure, the ventral margin of the latter is separated from the anterior median fissure by an intervening bridge of white matter, the anterior white commissure (com- iiiisstira aiiu-iitu alba < which connects the anterior columns of the' cord and provides an important pathway for fibres passing from one side to the other. A zone of mod- ified neuroglia immediately surrounding the central canal is known as the Substantia gelatinosa centralis < substantia THE CENTRAL NERVOUS SYSTEM. 1029 FIG. 886. 1C— Each crescent of gray matter is divisible into three parts — the ventral and the dorsal extremity, that project beyond the transverse gray commissure and constitute the anterior and posterior horns or cornua of the gray matter (columnae griseae), and the intermediate portion (pars intermedia) that connects the cornua and receives the commissure. The two horns differ markedly from each other and, although varying in details in different levels, retain their distinctive features throughout the cord. The anterior cornu (columna yrisca anterior) is short, thick and rounded, and separated by a considerable layer of white matter from the surface of the cord, through which the ventral root-fibres proceed to their points of emergence in the root-areas. The blunt tip of the anterior horn is known as the capnt cornu, and the dorsal por- tion by which it joins the commissure and the pars intermedia as the basis cornu. The posterior cornu (columna grisea posterior) presents a marked contrast in being usually relatively long, narrow and pointed, and in extending peripherally almost to the postero-lateral sulcus. The tip or apex of the dorsal horn is formed of a A -shaped stratum of peculiar character, the sub- stantia gelatinosa Rolandi, that appears lighter in tint (Fig. 885) and somewhat less opaque than the subjacent and broader portion of the horn, caput cornu, which it covers as a cap. More ventrally the posterior horn is usually somewhat contracted, to which portion the term, cervix cornu (cervix columnae posterioris) is applied. In the lower thoracic cord, however, this constriction is replaced by a slight bulging located on the mesial side of the junction of the posterior cornu with the gray commissure. This enlargement corres- ponds to the location of a longitudinal group of nerve- cells constituting the column of Clarke. The fairly sharp demarcation between the gray and white matter is interrupted along the lateral border of the crescent by delicate prolongations of gray matter into the surrounding lateral column (Fig. 888). The subdivisions of these processes unite to form a reticulum of gray matter, the meshes of which are occupied by longitudinally coursing nerve-fibres, the whole giving rise to an interlacement known as the processus or for- matio reticularis. Although to some extent present in the greater part of the cord, this structure is most marked in the upper cervical region, where it exists as a conspicuous net-work filling the recess that indents the lateral border of the pars intermedia and the neck of the posterior horn of the gray crescent. In the thoracic and upper parts of the cervical cord, therefore in regions in which the enlargements are wanting, the formatio reticularis is condensed into a compact process of gray matter that is directed outward (Fig. 885) and known as the lateral cornu (columna lateralis). IT— Taken as a whole, the gray matter, which in cross-sections appears as the H -shaped area formed by the two crescents and the commissure, constitutes a continuous column, whose irregular contour depends* not only upon the peculiar disposi- tion of the gray matter, but also upon the variations in its amount at different levels of the cord. Thus, at the level of the third cervical nerve the gray matter constitutes somewhat more than one-fourth of the entire area of the cord ; at that of the seventh nerve about one-third, while in the thoracic region, between the second and eleventh nerves, it is reduced to about one-sixth. At the last thoracic nerve it again forms one-fourth, and at the third and fifth lumbar two-fifths and three-fifths respectively. In the sacral cord the relative amount of gray matter increases until, at the level Diagram showing amount of gray and white matter in relation to entire area of cord, and relative lengths of cord-si-gments; the latter are indicated by divisions on left margin of figure— I C, I T, I L. I S, first segment of cervi- cal, thoracic, lumbar and sacral regions respectively ; dark zone next left bor- der represents the gray matter, light zone the white matter, outer dark zone the entire area of cord. (Donaldson.) io3o HUMAN ANATOMY. of the List sacra! nerve, it n-ach.-s three-fourths. The absolute amount of gray matter is greatest within tin- cervical and lumbar enlargements of the cord, where it is directly related to the large nerves supplying tli«- limbs. On comparing the tracts of white matter and the gray column it follows that while- in the lower third ot the lumbar cord these are of approximately equal area, below this level the -ray mat) Is the white. In the remaining regions, on the other hand, the white matter predominates, in the Beater part of the thoracic cord exceeding the gray from lour to live fold and in the cervical cord being from two to three times greater. The Central Canal. — Win-re well represented, the central canal (canalis cen- tralis'. the remains of the once conspicuous neural tube, appears as a minute opening in the -ray commissure, about .2 mm. in diameter and barely visible with the unaided eye. "in the child it extends the entire length of the cord and, below, ends blindly in the upper half <»f the tilum terminale. Above, it opens into the lower end of the fourth ventricle, from which it is prolonged downward through the lower half of the medulla oblongata into the spinal cord. In not over one-fifth of adult subjects, however, is the canal retained as a pervious tube throughout the cord, its lumen usually bring partially or completely obliterated for longer or shorter stretches, the lumen last disappearing in the lower part of the cord. Within the conus medullaris, the central (anal regularly exhibits an expansion, the sinus terminalis, that begins below the origin of the coccygeal nerve and extends caudally for from 8-10 mm., with a maximum frontal diameter of i mm. or over. The obli'. ration of the central canal, complete in about 50 per cent, of subjects beyond middle life (SchnI/), is to be regarded as a physiological accompaniment of advancing age. It is effected by displacement and proliferation of the ependyma-cells lining the canal, in conjunc- tion with ingrowth of the surrounding neurogliar fibres (Weigert). The form of the canal, as seen in cross-sections, is very variable and uncertain owing to the changes incident to the use of hardening fluids. In a general way when well preserved the lumen is round or oval and smallest in the thoracic region ; in some places, as in the upper cervical cord and in the lumbar enlargement, it is larger and often apjx.-ars pentagonal in outline, whilst in others the calibre may be reduced to a sagittal slit. The position of the central canal varies at different levels in relation to the ventral and dorsal surfaces of the cord. In the middle of the lumbar region it occupies approximately the centre of the cord, but above, in the thoracic and cervical segments, it lies much nearer the ventral than the dorsal surface, while below it gradually approaches the dorsal surface, but always remains closed. Mention may l>e made of a remarkable structure named Reissner's fibre, after its discov- erer, that as a longitudinal thread of great delicacy lies free within the central canal of the cord and the lower ventricle of the brain, extending from the cavity of the mesencephalon above to the lowest part of the cord-canal below. The interpretation of this structure as an artefact, which considering its extraordinary position is most natural, seems untenable in view of the positive testimony, confirming its existence as a preformed and true structure in many vertebrates, given by several subsequent observers and especially by Sargent. According to v and to Nicholls,1 the fibre is com '-rued in automatically regulating flexion of the body, by transmitting to the brain stimuli due to changes in tension. MICROSCOPICAL STkmVRE OF THE SPINAL CORD. The three chief components of the spinal cord — the nerve-cells, the nerve-fibres and the neiiroglia- -vary in proportion and disposition in the white and gfay matter. It i-, therefore, desirable t<> consider tin- general structure of the cord before describ- ing its detailed characteristics at different levels. The Gray Matter. — The mo-,t distinctive elements of the gray matter are the multipolarncrvt-celk which lie embedded within a complex sponge-like matrix formed by the various pt,. l.-ndrites. a \oiirs and collaterals — from other neurones, the supporting nniroglia and the blood vessels. In two localities— "immediately around the nial and rapping the dorsal conui— the grav matter varies in its appearance and . onstitution and exhibit-, the modifications peculiar to the central and Rolandic subst.mtia gelatinosa. (he details of which call for later description (page 1034). The nerve-cells of the anterior horn an- multipolar. in cross-sections the .'i.irly polygonal and in longitudinal sections fusiform in out- 1 . \iiatomischer An/ei-er, I'.d. xl., 1912. MICROSCOPICAL STRUCTURE OF SPINAL CORD. 1031 line. They may vary from .065-. 135 in diameter, unless unusually small, when they measure from .O3O-.o8o mm. (Kolliker). In a typical example, as represented by one of the ventral radicular cells giving origin to anterior root-fibres, from three to ten dendritic processes radiate in various planes, divide dichotomously with decreasing width and finally end in terminal arborizations. In contrast to the robust dendrites beset with spines, the axone is smooth, slender and directly continuous with the axis-cylinder of a root-fibre of a spinal nerve and unbranched, with the exceptions of delicate lateral processes that are given off almost at right angles. These processes, the collaterals, arise at a variable distance from the cell-body, but usually close to the latter and always before leaving the gray matter. They repeatedly divide and follow a recurrent course within the anterior horn. After appropriate staining the cytoplasm of the nerve-cells exhibits conspicuous accumulations of the deeply staining tigroid substance that lie within the meshes of the reticulum formed by delicate neurofibrillae, which not only occupy FIG. the cell-body but also extend into the various processes. The fibrillae, however, do not pass beyond the limits of the neurone to which they belong (Retzius). Each nerve-cell possesses a spherical or ellipsoidal nucleus, from .010 to . 020 mm. in its greatest diameter, which is en- closed by a distinct nuclear membrane and usually contains a single nucleolus, exceptionally two or three. Within the cytoplasm an accu- mulation of brownish- yellow pigment granules is usually present near one pole, often in the vicinity of the implanta- tion cone from which the axone springs. Inaddilion«:; ,, spicuous ventral radicular cells above described, the anterior horn contains other nervous elements, some of which, the com- missural cells, send their axones through the anterior commissure to the opposite half of the cord, while the axones of others, the strand-cells, pass into the columns of white matter of the same, less frequently- opposite, side. The commissural cells, which with few exceptions occupy the median portion of the anterior horn, resemble in size and contour the radicular cells, but differ from the latter in pos- sessing smaller nuclei. The majority of the dendrites are directed towards the inner part of the ventral cornu, but some pass into the gray commissure and a few end within the adjacent white matter. The axones traverse the anterior white commissure to gain the ventral column of the opposite side, in which they either divide T-like into ascending and descending fibres, or undivided turn brainward. The strand cells, variable in form and generally smaller than the root-cells, are only sparingly represented in the anterior horn. They are distinguished by the course of their axones, which usually pass to the anterior column of the same side. In some cases, however, Nerve-fibres of white matter Anterior root-fibres Portion of anterior cornu of gray matter, showing multipolar nerve-cells. X 120. 1032 HUMAN ANATOMY. the axone divides into two, r.irdy three, fibres, one of which crosses by way of the anterior white commissure to the opposite ventral column, while the other passes to the ventral column of the same side. As well seen in cross-sections, although the nerve-cells of the anterior horn are widely scattered they are nut uniformly distributed through the gray matter, but are collected into more or less definite groups that recur in consecutive sections. It is evident, therefore, that the cell-groups are nut limited to a single plane, but are continuous as longitudinal tracts or columns for longer or shorter stretches within the core of gray matter of the cord. The grouping of the nerve-cells of the anterior horn includes two general collections, a mesial group, containing many commissural cells, and a lateral group composed chiefly of ventral radicular cells. These collections, however, vary in extent and definition in different parts of the cord and, where well marked, are often FIG. 888. \ \ Cells of substantia gelatinosa Rolandi Posterior horn cells Accessory dorso- lateral groups Dorso-lateral group ._> :.- Ventro-lateral group Mesial group Transverse section of lower cervical cord, showing grouping of nerve-cells ; Nis>l staining. X to. made up of more than a single aggregation of cells. This feature is particularly evi- dent in the lateral collection, in which an anterior and a posterior subdivision are recogni/ed as the rfntro-laffm/ and the dorso-lati*ral ^ronp that occupy the corre- sponding angles of the anterior horn. The mesial collection, situated within the ventral angle, is likewise, but much less clearly, divisible into a rfHtro-wcsial and a dorso-nir-sitil i; roup. of which the latter is variable and at many levels wanting. In a r;il way the pronounced presence of these cell-groups influences the outline of the anterior horn, so that corresponding projections of the gray matter mark their position. This relation is conspicuously exemplified in the cervical and lumbo-sacral enlargements, in which the presence of la rye lateral cell-groups is directly associated with a marked increase in the transverse diameter of the anterior horn. Conversely, when these cell-columns become smaller or disappear, the corresponding elevations on the surface <>f the anterior horn diminish or are absent. Owing to such variations the contours of tli <>re are subject to constant and sometimes abrupt change. MICROSCOPICAL STRUCTURE OF SPINAL CORD. 1033 The ventro-median cell-column is the most constant, since, as emphasized by the pains- taking studies of Bruce,1 it is interrupted only between the levels of the fifth lumbar and first sacral nerve in its otherwise unbroken course through the length of the cord, as far as the level of the fifth sacral nerve. An augmentation of this tract in the fourth and fifth cervical segments is probably associated with the spinal origin of the phrenic nerve (Bruce). The dorso-mesial cell-column is much less constant, being represented only in the thoracic region, in a few cervical segments and at the level of the first lumbar nerve. In agreement with van Gehuchten and others, Bruce regards the continuity of the mesial group as presump- tive evidence of its close relation to the dorsal extensor muscles of the trunk. The ventro-lateral cell-column appears first at the level of the fourth cervical nerve, increases rapidly in the succeeding segments and fades away at the lower part of the eighth cervical segment. It reappears in the lumbar enlargement, reaching its maximum at the level of the first sacral nerve and, diminishing rapidly through the upper part of the second, disappears before the third sacral segment is reached. The dorso-lateral cell-column, in places the most conspicuous collection of the anterior horn, begins above at the lower part of the fourth cervical segment and, increasing rapidly, attains its greatest development in the neck in the fifth and sixth segments. It suffers a marked reduction at the level of the seventh cervical nerve, which is followed by a sudden increase in the next segment in which the column presents an additional collection of nerve-cells known as the accessory dorso-lateral or post-postero- lateral group. Below the level of the second thoracic nerve the dorso-lateral cell-column is unrepresented as far as the second sacral segment where it reappears, somewhat abruptly, and attains its maximum size in the fourth and fifth lumbar segments. The column then diminishes and ceases at the lower part of the third sacral seg- ment. Within the sacral cord, between the levels of the first and third nerve inclusive, the dorso-lateral cell-group is augmented by an accessory group. From the third lumbar to the sacral nerve-levels, an additional compact collection of nerve-cells occupies a more median position in the anterior horn and constitutes the central group. From the position of the greatest expansions of the lateral cell-columns — within the cervical and lumbo-sacral enlargements — it is evident that they ar£ associated with the large nerves sup- plying the muscles of the limbs. Further, according to Bruce, in a general way the size of the radicular cells bears a relation to that of the muscles supplied, the smaller dimensions of the cervical cells, as compared with those of the lumbo-sacral region, corresponding with the smaller size of the upper limb in comparison with that of the lower one. In addition to the nerve-cells assembled within the foregoing more or less well defined groups, some scattered cells are irregularly distributed through the anterior horn and do not strictly belong to any of the groups. Below the level of the first coccygeal nerve, the cells of the anterior horn become so diminished in n .mber, that they are no longer grouped with regularity, but, reduced in size, lie uncertainly distributed within the gray matter as far as the lower limits of the conus medullaris. The nerve-cells of the posterior horn are neither as large nor as regularly disposed as the anterior horn cells. Only in one locality, along the median border of the base of the posterior horn, are they collected into a distinct tract, the column of Clarke ; otherwise they are scattered without order throughout the gray matter of the posterior cornu. Since, however, the latter comprises certain areas, the cells of the posterior horn may be divided into (i) the cells of Clarke* s column, (2) the cells of the substantia gelatinosa Rolandi, and (3) the inner cells of the caput cornu. The cells of Clarke's column form a very conspicuous collection which extends from the level of the seventh cervical nerve to that of the second lumbar nerve and is best developed in the lower thoracic region of the cord. Although confined chiefly to the dorsal portion of the cord, and hence sometimes designated as the "dorsal nucleus," Clarke's column is represented to a slight degree in the sacral and upper cervical regions (sacral and cerrical nuclei (if Stilling) . In cross-sections the cell-column appears as a group of multipolar cells that occupy the mesial border of the base of the posterior horn and, where the column is best developed (opposite the origin of the twelfth thoracic nerve), correspond to an elevation on the surface of the gray matter. The cells usually are about .050 mm. in diameter, polygonal in outline and possess a relatively large number of richly branched dendrites that radiate chiefly within the limits of the group (Cajal). The axones commonly spring from the anterior or lateral margin of the cells and course ventrally for a considerable distance before bending outward toward the lateral column of white matter within which, as constituent fibres of the direct cerebellar tract (page 1044) , they turn brainward. 'Topographical Atlas of the Spinal Cord, 1901. 1034 HUMAN ANATOMY. The nerve-cells of the substantia gelatinosa Rolandi, also known as Gierke 's cells, include innumerable small stellate, less frequently fusiform or pear-shaped elements that measure only from .oo6-.o2o mm., although exceptionally of larger size. Their numerous short dendrites are irregularly disposed and branched. The axones, which always arise from the dorsal pole of the cell, are continued partly to the white matter of the posterior column, within which they divide into ascending and descending limbs, and partly to the gray matter itself, within which they run as longitudinal fibres. Under the name of the marginal cells are described the much larger (.035-.055 mm. ) nerve-cells which occupy the border of the substantia gelatinosa. They are spindle-shaped or pyramidal in form, their long axes lying parallel or the apices directed towards the Rolandic substance respectively, and constitute a one-celled layer enclosing the substantia gelatinosa, into which many of their tangentially coursing dendrites penetrate. Their axones •lirough the substantia gelatinosa and probably continue for the most part within the lateral column, although some enter the posterior column ( Cajal, Kolliker). The inner cells of the posterior horn are intermingled with numerous nervous elements of small size irregularly distributed within the head of the dorsal cornu. The inner cells proper are triangular or spindle-shaped in form and, on an average, measure about .050 mm.; they are, therefore, larger than the ordinary cells of the Rolandic substance. The dendrites arise IMG. 889. White matter of '.sl^^^^^B^ • - ' ^ posterior column -rvrfflMRIBEM&fc. , Cells of Clarke's column Suhstaiitii gelatinosa cetiiralis ^^ / Central camil Part of cross-section of cord, showing cells of Clarke's column in base of posterior horn. X no. from the angles or ends of the cells and diverge in all directions. The axones pass, either directly nr in curves, mostly into the lateral column of the same side; some, however, have been followed into the posterior or anterior columns of the same side (Kolliker), and, rarely, into tin- opposite anterior column (Cajal). Exceptionally type II cells— those in which the axone i-, not prolonged as the axis-cylinder of a nerve-fibre, but soon breaks up into an elaborate end arbori/ation confined to the gray matter are found within the gray matter of the posterior horn. Their number is, however, much less than often assumed (Xiehen). The nervous character of most of the cells seen within the substantia gelatinosa Rolandi lias l>'-en established only since the introduction of the C.olgi methods of silver-impregnation. I'r.-vloiislv. these elements u !••<] .is glia celK, ;m exceptionally large amount of m-nroglia in general being attributed to the Rolandic substance. It is now 'admitted that instead of such being the case, this region of the gray matter is relatively poor in neiirogliar elements and numerically rich in nerve-o-lls. The nerve-cells of the pars intermedia of the -ia\- matter, which connects the dorsal and ventral horns and lies opposite the grav commissure, may be broadly divided into two classes, the lateral and the middle cells, that occupy respectively the outer border and the more central area of this part of the gray matter of the cord MICROSCOPICAL STRUCTURE OF SPINAL CORD. 1035 Those of the first class, or intermedio-lateral cells, are associated with the formatio reticu- laris and its condensation, the lateral horn, and hence are often spoken of as the group or column of the lateral horn. These cells form a slender tract of small closely packed elements that is represented through almost the entire length of the cord, although best marked in the upper third of the thoracic region and partially interrupted in the cervical and lumbo-sacral segments. Where the formatio reticularis is condensed with a distinct lateral horn, as in the thoracic region, the cells occupy the projection, but elsewhere lie within the base of the gray net- work. As a continuous cell-column the tract extends from the lower part of the eighth cervical segment to the upper part of the third lumbar, being most conspicuous at the level of the third and fourth thoracic nerves (Bruce). Practically suppressed in the cervical region between the eighth and third segments, above the latter the column reappears along with the formatio reticularis. Below, it is again seen within the third and fourth sacral segments. The nerve-cells are multi- polar or fusiform in outline, from .OI5-.O45 mm. in their longest diameter, contain little pigment, and are provided with a variable number of dendrites, of which two are usually larger than the others. These arise from opposite poles of the cell and send branches, for the most part, into the adjacent white matter. The axones pass directly into the lateral columns and become ascending or descending fibres ; a few axones, however, enter the anterior column of the same side (Ziehen). The cells of the second class, or intermediate cells, are irregularly disposed and only in the upper part of the cord present a fairly distinct middle group (Waldeyer). They are polygonal or fusiform in outline, small in size (seldom exceeding .025 mm.) and provided with irregular dendrites. The axones are continued chiefly within the lateral column of the same side, although some pass to the anterior column and a few probably cross to the opposite side. A small number of isolated nerve-cells are usually to be found within the white matter, out- side but in the neighborhood of the gray core. These, the outlying cells of Sherrington,1 by whom they have been studied, occur most frequently in the vicinity of the more superficially placed cell-columns. Within the anterior columns they lie in the paths of the fibres proceeding to the anterior wrhite commissure ; in the lateral columns they are in proximity to the intermedio- lateral group of the lateral horn and formatio reticularis and to the cells of the substantia Rolandi ; and in the posterior columns, where they are relatively numerous, they are associated with the fibre-tracts leading to the column of Clarke. The outlying cells are regarded as elements displaced from their usual position during the course of the differentiation and growth of the white and gray matter. Similar displacement sometimes affects the cells of the spinal ganglia, which then may be encountered within the cord. The Neuroglia of the Gray Matter. — As in other parts of the cord, so in the gray matter the neuroglia is everywhere present as the supporting framework of the nervous elements, the cells and fibres. The gen- eral structureof neuroglia having been described (page 1004), it only re- mains to note here the special features of its arrangement within the gray matter. In general, the felt- work of the neu- rogliar fibrils is more compact than that per- meating the white matter, being somewhat denser at the periphery than in the deeper parts of the gray matter. There is, however, no hard boun- dary between the sup- porting tissue of the two, since numerous glia fibrils extend outward from the frame-work of the gray matter to be lost between the nerve fibres of the adjoining columns. This feature is marked in the anterior horn, where the glia fibrils form septa of considerable thickness that diverge into the surrounding columns ; further 1 Proceedings Royal Society, vol. 30, 1890. Posterior median I septum Paramedian septum subdividing posterior column •^L Lateral H column 'V y i JA 7 Anterior median fissure Anterior column Transverse section of cord slightly magnified, showing general arrangement of neuroglia. X 10. 1036 HUMAN ANATOMY. the conspicuous processes of the formatio reticularis and the projecting lateral horn consist largely of neuroglia. The larger nerve-cells and their robust processes are ensheathed by interlacements of neuroglia fibrilke. In the several parts of the posterior horn the amount of neuroglia varies. Thus, the apex consists almost exclusively of glia tissue, while within the Rolandic substance the number of glia fibres and cells is unusually small. Within the caput and remaining parts of the posterior horn the neurogliar elements are similar in quantity and disposition to those in the anterior horn. The ependyma cells lining the central canal of the cord are the direct descendants of the radially arranged embryonal supporting elements (page 1004) ; they may, therefore, be regarded as specialized neuroglia cells. Although most advantageously studied in the fcetus and the child, in favorable preparations from adult cords they are seen as a single row of pyramidal cells, from .030-.050 mm. long and from one-fourth to one-third as broad, whose bases are directed towards the lumen of the canal and beset with cilia. Their pointed distal ends, or apices, are prolonged into a long delicate ependyma! fibre, that in the adult is soon lost in the surrounding neuroglia, but in the fcetus extends through the entire thickness of the cord. The ependyma cells are not all of equal size, those occupying the ventral mid-line, especially in the cervical region, being about twice as long as those on the opposite wall of the canal. The epen- dymal fibres proceeding from these cells are of special length and thickness, the ventral ones con- verging to form a wedge-shaped mass that in the young subject continues as far forward as the bottom of the anterior median fissure. The dorsal ependymal fibres are prolonged through the gray commissure into the posterior median septum, some diverging into the columns of Goll. FIG. 891. Substantia gelatinosa centralis is the name given to a zone of peculiar trans- lucency that immediately surrounds the central canal. This annular area consists of modified neuroglia in which radial ependymal fibers are interwoven with circularly disposed neurogliar fibrillae, the whole giving rise to a compact stratum, interspersed with an unusual number of glia cells, upon which arrange- ment, in conjunction with the absence of nerve-fibres, the characteristic appearance of the gelatinous substance depends. In addition to the branched glia elements, a number of radially directed spindle cells are present in this zone ; they send delicate processes between the ependyma cells, of which they are probably outwardly displaced members. In marked contrast with the Ro- landic substance, which caps the posterior horn, the substantia gelatinosa centralis contains only a few small nervous elements, in recognition of which the term, sub- stantia gliosa centralis, has been proposed by Ziehen. The Nerve-Fibres of the Gray Matter.— Within all portions of the gray core a considerable part of the intricate ground-work in which the nerve-cells lie embedded is contributed by the processes of neurones situated at the same, different or even remote levels. These processes, which constitute the nerve-fibres, medullated and nonmedullated, that are seen traversing the gray matter, in all directions, include:(i) the collate- rals and the terminal branches of the dorsal root-fibres that enter the gray ma'trr ; •M Tve-fibres of the descending tracts that terminate in relation with the ventral (motor) horn cells ; (3) the axones and collaterals given off by the numerous pos- terior horn cells, that traverse the gray matter to and from the respective columns into which they pass. The dendritic processes, as well as the axones of the type II cells, also contribute to the sum of nervous fibrilla- encountered within the gray matter of the cord. WHITE MATTER OF THE SPINAL CORD. Tin predominating components of the white substance being the longitudinal nerve-fibres which pass fora longer or shorter distance up and down in the columns of the cord, in cross-sections the outer field, between the gray core and the periphery Central canal and surrounding substantia gelatinosa centralis, from child's cord ; canal is lint-d with ependvnia < rlK. outside of which lies neuroglia with glia cells. X 135. WHITE MATTER OF THE SPINAL CORD. of the cord, appears to be composed of innumerable, closely set, small cells, held together by delicate supporting tissue. These apparent cells are the medullated nerve-fibres cut transversely, in which the sectioned axis-cylinders show as deeply stained dots, that commonly lie somewhat eccentrically and are surrounded by deli- cate irregularly annular striations representing the framework of the medullary coat. The nerve-fibres of the cerebro-spinal axis are without neurilemma, the lack of this sheath being compensated by a slight condensation of the neuroglia around the fibres. Seen in transverse sections this investment appears as the ring that gives a definite outline to the fibre. The individual nerve-fibres vary greatly in size, even within the same tract large and small fibres often lying side by side. The smallest may be less than .005 mm. and the largest over .025 mm. In a general way, the diameter of the fibre bears a direct relation to its length, those Fig. 892. Trabecula of neuroglia Neuroglia cell Nerve-fibre Blood-vessel in pia Subpial layer of neuroglia Peripheral part of transverse section of spinal cord, showing nerve-fibres subdivided into groups by ingrowth of subpial layer of neuroglia. X 230. having an extended course being larger than shorter ones ; it follows that the fibres occupying the peripheral parts of the white matter, particularly in the lateral columns, are more frequently of large diameter than those near the gray matter. The immediate surface of the white substance beneath the pia mater is formed by a con- densed tract of neuroglia, the subpial layer, from .O2O-.O4O mm. in thickness, that is devoid of nervous elements and forms the definite outer boundary of the cord. This zone consists of a dense interlacement of circular, longitudinal and radial neuroglia fibrils among which numer- ous glia cells are embedded. From the deeper, surface of this ensheathing layer numerous bundles of fibrilke penetrate between the subjacent nerve-fibres to become lost in the general supporting ground-work. At certain places the bundles are replaced by robust septa by which the nerve-fibres are imperfectly divided into groups or tracts, as conspicuously seen in the pos- terior column where the paramedian septum effects an imperfect subdivision into the tract of Goll and of Burdach. The blood-vessels that enter the nervous substance from the pia, accom- panied by connective tissue, are surrounded by tubular sheaths of neuroglia, and the same is io38 IITMAX ANATOMY. true of the bundles of root-fibres of the spinal nerves. But apart from the connective tissue that enters with the blood-vessels, the amount of mesoblastic tissue concerned in the supporting framework of the cord is inconsiderable, according to some histologists, indeed, being practically nothing. Fibre-Tracts of the "White Matter.— Although microscopical examination of ordinary sections of the cord affords slight indication of a subdivision of the columns of white matter into arras corresponding with definite fibre-tracts, yet the combined evidence of anatomical, pathological, embryological and experimental investigation establishes the existence of a number of such paths of conduction. With few exceptions, they are, however, without sharp boundaries and illy defined, adjoining tracts often overlapping, and depend for their presence upon the fact that nerve-fibres having the same function and destination proceed in company from the same group of nerve-cells (nucleus) along a similar course. In addition to being pro- vided with paths of conduction necessary for the performance of its function as a centre for independent (reflex) impulses in response to external stimuli, the cord contains tracts that connect it with the brain, as well as those that bring the various levels of the cord itself into association. The white matter, therefore, contains three classes of fibres : ( i ) those entering the cord from the periphery and other parts of the body ; (2) those entering it from the brain ; and (3) those arising from the nerve-cells situated within the cord itself. The first two constitute the exogenous, the last the endogenous tracts. It is evident that some of these fibres constitute pathways for the transmission of impulses from lower to higher levels and hence form ascending tracts, while others, which conduct impulses in the opposite direction, form descending tracts. Since it is impossible to distinguish between these fibres by mere inspection of sections of the adult normal cord, and, moreover, extremely difficult and practically impossible to follow in such preparations the longer fibres throughout their course, advantage is taken of other means by which differentiation of individual tracts is feasible. Such means include chiefly the experimental and embryological methods. The experimental method depends upon the law discovered by Waller, more than half a century ago, that when the continuity of a nerve-fibre is destroyed, either by a pathological lesion or by the experimenter's knife, the portion of the nerve-fibre (the axone of a neurone) b'-yoml the break, and therefore isolated from the presiding nerve-cell, undergoes secondary degeneration, while the portion remaining connected with the cell usually undergoes little or no change. It should be pointed out, however, that occasionally the connected portion of the fibre, and even the nerve-cell itself, undoubtedly exhibits changes known as retrograde degen- eration, which, dependent upon the location of the lesion, may at times prove a source of error in deducing conclusions. If a lateral section of one-half of the cord of a living animal be made, and, after the expiration of from three to four weeks, transverse sections be cut and appropriately prepared (by the methods of Marschi or of Weigert), certain groups of nerve- fibres will present degenerative changes. It will be seen, however, that the degenerated tracts in si-ctions taken from above the lesion are not the same as those in sections from below the division, showing that certain fibres have been involved in opposite directions, those arising from nerve-cells lying below the lesion being affected with ascending degeneration, and those from cells situated above with descending degeneration. In this manner, by careful study of consecutive sections, much valuable information lias been gained as to the origin, course, ter- mination and function of many fibre-tracts within the central nervous system. The embryological method, also productive of important advances in our knowledge of the nervous pathways, is based on the fact, first demonstrated by Meckel, that the nerve-fibres of the central nervous system do not all acquire their medullary sheath at the same time. Taking advantage of such variation, as suggested by Meynert and later extensively carried out by Flechsig and others, upmi staining sections of embryonal tissue \\i:h reagents that color ially the medullary substance, it is possible to differentiate and follow certain fibre-tracts in the fu-tal cord with great clearness, since only those tracts are stained in which the myelin is already formed. It is of interest to note that, in a general way, the order in which the different strands of the cord acquire their medullary c. >. it accords witl'i the sequence in which nervous function is assumed by the firtus and child. Thus, the paths required for spinal reflexes (the tior and anterior root-fibres) are first to become medullated (fourth and fifth fu-tal months); those bringing into association the different segments of the cord next ( from the fifth to the seventh month \ acquire myelin; those connecting the cord with the cerebellum follow somewhat later, while those establishing relations with the cerebral cortex are last and do not begin to medullate until shortly before birth. WHITE MATTER OF THE SPINAL CORD. 1039 Based on the collective evidence contributed by these methods — anatomical, physiological, and developmental — it is possible to locate and trace with fair accuracy a number of fibre- tracts in the cerebro-spinal axis. Since they are undergoing continual augmentation or decrease, their actual area and position are subject to variation, so that the detailed relations in one region of the cord differ from those at other levels. The accompanying schematic figure, therefore, must be regarded as showing only the general relations of the most important paths of the cord, and not as accurately representing the actual form and size of the fibre-tracts! It must also be appreciated that the definite limits of these tracts in such diagrammatic FIG. 893. Association tracts Rubro-spinal tract Tecto-spinal tract Spino-thalamic tract Spino-olivary tract (Helweg) Vestibulo-spinal tract Cerebro-spinal tract' " Sulco-marginal fibres Diagram of spinal cord, showing position of chief tracts and relations of their component fibres to nerve-cells; 1-5, posterior root-fibres entering root- zone (R.Z.) and Lissauer's tract (L.)- open circles (o) indicate that fibres pass up and down ; e, c, collaterals from long ascending tracts (i, 2) to anterior root-cells; 3, fibres ending around cells of Clarke's column; 6, fibres forming direct cerebellar tract; 7, 8, fibres forming Cowers' tract; 9, 10, fibres from lateral and direct pyramidal tracts; n, n, anterior root-fibres ; V.F., ventral field; O.F., oval field; C.B., comma bundle. representations seldom exist in reality, since the fibres of the adjacent paths in most cases overlap, or, indeed, extensively intermingle, so that the fields seen in cross-sections may be shared by strands belonging to different fibre-systems. The Fibre-Tracts of the Posterior Column. — The subdivision of the posterior column of white matter by the paramedian septum into two general parts has been noted (page 1028). Of these the inner one is the postero-median fasciculus, or tract of Goll (fasciculus gracilis), and the outer one is the postero- lateral fasciculus or tract of Burdach (fasciculus cuneatus). These tracts are so intimately associated with the fibres entering by the posterior roots of the spinal nerves, that the general relations and behavior of these fibres must be considered in order to understand the composition of the posterior columns, as well as that of certain secondary paths. All sensory impulses that enter the spinal cord do so by way of the posterior root-fibres. The latter are the centrally directed processes (axones) of the neurones whose cell-bodies lie within the spinal ganglia situated on the dorsal roots of the spinal nerves. They convey to the cord the various impulses collected by the peripherally directed processes (the sensory nerves) from the integument, mucous membranes, muscles, tendons and joints from all parts of the body, with the exception of those served by the cranial nerves. The impulses thus conducted are transformed into the impressions of touch, muscle-sense, heat, cold and pain. The last being probably the result of excessive stimulation that by its intensity causes discomfort in various degrees, the existence of special paths for the conduc- tion of painful impressions is unlikely. It is evident that the larger part of the 1040 HUMAN ANATOMY. FIG. 894. sensory neurones lies outside the spinal cord ; it is, however, with the intramedullary portion of these neurones, as constituents of paths within the cord, that we are here concerned. On entering the spinal cord along the postero-lateral groove, the dorsal root- fibres for the most part penetrate the tract of Burdach, close to the inner side of the posterior horn. Some of the more external root-fibres, however, do not enter Bur- dach's tract, but form a small adjoining field, the tract of Lissauer, that lies im- mediately dorsal to the apex of the posterior horn. Soon after gaining the posterior column, with few exceptions, each dorsal root-fibre undergoes a >- or |— like divi- sii >n into an ascending and a descending limb, which assume a longitudinal course and pass upward and downward in the cord for a variable distance, the descending limb being usually the shorter. During their course from both, but particularly from the descending limb and from the proximal part of the ascending fibre, collateral branches are given off which bend sharply inward and pass horizontally into the gray matter to end chiefly in relation with the neurones of the posterior horn, from \vhich cells secondary paths arise. Not only the collaterals, but also the main stem-fibres of the descending and shorter ascending limbs end in the manner just described. In addi- tion to the short collaterals destined for the cells of the dorsal horn, others, the ventral reflex collaterals, pursue a sigmoid course, traversing the substantia gelatinosa Rolandi and the remaining parts of the posterior horn and the intermediate gray matter, to end in arborizations around the radicular cells of the anterior horn, and thus complete important reflex arcs, by which impulses transmitted through the dorsal roots directly impress the motor neurones. The latter are usually of the same side, but some collaterals cross by way of the anterior commissure to terminate in relation with the anterior horn cells of the opposite side. It is probable that a considerable number of such anterior horn reflex collaterals are given off from the fibres that ascend in the long tracts of the posterior column to the medulla oblongata. With possibly the exception of certain fibres which pass directly to the cerebellum (Hoche), all the sensory root-fibres (axones of neurones of the I order) end around the neurones situated either within the gray matter of the spinal cord or within the nuclei of the medulla ; thence the impressions are conveyed by the axones of these neurones of the II order to higher centers, to be taken up, in turn, by neurones of th«- III«or even higher order, in the sequence of the chain required to complete the path for the conduction and distribution of the impulse. Tin- most important groups of the collaterals and stem-fibres of the posterior roots are: 1. The long ascending tracts passing chiefly to the nuclei of the medulla. 2. The fibres passing t<> the cells of tin- column of Clarke. 3. Tin- collaterals passing to the anterior horn cells. 4. The fit ncs entering the posterior horn from the tract of Burdach and of ier to ,-nd a!..,m the neurones of the II order situated within the gray matter of the posterior horn and the intermediate gray matter. 'Hie direct ascending posterior tract includes the dorsal root-fibres that Uninterruptedly upward within the posterior column as far as the nuclei of the medulla. On entering the cord they lie at first within the tract of Burdach, but in their ascent are gradually displaced medianlv and dorsally by the continued addition of other root -fibres from the succeeding higher nerves. In consequence, in cross Diagram showing division of posterior root-fibres Into ascending and descending branches: long fibre sends collateral! to anu-rior root cells; other fibres end at different levels around cells in gray matter of posterior horn ; S. G., spinal ganglion. WHITE MATTER OF THE SPINAL CORD. 1041 sections of the cord in the cervical region the long fibres entering by the lower nerve- roots occupy the inner part of Coil's column. In the lumbar cord, they are excluded from the median septum by a narrow hemielliptical area, which with that of the opposite side forms the oval field of Flechsig. The fibres entering by the lower thoracic nerves lie more laterally, while those entering by the upper thoracic and cervical nerves appropriate the adjoining part of Burdach's tract, the lateral area of which, next the posterior horn, is occupied chiefly by the posterior root-fibres. It must be understood that while in a general way the fibres of the long ascending tracts have the disposition just indicated, they are so intertwined and mingled with the strands passing to and from the gray matter that the definite outlines of their conventional area, as represented in diagrams, are wanting. Collectively the fibres composing this tract are of medium or p-I(; small size, but acquire their medullary coat very early, myelination beginning about the fourth foetal month, although not completed until the ninth (Bechterew). The termination of the long ascend- ing fibres is chiefly in relation with the neurones within the lower part of the medulla — the fibres of Goll's tract end- ing about the cells of the nucleus gracilis and those of Burdach's tract about the cells of the nucleus cuneatus. From these stations paths of the II order convey the impulses to the cerebel- lum, by way of the inferior cerebellar peduncle, and to the higher sensory centres by way of the mesial fillet, as later described (page 1115). Whether certain of the component fibres of these ascending tracts are directly continued to the cerebellum, and perhaps to the mesial fillet, without undergoing inter- ruption in the nuclei of the medulla is still uncertain, although supported by the statements of Hoche, Kolliker, Solder and others. The root-fibres passing to Clarke's column occupy the middle and median part of Burdach's tract, mingled with those of the long ascending paths. After cours- ing longitudinally, usually for some distance, within the posterior column, they bend outward, and, sweeping in graceful curves, enter the gray matter to end about Clarke's cells. It is noteworthy that the level at which they end is often considerably higher than that at which the root-fibres enter the cord, an arrangement which explains the fact that lesions of the lowermost of these strands may be followed as ascending degenerations into the thoracic region (Mayer). On entering the gray matter the terminal arborization of a single root-fibre usually ends in relation with several neurones of Clarke's column (Lenhossek). The important sensory path of the II order, known as the direct cerebellar tract (page 1044), arises as the axones of these neurones. The anterior reflex fibres to the ventral horn are all collaterals, not continu- ations of the stem-fibres, far the greater part of which come from the fibres of the long ascending posterior tract. These collaterals penetrate the gray matter princi- pally at the median border of the head of the posterior horn, behind Clarke's column, but partly also through the substantia Rolandi, and thence pass ventrally or ventro-laterally, with a slightly curved or sigmoid course, towards the anterior horn. As they enter the latter, the collaterals diverge more and more and are distributed to the various groups of the anterior horn cells, chiefly in relation with the lateral groups of radicular cells from which the ventral root-fibres arise ; they thus establish direct reflex paths by which sensory impulses conveyed by the posterior root-fibres impress the motor neurones, while, at the same time, these impulses are transmitted 66 Section of spinal cord at level of second cervical seg- ment ; formatio reticularis fills bay between posterior and anterior cornua; substantia gelatinosa caps apex of pos- terior cornu. Drawn from Weigert-Pal preparation made by Professor Spiller. X 6. 1042 HUMAN ANATOMY. Section of spinal cord at level of sixth cervical segment ; anterior cornua are very broad ; obliquely cut bundles of posterior root-fibres lie in postero-lateral sulcus. Preparation by Professor Spiller. X 6. to higher levels by the ascending stem-fibres. Although the anterior reflex collat- erals are, for the most part, in relation with the cells of the same side, it is probable that some cross by way of the posterior commissure, and possibly also by the anterior bridge, to the opposite ventral horn cells. It is doubtful, on the other hand, whether either stem-fibres or collaterals of the posterior roots pass directly to the anterior column either of tin- same or opposite sides (Zichenj. The root-fibres passing to the posterior horn include those which pene- trate the substantia Rolandi, either as collaterals or stem-fibres of Burdach's or of Lissauer's tracts, to end FIG. 896. about the neurones within the Rolandic substance or within the head of the pos- terior horn. Their longitudi- nal course within Burdach's tract is ordinarily short ; they then bend horizontally and enter the gray matter of the posterior horn, within which they soon terminate in end-arborizations around the neurones of the II order. Some fibres, however, do not undergo T-division until after entering the posterior horn, where, within the Ro- landic substance or caput cornu, they then bifurcate, in some cases the ascending limbs pursuing a vertical course within the gray matter, particularly of the caput cornu, for some distance before ending about the head-cells of the posterior horn. The tract of Lissauer, or marginal zone, situated immediately behind the apex of the dorsal horn, receives the lateral group of the posterior root-fibres. These are all of unusually small size and, after a short longitudinal course in which the descending limbs predominate, they turn horizontally and, both as collaterals and stem-fibres, penetrate the substantia Rolandi, about whose cells and those of the caput cornu they end. From the foregoing description, it is evident that the dorsal root-fibres destined for the posterior horn terminate in relation with neurones of the II order represented chiefly by the cells of the substantia gelatinosa Rolandi, including the marginal cells, and the inner cells of the caput cornu. The secondary or endogenous tracts of the posterior column arise as axones from the neurones of the II order (the marginal cells, the cells of the substantia Rolandi and the head- cells) situated within the posterior horn and include ascending and descending paths. The ascending secondary tract is composed of the axones derived from the posterior horn cells of the same and, by way of the posterior commissure, opposite side, which pass into the posterior column. Tn a general way, they occupy the ventral .field, although sharing it with scattered strands of root-fibres and of descending endogenous fibres. The destination of the fibres of this ventral tract is uncertain, some fibres pursuing a short and others a longer course within the posterior column before entering the gray matter at higher levels to end in relation with the posterior horn-cells, or, perhaps, in some cases, with the neurones within the nuclei of the medulla (Rothmann). The descending secondary tracts, as shown by degenerations following lesions involving the posterior column, occupy varying but fairly well differentiated areas. In the cervical and upper thoracic con I the descending limbs of the long posterior fibres are collected into the comma bundle of Schnltze, which extends along the median margin of Burdach's tract. In the lower thoracic- and lumbar cord is formed an elongated half-ellipse along the posterior median septum which, with the corresponding bundle of the opposite side, produces the oval field of Flechsig. Still lower, in the sacral cord, fibres lie at the junction of the median septum and the posterior surface of the cord as the medio-dorsal triangular bundle of Gombault and Philippe. Additional descending endogenous fibres are scattered in the ventral field. It is WHITE MATTER OF THE SPINAL CORD. 1043 likely that these areas represent the principal aggregations of the downward coursing limbs of the axones, derived from the posterior horn-cells of the same and opposite sides. In the cervical region the descend- ing limbs of the posterior FIG. 897. root fibres appear as the comma tract; in the lower thoracic cord these are re- placed by, without being di- rectly continuous with, those forming the oval field, and these in turn by the axones of the triangular bundle. No one of these fields is exclusively devoted to the descending limbs of endogenous fibres, since in all the presence of exogenous posterior root- fibres has been demonstrated. The Fibre -Tracts of the Lateral Column. —These include : ( i ) the lateral pyramidal, (2) the direct cerebellar, (3) the ascending" antero-lateral Section of spinal cord at level of seventh cervical segment; anterior ,9 , , cornua are less robust ; root-zone is seen just behind Lissauer's tract. X 6. and (4) the lateral ground- Preparation by Professor Spiller. bundle. The lateral or crossed pyramidal tract (fasciculus cerebrospinalis lateralis) forms the chief path by which motor impulses originating in the cerebral cortex are conveyed to the spinal cord. It stands in close relation with the direct pyramidal tract of the anterior column. Both are continuations of the conspicuous pyramidal paths of the medulla oblongata and may be followed upward through the ventral part of the medulla, the pons and the cerebral peduncles into the white matter of the cerebral hemispheres and on to the cortical gray matter where, in the motor areas bordering chiefly the Rolandic fissure, lie the nerve-cells from which the pyramidal fibres arise. These fibres, therefore, are the axones of cortical motor neurones and extend without interruption from the superficial gray matter of the cerebral hemi- spheres to various levels in the cord, constituting long descending (corticifugal) motor tracts. On reaching the lower part of the medulla, from 80—90 per cent, of the component fibres of each pyramid cross to the opposite side by way of the decussation of the pyramids (page 1065) and, entering the cord, descend as the lateral pyramidal tract; the remaining fibres (on an average, about 15 per cent.) pass downward into the ventral column of the cord as the direct pyramidal tract. After decussating, the crossed pyramidal tract passes outward to enter the lateral column of the cord, thereby exchanging its former median and superficial position for a deeper and more lateral one. Since its fibres are continually entering the gray matter to end about the radicular cells from which the anterior root-fibres of the spinal nerves arise, the tract progressively loses in size as it descends, until, at about the level of the fourth sacral nerve, it ceases to exist as a distinct strand, although continued by small scattered bundles of fibres as far as the origin of the coccygeal nerve. This diminution is not regular, since in the sacral and lumbar enlargements the loss is more marked than elsewhere, on account of the relations of the tract-fibres to the large motor limb-nerves. The relations, as well as size, of the lateral pyramidal tract vary at different levels. As seen in cross-sections of the upper thoracic region of the cord, the tract occupies an area of considerable size, that mesially lies against the posterior horn and laterally is in contact with the direct cerebellar tract, by which it is excluded from the periphery. In front, where its limits are less definite, the tract extends ventrally for a variable distance into the lateral column, but seldom overreaches the plane of the gray commissure. With the diminution and disappear- ance of the direct cerebellar tract within the lower portions of the cord, the pyramidal field approaches and finally reaches the surface, which relation it retains as it grows smaller, the io44 HUMAN ANATOMY. reduction affecting the more deeply placed fibres. In consequence of these variations, the form of the pyramidal tract in cross-section changes from wedge-shape to triangular, with the base lying at the periphery and the apex directed FIG. 898. inward. During their descent the fibres of the pyramidal tract give off at different levels col- laterals, which bend horizontally inward and forward, enter the gray matter, and end in rela- tion with the anterior horn cells. A similar course is followed by the parent fibres on reach- ing the segment for which they are destined, the terminal part of the individual fibres sweeping in short curves through the intervening ground- bundle of the lateral column to gain the radicular cells around which they end. By means of its collaterals, each pyramidal fibre establishes rela- tion with several cord-segments. The fibres of this tract are relatively tardy in acquiring their medullary coat, which process does not begin until the last month of foetal life and is not com- pleted until after the second year. Section of spinal cord at level of sixth thoracic segment; slender posterior cornua covered with sub- »pi Almr** r»«»T-«»H«»11ar +ror«f ft- 1 nC direct CcrcDcliar IiaCL (traC- stantia gelatinosa; postero-lateral angle marks greatest f^s'ohr spmererior cornu> Preparation Ky Pro- tus spino-ccrebellaris dorsalis), is an as- cending path of the second order that establishes communication between the reception sensory cord-nucleus formed by Clarke's cells and the cerebellum. In cross-sections of the thoracic region, the tract forms a superficial flattened comet-shaped field that occupies the dorsal half of the lateral column, extending from the apex of the posterior horn forward along the periphery of the cord, to the outer side of the lateral pyramidal tract, to about the anterior plane of the gray commissure. Its ventral end, particularly in the lower cervical region, is broadest and projects somewhat into the lateral column in advance of the lateral pyramidal field. Although as a compact strand the direct cerebellar tract begins at the tenth thoracic segment, it is represented by isolated fibres in the lumbo- sacral region. The fibres collectively are large and become medullated about the sixth foetal month (Bechterew). In a general way the fibres having the longest course occupy the dorsal part of the tract and those having the shortest the ventral (Flatau). Arising as the axones of the cells of Clarke's column, the components of the tract pass in curves almost horizontally outward through the gray matter and lateral column to the peripheral field, on gaining which they bend sharply brainward and ascend without interruption to the medulla. Their further course includes the pas- sage through the dorso-lateral field of the medulla as far as the inferior cerebellar peduncle, by which the fibres reach the cerebellum to end in relation with the superior worm, on, probably, both the same and the opposite sides. The tract of Gowers (tractus spino-cerebellaris ventralis) constitutes another pathway of the II order, which connects the cord with the cerebellum and probably also establishes relations with the cerebrum. In cross-sections the tract appears somewhat uncertainly defined owing to the intermingling of its fibres with those of adjoining strands, but in tin: main it includes a superficial crescentic field that touches the direct cerebellar and lateral pyramidal tracts behind, extends along the margin of the cord for a variable distance, and usually ends in front in the vicinity of the ventral nerve-roots. The inner boundary, separating the tract in question from the lateral -round bundle, lacks in sharpness and is overlaid by the adjoining strands. H.-low, the tract appears about the middle of the lumbar region and continues throughout tin- remainder of the cord. As Gowers' tract ascends, it fails to show the considerable increase in si/e that mkdu be expected in view of the continual additions that it receives. In explanation of this, the probable mingling of some of its fibres with tho>e of the direct cerebellar tract, rather than their ending in the cord, seems the most plausible ( Xiehen ». The exa< t origin of the constituents of Gowers' tract is still uncertain, but it is very likely that its fibres are chiefly the axones of the neurones ( marginal and inner situated within the posterior horn, partly from the same and partly from the WHITE MATTER OF THE SPINAL CORD. 1045 opposite sides, with contributions, possibly, from the cells of the intermediate gray matter. After traversing the cord, the lateral field of the medulla, and the tegmental FIG. Section of spinal cord at level of lower part of fifth lumbar segment ; gray matter relatively large in amount ; anterior cornua bulky. Preparation by Professor Spiller. X 6. portion of the pons, the tract ascends the brain stem to the vicinity of the inferior cor- pora quadrigemina. Here the major part of the fibres turn backward and, by way of the superior cerebellar peduncle and the superior medullary velum, reach the cerebellum to end mostly in the superior worm, partly on the same side and partly crossed (Hoche). Possibly a part of the cerebellar contingent may share the path of the direct cerebellar tract and in this way reach the cerebellum by its inferior peduncle (Ziehen). It is possible that all fibres from Gowers' tract do not pass to the cerebellum, but that some continue upward to terminate in relation with the neurones of the superior corpora quadri- genyna and of the optic thalamus. The fibres of the tract acquire the medullary coat about the beginning of the eighth month of foetal life (Bechterew). The lateral ground-bundle (fasciculus lateralis proprius) of Flechsig includes the remainder of the lateral column. Much uncertainty prevails as to its detailed paths, but beyond question the composition of the ground-bundle is very complex and comprises a number of long exogenous paths that descend from the brain, as well as one long ascending and many shorter endogenous strands, both ascending and descending. These short tracts occupy chiefly the central parts of the lateral column and, in a general way, lie close to the gray matter, within an area between the ante- rior and posterior horns, known as the boundary zone. They are, however, not limited to this field, as not a few of their fibres lie scattered among the longer exogenous tracts occupying the more lateral portions of the ground-bundle. One long endogenous path, the spino-thalamic tract, is of unusual importance since it estab- lishes a direct sensory link between the cord and higher centres. This tract arises from the cells of the posterior horn of the opposite side, the axones crossing in the anterior commissure to pursue a course brainward within the antero-lateral ground-bundle. Although the fibres of this tract are scattered and not collected into a compact strand, their chief location is just medial to Gowers' tract. Associated with the fibres destined for the optic thalamus are others (tractus spino-tectalis} that end in the region of the corpora quadrigemina. The short endogenous tracts include both ascending and descending fibres which arise as the axones chiefly of the marginal and inner cells of the posterior horn, some coming from the opposite side by way of the posterior intracentral commissure. Entering the lateral column the axones undergo T-like division with ascending and descending limbs. The former pass upward for a distance that usually includes only from one to three segments, then bend inward and enter the gray matter to end probably in relation with other posterior horn cells. The down- wardly directed limbs form the descending endogenous fibres, which, in addition to occupying the boundary zone are also scattered among the longer tracts of the ground-bundle. After a relatively long course, they enter the gray matter to end probably in relation with the anterior horn cells. They are, therefore, regarded as establishing reflex-paths. Since these endogenous strands link together various levels of the cord, they are often collectively termed intersegmental association fibres. The exogenous tracts of the lateral ground-bundle are closely related with those found in the ground-bundle of the anterior column and what may be said of the former largely applies to the latter. Notwithstanding the study that these tracts have received, much uncertainty exists as to their exact origin and termination ; it may be stated in a general way, however, that they bring the higher sensory and coordinating centres into relation with the spinal cord and constitute, therefore, descending paths other than the Seciton of spinal cord at level of third sacral segment ; posterior cornua with substantia gelatinosn are relatively bulky. Preparation by Professor Spiller. X 8. 1046 HIM AX ANATOMY. FIG. 901. Section of spinal cord at level of fifth sacral segment ; anterior cornua small and inconspicuous. Prepara- tion by Professor Spiller. X 8. pyramidal tracts. Among those whose existence within the antero-lateral ground-bundle may be considered as established, are the following : 1. Rubro-spinal fibres from the cells of the red nucleus within the cerebral peduncles. 2. Tec to- spinal fibres from the cells of the anterior corpora quadrigemina. 3. Vestibulo-spinal fibres from the cells of the lateral vestibular (Deiters1) nucleus. 4. Olivo-spinal fibres from the cells of the inferior olivary nucleus. Of these strands, those from the red nucleus and corpora quadrigemina (tecto-spinal fibres), descend within the lateral ground-bundle, whilst the vestibulo-spinal fibres are particularly within the anterior ground-bundle. Although the latter includes the greater part of the vestibulo-epinal fibres, which occupy the ventral margin of the ground-bundle, perhaps similar fibres (tract us vestibulo-spinalis lateralis) descend within the lateral column mesial to the tract of Gowers. Associated with the spino-thalamic tract are fibres, which probably arise within the thalamus ; hence, thalamo-spinal fibres are recognized. For the most part, the exogenous strands are so intermingled and scattered, that they are without definite boundaries. An ex- ception is presented by the olivary fibres, which constitute a fairly distinct triangular bundle, known as Helweg's tract, situated at the periphery of the cord and just behind the ante- rior root-fibres. Concerning the exact relations of these fibres much uncertainty exists, since by some they are regarded as a descending (olivo-spinal) path and by others as an ascending (spino-olivary) one. It is probable that fibres course in both directions. Collectively, these scattered descending paths are of import- ance, since they bring the ventral horn cells under the coordinating influence of the higher centres. The Fibre-Tracts of the Anterior Column. — According to the simplest classification the anterior column includes two subdivisions : ( i ) the anterior pyra- midal tract and (2) the anterior grouna-bundle. The anterior pyramidal tract ( fasciculus cerebrospinalis anterior), also called the uncrossed or direct pyramidal tract, stands in complemental relation with the lat- eral pyramidal fasciculus, being composed of the pyramidal fibres that do not undergo decussation in the medulla oblongata. It usually contains about 15 per cent, of the pyramidal fibres, but may include a much larger proportion ; on the other hand, it may be entirely suppressed when, as rarely happens, total crossing occurs. The direct pyramidal tract occupies the inner part of the anterior column, forming a narrow area along the median fissure that extends from the white commis- sure behind to near the ventral margin of the cord. Ordinarily the tract ends below about the middle of the thoracic cord, but in exceptional cases, when a larger pro- portion of the pyramidal fibres than usual is included in the tract, it may extend as far as the middle of the lumbar enlargement, with corres- ponding increase in its cross area. If, on the other hand, the number of uncrossed fibres is unusually small, the tract may reach only as far as the cervical enlargement, with a reduction of its sagittal dimension. Although often spoken of as the ' ' uncrossed ' ' pyramidal tract, this characteristic applies only to the relation of the fibres at the decussation in the- medulla, since in their downward journey in the cord the great majority of the fibres traverse the anterior white commissure at appropriate levels to end in arborizations about the ventral root-cells of the anterior horn of the opposite side. It is highly probable, however, that some fibres do not undergo decussation, but terminate about the radicular cells of the same side. The anterior ground-bundle (fasciculus anterior proprius i, following the divi- sion of Flrchsii;, includes the remainder of the ventral column. In front, \\here its lateral limits are uneertain, it is continuous with the ground-bundle of the lateral col- umn, the two together being often with advantage regarded as constituting a single antrro-lnti-rnl tract. What has been >aid concerning the constitution of the lateral ground-bundle applies in the main to that of the anterior column, since, here as there, the re-ion bordering the gray matter contains chiefly the short endogenous strands, while the more peripheral (tarts of the ground-bundle are occupied by the long exogenous paths, intermingled, however, with the longer intrinsic fibres. FIG. 902. Section of spinal cord at level of lower part of coccygeaJ segment; differentiation of cor- nua is uncertain. Preparation by Professor Spiller. X 8. WHITE MATTER OF THE SPINAL CORD. 1047 The endogenous fibres arise as the axones, chiefly of the inner cells of the posterior horn, as well as from the cells of the intermediate gray matter (Ziehen), and in great measure cross by way of the anterior white commissure to the opposite anterior column. After undergoing T-division, their upwardly directed limbs constitute the ascending paths and those coursing downward the descending ones. While both sets of fibres for the most part pursue only a short path, that of the descending limbs is usually the longer, the fibres entering the gray matter to end in relation with the anterior horn cells of lower levels. They are, therefore, regarded as secondary reflex paths. The termination of the ascending limbs is uncertain, but probably is within the gray matter of the posterior horn. The exogenous tracts of the anterior ground-bundle, have been mentioned in connection with those of the lateral column. Certain endogenous fibres claim attention, which ascend partly intermingled with the fibres of the vestibulo-spinal tract and partly within the ventral portion of the anterior ground-bundle, although not grouped as a consolidated tract. These fibres belong to the important spino-thalamic system and take origin from the posterior horn- cells of the opposite side of the cord. After crossing by way of the white commissure, instead of cutting through the adjacent anterior horn and ascending amongst the constituents of Gowers' tract, the fibres in question arch ventrally and pass brainward intermingled with the vestibulo-spinal fibres. This part of the path connecting the spinal cord with the thalamus is sometimes noted as the anterior spino-thalamic tract and, according to some authorities, is concerned particularly in carrying impulses of pressure and touch. The anterior column also probably contains fibres that descend from the roof nucleus of the cerebellum and from the quadrigeminal bodies. Since most of such fibres occupy a ventro-median position, they have been designated the sulco-marginal tract. In recapitulation the chief fibre-tracts of the spinal cord may be grouped as follows: I. Within the Posterior Column — ^ Ascending Paths : Direct ascending posterior root-fibres. Ascending endogenous fibres. Descending Paths : Descending posterior root-fibres (comma tract). Descending endogenous fibres. II. Within the Lateral Column — Ascending Paths : Direct cerebellar tract. Gowers' tract. Spino-thalamic tract. Spino-tectal tract. Short endogenous fibres. Descending Paths : Lateral pyramidal tract. Indefinite exogenous tracts (including the rubro-spinal, quadri- gemino-spinal and olivo-spinal). Descending endogenous fibres. III. Within the Anterior Column — Ascending Paths : Ascending endogenous fibres from posterior horn cells. Ascending endogenous fibres from anterior horn cells. Descending Paths : Direct pyramidal tract. Vestibulo-spinal tract. Sulco-marginal tract Blood-Vessels of the Spinal Cord.— The arteries supplying the cord are from many sources — the vertebral, deep cervical, intercostal, lumbar, ilio-lumbar and lateral sacral of the two sides — since the vascular net-work within the pia accompanies the nervous cylinder throughout its length. Above and within the skull, the verte- bral arteries give off the two anterior and the two posterior spinal arteries, of which the latter retain their independence and descend upon the dorso-lateral surface of the cord, one on each side, in front of the posterior nerve-roots. The two anterior spinal arteries, on the other hand, soon unite (somewhere above the level of the third cervical nerve) into a single trunk, which descends along the ventral surface of the cord, just in front of the anterior median fissure. 1048 HIM AN ANATOMY. FIG. 903. Posterior sulcal Parasulcal Postero- lateral As these stems pass downward, they are joined and reinforced by the segmcntal spinal branches given off by the vertebral, intercostal, lumbar and lateral sacral arteries, which enter the spinal canal through the intervertebral foramina and, after piercing the dura and giving off small radicular branches to the nerve-roots them- selves, divide into ventral and dorsal branches that follow the respective nerve-roots to the cord, where they join with the longitudinal trunks which they thus assist in maintaining. By the junction of horizontal branches arising from these arteries, a series of complete annular anastomoses is formed around the cord, which is still further enclosed by additional vertical stems resulting from the union of upward and downward coursing twigs. In this manner, in addition to the large single anterior spinal trunk (tractns arteriosns spinalis anterior) in the mid-line in front and the paired postero-lateral trunk' (tractns arteriosus postero-latcralis spinalis') just in advance of the dorsal nerve-roots, smaller longitudinal arteries are formed at the side and in the vicinity of the nerve-roots. From the arterial net-work within the pia, the nervous tissue is supplied by pene- trating tu'iffs that enter the surface of the cord at various points. The gray matter receives its principal blood-supply from the series of anterior fissural arteries, over two hundred in number, which pass from the anterior spinal trunk backward within the median fissure to its bottom and there divide into right and left branches, which traverse the anterior white commissure to gain the gray matter on either side of the central canal. These vessels, the sulco-mar- ginal arteries, divide into ascending and descending branches that provide for the entire gray matter with the exception of the most peripheral zone. The latter, together with the white matter, receives its supply from the penetrating branches that come from the surrounding intrapial trunks and enter the surface of the cord. Unpaired horizontal twigs, the pos- terior sulcal arteries, follow the posterior median septum at different levels for some distance, but before reaching the posterior commissure usually break up into terminal ramifi- cations, some of which pass to the gray matter of the posterior horns. Communications exist between the penetrating twigs of the radicular arteries and the lateral branches of the anterior tissural. After entering the nervous tissue, however, each artery provides the sole supply for some definite part of the cord ; they are therefore "end-arteries," a fact which explains the extensive and elaborate system of vessels necessary to maintain the nutrition of the cord. _ The plexiform reins within tin- spinal pia an- formed by the union of the small radicles that rolled the blood from the intraspinal capillaries and, after an independ- OUTSC similar to that of the- arteries but not accompanying them, emerge at the surface of the ,,,1(1. From the venous net-work within the pia six main longitudinal trunks are differentiated These are :— the unpaired anterior median rein, in front of the corresponding fissure; the paired antero-lateral reins, just behind the ventral nerve-roots 4hese two seta receiving the tributaries emerging from the median fissure and in the vicinity of the anterior root-fibres ; the unpaired posterior median -ein, behind in the mid line ; and the paired postero-lateral rcitt.i, just behind the dorsal The blood is ronveved from these intrapial channels chiefly by the radicular reins, following tin- nerve-roots, which communicate with or terminate in the anterior and posterior longitudinal spinal reins within the vertebral .anal, from which the Penetrating artery' Anterior fissural Anterior Ascending branch spinal artery Part of transverse section of injected spinal cord showing vascular supply of white and gray matter. X 10. WHITE MATTER OF THE SPINAL CORD. 1049 intervertebral efferents carry the blood into the vertebral, intercostal, lumbar and lateral sacral veins. A part of the blood from the intrapial plexus is conducted upward by the anterior and posterior median veins into the venous net-work covering the pons and thence into the lower dural sinuses. Definite lymphatic vessels within the spinal cord are unknown. Development of the Spinal Cord. — A sketch of the general histogenetic processes leading to the differentiation of the neurones and the neuroglia has been given (page 1009) ; it remains, therefore, to consider here the changes in the neural tube by which the definite spinal cord is evolved. From the time of its closure, probably about the end of the second week of fcetal life, the neural tube presents three regions : — the relatively thick lateral walls and the thin ven- tral and dorsal intervening bridges, \hzfloor- and roof-plates, that in front and behind complete the boundaries of the canal in the mid-line. By the fifth week the lateral walls exhibit a distinct differentiation into three zones — the inner cpendymal layer, the middle nuclear layer and the outer marginal layer, surrounded by the external limiting membrane. In contrast to the other two, the marginal zone is almost devoid of nuclei and, beyond affording support and perhaps assisting in providing a medullary coat, plays a passive role in the production of the nervous elements. By this time the former general oval contour of the developing cord, as seen in cross-sec- tions, has become modified by the conspicuous thickening of the antero-lateral area of the nuclear layer into a prominent mass on each side, whereby the reticular marginal layer is pushed out- FIG. 904. Roof-plate FIG. 905. Roof-plate Dorsal zone Ventral root-fibres Neuroblasts Ependymal layer > Floor-plate Developing spinal cord of about four weeks. X 100. (His.} Floor-plate Ventral root-fibres Developing spinal cord of about five weeks. X 60. (His.) ward with corresponding increase in the width of the entire ventral part of the cord, which is now broadest in front. Within this thickened ventro-lateral part of the nuclear layer, later the anterior horn of gray matter, as early as the fourth week young neurones are seen from which axones grow outward through the marginal zone and pierce the external limiting membrane as the representatives of the anterior root-fibres of the spinal nerves. Postero-laterally the thin nuclear layer is covered by a somewhat projecting thickened area within the marginal layer, known as the oval bundle, whose presence is due to the ingrowth of the developing dorsal root- fibres from the sensory neurones of the spinal ganglion, which process begins as early as the end of the fourth week (His). Associated with these changes, the lumen of the cord becomes heart-shaped in consequence of a conspicuous local increase in its transverse diameter, with corresponding bulging of the lateral wall. In this manner a longitudinal furrow appears by which the side walls of the tube are differentiated into two tracts, the dorsal and the ventral zones (the alar and basal laminae of His). This subdivision is of much importance, since in the cord- segment, and also with less certainty in the brain-segment of the neural tube, these tracts are definitely connected with the root-fibres of the spinal nerves, the dorsal zone with the sensory and the ventral zone with the motor roots. In advance of the floor-plate the ventrally protruding halves of the cord include a broad and shallow furrow which marks the position of the anterior median fissure. During the sixth week the form of the tube-lumen becomes further modified by the elongation and narrow- 1050 HUMAN ANATOMY. ing of the dorsal part of the canal in consequence of the approximation of its walls, which in the course of the seventh week is closer and, by the end of the second month is completed by the meeting and fusion of the adjacent inner layers, with obliteration of the intervening cleft and the production of the posterior median septum in its place. Since the partition is formed by the union of the inner > epeiidymal) layers, it is probable that the septum is to be regarded -entially neurogliar in origin and character. It must be remembered, however, that a certain amount of mesoblastic tissue may be later introduced in company with the blood-vessels which subsequently invade- the septum. The remaining and unclosed part of the lumen for a time resembles in outline the conventional spade of the playing card, with the stem directed ventrally ; but later gradually diminishes in size and acquires the contour of the definite central canal. During these alterations in the extent and form of its lumen, the gray matter of the develop- ing cord markedly increases, especially behind where the posterior horn appears as a projection beneath the broadening mass of the ingrowing dorsal rt>ot-fibres. As \\\e posterior horn becomes better dcfuu-d, the root-bundle becomes meso-laterally displaced, lying behind the horn, and then constitutes the tract of Hnrdach. Coil's tract is formed somewhat later and at about the third month appears as a narrow wedge-shaped area that is introduced between the mid-line and Burdach's tract. Towards the end of the second month, the anterior white commissure is indicated by the oblique transverse ingrowth of axones into the most ventral part of the floor- plate as they make their way to the opposite side. Meanwhile the anterior median fissure has FIG. 906. FIG. 907. ('..•Irs tract Burdach's tract Posterior median septum Root-fibres Pla mater Anterior column Developing spinal cord of about seven and one-half weeks. • 44. {His.) Anteri with i.i.i Developing spinal cord of about three months. X 30. (His.) become deeper and narrower in consequence of the increased bulk of medio-ventral parts of the cord. As the tissute is thus differentiated the process of mesoblastic tissue, which from the earliest suggestion of the groove occupies the depression, is correspondingly elongated and affords a passage for the blood-vessels destined for the nutrition of the interior of the cord. I'ntil the third month the gray matter, derived from the nuclear laser, is much more voluminous than the surrounding inargiii.il layer, which, so tar as the contribution of nervous elements is nnicerned, is passixe, since its conversion into the white matter depends upon the ingrowth of ax. .nes from the neurones situated cither within or outside the cord. The dei'i-lopmt-nt of the individual fibre-tracts includes two stages, between the comple- tion of which a considerable, and sometimes a long, period intervenes. The first marks the invasion of the supporting tissue of the marginal /.one by the ingrowing axones as naked axis- rvlimlers ; the second witnesses the clothing of these fibres with myelin. The period betueen the appearance .if the tract and the development of the medullary coat is variable. In some as in the gieat < erebro-spiual motor paths, although the fibres grow into the cord during the fifth month of fatal life, myelination does not begin until shortly before birth and is not romplet.-d until alter the second vear. In other cases, as in the direct cerebellar, a period of three months, from the third to the sixth, elapses. It is probable that the acquisition of the medullary coat commences before the functional activity of the fibres begins, although such stimulation undoubtedly assists; further myelination proceeds gradually along the course of the fibres and in the direction of conduction. PRACTICAL CONSIDERATIONS: SPINAL CORD. 1051 Based on the observations of Flechsig, His, Bechterew, and others, the time of the appearance and of the development of the medullary coat of some of the fibres within the spinal cord may be given. Fibres of Appear Myelinate Anterior root about 4th week during 5th month Burdach's tract during 4th week end of 6th month Coil's tract about gth week beginning of 7th month Pyramidal tracts end of 5th month 9th month to 2nd^year Direct cerebellar tract beginning of 3rd month about 6th montli Gowers' tract during 4th month during 6th month The presence of the sinus terminalis (page 1030) in the cord at birth depends partly upon the persistence of the lumen of the central canal at the lower end of the conus medullaris and partly upon a proliferation of the wall-cells of the subjacent segment, followed by secondary- dilatation shortly before birth. During the early weeks of development, the neural tube extends to the lowermost limits of the series of somites ; but after differentiation of the root-fibres begins, the segment of the cord below the level of origin of the first coccygeal nerves is marked by feeble proliferation, the effects of which are soon manifest in the rudimentary condition of the caudal end of the cord. With the subsequent development of the other regions, this histological contrast becomes more evident, to which is soon added the conspicuous attenuation caused by the attachment of the lower end of the cord to the caudal pole of the spine, which elongates with greater rapidity than the contained nervous cylinder. In this manner the lowest segment of the cord, with its mesoblastic envelope, is converted into the delicate thread-like filum terminate, within whose upper half are found the remains of the rudimentary nervous tissue. PRACTICAL CONSIDERATIONS : SPINAL CORD. Congenital Errors in Development. — The spinal cord may be absent (^amyelia), or it may be defective in a certain portion (atelo myelid). In such conditions, however, the patient cannot live. The cord may be double from bifurcation {diplomyelia). A spina bifida is a congenital condition due to a deficiency in the vertebrae, almost always of the laminae and spinous processes. There is usually a protrusion of the contents of the spinal canal, although in some cases there is no protrusion, and in others the vertebral canal, or even the central canal of the cord may be open to the surface. Three varieties of tumors are described according to their contents. If the meninges only protrude from the canal in the form of a sac containing cerebro- spinal fluid, it is called a meningocele ; if the sac contains a portion of the cord also it is called a meningo-myelocele. In the third variety, syringo-myelocele, the cavity of the tumor is found to consist either of the dilated canal of the cord, so that the thinned-out substance of the cord is in the wall of the sac, or of a cavity in the cord tissue itself. This is the least common of the three forms. In the meningo-myelocele, which is the most common form, the cord becomes flattened out and attached to the posterior wall of the sac, but still has its central canal intact. The spinal nerves cross the sac to their corresponding intervertebral foramina. In this and in the syringo-myelocele there is frequently some degree of paralysis in the parts below from disturbance of the cord at the seat of the tumor. The most common seat of the defect is in the lumbo-sacral region. It is rare in other parts of the spine. Therefore, the bowels, bladder, and lower extremities are the parts most frequently affected. If the lesion is confined to the lower part of the sacral region, the extremities usually escape. Paralytic talipes is comparatively common. There is no sharp line of demarcation between the medulla oblongata and the cord. The beginning of the latter is variously given as at the origin of the first cervical nerve, the lower margin of the foramen magnum, or the decussation of the pyramids, the last being the more generally accepted. Since in the adult, the spinal cord ends below usually at the level of the disc between the first and second lumbar vertebrae, injuries of the spine below the second lumbar vertebra do not involve the cord. The membranes of the cord, however, containing cerebro-spinal fluid extend as far as the second or third sacral vertebra, so that at this level injuries with infection may cause fatal meningitis. I052 HUMAN ANATOMY. The bony canal is lined with periosteum, unlike the cranium, in which the external layer of the dura mater serves that purpose. The spinal dura is separated from the posterior common ligament, the ligamenta subflava, and the periosteum by a fatty areolar tissue containing a plexus of veins. Extensive extradural hemorrhage may, therefore, occur without serious pressure on the cord. The blood tends to sink by gravity, and later may produce symptoms of compression. The dura is thick and Strong and offers considerable resistance to the invasion of disease from with- out, even to tuberculosis with caries of the vertebrae, or to malignant tumors arising within the vertebne. Infections outside the spinal column, as in abscess of the back, or bed sores, may extend along the communicating veins, giving rise to extradural abscess and perhaps to extensive meningitis. The spinal cord, surrounded by cerebro-spinal fluid, hangs loosely within the dura, being attached to it only by the roots of the spinal nerves which receive invest- ments from the dura as they pass outward, by the ligamenta denticulata, and by the delicate fibres of arachnoid tissue extending from the pia to the dura. The cord is, therefore, not frequently injured from external violence. The numerous articulations of the vertebne and the elasticity of the ligaments and of the intervertebral discs permit the distribution of much of the force applied to the spine before it reaches the cord. The greater part of the cerebro-spinal fluid is contained in the subarachnoid space, which communicates freely with the same space in the cranium, and is con- tinuous with the ventricular fluid through the foramen of Majendie. The cord is exposed to the danger of penetration by sharp instruments only from behind, but even here the overlapping of the laminae and spinous processes offers an excellent protection. This protection is largely lacking above and below the atlas, and the risk there from such wounds is correspondingly greater. At lower levels in order that the canal may be reached, the vulnerating instrument must be directed in the line of the obliquity of the lamime, which will vary in the different portions of the spine, being greatest in the dorsal region. Concussion — shaking with molecular disturbance and without obvious gross lesion — of the cord, although more frequent than has been supposed, is rare because of (a) the arrangement of the different constituents of the vertebral column, which by means of its curves, the elastic intervertebral discs, its numerous joints, and the large amount of cancellous tissue in the vertebral bodies, is able to take up and distribute harmlessly forces of some degree of violence ; (fr) the situation of the cord in the centre of the column, where, as the most frequent serious injuries to the spine are caused by extreme forward flexion, it is somewhat removed from danger in accordance with a law of mechanics that " when a beam, as of timber, is exposed to breakage and the force does not exceed the limits of the strength of the material, one division resists compression, another laceration of the particles, while the third, between the two, is in a negative condition" (Jacobson) ; (c) the suspension of the cord in the surrounding cerebro-spinal fluid ("like a caterpillar hung by a thread in a phial of water" — Treves) by its thecal attachments and nerve-roots ; (d) its connection above with the cerebellum, itself resting on an elastic "water-bed" which minimizes the transmission downward of violence applied to the cranium. Many of the cases ivported as concussion are undoubtedly due to hemorrhage or other gros-, lesions of the cord. ( \uitns/\ni of the cord may occur from sprains, as in forced flexion of the spine. The most fn-quent and most serious cases are those due to fracture-dislocations of the spine, the cord being more or less crushed between the upper and lower fragments. It is s<> delicate a structure that it may be thoroughly disorganized without evident injury to tin- membranes or alteration of its internal form. The paralysis of the parts below will be- complete or partial according to whether the whole or only a part of the transverse section of the cord at the seat of injury is destroyed. Since when the lesion is complete everything supplied by the cord below the seat of the lesion i-> ]>araly/ed. the higher the injury to the cord the greater the gravity of the case. When the atlas ..r axis is fractured and displaced the vital centres in the medulla are in danger and death may result immediately. The phrenic nerves which arise chiefly from the fourth cervical segment, but partly from the third and fifth segments, are also paraly/rd and respiration ceases. PRACTICAL CONSIDERATIONS: SPINAL CORD. 1053 In. fracture-dislocations of the spine it is the body of the vertebra which is most frequently fractured, the ligaments yielding posteriorly and permitting the dislocation. The fractured edges of bone are, therefore, in front of the cord ; and, as the upper fragment passes forward, the anterior or motor portion of the cord is pressed and crushed against the sharp upper edge of the lower fragment. In partial transverse lesions of the cord the paralysis below the lesions affects, therefore, the motor columns of the cord more than the sensory columns which are in part posterior. The most frequent seat of fracture-dislocation of the spine is in the thoraco- lumbar region (page 145). Fortunately, it is this variety which offers the best prognosis, since the cord ends usually just below the lower border of the first lumbar vertebra, and the cauda equina being more movable and tougher than the cord itself, it can better evade the encroachment on the canal, although in spite of these facts, it is not infrequently injured in such lesions. The bodies of the lumbar vertebrae are the largest and most cancellous, the intervertebral discs the thickest and most elastic, so that crushing of them occurs with less tendency to invade the canal and injure the cord than in any other portion of the spine. In caries of the spine (Pott's disease) the lesion is situated in the bodies of the vertebrae, and therefore, in front of the cord. As the inflammatory exudate extends it will invade the spinal canal anteriorly, often producing an external pachymeningitis. The irritation and pressure resulting will again affect the motor portion of the cord, first producing a paralysis of motion in the parts below, varying in degree according to the amount of pressure on the cord. If sensation is impaired it is a later phenomenon and is due to greater pressure upon the cord, and in some cases to myelitis. The loss of motion is often the only effect produced. If the lower cervical region is involved by the lesion the phrenic nerves will escape paralysis, but the arms, trunk, bladder, rectum, and lower extremities will be affected. Since the intercostal and abdominal muscles are involved in the paralysis, breathing will be difficult and will depend upon the action of the diaphragm only. Thus as the lesion occurs at successively lower levels, the highest limits of the paralyzed area descend, and the expectation of life increases. In the cervical and thoraco-lumbar regions where the injuries to the spine and the cord are most frequent, are situated the two enlargements of the cord. The cervical begins at the fourth cervical vertebra, gradually reaches its largest diameter opposite the fifth and sixth vertebrae, and then gradually decreases to the first thoracic, where it merges into the thoracic portion of the cord. Only in the thoracic region does the circumference of the cord remain the same throughout. The lumbar enlargement is shorter than the cervical and begins opposite the tenth thoracic vertebra, gradually increases to the twelfth thoracic, after which it gradually decreases to the conus medullaris. The localization of lesions of the cord, producing symptoms of paralysis, will depend upon the height and extent of the paralyzed areas. It must be borne in mind that the nerve-roots arise from the cord usually at a level higher than the foramina through which they escape from the spinal canal. The first and second cervical nerve-roots pass out of the canal almost horizontally. The intraspinal course of the succeeding nerve-roots increases gradually in obliquity so that the spinous processes of the second, third and fourth vertebrae correspond approximately to the level of the third, fourth and fifth cervical nerve-roots. The seventh cervical spine corresponds to the first thoracic nerve-root. The spinous process of the fifth thoracic vertebra is on a level with the seventh thoracic nerve, and the spine of the tenth thoracic vertebra with the origin of the second lumbar nerve. The first lumbar nerve arises just below the ninth thoracic spine, the second lumbar nerve opposite the tenth thoracic spine, the third and fourth lumbar nerves opposite the eleventh spine, and the fifth lumbar and the first sacral nerves between the eleventh and twelfth thoracic spines. Only the spinous processes can be our surface guides, and it must be borne in mind that they are not always on the level of their corresponding vertebrae. Briefly, it may be said that the eight cervical nerves arise from the cord between the lower margin of the foramen magnum and the sixth cervical spine, the first six thoracic 1054 lir.MAX ANATOMY. nerves between the latter spine and the fourth thoracic, the lower six thoracic nerves between the fourth and ninth dorsal spines FIG. 908. First cervical vertebra Skull First thoracic vertebra First thoracic spine acral vertebra the five lumbar nerves opposite the ninth, tenth and eleventh spines, and the five sacral nerves opposite the twelfth thoracic and the first lumbar spine. A convenient rule to locate the levels of origin of the nerve-roots, applicable to the prelumbar nerves, is given by Ziehen as follows : — For the cervical nerves, subtract one from the number of the nerve, the remainder indicating the correspond- ing spinous process ; for the upper (1-V; tfroracic nerves subtract one ; for the lower (YI-XII) thoracic nerves subtract two. All the cer- vical nerves pass out through the intervertebral foramina above the vertebrae after which they are named, except the eighth cervical, which emerges between the seventh cer- vical and the first dorsal vertebrae. All the other spinal nerves escape below the vertebrae from which they are named. Since the nerve-roots pass a considerable distance down- ward within the spinal canal before leaving it, it follows that a lesion of the cord at a given level, as from a fracture-dislocation of the spine, may be associated with a paralysis of the nerve-roots passing out at or below that level, and arising from the cord at a higher point. This must be taken into account in determining the seat of the lesion, since when the nerve-roots are not involved the lesion will be as much higher than its corresponding inter- vt rtebral foramina (as indicated by the upper limits of the paralyzed area) as the length of the intraspinal course of the corresponding nerve- roots. Each root-cell in the anterior horn of gray matter is connected with a motor fibre, which passes out in the anterior root of a spinal nerve to its muscle. Motor impulses originating in the cortex of the brain, pas- downward along the antero- lateral columns of the cord, chiefly in the lateral pyramidal tract. They first traverse the ganglion cells of the anterior horns before passing out in the anterior or motor roots These ganglion cells constitute, at least functionally, the trophic of the anterior horns, therefore, besides causing First lumbar spine Sacrum Coccyx < 1'i.iKram, based on 1< •'. showing illations of •trln. i- tu Irvi-ls at which siniial nerves escape troni \ -i-itfln.il iaiial. to their destination. centres for the must Us. THE BRAIN. 1055 paralysis (polio-myelitis}, will lead to atrophy of the corresponding muscles. The vasomotor centres are also in the anterior horns, probably in the intermedio-lateral tract. Sensory impulses pass to the posterior horns through the posterior roots, and some of them soon cross to the opposite side of the cord, others ascending in the posterior column. The lemniscus is probably the chief sensory tract in the medulla oblongata, pons, and cerebral peduncles. Every segment of the spinal cord contains centres for certain groups of muscles, and for reflex movements associated with them. A reflex begins in the stimulation of a sensory nerve. The impulse thus created passes to a centre in the cord and thence is transmitted to a motor nerve, thus producing a contraction of the muscle supplied by that nerve. The complete path of this impulse is called a reflex arc. The sensory impulse may be transmitted to different segments of the cord and thence out through the corresponding motor roots. Thus a complicated reflex arc is produced. It is to be assumed, however, that the impulse will take the shortest route, so that simple reflexes will have their reflex arc chiefly in those segments of the cord in which the posterior root enters. Each segment of the cord is connected with fibres from the brain to which must be ascribed the function of reflex inhibition. If the inhibitory fibres are irritated, the reflexes are impaired from stimulation of inhibition. If the conductivity of these fibres is destroyed, the reflexes are increased; but if the reflex arc is broken at any point, the reflexes are lost. Among the most important of these are the skin and tendon reflexes. The centres for the bladder, rectum, and sexual apparatus, are located in the sacral segment of the spinal cord at and below the third sacral segment. *They regulate the functions of these organs and are associated in some unknown way with the brain. (See mechanics of urination, page 1914). Hcemato-rhachis, or hemorrhage into the membranes of the cord (extramedullary hemorrhage), may result from an injury to the spinal column, as a fracture or a severe sprain. Th'e bleeding may be from the plexus of veins between the dura and bony wall of the canal (most frequent), or from the vessels between the dura and the cord. In either case the symptoms will be much the same. There will be a sudden and severe pain in the region of the spine, diffused some distance from the seat of the in- jury, due to irritation of the meninges, and pain transferred along the distribution of the sensory nerves coming from the affected segments of the cord, accompanied by abnormal sensations, as tingling and hyperaesthesia. In the motor distribution there will be muscular spasm, or sometimes a persistent contraction of the muscles. Gen- eral convulsive movements, retention of urine, and, later, symptoms of paralysis may appear, but as a rule the latter is not complete. Hcemato-myelia, or hemorrhage into the substance of the cord (intramedullary hemorrhage) from traumatism, usually occurs between the fourth cervical segment of the cord and the first dorsal (Thorburn), and is commonly due to forced flexion of the spine, which is most marked in this region, as in falls on the head and neck. The cord has been crushed in such accidents without fracture of the spine and with only temporary dislocation. The hemorrhage is usually chiefly in the gray matter and may be only punctate in size, or may be large enough to extend far into the white matter, or even outside the cord into the subarachnoid space. The symptoms usually appear immediately after the injury and are bilateral, suggesting a total transverse lesion. There will be much pain in the back, occasionally extending along the arms or around the thorax. Spasms, rigidity, and paralysis rapidly ensue, with loss of the reflexes in the segment of the cord involved. There may be the same dissociation of sensation as in syringomyelia when the hemorrhage is confined to the centre of the cord. THE BRAIN. The brain, or the encephalon, is the part of the cerebro-spinal axis that lies within the skull. It is produced by the differentiation of the cephalic segment of the neural tube. Although the brain is often of great relative bulk and high complexity, as in man and some other mammals, it must not be forgotten that the spinal cord is the 1056 HUMAN ANATOMY. fundamental and essential part of the nervous axis and that the degree to which the brain is developed is, in a sense, accidental and dependent upon the necessities of the animal in relation to the exercise of the higher nervous functions. In the lowest vertebrates, the fishes, in which association of the impressions received from the outer world is only feebly exercised, those parts of the brain rendering such functions possible, as the cerebral hemispheres, are very imperfectly represented. On the other hand, in man, in whom the capacity for the exercise of the higher nervous functions involving association is conspicuous, the antero-superior parts of the brain, the pallium, as the regions particularly concerned are called, are so enormously developed that the human brain is thereby distinguished from all others. Whether of low or high development, all brains are evolved from certain fundamental parts, the brain-vesicles, differentiated in the head-end of the embryonic neural canal ; the underlying conception of the brain, therefore, is that of a tube, bent and modified to a variable degree by the thickening, unequal growth and expansion of its walls. Even when most complex, as in man, the adult organ exhibits unmistakable evidences of subdivision corresponding more or less closely with the primary brain-vesicles, and contains spaces, the ventricles, that represent the modified lumen of these segments. FIG. 909. Orbital surface of froiual lobe Optic commissure Optic tract Cerebral peduncle Interpeduncular space Medulla Cerebellum Olfactory tract Stalk of pituitary body Tuber cinereum Mammillary bodies Cerebral peduncle Temporal lobe Pons Cerebellum Occipital lobe Spinal cord Simplified drawing of brain as seen from below, showing relations of brain-stem to spinal cord and cerebrum. I 'i vparatory to entering upon a description of the fully formed brain, it is desirable to Consider brielly tin- broad plan according to which the organ is laid down and the general line* along which its evolution proceeds. Before doing so, however, it will In- necessary to take a general survey of the relations of the several divisions composing the brain. 1 >( muled of its inv, sting membranes and tin- attached cranial nerves, and viewed from below i Fig. 909 I, the encephalon is seen to consist of a median brain-stem, that interiorly is directly continuous with the spinal cord through the foramen magnum and al.ove di\ides int.. two diverging arms that disappear within the large overhang- ing m. is, ,,f the cerel.nim. The brain-sti-m includes three divisions, the inferior of which, the medulla oblon^nt^, is the uninterrupted upward prolongation of the spinal .md above i> limited by the projecting lower border of the quadrilateral mass THE BRAIN. of the next division, the pons Varolii. Beyond the upper margin of the pons the brain-stem is represented by a third division that ventrally is separated by a (hep recess into two diverging limbs, the cerebral peduncles, or critra cercbri, to corre- spond with the halves or hemispheres of the cerebrum, each of which receives one of the crura and in this manner is connected with the lower levels of the cerebro- spinal axis. The greater part of the medulla and pons is covered dorsully by the cerebellum, whose large lateral expansions, or hemispheres, project on either side as conspicuous masses, distinguished by the closely set plications and intervening fissures that mark their surface. Of the five component parts of the brain — medulla, pons, cerebral peduncles, cerebrum, and cerebellum — the last two are coated with the cortical gray matter, in which, broadly speaking, are situated the neurones that constitute the end-stations for the sensory impulses conveyed by the various corticipetal paths and the centres controlling the lower-lying nuclei of the motor nerves. .The brain-stem, on the other hand, whilst containing numerous stations for the reception and distribution of sensory impulses, is primarily the great pathway by which the cerebrum and the cerebellum are connected with each other and with the spinal cord. Viewed in a mesial sagittal section (Fig. 910), each of these divisions is seen to be related to some part of the system of communicating spaces that, as the lateral and third ventricles, the aqueduct of Syhmis and the fourth ventricle, extend from the cerebral hemispheres above, through the brain-stem and beneath the cerebellum, to the central canal of the spinal cord below. Since the lateral ventricles are two in number, in correspondence with the cerebral hemispheres in which they lie, their position is lateral to the mid-plane and hence only one of the openings, fat foramina of Monro, by which they communicate with the unpaired and mesially placed third ventricle, is seen in sagittal sections. Both the roof and the floor of the irregular third ventricle are thin, whilst its lateral walls are formed by two robust masses, the optic thalami, the mesial surface Corpus callosu Septum lucidum FIG. 910. Frontal lobe, mesial surfa Anterior commissure Foramen of Monro Lamina cinerea Optic commissure Floor of third ventricle Mammillary body Aqueduct of Sylvius Pons Fo Optic thalamus, dorsal surface Lateral wall of third ventricle (optic thalamus) Cerebral peduncle Roof of Sylvian aqueduct Occipital lobe -Superior medullary velum White core of cerebellum Inferior medullary velum Spinal cord • Simplified drawing of brain as seen in mesial section, showing relation of brain-stem, cerebrum and cerebellum, and ventricular spaces. of one of which forms the background of the space when viewed in sagittal section. The roof vi the ventricle is very thin and consists of the delicate layer of cpcndvwa, as the immediate lining of the ventricular spaces is designated, supported by the closely adherent fold of pia mater which in this situation pushes before it the neural wall and contains within its lateral border a thickened fringe of blood-vessels, the 6? io58 HUMAN ANATOMY. choroid plexus. The two structures, the ependyma and the pia mater together, constitute the membranous velum intcrpositum that forms the roof of the ventricle and lies beneath the triangular/6>/v//.i , whose vaulted form is suggested by the arching ridge that descends in front of the thalamus and marks the position of the anterior pillar of the fornix. Behind, just over the upper end of the Sylvian aqueduct, lies the cone-shaped pineal body that belongs to the third ventricle, from which it is an outgrowth. The floor of the ventricle is also, for the most part, relatively thin and irregular in contour. It corresponds to the median part of the lozenge-shaped area, the interpeduncular space, which, seen on the inferior surface of the brain, is bounded behind by the anteriorly diverging cerebral peduncles and in front by the optic chiasm and the posteriorly diverging optic tracts. The posterior half of this area includes the deep triangular recess at the bottom of which are seen the numerous minute open- ings of the posterior perforated space through which small branches of the posterior cerebral arteries pass to the optic thalamus and the crura. Passing forward, the paired corpora mammillaria, the tuber cinereum, the stalk of the pituitary body occupy successively the interpeduncular space. Anteriorly, between the trans- versely cut optic chiasm below and the recurved portion of the great arching com- missure, the corpus callosum, above, the third ventricle is closed by a thin sheet of nervous substance known as the lamina cincrea. Through the foramina of Monro the lateral ventricles open into the third, and the latter communicates with the fourth ventricle by way of the Sylvian aqueduct. This narrow canal is surrounded below and laterally by the dorsal part or tegmentum of the cerebral peduncles ; above it lies a plate of some thickness the dorsal surface of which is modelled into two pairs of rounded elevations, the superior and inferior corpora quadrigemina. In sagittal section, the fourth ventricle appears as a triangular space, the anterior or basal wall being formed by the dorsal surface of the pons and medulla and the posteriorly directed apex lying beneath the cerebellum. The upper half of the thin tent-like roof of the ventricle is formed by the superior medullary velum, a thin layer of white matter that stretches from beneath the inferior corpora quadrigemina to the cerebellum. A similar lamina, the inferior medullary velum extends from the cerebellum downward, but before reaching the dorsal surface of the medulla becomes so attenuated that this part of the ventricular roof, known as the tela chorioidea, consists practically of the pia mater, although the ependyma excludes the vascular membrane from actual entrance into the ventricle. The pia, however, pushes in the ependyrnal layer and in this manner produces the vascular fringes known as the choroid plexus of the fourth ventricle. When viewed from behind, the ventricle exhibits a rhomboidal outline, the lateral boundaries above being formed by two arms, the superior cerebellar peduncles, that divergingly descend from the sides of the corpora quadrigemina to the cerebellum. Similar bands, the inferior cercbcllar peduncles, convergingly descend from the cerebellar hemispheres to the posterior columns of the medulla and form the lower lateral boundaries of the fourth ventricle. Seen from directly above (Fig. 984), the cerebrum, divided into its hemi- spheres by the deep sagittal fissure, is the only part of tin- brain visible, the other four divisions being masked by the enormously developed overhanging cerebral mantle. The effects of this expansion in displacing base-ward parts which, temporarily in man and permanently in the lower vertebrates, occupy a superior position, are conspicuous when the sagittal section of the developing (Fig. 913) and that of the fully formed human brain (Fig. 910) are compared. It should be noted, that although in the latter the brain-stem and the cerebellum are completely overhung by the cerebral hemispheres, they still are in relation with the free surface of the brain, and by passing beneath the posterior part of the cerebrum the dorsal surface of the cerebellum and of the brain-stem may be reached without mutilation of the nervous tissue. THE GENERAL DEVELOPMENT OF THE BRAIN. Even before complete closure of the anterior end of the neural tube, which takes place probably shortly after tin- end of the second week of foetal life, the cephalic region of this tube, slightly flattened from side to side, exhibits the results GENERAL DEVELOPMENT OF THE BRAIN. 1059 of unequal growth in two slight constrictions separating three dilatations known as the primary brain-vesicles. The posterior of these, the hind-brain,1 is much the longer, exceeding the combined length of the other two (Fig. 911); after a short time when viewed from behind it presents an elongated lozenge-shaped form and, hence, is also called the rhombencephalon. The middle vesicle, the mid-brain, or mesencephalon, is conspicuous on account of its rounded form and prominent position, lying, as it does, over the marked primary flexure which the head-end of the neural tube very early exhibits. The anterior vesicle, known as the fore-brain, or prosencephalon, at first is small and rounded, but soon becomes modified by the appearance, on either side, of a hollow protuberance, the optic vesicle, that pushes out from the lower lateral wall. For a time the optic vesicle communicates with the main cavity of the fore- brain by a wide opening. This gradually becomes reduced and constricted until the FIG. 911. Fore-brain Pallium Mid-brain Optic vesicle Fore-brain (thalamic region) Mid-brain Hind-brain Reconstruction of brain of human embryo of about two weeks (3.2 mm.); A, outer surface; £, inner surface; np, neural pore, where fore-brain is still open ; cs, anlage of corpus striatum ; or, optic recess leading into optic vesicle; At, hypothalamic region. (His.) evagination is attached by a hollow stem, the optic stalk, which later takes part in the formation of the optic nerve that connects the eye with the brain, the vesicle itself giving rise (page 1482) to the nervous coat of the eye, the retina. By the time the optic evagination is formed, the front part of the fore-brain shows a slight bulging, narrow below and broader and rounded above, and separated from the optic outgrowth by a slight furrow. This is the first suggestion of the anlage of the hemisphere or pallium (His). The latter soon gives rise to two rounded hollow protrusions, one on either side of the fore-brain, that rapidly expand into the conspicuous primary cerebral hemispheres. The lower part of the fore-brain includes the region that later, after differentiation and outgrowth from the hemisphere, receives the nerves of smell and is known as the rhinencephalon. A slight ridge (Fig. 911, B\ projecting inward from the roof of the fore-brain, suggests a subdivision of the general space into a posterior and an anterior region. 1 This use of the term hind-brain is at variance with its older significance, still retained by some German writers, as indicating the upper division (metencephalon) of the posterior primary vesicle. In view, however, of the now general application of fore-brain and mid-brain to the other primary vesicles, it seems more consistent to include hind-brain in the series, as has been done by Cunningham, with a distinct gain not only in convenience, but in avoiding terms which in their Anglicised form are at best awkward and unnecessary. io6o !H MAN ANATOMY. The latter, the outwardly bulging pallium or hemisphere-anlage, is limited below by the optic recess, the entrance into the optic vesicle, and, farther front, by a flattened triangular elevation that marks the earliest rudiment of the corpus striatitm. The posterior or tkalamic region extends backward to the mid-brain, from which it is separated by the slight external constriction and corresponding internal ridge. During the fourth week the demarcations just noted become more definite, so that the primary anterior vesicle is imperfectly subdivided into two secondary compart- ments, the telencephalon, conveniently called the cud-brain, and the dienceph- alon. Considered with regard to the details presented by the interior of the fore- brain, the four areas recognized by His are evident. These are (Fig. 912) the region of the pallium and of the corpus striatian, respectively above and below in the telencephalon, and the region of the thalamus and of the hypothalanius respec- tively above and below in the diencephalon. Between the protruding hemispheres, the telencephalon is closed in front and below by a thin and narrow wall, the lamina tcrnihialis, which defines the anterior limit of the brain-tube. While the more detailed account of the further development of these regions will be given in connection with the description of the several divisions of the brain, FIG. 912. Mid-brain Mid-brain Diencephalon Thalamus bf Telencephalon Pallium Spinal cord Spinal cord Reconstruction of brain of human embryo of about four weeks (6.9 mm.); A, outer surface; B, inner surface; /, isthmus; os, aperture of optic stalk ; c/>, cerebral peduncle; cf, cervical flexure; bf, cephalic flcxuu-. I )ia\vn from His mod. 1. it may be pointed out here, in a general way, that tin- pallium _^ives rise to the con- spicuous cerebral hemispheres, which, joined below by a common lamina, expand out- ward, upward and backward and rapidly dwarf the Other parts of the brain-tube which an- thus gradually covered over. Thestriate area thickens into the corpus Stratum, which appears as a striking prominence on the outer and lower wall of each lateral ventricle. The latter represents a secondary extension of the original cavity of the fore-brain enclosed by the developing cerebral hemisphere, and at first is large and thin-walled and communicates by a wide opening with the remainder of the brain- vesicle. The unequal growth and thickening, which subsequently modify the BUITOUnding walls, reduce this large- aperture until it persists as the small foramen of Monro, by which the lateral ventricle communicates with the third ventricle. The latter represents what is left of the cavity of the fore-brain and, therefore, the com- GENERAL DEVELOPMENT OF THE BRAIN. • 1061 bined contribution of the telencephalon and diencephaion. During- the fifth week the diencephaion expands into a relatively large irregular space (Fig. 913), whose roof and floor are thin and whose lateral walls are thickened by the masses of the developing thalami. The hypothalamic region becomes the most dependent part of the fore-brain and gives rise to the structures that later occupy the inter- peduncular space on the base of the brain. The roof of the diencephaion remains thin, does not produce nervous tissue and, in conjunction with the ingrowth of the vascular pia mater, forms the velum interpositum and its choroid plexuses. The pineal body.and the posterior lobe of the pituitary body arise as outgrowths from the roof and floor of the diencephaion respectively. The mid-brain, or mesencephalon, at first large and conspicuous on account of its elongation and prominent position at the summit of the brain-tube, does not keep pace with the adjoining vesicles, and in the fully formed brain is represented by the parts surrounding the aqueduct of Sylvius. Neither does it subdivide, but, while its entire wall is converted into nervous tissue, retains its primary simplicity to a greater degree than any of the other brain-segments. The lateral and ventral walls of the mid-brain contribute the cerebral peduncles' ; its roof gives rise to the corpora quadrigemina ; and its cavity persists as the narrow canal, the aqueduct of Sylvius, that connects the third and fourth ventricles. The posterior vesicle, the hind-brain, or rhombencephalon, the largest of the primary brain-segments, is the seat of striking changes. These include thicken- ing and sharp forward flexion of the ventro-lateral walls, in consequence of which the floor of the space becomes broadened out opposite the bend and assumes a lozenge- shaped outline. The hind-brain is conventionally subdivided (Fig. 913) into a superior part, the metencephalon, and an inferior part, the myelencephalon. Its cavity, common to both subdivisions, persists as the fourth ventricle. The extreme upper part of the metencephalon, where it joins the mid-brain, early exhibits a constriction, which by His has been termed the isthmus rhom- bencephali and regarded as a distinct division of the brain-tube. In the fully formed brain, the isthmus corresponds to the uppermost part of the fourth ventricle, just below the Sylvian aqueduct, roofed in by the superior medullary velum that stretches between the superior cerebellar peduncles. The thickened and markedly bent ventro- lateral wall of the metencephalon gives rise to the pons Varolii, whilst in the roof of the ventricle appears a new mass of nervous tissue, the cerebellum. The myelencephalon, soon limited below by the cervical flexure, shares in the ventral thickening seen in the preceding division. Its floor and particularly its sides, the latter at the same time spreading apart, form the medulla oblongata, which below gradually tapers into the spinal cord. Its roof, in which thinness is always a prominent feature, becomes more attenuated as development proceeds and is converted into the inferior medullary velum and the tela chorioidea that close in this part of the fourth ventricle. The subsequent invagination of this membranous portion of the ventricular roof by the pia mater brings about the production of a choroid plexus similar to that seen in the roof of the third ventricle. From the foregoing sketch of the changes affecting the embryonic brain-tube, it is evident that the anterior and posterior primary vesicles undergo subdivision, while the mid-brain remains undivided, five secondary brain-vesicles — the telencepha- lon, the diencephaion, the mesencephalon, the metencephalon and the myelencepha- lon— replacing the three primary ones. In consequence of the unequal growth of various parts of the cephalic segment of the neural tube, the latter becomes bent in the sagittal plane at certain points, so that, when viewed from the side, the axis of the developing human brain describes an S-like curve (Fig. 912). These flexures, to which incidental reference has been made, bring about a disturbance, for the most part temporary, in the relations of the brain-segments, which in the lower vertebrates follow in regular order along an axis practically straight. In the developing human brain, in which they are most conspicuous, there are three flexures — the cephalic, cervical, and pontile. The first of these, the cephalic flexure which appears towards the end of the second week and before the neural tube has completely closed, is primary and involves the entire head. It takes place in the region of the mid-brain and lies io6a HUMAN ANATOMY. Telencephalon Corpus striatum Optic recess Mesencephalon Isthmus Metencephalon Myelencephalon above the anterior end of the primary gut-tube and of the notochord. At first the axis ot the fore-brain lies about at right angles with that of the rhombencephalon, (Fig. 911) but, with the in- FIG. 913. creasing size of the middle an(j anterior vesicles, the angle of the flexure becomes more acute until the long axis of the fore-brain and of the rhombencephalon are almost parallel (Fig. 912). During the fourth week a second ventral bend, the cervical flexure, appears at the lower end of the hind- brain and marks the separa- tion of the encephalic from the spinal portion of the neural tube. The cervical flexure, which also involves the head, is most evident at the close of the fourth week, when it is almost a right angle ( Fig. 912); after this it becomes less pronounced in consequence of the elevation of the head which succeeds the period when the embryonic axis is most bent. The third flexure appears about the fifth week in the part of the metencephalon in which the pons is later developed and, hence, is termed the pontile flexure. It concerns chiefly the ventral wall, which is in consequence for a time ventrally doubled on itself ; subsequently this flexure almost entirely disappears. In contrast to the preceding bends, this flexure is only partial and involves chiefly the ventral and only slightly the dorsal wall of the neural tube ; on the exterior of the embryo its presence is not detectable. The developmental relations of the chief parts of the fully formed brain to the embryonic brain-vesicles are shown in the accompanying table. TABLE SHOWING RELATIONS OF BRAIN-VESICLES AND THEIR DERIVATIVES. \ Ventral Dorsal zone of brain-wall Diagram showing five cerebral vesicles and dorsal and ventral zones of their wall ; based on brain of embryo of four and one-half weeks. (//«.) PRIMARY SEGMENT SECONDARY SEGMENT DERIVATIVES CAVITY Anterior vesicle Telencephalon Cerebral hemispheres Ollaciory lobes Corpora striata Lateral ventricles ) _ j_ For.unmaof Monro/sccondar>' Anterior pan of third ventricle Prosencephalon or Fore-brain Diencephaion Optic thalami Optic nerves and tracts Sulith:il,miu- u-i;menta liiterpfdmicular structures Pineal and pituitary bodk-s Posterior part of third ventricle Middle vesicle Meiencephalon or Mid-brain McsrtiiTphalon Cerebral peduncles Coipora <|uadrij;i-niina Aqueduct of Sylvius Isthmus Superior cerebellar peduncles Supt'ior medullary velum Rhombencephalon or Metencephalon Pons Cerebellum Fourth ventricle I linil ln.iin Myrli-nri-plialon Medulla lutVrior medullary velum Notwithstanding the great changes in position and relation which many parts of the human brain suffer during development, chiefly in consequence of the enormous expansion of the pallium and the correspondingly large size of its commissure, the GENERAL DEVELOPMENT OF THE BRAIN. 1063 Epithalamus Thalamus / Metathalamus corpus callosum, the fundamental relationships indicated by embryology are of such value that, even in the description of the adult organ, grouping of the various parts of the brain upon a develop- mental basis is found advan- FIG. 914. tageous. Although strict adherence to such a plan would be at times inconven- ient, and, therefore, will not be followed, constant refer- ence to primary relations is imperative. It will be con- venient, therefore, at this place, to call attention to the accompanying outline diagrams which illustrate the principles established by His in his epoch-making studies of the human brain. In addition to showing the five cerebral vesicles, Fig. 913 indicates the relative position and extent of the two fundamental subdivisions of the lateral walls of the neural tube, the dorsal or alar and the ventral or basal laminae, which play such important roles in the differentiation of the various parts of the brain-stem. Fig. 914 shows a later stage, in which the genetic relations of all the more important parts of the brain may be recognized. The greatest complexity is presented in the development of the derivations of the fore-brain, particularly of those which are differentiated from the diencephalon and later are found connected with the third ventricle. In order to set forth the developmental relations of the fore-brain, the following table from His, slightly modified, will be of service : Rhinencephalon Pars optica hypothalami Pars mammillaris hypothalami esencephalon Pedunculi cerebri Isthmus erebellum Pons Medulla Dorsal zone Ventral zone Diagram showing chief derivatives from cerebral visicles brain of embryo of third month. (His.) based on Fore-Brain or Prosencephalon (Pallium Hemisphaerium •< Corpus striatum TELENCEPHALON*; (Rhinencephalon Pars optica hypothalami DIENCEPHALON Pars mammillaris hypothalami f Thalamus Epithalamus Habenula Thalamencephalon Corpus pineale Commissura post. Metathalamus Corpora geniculata PARTS OF THE BRAIN DERIVED FROM THE RHOMBENCEPHALON. THE MEDULLA OBLONGATA. The medulla oblongata, sometimes called the bulb and usually designated by the convenient but indefinite name " medulla," is the direct upward prolongation of the spinal cord. It begins at the decussation of the pyramids below, about on a level with the lower border of the foramen magnum, and ends at the lower margin of the pons above and is approximately 2.5 cm. (i in) in. length. Its general form is tapering, increasing in breadth from the transverse diameter of the cord ( 10 mm. ) below, to almost twice as much (18 mm.) above, and in the antero-posterior dimen- sion from 8-15 mm. Its long axis corresponds very closely with that of the cord and is, therefore, approximately vertical. The medulla, surrounded by the pia and arach- noid, lies behind the concave surface of the basilar portion of the occipital bone, with its dorsal surface within the vallecula between the hemispheres of the cerebellum. Superficially, in many respects the medulla appears to be the direct continuation of the spinal cord. Thus, it is divided into lateral halves by the prolongation of the anterior and posterior median fissures ; each half is subdivided by a ventro-lateral and a dorso-lateral line of nerve-roots into tracts that seemingly are continuations of 1064 HTM AN ANATOMY. the anterior, lateral and posterior columns of the cord. This correspondence, how- ever, is incomplete and only superficial, since, as will be evident after studying the internal structure of the medulla, the components of the cord, both gray and white matter, are rearranged or modified to such an extent that few occupy the same posi- tion in the medulla as they do in the cord. The anterior median fissure is interrupted at the lower limit of the medulla, for a distance of from 6-7 mm. , by from five to seven robust strands of nerve-fibres that pass obliquely across the furrow, interlacing as they proceed from the two sides. These strands constitute the decussation of the pyramids (decussatio pyramidum), whereby the greater number of the fibres of the important motor paths pass to the opposite sides to gain the lateral columns of the cord, in which they descend as the lateral pyramidal tracts. The fibres that remain uncrossed occupy the lateral por- tions of the pyramids and, converging towards the median fissure, descend on either side of the latter within the anterior columns as the direct pyramidal tracts. The FIG. 915. Optic tract Mammillary body Pons (basilar groove) Middle cerebellar peduncle Anterior median fissure Cerebellum Root-bundles of ninth and tenth nerves Infundibulum Cerebral peduncle Interpeduncular space Tiigeminal nerve Middle cerebellar peduncle Inferior cerebellar peduncle (Restiform body) Olivary eminence Arcuate fibres Pyramidal decussation Root-bundles of twelfth nerve Anterior roots of first spinal nerve Brain-stem viewed from in front, showing ventral aspect < f medulla, pons and mid-brain. decussation varies in distinctness, sometimes the component strands being so buried within the fissure that they are scarcely evident, or even not at all apparent, on the surface and can be satisfactorily seen only when the lips of the groove are separated. Above the decussation the anterior median fissure increases in depth in conse- quence of the greater projection of the bounding pyramidal tracts. Its upper end, just below the inferior border of the pons, is marked by a slightly expanded triangular depression, the foramen cffcunt. The posterior median fissure, the direct continuation of the corresponding ur< H>ve on the cord, extends along only the lower half of the medulla, since above that limit it disappears in consequence of (a) the separation and divergence of the dorsal tracts of the bulb, which l>do\v enclose the fissure-, to form the lower lateral boundaries of the lozenge-shaped fourth ventricle (fossa rhomboidalis), and (fi) the gradual backward displacement of the central canal within the closed part of the medulla until, at the lower angle of the ventricle, it opens out into that space. Kach half of the medulla is superficially subdivided into three longitudinal tracts or areas by two grooves situated at some distance to the side of the ventral and dorsal median fissures respectively. ( )ne of these, the antero-lateral furrow, marks the line of emergence of tin- root-fibres of the hypoglossal nerve, which, being entirely THE MEDULLA OBLONGATA. 1065 FIG. 916. Cerebral cortex motor, correspond to the ventral roots of the spinal nerves with which they are in series. The other groove, the postero-lateral furrow, continues upward in a general way the line of the dorsal spinal root-fibres and marks the attachment of the fibres of the ninth, tenth and bulbar part of the eleventh cranial nerves. Unlike the posterior root-fibres of the cord, which are exclusively sensory, those attached along this groove of the medulla are partly efferent and partly afferent, the fibres belong- ing to the spinal accessory being entirely motor, while those of the glosso-pharyngeal and the pneumogastric include both and, therefore, are mixed. The Anterior Area. — This subdivision of the medulla, also known as the pyra- mid, includes the region lying between the anterior median fissure and the antero- lateral furrow. Superficially it appears as a slightly convex longitudinal tract, from 6-7 mm. in width, that continues upward the anterior column of the cord. Each pyramid constitutes a robust strand, which below beginsat thedecussationand, increas- ing slightly as it ascends, above disappears within the substance of the pons. Just before its disappearance, or, strictly speak- ing, after its emergence, the pyramid is slightly contracted on account of the increased width of the bounding furrows. Its chief components being the descending motor paths formed by the cortico-spinal fibres, of which approximately four-fifths pass to the opposite side by way of the decussation to gain the lateral pyramidal tract, it is evident that only to the extent of the direct pyramidal fasciculus and, for a short distance, the anterior ground-bundle, are its constituents represented in the anterior column of the spinal cord. The fibres destined for the direct pyramidal tract, which above the decussa- tion occupy the lateral part of the pyramid, gradually converge toward the mid-line as the decussating fibres disappear, until, at the lower limit of the crossing, they lie next the median fissure, which position they retain in their further descent within the cord. The space thus afforded at the lower end of the medulla, to the outer side of the uncrossed fibres, is occupied by the prolongation of the anterior ground- bundle, which, however, soon suffers displacement as it encounters the pyramid. The ground-bundle lies at first to the outer side of the strands of decussating fibres and then behind the pyramid; higher, it is pushed backward towards the mid-line by the appearance of the inferior olive and the mesial fillet until, finally, it is continued as the posterior longitudinal fasciculus at the side of the median raphe beneath the gray matter covering the floor of the fourth ventricle. The proportion of the pyramidal fibres taking part in the motor decussation is not always the same, from 80-90 per cent, being the usual number. Vary rarely all the fibres cross, with suppression of the direct pyramidal tracts — an arrangement found normally in many lower animals. On the other hand, the direct pyramidal tracts may appropriate an unusually large number of the fibres, even to 90 per cent, of the entire pyramid, the crossed tract, however, never being entirely unrepresented. Ordinarily the tracts of the two sides are approximately of equal extent, but occasion- ally they may be asymmetrical, in which case the excess of the one is offset by a corresponding diminution in the fasciculus of the opposite side (Flechsig). Pyramidal decussation Lateral pyramidal tract Direct pyramidal tract Spinal nerve Diagram showing course and decussation of cortico- spinal (pyramidal) tract ; M, medulla; P, pons; CP, cerebral peduncle; T, thalamus; C, L, caudate and lenticular nuclei ; CC, corpus callosum. 1066 HUMAN ANATOMY. The Lateral Area. — This region is defined on the surface by the antero-lateral and postero-lateral furrows in front and behind respectively, and includes a narrow strip on the lateral aspect of the medulla. Below, the tract is continuous with the lateral column of the cord, a resemblance which is, however, only superficial since within the medulla the large crossed pyramidal tract no longer lies laterally but within the anterior area of the opposite side. The upper part of the lateral area is conspicuously modified by the presence of an elongated oval prominence, the olivary eminence (oliva), produced by the underlying corrugated lamina of gray matter composing the inferior olivary nucleus. The olive- measures about 13 mm. in length and about half as much in its greatest width. Its upper end, more prominent and slightly broader than the lower, is separated from the inferior border of the pons by a deep groove, which medially joins the furrow occupied by the hypoglossal root- fibres and laterally is continuous with a broad depressed area, the paraolivary fossa, that separates the olive from the restiform body and lodges the fibres of the glosso- pharyngeal and pneumogastric nerves. The demarcation of the lower tapering end of the olive is somewhat masked by the anterior superficial arcuate fibres, which cover for a variable distance the inferior part of the olive in their course backward to gain Thalamus Pulvinar Median geniculate body Inferior brachium Superior colliculus Cerebral peduncle Inferior colliculus Superior cerebellar peduncle Superior medullary velum Middle cerebellar peduncle Line of attachment of roof of IV ventricle Inferior cerebellar peduncle (restiform body) Clava Tuberculum cuneatum Tuberculum Kolandi FIG. 917. Lateral geniculate body- Superior brachium Mesial root of optic tract Anterior perforated space Optic tract Lateral olfactory root Optic nerve Optic commissure •Tuber cinereum Mammillary body -Olivary eminence • Arcuate fibres Lateral area of medulla Brain-stem viewed from the side, showing lateral aspect of medulla, pons, and mid-brain. the restiform body. The components of the lateral column of the cord traceable into the medulla — the direct cerebellar and Gowers' tract and the long paths of the lateral ground-bundle — for the most part, with the exception of the direct cerebellar tract, pass beneath or to the outer side of the olive. The superficially placed direct cere- bellar tract gradually leaves the lateral area and passes outward and backward to join the inferior cerebellar peduncle by which it reaches the cerebellum. The Posterior Area. — The posterior region of the medulla is bounded laterally by the fibres of the ninth and tenth nerves ; and mesially, in the lower half of the bulb, by the posterior median fissure and, in the upper half, by the diverging sides of the fourth ventricle. Below, the posterior area receives the prolongations of the tracts of (".oil and of Burdarh, which within the medulla are known as the funic- ulus gracilis and funiculus cuneatus respectively, and are separated from each otln-r by the parumedian sulcus. 1'x -inning with a width of about 2 mm., the i;ra cile hiniculua increases in breadth as it ascends until, just before reaching the lower end of the fourth ventricle, it expands into a well-marked swelling, the clava, about 4 mm. wide, which is caused by a subjacent accumulation of j^ray matter. Then, diverging from its fellow of the opposite side to bound the ventricle, after a short course it loses its identitv as a distinct strand and becomes continuous with the THE MEDULLA OBLONGATA. 1067 inferior cerebellar peduncle or restiform body. The expansion within the upper part of the funiculus gracilis, the clava, contains the nucleus gracilis ( nucleus funiculi gracilis), the reception station in which the long sensory fibres of Goll's tract are interrupted. The triangular interval included between the gracile funiculi, where these begin to diverge, corresponds to the level at which the central canal of the cord ends by opening out into the fourth ventricle. A thin lamina, the obex, closes this interval and is continuous with the ventricular roof. Along the outer side of the gracile fasciculus and separated from it by the para- median furrow, extends a second longitudinal tract, the funiculus cuneatus, which at the lower end of the medulla receives the column of Burdach. Slightly above the lower level of the clava, the cuneate strand also exhibits an expansion, the cuneate tubercle (tuberculura dnereum), that is less circumscribed, but extends farther upward than the median elevation. Beneath this prominence lies an elongated mass of gray matter, the nucleus cuneatus( nucleus funiculi cuneati), around whose cells the long sensory fibres of Burdach' s tract end. Still more laterally, between the roots of the ninth and tenth nerves and the cuneate strand, the posterior area of the medulla presents a third longitudinal eleva- tion, the funiculus of Rolando. The latter is caused by the increased bulk of the FIG. 018 Inferior colliculus Cerebral peduncle Median fossa Median sulcus Middle cerebellar peduncle Acoustic striae Acoustic trigone Restiform body Attachment of ventricular roof Obex Funiculus cuneatus Frenulum Superior trochlear nerve Cerebellar peduncle Floor of fourth ventricle Fovea superior Eminentia teres Trigonum hypoglossi Trigonum vagi (fovea inferior) Funiculus separans Area postrema Funiculus gracilis Lateral area Medulla and floor of fourth ventricle seen from behind, after removal of cerebellum and ventricular roof. X i'A- underlying substantia gelatinosa that caps the remains of the posterior horn of gray matter, and is overlaid by a superficial sheet of white matter composed of the longi- tudinal fibres of the descending root of the trigeminal nerve. While, therefore, the tubercle of Rolando is produced by the exaggeration of gray matter represented within the spinal cord, the gracile and cuneate nuclei are new stations in which the posterior root-fibres not interrupted at lower levels end, and from which the sensory impulses collected by the cord are distributed to the cerebellum and the higher centres by neurones of the second order. The upper half of the posterior area of the medulla is modified by the presence of the fourth ventricle, the lower lateral boundary of which it largely forms, into a robust rope-like strand that diverges as it ascends. Above, it abuts against and fuses with the lateral continuation of the pons and then, bending backward, enters the overhanging cerebellum as the inferior cerebellar peduncle. This strand, also known as the restiform body (corpus restiforme), is seemingly the direct prolongation of the gracile and cuneate funiculi. Such, however, is not the case, since the fibres ' passing from these tracts to the cerebellum by way of the restiform body are the axones of the gracile and cuneate nuclei and, therefore, new links in the chain of conduction. io68 HUMAN ANATOMY. The inferior cerebellar peduncle is the most direct path by which the cerebellum is connected with the medulla and the spinal cord. In addition to the tracts originating in the cord and destined for the cerebellum (the direct cerebellar and possibly part of Cowers' tract), it comprises probably fibres passing in both direc- tions; that is, from the cells within the medulla to the cerebellum, and from the cerebellar cells to the medulla. A more detailed account of these components will be given in connection with the structure of the medulla (page 1072). Upon close inspection of the surface of the medulla, the direct cerebellar tract is seen as an obliquely coursing band that at the lower level of the olive leaves the lateral area and gradually passes backward, over the upper and outer end of the Rolandic tubercle, to join the restiform body, within which it continues its journey to the cerebellum. The anterior superficial arcuate fibres also enter the restiform body, after sweeping around the inferior pole of the olive, or crossing its surface, and the upper part of the funiculus of Rolando. Additional contributions, the posterior superficial arcuate fibres, proceed to the restiform body from the gracile and cuneate nuclei of the same side. Just before bending backward to enter the cerebellum, the restiform body is crossed by a variable number of superficial strands, the striae acusticae, ihat may be traced from the floor of the fourth ventricle and around the inferior peduncle to the cochlear nucleus. INTERNAL STRUCTURE OF THE MEDULLA OBLONGATA. As already pointed out, the correspondence between the spinal cord and the medulla is only superficial, sections across the medulla revealing the presence of con- siderable masses of gray matter and important tracts of nerve-fibres not represented FIG. 919. Fig. 92* Fig. 921 Fig. 920 Ventral (A ) and dorsal (S) aspects of brain stem, showing levels of sections which follow. in the cord, as well as the rearrangement, modification or disappearance of spinal trarm \\hidi an- prolonged into the bulb. In consequence, the medulla, even at its lower end, presents new features, and towards its upper limit varies so greatly in>m the cord that t>ut slight resemblance to the latter is retained. The character- istic features displayed by transverse sections of tin- medulla at different levels depend upon tin- changes induced by four chief factors: — (i) the decussation of the pyramids, (z> the appearance of the dorsal nuclei, (3) the production of the' formatio n-ticularis, and (4) the opening out of the fourth ventricle. THE MEDULLA OBLONGATA. 1069 Funiculus icatus The effects of the decussation of the pyramidal tracts, assuming for convenience that the latter pass from below upward, are conspicuous when followed in consecutive transverse sections from the spino-bulbar junction FIG. 920. . r. ' _^ / Fumculus gracihs cerebralward. I he first suggestion of the decussa- tion appears (Fig. 920) as strands of nerve-fibres, that pass from the field of the lateral pyramidal tract in the lateral column obliquely through the adjacent ante- rior horn of gray matter and across the bottom of the an- terior median fissure to gain the opposite anterior col- umn. At a slightly higher level, where the decussation is fully established (Fig. 921), the large strands of obliquely sectioned fibres are seen cutting through the gray matter, partly filling the median fissure, and collecting on either side of the latter as the large ventral bundles which thence upward constitute the prominent pyramidal fields. In consequence of the greater space required by the pyramids, the isolated anterior horns of the gray matter, cut off by the crossing strands, and the adjacent anterior ground-bundle are displaced laterally and at first lie to the outer side of the decussated fibres. Later, the ground-bundle assumes a position behind the pyramid and eventually becomes continuous with the posterior longitudinal fasciculus (page in 6). The detached anterior cornu of the gray matter is pushed outward and backward and gradually becomes broken up by and interspersed among the fibres of the formatio reticularis. The Posterior Nuclei and the Arcuate Fibres. — The robust tracts of white matter (nerve-fibres) prolonged into the gracile and cuneate funiculi from the tracts of Goll and of Burdach become invaded by new masses of gray matter, the nucleus gracilis and cuneatus. The gracile nucleus, the first encountered, begins as a narrow area of gray Anteri con Transverse section of medulla at level A, Fig. 919; beginning of pyramidal decussation. Weigert-Pal staining. X 5/^- Preparation made by Professor Spiller. FIG. 921. Nucleus gracilis Funiculus cuneatus Spinal root of V nerve Pyramidal decussation Transverse section of medulla at level B, Fig. 919; pyramidal decus- sation well established ; posterior cornua are posterior columns. X displaced laterally by Preparation by Professor Spiller. matter within the correspond- ing strand, on a level with the pyramidal decussation (Fig. 921). It rapidly in- creases in bulk, until it not only invades the entire funiculus gracilis, but also joins the gray matter sur- rounding the central canal. The superficial stratum of spinal fibres gradually dimin- ishes as more and more of its components end around the cells of the gracile nucleus, until, finally, all are inter- rupted. Meanwhile the cuneate nucleus appears within the funiculus cuneatus as a dorsally directed club- shaped mass of gray mat- ter (Fig. 922) which soon becomes a prominent mottled area, sharply defined by the overlying stratum of Burdach fibres. The cuneate nucleus extends to a higher level than the nucleus 1070 HI MAN ANATOMY. gracilis and, even after the disappearance of the latter, continues as a striking collec- tion of gray matter beneath the dorsal surface of the medulla, from which it is separated by the posterior superficial arcuate fibres. Within the upper part of the fasciculus cuneatus the gray matter becomes subdivided into two masses (Fig. 924), the more superficial and continuous of which is called the nucleus cuneatus externus, and the deeper and more broken one, the nucleus cuneatus intcrnus. Owing to the increased bulk of the fasciculi of the posterior area occasioned by the appearance and expansion of the contained nuclei, the dorsal horns of the gray matter are displaced laterally and forward, so that they come to lie on a level with the central canal. Meanwhile the posterior cornua themselves, especially the capping substantia gelatinosa, materially gain in bulk and now appear as two club-shaped masses of gray matter that cause the dorso-lateral projections of the Rolandic tubercles seen on the FIG. 922. Nucleus gracilis Funiculus gracilis Funiculus cuneatus _, / Spinal root of V IHTVC— _^ ^^9j Substantia gelatinosa-j3B^H Accessory olivary nucleus Antero-lateral ground-bundle^ Nucleus cuneatus rCentral gray matter arcuate fibres Fibres of XII nerve Sensory decussation Anterior superficial arcuate fibres' •Pyramidal tracts Transverse section of medulla at level C, Fig. 919, showing sensory decussation, posterior nuclei and pyramidal tracts. X 5%. Preparation by Professor Spiller. surface. Beneath the latter and closely overlying the outer border of the extensive area of the substantia gelatinosa, a crescentic tract of the longitudinally coursing nerve- fibres marks the position of the descending root of the trigeminal nerve (Fig. 922). The chief purpose of the gracile and cuneate nuclei being the reception of the long sensory tracts continued from the cord and the distribution of impulses so received to the cerebellum and to the higher centres, it is evident that new paths of the second order must arise within these nuclei. About on a level with the- upper limit of the pyramidal or motor decussation, fibres emerge from the gracile and cuneate nuclei, sweep forward ami inward in bold curves and cross the median raphe to the opposite side < >f the medulla, immediately behind the pyramids (Fig. 922). They then turn sharply upward and form the beginning of the important sensory pathway known as the median fillet (Icmnisctis nicdialis i that connects the medullary nuclei with the higher centres, as the superior corpora quadrigfemina and the optic thalamus. The first fibres that emerge in this manner from the gracile and cuneate nuclei constitute a fairly well delined strand to which the name sensory decussation or decussation of the fillet is given. It must not be supposed, however, that with this decussation the crossing ceases, for, quite the contrary, it is only the beginning of an extended series of sensory fibres that pass across the raphe at various levels throughout the brain-stem. As many longitudinally coursing fibres are encountered by tho>e sleeping from side- to side, an interweaving of vertical and horizontal fibres occurs, which results in the production of the characteristic formatio reticularis that constitutes a large part of the medulla, as well as of the dorsal or tegmental portions THE MEDULLA OBLONGATA. 1071 Nucleus cuneatus Nucleus gracilis Fibres from Coil's tract Fibres from Burdach's tract Post, superficial arcuate of the pons and cerebral crura. A feeble expression of a somewhat similar structure is seen in the reticular formation within the lateral column of the spinal cord. The Arcuate Fibres. — These originate as the axones of the cells of the gracile and cuneate nuclei and include three sets. The first, the deep arcuate fibres, turn sharply brainward after crossing the raphe and FIG. 923. constitute the chief con- stituents of the mesial fillet. The second set, the anterior superficial arcuate fibres, also cross the mid-line, but these, instead of turning upward, pass forward, enter through the pyra- mid or along its median aspect, and, gaining the surface, sweep over the pyramid and olivary emi- nenceand thenceproceed backward to the restiform body and on to the cere- bellum. An oval collection of small fusiform nerve-cells, the arcuate nucleus (nucleus arcuatus) lies in the path of these fibres, at first on the ventral surface of the pyramid and then along the median fissure. Whilst some additional arcuate fibres arise from the cells of the nucleus, the majority sweep by without interruption. The third set, the posterior superficial arcuate fibres, proceed from the cells of the gracile and cuneate nuclei of the same side and pass beneath the ventricular floor to the adjacent restiform body and thence to the cerebellum. Deep arcuate Anterior superficial arcuate Arcuate nucleus Diagram illustrating source and path of arcuate fibres; RB, restiform body; P, pyramidal tract ; O, inferior olivary nucleus. FIG. 924. Nucleus gracilis Nucleus cuneatus internus Nuc. cuneatus 'externus Funiculus cuneatus - Fasciculus solitarius Nucleus lateral! Nucleus ambiguu Decussation of fillet fibres Median fillet Pyramidal tract Anterior superficial arcuate fibres 5\ Deep arcuate fibres 33- and f ormatio fflj reticularis Dorsal access, olivary nucleus Inferior olivary nucleus Mesial access, olivary nucleus Root-fibres of hypoglossal Arcuate nucleus Transverse section of medulla at level D, Fig. 919, showing posterior nuclei, inferior olivary nuclei, formatio reticularis and dorsal displacement of central canal. X 5%. Preparation by Professor Spiller. The Olivary Nuclei. — These include, in each half of the medulla, three masses of gray matter — the inferior olivary nucleus and the two accessory olivary nuclei. Beneath the prominent olivary eminence lies a corrugated sack-like lamina of gray IOJ2 HUMAN ANATOMY. FIG. 925. Dorsal Ventral Dorso-lateral aspect of inferior olivary nucleus as reconstructed by Dr. Florence R. Sabin. X 5. matter, the inferior olivary nucleus (nucleus olivaris inferior), which in favorable transverse sections appears as a conspicuous sinuous C-like figure. The nucleus resembles a greatly crumpled bag, of which the closed end lies beneath the corresponding superficial protuberance and the mouth, or hilum, looks mesially and somewhat dorsally. When reconstructed and viewed from the side (Fig. 925), the plications of the lateral and dorso-lateral surfaces display a general antero-lateral disposition. On the ventral surface the grooves radiate from the ventral border of the hilum (Sabin). The greatest length of the inferior olivary nucleus is from 12-15 mm., its transverse diameter is about 6 mm., and its vertical one about one millimeter less. The somewhat compressed hilum measures sagittally from 8—9 mm. The plicated lamina of gray matter composing the wall of the sac is from .2-. 3 mm. in thickness and contains numerous small irregularly spherical nerve-cells, each provided with a variable number of dendrites and an axone, embedded within a compact feltwork of neuroglia fibres. The interior of the gray sac is filled with white matter consisting of nerve-fibres that, for the most part, stream through the hilum and thus constitute the olivary peduncle. These strands, known as the cerebello -olivary fibres, connect the cerebellar cortex with the inferior olivary nucleus and probably pass in both directions. Many fibres, the axones of the olivary neurones, issue from the hilum on the one side, cross the mid-line and, sweeping through the opposite olivary nucleus either by way of the hilum or directly traversing the gray lamina, continue their course to the restiform body and thence to the cerebellum. Other fibres originate in the cells of the cerebellar cortex and proceed in the opposite direction along the same pathway to end in relation with the cells of the inferior olivary nucleus. The further links in the chain of conduction are uncertain ; according to Kolliker it is prob- FlG- 926. able that from some of ««*s2 the olivary cells, fibres pass downward into the antero-lateral ground- bundle of the cord. The accessory olivary nuclei are two irregular plate-like masses of gray matter that lie respectively mesially and dorsally to the chief olive. The first of these, the mesial accessory olivary nu- cleus ( nucleus olivaris accessorins mesialis) -^agittally placed lamina, from 10-1 I mm. length, which lies ** ; * '%m in matter Core of \vhitc ni.ttu-r between the tract of the fillet and the root ti lues of the hypoglossal nerve. It extends be- low the inferior olive and, therefore, is encountered in transverse sections at a lower level — immediately above the pyramidal decussation — than the main nucleus. According to the recon- structions of Sabin, the nucleus comprises three dorso-ventral columns of cells, of Section of inferior olivarv nucleus, showing plicated sheet of gray substance traversed by strands of cerebi-llo-olivary fibres. X 100. THE MEDULLA OBLONGATA. 1073 which the lower and middle are continuous and the upper is unconnected, and four small isolated masses of gray matter along the dorsal border of the nucleus. The inferior or spinal end of the nucleus is thickened and bent outward, so that its plane is oblique and parallel with the ventral surface of the chief olive. Higher, when the latter is well established, the mesial accessory nucleus is represented by a narrow broken tract, that corresponds more closely with the sagittal plane. In this situa- tion the nucleus lies between the fillet and the inner end of the chief olive and across Dorsal nucleus of vagus FIG. 927. Ventricular roof Fuuiculus cunea- tus, overlaid by restiform body Fascicu]us solitarius Substantia gela- tinosa overlaid by root of V Nucleus ambiguus- Nucleus lateralis Nucleus cuneatus Post, longitudinal fasciculus Root-fibres of XII Inferior olivary 'nucleus Tract of mesial fillet Pyramidal tract Anterior superficial arcuate fibres Transverse section of medulla at level E, Fig. gig ; central canal has opened into fourth ventricle ; restiform body appearing. X 5- Preparation by Professor Spiller. its hilum. The dorsal accessory olivary nucleus (nucleus olivaris accessorius dorsalis) is less extensive than the median, measuring about 9 mm. in length, and lies close to and behind the posterior lip of the hilum of the inferior olive. The Central Gray Matter. — As pointed out, within the closed part of the medulla the central canal and the surrounding gray matter are gradually displaced dorsally in consequence of the increasing space required by the pyramid, the fillet tract and the posterior longitudinal fasciculus, three paired tracts of longitudinally coursing fibres that lie close to the median raphe and enlarge as they are followed upward. When the central canal opens out into the fourth ventricle, the sur- rounding gray matter is correspondingly spread out and forms the lining of the ventricular floor. Within this gray sheet and near the mid-line, on each side, is seen the group of cells constituting the hypoglossal nucleus from which the fibres of the twelfth cranial nerve arise. These strands take a direct ventro-lateral course through the medulla and emerge on the surface in the groove between the pyramid and olivary eminence. Slightly more lateral, and to the outer side of the hypoglossal nucleus, another group of cells marks the position of the elongated vago-ghsso- pharyngeal nucleus, partly sensory and partly motor, belonging to the tenth and ninth cranial nerves. The fibres of the vagus traverse the medulla laterally and meet the surface at the junction of the lateral and posterior areas. In this way the diverging fibres of the tenth and twelfth nerves subdivide each half of the medulla into three triangular areas — a mesial, a lateral and a posterior (Flechsig). Viewed in transverse sections through the upper third of the medulla, the poste- rior area — the space between the vagus fibres and the dorsal surface of the medulla — is seen to contain a number of important fibre-tracts. ( i ) The restiform body appears 1074 HUMAN ANATOMY. as a large irregularly crescentic tract of transversely cut fibres that occupies the greater part of the periphery. ( 2 ) The descending root of the vestibular nerve is seen to the inner side of the dorso-mesial border of the restiform body as a field of loosely grouped bundles of cross-sectioned nerve-fibres. (3) The fasciculus solitarius, or Fasciculus solitarius FIG. 928. Dorsal nucleus of X , Nucleus of XII I Post. long. \ fasciculus Ventricular roof Nucleus ambiguus Root-fibres of >Restiform body Descending vestibular root Gray column of vestibular root Form.retic.grisea Form, retic. alba Interolivary stratum median fillet) Inferior olivary nucleus Pyramidal tracts ^"Tir-v '^JSJjj&iSs Transverse section of medulla at level F, Fig. 919; ventricular floor is wide; restiform body well established ; descending root of vestibular nerve is seen. X 5. Preparation r>y Professor Spiller. descending root of the vagus and glosso-pharyngeal nerves, shows as a conspicuous transversely cut bundle which lies ventro-mesially to the vestibular root. (4) The descending root of the trigeminal nerve is easily identified as a superficial crescentic field that on its mesial aspect encloses the remains of the substantia gelatinosa Rolandi. The lateral area, between the diverging vagus and hypoglossal root-fibres, is chiefly occupied, in addition to (i) the inferior olivary and (2) dorsal accessory olivary nucleus, by the feltwork of fibres producing the reticular formation. In con- trast to that within the FIG. 929. anterior area, the retic- ulum within the lateral area contains a con- siderable amount of diffuse gray matter be- tween its fibres, and, hence, is known as (3) the formatio rcticularis grisca. Accessions to the irregularly distrib- uted nerve-cells occur as two moredefinitecol- lections ; one of these, (4) the nucleus aw- biguns. consists of an inconspicuous group of laroe cells lying about the middle of the gray reticular substance and is of importance as the nucleus of origin of at least part of the motor fibres of the vagus nerve. The other (5 ), the nucleus latrra/is, includes an uncertain aggregation of medium sized cells, situated near the periphery and ventral Nerve-cell Transverse fibres Longitudinal «_ *?fyf • «•<,*•• fibres \ ' ' . "''"-. •••"**" -~ Portion of formatio reticularis gris«:i. showing nerve-cells and interlacing tt;msverse and longitudinal fibres. X 13°- THE MEDULLA OBLONGATA. 1075 from the trigeminal root. A separate group of somewhat larger cells, nearer the ventral border of the trifacial root, has been designated the nucleus lateralis dorsalis, and by Kolliker regarded as belonging to the origin of the spinal accessory nerve. Cochlear fibres crossing Descending root of restiform body vestibular nerve FIG. 930. Strise icusticae Median Nucleus vestibular of IX nucleus Deiters' nucleus Restiform body ong. isciculu Root of IX nerve Substantia gelatinosa Formatio reticularis alba Tract of mesial fillet Inferior olivary body "\ Fibres of IX nerve Spinal root of V neive Substantia gelatinosa Formatio reticularis grisea x^ X/^>" '-. •• ^^^^^^^~~ Pyramidal tracts Transverse section of medulla at level G, Fig. 919; ventr.il part is narrower, whilst dorsal part is expanded owing to increased size of restitoim uodies. X 4- Preparation by Professor Spiller. In a general way the cells of these nuclei (ambiguus and lateralis) of the Substantia grisea may be regarded as the analogues of the lateral horn-cells of the cord, just as those of the hypoglossal nucleus resemble the anterior root-cells of the spinal nerves. The anterior area, between the mid-line and the hypoglossal root-fibres, is occupied ventrally by ( i ) the pyramidal tract, which appropriates the entire width of the field with the exception of a very narrow peripheral zone that intervenes FIG. 931. , „ Nerve- Longitudinal Transverse cell fibres fibres Median raphe *-Cli I1U1C3 Illylvj Median raphe 'ortion of transverse section of medulla showing median raphe and adjacent formatio reticularis alba. X 130- between the pyramidal fibres and the surface along the median fissure and the ventral aspect of the medulla. This zone is traversed by (2) the anterior superficial arcuate fibres, among which is lodged an irregular column of nerve-cells that constitute (3) 1076 HUMAN ANATOMY. the arcuate nucleus. The latter lies at first chiefly on the ventral and, higher, on the mesial aspect of the pyramidal tract. The cells of this nucleus, small and fusiform, are the origin of not a few of the superficial arcuate fibres, although those from the dorsal nuclei continue their course over the nucleus without interruption. At the upper end of the medulla, the cells of the arcuate nucleus increase in number and mingle with those of the nucleus of the raphe and the pontile nucleus. Dorsal to the pyramid and immediately next the mid-line lies (4) the compact tract of the median fillet, composed of longitudinal fibres that are the upward continu- ation of the deep arcuate fibres, which, from the sensory decussation to the upper limit of the cuneate nucleus, bend sharply brainward after crossing the mid-line. The fillet-tracts are also known as the intcrolivary stratum, as they constitute a compact and laterally compressed field between the inferior olivary nuclei. Lateral to the fillet, between the latter and the hypoglossal fibres, lies (5) the mesial accessory olivary nucleus. (6) The posterior longitudinal fasciculus appears in cross-section as a compact oval or laterally flattened strand, which lies next the raphe and immediately beneath the gray matter covering the floor of the fourth ventricle. This important path will be later described (page 1116). The remaining space of the anterior compartment, between the pyramid and the ventricular gray matter, is occupied by the formatio reticularis alba, so designated in distinction to the formatio grisea on account of its meagre number of nerve-cells, since, with the excep- tion of those scattered in the immediate vicinity of the mid-line (nucleus raphe), few cells are present. The Formatio Reticularis. — Repeated mention has been made of the reticu- lar formation produced by the interweaving of the horizontal and vertical fibres. Whilst particularly conspicuous within the medulla at the levels occupied by the gracile, cuneate and inferior olivary nuclei, on account of the prominence of the arcuate and cerebello-olivary fibres, the formatio reticularis does not end with the disappearance of these nuclei and fibres, but is prolonged upward, although less marked, by transversely coursing fibres derived from the reception-nuclei of various cranial nerves — the vagus, glosso-pharyngeal, auditory, facial, and trigeminal — from whose neurones axones of the second order arise that sweep across the mid-line to join chiefly the fillet tract or to end, perhaps, about nerve-cells of other nuclei. In this manner the formatio reticularis finds representation within the dorsal or tegmental areas of the pons and the cerebral crura. The longitudinal fibres within the formatio reticularis grisea are derived from many sources. Some are the continuation of Cowers' tract ; some belong to the long strands concerned in establishing reflex paths connecting the corpora quadrigemina, nucleus rubrum, vestibular and olivary nuclei with the spinal cord ; some are the axones of tegmental neurones and pursue shorter courses, both descending and ascending, as association fibres linking together different levels of the brain-stem ; while still others are the prolongations of the spino-thalamic and other long tracts of the antero-lateral ground- bundle of the cord. The longitudinal fibres of the formatio alba are chiefly the components of the mesial fillet and of the posterior longitudinal fasciculus with, possibly, the addition of short association fibres proceeding from the nerve-cells that are found within tin- anterior area. The details of a transverse section passing just beneath the lower border of the pons (Fig. 932) vary considerably from those of the level shown in Fig. 930. The ventral half of the medulla has lost in width in consequence of the disappearance of the superficial olivary emi- nence, the inferior olive being at this level represented by only a few irregular plications. The pyramids, likewise, are narrower, and separated In the broadened anterior median fissure. The mesial fillet and the posterior longitudinal fasciculus are now widely separated by the inter- vening nucleus centralis inferior that appears between them along the raphe. The nuclei of the hypoglossal and glosso-pharyngeal nerves are no longer seen, but instead, along the floor of the ventricle underlying the area acustica, appears a large triangular mass of gray matter, the inesiti/ -'t-stihiilar iimfftts. Kxternal to the latter the lateral or /V/Av.v' nucleus and the descending or spinal vestibular root lie close to the restiform body, which in transverse section presents a bean-shaped outline. Between the restiform body and the descending trigeminal root, the fibres of the mesial or resti/m la r part of the auditory nerve pass backward to gain the vestib- ular nuclei. The outer surface of the restiform body is closely related to a considerable THE PONS VAROLII. 1077 tract of gray matter that collectively constitutes the reception-nucleus of the cochlear division of the auditory nerve. This ganglion is subdivided into a superior and an inferior portion, these being the dorsal cochlear nucleus and the ventral cochlear nucleus respectively. They both receive the fibres of the cochlear or lateral division of the auditory nerve. The ventral cochlear nucleus is the starting point of a tract of transverse fibres, that pass horizontally inward, many traversing the fillet and crossing the raphe, and intermingle with those from the opposite side. They thus form a broad strand, the cot-pus trapezoides, that within the pons occupies the lower limit of the tegmental region, which it separates from the ventral. In Fig. 932 FIG. 932. Descend- Gray substance Substantia ing root of floor Nucleus 01 gelatinosa ofVUI ofventricle facial Mesial vestibular nucleus Post. long, fasciculus Betters' nucleus Vestibular nerv Restiforra body \ Restiform body Cochlear nerve Cochlear nerve and ventral cochlear nucleus Spinal root of V Trapezoidal fibres' Inferior olivary nucleus Median fillet Pyramidal tract ' Transverse section of medulla at level H, Fig. 919 ; pyramids are small and inferior olivary nuclei are disappearing; roots of auditory nerve are entering in relation to restiform bodies. X 4. Preparation by Professor Spiller. only the beginning of this tract is visible, but slightly higher, in the pons (Fig. 933), the trapezoidal fibres are shown in force. Strands of fibres from the cochlear nuclei arch over the restiform body and proceed beneath the ventricular floor to the mid-groove ; these mark the course of the strife acusticce seen crossing the ventricle. Ventro-mesial to the spinal root of the trigeminus and the associated Rolandic substance the nucleus of the facial nerve appears as an irregularly oval and somewhat broken group of large stellate cells, from which the strands of root-fibres pass dorso-medially. THE PONS VAROLII. Viewed from in front, the pons appears as a quadrilateral prominence on the ventral aspect of the brain, interposed between the medulla oblongata below, the cerebral peduncles above, and the cerebellar hemispheres at the sides. Its lower and upper limits are well defined by grooves that separate the corresponding borders from the adjacent divisions of the brain -stem, and between these boundaries the pons measures from 25-28 mm. in the mid-line. Laterally, however, its limits are unmarked, as here the mass of the pons narrows and is directly continued on each side as a robust arm which sweeps downward and backward into the cerebellum as the middle cerebellar peduncle. The fibres of the trigeminal nerves, which are attached near its upper and lateral margins, are taken as the conventional lateral limits of the pons, the transverse diameter measured between these points being about 30 mm. The ventral surface of the pons, strongly convex transversely and less so in the opposite direction, lies behind the basilar process of the occipital bone and the dorsum sellae. It is marked by a shallow median groove (sulcus basilaris), which broadens as it ascends and lodges the basilar artery and is bounded on each side by a slight longitudinal elevation. Where the latter meets the medulla, the pyramid is seen to plunge into the pons beneath its transversely striated surface. The longitudinal 1078 HUMAN ANATOMY. ridges are produced by the underlying pyramidal tracts in their journey through the pons from the cerebral peduncles to the medulla. The transverse striation indicates the general course of the superficial fibres towards the cerebellum. The lateral surface, continued from the ventral without interruption, above is rounded and sloping and separated from the cerebral peduncles by a distinct furrow. Below, it passes insensibly into the middle cerebellar peduncle, into which the lower and lateral part of the pons is prolonged. Whilst the superficial striation in a general way follows the contour of the pons, a broad band (fasciculus obliquus pontis) from the upper part of the ventral surface sweeps obliquely backward and downward and overlies the more horizontally directed middle and lower fibres. The free portion of the dorsal surface of the pons contributes the upper half of the floor of the fourth ventricle and is, therefore, not visible until the roof of that cavity is removed. Above the middle peduncle, the sides of the pons are blended with the overlying superior cerebellar peduncles, which, in conjunction with the intervening superior medullary velum, complete dorsally the ring of tissue sur- rounding the narrowed superior end of the fourth ventricle. INTERNAL STRUCTURE OF THE PONS VAROLII. Viewed in transverse sections the pons is seen to include two clearly defined areas, the ventral and the dorsal (Fig. 933). The ventral part (pars basilaris ) presents a characteristic picture in which the large pyramidal tracts are covered in FIG. 933. Abducent fibres Inferior cerebellar I«diinc.le Facial fibres Substantia gelatinosa Spinal root of V Facial nucleus Trapezoidal fibres Superior olive Pyramidal tracts Inferior cerebellar peduncle Nucleus Post, long Nucleus of VI fasciculus of VI x ^^^HKitinerging facial fibres • '/^Vcstibular fibres TT • Spinal root of V .Olivary peduncle _ Superior olive Formatio reticularis -£, oftegmentum Transverse fibres Transverse section of pons at level I, Fig. 910; showing general subdivision into ventral and dorsal (tegmental) areas and nuclei of sixth and seventh nerves. X 3. and excluded from the surface by a conspicuous layer of superficial transverse fibres ( stratum supiTiiciale pontis ), that laterally sweep backward into the cerebellar peduncle and are traversed by the root-fibres of the >c\i nth ami eighth nerves. The pyra- mids no longer appear as compact fu Ids, but are broken uj> into smaller bundles by the transverse strands of ponto-cereU liar fibres. This subdivision becomes more marked at higher levels of the pons (Fig. 936), in which the interweaving of the longitudinal and transverse bundle.-, produces a coarse feltwork (stratum complexum). At the upper border of the pons, the scattered pyramidal bundles become once more collected into two compact strands, which are continued into the central part of the crusta <>f the cerebral peduncle. The dorsal limit of the ventral field is occupied In a well marked deeper layer of transverse fibres (stratum profundum pontis). A considerable am< Hint of gray matter, collectively known as the pontile nucleus THE PONS VAROLII. 1079 Portion of cross-section of pons, showing cells of pontine nucleus. X 300. (nucleus pontis) is distributed within the interstices between the bundles of nerve- fibres. The cells of this nucleus, small in size and stellate in form, are closely related to the ponto-cerebellar fibres of the same and of the opposite side, many constituting stations of interruption in the cortico-cerebellar paths. The dorsal or tegmental part of the pons (pars dorsalis pontis) resembles to a considerable extent in its general structure the formatio reticularis grisea of the medulla, consisting for the most part of a reticulum of transverse and longitudinal fibres, interspersed with nerve-cells, on each side of the median raphe. The appear- ance of certain new masses of gray matter and of nerve- FIG- 934- fibres, together with changes in the position of the fillet, produce details that vary with the level of the section. When this passes above the lower margin of the pons (Fig- 933). two diverging and obliquely cut strands of fibres, coursing from the ventricular floor towards the ventral aspect, mark the root- fibres of the sixth and seventh cranial nerves and divide the dorsal region, on each side, into three areas. The middle area, between the abducent fibres mesially and the facial fibres laterally, contains three important collections of nerve- cells. One of these, the nu- cleus of the sixth nerve, lies close to the floor of the ventricle and beneath the rounded prominence of the eminentia teres, which it helps to produce, and gives origin to the root-fibres of the abducent nerve. These fibres take an obliquely ventral path, slightly bowed towards the raphe, and cut through not only the dorsal but also the ventral part of the pons to gain its lower border, along which they emerge a few millimeters from the mid-line. In favorable sections the nucleus of the sixth is seen separated from the floor of the fourth ventricle by the arching fibres of the facial nerve. Another conspicuous nucleus of the middle area, the superior olive (nucleus olivaris superior), lies near the ventral limit of the tegmental area, partly lodged within an indentation on the dorsal surface of the conspicuous tract of transverse fibres, known as the corpus trapezoideum, that extends from the ventral cochlear nucleus medially and materially aids in defining the ventral boundary of the dorsal area. The superior olive (Fig. 933) is an irregularly spherical collection of nerve-cells, interposed in the path connecting the auditory nuclei with the cerebral cortex, and closely related with the tract of the lateral fillet (page 1082). In addition to contrib- uting numerous fibres to the latter, the superior olive sends others to the abducent nucleus which are seen as delicate strands, the peduncle of the superior olive, that pass towards the nucleus of the sixth nerve and bring this centre into relation with auditory impulses. A small collection of nerve-cells between the fibres of the trape- zoidal tract, ventro-medial to the superior olive, constitutes the nucleus trapezoideum. Close to the medial border of the superior olive a small oval bundle of longitudinal fibres, the central tcgmental fasciculus, is sometimes seen. These fibres are probably derived from the olivary nucleus (Obersteiner). The facial nucleus, a conspicuous but broken oval mass of gray matter (Fig. 933), includes several groups of large stellate cells that lie dorso-lateral to the superior olive and to the inner side of the emerging facial fibres. . From the cells of this nucleus the loosely collected root-fibres of the facial nerve pass back- ward and inward to reach the floor of the fourth ventricle. Here they converge into io8o HUMAN ANATOMY. a compact strand that, as the ascending portion of the nerve, courses beneath the eminentia teres seen on the ventricular floor, close to the mid-line, until it bends outward and, arching around the abducent nucleus, continues ventrally as the emerging root-fibres. The ventral part of the inner area and the adjoining part of the middle one are occupied by the field of the mesial fillet which, at the level under consideration, no longer has its longest axis directed dorso-ventrally, but approximately horizontal. Tin- tract now appears as a modified oval, somewhat compressed from before back- ward, the thicker inner end of which reaches the raphe while the tapering outer end lies near the superior olive. The posterior longitudinal fasciculus is seen as a com- pact strand, immediately beneath the gray matter of the ventricular floor and at the side of the raphe. To the outer side of the emerging facial fibres, and therefore in FIG. 935. Mesencephalic root of V Posterior longitudinal fasciculus Superior cerebellar peduncle Inferior cerebellar peduncle Sensory trigeminal nucleus ^ Middle cerebellar peduncle Motor trigeminal nucleus Motor fibres of V Trigeminal nerve Superior olive Median fillet Deep transverse pontine fibres Pyramidal tracts Middle transverse pontine fibres Transverse section of pons at level J. Fig. 919, showing root of trigeminal nerve with its nuclei. Preparation by Professor Spiller. X3- the lateral pontine area, appear the substantia gclatinosa and the associated spinal root of the trigeminal nerve. Just behind the latter the descending vcstibular root Hi -s close to the inner side of the restiform body. The collection of nerve-cells marking Deiters- nucleus is seen beneath the ventricular floor in close relation with the descending •vestibular root. Sections passing at the level of Fig. 935, and, therefore, about three millimeters above that of Fig. 933, show interesting details connected with the nuclei and roots of the trigeminal nerve. At this level the nuclei and roots of the sixtli and seventh nerves are no longer seen. The median fillet appears on each side as a compressed oval, the long axis of which is liori- /ontal and whose inner end almost touches the raphe. Just above the outer end of the fillet, the cerebral extremity of the superior olive is still visible, to which a few strands of transverse fibres the last of the trape/oid body- -pass. The lateral boundary of the ventral part of the pons is defined by a hugh tract of obliquely rut fibres that marks the entering sensory root of the trigeminal nerve. On following this tract dorsally it is seen to enter a large mass of gray matter, the sensory nucleus of the trigeminal nerve. This ganglion, composed of closely packed small multipolar cells, corresponds to an accumulation of the substantia gclatinosa, which, it will be remembered, is to be seen in all the preceding lower levels intimately related THE PONS VAROLII. 1081 to the descending or spinal root of the fifth nerve. A second and more compact ganglion, the motor nucleus of the trigeminus, lies to the inner side and slightly farther back. It contains large multipolar cells, extends to a somewhat higher level than the sensory nucleus, and is separated from the latter by a strand o'f fibres which arch over the motor nucleus and then pass mesially beneath the ventricular floor to the raphe, where they cross to the motor nucleus of the oppo- site side. These fibres are part of the crossed constituents of the motor trigeminal root. Additional components of the latter, the descending or mesencephalic root, are seen in the interval between the superior cerebellar peduncle and the lateral angle of the ventricle. The motor root itself is represented by several inconspicuous and broken strands of fibres that emerge from the motor nucleus and lie close to the inner side of the large sensory root. Lateral to the sensory nucleus and root of the fifth, and therefore beyond the conventional limits of the pons, the section includes the three large fibre-tracts of the three cerebellar peduncles. The most anterior of these is the middle peduncle into which the corresponding ventral part of the pons is continued. The next and middle tract, joining the tegmentum to the Fourth ventricle FIG. 936. Superior cerebellar penduncle Substantia ferruginea Tegmental area Lateral fillet Nucleus 01 lateral fillet Mesial fillet Lingula overlying superior medullary velum Floor of fourth ventricle Mesencephalic root of trigeminu Posterior longitudinal fasciculus Nucleus centralis superior Gray matter of pontine — nucleus isverse pontine fibres Pyramidal tracts Raphe Transverse section of pons at level K, Fig. 9:9. showing fourth ventricle closed by superior cerebellar penduncles and superior medullary velum. X 3. Preparation by Professor Spiller. outer side of the sensory trifacial nucleus, is the now obliquely cut inferior peduncle or resti- forrn body. The third and dorsal tract is part of the superior peduncle, which being crescentic in cross-section, is here represented by its ventral edge. The three peduncles are thus intimately related as they pass into the central core of white matter of the cerebellum. In sections passing at levels above the middle cerebellar peduncle (Fig. 936), the ventro- lateral surface of the pons is free and unattached and passes over the rounded dorso-lateral border onto the free posterior surface of the projecting part of the pons. Behind, the latter is blended with the robust arms, the superior cerebellar peduncles, that form the lateral walls of the upper part of the narrowing fourth ventricle. This latter space is roofed in by the superior medullary velum which stretches across the ventricle between the superior peduncles and on its upper surface supports the thin lamina of cerebellar cortical gray matter belonging to the lingula of the superior worm. The floor of the ventricle is grooved in the mid-line by a furrow bounded on each side by an elevation — the upward prolongation of the eminentia feres. The depression at the lateral angle of the ventricular floor is the upper part of the/ovea superior. Beneath the latter are grouped the deeply pigmented nerve-cells of the substantia ferruginea that, seen through the intervening layer of tissue, confer the characteristic bluish tint of the 1082 HUMAN ANATOMY. locus caeruleus to this part of the ventricle (page 1095). Mesial to these cells the posterior longi- tudinal fasciculus shows, in transverse section, as a triangular field close to and on each side of the raphe. The most conspicuous feature of the dorsal part of the section is the comma-shaped fibre- tract of the superior ccrebellar peduncle (brachium conjunctivum). The thicker part of the tract lies dorsally and its thinner edge cuts into the lateral part of the posterior area of the pons about half way between its dorsal and ventral boundaries. Between the cerebellar tract and the lateral angle of the ventricle, a slender crescentic strand of transversely cut fibres marks the descending motor or mcsencephalic root of the trigeminal nerve. The tract of the median fillet no longer touches the raphe, but lies as a compressed and horizontally elongated oval along the ventral border of the dorsal field. The three-cornered area included between the outer end of the mesial fillet, the cerebellar arm and the surface, contains a curved triangular tract that sweeps backward and insinuates its pointed dorsal extremity along the outer side of the cere- bellar strand. This tract is the lateral fillet (lenmiscus lateralis), an important part of the pathway by which auditory impulses are carried from the reception-nuclei of the eighth nerve to the inferior corpora quadrigemina, the internal geniculate body and the cerebral cortex. A collection of small nerve-cells, embedded within the outer angle of this tract, gives rise to a number of its component fibres and is, therefore, known as the nucleus of the lateral fillet (nucleus lemniscus lateralis). An additional group, between the lateral fillet and the cerebellar tract, constitutes the nucleus teginenti lateralis (Kolliker). The remainder of the tegmental area is occupied by the formatio reticularis. THE CEREBELLUM. The cerebellum — the "little brain," in contrast to the cerebrum or "great brain" — is placed in the posterior fossa of the skull and beneath the tent-like shelf of dura, the tentorium, which separates it from the overlying posterior part of the FIG. 937. Pons Anterior crescentic lobule Great horizontal fissure Middle cerebellar 'peduncle Medulla Accessory flocculus Flocculus Biventral lobule Postero-suw-rior lobule Postero-inferior lobule Tonsil Pyramid Posterior cerebellar notch Tuber Cerebellum viewed from in front and below ; pons and medulla occupy greater part of vallecula and mask worm. cerebral hemispheres. It lies behind the pons and medulla and the fourth ventricle, with the roof of which space it is intimately related. By means of its three peduncles — inferior, middle and superior — the cerebellum is connected with the medulla, the pons and the mid-brain respectively. The general form of the cerebellum is that of an ellipsoid, compressed from above downward and constricted, save on the dorsal aspect, by a median groove of varying proportions. Its greatest dimension is the transverse diameter, about 10 cm. (4 in. ) ; its least is the vertical ( 3 cm. ), whrle in the sagittal direction the cerebellum measures about 4 cm. in the mid-line and about (> cm. at the side. The cerebellum weighs about 140 gin. (5 oz. ) and constitutes approximately one-tenth of the entire brain-weight The conventional division into a narrow median part, the worm, and the two lateral expanMons, the hemispheres, while convenient for the description of the cerebellum of man, is not warranted by recent comparative and developmental THE CEREBELLUM. 1083 FIG. 938. • Sylvian aqueduct Quadrigeminal plate Superior medullary velum Lobulus centralis Lingula Culmen studies (Stroud, Elliott Smith, Bradley, Bolk and others), since some details given prominence in human anatomy are of secondary importance, and others of greater morphological significance are only slightly emphasized. The surface of the cerebellum is divided by the deeper fissures into more or less well defined areas, the lobules, each of which is subdivided by shallower clefts into narrow tracts, the folia, from 2-4 mm. in width, that usually pursue a curved course within a given lobule and, in a general way, run parallel to one another and to the sulci bounding the tract. On separating the plate-like folia, or on making a section across the plications (Fig. 943), it will be seen that the pattern of the folia is greatly extended by the presence of numerous additional furrows on the deeper and hidden aspects of the leaflets, which are, therefore, ordinarily invisible from the surface. Whether free or sunken, the exterior of the cerebellum is everywhere formed by a cortical layer of gray matter, from 1-1.5 mm- thick, that encloses a medullary layer of white matter of variable thickness. Owing to this arrangement, sagittal sections of the cerebellum expose an elaborate system of branching tracts of white and gray matter, designated as the arbor vita (Fig. 938). The general ellipsoidal mass of the cerebellum, comprising the narrow central vermis and the expanded lateral hemispheres, presents a superior and an inferior sur- face and rounded anterior and posterior borders. Of these the anterior border is indented by a wide groove, the anterior notch (incisura cerebelli anterior), which is much larger than the posterior and bounded laterally by the cerebellar hemispheres and behind by the anterior part of the worm. It is occupied by the inferior corpora quadrigemina and the superior cerebellar peduncles and intervening superior medul- lary velum. The posterior border is interrupted by a smaller median indentation, the posterior notch (incisura cerebelli posterior), which is bounded on each side by the hemispheres and at the bottom by the hind part of the worm, and contains the crescentic fold of dura known as the falx cerebelli. The upper surface of the cerebellum is modelled by the overlying tentorium and presents a slight median trans- versely furrowed ridge that cor- responds to the upper surface of the middle division, orworm, and is known as the vermis superior. The most elevated part of this surface 1 a short distance behind the anterior notch. From this point, designated the mon- ticulus the upper surface slopes gradually downward on each side to the lateral margin's of the hemispheres, whilst it falls off more rapidly towards t e po: The lower surface of the cerebellum is much less regular, owing to the pres- ence of a wide median groove, the vallecula, that is bordered laterally by the rounded hemispheres and is continuous in front and behind xyith the anterio posterior notches. The bottom of the vallecula is occupied by the irregular ndge-hke surface of the middle lobe which is here known as the vermis inferior, of the valley receives the dorsal surface of the medulla. The cerebellum is incompletely divided into an upper and a lower part by a deep cleft the great horizontal fissure (sulcus horizontalis cerebelli). The sulcus beei'ns in'front, at the side of the middle cerebellar peduncle, by the juncUon of two divereincr limbs that embrace the three cerebellar peduncles. It passes usually con- tinuously around the circumference of the cerebellum, but sometimes is interrupted !ore of white matter Pons Tela cho Medulla Mesial sagittal section of brain-stem and cerebellum, showing fourth ventricle, Sylvian aqueduct, and cerebellar worm. 1084 HUMAN ANATOMY. on the worm, and cuts deeply into the lateral and posterior portions of the hemispheres and the worm behind. It is, however, visible on the upper aspect of the cerebellum only for a short distance as it approaches the posterior notch, the remainder of its course being masked by the overhanging border of the hemisphere. Although of cardinal importance in the usual description of the human cerebellum, the great horizontal sulcus is of secondary morphological significance, being a secondary fissure that is developed relatively late in man and feebly or not at all in many other animals. Both the vermis and the hemispheres are subdivided into tracts, or lobules, by the deeper fissures ; these are grouped into lobes, in the conventional division of the human cerebellum, by regarding each median division of the worm as associated with a pair of lateral lobules, one for each hemisphere. LOBES AND FI^IKKS OF THE UPPER SURFACE. — The subdivisions of the superior worm are, from before backward : — (i) the lingula, (2) the lobitlus centra/is, (3) the culmen, (4) the clivus, and (5) the folium cacuminis. With the exception of the lingula, which usually is unprovided with lateral expansions, these median tracts are connected respectfully with (2) the ala lobuli centralis, (3) the anterior crescentic lobule, (4) the posterior crescentic lobule, (5) the postero-superior lobule. Lobus Lingulae. — The lingula, the extreme anterior end of the superior worm, is not free, but lies attached to the upper surface of the superior medullary velum, covered by the over- hanging adjacent part, lobulus centralis, of the worm, which must be displaced to expose the FIG. 939. Ala lobuli centralis Anterior notch Lobulus centralis Anterior crescentic lobule Posterior crescentic lobule Postero-su Postcentral fissure Culmen Preclival fissure Postclival fissure Clivus Great horizontal fissure Postero-inferior lobule Folium cacuminis Tuber Cerebellum viewed from above. structure in question. The lingula consists of a tongue of gray matter, composed of five or six rudimentary transverse folia, that overlies the median and lower part of the superior medullary \i him and, therefore, is behind the upper part of the fourth ventricle (Fig. 938). Occasionally the lingula is prolonged laterally by rudimentary folia onto the superior cerebellar peduncles, in which case these extensions, known as the alae lingula; (vincula lingulae) are reckoned as the lateral divisions of the lobus lingulie. Lobus Centralis. — The median part of the subdivision includes the second segment of the upper worm, the central lobule i lolniliis ojinralisK that lies chiefly at the bottom of the anterior notch and is visible to only a very limited extent on the upper surface of the cerebellum. The central lobule consists of from i.s-iS folia, but not infrequently is divided into two sets of leaflets, which then are collectively someu hat more numerous. It is separated from the lingula by the precentral fissure and from the culmen by the postcentral fissure. On each side the central folia are prolonged into a triangular tract that curves along the side of the anterior notch, form- ing a lateral wing-like lobule, the ala (nla lotmli centralis). The two alae, in conjunction with the median worm-segment, constitute the lobus centralis. Lobus Culminis. — The third division of the upper worm includes the most prominent part of the upper surface of the hemisphere and, being the crest or summit of the general elevation, THE CEREBELLUM. 1085 the monticulus, is called the culmen i oilmen monticuli). It is formed by a half dozen or more longer and shorter folia that laterally are continuous with a lunate area of the hemisphere known as the anterior crescentic lobule (pars anterior lobuli quadrangularis). The latter is the most anterior division of the upper surface of the hemisphere and is a broad crescentic tract limited behind by the preclival fissure(sulcus superior anterior). The two anterior crescentic lobules and the culmen constitute the lobus culminis. Lobus Clivi. — The fourth segment of the superior worm slopes rapidly downward from the culmen and receives the name clivus (declive monticuli). It is separated from the preceding part of the worm by a deep cleft, the central part of the preclival sulcus, which on account of its mor- phological importance has been called the fissura prima (Elliot Smith). Laterally the clivus is connected on each side with the posterior crescentic lobule (pars posterior lobuli quadrangularis) which resembles the lobule in front and is separated from the one behind by the postclival fissure (sulcus superior posterior). The clivus and the two posterior crescentic lobules constitute the lobus clivi. The two crescentic lobules, the anterior and posterior, are regarded by German anatomists as constituting one tract, the lobulus quadrangularis, of which the crescentic lobes then become the pars anterior and pars posterior respectively. Lobus Cacuminis. — The fifth and last segment of the superior worm, the folium cacuminis (folium vermis), varies greatly in its details. It consists of a narrow plate that lies between the clivus above and the tuber below and includes usually only one or two, exceptionally as many as five or six, small folia. Sometimes it reaches the level of the adjoining parts of the worm, of which it forms the posterior end ; at other times it is so sunken and buried that its presence can be demonstrated only after separating the clivus and tuber, with either of which it is occasionally joined. At best it is insignificant in comparison with the large crescentic tracts, the postero-superior lobules, that it connects. The postero-superior lobule (lobulus semilunaris posterior) includes the remainder of the upper cerebellar hemisphere of which it forms the most expanded and lateral tract. In front it is separated from the posterior crescentic lobule by the postclival fissure and behind is limited by the great horizontal sulcus, which it overhangs at the side. The folium cacuminis and the two postero-superior lobules constitute the lobus cacuminis. LOBES AND FISSURES OF THE LOWER SURFACE. — The inferior surface of the cerebellum is modified by a wide depression, the vallecula, in the broader upper half of which the posterior surface of the tapering medulla oblongata is received. The bottom of the valley is occupied by the irregular projection of the inferior worm, which, when the brain-stem is in place, is covered and not seen, except at its posterior third (Fig. 940). After removal of the pons and medulla by cutting through the cerebellar peduncles and the medullary vela, not only the entire inferior worm is exposed, but also the lobulus centralis and its alae are seen to good advantage. The inferior worm is separated on each side from the adjacent surfaces of the cerebellar hemispheres by a groove, the sulcus valleculae, that is deepened in its anterior third by the close apposition of its lateral boundary (the tonsil) with the worm. The connections between the divisions of the inferior worm — from before back- ward (i) the nodule, (2) the uvula, (3) the pyramid and (4) the tuber — and the related parts of the hemisphere are less evident and direct than on the upper surface of the cerebellum. The inferior surface includes four lobules which, from before backward, are: (i) the flocculus, (2) the tonsil, (3) the biventral lobule and (4) the postero-inferior lobule. Lobus Noduli. — The nodule (nodulus^, the most anterior segment of the inferior worm, varies much in size and form, but frequently appears as a rounded triangular prominence, made up of about a dozen folia, that are limited at the sides by the sulcus valleculse and behind by the postnodular fissure. The relation of the nodule to the inferior medullary velum is somewhat analogous, but less intimate, to that of the lingula to the superior velum. The two structures are more or less extensively united, and the nodule thus excluded from the fourth ventricle by the inferior velum that passes beneath the inferior worm to the apex of the posterior recess of the ventricle (Fig. 938). The division of the hemisphere associated with the nodule, the flocculus, lies at some distance from the worm and appears, on either side of the cerebellum, as a wedge-shaped group of short irregular folia that project between the middle cerebellar peduncle and the anterior border of the hemisphere. When well developed it may touch the adjacent margin of the anterior crescentic lobule of the upper surface. In addition to the chief floccules, io86 HUMAN ANATOMY. composed of from ten to twelve leaflets, a second and smaller set, known as the paraflocculus or accessory flocculus, lies behind and lateral to the main group, often completely buried beneath the overhanging margin of the biventral lobule. In the embryo and in many mammals, the paraflocculus is of considerable size and then shares the relatively much greater development of the flocculus than seen in the adult human brain. The connection between the flocculus and the nodule is established by the lateral part of the inferior medullary velum, which constitutes the peduncle of white matter for the floccular folia. In this manner the nodule and the two flocculi, with the intermediate part of the medullary velum, constitute the lobus noduli. Lobus Uvulae. — The uvula, the next part of the inferior worm, is laterally compressed between the deeper parts of the two tonsils. It varies in form and often appears as a narrow ridge-like structure, triangular on section, of which the median crest alone is seen when the tonsils art- in place. The uvula is limited in front by the postnodular fissure, and behind by the prepyramidal, which laterally, as the post-tonsillar fissure, curves outward along the postero- lateral border of the tonsil. The free median surface of the uvula is usually cleft into two or three major subdivisions, which in turn are scored by shallower incisions, so that from six to ten leaflets are present. Some two dozen additional folia mark the hidden lateral surfaces, the entire number being thus usually raised to thirty or more. The tonsil or amygdala (tonsilla), the segment of the hemisphere associated with the uvula, is a pyramidal mass lying between the worm and the biventral lobule and forming the central zone of the general quadrant embracing the lower surface of the entire hemisphere. The free convex inferior surface of the tonsil is irregularly triangular in outline and bounded by a rela- tively straight median margin (along the sulcus valleculse), an outwardly arched postero-lateral FIG. 940. Lobulus centralis Superior cerebellar peduncle Middle cerebellar peduncle Inferior medullary1 velum Great horizontal fissure Postero-inferior lobule Biventral lobule Ala lobuli centralis Superior medullary velum Fourth ventricle Nodule Accessory flocculus Flocculus Uvula Posterior notch Tonsil Inferior aspect of cerebellum, after removal of pons and medulla. Pyramid border (along the curved posttonsillar fissure) and a notched anterior edge. This, the chief surface, is marked by a straight furrow that extends from the indentation on the anterior border backward and inward and marks a line along which the curved folia, from nine to fourteen in number, abut. Of the other surfaces bearing folia— the median, posterior and lateral — that directed towards the uvula (median) alone is entirely unattached, the others, with the superior, receiving the stalk of white matter. The deeper part of the tonsil is subdivided, so that on removing the larger and more superficial portion of the amygdala a buried and accessory segment of its mass often remains. Beneath (really above) the tonsil, a narrow tongue, marked with short transverse folia, stretches from the posterior part of the uvula across the root (.! the space occupied by the tonsil to the upper and lateral part of the amygdala. This tract, known as the furrowed band (alac uvulae) connects the worm with the hemisphere and thus joins the uvula and the two tonsils into the lobus uvula' The posterior border of the furrowed band is free, whilst its anterior one is continuous with the inferior medullary velum. After removal of the tonsil by cutting through its supero-lateral stalk, a deep recess is left, which is bounded medially by the uvula and laterally by the biventral lobule and roofed in by the furrowed band and the inferior velum. To this space the older anatomists gave the name, "bird's nest" (nidus ari*\. Lobus Pyramidis.— The pyramid (pyramis) the segment of the inferior worm lying behind the uvula and in front of the tuber, is partly covered by the tonsils. Posterior to the latter THE CEREBELLUM. 1087 it is seen at the bottom of the vallecula between the median areas of the biventral lobules, where it forms the most prominent division of the worm. It is an elongated club-shaped mass, attached by a narrow stalk and separated from the adjacent parts of the worm by the prepyramidal and postpyramidal fissures and from the hemispheres by the sulci valleculie. The convex inferior surface usually presents from 5-8 superficial folia, those towards the uvula being longer than those directed towards the tuber. After removal of the tonsil, a narrow band, the connecting ridge, is seen passing, on each side, from the anterior part of the pyramid to the adjacent mesial end of the biventral lobe, which, in this manner, is brought into relation with the worm. The biventral lobule (lobulus biventer) ordinarily consists, as its name implies, of two sub- divisions, which together appear on the surface as a curved zone, the extremities of which are more contracted than the intermediate tract that attains a breadth of 15 mm. and more. The details of form and foliation are quite variable, the lobule being not only sometimes much broader than usual, but farther subdivided, so that three, instead of two, tracts are included. The broader outer end of the lobule reaches the anterior margin of the hemisphere, and the narrowed inner end the vallecula, in consequence of which the component superficial concentric leaflets, some twelve to sixteen in number, are compressed and thinner as they approach the sulcus valleculce. The biventral lobule is separated from the tonsil, around which it curves, by the lateral extension of the prepyramidal or post-tonsillar fissure and is limited behind by the arched postpyramidal fissure. Lobus tuberis. — The tuber (tuber vermis) forms the most posterior division of the inferior worm and lies beneath the great horizontal fissure when that sulcus is continuous across the mid-line. When the folium cacuminis is small and buried, the tuber comes into close relation with the lower end of the clivus, the three divisions of the worm just mentioned all springing from a common stalk of white matter. The tuber is of a general FIG. 941. Superior worm Roof of fourth ventricle (lobulus centralis) Superior cerebellar peduncle^ \^ _^gaeg^gg^_^-^'^^ ^ Nodule Middle cerebellar peduncle. Flocculus^ ^*^/7 /i098HK<*3iE£^:l3^rT>w ^—-Inferior medullary velum Great ~~" ' "* --— -* — •- " ' " -— -~-" horizontal fissure" _ -Furrowed band Postero-inferior lobule' „ __ _— ^ — Pyramid ^K A" ... •••..*. Biventral lobule Position of removed tonsil ,^_ _^^ Cerebellum, seen from below after removal of tonsils. conical form, with the base directed towards the pyramid, from which it is separated by the postpyramidal fissure, and its apex projec'insr into the posterior cerebellar notch. ^It presents a few, from 2-4, superficial folia, which model the posterior pole of the worm, as viewed from behind and above. The tuber is directly connected on each side with a considerable crescentic tract, the postero-inferior lobule (lobulus semilunaris inferior),' that is limited in front by the lateral extension of the postpyramidal fissure f sulcus inferior anterior) and behind by the great hori- zontal fissure. After emerging from the sulcus valleculae, the folia rapidly expand into a lunate tract, from 15-25 mm. in its widest part, that forms the immediate posterior border of. the hemisphere. The postero-inferior lobule is usually described as divided into two parts, an anterior and a posterior, by the postgracile fissure (sulcus inferior posterior), but quite frequently further subdivision of the superficial folia, from i2-iS in number, results in defining three sublobules. The anterior of the two conventional subdivisions is a narrow tract of fairly uniform width to which the name lobulus gracilis is applied. The lunate posterior area, much less regular in contour and foliation, is known as the inferior crescentic lobule (lobulus semilunaris inferior) and sometimes presents evidence of subdivision into two secondary crescentic areas. The postero-inferior lobules and the tuber constitute the lobus tuberis. io88 HUMAN ANATOMY. In recapitulation, the foregoing cerebellar lobes, with their component worm-segments and associated hemisphere-tracts, and the intervening fissures may be followed in order, from the anterior and superior end of the worm to its front and lower pole. Although not agreeing with a morphological division, such grouping ' is convenient as applied to the adult human cerebellum. THE LOBES OF THE CEREBELLUM. WORM HEMISPHERE LOBE Lingula (Vinculum lingulae) Lobus lingulae — Sulcus precentralis — Lobulus centralis Ala lobuli centralis Lobus ccntralis - Sulcus postcentralis - Culmen monticuli Lobulus lunatus anterior Lobus culminis - Sulcus preclivalis — Clivus monticuli Lobulus lunatus posterior Lobus clivi — Sulcus postclivalis — Folium cacuminis Lobulus postero-superior Lobus cacuminis Sulcus horizontalis Tuber vermis Lobulus postero-inferior Lobus tuberis — Sulcus postpyramidalis — Pyramis Lobulus biventer Lobus pyramidis Sulcus prepyramidalis — Uvula Tonsilla Lobus uvulae - Sulcus postnodularis - Nodulus Flocculus Lobus noduli Architecture of the Cerebellum. — With the exception of where the robust peduncular collections of nerve-fibres' enter the hemispheres and immediately above the dorsal recess of the fourth ventricle, the cerebellum is everywhere covered by a continuous superficial sheet of cortical gray matter which follows and encloses the sub- divisions of the white core. The latter, as exposed in sagittal sections of the hemi- sphere, is seen to be a compact central mass of white matter, from which stout stems radiate into the various lobules. From these, the primary stems, secondary branches penetrate the subdivisions of the lobules, and from the sides of these, in turn, smaller tracts of white matter, the tertiary branches, enter the individual folia. Over these ramifications of the white core, the cortical gray matter stretches as a fairly uniform layer, about 1.5 mm. thick, that follows the complexity of the folia and fissures. The resulting arborization and the contrast between the white and gray matter are particularly well shown in sections passing at right angles to the general direction of the folia. This disposition is especially evident in median sagittal sections (Fig. 938), where the less bulky medullary substance of the worm, also known as the corpus trapezoideum, and its radiating branches produce a striking picture, to which the name, arbor vita1 cerebelli, is applied. The Internal Nuclei. — In addition to and unconnected with the cortical layer, four paired masses of gray matter, the internal nuclei — one of considerable size and three small — lie embedded within the white matter. The dentate nucleus (nucleus dentatusX or corpus dentation, the largest and most important of the internal nuclei, consists of a plicated sac of j^ray matter (Fig. 951) and resembles in many respects the inferior olivary nucleus. Like the latter, it is a crumpled thin lamina of gray matter which is folded on itself into a pouch, enclosing white matter, through whose medially directed mouth, termed the hilitm, emerge many fibre-constituents of the superior cerebellar peduncle. The dentate nucleus never encroaches upon the core of the worm, but lies embedded within the anterior part of the median half of the hemisphere, with its long axis 'Modified from Schafer and Thane in Quain's Anatomy, Tenth Edition. THE CEREBELLUM. 1089 directed forward and somewhat inward and, therefore, slightly oblique to the sagittal plane. Anteriorly the nucleus reaches the level of a frontal plane passing through the precentral fissure ; laterally it extends to about the middle of the hemisphere (Ziehen) ; whilst medially its postero-inferior end comes into such close relation with the fourth ventricle that a slight elevation, eminentia nuclei dentati, is produced on the lateral ventricular wall. In its longest (antero-posterior) dimension the nucleus measures from 15-20 mm., and in breadth about half as much. Of the other paired internal collections of gray matter — the nucleus fastigii, the nucleus emboliformis and the nucleus globosus — the nucleus fastigii, or the roof nucleus, is the best defined. It lies within the core of the worm, in the lower part of the corpus trapezoideum, very close to the mid-line and to its fellow of the oppo- site side. In its general form the nucleus is egg-shaped, with the posterior pole somewhat prolonged, and in its sagittal diameter measures about 10 mm. and in the transverse dimension about half as much. The nucleus extends from the base of the Nucleus fastigii Nucleus globosus Rest i form body (external division) Superior worm Decussation of roof-nuclei Fourth ventricle Nucleus dentatus Restiform body (internal division) Ventral cochlear nucleus Flocculus Posterior longitudinal fasciculus Pyramidal tracts Vestibular nerve Inferior olivary nucleus Section across upper part of fourth ventricle, showing internal cerebellar nuclei ; new-born child. Weigert-Pal staining. Preparation by Professor Spiller. lingula to the stem of the pyramid, and in frontal sections (Fig. 942) appears circu- lar in outline and closely related with fibre-tracts that in part end in the nucleus of the opposite side. The nucleus emboliformis, or embolus, is an irregular wedge-shaped plate of gray matter that partly closes the hilum of the dentate nucleus, in much the same manner that the median accessory olivary nucleus obstructs the mouth of the chief olivary nucleus. In its sagittal diameter it measures about 15 mm. , and in the vertical one approximately one-fourth as much ; it decreases in thickness from about 3 mm. in front to a slender wedge behind. The embolus rests upon the superior cerebellar peduncle, its front end extending to within a few millimeters of the precentral fissure and its posterior pole reaching almost as far back as the dentate nucleus, with which it is united by a limited connection. The nucleus globosus lies close to the medial side of the embolus, between the latter and the roof nuclei. In its general form the nucleus is comparable to a sphere attached to a sagittally directed stalk (Ziehen). The globular head, about 5 mm. in diameter and somewhat transversely compressed, lies above the tonsil and is continuous with the stalk that extends backward for a distance of about 8 mm. By means of uncertain and limited attachments the nucleus globosus is loosely connected with the roof nucleus and the embolus, and also joins the postero-inferior 1 090 HI' MAN ANATOMY. Molecular layer Granule layer White matter ^•— Cells of Purkinje part of the dentate nucleus. Since the latter and the embolus are likewise slightly connected, it is evident that all four internal nuclei are more or less continuous masses of .gray matter. In structure the internal nuclei differ markedly from the cerebellar cortex, since in the main they are composed of irregularly disposed nerve-cells of one kind interspersed with numerous nerve-fibres. The dentate nucleus contains cells from .020-.030 mm. in diameter whose bodies are angular or stellate in outline and pig- mented in varying degrees. Their processes are usually so disposed that the axones pass into the medullary substance enclosed by the plicated lamina and the dendrites into the surrounding white matter of the hemisphere. Numerous fibres enter the dentate body from without, many being the axones of the Purkinje cells, and break up into a rich plexus within the folded sheet of gray substance. Since the nucleus emboliformis and the nucleus globosus are only incompletely isolated parts of the dentate nucleus, their structure corresponds closely with that of the chief mass. The roof-nuclei, on the FIG. 943. contrary, possess cells of much larger size (.040 to .080 mm.), more rounded form and greater uniform- ity in tint, although their general yellowish brown color implies less intense pigmentation. Numerous strands of nerve-fibres sub- divide the nucleus into secondary areas, while some large transversely coursing bundles establish a decussation with the roof- nucleus of the opposite side. The Cerebellar Cortex. — When the folia are sectioned at right angles to their course, each leaflet composing the characteristic arborization is seen to consist of a cen- tral tract of white medul- lary substance, covered in by the continuous super- ficial sheet of cortical gray matter. The latter, usually somewhat less than one millimeter in thickness, includes two very evident strata — the outer and lighter molecular layer and the inner and darker grannie layer. The molecular layer is of uniform thickness, about. 4 mm., and contains three varieties of nerve-cells — the Purkinje cells, the basket cells and the small cortical cells. The Purkinje cells, the most distinctive nervous elements of the cerebellum, occupy the deepest part of the molecular layer, where they are disposed in a sing It- row along the outer boundary of the subjacent granular layer. The cells are most numerous and more closely placed upon the summit of the folium and fewer and more scattered aloni; tin- fissures, in which situation they are also often of less typical pyriform shape. They possess a large flask-like body, about .060 mm. in diameter, from the pointed and outwardly directed end of which usually one, sometimes more, robust dendritic process arises. The chief process, relatively thick and very short, soon divides into two branches, which at first diverge and run more or less horizontally and then turn sharply outward to assume a course vertical to the surface and undergo repeated subdivision. Tin- arrangement of the larger dendrites is very striking and recalls the branching of the antlers of a deer. The smaller processes arise at varying jji1 -,..!__ ..---' ^ \ Central limb of white matter Transverse section of cerebellar folium, showing relations of cortex to underlying white matter. X 10. THE CEREBELLUM. 1091 and often acute angles, the completed division resulting, as displayed by silver impregnations (Fig. 944), in an arborization of astonishing richness and extent that often reaches almost to the outer boundary of the molecular layer. The dendritic ramification of each cell is limited, however, to a narrow zone extending across the folium and, hence, when examined in sections cut parallel with the plane of the folium, these expansions are found to be confined to tracts separated by zones of the molecular layer that are uninvaded by the dendrites of the Purkinje cells. The axones of the latter arise from the rounded basal or deeper end of the pyriform body and at once enter the granular layer, which they traverse to gain the white medullary core of the folium. In their course the axones give off a few recurrent collaterals that end within the molecular layer in the vicinity of the bodies of the cells of Purkinje. The stellate or basket cells lie at different planes, but chiefly within the deeper half of the molecular layer. They possess an irregular stellate body, from .OIO-.O2O mm. in diameter, from which several dendrites radiate. Their chief feature of interest is the remarkable relation of FIG. 944 the axone, which extends across the folium in an approximately horizontal plane along and to the outer side of the row of the Purkinje cells. During this course the axone gives off from three to six collaterals that descend to the cells of Purkinje, whose bodies they sur- round and enclose with a basket - like arborization, the terminal ramification of the main process itself ending in like manner. By means of this arrange- ment each basket cell is brought into close relation with several of the larger elements. Purkinje cell from silver preparation of cerebellar cortex; A, axone. X 120. The small cortical cells occur at all depths, but are most numerous in the more superficial planes, in which they appear as diminutive multipolar elements with radiating dendrites and axones of uncertain destination. The granule layer, of a rust-brown tint when fresh and deeply colored in stained preparations, is thickest on the summit of the folia and thinnest opposite the bottom of the sulci. While sharply defined from the overlying molecular layer, it is less clearly distinguished from the medullary substance. The granular layer con- tains two varieties of nerve-cells — the granule cells and the large stellate cells. The granule cells are very small (.ooy-.oio mm.) and numerous and so closely packed that they confer upon the stratum its distinctive density. They are provided with from three to six short radiating dendritic processes that end in peculiar claw- like arborizations in relation with other granule cells. The axones, directed towards the surface, enter the molecular layer, within which, at various levels corresponding to the depth of the cells, they undergo T-like division. The two resulting branches run horizontally and lengthwise and in the folium— that is, parallel to the surface and at right angles to the plane of expansion of the dendrites of the Purkinje cells, through the arborizations of which they find their way and with which they probably come into close relation. The large stellate cells are present in varying number, but are never They lie close to the outer limit of the granule layer and possess a cell-body c ous. . uncertain and irregular form, from .030-. 040 mm. in diameter, from which usually IOQ2 HUMAN ANATOMY. several richly branched dendrites pass in various directions, but largely into the molecular layer. The axone is most distinctive, as very soon after leaving the cell it splits up into an arborization of unusual extent and complexity, which, however, is confined to the granular layer. These cells, therefore, belong to those of type II (page 998). Since by their processes they are brought into intimate relation with a number of other neurones, the elements under consideration are probably of the nature of association cells. The nerve-fibres encountered within the cerebellar cortex (Fig. 945) comprise three chief varieties, (i) The first of these includes the axones of the cells of Purkinje which contribute an inconsiderable portion of the fibres passing from the cerebellar cortex to other parts, either of the cerebellum itself or of the cerebrum and brain-stem. (2) The moss-fibres destined especially for the granular layer, which upon enter- FIG. 945. Molecular layer Granule layer White matter Moss-fibres Purkinje cells Climbing fibres Diagrammatic reconstruction of part of folium, illustrating relations of nerve-cells and fibres of cerebellar cor- tex; folium is shown cut transversely and longitudinally ; a, Purkinje cells; b, granule cells; r, small cortical cells; d, basket cells ; f, large stellate cells. ing the latter break up into a number of branches that bear, either at the points of division or at their ends, thickenings from which bundles of short diverging twigs are given off. By this arrangement each moss-fibre ends in relation with a large number of granule cells. (3) The climbing-fibres, so named (Cajal) on account of their tortuous and vine-like course, ascend through the granular to the molecular layer, to which they are chiefly if not exclusively distributed, where they entwine and ding to the primary and secondary dendritic processes of the Purkinje cells. Additional fibres encountered within tin- granule layer are, evidently, the axones of the granule cells and the collaterals of the cells of Purkinje, whilst a large propor- tion of the fibres within the molecular layer are formed by the ramifications of the axones of the granule cells and of the basket cells. The neuroglia forms a supporting framework of considerable density both within the white matter and the cortex. As seen in preparations colored with the usual nuclear stains, the neiirogliar elements are conspicuous within the granule layer, to whose numerous small nuclei they contribute no small part. The cells occupying the miter /one of the granule layer exhibit a peculiar arrangement of their that in a measure recalls the disposition of those of the Purkinje cells. In THE CEREBELLUM. 1093 addition to a short and uncertain centrally directed process, the irregular cell gives off a brush-like group of fibrillae which penetrate the molecular layer, seldom branch- ing, as far as the free surface of the folium, when they end beneath the pia in expan- sions that become condensed and unite into a delicate limiting membrane. The radial disposition of the neuroglia fibres, as well as of the Purkinjean dendrites, climbing fibres and the larger blood-vessels, confer upon the molecular layer a vertical striation that is often marked. The Medullary Substance. — The white matter composing the core of the cerebellar hemispheres exhibits several fairly definite subdivisions, among which may be distinguished : 1. The subcortical layer, from .2-.5 mm. in thickness, that extends beneath the granule layer, parallel to the surface, and sweeps around the bottom of the deeper fissures. Within the series of festoons thus formed lie the association tracts that connect the folia and lobules of the same hemisphere. 2. The commissural tracts, of which the larger lies in front of the dentate nucelus and the smaller behind this nucleus, are continued across the mid-line and into the opposite hemisphere as the anterior (superior) and the posterior (inferior) cerebellar decussations. 3. The peridentate stratum that comprises a fibre-complex that surrounds the nucleus d^ntatum. Within the medullary substance of the worm, lie : 1. The superior cerebellar commissure, a robust tract of transversely coursing fibres that passes in front of the roof-nucleus and, beyond the worm, expands on each side into the main limbs of the medullary tree. It is chiefly by the decussating fibres within this commissure that the cortex of the two hemispheres is connected. 2. The inferior cerebellar commissure passes behind the roof-nucleus and consists of a number of small transversely coursing bundles. 3. The decussation of the roof -nuclei constitutes a commissural and decussating tract distinct from that of the cerebellar commissures just described. The rounded bundles traverse the roof-nucleus, particularly its superior (anterior) part, more distally skirting its dorsal margin and, still farther backward, invading the beginning of the horizontal medullary limb. 4. The median sagittal bundle extends from the superior medullary velum beneath the roof- nucleus into the medulla of the worm ; above, these fibres are continued upward through the velar frenum and into the inferior quadrigeminal colliculus. In addition to the foregoing tracts, the central parts of the branches of the medullary tree, not only of the hemispheres but also of the worm, are occupied by longitudinally coursing fibres that pass directly into the white core, and thence are continued into the cerebellar peduncles as the afferent and efferent paths by which the cerebellar cortex is brought into relation with other parts of the brain and spinal cord. FIBRE-TRACTS OF THE CEREBELLAR PEDUNCLES. Repeated mention has been made of the three robust arms of white matter, the peduncles, that enter the medullary substance of the cerebellum and serve to transmit the fibre- tracts that connect the cerebellum with the cerebrum, the brain-stem and tKe spinal cord. The general features of the inferior, middle and superior cerebellar peduncles are described in connection with the medulla, the pons and the mid-brain respectively. It will be convenient in this place, in connection with the cerebellum, to consider more in detail the constituents of these important pathways. The Inferior Cerebellar Peduncle. — This robust stalk (corpus restiforme), also known as the restiform body, includes not only the tracts connecting the cere- bellum with the spinal cord, but also those that link the cerebellum and the medulla. Two divisions, the spinal and the bulbar, are therefore often recognized. The chief constituents of the inferior peduncle are : 1. The direct cerebellar tract, the fibres of which arise from the cells of Clarke's column, course through the lateral part of the inferior peduncle and end in the cortex of the anterior part of the superior worm on the same side, some fibres reaching the opposite side of the worm by way of the superior commissure. 2. The arcuate fibres (anterior and posterior superficial), from the gracile and cuneate nucle of the opposite and the same side. Additionally, perhaps, some fibres are continued, without inter- ruption in the medullary nuclei, from the posterior fasciculi of the cord. All of these, direct and indirect, end chiefly within the cortex of the superior worm of the same and the opposite side. 1094 HUMAN ANATOMY. 3. The olivo-cerebellar fibres, chiefly from the opposite inferior olivary nucleus but to a limited extent also from the nucleus of the same side. They contribute in large measure to the formation of the lateral part of the restiform body and, on reaching the cerebellum, end within the cortex of the hemisphere and worm, as well as within the fibre-complex enveloping the nucleus dentatus. Whilst for the most part afferent, it is probable that some of the fibres within the tract are efferent and hence conduct impulses in the contrary direction. 4. Fibres from the nucleus lateralis of the medulla, which pass to the cortex of the cere- bellar hemisphere. 5. Fibres from the arcuate nucleus, which pass to the cerebellar cortex. 6. The nucleo-cerebellar tract, comprising fibres from the cells within the reception-nuclei of the trigeminal, facial, vestibular, glosso-pharyngeal and vagus nerves. The tract occupies the median part of the peduncle and ends chiefly in the roof-nucleus of the same and of the opposite side. 7. (Jther fibres pass in reversed direction from the roof-nucleus to the dorso-lateral (Deiters1 ) vestibular nucleus of the auditory nerve and thence, as the vestibulo-spinal tract, descend through the medulla into the antero-lateral column of the cord. 8. Additional vestibular (and, possibly, other sensory) fibres pass without interruption by way of the restiform body to the roof-nuclei and constitute the direct sensory cerebellar tract of Edinger. The Middle Cerebellar Peduncle. — The middle peduncle (brachium pontis), which continues the pons laterally into the medulla of the cerebellum, transmits the fibres whereby the impulses arising within the cerebral cortex are conveyed to the cerebellum. It does not establish direct connections between the cerebellar hemi- spheres, as it might be supposed to do from its transverse position and intimate relation with the cerebellar hemisphere, such bonds from side to side passing exclusively by way of the commissures within the worm. The chief constituents of the middle peduncle are : 1. The continuations of the fronto-cerebellar and temporo-occipito-cerebellar tracts, the fibres of which arise from the cortical cells within the frontal, temporal and occipital lobes respectively, descend through the internal capsule and the cerebral crus, and end around the cells of the pontile nucleus. From the latter cells arise the ponto-cerebellar fibres, the imme- diate constituents of the middle peduncle, that for the most part cross the mid-line and traverse the peduncle to be distributed to all parts of the cortex of the hemispheres and of the worm and, possibly, also to the nucleus dentatus. A small number of these fibres do not decussate, but pass from the pontile cells to the cerebellar cortex of the same side. It should be remembered that the pontile nuclei are also influenced by cortical impulses that descend by way of the pyram- idal tracts, since numerous collaterals from the component fibres of these motor paths end around the pontile cells. 2. Efferent cerebello-pontile fibres, distinguished from the afferent fibres by their larger diameter, originate as axones of the Purkinje cells and pass from the cerebellar cortex through the middle peduncle into the dorsal part of the pons, where, after crossing the mid-line, they are believed (Bechterew) to end within the tegmentum in relation with the cells of the nucleus reticularis tegmenti close to the raphe. The assumption, often made, that many of the efferent cerebello-pontile fibres end around the cells of the nucleus pontis, lacks the support of the more recent observations. The Superior Cerebellar Peduncle. — The superior peduncle (brachium con- junctiviini) forms, with its fellow of the opposite side, the important pathway by which the cerebellar impulses arc transmitted to the higher centres and, eventually, to the cerebral cortex, as well as indirectly to the spinal cord. Its chief constituents are (i) the cerebello-rubral and (2) the cerebello-thalamic fibres collectively known as tin: cerebello-tegmental tract. Tin- principal components of the latter are the fibres arising from the rolls of the dentate nucleus, which, emerging from the hilum of the corpus dentatum and receiving augmentations from the roof-nucleus and, probably, to a limited extent from the cortex of the worm, become consolidated into the rounded arm that skirts the supero-laterul boundary of the fourth ventricle. Converging with the tract of the opposite side towards the mid-line, the peduncle sinks ventrally and disappears beneath the corpora quadri- gemina, many of its fibres continuing their course through the tegmentum of the cerebral peduncle into the subthalamic region and the thalamns. On reaching a level corresponding to that of the upper third of the inferior colliculi of the quadrigeminal bodies, the tracts of the two sides meet and begin to inte: mingle, the decussation of the superior peduncle (Fig. 960) thus estab- THE CEREBELLUM. 1095 lished being best marked opposite the superior colliculi. Above this clecnssation, which, how- ever, does not involve all of its fibres, since some ascend on the same side, the cerebello-tegmental tract is in large measure interrupted in the red nucleus (nucleus tegmenti ruber), that lies within the upper part of the tegmental area of the cerebral crus (page 1114). The fibres not ending around the cells of this nucleus are continued through the subthalamic region into the thalamus, in relation to the cells of which they terminate. Of those ending within the red nucleus, the majority transfer their impulses to fibres that arise from the rubral neurones and thence proceed to the thalamus in company with the unin- terrupted fibres. From the thalamus the impulses are pIG 6 carried by the thalamo-cortical paths (page 1122) to the cere- bral cortex, the cells of which are thus influenced by the coordinating reflexes of the cerebellum. A considerable part of the impulses conveyed to the red nucleus is diverted by the axones of some of its neurones into an entirely different path, namely, the rubro-spinal tract, which decussates and carries impulses from the cerebellum through the brain-stem and antero-lateral column of the cord to the anterior root-cells of the spinal nerves. From the foregoing descriptions it is evident that by means of its peduncles the cerebellum receives no small part of the sensory impulses collected by the spinal and cranial nerves and, in turn, issues the impulses necessary to maintain coordination and equilibrium. Such impulses may be entirely reflex, as in the case of movements per- formed automatically, in which instance the circuit is (a) from the spinal cord and the medulla, directly or indirectly, to the cerebellum chiefly by way of the tracts within the inferior cerebellar peduncles ; (b) from the cerebellum to the motor root-cells within the brain-stem and the cord by way of the cerebello-vestibulo- spinal tract and the cerebello- rubro-spinal tract. When the necessity arises for voluntary efforts in maintaining equilibrium, the circuit includes impulses from the cerebral cortex, in which case the cerebello- rubro - thalamo - cortical tract Pos Diagram illustrating chief components of cerebellar peduncles ; fibres passing by inferior peduncle (IP) are red ; those by superior peduncle (SP) are blue; those by middle peduncle (MP) are black; C, cerebrum; T, thalamus; 1C, internal capsule ; R, red nucleus; Cb, cerebellum ; d. dentate nucleus; p, pontine nucleus; v, 1, o, vestibular, lateral, and inferior olivary nuclei; s, reception nuclei of sensory nerves; Sg spinal ganglion; i. 2, cerebello-rubral fibres, one of which (4) is continued downward as rubro- spinal tract; 3, cerebello-thalamic ; 5, rubro-thalamic ; 6, thalamo-cortical; 7, fronto-pontine ; 8, temporo-occipito-pontine ; 9, 10, ponto-cerebellar fibres. and the cortico-spinal tract form the most direct path. As accessory to this an indirect path, impulses by way of the cortico-ponto-cerebellar and the cerebello-rubro-spinal tracts, may be assumed as probably taking part in securing the necessary motor balance. 1096 HUMAN ANATOMY. THE FOURTH VENTRICLE. The fourth ventricle (vcntriculus quartus), the persistent and modified hind-brain segment of the primary neural canal, is an irregular triangular space between the pons and the medulla in front, and the inferior cerebellar worm and the superior and inferior medullary vela behind. The lateral boundaries are contributed by the supe- rior and inferior cerebellar peduncles. Its long axis is approximately vertical and about 3 cm. in length, measured from the lower extremity, where the ventricle is directly continuous with the central canal enclosed within the medulla and spinal cord, to the upper end, where it passes into the aqueduct of Sylvius. Its width is greatest (about 2.75 cm.) somewhat below the middle, where this dimension is increased by two lateral recesses, one on each side, that continue the cavity of the ventricle over the restiform body. The Floor of the Fourth Ventricle. — The floor of the ventricle, really its anterior wall, when viewed from behind after removal of the cerebellum and the medullary vela, appears as a lozenge-shaped area (fossa rhomboidea). The upper half of the floor is formed by the dorsal or ventricular surface of the pons and is bounded FIG. 947. Sylvian aqueduct Superior posterior recess IV ventricle Lateral recess Posterior commissure Sylvian aqueduct Isthmus Superior median sulcus Superior lateral sulcus Foramen of Luschka Superior posterior recesses Lower end of ventricle containing foramen of Magendie Cast of cavity of fourth ventricle; A, from the side ; £, from above. X J. (Retzius.) laterally by the upwardly converging superior cerebellar peduncles. The lower half is formed by the ventricular surface of the open part of the medulla and is bounded by the downwardly converging inferior cerebellar peduncles and the clavae. The narrow lower angle of the rhombic area, long known as the calamus scriptorius, corresponds to the interval between the clavae, where the central canal of the cord communicates with the fourth ventricle. The upper angle, situated beneath the superior medullary velum and, therefore, described by some anatomists as belonging to the isthmus of the hind-brain (rhombencephalon), marks the lower end of the Sylvian aqueduct. The length of the rhombic fossa is about 3 cm., and its breadth, greatest at the level of the auditory nerve, is about 2 cm. In consequence of the elevation of its lateral boundaries, the floor appears sunken and corresponds approximately with the frontal plane, being almost vertical. It is divided into symmetrical lateral portions by a median groove (sulcus medianus longi- tudinnlis sinus rhomboidnlis), and into an upper and a lower half by transverse mark- ings, the acoustic striae (striae acusticae), which on each side arise from the nuclei of the cochlear nerve, wind over the restiform body and cross the floor of the ventricle to disappear within the median furrow. At its lower end, where it sinks into the central canal of the cord, the median groove becomes somewhat wider, the resulting depression being sometimes designated the vcntriculus Arantii. Roofing in the ventricle at this point and brid^in^ the cleft separating the posterior columns, lies a thin triangular sheet of loose vascular tissue, the obex, which laterally is continuous THE FOURTH VENTRICLE. IO97 with the delicate roof-membrane, known as the tela chorioidea. Toward its upper end the longitudinal furrow presents a second expansion, the fossa mediana. The acoustic stnae vary greatly in distinctness and arrangement, sometimes appearing as well-marked bands that cross the ventricular floor with little divergence, or they may constitute a fan-shaped group in which the strands may be irregularly disposed or even overlap ; in other cases they may be much less distinct on one side, or so feebly marked on both as to be unrecognizable. Quite frequently one band diverges from the others and crosses the floor obliquely upward and outward. This strand spe- cially designated as the conductor sonorus, is seldom equally distinct on the two sides, being usually better seen on the left. The inferior division of the ventricular floor, that lying below the acoustic striae, presents three general fields of triangular outline. The one next the median groove, with its base above and its apex directed towards the lower angle of the ventricle, which it almost reaches, is the trigonum hypoglossi, so called from the fact that it partly overlies the nucleus of the twelfth nerve. Lateral from the last FIG. 948. v Corpora quadrigetnina A ) Sylvian aqueduct ak. Fovea superior \^ J$ ^/jt- HJ II ~fc^~TT*^- Superior cerebellar peduncles Acoustic striae J^S?sS '"' /I I \S? $>^ Eminent* teres White core of c^iebellum . Trigonum acustici ^- _ • . . T- ^ ' Clava Trigonum vagi - ' Funiculus gracilis Floor of fourth ventricle exposed after removal of its roof by frontal section. named area is a somewhat depressed triangular field of darker color, the apex of which is placed above, near the acoustic striae, and the base below ; this field is known as the ala cinerea, from the dark tint imparted to it by the pigmented cells lying beneath, and as the trigonum vagi, in recognition of the subjacent glosso-pharyngeo-vagus nucleus. The remainder of the inferior division of the ventricular floor includes an elevated triangular field, the trigonum acustici, that is part of the larger tract, the area acustica, which occupies not only the lateral angle of the rhomboidal fossa, where it is crossed by the acoustic striae, but also the adjacent portion of the superior division of the ventricular floor. Laterally, the acoustic area presents a distinct elevation, the tuberculum acusticum, which, together with the adjacent part of the trigonum acustici, is related to the nuclei of the cochlear nerve ; the more median portion of the acoustic area, on the other hand, belongs to the vestibular division. The superior division of the ventricular floor, above the acoustic striae, is marked on each side of the median groove by a prominent elevation, the eminentia teres, which below is continuous with the trigonum hypoglossi and above narrows and fades away towards the floor of the Sylvian aqueduct. Laterally the eminence is bounded by a depressed area, the fovea superior, which is the expanded upper part of a second longitudinal furrow, the sulcus lateralis, that defines the outer limit of the eminentia teres and below is continued into the depressed trigonum vagi, to which the name, fovea inferior, is sometimes applied. Above and to the outer side of the 1098 HUMAN ANATOMY. superior fovea, the ventricular floor presents a slightly sunken field, the locus cceruleus, which extends upward to the Sylvian aqueduct and in fresh preparations possesses a bluish gray tint in consequence of the deeply pigmented cells of the underlying substantia ferruginea (page 1081) showing through the ependymal layer. The accurate description of the surface markings of the ventricular floor given by Retzius,1 has been supplemented by Streeter's2 careful study of the relation of these details to the under- lying structures. The most important results of these observations, which have materially advanced our understanding of this important part of the brain-stem, may here find mention. The trigonum hypoglossi is seen, especially when examined under fluid with a hand-lens, to include two subdivisions, a narrow median and a broader lateral. The first of these is con- vex, about 5 mm. long by i mm. wide, and corresponds to the rounded upper end of the nucleus of the twelfth nerve ; it is, therefore, appropriately called the eminentia hypoglossi (Streeter). The entire hypoglossal nucleus, however, is of much larger size (about 12 mm. long by 2 mm. wide) and extends some 5 mm. below the tip of the calamus scriptorius, ventral (anterior) to the FIG. 949. Colliculus inferior IV. nerve >* Superior cerebellar peduncle ^_^ \^^ - ^S Superior fovea -£. — __ —Jfcl( ^^"^ ^^ Area n. abducentis Trigonum acustici — ^-^*"^^^t"^^«B HC^^Funiculus solitarius Trigonum hypoglossi — ^^^^^P^^^^^f^A """-—Area n. vagi Floor of the fourth ventricle; areas corresponding to nuclei of nerves are shown on right half of figure. X J. (Streetft.) vagus nucleus and nucleus gracilis. Lying immediately above the hypoglossal eminence is a second and somewhat less pronounced elevation, formed by the nucleus funiculi teretis and meas- uring nearly 6 mm. in length by i mm. in breadth. Lateral to these two median elevations and limited externally by the ala cinerea, lies a wedge-shaped field that is insinuated between the hypoglossal eminence and the vagal trigone. It stretches from the acoustic striae above to the nib of the calamus scriptorius below. This field, named the area plmnifonnis by Retzius on account of its feather-like markings, is regarded by Streeter as corresponding to a group of cells, the nucleus intercalatus, that occupies a superficial position in the ventricular floor and partly overlies the hypoglossal nucleus. The fovea vagi (ala cinerea), which lies lateral to the nucleus intercalatus, corresponds to the middle and superficial third of the vago-glosso-pharyngeal nucleus, the entire extent of tin- latter including a tract measuring about 13 mm. in length by 2 mm. in breadth, that stretches rrom beneath the vestibular nucleus above to over 2 mm. beyond the inferior angle of the ventricle. The lower third of the area of the va.nus nucleus is partly within the ventricle; immediately above the obex this intraventricular portion is covered by a layer of loose vascular tissue and appears as an upwardly diverging pointed field, area postrema of Retzius. This is separated from the ala cinerea by a translucent ridge, the funiculus separans, composed of thickened ependymal neiiro^lia (Streeter). . 1 Das Menschenhirn, 1896. 1 Amer. Journal of Anat. Vol. II, 1903. THE FOURTH VENTRICLE. 1099 Corpora quadrigemin cerebellar ped Superio Telacho and choroid pie Dentate nucleus The prominence of the eminentia teres is due to the underlying nucleus of the sixth nerve, enclosed by the knee of the facial ; for it, therefore, Streeter proposes the name eminentia abdu- centis. The longitudinal ridge that continues upward and bounds the median fovea, the last cited author interprets as due to a field of gray matter, thin in the vicinity of the abducent eminence and thicker above, to which the name nucleus incertus is applied. Lateral to the nucleus incertus and the facio-abducent eminence, lies the fovea anterior, which elongated and depressed area (nearly 6 mm. long by i mm. wide ) is due to the exit of the root of the fifth nerve ; it may, therefore, be called the fovea trigemini. The median portion of the elevated acoustic area includes the elongated and irregularly lozenge-shaped vestibular area, that measures about 16 mm. in length by 4 mm. in breadth and extends from the fovea anterior (trigemini) to the nucleus gracilis. The lateral part of the area acustica is occupied by the cochlear area, which stretches into the recessus lateralis and overlies the nucleus cochlearis. The Roof of the Fourth Ventricle. — Viewed in median sagittal section (Fig. 938), the roof of the fourth ventricle appears as a tent-like structure, whose walls, where they come together, bound a space, the recessus tecti, that penetrates the cerebellar medulla between the superior FIG- 95°- and inferior worm. The upper wall of the tent is formed by the superior med- ullary velum, the triangular sheet of whitematterstretch- ing from beneath the quadrigeminal bodies above to the medullary substance of the cerebellum below, and is over- laid by the rudimen- tary cerebellar folia of the lingula. It must be understood that the ventricular surface of the velum is clothed by the ependyma as are all other parts not only of the fourth ventricle but of all the ventricular cavities. Laterally the superior medullary velum is attached to the superior cerebellar peduncles, which to a limited extent share in closing in this part of the ventricle (Fig. 936). The lower half of the roof comprises two parts, an upper and thicker crescentic plate of white matter, the inferior medullary velum, and a lower and extremely thin membrane, the tela chorioidea. Medially the inferior medullary velum is attached for some distance to the front and lower surface of the nodule, which it excludes, strictly regarded, from the ventricle, whilst laterally the velum is prolonged^ to the flocculus, its fibres becoming continuous with the white core of this subdivision of the cerebellum. The nervous constituents of the velum extend only as far as its crescentic lower border, beyond which the roof of the ventricle, in a morphological sense is formed by the ependymal layer alone. This, however, is supported by a backing of pial tissue, which, in conjunction with the ependyma, forms the tela chorioidea. On nearing the lower angle of the ventricle, the roof presents a trian- gular thickening, the obex, that closes the cleft between the clavae and lies behind (above) the nib of the calamus scriptorius. On each side the obex, which consists of a layer of white matter fused with the underlying ependyma, is continuous with the slightly thickened margin of the roof the tsenia ventriculi, whose line of attachment passes from the clava upward and outward over the cuneate tubercle of the medulla and the restiform body and farther upward runs obliquely across the dorsal surface of this peduncle to close in the Ut White core of cerebellum Dorsal portion of preparation shown in Fig. 948 ; roof of fourth ventricle is seen from below. IIOO HUMAN ANATOMY. recess — one of the pair of diverticula that overlie the inferior cerebellar peduncles and add materially to the transverse dimension of the ventricle. After enclosing the lateral recess the taenia leads to the stalk of the flocculus and the inferior velum. Within the triangular field of the teia chorioidea, the pia mater takes advantage of the attenuation of the ventricular wall to effect imaginations by which its blood- vessels apparently gain entrance into the ventricle. Such invaginations, known as the choroid plexus of the fourth ventricle, occur in the ventricular roof on each side and in the immediate vicinity of the mid-line, where they appear as parallel villous or fringe-like stripes, the median plexus, which extends upward from near the obex to the inferior medullary velum. Opposite the nodulus they FIG. 951. Nucleus gl Fourth ventricle Lateral recess of ventricle"*^ ^QSt Inferior olivary nucleus Choroid plexus Posterior longitudinal fasciculus Pyramidal tracti Section across lower third of fourth ventricle, showing internal cerebellar nuclei, choroid plexus, lateral recesses and medulla ; new-born child. X 3/i- Preparation by Professor Spilfer. diverge and, as the lateral plexuses, invaginate the wall of the lateral recesses. The vascular complex lies within the fold of pial tissue, the space between the pial layers being occupied by prolongations of the arachnoid. Notwithstanding its conspicuous thinness during the first half of fcetal life, the tela chorioidea suffices to completely close the ventricle. From about the fifth month, however, the delicate membrane is perforated by an aperture that remains throughout life. This opening, the foramen of Magendie (apertura mcdialis vcntriculi quart!) lies immediately above the obex and between the strands of the choroid plexus. Two additional clefts, the foramina of Luschka (aperturac laterales), usually exist, one on each side, in the wall of the lateral recesses in the neighborhood of the vago- glosso-pharyngeal nerves. By means of these three openings, and probably by these alone, the system of ventricular cavities and the central canal of the spinal cord are brought into communication with the subarachnoid lymph-space. A path is thus provide. 1 by u hich the cerebro-spinal fluid, secreted within the lateral, third and fourth ventricles by tin- various rhoroid plexuses, constantly escapes and thereby prevents undue accumulation and distension within the cavities of the brain and spinal cord. THE DEVELOPMENT OF THE HIND-BRAIN DERIVATIVES. In the general sketch of the development of the brain previously given (page 1061), it was pointed ,,ut that the hind-brain, or rhombcnrcphalon, includes two subdivisions, the myclfHceph- alon and the wff,-m;-f>hii/on, the extreme upper part of the latter being designated the isthmus. It has be<-,i further noticed that tV junction of the cord and brain-segments of the neural tube corresponds with the conspicuous cervical flexure, whose early appearance is followed by an DEVELOPMENT OF HIND-BRAIN DERIVATIVES. 1 101 FIG. 952. outward bending of the lateral walls of the brain-vesicle and the stretching and flattening of tin- roof-plate. In consequence of these changes the roof of the rhombencephalon becomes reduced to an attenuated sheet which, when viewed from above, appears as a lozenge-shaped membrane that closes in the subjacent cavity, the subse- quent fourth ventricle. It has also been pointed „ out (page 1049) that the relatively thick lateral walls of the neural tube exhibit, even within the cord-segment, a differentiation into a dorsal and a ventral zone (the alar and basal lamina? of His), which subdivisions are associated with the sensory and motor root-fibres of the nerves respectively. Similar relations, in a more pro- nounced degree, are evident within the brain- stem and are of much interest as indicating the morphological correspondence of the purely motor nerves (the third, fourth, sixth and twelfth) on the one hand, and of the mixed nerves (the fifth, seventh, ninth and tenth) on the other. The Medulla. — The great preponderance of the nervous matter along the floor of the fourth ventricle, as represented by the medulla, is due primarily to the outward bending of the lateral walls of the myelencephalon, supple- mented by the accession of large tracts of nerve-fibres that later grow in from other parts of the cerebro-spinal axis. In consequence of the former change, the dorsal zones of the side-walls are gradually displaced laterally ; at the same time they become partly folded on themselves to produce along their outer margin the rhombic lip (His), which is directly continuous with the expanded and thin roof-plate. Later, the dorsal zones come to lie almost horizontally, their ventricular surface corresponding with that of the ventral laminae, in conjunction with which the floor of the definitive fourth Mid-brain Right hemisphere Inferior colliculus Roof-plate Cerebellum Cavity of hind-brain Lateral recess Rhombic lip Attachment of roof Medulla Reconstruction of brain of human embryo of 22.8 mm., showing hind-brain and part of mid-brain viewed from behind. X 12. Drawn from model made by Dr. Ewing Taylor. Pineal body FIG. 953. Superior colliculus Cavity of i Inferior colliculus Corpus striatum Cut Worm of cerebellum Hemisphere of cerebellum Lateral recess Cavity "f hind-brain (IV ventricle) Roof of hind-brain, lower part Reconstruction of hind-brain of human embryo of about three months (50 mm.), viewed from side and behind. Drawn from His model. ventricle is later formed. Coin- cidently with the outward mi- gration of the dorsal laminae, the ventral zones also thicken and assume a much more hori- zontal position, with their inner ends separated superficially by a median furrow and, deeper, by the compressed remains of the floor-plate. Very early and before the flattening out of the myelencephalon has advanced to any marked extent, the de- marcation between the dorsal and ventral zones is evident as a lateral longitudinal groove on the ventricular surface of the myelencephalon. Indica- tions of this division persist and in the adult medulla are represented by the fovea in- ferior and the sulcus lateralis seen on the floor of the fourth ventricle. As in the cord-seg- ment, so in the myelencepha- lon the lateral walls are the only regions of the neural tube in which neuroblasts are devel- oped, the roof-plate and the floor-plate containing spongioblasts alone. Very early and before the flattening out of the myelencephalon has advanced to any marked extent, within the ventral zones and close to the mid-line, appear groups of neuroblasts, from which axones grow ventrally to form the root-fibres of the motor (hypoglossal) nerves. Sensory 1 102 HUMAN ANATOMY. FIG. fibres are also early represented by bundles which grow centrally from the ganglion of the vagus towards the developing medulla, upon whose surface, opposite the junction of the dorsal and ventral zones, they appear as a flattened oval bundle (fasciculus solitarius). For a time super- ficial and loosely applied, this bundle gradually becomes more deeply placed in consequence of the extension, ventral folding, and final fusion of the rhombic lip with the remainder of the dorsal zone. Subsequently the fasciculus solitarius becomes still farther removed from the surface by the ingrowth of tracts of nerve-fibres from the neuroblasts of the rhombic lip and from other sources until, finally, the bundle comes to lie beneath the ventricular floor where its position permanently indicates the junction between the original dorsal and ventral zones of the medul- lary wall. In a similar manner the sensory fibres of the trigeminal nerve are applied to the sur- face of the developing pons ; since, however, the bundle is attached after consolidation of the dorsal zone of the medulla has begun, the descending trifacial fibres retain the relatively superficial position characterizing the spinal root, while the descending root (fasciculus solitarius) of the glosso-pharyngeo-vagus lies more deeply placed. Subsequent to the invasion of the medulla by the sensory parts of this nerve, the outgrowth of the axones from the neuroblasts constitut- ing the nucleus of origin provides its motor root- fibres. The rhombic lip is a region of much impor- tance, since from the neuroblasts which appear within it are derived the cells of the reception nuclei (substantia gelatinosa) of the sensory cranial nerves, of the nuclei of the posterior col- umns, of the inferior and accessory olivary nuclei and of the arcuate nucleus. From the neuro- blasts many axones grow medio-ventrally, pierce the median spongioblastic septum derived from the primary floor-plate, which later becomes the median raphe, and gain the opposite side and thus establish the systems of arcuate fibres. Other axones grow dorsally and take part in even- tually producing the fibre-tracts connecting the olivary, dorsal and arcuate nuclei with the cere- bellum. It is evident that the development of the myelencephalon primarily contributes the nerv- ous substance that becomes the dorsal part of the medulla and underlies the fourth ventricle. Later the closed part of the medulla, which at first is wanting, as well as the conspicuous pyramidal tracts, are added as the strands of ascending and descending fibres grow into the medulla from the spinal cord and from other parts of the brain. In this manner the important tracts of the posterior columns and the spinal constitu- ents of the restiform body and of the brain-stem are added and, still later, the bulky pyramids take form when the cerebro-spinal paths are established. In accord with the falling apart and thick- ening that affect the lateral walls of the mye- lt -IK •( -pnalon and lead to the production of the medulla, tin- roof-plate of the brain-vesicle becomes flattened and laterally expanded to keep pace with the increasing width of the ventricular floor. In consequence, the roof-plate is converted into a rhomboidal sheet of great delicacy, the pritnary rr///;;/, which historically consists of little more than the layer of ependymal cells. These, however, soon come into close- relation with the overlying mesoblastic tissue from which the pia is differentiated. I hiring the third month a transverse fold, the plica chorioidea, appears in the roof-sheet, near the posterior limit of the developing cerebellum (Fig. 955, B). Into this <• sections of hind-brain of human embryos, showing three stages in development of medulla; -4, about four and a half \\ • >ut six weeks; C, nbout eight weeks; rf>. loof-platc ; r, raphe; d, y, dorsal (alar) and ventral (basal) lamin:r ; >l, rhombic lip; /» , lateral recess; /s, fasciculus solitarius; rr, :tn t...,!v; i ;;. hvpnglossal nerve; sv, spinal root of trigeminus ; in, inferior olivary nucleus. (His.) DEVELOPMENT OF HIND-BRAIN DERIVATIVES. 1103 duplicature, directed towards the brain cavity, the mesoblast grows and later develops blond- vessels, and is converted into a vascular complex that eventually forms the choroid plexus of the fourth ventricle. From the manner of its development, it is evident that the plexus is excluded by the ependymal layer from the ventricular space, outside of which the pial blood-vessels, therefore, really lie. The conversion of the upper part of the primary velum into the thicker definite inferior medullary velum follows the addition of nervous substance during the develop- ment of the cerebellum. Similar thickening of the roof-sheet at the lower angle of the ventricle results in the production of the obex and the taenise. The Pons.— The pons arises as a thickening of that part of the metencephalon which forms the anterior wall of the pontine flexure. In its essential phases the development of the pons probably closely resembles that of the medulla, since the early metencephalon presents the same general features as does the myelencephalon. Thus, the ventral zones of its lateral walls play an active role in the production of the tegmental portion of the pons and the nuclei of origin of the motor root-fibres of the fifth, sixth and seventh nerves, whilst the floor-plate becomes the raphe. In addition to providing the reception-nuclei of the sensory cranial nerves, and, per- haps, the pontine nuclei, the dorsal zones contribute the neuroblasts which pIG> 955- become the nervous elements of the c cerebellum. As in the medulla, so in the pons the great ventral tracts are secondary and relatively late additions to the tegmentum, which must be re- garded as the primary and oldest part of this segment of the brain-stem, the bulky ventral nervous masses taking form only after the appearance of the cerebro-spinal and cerebro-cerebellar paths. In a manner analagous to that by which the sensory part of the vagus is at first loosely applied and later in- corporated with the medulla, the sen- sory fibres of the trigeminus are for a time attached to the surface of the dorsal zone of the pons, subsequently becoming covered in and more deeply placed by the addition of peripheral tracts. Likewise the fibres of the audi- tory nerve come into relation with the superficially situated reception-nuclei of "the cochlear and vestibular nerves. The Cerebellum. — The develop- ment of the human cerebellum pro- ceeds from the roof-plate and adjacent parts of the dorsal zones of the lateral walls of the metencephalon. In an embryo 22.8 mm. long, the cerebellar anlage consists of two lateral plates connected by a narrow thin intervening lamina representing the roof-plate (Fig. 952). After the apposition of the lateral plates, which soon occurs, this bridge disappears, the developing cerebellum for a time appearing as an arched lamina enclosing the upper part of the cavity of the hind-brain (Kuithan1). The subsequent development of the human cerebellum has been recently carefully studied by Bolk 2 in a series of about forty fcetuses, hardened in formalin and ranging from 5 to 30 cm. in their entire (crown-sole) length. The following account is based largely on these investigations. In a fcetus of 5 cm., about nine weeks old, the cerebellar anlage is represented by a horseshoe- shaped thickening of the metencephalic roof, the cerebellar lamina, whose upper margin is con- nected by a conspicuous fold with the mid-brain and whose lower border has attached to it the primary velum — the thin rhomboidal roof-plate of the myelencephalon. Median sagittal section of the cerebellar lamina at this stage (Fig. 955, A) shows its form to be asymmetrically biconvex, the more convex surface encroaching upon the brain-cavity. In a slightly older fcetus ( Fig. 955, £) the cerebellar lamina has become triangular, in section presenting a superior, an anterior, and an inferior surface. From its attachment along the superior margin of the lamina the inferior velum dips forward toward the pontine flexure and, forming a transversely cresentic 1 Miinchner med. Abhand., 1895. *Petrus Camper, 36 Deel, 1905. Median sagittal sections showing four early stages of develop- ment of human cerebellum, from fcetuses from 5 to 9 cm. long; nib, mid-brain ; c, cerebellum ; sv, iv, superior and inferior medul- lary velum ; vc, ventricular cavity ; d, cavity of diencephalon ; p, pons ; m, medulla ; s, spinal cord ; if, incisura fastigii ; /, sulcus stnodularis. (Drawn from figures of primarius ; j>, sulcus post of Bolk.) 1 104 HUMAN ANATOMY. fold, the plica, chorioidea, bounds a narrow recess that extends along the inferior surface of the cerebellar lamina. This recess is only temporary and is soon obliterated by the subsequent at- tachment of the roof-membrane to the inferior surface of the cerebellar lamina. The succeeding stage (Fig. 955, C) emphasizes the alteration in the planes of the cerebellar surfaces, the former superior now becoming the anterior, the anterior the inferior, and the inferior the posterior. From the posterior margin of the dorsal surface the choroid fold dips into the brain-cavity. Between the mid-brain and the cerebellum now stretches the first definite indication of the later superior medullary velum. In agreement with His, Bolk recognizes that the former intraven- tricular (inferior) surface has now become an extra ventricular one and that the permanent attach- ment of the plica chorioidea corresponds to a secondary and not to the primary line of union. The stage represented in Fig. 955, D is important, since it marks the beginning of the first fissures. One of these, the sulcus primarius (the fissura prima of Elliot Smith), appears as a transverse groove on the upper part of the anterior surface and thus early establishes the funda- mental division of the cerebellum into an anterior and a posterior lobe. The other fissure appears in the median area near the posterior margin of the cerebellum and is the sulcus post- nodularis. On each side (Fig. 956, A ) an additional fissure cuts oft" a narrow tract that embraces the postero-lateral area of the cerebellum. This fissure, \3oKSblcvsjloccularis, for a time remains ununited with the postnodular sulcus ; but later, with its fellow, it becomes continuous with the postnodular sulcus and thus defines a narrow band-like tract, the median part of which 5 4 Six stages in development of human cerebellum, from feet uses of 9 (A), 13 (B), 15 (C), 22 (D), 25(E),a.nd 32 cm. ) length ; /, sulcus primarius (preclival) ; 2, s. floccularis ; j, s. postnodularis ; 4, s. Infrapyramidalis ; 5, s. iriinr posterior (postrlival ) ; h. jjrcat horizontal fissure; mh, mid-brain; r, roof-membrane; Ir, lateral (su',l,iur" osterior ( -«t, ", Modulus ; ", uvula ; f. pyramis; t, tuber;/, folum. {Drawn from figures of Bolk.) eventually becomes the nodule, the lateral portions the flocculi, whilst the intervening strips become the floccular peduncles and part of the inferior medullary velum. The diverticulum bounded on each side by the floccular area is the beginning of the lateral recess of the fourth ventricle and is early filled by the rapidly growing choroid plexus. A shallow transverse groove, the incisiini /ns/i^ii, just suggested in Fig. 955, C but distinct in the succeeding sketch, marks the beginning of the tent-like recess that later conspicuously models the roof of the fourth ventricle. Coincidently with and about midway between the fissures just described, a third furrow appears on the posterior cerebellar lobe. This is the /V.V.SWY/ secunda (Elliot Smith) or the infmpyraiindal sitlcus. Very shortly a fourth groove appears behind the sulcus primarius and marks the beginning of the />r,-f>ynunittii/ fissure. In this manner the median tract of the posterior lobe is early subdivided by three fissures into four areas, which, from behind toward the sulcus primarius, give rise to the nodule, the uvula, the pyramid and a still undiffereiuiated zone. i'>y the subsequent appearance of additional furrows, this narrow zone gives origin to the tuber, the folium cacuminis and tin- clivus. Meanwhile on the anterior lobe of the cerebellum three short transverse fissures appear, by which the anterior end of the worm-tract is broken tip into areas that, while establishing subdivisions of morphological value (Bolk), are later li.st in th<- uncertain foliation of the lingula and lobulus ceiitralis of the mature cerebellum. After the fundamental subdivision of the median area (worm) has been accomplished, tin- lateral masses (hemispheres) of the cerebellum become subdivided into definite tracts (lobules) by fissures that appear during the fourth and fifth months of foetal life. The lateral extensions THE MESENCEPHALON. 1105 of the sulcus primarius— itself the later preclival fissure— separate the anterior and posterior crescentic lobules. During the fourth month the postlunate fissure appears, in each hemisphere, on the upper surface of the posterior lobe. By the extension and medial union of these sulci, for a time separate, are established the posterior limit of the clivus (postclival fissure] and the demarcation between the posterior crescentic and the postero-superior lobule. The post-tonsillar fissure bounds the conspicuous elevation of the tonsil behind and medially joins the infrapyram- idal (later prepyramidal ) sulcus. The parapyramidal fissure defines the up|>; r (posterior) limit of the biventral lobule and unites with the suprapyramidal (later postpyramidal) fissure. The great horizontal fissure, so conspicuous in the mature cerebellum, appears relatively late, about the end of the fifth month, and is at first represented by a shallow transverse median fur- row that lies immediately in front of the suprapyramidal fissure (Bolk), an origin at variance with the generally accepted formation of the horizontal fissure by the union of t\vo lateral sulci, that grow medially from the hemispheres and meet in the worm. The early fissure having such history, Bolk identifies as the postlunate (sulcus superior posterior) and not as the horizontal. This author also emphasizes the fact that at the sixth fcetal month the folium cacuminis is, as a rule, not only defined, but forms a well-marked superficial tract that connects the adjoining lateral tracts (postero-superior lobules) . This part of the worm, however, does not keep pace with the cortical expansion of the surrounding parts and, hence, becomes overgrown by these and sinks into the relative insignificance that distinguishes this part of the worm in the fully matured cerebellum. In consequence of the rapid growth and expansion of the peripheral portions of the human cerebellum, some fissures of secondary morphological importance, as the horizontal, become excessively deepened and more conspicuous in man than those of fundamental significance, as the sulcus primarius (preclival) and the postnodular fissures. This cortical expansion, especially within the superior region, likewise brings about prominent changes in the position of the segments of the worm, so that eventually those which primarily lay behind later come to lie below, the divisions of the conventional upper and lower worm of the mature cerebellum following along the C-like curve seen in sagittal sections. The histogenesis of the cerebellar cortex probably primarily proceeds from the invasion of the cellular lamina by the cells of the dorsal zones of the lateral walls of the metencephalon, as well as directly from these zones themselves. The earliest differentiation results in the production of three strata : (a) the inner ependymal layer, and (6) the middle mantle layer, and (c) the outer marginal layer. Of these the mantle layer is the thickest and richest in cells, from which both neuroblasts and spongioblasts arise, although their differentiation occurs relatively late. The Purkinje cells, early distinguishable by their large clear nuclei, appear during the sixth fcetal month, but for some time lack their characteristic processes. Likewise from the mantle layer are derived the earliest constituents of the granule layer. Meanwhile within the marginal layer, immediately beneath the external surface of the cerebellum, an additional and temporarily con- spicuous cell-stratum, the external granule layer, becomes a prominent feature of the develop- ing cerebellar cortex. This layer soon exhibits a subdivision into two zones of which the outer contains many dividing cells, while the inner is almost free from karyokinetic figures. During the later months of foetal life the inner sublayer disappears and at birth the outer one is greatly reduced ; finally, this also disappears, so that after the earliest years of childhood the external granule layer is no longer seen. The chief factor in this reduction and eventual obliteration of this stratum is, according to Cajal, the gradual transformation of its neuroblasts into nerve- cells that recede from their peripheral position to assist in the completion of the granule layer, as whose small and characteristically branched elements they persist. Other neurones of the external granule layer are transformed into the basket cells and the large stellate cells. The neuroglia of the cerebellar cortex is derived chiefly from the spongioblastic elements of the inner or ependymal layer, the conversion of the cells of the outer granule layer into the supporting tissue, as sometimes assumed, being unlikely (Ziehen). Since the molecular layer is composed to a considerable extent of the dendritic processes of the Purkinje cells, the development of the outer division of the cerebellar cortex is complete only after the growth of such processes, as well as of the climbing fibres from the white core, has taken place. The production of the superior cerebellar peduncles and of the definite superior medullary velum is dependent upon the development of the fibres that pass from and to the dentate nucleus and the cerebellar cortex— an invasion that occurs during late fcetal and early post- natal life. THE MESENCEPHALON. Notwithstanding its considerable size and prominent position in the embryo, in its mature condition the mesencephalon, or mid-brain, forms the smallest and least con- spicuous division not only of the brain-stem but also of the entire brain. Neverthe- less, the many fundamental tracts which it contains, as well as the new_ paths and combinations which arise within its substance, confer on the mid-brain an importance 70 uo6 HUMAN ANATOMY. not suggested by its size. Its upper limit corresponds with an oblique plane passing through the base of the pineal body and the posterior border of the corpora mam- millaria ; its lower one is indicated on the ventral surface by the upper border of the pons and on the dorsal aspect by the upper margin of the superior medullary velum. As seen in sagittal sections (Fig. 938,) the mid-brain is about n mm. in length, although when measured on the ventral surface it is slightly shorter (9 mm.) and on the dorsal aspect a little longer (13 mm.). Its greatest breadth is approximately 23 mm. The mid-brain is traversed longitudinally by a canal, the Sylvian aqueduct, which, however, lies rrtuch nearer the dorsal than the ventral surface of the brain-stem. When the several parts of the brain are undisturbed, only a portion of the ventral aspect of the mid-brain can be seen. Its dorsal and lateral surfaces are hidden by the overhanging cerebral hemispheres, the splenium of the corpus callosum and the pulvinar of the thalamus being in close relation with these surfaces respectively. Notwithstanding its ventral position and apparent removal from the exterior of the brain behind, the dorsal surface of the mid-brain is, in fact, directly continuous with FIG. 957. Thalamus Trigonum habenulae Pulvinar Colliculus superior Cerebral peduncle Fourth nerve Pons Superior cerebellar peduncle Tsenia thalami Commissura habenulae Pineal body Median geniculate body Brachiuin inferior Colliculus inferior Frenulum veil Lingula Cerebellum, cut surface Mid-brain viewed from behind ; upper part of cerebellum has been removed to expose superior medullary velum with lingula. and a part of the free posterior surface of the brain. It is, therefore, covered with the pia mater, as may be demonstrated by drawing aside the overhanging cerebral hemispheres. In situ the mid-brain occupies the opening bounded by the tento- rium and thus connects the divisions of the brain which lie within the posterior cra- nial fossa (cerebellum, pons and medulla) with those (cerebral hemispheres) that lie above. Its cavity, the Sylvian aqueduct, establishes direct communication between the third and fourth ventricles. The mid-brain includes two main subdivisions, a smaller dorsal part, the qwuirigtminat plate, which roofs in the Sylvian aqueduct and bears the corpora quaorigemina, and a much larger ventral part, made up by the « rebral peduncles. The quadrigeminal plate lies behind the plane of the roof of the Sylvian aqueduct and extends from the base of the pineal body above to the upper margin of the anterior medullary velum below. Its dorsal surface is subdivided into four white rounded elevations, tin- corpora quadrigemina, by two grooves, one of which is a median longitudinal furrow and the other a transverse furrow that crosses the first one at right angles and slightly below its middle point. The upper part of the longi- tudinal groove, between the upper pair of elevations, broadens into a shallow trian- gular depression, the pineal fossa (triuonum subpineale) in which rests the pineal body. Below, the mid-furrow ends at the base of the frenum of the superior medul- lary velum. THE MESENCEPHALON. 1107 The elevations forming the upper pair of quadrigeminal bodies, the colliculi superiores, are the larger and more conspicuous, and measure from 7-8 mm. in length, about 10 mm. in breadth, and 6 mm. in height. Laterally each superior col- liculus is continued into an arm, the superior brachium (brachiura quudriucminum superius) which is denned by a groove above and below, and passes upward and outward, between the optic thalamus and the median geniculate body, to be lost within an indistinctly circumscribed oval eminence, the lateral geniculate body (corpus geniculatum laterale), which lies beneath the pulvinar. In like manner, each of the smaller lower pair of quadrigeminal bodies, the colliculi inferiores, (abojt 6 mm. in length by 8 mm. in breadth and 5 mm. in height) is prolonged laterally into the inferior brachium (brachiura quadrigeminum inferius), which in turn ends in the sharply denned median geniculate body (corpus geniculatum mediate), an oval elevation about 10 mm. in length. Ventrally the quadrigeminal plate becomes directly continuous with the adjacent part of the cerebral peduncles. The cerebral peduncles (pedunculi cerebri), also called the cerebral crura, constitute the bulky ventral part of the mid-brain. Dorsally, the two peduncles are fused into a continuous tract, the tegmentum, which contributes the side-walls and floor of the Sylvian aqueduct and blends on each side with the overlying quadri- geminal plate. Ventrally the peduncles are unfused and appear on the inferior sur- face of the brain as two robust stalks (Fig. 993). These emerge from the upper border of the pons and pass, diverging at an angle of from 70-85°, upward and out- ward to enter, one on each FIG. 958. V-, Pulvinar / \ (7^ Superior colliculuss. *•» i Superior brachium / Median geniculate body / / . Lateral geniculate body f / Tractus transversus Cerebral peduncle / Optic tract Inferior colliculus Superior medullary velum Superior cerebellar peduncle Lingula Middle cerebellar peduncle, cut Dorso-lateral aspect of mid-brain. side, the cerebral hemi- spheres just where the peduncles are crossed by the outwardly winding optic tracts. At the pons each peduncle possesses a breadth of from 12-15 mm., which increases to from 18-20 mm. at the upper end of the stalk ; the borders of each peduncle are, therefore, not quite parallel, but slightly di- verging. Neither are the mesial margins of the pe- duncles in contact as they issue from the pons, but separated by an interval of about 3 mm. This distance increases until at their upper ends the peduncles are about 13 mm. apart. Superficially each peduncle is formed by strands of fibres which do not pursue strictly longitudinal course, but wind spirally from within outward ; in consequence of this arrangement the surface of the peduncle presents a characteristic twisted or rope-like striation. The regularity of this marking is sometimes disturbed by a faintly defined strand of fibres (tractus peduocularis transversus), that winds over t median border and ventral surface of the peduncle, passes upward and outward across the lateral surface of the mid-brain, to be lost in the vicinity of the medial geniculate bodv The depressed triangular area included between the diverging peduncles is the inter'peduncular fossa, the floor of which is pierced by numerous minute openings that transmit small blood-vessels, and hence is known as the Posterior perforated substance. The blunted inferior angle of the fossa, immediately above the pons corresponds with a depression, the recessus posterior; another, but less ma depression the recessus anterior, is bounded by the postero-median surface the mammillary bodies. A shallow lateral groove (sulcus mcsencephah laterahs extends ilone the outer surface of the peduncle, whilst along its inner aspect, and therefore looking into the interpeduncular fossa, runs the median or oculomotor groove (sulcus nervi oculomotor!!), that is more distinct than the lateral furrow and no8 HUMAN ANATOMY. marks the line along which the root-fibres of the third cranial nerve emerge. On transverse section (Fig. 963) these furrows are seen to correspond with the edges of a crescentic field of deeply pigmented gray matter, the substantia nigra, by which each peduncle is subdivided into a dorsal portion, the tegmentum, and a ventral part, the crusta (basis pedunculi). The latter lies ventral to the superficial lateral and median furrows, and contributes largely to the bulk of the free part of the peduncle. When traced upward it is found to enter the cerebral hemisphere and become continuous with the internal capsule. It contains the great motor tracts and i»the chief pathway by which efferent cortical impulses are transmitted to the lower lying" centres. The tegmentum, on the contrary, in a general way is associated with the sensory tracts, and, above, enters the subthalamic region (page 1127). The dorso-lateral surface of the mid-brain, just where it passes into that of the superior cerebellar peduncle, shares with the latter a triangular area, the trigo- FIG. 959. Emerging fibres of fourth nerve Fourth nerve, Lateral fillet Posterior longitudinal fasciculus Tegmental field Mesial fillet Decussation of fourth nerve Sylvian aqueduct Central gray substance Mesencephalic root of trigeminus Substantia ferruginea Superior cerebellar peduncle Decussation of cerebellar peduncle Pyramidal tracts Transverse fibres Transverse section of brain-stem at level L (Fig. 919), junction of pons and mid-brain ; superior cerebellar pedun- cles are beginning to decussate; trochlear decussation seen above Sylvian aqueduct. Weigert-1'al Staining Preparation by Professor Spiller. num lemnisci, which, as implied by its name, is related to the underlying and here superficially placed tract of the fillet (lemniscus). Above, this area extends as far as the inferior brachium and is limited in front by the sulcus mesencephali lateralis, whilst behind it is defined from the superior cerebellar peduncle by a slight furrow ( sulcus limitans posterior). When closely examined the triangular field is seen to be subdivided by a faint groove into an upper and a lower area, which correspond with the underlying fibres of the lateral and of the mesial fillet respectively. A superficial strand of fibres, the tractus peduncularis transversus, is sometimes seen -ing the lateral surface of the mid-brain. It appears on the dorsal aspect of the latter, between the inferior brachium and the median geiiiculate body, winds around the 1. itci •« i -ventral surface of the peduncle and disappears in the vicinity of the mammillary body. According to Marburg, the strand establishes a connection between the cells of the retina and a nucleus in the floor of the third ventricle and represents, in a rudimentary condition, the basic optic root found in many animals. The Sylvian aqueduct (aquaeductus cercbri) represents the cavity of the middle brain-vesicle and, therefore, is lined with an ependymal layer continuous above and below with that clothing the interior of the third and fourth ventricles. As seen in THE MESENCEPHALON. 1109 cross-sections, (Fig. 960) its outline in a general way is triangular, with the base above and the apex directly below ; but the contour of the canal varies at different levels, being triangular near its extremities and irregularly cordiform or elliptical in the intervening part of its course. INTERNAL STRUCTURE OF THE MESENCEPHALON. Disregarding the several small nuclei, the nuclei of the corpora quadrigemina and the red nuclei, the gray matter within the mesencephalon is disposed as three tracts that extend the entire length of the mid-brain. These are the tubular mass of the central gray matter, which surrounds the aqueduct, and the two crescentic columns of the substantia nigra, which subdivide the peduncles into the tegmental and basal portions. The central gray matter (stratum griseum centrale) completely encloses the cavity of the mid-brain and hence is often called the Sylvian gray matter. It contains numerous irregularly scattered nerve-cells of uncertain form and size, and, along its ventral border, the nuclei of origin of the oculomotor and trochlear nerves ; within its lateral parts lie the nuclei from which proceed the fibres of the mesencephalic roots of the trigeminal nerves. FIG. 960. Inferior colliculus Mesencephalic root of trigeminus Lateral fillet Fibres of fourth nerve Nucleus of fourth nerve Mesial fillet Sylvian aqueduct Central gray substance Posterior longitudinal fasciculus uiitain decussation Mesial fillet Cerebellar peduncle Decussation of cerebellar peduncle Transverse section of dorsal part of mid-brain through lower end of inferior colliculi, at level M (Fig. 919) showing nucleus of trochlear nerve, and decussation of cerebellar peduncle. Weigert-Pal staining. X 3&. Preparation by Professor Spiller. The substantia nigra is disposed as two irregular crescentic columns of dark gray matter that separate the tegmentum from the crustae of the peduncles. The substance begins below at the upper border of the pons and continues uninterruptedly through the length of the mid-brain into the subthalamic region of the diencephalon, where it gradually disappears. The deep color of this tract is due to the conspicuous pigmentation of its numerous nerve-cells. These cells are of medium size and of various form, spindle-shaped elements, interspersed with some of stellate and a few of pyramidal form, predominating. They enclose considerable accumulations of dark brown pigment that render the cells unusually conspicuous. During the earliest years of childhood the pigmentation is absent or very slight, but after the sixth year it is marked, and by the seventeenth has acquired its full intensity. Seen in cross-sections (Fig. 961), the convexity of each column, directed forward and out- ward, is not uniform, but broken into irregular scallops by processes of gray matter that penetrate the subjacent crusta. The concave dorsal margin, on the contrary, is unbroken and even. The horns of the crescentic areas, of which the median is somewhat the thicker, approach the free surface along the bottom of the superficial 1 1 io HUMAN ANATOMY. lateral and median grooves of the mid-brain. Concerning the functions and connec- tions of the neurones within the substantia nigra very little is known. The Quadrigeminal and Geniculate Bodies. — The inferior colliculus consists chiefly of a biconvex (in section oval) mass of gray matter, the nucleus colliculi inferioris, in which many nerve-cells of varying form and mostly of small size lie embedded within a complex of nerve-fibres. The lower end of the nucleus stands in intimate relation with the acoustic fibres composing the lateral fillet, many of which enter the ventral aspect of the nucleus colliculi to end around its cells, whilst a considerable number pass superficial to the nucleus and thus form an external fibre- layer that intervenes between the gray nucleus and the surface. Although many of these external fillet-fibres enter the colliculus at higher levels, not a few continue, by way of the inferior brachium, to the median geniculate body, around whose neu- rones they end. A much smaller and less well defined tract of fillet-fibres passes to the mesial side of the nucleus, the ventral margin of which is thus embraced (Fig. 960) by the diverging but unequally robust fillet-strands that in this manner partially encapsulate the collicular nucleus. From the supero-lateral parts of the nucleus fibres proceed which, in conjunction with those continued from the lateral fillet, form the chief constituents of the inferior brachium. A part of this arm, how- ever, is composed of strands of fibres that pass from the cerebral cortex (especially the temporal) to the inferior colliculus. Towards the upper pole of the nucleus some loose strands of fillet-fibres, probably along with commissural fibres uniting the inferior colliculi, cross the mid-line and establish a decussation. The internal or median geniculate body (corpus geniculatum mediate), although genetically belonging to the diencephalon, is so closely related to the inferior colliculus as to require description in this place. It consists of a superficial layer of white matter composed of fibres from the inferior brachium, which pass outward as continuations of the lateral fillet, as axones of the cells of the inferior colliculus, or as fibres forming the mesial root of the optic tract, also known as the inferior commissure of Gudden. Within this fibre-capsule lies an oval mass of gray matter, the nucleus corporis geniculati medialis, from whose cells axones proceed chiefly towards the cerebral cortex in continuation of the auditory paths of which the inferior colliculus and the median geniculate body are important stations. Connections of the Inferior Colliculus and Median Geniculate Body.— Mention has been made, when describing the reception-nuclei of the cochlear portion of the auditory nerve (page 1076) , that the tract of the lateral fillet takes origin to an important extent from the cells of these nuclei, and, further, (page 1082), that the fillet-fibres end around either the cells of the inferior colliculus, or those of the median geniculate body. It is evident, therefore, that these parts of the mid-brain stand in intimate relation with the parts concerned in conveying auditory impulses. The more detailed account of the chaining together of the neurones forming such paths is deferred until the auditory nerve is considered (page 1257). The connection of the fibres com- posing the median root of the optic tract with the median geniculate body and the inferior collic- ulus has been established beyond doubt ; further, that this part of the optic tract is not concerned in conducting visual impulses, is shown by the fact that these fibres remain unaffected under conditions (after removal of the eyes) that lead to degeneration of the fibres of retinal origin. The destination and significance of the fibre-systems included within the median root of the optic tract are only imperfectly understood, but it may be accepted as certain that they can no longer he regarded as merely establishing a bond between the median geniculate and indirectly the inferior quadrigeminal bodies of the two sides, as implied by the name commissure, since many of these fibres are probably directed after decussation to the lenticular nucleus (globus pallidus), while others possibly may end on the same side in the subthalamic nucleus ( page 1128). The gray matter of the inferior colliculus, like that of the superior, gives rise to fibres of the tecto-bulbar and tecto-spinal tracts, presently to be described (page mi). The superior colliculus is composed of a number of alternating layers of white and gray matter. The latter, however, is not aggregated into a definite nucleus, as in the case of the inferior colliculus, but is broken up into uncertain zones by the tracts of nerve-fibres. Although as many as seven layers have been described, some of these are so blended that only four well-defined strata can be readily distinguished. From the surface inward these are : THE MESENCEPHALON. mi 1. The stratum zonale, a thin peripheral fibre-layer that occupies the surface of the collic- ulus, whose components are fibres derived, in great part at least, from the optic tract. 2. The stratum cinereum, which is not uniform, but thickest and most marked over the convexity of the colliculus, and appears, therefore, crescentic in transverse sections. The nerve- cells contained in this cap-like sheet are small and relatively few, their axones passing for the most part towards the deeper layers, whilst their dendrites are directed peripherally. The stratum is by no means composed entirely of gray matter, but is invaded by many medullated nerve-fibres. 3. The stratum opticum, which consists of a complex of gray matter and nerve-fibres, the latter including strands derived from the optic tract, which gain the side of the colliculus by way of the superior brachium (page 1107) as direct continuations of the optic fibres, or after interruption in the lateral geniculate body. That this stratum includes other fibres, is shown by the incomplete involvement of the layer in conditions producing degeneration of the FIG. 961. Sylvian aqueduct Inferior colliculus Central gray substance Inferior brachium Posterior longitudinal fasciculus Tegmental field Lateral sulc Fountain decussation Mesial fillet Decussation of cerebellar peduncles Substantia"\j nigra, separat- ing crusta from tegmentum Motor tracts ' Cerebellar peduncle Pontine Interpeduncular Substantia fibres space nigra Crusta of peduncle Transverse section of mid-brain at level N (Fig. 919); decussation of cerebellar peduncles is just ending. Weigert-Pal staining. X 3. Preparation by Professor Spiller. optic paths, as well as by the prominence of parts of the stratum in animals possessing only rudimentary visual paths (Edinger). The stratum opticum, however, consists by no means exclusively of fibres, but contains, especially in its deeper part, numerous nerve-cells of large size, around which the end-arborizations of the optic fibres terminate. 4. The stratum lemnisci, which likewise includes masses of gray matter intersperse between the strands of nerve-fibres. The latter are chiefly from that part of the median fi which terminates within the superior colliculus ; a certain number of the fibres, however, are probably derived from the lateral fillet, which, while having its principal quadrigemmal relation with the inferior colliculus, also sends a small contingent to the upper body. of the fillet-layer contains a considerable amount of gray matter, in which numerous nerve-' usually of small size, are irregularly distributed. In addition to receiving optic and fillet-fibres, the gray matter of the colliculus gives origin tc an important system of descending fibres which establishes connections between the mid and the lower levels of the brain-stem and the spinal cord. These fibres emerge from the v tral border of the colliculus as radially disposed strands which, on neanng the gray n surrounding the aqueduct, turn ventrally. The more laterally situated fibres, reinforced by those from the opposite side, descend within the tegmental field to end parti v in relation w the nuclei within the brain-stem (tractus tecto-bulbaris lateralis) and partly within the spin (tractus tecto-spinalis lateralis). The medially situated fibres sweep around the Sylv matter and, for the most part, cross the raphe immediately ventral to the posterior tonptudir fasciculus thus establishing the fountain decussation of Meynert (Fig. 960). 1 1 12 HUMAN ANATOMY. of these fibres is downward through the brain-stem and into the anterior column of the cord (tractus tecto-spinalis medialis). Whether these fibres are interrupted in small secondary nuclei within the tegmentum, or pass unbrokenly from the collicular cells to the cord is undetermined. It is probable that, as constituents of a spino-tectal path, fibres also ascend from the spinal cord to the quadrigeminal bodies. According to Kolliker, some of the radial fibres are traceable through the tegmentum, passing to the outer side of the red nucleus and piercing the tract of the median fillet, and into the substantia nigra, whose cells they probably join as axones. The commissure of the superior colliculi is formed by fibres that cross the mid-line to the opposite quadrigeminal body and probably includes, in addition to the axones of cells within the1 colliculi themselves, fibres from the fillet and optic tracts. The most important connections of the superior colliculus, as may be anticipated from the foregoing description of its structure, are : i. With the optic tract, without interruption in the lateral geniculate body, by way of the superior brachium. Such fibres serve a special purpose, namely, to carry stimuli which excite pupillary reflexes, by transference to the oculomotor nucleus. 2. With the posterior sensory columns of the spinal cord, indirectly by way of the median fillet. 3. With the cochlear nuclei by way of the lateral fillet, thus establishing a path for audito-visual reflexes. 4. With nuclei of the third, fourth and sixth cranial nerves, controlling the eye-muscles, especially the oculo- motor, by way of the posterior longitudinal fasciculus. 5. With the lower levels of the brain- stem and the spinal cord by way of the tecto-bulbar and tecto-spinal tracts. The lateral geniculate body belongs to the diencephalon and may be regarded as a special- ized part of the optic thalamus ; the consideration of its structure therefore, properly falls with that of the metathalamus (page 1126). The Tegmentum. — The tegmental region of the mid-brain includes, as seen in transverse sections (Fig. 961), the U-shaped area extending from the quadri- geminal bodies behind to the crescents of the substantia nigra in front. In the vicin- ity of the central gray matter that surrounds the Sylvian aqueduct, the tegmentum consists chiefly of a foundation resembling the formatio reticularis seen at lower levels. This substance is produced by the intermingling of transverse or arcuate and longitudinal fibres and a meagre amount of gray matter with irregularly distributed nerve-cells, that fills the interstices between the strands of nerve-fibres. The more lateral and ventral parts of the tegmentum are to a large extent occupied by the prominent fibre-tracts belonging to the fillets and to the superior cerebellar peduncles, or by collections of gray matter, as the red nuclei. Special groups of nerve-cells and of nerve-fibres mark the origin and course of the oculomotor and trochlear nerves. The details of the tegmentum vary with the level of the plane of section. Thus, at the lower end of the mid-brain the tracts of the cerebellar peduncles approach the mid-line as they ascend and those of the fillets assume a more lateral position ; whilst at higher levels these tracts, which lower in the mid-brain are so conspicuous, either terminate to a large extent, or become so broken up as to no longer form impressive bundles. In sections passing through the lower pole of the inferior quadrigeminal bodies (Fig. 960), the zone overlying the substantia nigra is occupied to a great extent by the median fillet, which here appears as a broad but thin crescentic or comma-shaped field, whose outer and thicker end lies at the periphery and abuts against the base of the dorsally arching tract of the lateral fillet. At the inner end of the median fillet, near the mid-line, an isolated group of obliquely cut fibres sometimes indicates the position of the lemnisco-crustal bundle that appears ventrally among the robust strands of the crusta. Taken together, the two fillets form a compact tract, the outer contour of which, at the level now considered, resembles a horizontally placed Gothic arch, the summit of the curve lying at the surface and the lower and upper limits of the arch bring thr mrdian and lateral fillets respectively. The lateral fillet continues the sweep of the fillet-stratum along the periphery of the tegmentum until it embraces the lower pole of the inferior collie ul us in the manner previously (It-scribed (page mo). Dorsal to the tract of the median fillet, and separated from the latter by a thin layer of com- pact foundation-substance, the ventral tegmental field, lies the broad curved band formed by the blending of the two superior cerebellar peduncles. At lower levels (Fig. 936) these stalks are separate and appear as laterally placed and conspicuous crescentic areas of transversely cut fibres ; but opposite the lower limit of the inferior quadrigeminal bodies the ventral ends of these crescents meet at the mid-line and interlace to form the decussation of the cerebellar peduncles. At a slightly higher level, after their decussation has been almost completed (Fig. 961), the i-rrebrllar peduncles appear as prominent rectangular fields, with rounded comers, on each side of and close to the mid-line. These fields of transversely cut fibres represent the peduncles THE MESENCEPHALON. 1113 as they pass upward to the red nuclei, in which a large number of their component fibres end. On each side of the median raphe of the tegmental field and above (behind) the peduncular tract, is seen the posterior longitudinal fasciculus, which here, broader than in the pons, passes close to the ventral side of the nucleus of the trochlear nerve. The attenuated crescentic tract of transverely cut fibres which lies along the lateral margin of the central gray substance, medial to the nucleus of the inferior colliculus, represents the mesencephalic root of the trigeminal nerve. In sections taken slightly below the level of the trochlear nucleus, irregular bundles of obliquely cut fibres mark the dorsally directed course of the fourth-nerve to gain its decussation in the roof of the aqueduct at the lowest limit of the mesencephalon (Fig. 959). FIG. 962. Caudate nucleus Anterior nucleus Thalamus Ventral nucleus Pulvinar Posterior commissure Pineal bo Sylvian gray matte Corpora qu: Posterior longitudinal fasciculus Superior medullary velum Cerebellum Fourth ventricle Fibres of abducent nerve Internal capsule External medullary lamina Lenticular nucleus Stratum medullare hypothalamicum Tuber cinereum Optic commissure Crustaand substantia nigra (latter to left) Red nucleus Fibres of oculomotor nerve Superior cerebellar peduncle Ponto-cerebellar tracts Pyramidal tracts Mesial fillet Floor of IV ventricle (medulla) Formatio reticularis Internal arcuate fibres Nucleus cuneatus Nucleus gracilis Posterior fasciculi Inferior olivary nucleus Sagittal section of brain-stem ; plane of section is somewhat lateral to mid-line. by Professor Spiller. X j. Preparation As seen in cross-sections passing through the superior quadrigeminal bodies, the details of the tegmentum differ considerably from those at the levels previously stated, is no longer present as a distinct field, since with the exception of a few strands that are con- tinued into the superior colliculus, its fibres end within the lower colliculus or pass into tl inferior brachium. The median fillet now shows (Fig. 963) as a somewhat attenuated crescer field lying to the inner side of the obliquely cut inferior brachium, in consequence of many o its fibres having ended within the lower part of the superior colliculus, the more dorsally situatec of those remaining being seen within the upper colliculus as the stratum lemmsci. HUMAN ANATOMY. The most conspicuous object within the tegmentum in the superior half of the mid-brain is a large round reticulated field on each side of the median raphe, which marks the position of the red nucleus (nucleus ruber). This body, also called the nucleus tegmenti, is of an irregular ovoid form (Fig. 963) and of a reddish tint when seen in sections of the fresh brain. Its lower limit corresponds with the level of the lower margin of the superior colliculus, whilst its upper pole extends into the subthalamic region. Its diameter increases towards the upper end and its long axis converges as it ascends, so that the upper enlarged portions of the two nuclei lie close to the mid-line and nearer each other than do the lower poles. Each nucleus consists of a complex of gray matter and nerve-fibres. The latter preponderate below, where the red nucleus receives the fibres of the superior cerebellar peduncle, and are much less numerous above, since many fibres come to an end around the rubral cells. These elements are very variable in shape and size (.020. -060 mm.), but are most often irregularly triangular or stellate. The red nuclei constitute not FIG. 963. Optic fibres joining superior colliculus Opttc thalamus Part of median yeniculate nucleus Median kreniculate bodj Root-fibres of_ third nerve Emerging fibres of oculomotor nerve Inter- peduncular space Red nucleus Lateral geniculate body Stratum intermedium Crusta of cerebral peduncle Substantia nigra Transverse section of mid-brain at level O (Fig. 919), passing through superior colliculus and gen iculate bodies; red nucleus, and nuclei and root-fibres of oculomotor nerve. Weigert-Pal staining. X 3- Preparation by Professor Spiller. only important stations in the path connecting the cerebellum and spinal cord, but also probably contribute links in chains uniting the cerebral cortex and the internal nuclei with the cord. Whilst some of the constituents of the superior cerebellar peduncle pass around the red nucleus and continue as cerebcllo-ihalamic fibres uninter- ruptedly to the optic thalamus, the majority of the fibres of this arm end around the cells of the nucleus. Of these many give off axones that proceed brainward as rubro- thalamic fibres ; others emerge from the ventro-medial surface of the nucleus, cross the mid-line (decussation of Forel) and bend downward as the rubro-spinal tract. The latter descends within the tegmentum of the mid-brain and pons, traverses the medulla and finally enters the lateral column of the cord as one of the important but uncertainly defined descending tracts. Other fibres enter the red nucleus on its lateral aspect and establish connections between the cerebral cortex (Dejerine), and probably also the corpus striatum (Edinger), and the nucleus. From the cells of the latter tin- path is continued by fibres which join the rubro-spinal tract, and in this manner establish an indirect motor path that supplements the cortico-spinal tracts identified with the pyramidal. THE MESENCEPHALON. 1115 The Crusta. — The crusta, or pes pedunculi, appears in transverse sections (Fig. 963) as a bold sickle-shaped field that occupies the most ventral portion of the mid-brain. It consists chiefly of longitudinally coursing fibres which, having traversed the internal capsule, are passing from various parts of the cerebral cortex to lower levels in the brain-stem and the spinal cord. The longitudinal fibres are separated into bundles by the invasion of numerous strands from the fibre-complex, known as the stratum intermedium, which lies along the ventral border of the substantia nigra. The fibres of the crusta comprise three general sets: the cortico-pontile, the cortico-bulbar, and the cortico- spinal. The cortica-pontile fibres include those passing from the cells of the cerebral cortex to the cells of the pontile nucleus as links in the cortico-cerebellar paths. They are represented by the fronto-pontile and the temporo-occipito-pontile tracts, which occupy approximately the median and lateral fifths of the crusta respectively. The cortico-bulbar-fibres include the efferent strands which pass from the motor areas of the frontal lobe to the nuclei of the motor fibres originating in the bulbar portion of the brain-stem (trigeminal, abducent, facial, glosso-pharyngeal, vagus and hypo- glossal nerves). These tracts occupy something less than the fifth of the crusta lying next the fronto-pontine tract. The cortico-spinal fibres include the great motor strands which, as the pyramidal tracts, are so conspicuous at lower levels. These tracts share with the fronto-bulbar paths the middle three-fifths of the crusta, appropriating approximately the lateral three-quarters of this area (Fig. 1012). The Median Fillet. — Repeated reference has been made to the median fillet (lemniscus medialis) in the preceding descriptions of the brain-stem ; a general con- sideration of this important sensory tract may here be given. It begins at the lower part of the medulla, about on a level corresponding with the upper limit of the pyramidal decussation, as axones of the cells within the nucleus gracilis. These sweep ventro-medially as the deep arcuate fibres, for the most part cross the raphe, and bend sharply brainward. Succeeding the condensation of the fillet-fibres into the sensory decussation (Fig. 922) which marks the lowest limit of the tract, the fillet receives continuous additions of arcuate fibres from the gracile and cuneate nuclei so long as these collections are present. On reaching the inferior olivary nuclei in its journey brainward, the fillet forms a laterally compressed tract, the inierolivary stratum, lying immediately dorsal to the pyramids (Fig. 928). Towards the upper end of the pons, the fillet gradually exchanges its sagittal plane and median position for an obliquely horizontal disposition, with an increasing tendency to migrate laterally. The fibres arising from the nucleus cuneatus, which below occupied the ventral part of the fillet, now constitute the lateral part of the tract, whilst those from the nucleus gracilis form its medial portion. Within the mid-brain the'median and the lateral fillets form a continuous crescentic tract which, within the upper part of the tegmentum and after the disappearance of the acoustic paths, is represented chiefly by the superficial and laterally placed tract which the median fillet has now become. A considerable part of its fibres end around the cells of the deeper gray stratum of the superior colliculus, some passing over the aque- duct to the colliculus of the opposite side. The remaining fibres continue upward through the tegmentum, lateral and dorsal to the red nucleus, and the subthalamic region to terminate chiefly in relation with the cells within the ventral part of the optic thalamus. After such interruption the impulses are carried by fibres arising within the thalamus to various parts of the cerebral cortex. Whether fillet-fibres gain the cortical gray matter without interruption within the thalamus is uncertain. Other fibres, said to be derived from the cuneate nucleus, end in the corpus subtha- lamicum and the lenticular nucleus (globus pallidus), from whose cells a certain num- ber of fibres proceed by way of a strand placed above the optic chiasm, the com- missure of Meynert, to the globus pallidus of the opposite side, are traceable into the posterior commissure of the brain and into the mammillary yThe constituents of the median fillet, however, are by no means restricted to the fibres arising from the gracile and cuneate nuclei of the posterior columns, but include numerous important accessions from the reception-nuclei of all the sensory cranial nerves connected with the brain-stem. From the cells within the more in6 HUMAN ANATOMY. FIG. 964. Superior collicul y part) extensive of such nuclei, as those within the column of substantia gelatinosa accom- panying the spinal root of the trigeminus, numerous arcuate fibres sweep towards the raphe and, with few exceptions, cross to join the median fillet of the opposite side. In this manner provision is made for the transmission to the higher receptive centres of sensory impulses collected not only by the strands of the posterior column of the cord, but also by the sensory fibres of the cranial nerves attached to the brain-stem. Although the principal components of the fillet-tract are the bulbo-tecto- thalamic strands, some fibres running in the opposite direction are also present. Some of these probably arise from cells within the optic thalamus and the corpora quadrigemina. Others are efferent strands which establish connections between the cortical gray matter and the nuclei of the motor cranial nerves, especially the facial and hypoglossal. These cortico- bulbar tracts descend within the crusta to the lower end of the cerebral peduncle ; then, leaving the latter, they traverse the stratum intermedium and in the upper part of the pons join the median fillet and descend within its ventro - median part as far as the superior end of the hypoglossal nucleus. During their course, the fibres of this crustal fillet, as it is called, for the most part undergo decussation on reaching the levels of the motor nucleus for which they are destined ; some fibres, however, possibly end around the cells of the nucleus of the same side. The Posterior Longi- tudinal Fasciculus. — This bundle (fasciculus loogitttdinalia rncdialis) is an association path of fundamental importance, be- ing present in all vertebrates. As a distinct strand it begins in the superior part of the mid-brain and thence is traceable as a con- tinuous tract through the teg- mental region of the pons, the dorsal and lateral ventral field of the medulla into the anterior ground-bundle of the spinal cord. Throughout the greater part of its course through the brain-stem, its position is constant, the fasciculi of the two sides lying close to the median raphe and immediately beneath the gray matter flooring the Sylvian aqueduct and the fourth ventricle (Figs. 959, 961). In the lower part of the medulla, the bundle gradually leaves the ventricular floor and rests upon the dorsal border of the median fillet, and, at the level of the pyramidal decussation, where the fillet no longer intervenes, lies behind the pyramid and at some distance from the mid-line. Lower, it assumes a more ventral position, to the medial side of the isolated anterior cornu, and, finally, enters the anterior column of the cord to be lost within the upper part of the ground bundle. The fasciculus includes association fibres of varying lengths, some of which are ascending and others descending paths. The constitution of the bundle is, there- fore, continually changing, the loss of certain fibres being replaced by the addition of others. Its tibn-s arc among the very first in the brain to become medullated, and begin to acquire this coat during the fourth foetal month (Hosel). Sensory decussation Posterior nuclei Spino-thalamic Spinal ganglion Diagram showing chief afferent constituents of median fillet. THE MESENCEPHALON. 1117 FIG. 965. Notwithstanding the admitted importance of the tract and the prolonged study that it has received, much remains to be determined concerning the source and connections of the many constituents which undoubtedly go to form the bundle. Among the more certain of these com- ponents the following may be mentioned : 1. At the upper end of the fasciculus a considerable number of fibres arise from the cells of the nucleus of the posterior commissure, or DarkschewitscK1 s nucleus, which lies in advance of the oculomotor nucleus, within the gray matter surrounding the superior end of the Sylvian aqueduct. According to Edinger an additional contingent takes origin from a nucleus (n. fas- ciculi longitudinalis medialis) within the gray matter of the floor of the third ventricle in the vicinity of the corpus mam- millare. The contributions from both these sources join the fasciculus as crossed fibres from the nuclei of the opposite side. 2. The fibres arising from the vestibular (Deiters') nucleus constitute an important element of the posterior longitudinal bundle, since they establish reflex paths for equilibration impulses. These fibres, both crossed and uncrossed, join the fasciculus and pass in both directions. Those passing brainward have as their chief objective point the oculomotor nucleus, although the nuclei of the sixth and fourth nerves receive fibres or collaterals. In this manner the filaments supplying the various ocular muscles are brought under the influence of the vestibular impulses. It is probable that the facial nucleus likewise receives collaterals, if not main stems, of the vestibulo-nuclear fibres. 3. Upon clinical and experimental evidence, it may be assumed that fibres pass by way of the longitudinal bundle from the abducent nucleus to that part of the oculomotor nucleus sending fibres to the internal rectus muscle of the opposite side (perhaps also from the nucleus of the third nerve to that of the abducens of the same side), by which arrangement the harmonious action of the internal and external recti muscles is insured. Basing their conclusions upon similar evidence, many anatomists accept the existence of fibres which pass by way of the posterior longitudinal bundle from the oculomotor nucleus to the cells of the facial nucleus (page 1251) from which proceed the fibres supplying the orbicularis palpebrarum and the corrugator supercilii. In this manner the coordinated action of these muscles and the levator palpebrae superioris is explained. A similar connection is probably established by the posterior longitudinal bundle between the nucleus of the hypoglossal and that of the facial nerve, whereby the closely associated movements of the lips and tongue are assured. That the function of the posterior fasciculus is by no means limited to association of the nuclei of the ocular nerves is evident from the fact that in animals or individuals in which such centres are wanting (due to absence or imperfect development of the visual organs) the bundle is nevertheless well represented. 4. Fibres arise from the reception-nuclei of the remaining sensory nerves of the brain-stem and pass to the posterior longitudinal fasciculus of the same and the opposite side. On enter- ing the bundle, they course in both directions and by means of their collaterals and stem-fibres send end-brushes to the nuclei of the motor nerves, in this manner establishing direct reflex areas between the afferent and efferent paths. Strictly considered, it is probable that the fibres establishing connections with the nuclei of the sensory nerves constitute a small separate tract, lying within the central gray matter dorso-lateral to the posterior longitudinal bundle. This path has been called the fasciculus longitudinalis dorsalis of Schiitz, while the main bundle is then termed the fasciculus longitudinalis medialis. In order to avoid confusion, both sets of fibres are here regarded as parts of one path, the pos- terior longitudinal bundle. DEVELOPMENT OF THE MESENCEPHALON. Of the three primary cerebral vesicles, the mid-brain undergoes least change. Although much smaller than either of the other segments of the brain-tube, its prominent position, lying as it does at the summit of the cephalic flexure, makes it conspicuous in the early developing brain. During the enormous expansion upward and backward incident to the development of the cerebral hemispheres in man, the mid-brain becomes covered in and deposed to a dependent position and a relatively small size. For a time possessing a spacious cavity, it fails to keep pace with the growth of the adjoining parts ; its walls thicken and its lumen becomes eventually reduced to the narrow Sylvian aqueduct. Diagram showing chief constituents of posterior longitudinal fasciculus. Ill, IV, VI, VII. XII, nuclei of respective nerves; D.vtstihmar (Deiters'; nucleus; CN, common nucleus of posterior commissure and posteiior longitudinal fasciculus. in8 HUMAN ANATOMY. The dorsal zones of the lateral wall of the mid-brain give rise to the quadrigeminal plate, whose external surface is at first smooth but later marked by a temporary median longitudinal ridge. About the third fcetal month, with the exception of its lower end, which persists as the frenulum veli, this ridge is succeeded by a longitudinal groove bounded on either side by an elevation. The elevations of the two sides mark the appearance of the corpora bigemina, cor- responding to the optic lobes of the lower vertebrates. During the fifth month, an obliquely transverse furrow forms on each side, by which the paired elevations are subdivided into four eminences, the corpora quadrigemina. About this time the corpora geniculata, which however belong developmentally to the diencephalon, are also differentiated and for awhile are rela- tively very large and prominent. The ventral zones greatly thicken and give origin to the tegmentum, including the nuclei of the oculomotor and of the trochlear nerves and, perhaps, the red nuclei, and the mantle layer of the cerebral peduncles with the interpeduncular substance. The floor-plate becomes com- pressed between the expanding ventral zones of the lateral walls and probably is represented by the raphe. Since the fibre-systems of the crustae are, for the most part, derived from sources outside the brain-stem, their appearance within the peduncles follows a secondary ingrowth, and only after such invasion do the cerebral crura present their characteristic ventral prom- inence. The cortico-pontile tracts share with the pyramidial fibres the characteristic of tardy myelination, since they do not acquire their medullary coat until some time after birth. Among the earliest of the cortico-bulbar fibres to become medullated (a few weeks after birth ) are those destined for the motor cranial nerves by way of the crustal or pyramidal fillet of Flechsig. According to Kolliker, the stratum intermedium, which is closely related to the substantia nigra, not only in position but also by the destination of many of its fibres, contains a consider- able number of medullated fibres by the ninth foetal month. THE FORE-BRAIN. It will be recalled that the fore-brain, the anterior primary cerebral vesicle, gives rise to two subdivisions, the tclencephalon and the diencephalon (page 1060). Since the latter lies immediately in front of the mid-brain, in following the order in which the brain-segments have been described, the diencephalon next claims attention. THE DIENCEPHALON. Strictly considered upon the basis of the classic subdivision suggested by His, the diencephalon, or inter-brain, includes (i) a large dorsal portion, the thalamen- cephalon and (2) a small ventral portion, the pars mammillaris hypothalami, together with (3) the enclosed remains of the posterior part of the cavity of the fore-brain, as represented by the greater part of the third ventricle. The thalamen- cephalon, in turn, includes : (a~) the thalamus, (6) the epithalamus, comprising the pineal body, the habenular region and the posterior commissure, and (c*) the meta- thalamus, including the corpora geniculata. Since, however, the description of the third ventricle and its surrounding structures — the essential features of this segment of the adult brain — requires the inclusion of parts belonging to the telencephalon (pars optica hypothalami), it will be more convenient to disregard their strict developmental relations and include the representatives of the pars optica in the consideration of the diencephalon. The Thalamus. — After removal of the overlying structures— the corpus callo- sum, the fornix and the velum interpositum — the thalami (thalami), also called the optic thalami, are seen as two conspicuous masses of gray matter separated by a narrow cleft, the third ventricle. Each thalamus is an ovoid ganglionic mass, blunt wedge-shaped, as seen in cross-sections (Fig. 967), whose long axis extends from the narrow anterior pole backward and outward. Of its four surfaces, the lateral and ventral are blended with the surrounding nervous tissue, and the mesial and dorsal are to a large extent free. The large superior surface is irregularly triangular in outline, slightly convex in the frontal plane and markedly so in the sagittal, and covered with a thin layer of nerve-fibres, the stratum zonale, which imparts a whitish eolor. This stratum is composed of fibres which are traceable on the one hand to the optic tract, and on the other to the optic radiation in the hind part of the internal capsule. Laterally, the superior surface is separated from the caudate nucleus by a groove which obliquely crosses the floor of the lateral ventricle and lodges a narrow hand of fibres, the taenia semicircularis (stria terminal's) and, in its anterior part, the vein of the corpus striatum. In its front half, where it bounds the THE DIENCEPHALON. 1119 ventricle, the inner border is sharply defined from the mesial surface by a delicate but well defined ridge, tsenia thalami, produced by the thickening of the ependyma of the third ventricle, along its line of reflection onto the membranous roof, and the underlying strand of nerve-fibres, the stria medullaris. Traced backward, the tsenia thalami becomes continuous with the stalk of the pineal body. Between this ridge and the diverging mesial border of the upper surface of the thalamus, is included a narrow, depressed triangular area, known as the trigonum habenulae. It lies on a distinctly lower level than the adjoining convex upper surface of the thalamus. Since it contains a special nucleus and belongs to the epithalamus, its description will be deferred until that region is considered (page 1123). The upper surface is not quite even, but subdivided by a shallow oblique furrow, which runs from before backward and outward and marks the position of the overlying lateral border of the fornix. External to this furrow lies a free marginal zone that forms a part of the floor of the lateral ventricle ; internal to it is an attached inner zone over which the velum interpositum is united to the thalamus. By the attachment of this FIG. 966. Corpus callosum Septum lucidum Tsenia semicircularis and vena terminalis Tzenia chorioidea Furrow for fornix Tsenia thalami Trigonum habenulae Pulvinar Corpora quadrigemina Thalami, caudate nuclei and ventricles viewe< velum interpositum ; third ventricle sho Caudate nucleus Anterior pillars of fornis Foramen of Moiiro — -Anterior commissure Middle commissure in III ventricle Thalamus Posterior commissure Pineal body Lingula ed from above after removal of corpus callosum fornix and ws as narrow cleft between mesial surfaces of thalami. sheet to the fornix above and to the thalamus below, direct communication between the third and lateral ventricles is shut off save through the foramen of Monro. In front the superior surface ends on the rounded elevation (tubcrculum anterms thalami) which marks the anterior pole of the ganglion, while behind it goes over onto the prominent posterior projection, the pulvinar, which overhangs the superior brachium and the corpora geniculata. The mesial surface forms the greater part of the lateral wall of the third ventricle. It is covered by a layer of gray matter prolonged from the central gray of the Sylvian aqueduct, over which stretches immediate lining of the ventricle, the ependyma. The upper boundary of the mesi surface is sharply defined by the tasnia thalami, which behind is continuous With tt stalk of the pineal body (Fig. 966). Its lower limit is indicated by an c furrow the sulcus hypothalamicus, which separates the thalamic from tl hypothalamic regions. Somewhat in advance of their middle, the mesial surfaces of the two thalami are connected by a bridge of gray matter, known as the middle commissure (massa intermedia), usually about 7-8 mm. in diamete oval in se-tion but very variable in thickness and form. From the meagre num of medullated nerve-fibres that it contains, its importance, at least in to be small The lateral surface of the thalamus is inseparably blended with the adjacent thick and conspicuous stratum of white matter the internal ca] which intervenes between the thalamus and the more laterally placed lenticular II2O HUMAN ANATOMY. nucleus, and establishes the important pathway transmitting the fibre-tracts con- necting the cerebral cortex with the thalamus and with the lower levels by way of the crusta of the cerebral peduncle. Since the innumerable fibres which pass to and from the thalamus along its ventro-lateral surface interlace, this surface is covered by a distinct reticulated stratum, to which the name external medullary lamina is applied. The ventral surface is also attached, but instead of being united with the internal capsule, as is the lateral, it rests upon and is intimately blended with the upward prolongation of the tegmental portion of the cerebral peduncle, here known as the subthalamic tegmental region, presently to be described (page 1127). FIG. 967. Corpus callosum Choroid plexus Foruix T;i:iiia thalami Middle commissure Third ventricle Mammillo-thalamic tract Marnmillary body Amygdaloid nucleus Caudate nucleus Thalamus. mesial nucleus Thalamus, lateral nucleus Lenticular nucleus Subthalamic nucleus Optic tract Tail of caudate nucleus Inferior horn of lateral ventricle Hippocampus, cut obliquely Crusta of cerebral peduncle Frontal section of brain passing through thalami, middle commissure and inammillary bodies. Structure of the Thalamus. — Although composed chiefly of gray matter, the thalamus is partially surrounded and penetrated by tracts of white matter. In addition to being invested on its superior and ventro-lateral surfaces by the stratum zonale and the external medullary lamina respectively, the general ganglionic mass is subdivided by a vertical internal sheet of fibres, continuous with the stratum /onale and known as the internal medullary lamina, into three fairly marked nuclei, the anterior, the mesial and the lateral ( Fig. 967). Of these the lateral nucleus is much the largest and is included between the external and internal medullary lamina-. Whilst the lateral nucleus does not reach as far forward as the anterior pole of the thalamus, its caudal extremity includes the entire pulvinar. The lateral nucleus consists histologically of an intricate complex of nerve-fibres and cells. The latter are in general of the multipolar type, although very variable as to details of form and size. Two principal types are recogni/ed by Kolliker, the one being elongated or fusiform and possessed of relatively few branches, and the other being stellate and provided with richly branched dendrites. Many of the fibres represent paths ending within the thalamus and therefore terminate in arborizations around the thalamic cells ; others are the axones of such cells and pass to various parts of the cortex or other parts of the brain. The histological characteristics of the lateral nucleus, in the THE DIENCEPHALON. II2I main hold good for the other nuclei, although the lateral nucleus is particularly rich in fibres, and therefore of a paler tint, on account of its close relations to the internal capsule and the tegmentum of the cerebral peduncle. The mesial nucleus lies between the central gray matter of the ventricular wall and the internal medullary lamina, and is separated by the latter from the lateral nucleus. Its caudal end is bordered internally by the ganglion habenulae, and, behind, by the pulvinar. The anterior nucleus, the smallest of the three, is a wedge-shaped mass, whose rounded base looks forward and corresponds to the anterior tubercle, and whose apex is directed backward and lies between the front ends of the mesial and lateral nuclei, separated from these by the internal medullary lamina, which divides into two diverging levels that embrace the anterior nucleus. In addition to its contribution of radiating fibres which take part in the production of the thalamic radiation, the anterior nucleus contains a compact bundle of fibres traceable into the mammillary body on the base of the brain. These are the constituents of the mam- millo-thalamic tract, or bundle of Vicq d1 Azyr, by which a large part of the fibres For nix Choroid plexu Lateral ventricle Stratum zonale Caudate nucleus Genu of internal capsule Thalamus, mesial nucleu Thalamus, lateral nucleus Internal capsule Putamer Globus pallidus Anterior pillars of fornix' ""' "v Lamina cinerea Gyms callosus Cingulum Corpus callosum .Striate vein Caudate nucleus Tsenia semicircularis Internal medullary lamina External medul- ary lamina tlammlllo- thalamic tract -Putamen Globus pallidus Thalamo-tegmental tract •Olfactory fibres Anterior commissure Oblique frontal section through thalamus and anterior commissure ; Weigert-Pal staining. X J. Preparation by Professor Spiller. coursing within the anterior pillar of the fornix are carried to the thalamus (page 1159). The entire ventral part of the thalamus is occupied by an illy- defined mass of gray matter, known as the ventral nucleus, which lacks sharp definition from the overlying nuclei and in fact is continuous with the lateral nucleus. The ventral nucleus presents a differentiation into the nucleus centralis of Luys, w-hich occupies a mesial" position and appears round in section (Fig. 970), and receives fibres from the red nucleus and the posterior commissure, and the nucleus arciformis, which lies ventro-lateral to the preceding nucleus and is crescentic in outline. The ventral nucleus is of importance, not only because it receives the great sensory paths, but also on account of its phylogenetic rank, since, according to Edinger, it, together with the ganglion habenulas, represents the oldest of the thalamic nuclei and is found through- out the vertebrate series. Connections of the Thalamus.— Broadly considered, the thalamus may be regarded as a great ganglionic internode interposed in the corticipetal paths around whose cells most of the constituents of the important secondary paths conveying afferent impulses from the spinal cord, the brain-stem and the cerebellum end. 1122 HUMAN ANATOMY. and from whose cells corticipetal fibres pass to all parts of the cerebral cortex and to the corpus striatum. Further, it must be understood that the thalarnus receives fibres from all parts of the cerebral cortex, and, lastly, that from it proceed efferent fibres to the lower centres within the brain-stem and the cord. It is evident, therefore, that the connections of the thalamus are very intricate and far reaching. FIG. 969. i. The lower thalamocipetal tracts include : (a) those passing directly from the spinal cord, as the spino-thalamic and possibly a part of Gowers' tract ; (d) those passing from the various nuclei by way of the median fillet ; (c) those passing from the cerebellum, either directly, as the cerebello-thalamic tract, or, after interruption in the red nucleus, as the rubro-thala- mic; (d) probably other tracts which arise within the tegmen- tal area of the brain-stem. The fibres from the various sources enter the under surface of the thalamus to end within the ven- tral nucleus, or by means of the internal medullary lamina to be distributed to the other nuclei. 2. The thalamic radiation comprises the fibres which stream from the latero-ventral surface of the thalamus to all parts of the hemisphere ( thala mo-c ortical ) , some crossing by way of the corpus callosum to the oppo- site side, as well as those which pass in the opposite direction ( cortico-thalamic ) towards the ganglion. Although as they traverse the external medullary lamina the fibres are not particu- larly grouped, their various rela- tions to the cortex or other parts are established by different and more or less definite paths. These are designated as the stalks of the thalamus, of which a frontal, a parietal, an occipital and a ventral are conventionally distinguished. The anterior or frontal stalk emerges from the fore-part of the lateral surface of the thalamus, traverses the an- terior part of the internal capsule between the caudate and lentic- ular nuclei, to which it distributes fibres, and finally gains the cortex of the frontal lobe. From the cells of this region, cortico- thalamic fibres follow in reversed order the paths just mentioned, thus establishing a double relation between the cortex and the basal ganglion. In addition to the preceding cortico-thalamic fibres, the antero-ventral part of the thalamus receives a strand from the cortex of the olfactory bulb. The parietal stalk leaves the lateral surface of the thalamus and enters the internal capsule and often the lenticular nucleus, in its course to the parietal cortex. Other corticipetal fibres, destined for the parietal and adjacent parts of the frontal lobe, are the continuations of the path of the mesial fillet. To a large extent these fibres pass from the ventral thalamic nucleus outward to the under surface of the lenticular nucleus, then bend upward and traverse the lenticular nucleus by way of the medullary- stri.-e or the globiis pallidus to gain the cortex. Other fibres continue the filler- path by filtering the internal capsule and thus, perhaps, directly "proceed to the cortex. The occipital stalk includes the fibres that connect the thalamus with the visual cortical areas of the occipital and parietal lobes. They issue from the lateral surface of the pulvinar, and as the Spino-thalamic Diagram showing chief connections of thalamus ; black fibres rep- resent afferent tracts ending in thalamus and thalamo-ionical paths ; red fibres are the cortico-thalamic and strio-thalamic paths; Th, thal- amus; C, L, caudate and lenticular nuclei; C, C, corpus callosum; F,P, T, O} frontal, parietal, temporal and occipital lobes; Fx, fornix; Af, mammillary body; fii, cerebral peduncle; SC, /C, superior and in- ferior colliculi ; K, red nucleus; fs, pons ; /, frontal stalk ; 2, parietal stalk ; 3, 4, lenticular and temporal parts of ventral stalk ; 5, occipital stalk. THE DIENCEPHALON. 1123 optic radiations sweep outward and backward around the posterior horn of the lateral ventricle to end in the cortex. The ventral stalk is complex in its relations, since its fibres include two systems. Emerging from the fore-part of the ventral surface of the thakimus, from the lateral and mesial nuclei, the stalk passes downward and outward beneath the lenticular nucleus. Its lower part, known as the ansa peduncularis, continues laterally into the cortex of the temporal and of the central lobe ; its upper part, the ansa lentit'itlaris, closely skirts the adjacent border of the lenticular nucleus which it enters to gain the putamen, or, continuing through the lenticular nucleus by way of the medullary laminae, to reach the caudate nucleus. Under the name tractus strio-thalamicus, are included the fibres which pass from the caudate nucleus and the putamen to the thalamus, subthalamic body and red nucleus, a small number of fibres probably entering the thalamus from the caudate nucleus by the more direct route of the internal capsule. 3. The stratum zonale, the thin layer of white matter which covers the superior aspect of the thalamus, consists in large part of thalamocipetal fibres derived from the optic tract or the optic radiation. Those from the lateral root of the tract superficially cross the external genic- ulate body and spread over the thalamus, while those from the occipital cortex by way of the optic radiation invest the pulvinar. Other contributions to the stratum zonale include fibres from the temporal cortex by way of the ventral stalk. The Epithalamus. — Under this subdivision of the thalamencephalon are included: (i) the trigonum habenulcc, (2) the pineal body, and (3) the posterior commissure — all structures closely associated with the superior and posterior boun- daries of the third ventricle. FIG. 970. Veins of Galen in Corpus velum interpositum Stria Fornix callosum I medullaris Ganglion habenulse Lateral ventricle Caudate nucleus Thalamns. f'\ ventral nucleus ~^~t7 Thalamus, mesial nucleus' External medullary lamina \ Internal capsule Crusta of cerebral peduncles Red nucleus Optic tract ' N^y Subthalamic nucleus Oblique frontal section through thalamus and subthalamic region ; Weigert-Pal staining. X i- Preparation by Professor Spiller. The trigonum habenulae is the narrow triangular area lying between the sharply denned edge (tasnia thalami) of the ventricular wall internally _ and the diverging mesial border of the upper surface of the thalamus externally (Fig. 966). Its surface is depressed and at a lower level than that of the thalamus and behind i continuous with a mesially curving strand, the pineal peduncle. Beneath ridge of thickened ependyma marking the tsenia thalami, lies a distinct stranc of nerve-fibres, the stria medullaris, while at a still deeper level and coverec the superficial fibres is situated an aggregation of small nerve-cells, known as t ganglion habenulze. The source of the fibres composing the stria medullaris anc the connections of the ganglion habenulae are still uncertain It is probable, how- ever that many components of the stria are associated with the olfactory centres and include : (i) olfado-habenular fibres, which arise from cells within the septi 1 1 24 HUMAN ANATOMY. lucidum and the olfactory area, and (2) cortico- habenular fibres, which spring from the cortical cells within the hippocampus or the adjacent region, and by way of the fornix and its anterior pillar are carried to the fore-end of the thalamus, whence they pass backward within the medullary stria. (3) Other thalamo-habenular fibres also probably join the stria medullaris from the interior of the thalamus. Whilst many of the fibres composing the stria end around the cells of the ganglion habenulae, some continue backward, without interruption, within the strand known as the peduncle of the pineal body, cross to the other side in the bundle bearing the name, commissura habenulae, and end in relation with the cells of the opposite habenular nucleus. The ganglion habenulae (Fig. 970), in turn, gives origin to an important bundle, the fasciculus retroflexus of Meynert, which arches down- ward and backward, passing at first between the central gray matter of the third ventricle and the thalamus proper, and later to the medial side of the red nucleus, to reach the base of the brain, and for the most part to end around the cells of the interpeduncular ganglion. This nucleus, which in many animals is a well-defined collection of cells, in man is represented by a more scattered median cell-group within the posterior perforated substance close to the anterior border of the pons. The fasciculus, also termed the habenulo- peduncular tract, receives contribu- tions from the ganglion habenulse of both sides, some fibres having crossed in the habenular commissure ; although the majority of its fibres end, mostly crossed, in the interpeduncular ganglion, not a few may be traced farther caudally within the tegmentum of the brain-stem (Obersteiner), as may also the fibres from the cells of the ganglion interpedunculare. The Pineal Body. — The pineal body (corpus pineale), also often called the epiphysis, is a cone-shaped organ, from 8-10 mm. in length, attached to the posterior extremity of the roof of the third ventricle. It is slightly compressed from above downward and FIG. 971. rests, with its apex pointing backward, on the dorsal aspect of the mid-brain in the trian- gular pineal depression between the superior corpora quadrigemina (Fig. 966). Its base, as its anterior end is called, is attached above to the commis- sura habenulae, from which on each side a narrow but distinct ric'^e, the pineal stalk, curves forward to be- come continuous with the stria medullaris. Pelc >w, i'.i base is united with the posterior c< in- missure of the brain overlying the entrance into the Sylvian aque- duct. Between the habenular and posterior commissures a small pointed diverticulum, \\\v pineal recess, extends from the third ventricle for a very short distance into the pineal body, and thus recalls the early condition in which the organ is developed as a tubular outgrowth in the roof-plate of the diencephalon. This relation to the thin ventricular roof the body retains, its apex later becoming closely surrounded by and embedded within the loose vascular tissue of the pia mater. The structure of the pineal body, as seen in cross-section ( Fig. 971), includes a reticular framework of connective tissue trabeculae, whose meshes are filled with <$•$ Connective ti-siir septa Section of pineal body showing calcareous concretions or brain-sand. X 130. THE DIENCEPHALON. II25 rounded or sometimes elongated epithelial cells, which often contain brownish pig- ment. With the exception of a few nerve-filaments in the anterior part probably sympathetic in origin and destined for the blood-vessels, and a dense net-work of neurogha fibres in the under part, the pineal body contains no ele- 1' *<*'•• 972. ments of a nervous character, nerve- Lenticular area 111* i icctin«il srcei cells being absent. Quite com- monly the adult organ encloses a variable number of concretions, often called brain-sand {acervulus} , which consist of laminated particles composed of calcium carbonate and phosphate mingled with or- ganic material. They may be of microscopic dimensions, or reach the size of a millet seed, and by aggregation assume a mammillated form. Blood-vessel Diverticulum dividing into tubules Sagittal section of pineal organ of lizard (Lacerta agilis) embryo. X 175. The significance of the pineal body long remained an unsolved riddle and served as the theme for unrestrained speculation. The em- bryological and comparative studies of Graaf, Spencer and others have shown that in many of the lower animals, especially in the reptiles (lizards), the pineal body reaches a high degree of development and is a flattened cup-shaped organ connected with the brain by a stalk containing nerve-fibres. The structural resemblances to the invertebrate visual organ suggested a possible similarity of purpose in the higher types, an assumption that was strengthened by the fact that in certain lizards the pineal body not only is borne by a stalk but reaches an interparietal subcutaneous position on the head by passing through or lying within a special foramen in the skull. The organ was, therefore, designated the pineal eye, although probably in no existing animal a functionating structure. While such a superficial position in the adult is very exceptional, the embryonic relations in many reptiles (Fig. 972) are very suggestive of the probable significance of the pineal body, at least in such form as a rudimentary sense organ, although not necessarily an eye. These conclusions are likewise suggestive in forming our conceptions concerning the pineal body in man, which is now by many regarded as representing a very imperfectly developed and greatly modified sensory structure. Although strictly belonging to the telencephalon, men- tion may here be made of a second evagination, know as the paraphysis, which arises from the roof-plate of the fore-brain. The pouch appears in advance of the pineal outgrowth and is a temporary structure, seemingly being in nature comparable to an outwardly directed choroid plexus. The paraphysis has been described in the lower vertebrates, including reptiles and birds, in some mammals and, indeed, according to the observations of Francotte and of Ewing Taylor, it is not improbable that a corresponding evagination is recognizable in the early human embryo. FIG. 973. Small portion of pineal body, showing constituent cells more highly magnified. X 600. The posterior commissure (commissura poste- rior cerebri) is a narrow but distinct cord-like band of white matter which overlies the superior entrance into the Sylvian aqueduct (Fig. 976) and is partially masked by the habenular commissure and pineal peduncle above. Behind and laterally it is continuous with the superior colliculi. The commissure provides the paths by which fibres from various sources undergo median decussation, but the details and connections of its component fibres are only imperfectly understood. Among its probable constituents are: (i) fibres originating in the nucleus of the posterior commissure and also from the nucleus of the posterior longitudinal fasciculus (nucleus fasciculi longitudinalis posterior), which occupies the gray matter of the floor of the third ventricle near the mammillary bodies (page 1117); (2) fibres from the posterior part of the thalamus of the 1126 HUMAN ANATOMY. opposite side which descend within the tegmentum, lateral and ventral to the posterior longitudinal fasciculus ; (3) fibres which cross to join the fasciculus retro- flexus ; (4) fibres from the median fillet and (5) from the superior cerebellar peduncle which traverse the commissure to reach the opposite thalamus ; (6) per- haps fibres from the deeper gray stratum of the corpora quadrigemina to the cerebral cortex of the other side. Its presence in all vertebrates and the very early acquisition of a medullary coat by its fibres indicate, as pointed out by Edinger, the fundamental character of the commissure. The Metathalamus. — This subdivision of the thalamencephalon includes em- bryologically both the median and lateral geniculate bodies. Since in the fully formed FIG. 974. Corpus callosum Choroid plexus Nucleus habemike Substantia nigra Oculomotor nerve Crusta of cerebral peduncle Caudate nucleus Thalamus Subthalamic region Choroid plexus in inferior horn of lateral ventricle Caudate nucleus, tail Hippocampus, obliquely cut Gyrus dentatus Gyrus hippocampi, bounding inferior fissure leading into choroidal plexus Frontal section of brain passing through thalami, subthalamic region and cerebral peduncles; inferio horn of lateral ventricle with hippocampus in section also seen. brain the former are closely associated with the inferior colliculi and their arms, inferior brachia, they may be conveniently described in connection with the mid- brain, as has been done (page mo). The lateral geniculate bodies, (corpora gcniculata latcrales), one on each side, are two fusiform elevations, about 10 mm. in length and half as much in width, which project from the outer and under surface of the posterior part of the thalamus (Fig. 958). They are so buried within the thalamus that they are much less distinct than the median geniculate bodies. In front they receive the outer division of the optic tracts, while behind they are connected by the superior brachia with the superior corpora quadrigemina. In structure the lateral geniculate body consists of alternating layers of white and gray matter. The former, somewhat thinner than the gray substance, is, to a large measure, the optic fibres, many of which end around the celb wi'hin tin- gray laminae. Other fibres of the optic tract continue without interruption into the superior braehium and so to the upper colliculus, while a certain number end within the thalamus, and in their course over the- surface of the latter take part in the production of the stratum xonale (page 1118). From many of the cells within the geniculate body, fibres proceed by way of the optic radiations to the cerebral cortex. THE DIENCEPHALON. 1127 Then, too, many corticifugal fibres course in the opposite direction as the axones of the cortical cells, and end in relation to the geniculate neurones, thus establishing a double relation between the lateral geniculate body and the occipital cortex. The Hypothalamus. — Although, strictly regarded according to its develop- mental relations, the diencephalon claims only the posterior or mammillary part of the hypothalamus, it is desirable to consider at this time the derivations of the entire hypothalamic subdivision of the fore-brain. Under the above heading will be de- scribed, therefore, the structures lying within or forming the floor and the anterior wall of the third ventricle, including the subthalamic region. The subthalamic region in its developmental relations stands, as it were, as a link connecting the diencephalon and the mid-brain. The subthalamic region is the upward prolongation of the tegmentum of the cerebral peduncles and occupies, on each side of the mid-line, the triangular area between the thalamus above and the internal capsule and its continuation, the crusta of the peduncle, below (Fig. 974). It is insepa- FIG. 975. Red nucleus Substantia nigra Choroid plexus Fornix Pulvinar Lateral geniculate body (leader crosses cut tail of caudate nucleus) Median geniculate body Hippocampus Superior cerebellar peduncle Frontal section of brain passing through posterior poles of thalami, pineal body and brain-stem. rablv blended with the ventral surface of the thalamus, which thus obliquely. overlies the termination of the tegmental or sensory portion of the cerebral stalk. this area the important thalamocipetal paths of the fillet and of the superior cerebellar peduncles reach the thalamus, and within it are seen the upper extremiti chief eanglia of the mid-brain, the substantia nigra and the red nucleus, and mass of gray matter, the corpus subthalamicum. The substantia nigra presents the same characteristics here as in the peduncle, being conspicuously dark and over! vm* the crustal fibres. As it ascends, it decreases in bulk from within outward until, at the level of the mammillary body, the substantia nigra is no longer recogn The connections of the cells within the substantia nigra are imperfectly understood but it is probable that they receive many fibres from the caudate nucleus and putamen and, perhaps, also from the frontal cortical areas From the cells < other hand, fibres pass into the tegmentum and into the crusta lower levels According to Bechterew, some fibres join the fillet-tract and relch the superior quadrigeminal bodies. At first the red nucleus is a very prominent Kre in frontal » sections of the subthalamic region (Fig. 97o), appearing 1128 HUMAN ANATOMY. as a circular area of gray matter enclosed by a zone of cerebello-thalamic fibres ; farther forward it, too, gradually diminishes and disappears at a level somewhat behind that of the corpora mammillaria. The connections of the red nucleus have 1)( ( n considered in connection with the superior cerebellar peduncle (page 1095) ; suffice it here to recall its twofold significance as an interruption station for many of the cerebello-rubro-spinal and for the cerebro-rubro-spinal tracts. The corpus subthalamicum (nucleus hvpothalamicus), or nucleus of Luys, is a mass of deeply tinted gray matter peculiar to the subthalamic region and unrepresented in the mid-brain. It appears in cross-section (Fig. 970) as a small biconvex area, immediately dorsal to the tract of crustal fibres and lateral to the red nucleus and the substantia nigra. As the latter diminishes, the subthalamic nucleus expands to take its place and, where fully represented, measures from 3-4 mm. in thickness and from 10-12 mm. in its lengest diameter, and extends superiorly considerably beyond the level of the red nucleus. Histologically the subthalamic body is distinguished by a dense net-work of fine medullated nerve-fibres, enclosing pigmented multipolar nerve- cells of medium size, and by an unusually close mesh-work of capillary blood-vessels. The dorsal surface of the nucleus is defined by the overlying lateral part of the field FIG. 976. Septum lucidum Choroid plexus Foramen of Monro Genu of corpus callosum — \ Rostrum of corpus callosum Anterior commissure Lamina cinerea Optic recess Optic commissure Anterior lobe of pituitary body Posterior lobe of pituitary body Infuiidibulum Body of fornix Velum interpositum covering Thalamus. [thalamus mesial surface Tsenia thalami plenium Commissura habenuhe Pineal recess Mammillary body . Anterior pillar of fornix Tuber cinereum Right lateral wall of third ventricle : velum interpositum covers superior surface of thalamus. of Forel, as the stream of fibres passing between the red nucleus and the thalamus and the internal capsule is called. From the ventral surface of the nucleus, fibres pierce the adjacent crusta and join the ansa lenticularis to gain, probably, the globus pallidus ; other perforating fibres perhaps connect the subthalamic body with Meynert's and Gudden's commissures (Obersteiner). The ventro-medial c-nds of the bodies of the two sides are connected by a bridge, the commissura hypothalamica, which traverses the floor of the third ventricle above the mammillary bodies. In addition to connecting the two subthalamic nuclei, the commissure contains decussating fibres from the anterior pillars of the fornix and, according to Edinger, probably fibres from the fore-end of the posterior longitudinal fasciculus. The corpora mammillaria (corpora mamillaria"), also called the corpora albi- (luitnt, aivtuo hemispherical elevations, about 5 nun. in diameter, which lie close to the mid-line within tin- interpeduncular space on the basal surface of the brain (Fig. 993). rii.-y are almost but not quite in contact, being separated by a narrow interval which immediately behind the little bodies deepens into the anterior recess marking the front end ^of the shallow median furrow that grooves the posterior perforated sub- stance. The posterior surfaces of the mammiilary bodies indicate the anterior limit of the ventral surface of the mid-brain. When examined in section (Fig. 970), THE DIENCEPHALON. 1129 each body is seen to be composed of an outer layer of white matter enclosing a core of gray substance, known collectively as the nucleus mammillaris. The latter is subdivided into a medial and lateral part by fibres from the downward arching ante- rior pillar of the fornix, which penetrate the gray matter as well as invest to a large- extent its exterior. Only a part of (i) the > fornix fibres, however, end directly in the mammillary nuclei, since some pass above and behind the ganglion to gain the hypothalamic commissure (page 1128) and, after decussation, to end in the mam- millary body of the opposite side. From the dorsal part of the medial nucleus, distinguished from the lateral one by its larger nerve-cells, emerges a distinct and compact bundle of fibers (Fig. 967), which on clearing the nucleus, separates into two strands. One of these, known as (2) the mammillo-thalamic tract, or the bundle of Vicq d1 Azyr, courses upward and forward, and ends within the anterior nucleus of the thalamus ; in this manner it completes the paths by which the cortical olfactory centres within the hippocampus major are connected (by way of the fimbria, body and anterior pillar of the fornix and the mammillo-thalamic strand) with the thalamus (Fig. 1049). That fibres pass between the latter and the mammillary nucleus in both directions, is shown by the fact that destruction of either of these centres is fol- lowed in turn by ascending or descending degeneration of the fibres. (3) The other part of the bundle issuing from the mammillary nucleus arches backward and downward and, as the mammillo-tegmental tract, is traceable into the tegmentum of the mid-brain to the vicinity of the inferior colliculus. (4) Under the name, pedun- culus corporis mammillaris, another mammillo-tegmental tract is described. This strand springs from the lateral mammillary nucleus, and, coursing backward and downward along the medial margin of the crusta, enters the tegmentum. Its des- tination is uncertain, but according to Kolliker the tract probably ends in the central gray matter surrounding the Sylvian aqueduct in proximity with the trochlear nucleus. Other, but much less well established, strands have been described by Lenhossek as proceeding forward from the peripheral layer of the mammillary body over the tuber cinereum. Concerning their further course little is known with certainty. The tuber cinereum is the first of a series of median outpouchings which model the thin sheet of gray matter constituting the floor and the anterior wall of the third ventricle and belong to the pars optica of the hypothalamus. As seen from the exterior (Fig. 993), the tuber cinereum is a median elevation placed between the mammillary bodies behind and the optic chiasm in front, and the cerebral peduncles and the optic tracts at the sides. Together with the infundibulum, it forms the most dependent part of the third ventricle and consists of a thin layer of gray matter, less than 1.5 mm. thick, that is continued forward as the attenuated extension of the im- portant sheet found within the mid-brain and fourth ventricle. In addition to the fibre- strands coming from the mammillary bodies noted by Lenhossek, this investigator and Kolliker credit the tuber cinereum with possessing small paired composite gang- lia, the nuclei tuberis and the nuclei supraoptici of Kolliker. Concerning their con- nections nothing is definitely known. The anterior part of the tuber, immediately behind the optic chiasm, descends abruptly and somewhat forward to form a funnel- shaped stalk, the infundibulum, to whose lower end or apex is attached the pos- terior lobe of the pituitary body (Fig. 976). Although in the very young child the infundibulum retains to some extent its original character as a hollow outgrowth from the ventricle, in the mature subject this cavity, the recessus infundibuli, has mostly disappeared and the stalk is solid, save for a slight diverticulum within its upper and widest part. The posterior part of the tuber cinereum, between the root of the infundibulum and the mammillary bodies, exhibits occasionally in the adult brain, and almost con- stantly in that of the foetus, a small rounded median projection, flanked on each side by a slight elevation. To this modelling Retzius has applied the name, eminentia saccularis in recognition of its similarity to the evagination (saccus vasculosus) found in fishes. The eminence encloses a shallow pouch, recessus saccularis, which opens into the third ventricle. The pituitary body (hypophysis cerebri) is attached to the dependent tip of the infundibulum, and, closely invested by a loose sheath of connective tissue, hangs 1 130 HUMAN ANATOMY. within the pituitary fossa on the base of the skull, just in advance of the dorsum sellae (Fig. 996). Above, the fossa is closed by a special partition of dura, the diaphragma sellte, through an opening in which the infundibulum passes to the mushroom-shaped organ. The pituitary body consists of two principal parts, of which the so-called anterior lobe is much the larger and of a darker grayish red color. The boundary between the anterior and posterior lobes is occupied by a zone of modified glandular tissue, the pars intermedia, which extends for a variable distance along the ventral surface of the posterior lobe towards the infundibulum. The two lobes are not only dis- tinct as to structure and probably function, but are developed from entirely different regions. The anterior lobe is formed as an outgrowth from the oral diverticulum, while the posterior lobe first appears as a ventral evagination from the diencephalon (Fig. 1532). The anterior lobe, glandular in character, has been described in con- nection with the Accessory Organs of Nutrition (page 1806) and, therefore, calls for no further consideration in this place. FIG. 977. Pars intermedia 'osterior or cerebral lobe Blood-sinus Connective-tissue trabecula Transverse section of pituitary body, showing relation of anterior (oral) and posterior (cerebral) lobes. X 7. The posterior lobe of the pituitary body is lighter in color and softer in con- sistence and directly attached to the floor of the third ventricle by means of its stalk. the infundibulum. During the early stages of its development, this lobe is repre- sented by a tubular outgrowth whose walls partake of the general character of the adjacent brain-vesicle. Later the lumen within the lower end of the diverticulum dis- appears in consequence of thickening and approximation of its walls, a funnel-shaped recess of variable depth within the infundibulum alone remaining. In the adult con- dition, the posterior or cerebral lobe retains few histological features suggesting its nervous origin. Of the demonstrable interlacing fibres, with fusiform enlargements and elongated nuclei, none can be identified as nerve-fibres, while of the numerous cells which the lobule contains, only a few of large size and pigmented cytoplasm uncertainly resemble nervous elements. With the exception of possibly neurogliar cells, the existence of definite nervous tissue within the cerebral lobe of the mature human hypophysis is doubtful. The optic tracts and commissure are elsewhere described (page 1223), suffice it at this place to mention their relation to the interpeduncular structures. The optic tracts diverge backward and wind around the ventral surface of the cere- bral peduncles (Fig. 993). Their medial ends are fused into a transversely flattened white band, the optic commissure or chiasm. The latter is connected with the front surface of the tuber cinereum, whilst above the chiasm the anterior wall of the ventricle consists of a delicate sheet of gray matter, the lamina cinerea (lamina terminalis). This structure lies in the mid-line, passes almost vertically upward, with a slight forwardly directed curve, and becomes continuous with the rostrum of the corpus THE DIENCEPHALON. 1131 callosum. Just before meeting the latter, the lamina passes in front of the anterior commissure of the brain (Fig. 976). The Third Ventricle. — The third ventricle (ventriculus t^aius cerebri; is the narrow cleft-like space that separates the medial surfaces of the thalami (Fig. 966). It is somewhat broader behind and much deeper in front, where it comes into close relation with the exterior of the brain, the interpeduncular lamina alone intervening. Seen from the side, as in mesial sagittal sections (Fig. 996), the outline of the ventricle is irregularly comet-shaped, with the broader end above and behind and the blunted point directed downward and forward (Fig. 978). Behind, it communicates with the Sylvian aqueduct, and through this canal indirectly with the fourth ventricle; anteriorly it connects with the two lateral ventricles by means of the foramina of Monro. Its sagittal diameter, measured between the anterior commissure and the base of the pineal body, is approximately 2.5 cm. The lateral wall of the ventricle (Fig. 976) is formed chiefly by that part of the thalamus which lies below the level of the taenia thalami. On this surface, slightly in advance of the middle, is seen the small oval field of the middle commissure, and in front of this the downward curving elevation produced by the anterior pillar of the fornix. Between the latter and the prominent anterior tubercle of the thalamus lies the foramen of Monro (foramen interventriculare), which establishes communication between the third and the cor- FIG. 978. Pineal recess Suprapineal recess Posterior commissure Sylvian aqueduct Mammillary body' Infundibulum' Middle commissure Foramen of Mouro Anterior commissure Optic recess Optic chiasm Cast of third ventricle, viewed from the side. X 4- (.Retains.) responding lateral ventricle, and transmits the trunk formed by the union of the vein of the corpus striatum and the choroid vein. A shallow furrow on the ventric- ular wall, the sulcus hypothalamicus leads from the foramen backward and some- what downward (Fig. 976). It is of importance as indicating, even in the adult brain, the demarcation between the thalamencephalon and the hypothalamus — parts derived respectively from the dorsal and ventral zones of the embryonic brain-vesicle. The roof of the ventricle extends from the foramina of Monro, bounded above and in front by the arching pillars of the fornix, to the pineal body behind, over which it pouches out into the suprapineal recess, as the little diverticulum overlying the body is termed. The immediate and morphological roof consists of the delicate ependymal layer, which is attached to the taenia thalami on each side and, stretching across the interthalamic cleft, closes in the ventricle. The ependymal layer how- ever, is backed by a vascular fold of pia mater, which, in conjunction with the epithelial layer, constitutes the velum interposition. This structure is more fully described in connection with the lateral ventricles (page 1162); but its relation to the third ventricle finds appropriate mention at this place. As in the roof of fourth ventricle and in the lateral ventricles, so in the third does the vascular t of the pia mater invaginate the ependymal layer to form vascular fringes whicl project into the ventricle (Fig. 974). A double line of such imaginations hangs from the roof of the third ventricle and constitutes the choroid plexus of that space Since the ependyma everywhere covers these pial processes, it is evident that I fringes are, strictly regarded, outside the ventricle and excluded by the continuous layer of the epithelium. 1132 HUMAN ANATOMY. The posterior wall of the third ventricle is very short and includes the base of the pineal body, with the opening into the minute pineal recess, the posterior com- missure and the orifice leading into the Sylvian aqueduct. The floor slopes rapidly downward and forward (Fig. 976) and comprises a small part of the tegmentum of the cerebral peduncles, the posterior perforated substance, the mammillary bodies, and the tuber cinereum with the infundibulum — structures already described and included within the interpeduncular area on the base of the brain. Corresponding with the position of the superficial elevation, the ventricle exhibits the diverticulum of the infundibulum. The optic chiasm marks the anterior limit of the floor and the beginning of the anterior wall. Immediately above the chiasm the anterior wall exhibits a diverticulum, the optic recess, from which the lamina cinerea ascends to join the rostrum of the corpus callosum, in its course passing close to and in front of the anterior commissure. The latter structure shows on the front wall of the FIG. 979. Cavity in septum lucidum Corpus callosum, cut Caudate nucleus Internal capsule Putamen of lenticular nucleus Cut anterior end of fornix Anterior pillars of fornix Anterior commissure Lateral ventricle Ependyma covering ta.-nia semicircularis and vena terminalis Lamina cinerea, above__-— — ' Thalamus. anterior tubercle \ X Foramen of Monro Optic chiasm Lamina cinerea Portion of frontal section of brain passing through foramina of Monro, showing anterior wall of third ventricle modelled by anterior commissure and pillars of fornix. ventricle as a transverse ridge between the descending and slightly diverging anterior pillars of the fornix (Fig. 979). Although distinctly modelling the ventricular walls, all of these bands are excluded from the ventricle by its ependymal lining. THE TELENCEPHALON. The telencephalon, or end-brain, consists of two fundamental parts, the hemi- sphaerium and the pars optica hypothalami. The latter includes: (i) the lamina cinerea (terminalis}, (2) the optic commissure, (3) the tuber cinereum and (4) the pituitary body, all of which have been already considered, as a matter of con- venience, in connection with the diencephalon and the third ventricle. The hemi- sphere comprises: (i) the pallium, (2) the rhinniccphalon, and (3) the corpus strnition. The first of these subdivisions undergoes such enormous development in the anthropoid apes and in man, that the pallium' becomes the dominating factor and, expanding upward, laterally and backward as the great cerebral mantle, not only forms the chief bulk of the cerebrum, but overlies the derivatives of the other brain- segments to such an extent that these parts are to a large measure covered and deposed from their primary position on the free dorsal surface of the brain. In conse- quence in man, in whom the pallium reaches its highest development, the thalami, corpora quadrigemina and the cerebellum are masked by the hemispheres and occupy topographically a dependent position. The rkivencephalon, on the contrary, is in man only feebly developed and rudimentary in comparison with the conspicuous and bulky corresponding structures possessed by animals in which the sense of smell is highly developed. The corpus striatum, consisting of two large masses of gray THE TELENCEPHALON. 1133 matter, the caudate and the lenticular nucleus, represents the internal nucleus of the end-brain. Certain commissural structures, as the corpus callosum, the anterior com- missure and \hefornix are to be regarded as secondary and as serving to connect the halves of the great brain. The immediate free or outer surface of the pallium is everywhere formed by a thin peripheral layer of cortical gray matter, which, as an unbroken sheet, clothes the various ridges and intervening furrows — the convolutions and fissures — which model the exterior of the cerebrum and provide the necessary extent of surface. Beneath the cortical gray substance lies the u •////<• matter, which constitutes the bulk of the hemisphere and consists of the tracts of nerve-fibres pass- ing to and from the cortex, as well as of those connecting the various regions of the cortex with one another. Embedded within the core of white matter and lying much nearer the basal than the superior surface of the hemisphere (Fig. 1009), the corpus striatum is closely related to the ventricular cavity by means of the caudate nucleus on the one hand, and to the cortical gray matter by the lenticular nucleus on the other. In view of the rudimentary condition of the rhinencephalon and the over-shadowing development of the pallium in man, it is usual and convenient to regard most of the parts derived from the telencephalon as belonging to the hemispheres, the latter term being used in a less restricted sense than warranted by a precise interpretation of its developmental significance. THE CEREBRAL HEMISPHERES. Viewed from above, the human brain presents an ovoid form, the narrower end being directed forward and the broader backward, the greatest width corresponding with the parietal eminences (Fig. 984). The convex surface formed by the hemispheres is divided by a deep median sagittal cleft, the longitudinal fissure (fissura longitudinalis cercbri), that, for a distance less than one-third of its length anteriorly and more than one-third posteriorly, completely separates the hemi- spheres. In its middle third or more, the fissure is interrupted at a depth of about 3.5 cm. by the arched upper surface of the corpus callosum, the chief connection between the hemispheres. The upper and back part of the longitudinal fissure, throughout its length, is occupied by the sickle-shaped mesial fold of dura mater, the falx cerebri, which incompletely subdivides the space occupied by the cerebrum into two compartments. Under the name, transverse fissure (fissura transversa cerebri), is sometimes described the deep cleft which separates the postero-inferior surface of the hemisphere from the cerebellum, the corpora quad- rigemina and the pineal body. This cleft, so evident after the brain has been removed from the skull, when the parts are in situ is filled behind by the tentorium cerebelli and in front by a fold of pia. The hemispheres are advantageously studied after being separated from each other by sagittal section, and from the brain-stem by cutting across the mid-brain. When examined after such isolation, especially when .hardened before removal from the skull, each hemisphere presents a dorso-lateral, a mesial and an inferior surface. The dorso-lateral surface (Fig. 980) is convex both from before backward and from above downward and closely conforms to the opposed inner surface of the cranial vault. The mesial surface (Fig. 987) is flat and vertical and bounds the longitudinal fissure. It is in contact with the sagittal fold of dura, the falx cerebri, except in front and below where the partition is narrow; here the mesial surfaces of the hemispheres, covered of course by the pia and arachnoid, lie in apposition. The inferior surface (Fig. 989) is irregular, its approximate anterior third resting in the anterior cerebral fossa of the cranial floor, the middle third in the lateral part of the middle fossa, whilst the posterior third is supported by the upper aspect of the tentorium, which separates it from the subjacent cerebellum. At the juncture of its anterior and middle thirds, the inferior surface of the hemisphere is crossed transversely, from within outward, by the stem of the Sylvian fissure and thus subdivided into an anterior and a posterior tract. The former and smaller, known as the orbital area, rests upon the orbital plate of the frontal bone and is' modelled by this convex bony shelf into a corresponding slight con- cavity from side to side. The tract behind the deep Sylvian cleft is at first convex "34 HUMAN ANATOMY. .and rounded, as it lies within the middle fossa, but traced backward it passes insensibly into the tcntorial area, supported by the tentorium cerebelli. This area is concave from before backward and directed inward as well as downward, in correspondence with the characteristic curvature of the tent-like dural septum. The borders separating the surfaces of the hemisphere are the dorso-mesial, the infero-lateral and the infero-mesial. The dorso-mesial border intervenes between the mesial and lateral surfaces and, therefore, follows the arched contour of the hemisphere beneath the vaulted calvaria. The infero-lateral border, between the lateral and inferior surfaces, is better defined in front, where it separates the orbi- tal area from the external surface as the arched superciliary border (Cunningham), than behind, where it is so rounded off as to scarcely be recognizable as a distinct margin. The infero-mesial border intervenes between the mesial and the inferior surface of the hemisphere. It is well marked in front, where it limits the orbital area mesially, and again behind, where it corresponds to the line of juncture between FIG. 980. Lateral aspect of left cerebral hemisphere; dorso-median surface is somewhat foreshortened ; red lines indicate boundaries separating parietal, teniporal and occipital lobes ; r, Rolandic fissure ; j. g .. i. g., its superior and inferior genu; S't S*, S3, S* asc., vertical, horizontal, posterior and ascending limbs of Sylvian fissure; /. p. c., s. p. c., inferior and superior precentral; s/., if., superior and inferior frontal ; p. m., param'edian ; »/./., mid-frontal ; , />-, f>3, />', inferior, superior, horizontal and occipital limbs of inter-parietal ; p. o., parieto-occipital ; /', f1 asc., superior temporal and its upturned limb; f-, f- asc., middle temporal and its upturned limb;/, o., transverse occipital; /. o., lateral occipital; A., arm centre; .5. T. O.,pars basahs, tnangularis and orbitalis ; Arc. p.-o., arcus parieto-occipitalis. the falx cerebri and the tentorium and marks the division between the mesial surface and the tentorial area. This margin has been designated the infernal occipital border by Cunningham. The extreme anterior end of the cerebral hemisphere is known as the frontal pole (pohis frontalis), and the most projecting part of the posterior end as the occipital pole ( polus occipitalis), while the tip of the subdivision of the hemisphere which projects below the Sylvian fissure constitutes the temporal pole (polus tem- I'oialis). A short distance behind the latter, the inferior surface exhibits a well ^•\\\\v<\ pctrosal depression (impressio petrosa >; this is caused by the elevation cross- ing the petrous portion of the temporal bone which corresponds to the position of the superior semicircular canal. Under favorable conditions of hardening, tin- infero- inesial aspect of the occipital pole sometimes displays a broad shallow groove which marks the commencement of the lateral sinus. The groove is usually better marked on the right side than on the left, in accordance with the larger size of the ri^ht sinus as commonly found ; occasionally these relations are reversed, and frequently n<> groove is n ( ..-ni/able on the side of the smaller sinus. In brains hardened in situ, the gently arching curve of the hind-half of the infero-lateral border of the hemi- sphere is interrupted I,\ a more or less evident indentation, the preoccipital notch i incisiua prauiccipiialis >, at a point about 3.75 cm. (l^ in.) in front of the occipital p.>le i Fig. 980). This notch, prominent in the child but later variable in THE TELENCEPHALON. 1135 its distinctness, is produced by a fold of dura over the parieto-mastoid suture and above the highest part of the lateral sinus (Cunningham). It is of importance in the topography of the brain, since it is often taken as the lower limit of the parieto- occipital line, establishing the conventional division on' the lateral surface of the hemisphere between the parietal and occipital lobes (page 1143). The complex modelling of the surface of the cerebral hemispheres, the charac- teristic feature of the human brain, is produced by the presence of irregular eleva- tions, the convolutions or gyri, separated by the intervening furrows, the fissures or sulci. Although presenting many variations in the details of their arrangement, not only in different individuals but even in the hemispheres of the same brain, the convolutions and fissures of every normal human brain are grouped according to a general and definite plan to which the brain-patterns, whether elaborate or simple, in the main conform. The fissures differ greatly not only as to their depth as observed in the fully formed brain, but also as to their relation with the developing hemi- sphere, a very few, known as the complete fissures, involving the entire thickness of the wall of the cerebral vesicle and in consequence producing corresponding eleva- tions on the internal surface of the ventricular cavities. Of such total sulci the most important permanent ones are : ( i ) the hippocampal fissure, which produces the pro- jection known as the hippocampus major within the lateral ventricle ; (2) the ante- rior part of the calcarine fissure, which gives rise to the calcar avis ; and (3) the fore-part of the collateral fissure, which is responsible for the variable collateral emi- nence. The choroidal and the parieto-occipital fissure are also complete fissures of foetal life, but give rise to invaginations which do not permanently model the ventric- ular walls. The remaining furrows merely impress the surface of the hemispheres and are termed incomplete fissures. Their depth varies, in some cases being only a few millimetres and in others as much as 2.5 cm., with an average of aboutai cm. The height of the convolutions usually exceeds their width, the latter, in turn, being commonly somewhat greater at the surface than at the bases of the gyri. It is evi- dent, therefore, that the convoluted condition of the hemispheres provides a greatly increased area of cortical gray matter without unduly adding to the bulk of the brain, the extent of the sunken surface being estimated as twice that of the exposed. The larger and longer adjacent convolutions are frequently connected by short ridges, the annectant gyri, which have no place in the typical arrangement. They may cross the bottom of the intervening fissure and ordinarily be entirely hidden from view (gyri profundi); or they may be superficially placed (gyri transitivi) and materially add to the complexity of the surface configuration. The cause and origin of the cerebral convolutions are still subjects for discussion. The fact, that at the time the fissures begin to appear, towards the end of the fifth fetal month, the surface of the young brain is not in close contact with the cranial wall, disproves the assumption that the latter is directly responsible for the production of the fissures and convolutions. It is probable that the immediate cause of the surface modelling must be sought in the unequal growth and consequent localized tension which affect the hemispheres, excessive growth in the longitudinal axis resulting in transverse furrows, and that in the opposite axis producing fissures extending lengthwise. Whether the excessive expansion is caused by increase in the gray or white matter is uncertain, although local augmentation of the cortical gray substance is prob- ably the more important factor. After the beginning of the eighth month, when the growing brain comes into contact with the cranial wall, the convolutions, which before were to a large extent unrestrained and therefore relatively broad and rounded, suffer compression, the results of which are seen in the flattening and closer packing of the gyri and the narrowing and deepen- ing of the intervening fissures. By the end of foetal life the salient features of the plan of arrangement have been established, although the final details of the brain-pattern are not acquired until sometime after birth. The Cerebral Lobes and Interlobar Fissures. — For the purposes of description and topography, the cerebral hemispheres are subdivided into more or less definite tracts, the lobes, by certain sulci, appropriately known as the inter- lobar fissures. With few exceptions, however, the lobes so defined have little fundamental importance, since their recognition is warranted by convenience and not by morphological significance, in most cases the conspicuous limiting sulci being of 1136 HUMAN ANATOMY. secondary importance, while those of primary value are comparatively obscure in the fully formed human brain. The interlobar fissures, six in number, are : ( I ) the fissure of Sylvius, (2) the central fissure, (3) the parieto-occipital fissure, (4) the collateral fissure, (5) the calloso-marginal fissure and (6) the limiting sulcus of Reil. The lobes marked off by these fissures with varying degrees of certainty are : ( i ) \hzfrontal, (2) \\vtparietal, (3) the temporal, (4) the occipital, (5) \hvlimbic, and (6) the insula. An additional division, (7) the olfactory lobe, although of. impor- tance as representing the peripheral part of the rhinencephalon of osmatic animals (as those possessing the sense of smell in a high degree are called), is not related to the foregoing sulci and comprises the rudimentary olfactory bulb and tract and associated parts (page 1151). It will be of advantage to describe the interlobar fissures as pre- paratory to a detailed consideration of the lobes. The fissure of Sylvius (fissura cerebri lateralis) is the most conspicuous fissure of the hemisphere. It begins on the inferior surface of the brain in a depression, the valltrula Sylvii, which opens out on the anterior perforated space. The first part of the fissure, its stem, passes horizontally outward to the lateral surface of the hemi- sphere, forming a deep cleft which separates the orbital area from the underlying tem- FIG. 981. Rolandic fissure Inferior precentral sulcus Inferior frontal sulcus Posterior limb Orbital surface Horizontal limb Portion of lateral surface of right hemisphere, showing ascending, horizontal and posterior limbs of Sylvian fi--.iitc radiating from Sylvian point. B, T, O, pars basalis, triangularis and orbitalis of inferior frontal gyrus ; ST, superior temporal gyrus. poral pole. On reaching the surface at the Sylvia n point, the fissure divides (Fig. 981) into (a) a short anterior horizontal branch, (6) a somewhat longer anterior ascending branch, and (c*) a long posterior branch. The anterior horizontal branch (ramus anterior horizontalis), about 2 cm. in length, extends forward into the inferior frontal gyrus parallel to and just above the infero-lateral border, and forms the lower limit of the pars triangularis i page 1141 ). The anterior ascending branch (ramus anterior ascendens i passes upward and slightly forward into the hind-part of the inferior frontal convolution for a distance of about 3 cm. The frequently observed variations in the relation and arrangement of the anterior branches of the Sylvian fissure — the ascending and horizontal limbs in many cases arising from a common arm, sometimes being fused into a single sulcus. Of .i-^ain being absent — are due to atypical growth of the opercula, particularly of the frontal. 'Fite posterior branch ( ramus posterior), the main continuation of the fissure and about 8 cm. in length, is directed horizontally backward, with a slight inclination upward. It forms a very evident boundary between the anterior parts of the parietal and temporal lobes which it separates by a deep cleft that usually ends behind in an ascending limb surrounded by the angular gyrus (Fig. 980). Not infrequently the fissure ends by dividing into two short arms, one of which penetrates the parietal lobe while the other arches downward into the temporal lobe. THE TELENCEPHALON. H37 FIG. 982. The form and relations of the fissure of Sylvius are so dependent upon the growth of the surrounding parts, that a sketch of the development of this region of UK- hemisphere is necessary for an understanding of the significance of this conspicuous sulcus. During the third fu-tal month the lateral surface of the cerebral hemisphere presents a crescentic depressed area, the fossa Sylvii, whose floor corresponds to the insula or island of l\cil. The latter is seen in the adult brain, on separating the margins of the Sylvian fissure, as a sunken area which is completely hidden by the overhanging parts, the opercula insulae, of the surrounding lobes (Fig. 990). During the fifth month the former shallow crescentic Sylvian fossa gives place to a more definitely walled triangular depres- sion, which, during the succeeding month, begins to be enclosed by the formation of the opercula. The details of this process have been carefully studied by Cunningham1 and more recently by Retzius.2 The opercula which bound the triangular fossa, named from the regions which contribute them and at first three in number, are the upper or parieto-frontal, the lower or temporal, and the anterior or orbital. The upper and lower walls first cjme in contact and thereby form the posterior limb of the Sylvian fissure. Later the angle between the upper and front walls of the fossa becomes modified and is finally obliterated by the appearance of a wedge-shaped projection, later the frontal operculum, which insinuates itself between the adjacent end FIG. 983. Inferior precentral Rolandic fissure Svlvian fossa Temporal lobe Left hemisphere of brain of five months foetus ; three-fourths natural size. Inferior Interparietal Orbital operculum Olfactory bu Sylvian fissure Superior temporal sulcus Lateral surface of left hemisphere of eight months foetus ; insula is partly covered by opercula; three-fourths natural size. (Retzius.) of the parieto-frontal and the orbital opercula. The orbital and particularly the frontal operculum are late in their differentiation and growth, and not until towards the second year after birth do they come into apposition with each other and the remaining opercula to complete the curtain that overhangs the insula. Along with the closure of the front part of the Sylvian fossa, the dif- ferentiation of the anterior limbs of the fissure pro- gresses, since upon the adequate growth of the frontal operculum depends the production of a distinct pars triangularis and of two separate anterior branches. Faulty development of this intermediate part of the opercular wall accounts for the Y or I form, as well as the occasional absence, of the anterior limbs. The central fissure (sulcus centralis), or fissure of Rolando, extends transversely across the upper half of the convex dorsal surface of the hemisphere and therefore, with the bordering precentral and postcentral convolutions, interrupts the general longitudinal course of the gyri and sulci. Bearing this peculiarity in mind, the fissure is readily identified even in brains exhibiting an elaborate and complex modelling. It begins above on the supero-mesial margin of the hemisphere, a short distance behind the middle of the border, and descends with a slight general forward obliquity to the vicinity of the posterior limit of the fissure of Sylvius, above whose mid-point it usually ends. Its upper extremity usually extends over the supero- mesial border of the hemisphere and, passing obliquely backward, cuts for a short distance into the marginal gyrus of the mesial surface (Fig. 987). Its lower ex- tremity usually ends short of the Sylvian fissure, but occasionally (rarely) opens into this cleft. It constitutes a very definite boundary on the external surface of the hemisphere between the frontal and parietal lobes. Although passing obliquely downward and forward, the course of the central fissure is by no means straight 1 Contribution to the Surface Anatomy of the Cerebral Hemispheres, Irish Academy, 189^. 2 Das Menschenhirn, 1896. H38 HIM AN ANATOMY. owing to a marked angular backward projection of the substance of the precentral convolution, situated about the junction of the upper and middle thirds of the fissure. In consequence, the fissure presents in this part of its course a distinct curve, with the concavity directed forward, the upper and lower limits of this bend consti- tuting the superior and the inferior genu respectively (Fig. 980). The cortical tissue filling this recess is of importance, since it represents the part of the precentral gyrus devoted to the motor centre for the arm. Below the inferior genu the fissure descends almost vertically, its lower end often bending slightly backward. The angle which the general direction of the central fissure makes with the mesial plane in the adult brain is on an average 71.7° (Cunningham), the Rolandic angle, as it is called, of the two sides subtending therefore about 143° (Fig. 984). FIG. 984. Superior aspect of cerebral hemispheres; LF, longitudinal fissure; r., r, Rolandic fissure; JP% im, paramedian ; />, />-, p'-\ />*, inferior, superior, horizontal and occipital limbs of interparietal ; p-o. parieto-occipital ; t.o., /.<>., transverse and lateral occipital; Sasc, ascending limb of Sylvian ; t*asc., t-asc.. ascending limbs of superior and middle temporal. Since the central fissure is usually developed from two separate parts, a longer lower and a short upper (Cunningham, Ret/ius) which later become continuous, a deep annectant gyrus is generally found crossing the bottom of the sulcus at the junction of its upper and middle thirds. In exceptional cases the original separation is continued by the deep annectant gyrus maintaining its superficial relations, the adult fissure then being interrupted by the bridge which ordinarily is limited to the bottom of the cleft. As a variation of very great rarity, completed doubling of the central fissure has been observed. The parieto-occipital fissure < tissura parieto-occipitalis) is seen chiefly on the medial surface . .f the hemisphere (Fig. 987), where it appears as a deep cleft which extends from a point on the supero-mesial border of the hemisphere, about 4 cm. in front of the occipital pole, downward and forward. This inner part of the fissure, THE TELENCEPHALON. II39 the so-called internal parieto-occipital fissure, separates the mesial surfaces of the parietal and occipital lobes and ends below by joining the calcarine fissure, the two sulci together forming a >- whose posteriorly directed diverging limbs in- clude a wedged-shaped portion of the occipital lobe known as the cuncus. The parieto-occipital fissure is continued without interruption across the upper margin of the hemisphere and onto the external surface for a short distance. This outer exten- sion, usually only from 12-15 mm. in length, constitutes the external parieto-occipital fissure and terminates after its limited transverse course in a bowed convolution, the arcus parieto-occipitalis, which surrounds and separates its end from the occipital 'part of the interparietal fissure. Although sometimes ending in two short and somewhat open branches, the external limit of the parieto-occipital fissure is usually relatively inconspicuous; notwithstanding, the sulcus is of much importance as affording a readily recognized upper limit of the conventional boundary line between the occipi- tal and the parietal and temporal lobes. In the fcetal brain the parieto-occipital sul- cus produces a distinct imagination of the wall of the cerebrum and corresponds, therefore, to a complete fissure. In the adult brain, however, all trace of this infold- ing has disappeared in consequence of the growth and thickening of the ventricular wall which subsequently takes place (Cunningham). The collateral fissure (fissura collateral) is a well marked sulcus on the inferior surface of the hemisphere. It begins behind a little to the outer side of the occipital pole and extends forward, crossing the tentorial area parallel with, below and lateral to, the calcarine fissure, until opposite the posterior end of the corpus callo- sum, where it meets the hippocampal gyrus. It is then directed slightly outward, forming the lateral boundary of the last-named convolution, over the temporal area well toward the temporal pole, near which it either embraces or joins with a short curved furrow, the indsura temporalis, which, in conjunction with the collateral fissure, separates the lower or hippocampal part of the limbic lobe from the temporal lobe. According to Cunningham, the collateral fissure is at first represented by three distinct parts — a posterior or occipital, an intermediate and a temporal — which later become one continuous furrow. Of these three primary divisions, the interme- diate, and usually also the temporal, are complete fissures', producing respectively the collateral protuberance and the collateral eminence seen in the lateral ventricle (page 1164). The occipital portion of the fissure is never complete and, therefore, does not give rise to any elevation. The calloso-marginal fissure (sulcus cinguli) is the most conspicuous sul- cus on the mesial surface of the hemisphere, where it appears as a curved furrow running above and concentric with the arched upper surface of the corpus callosum. It begins in front below the fore-end of this bridge, just above the anterior perforated space, sweeps around the genu of the corpus callosum and arches backward above the latter structure almost as far as the splenium, where it turns upward (raraus mar- ginalis) and reaches the supero-mesial border of the hemisphere a short distance be- hind the overturned end of the Rolandic fissure. By its course the calloso-marginal sulcus marks off on the anterior two-thirds of the mesial surface of the hemisphere the marginal convolution of the frontal lobe from the callosal gyrus of the limbic lobe, the somewhat uncertain posterior boundary of the latter beyond the sulcus being indicated by the inconspicuous postlimbic fissure, which arches downward concen- trically with the splenium. The frequent variations in the details of the calloso- marginal fissure depend upon irregularities in the arrangement and fusion of the three separate furrows by the union of which a continuous sulcus is formed. The limiting sulcus of Reil (sulcus circularis Reili) is a shallow furrow that incompletely surrounds the insula and imperfectly separates this buried portion of the cerebral cortex from the deeper parts of the enclosing opercula. The sulcus consists of three parts — a superior, separating the island from the parietal and fron- tal lobes, an anterior, intervening in front between the insula and the frontal lobe, and a posterior, imperfectly separating the hind part of the island from the limbic lobe. THE LOBES OF THE HEMISPHERES. The Frontal Lobe. — The frontal lobe (lobus frontalis) is the largest of the subdivisions of the hemisphere and includes approximately one-third of the hemi- 1140 HUMAN ANATOMY. cerebrum. It appears on each of the three aspects of the hemisphere and has, therefore, a dorso-lateral, a mesial and an inferior surface. On the external surface of the hemisphere it is bounded behind by the central fissure, which separates it from the parietal lobe, and below by the fore-part of the Sylvian fissure, which intervenes between it and the temporal lobe. On the mesial surface the frontal lobe includes an irregular -> , marked off by the calloso-marginal sulcus, the longer upper limb ending behind the central fissure. On the inferior surface of the hemisphere, the frontal lobe includes the concave orbital area, bounded behind by the transversely directed stem of the Sylvian fissure, which sulcus thus separates it from the temporal lobe. The principal fissures on the dorso-lateral surface of the frontal lobe are: ( i ) the inferior precentral, (2) the superior precentral, (3) the superior frontal and (4) the inferior frontal. The inferior precentral sulcus consists of a longer vertical and a short transverse limb and has a general ~| or T form. The vertical limb begins above the fissure of Sylvius and in front of the central fissure and extends upward parallel to the latter and separated from it by the lower part of the precentral convolution. The horizontal limb passes obliquely forward and upward and cuts for a variable distance into the middle frontal convolution. Fre- quently the inferior precentral sulcus is directly continuous with the inferior frontal furrow; sometimes it opens below into the Sylvian fissure and above may join the superior. The superior precentral sul- cus prolongs upward the anterior boundary of the precentral convolu- tion. It lies parallel with the upper half of the Rolandic fissure, but does not usually, although sometimes reach the upper margin of the hemisphere. Al- most constantly it receives the poste- rior end of the superior frontal sulcus with which it forms a — | shaped furrow. The superior frontal sulcus FIG. 985. Anterior aspect of cerebral hemispheres, hardened in skull; sf, i/, superior and inferior frontal fissures; /»;., paramedian; in.f, mid-frontal; /-»/., fronto-marginal. extends forward from the preceding fissure with a course which corresponds in general with the supero-mesial border of the hemisphere and thus marks off a longitudinal marginal tract, the superior frontal convolution. Anteriorly the superior frontal may join the median frontal sulcus, while its posterior end may incise the precentral convolution. Often the course of the fissure is interrupted by superficial annectant gyri which connect the adjacent borders of the upper and middle frontal convolutions. The inferior frontal sulcus begins behind in the interval between the hori- zontal and vertical limbs of the inferior precentral furrow, or in confluence with one of these. In its general course it arches forward and downward towards the anterior <>r superciliary margin of the hemisphere and terminates a short distance behind this border by bifurcating into a transverse limb. The line of the fissure is often obscured by superficial annectant gyri and complicated by small secondary furrows which pass from it into the bordering middle and inferior frontal convolutions. The convolutions on the dorso-lateral surface of the frontal lobe are the pre- central, the superior frontal, the middle frontal and the inferior frontal. The precentral gyrus ( »yrus contrails anterior), also known as the ascending jiontnl, is bounded behind by the central fissure and in front by the superior and inferior pmvntral sulci. Below it is limited by the Sylvian fissure, whilst its upper end is continuous with the paracentral lobule of the mesial surface. Anteriorly it is connected with all three frontal convolutions. A short distance above its middle, it sends backward a conspicuous projection, triangular or rounded in outline, which encnui lic^ upon the postcentral gyrus and correspondingly modifies the line of the THE TELENCEPHALON. I14I Rolandic fissure. The observations of Mills and of Griinbaum and Sherrington emphasize the predominating importance of the precentral convolution as containing the important cortical motor areas (page 1211), the backward projection just noted containing the centres controlling the muscles of the upper extremity The superior frontal gyrus lies between the supero-mesial border of the hemi- sphere and the superior frontal sulcus. Since its course corresponds with the tipper margin of the hemisphere, it is much longer than the other frontal convolutions on the external surface and reaches the frontal pole. It is continuous with the marginal gyrus, which, in fact is only its mesial part. Behind, it joins the precentral convolu- tion by a narrow bridge between the upper end of the precentral sulcus and that of a branch from the calloso-marginal fissure. The superior frontal convolution notwith- standing its meagre width, is frequently imperfectly divided into an upper and a lower part by a series of shallow longitudinal furrows collectively termed the Para- median sulcus. The latter is regarded as a distinctive feature of the human brain and is found relatively deep and well marked only in the brains of the higher races' The middle frontal gyrus, the broadest of the three, extends forward parallel with the upper frontal convolution well towards the frontal pole. It is bounded FIG. 986. Inferior frontal sulcus Inferior precentral sulcus \ \ I _ Rolandic fissure Ascending limb Orbital surface Horizontal limb/ Posterior limb Portion of lateral surface of left hemisphere, showing pars basalis (/?), triangularis (7") and orbitalis (O) of inferior frontal gyrus, known as Broca's convolution : ST., superior temporal gyrus. above and below by the superior and the inferior frontal sulcus and, in man and the anthropoid apes, is almost constantly subdivided into an upper and a lower sub- lower subdivision by the 'mid-frontal sulcus (sulcus frontalis medius). The latter is often broken by annectant gyri into two or more pieces and in front usually bifurcates to form the fronto-marginal sulcus (sulcus transversus anterior), which runs across the hemisphere a short distance above the superciliary margin. The inferior frontal gyrus, the shortest of the three, lies below the inferior frontal sulcus and arches forward and downward around the anterior limbs of the Sylvian fissure. Below and behind it is connected with the lower end of the pre- central convolution by a narrow bridge enclosing the lower end of the inferior pre- central sulcus. By the ascending and horizontal limbs of the Sylvian fissure the inferior frontal gyrus is incompletely divided into three portions — the pars basalis, the pars triangularis and the pars orbitalis (Fig. 986). The pars basalis (pars opercularis) occupies the posterior part of the convolution and lies between the inferior precentral sulcus and the ascending Sylvian limb. It forms the fore-part 'of the fronto-parietal operculum and is indented by an inconspicuous although constant furrow, the sulcus diagonalis, which extends obliquely downward and forward across the gyrus for a variable distance. Although usually distinct, the diagonal sulcus may join the inferior precentral (Fig. 986), the inferior frontal or the Syhian fissure. The pars triangularis is the wedge-shaped tract included between the two limbs of the Sylvian fissure. Its base is directed upward and forward and its n42 HUMAN ANATOMY. apex towards the Sylvian point. The pars orbitalis lies below the horizontal limb and is continued around the margin of the hemisphere onto the orbital surface of the frontal lobe. It is evident, from the description of the boundaries of the Sylvian fissure already given (page 1137), that the preceding subdivisions of the inferior frontal gyrus correspond with certain of the opercula — the pars basalis with the anterior part of the fronto-parietal, the pars triangularis with the frontal and the pars orbitalis with the orbital operculum. The posterior extremity of the inferior frontal gyrus on the left side is known as Broca s convolution and has long been regarded as the centre for the movements for articulate speech, although the accuracy of this view has been questioned. According to Marie, Broca' s convolution has no relation with speech, a conclusion, however, so far not convincingly supported. The convolution is sometimes better developed on the left than the right side of the brain, the pars triangularis particularly being increased. As previously noted, the development of this wedge — the frontal operculum — bears a direct relation to the degree of independence of the two anterior limbs of the Sylvian fissure. The mesial surface of the frontal lobe ( Fig. 987), includes only one convolution, the marginal gyrus, which lies between the dorso-mesial margin of the hemisphere and the calloso-marginal sulcus (page 1139), and by the latter is separated from the limbic lobe. It is —>-shaped and directly continuous with the superior frontal gyrus above and with the gyrus rectus on the orbital surface below. Its posterior end is almost completely cut off from the rest of the gyrus by an ascending limb (sulcus para- centralis) from the calloso-marginal sulcus, the portion so isolated forming the front part of the paracentral lobule, which is bounded behind by the upturned end (ramus marginalis) of the calloso-marginal sulcus and contains, near its hind border, the termination of the fissure of Rolando. By means of an annectant convolution passing below the last-named furrow, the frontal part of the paracentral lobule is con- tinuous with the part contributed by the parietal lobe. The middle of the mar- ginal gyrus is often incompletely subdivided by a shallow longitudinal groove, the mesial frontal sulcus, into an upper and a lower tract, whilst its anterior and lower end is uncertainly cleft by two or three short downward curving furrows, the sn/ci rostrales. The orbital surface of the frontal lobe is marked by two fissures, the olfactory and the orbital and by three chief convolutions, the inner, the middle and the outer orbital. Although such division is convenient for the purposes of description, it must be remembered that these orbital gyri are not separate convolutions, but largely the inferior portions of the upper, middle and lower frontal convolutions of the outer surface of the lobe. The olfactory sulcus lodges the olfactory bulb, tract and tubercle, and ex- tends parallel with, or inclined somewhat towards the great longitudinal fissure. Its course being straight, the sulcus marks off a narrow strip, about i cm. in width, along the mesial border of the lobe. This area, although specially designated as the gyrus rectus, is only a part of the broader longitudinal tract which corresponds to the orbital surface of the superior frontal convolution. The orbital sulcus includes a number of furrows whose arrangement is very variable, not only in different brains but often on the two sides of the same brain. In the disposition assumed as the typical one, which, however, is far from constant, the orbital sulcus consists of \WQ* longitudinal limbs, connected by a shorter trans- verse arm, the three furrows forming a common fissure which corresponds more or less closely with the letter H. In many cases, however, the sulcus more nearly re- sembles an X or K, or it may be still further modified by the presence of additional secondary grooves of variable number and length. Assuming the conventional H- fonn to exist, the orbital surface is divided into three longitudinal tracts, the inner, middle and outer orbital gyri, by the long limbs < sulcus orliitalis interims et exter- nus). The inner tract is subdivided by the olfactory sulcus into the gvrns rectus, above mentioned, and an outer part, the ^yrns orb;talis interims in the more restricted sense. The middle orbital gyrus is subdivided by the curved transverse limb i sulcus orbitalis tiaiis\ci MIS ) into the- anterior and the posterior orbital »yrns, which lie respectively in front and behind the tr.m-vcrse furrow. In many cases the latter cuiYea outward and backward until it almost reaches the Sylvian fissure. THE TELENCEPHALON. 1:43 The Parietal Lobe. — This division includes a considerable part of the hemi- sphere and presents two surfaces, an external and a mesial. The external surface, much the more extensive and irregularly quadrilateral in outline, is bounded above, in front and partially below by well marked fissures, but behind and postero-infe- riorly its limits from the occipital and temporal lobes are defined for the most part by imaginary lines. Its upper boundary corresponds with the supero-mesial border of the hemisphere ; its anterior boundary is the central fissure, by which the pari- etal lobe is completely separated from the frontal except below, where the postcen- tral gyrus is continuous with the precentral by the bridge closing the lower end of the Rolandic fissure. Its posterior boundary, which separates the parietal from the occipital lobe, is largely conventional and indicated by a line drawn from the point where the parieto-occipital fissure cuts the upper margin of the hemisphere to an in- dentation, the preoccipital notch (page 1134), which grooves the infero-lateral border of the hemisphere at a point from 3.5-4 cm. in front of the occipital pole. Its inferior border, between the parietal and the temporal lobes, is definite where formed by the posterior limb of the Sylvian fissure. Beyond the upturned end of the latter, FIG. 987. Infero-mesial aspect of left cerebral hemisphere; cm., calloso-marginal fissure; ros., rostral; r., overturned end of Rolandic ; p. /., post-limbic; i. p-o., internal parieto-occipital ; f>, cal., a. cal., posterior and anterior calcarinej p. col., a. col., posterior and anterior collateral ; z'. t., incisura temporalis or rhinial; o-t., occipito-temporal. the parietal and the temporal lobes are continuous and their separation is conven- tionally assumed to be made by an arbitrary line prolonged backward in the direc- tion of the posterior limb of the Sylvian fissure until it meets the parieto-occipital line previously described. The external surface of the parietal lobe is subdivided by a composite fissure, the interparietal sulcus, into three general tracts, the postcentral, the superior pari- etal and the inferior parietal gyrus. The interparietal sulcus, especially described by Turner, starts in the antero- inferior angle of the lobe a short distance above the Sylvian fissure, with which it is rarely continuous, ascends for about an inch parallel with the central fissure, and then sweeps backward and slightly upward across the parietal into the occipital lobe. The interparietal sulcus is developed as four originally distinct parts, which in the fully formed brain, notwithstanding their usual fusion, are recognized as the inferior and the superior postcentral sulcus and the horizontal and occipital limbs (Cun- ningham). The inferior postcentral sulcus lies behind and parallel with the lower part of the central fissure. Although in most cases continuous with either the superior postcentral sulcus (in 72 per cent, according to Retzius1), or with the horizontal limb 1 Biologische Untersuchungen, VIII., 1898. 1 144 HUMAN ANATOMY. (66 per cent.), or with both (55 per cent.), the inferior limb may remain ununited (17 per cent.). When joined, the two limbs together form a continuous postcentral sulcus which parallels the fissure of Rolando and bounds the postcentral convolution behind. In rare instances the inferior postcentral sulcus opens below into the Sylvian fissure. The superior postcentral sulcus lies behind and parallel with the upper part of the fissure of Rolando, gaining the superior margin of the hemisphere between the incisions of the Rolandic fissure and the upturned end of the calloso-marginal sulcus. Although in 59 per cent, of the brains studied by Retzius the fissure was confluent with the horizontal limb, in 24 per cent, it remained isolated. The horizontal limb passes backward and slightly upward and separates the superior and inferior parietal convolutions from each other. It is usually continuous in front with one or the other or with both postcentral sulci and behind with the FIG. Lateral aspect of left side of brain. LF, longitudinal fissure; >-., ;•., r.. Rolandic fissure; i- PC., s. f>c., inferior and superior precentral; sf., if., superior and inferior frontal; Sj>, S. asc., posterior and ascending limbs of Sylvian fissure; /', p*, A1./4, inferior, superior, horizontal and occipital limbs of interparietal ; p-o. parieto-occipital ; /. o., I. o., transverse and lateral occipital ; /', tlasc., superior temporal and its upturned limb; <*, t-asc., middle temporal and its upturned limb. posterior or occipital limb. As a rule it joins a continuous postcentral sulcus, in which case the three furrows form a | — shaped fissure, which subdivides the parietal lobe into its three main convolutions. The occipital limb is usually attached to the horizontal one and then directly prolongs the interparietal sulcus into the occipital lobe. Sometimes, however, it retains its original independence and is separated from the ramus horizontalis by a deep annectant gyrus. It is irregularly curved and marks the lower boundary of the gyrus, the arcus parieto-occipitalis, which receives the outer end of the parieto- occipital fissure. Beyond the line of this furrow, the sulcus lies in the occipital lobe and behind the arcus parieto-occipitalis ends by bifurcating into two widely divergent arms, which constitute the transverse occipital sulcus. The chief con-solutions on the external surface of the parietal lobe are three — the postcentral, the superior parietal and the inferior parietal. The postcentral gyrus, also called the ascending -parietal, forms the posterior wall of the fissure of Rolando, and itself is bounded behind by the postcentral sulcus, either l>y the continuous fissure or by its two divisions. The lower end of the gyrus is connected with the precentral convolution in front and with the inferior parietal one behind by the annectant ^yri closing the lower ends of the central and postcen- THE TELENCEPHALON. II45 tral sulci respectively. Above, the convolution is continuous with the pri-cvntral lobule of the mesial surface between the terminations of the calloso-margin;il and tin- Rolandic fissures. In its width and general oblique course across the' hemisphere the postcentral convolution strongly resembles the precentral gyrus and with the latter and the three associated sulci— the precentral, central and postcentral— forms a conspicuous feature in the modelling of the external surface of the hemisphere and affords a ready means of locating the Rolandic fissure. The superior parietal gyrus is the triangular tract lying between superior postcentral sulcus, the horizontal limb of the interparietal sulcus and the supero- mesial border of the hemisphere. Behind, it is limited by the overturned outer end of the parieto-occipital fissure, around which, however, it is continuous with the occipital lobe by means of the curved convolution, the arcus parieto-occipitalis. Farther forward it is frequently deeply incised by an ascending branch from the inter- parietal sulcus. It is connected with the postcentral gyrus around the upper end of the superior postcentral sulcus and, in those cases in which the last-named sulcus fails to unite with outer segments of the interparietal fissure, additionally joins the post- central gyrus about the inferior postcentral sulcus. The inferior parietal gyrus is included between the curved interparietal sulcus and the conventional lower boundary of the lobe. Since only the front end of this boundary is defined by a groove, its greater part being the arbitrary line above described, it follows that behind the Sylvian fissure the inferior parietal convolution is continuous with the subjacent temporal gyri. The convolution is cut into from below by the upturned end of the Sylvian fissure and the terminations of the first and second temporal sulci and by these incisions is somewhat uncertainly subdivided into three parts, the supramarginal, the angular and the postparietal gyri (Fig. 988). The supramarginal gyrus arches around the upturned extremity of the Sylvian fissure. It lies behind and below the front part of the r.nterparietal sulcus, around whose lower end it joins the postcentral gyrus, whilst below it is continuous with the superior temporal and behind with the angular gyrus. The angular gyrus surmounts the upwardly directed end of the superior temporal sulcus and below is prolonged into the superior and middle temporal convolutions. It is commonly imperfectly sepa- rated from the postparietal gyrus by a shallow furrow. The postparietal gyrus bends over the obliquely vertical extremity of the middle temporal sulcus and below joins the middle and inferior temporal convolutions. It lies approximately opposite the arcus parieto-occipitalis from which it is separated by the occipital branch of the interparietal sulcus. The mesial surface of the parietal lobe includes an irregularly quadrate area ex- tending from the internal limb of the parieto-occipital sulcus behind to the line of the Rolandic fissure in front; below it is imperfectly defined from the limbic lobe by the calloso-marginal sulcus, to a very slight extent, and its continuation, the post-limbic furrow. By far the greater part of this surface is embraced by the quadrate lobule or precuneus, an irregularly quadrilateral area (Fig. 987) limited in front by the upturned terminal limb of the calloso-marginal and behind by the parieto-occipital sulcus. The lobule, the mesial aspect of the superior parietal convolution, is usually marked by one or more furrows, the precuneate sulci, which incise the upper margin of the hemisphere and extend for a short distance onto the outer surface. The Occipital Lobe. — The occipital lobe is pyramidal in form and includes the occipital pole and the adjacent parts of the hemisphere. It is represented on all of the aspects of the hemisphere and possesses, therefore, a lateral, a mesial and an inferior or tentorial surface. A well-marked occipital lobe is found only in the brain of man and of the anthropoid apes and is developed as a backward prolongation of the parietal and temporal lobes, from which, therefore, it is but imperfectly sepa- rated. On the mesial surface its extent is definitely limited by the internal parieto- occipital sulcus, by which it is cut off from the quadrate lobule or precuneus of the parietal lobe. On the lateral surface, on the contrary, it is continuous with the pari- etal and temporal lobes, its anterior boundary being arbitrary and indicated by the parieto-occipital line drawn from the overturned limit of the parieto-occipital sulcus above to the preoccipital notch below. On the inferior or tentorial aspect its demar- cation is even more uncertain, the occipital, limbic and temporal lobes being here 1146 HUMAN ANATOMY. directly continuous, and depends upon the recognition of an arbitrary line which may be drawn, as suggested by Cunningham, from the preoccipital notch on the infero-lateral border to the isthmus of the limbic lobe, just below the splenium of the corpus callosum. The external surface of the occipital lobe is modelled by two well-defined fissures, the transverse occipital and the lateral occipital, and by two somewhat uncertain convolutions, the superior and the inferior occipital (Fig. 988). The transverse occipital sulcus is, as above pointed out, the widely diver- gent terminal bifurcation of the interparietal fissure, whose last segment beyond the outer end of the parieto-occipital sulcus enters the occipital lobe to end in the manner just indicated. FIG. 989. Inferior aspect of cerebral hemispheres, i.o., /. shaped sulcus, between whose diverging limbs lies the triangular cuneus. Although usually appearing as one continuous fissure, the parieto-occipital and calcarine sulci are incompletely separated by a deep annectant gyrus, which connects the cuneus with the limbic lobe. The calcarine fissure itself is subdivided by a second sunken gyrus into an anterior and a posterior part. The latter, the posterior calcarine fissure, is shorter and shallower than the front part and is not a total fissure. The other portion, the anterior calca- rine fissure, is not only the deeper but completely invaginates the brain- wall, thereby giving rise to the elevation known as the calcar avis, seen on the inner boundary of the posterior horn of the lateral ventricle. The cuneus forms the chief part of the mesial aspect of the occipital lobe. It is triangular in outline and lies between the parieto-occipital sulcus in front and the posterior limb of the calcarine fissure below, whilst above and behind it reaches the superior border of the hemisphere (Fig. 987). Its surface is frequently impressed by one or more shallow vertical furrows. The lingual gyrus, also called the infracalcarine ', is the irregular elongated tract bounded mesially and above by the calcarine fissure, and laterally and below by the collateral (Fig. 989). Its rounded hind-end lies in the occipital lobe, whilst its tapering and greatly narrowed front-end is continuous with the hippocampal convo- lution. The gyrus fits into the angle between the falx cerebri and the tentorium and therefore bears the internal occipital border of the hemisphere and appears on both the mesial and the tentorial surfaces. It is usually modelled by irregular shallow furrows which break up the larger tentorial aspect into uncertain secondary gyri. The inferior or tentorial surface of the occipital lobe is continuous with the more extensive similar surface of the temporal lobe resting upon the tentorium. In addi- tion to the tentorial part of the lingual gyrus, this aspect of the lobe is occupied by the posterior part of the occipito-temporal gyrus. The latter includes an irreg- ular fusiform tract, bounded by the collateral fissure internally and by the inferior temporal sulcus laterally (Fig. 989). As expressed by its name, the occipito- temporal convolution belongs partly to the occipital and partly to the temporal lobe and extends from the occipital to the temporal pole. Its surface is broken by a number of irregularly disposed furrows which add to the uncertainty of its outer boundary. The Temporal Lobe. — The temporal lobe includes the irregularly pyramidal division of the cerebral hemisphere, whose apex is lodged within the middle fossa of the skull and whose succeeding part forms the conspicuous dependent mass seen on the infero-lateral surface of the hemicerebrum. In front it is separated from the frontal lobe by the stem of the Sylvian fissure; above it is marked off from the pari- etal lobe by the posterior limb of the Sylvian fissure and the arbitrary line prolonged backward in the direction of this sulcus; externally and below it is defined by the infero-lateral border of the hemisphere; and mesially it is separated from the limbic lobe by the collateral fissure. Its posterior border, however, on both the lateral and the inferior (tentorial) surface is arbitrary and indicated by the lines already men- tioned (pages 1 1 43 and 1146) which afford the conventional demarcation between the occipital and temporal lobes. The temporal lobe presents three surfaces, the convex lateral, the inferior (largely tentorial), and the buried superior or opercular. Of these the lateral and inferior are separated by a border so broad and rounded that the surfaces pass insen- sibly into each other. Its tip corresponds with the temporal pole of the hemisphere and underlies the posterior part of the orbital surface of the frontal lobe, which it partially masks. The lateral surface of the temporal lobe is modelled by two fissures, the superior and the middle temporal, and three convolutions, the superior, the middle and the 1148 HUMAN ANATOMY. inferior temporal (Fig. 988), all of which correspond in the general direction of their course with the posterior limb of the Sylvian fissure and extend backward and slightly upward. The superior temporal sulcus, also called the parallel sulcus in recognition of the similarity of its course with that of the posterior limb of the Sylvian fissure, is the first in the series of longitudinal furrows, the third of which appears not on the outer, but on the inferior aspect of the lobe. It begins near the temporal pole, runs parallel with the posterior limb of the Sylvian fissure and ends by cutting upward into the inferior parietal convolution, whose angular gyrus surrounds the upturned extremity of the sulcus. The middle temporal sulcus, the second in the series, lies below the pre- ceding fissure, whose direction in a general way it follows. It is, however, much less certainly marked and in most cases is not a continuous furrow, as is the superior sulcus, but broken by superficial annectant convolutions into a number of separate pieces, the exact sequence of which is often difficult to follow. The upturned end of the middle temporal sulcus cuts into the lower parietal convolution towards the pos- terior limb of the interparietal sulcus (Fig. 988) from which, however, it is separated by the arching postparietal gyrus. FIG. 990. Rolandic fissure Right cerebral hemisphere, with opercula displaced to expose island of Reil. The superior temporal gyrus intervenes between the posterior limb of the Sylvian fissure and the superior temporal sulcus. Its lower end lies at the temporal pole, whilst above the tract is continuous with the supramarginal and angular gyn of the parietal lobe. The middle temporal gyrus, between the upper and middle temporal sulci, is connected with the subjacent convolution by the bridges which interrupt the sec- ond temporal furrow. Above and behind it is continuous with the angular and postparietal convolutions. The inferior temporal gyrus occupies the rounded mfero-lateral margir the hemisphere, and appears on both the lateral and the inferior surface of the lobe, being continuous with the occipital lobe behind (Fig. 988). Its upper boundary, formed by the middle temporal sulcus, is indistinct ; its lower and mesial limit is defined by the inferior temporal sulcus, which separates it from the occipit temporal gyrus. The inferior surface of the temporal lobe is rounded in front, where it rests in the anterior cerebral fossa, but behind is modelled by the upper surface of the ten- torium cerebelli and is, therefore, concave from before backward and slightly convex from side to side. It presents one fissure, the inferior temporal, and one convolu- tion, the anterior part of the occipito-temporal. The inferior temporal sulcus, also called the occifiito-tcmfioral* courses tudinally a short distance' internal to the infero-lateral border of the hemisphere and THE TELENCEPHALON. 1149 separates the inferior temporal from the occipito-temporal gyrus. Although for the greater part of its extent on the temporal lobe, it is not confined to this, but continues backward into the occipital lobe which, therefore, claims it as one of its furrows. The sulcus is rarely continuous, usually being broken by annectant gyri into a posterior, a middle and an anterior segment. The occipito-temporal gyrus (gyrus fusiformis) is, as its names imply, a fusiform tract belonging partly to the occipital and partly to the temporal lobe (Fig. 989). Its two ends, in front and behind, are pointed and connected by a broader intervening tract, which is commonly broken up by secondary furrows. The temporal division of the gyrus, including approximately its anterior two-thirds, is embraced between the converging collateral fissure mesially and the inferior temporal sulcus laterally ; its conventional posterior limit is the line drawn from the preoccipital notch to the isthmus of the limbic lobe, immediately beneath the hind-end of the corpus callosum. The superior surface of the temporal lobe is directed towards the insula and is therefore an opercular aspect. On separating the walls of the Sylvian fissure to expose it, this buried surface of the temporal lobe often exhibits several shallow transverse furrows and indistinct gyri, the deep aspect of the temporal pole being similarly indented. Rolandic fissure Sulcus subdividing precentral lobule / Cut surface of frontal lobe V /^ ^/ Sulcus centralis^ ^ T T»" *« Sulcus drcularfs Gvri breves Sulcus cetitralis insulse Gyrus longus Temporal Apex Limen lobe, cut Island of Reil exposed after cutting away surrounding parts of right cerebral hemisphere. The Insula.— The insula, or island of Reil, sometimes also called the central lobe is in the human brain, entirely concealed within the Sylvian fissure by the approximation of the overhanging opercula. The manner in which the latter are developed from the wall surrounding the early Sylvian fossa has been described (page in?) ; it remains here to note the chief features of this region in the adu brain On examining the relations of the insula, as seen in frontal sections of brain' (Fig. 967), it will be noted (a) that the shell of cortical gray matter cove ing the sunken convolutions is directly continuous along the Sylvian fissure with t covering the convolutions on the freely exposed parts of the hemisphere ; (*) the insular cortex lies close to the underlying mass of gray matter, the len division of the corpus striatum, a narrow tract of white matter, the external caps! alone intervening. Since the corpus striatum is one of the earliest of the funda- mental parts of the telencephalon to be developed, it is probable that its close pri- mary relation to the surface of the hemisphere is largely responsible for the failure c the overlying cortex to keep pace with the general expansion of the adjoining parts. When exposed by separation or removal of the surrounding opercula (1 QQI) the insula appears as a triangular convex field composed of a group «»t radi- ating convolutions, whose broader ends lie above and pointed ones 1 1 50 HUMAN ANATOMY. dependent apex of the insula lies close to the anterior perforated space, with the gray matter of which the cortical sheet of the island is continuous by way of a transitional area, known as the limen insula:, where the limiting sulcus of the island is incomplete. In addition to being imperfectly separated from the surround- ing opercula by the curved limiting sulcus {sulcus circularis insula}, the island is divided into an anterior and a posterior part by the sulcus centralis insulce. This furrow continues in a general way the downward and forward direction of the fissure of Rolando, the deeper part of which is seen above the island (Fig. 991). The anterior part, or precentral lobule, is subdivided by two, sometimes by three, shallow grooves into three or four short downwardly converging ridges, the gyri breves, of which the front one is connected with the deeper part of the inferior frontal convolution by a small arched annectant gyrus transversus. The hind-part of the island, the postcentral lobule, includes a longer wedge-shaped tract, the gyrus longus, which below is continuous with the limbic lobe. The gyrus longus is frequently subdivided by one or more shallow furrows into secondary ridges. The Limbic Lobe. — The limbic lobe (gyrus fornicatus) appears on the mesial and inferior surfaces of the hemisphere (Fig. 987) as an elongated o-shaped tract, Splenium of corpus callosum Callosal gyrus FIG. 992. Fornix, body Thalmus, partly cut away Septum lucidum Fasciola cinerea1 Calcarine fissure Isthmus Rhinal fissure Uncus Collateral fissure Gyrus dentatus Gyrus Firnbria hippocampi Portion of infero-mesial surface of left hemisphere, showing lower part of limbic lobe and adjacent structures. whose ends lie closely approximated with each other and with the anterior per- forated space. These extremities are further intimately associated with the two limbs of the olfactory tract, in this manner the limbic and olfactory lobes becoming, at least topographically, continuous. The limbic lobe comprises two parts, an antero- superior and an inferior, of which the former, the callosal gyrus, lies concentric with the upper surface of the corpus callosum, and the inferior part, the hippo- (diHpal gyrus, forms the mesial tract of the tentorial surface of the hemisphere. The limbic lobe is separated from the adjacent convolutions by the calloso-marginal sulcus in front and above, by the postlimbic sulcus behind, and by the anterior part of the collateral fissure below. Its demarcation from the anterior part of the temporal lobe is effected by the inconspicuous rhinal sulcus (fissura rhinica), or incisura temporalis, which feeble furrow in man represents the important and fundamental ectorhinal fissure of the lower animals. The callosal gyrus (jjyms cinguli), also called \\\z gyrus fornicatus (not to be mistaken, however, with the same name as applied to the entire limbic lobe), begins at the anterior perforated space, below the recurved rostrum of the corpus callosum. Thence it winds around the genu of the latter and follows the convex dorsal surface of the corpus callosum, separated however from it by the narrow callosal sulcus (sulcus corporis callosi ). On reaching a point just below the splenium, around which THE TELENCEPHALON. 1151 it bends, the callosal gyrus is markedly reduced in width by the encroachment of the calcarine fissure, the narrowed tapering tract thus formed being the upper part of the isthmus (isthmus gyri fornicati), which below joins the similarly reduced upper end of the hippocampal convolution and so establishes the continuity between the two parts of the lobe. The hippocampal gyrus (gyrus hippocampi) curves forward from the isthmus along the mesial border of the tentorial surface of the hemisphere towards the apex of the temporal lobe, which, however, it fails to reach (Fig. 992). Its anterior extremity is distinctly thickened and forms a rounded hook-like projection, the uncus, which is recurved and directed backward and inward. The uncus is separated from the apex of the temporal lobe by the incisura temporalis (fissura rhinica), whilst the hippocampal convolution is marked off laterally by the anterior part of the collateral fissure. Although blended with the gyrus hippocampi and seemingly a part of the limbic lobe, the uncus, strictly considered, belongs to the rhinencephalon and not to the limbic lobe (Turner, Elliot Smith). The posterior end of the hippocampal convolution is incised by the anterior extremity of the calcarine fissure and so divided into two parts ; of these the upper aids in forming the isthmus and is continuous with the callosal gyrus, whilst the lower one blends with the front part of the gyrus lingualis of the occipital lobe. The Rhinencephalon. — Although a division of fundamental importance and differentiated at a very early period in the development of the human telencephalon, in the brain of man it is represented by structures, which to a great extent are rudi- mentary and feeble expressions of the bulky corresponding parts in the brains of many of the lower animals. Its small size in man, as compared with the voluminous structures seen in some mammals in which the rhinencephalon constitutes a large part of the entire hemisphere, is no doubt associated with the relatively feeble olfac- tory sense possessed by man. It is probable, however, that other and unknown factors are responsible for the development of this part of the hemisphere to a degree disproportionate to the olfactory capacity of the animal, as strikingly observed among the lower vertebrates. The conclusions deduced from comparative studies empha- size the fundamental character of the rhinencephalon as phylogenetically being the oldest part of the hemisphere. Indeed of such primary morphological significance is the rhinencephalon that it is termed the archipallium, as distinguished from the neopallium, which comprises almost the entire remainder of the hemisphere with the exception of its nucleus, the corpus striatum. As seen in the human brain, the rhinencephalon includes the rudimentary olfac- tory lobe — represented by the olfactory bulb, the olfactory tract with its roots, the olfactory trigone, and the parolfactory area — and the uncus and a number of acces- sory parts, including the anterior perforated space, the gyrus subcallosus, the sep- tum lucidum, the fornix, the hippocampus and the gyrus dentatus. Some of these accessory structures can be understood only after their relations to other parts of the brain have been considered. Deferring the details of certain of these struc- tures, as the septum lucidum, the fornix, and the hippocampus major, until the lateral ventricles are described (page 1160), it will suffice for the present to point out their general features as related to the rhinencephalon. The Olfactory Lobe. — This division of the adult human brain is small and rudimentary and comprises the olfactory bulb, the olfactory tract, the olfactory trigone and the parolfactory area (Fig. 993). Of these all but the last lie on the inferior surface of the brain, whilst the parolfactory area occupies a small space on the mesial aspect of the hemisphere. The olfactory bulb (bulbus olfactorius) is an elongated irregularly oval swell- ing, about 10 mm. long, from 3-4 mm. wide and about 2.5 mm. thick, which behind is continuous with the olfactory tract and below receives the olfactory filaments. Its upper surface underlies the olfactory sulcus of the orbital aspect of the frontal lobe, and its under one rests upon the cribriform plate of the ethmoid bone, through the apertures of which the bundles of the olfactory nerve-fibres ascend from the nasal mucous membrane to the bulb. The structure of the olfactory bulb shares the general rudimentary condition which charac- terizes the lobe in man, the bulb having lost the central cavity ( ventriculus bulbi o/fac/orti), 1152 HUMAN ANATOMY. which in many animals is continuous with the fore-part of the lateral ventricle, as well as some of the six layers that may be typically represented, as in the dog's bulb. The ventral aspect of the bulb, receiving the olfactory nerves, retains most completely its nervous character and pre- sents three chief strata (Fig. 995). (i) The stratum of olfactory fibres appears as a narrow zone made up of the irregularly intermingled bundles of axones of the olfactory cells situated within the olfactory area of the nasal mucous membrane. This layer is succeeded by a broader tract, (2) the stratum of the mitral cells, so named on account of the numerous nerve-cells of peculiar bishop's-hat form which occupy its upper border. Along its lower margin extends a narrow zone of large spherical masses, the olfactory glomeruli. These bodies, from .o65-.o9o mm. in diameter, consist of an intricate complex formed by the intertwining of the richly branching axones ascending from the olfactory cells and of the dendrites descending from the mitral cells. The interval between the upper and lower margins of the second stratum is occupied by the molecular layer, composed of small nerve-cells whose dendrites also enter the glomeruli. (3) The stratum of central fibres includes the centrally directed axones of the mitral and other nerve-cells which constitute the second link in the complicated paths by which the olfactory stimuli are carried to the cortical areas. The outer zone of this stratum is known FIG. 993. Olfactory bulb Olfactory tra Mesial olfactory stria Lateral olfactory stria Island of Reil .^_ Anterior perforated space Cut surface of temporal lobe Cerebral peduncle crossed by optic tract Lateral geuiculate body Olfactory sulcus Parolfactory area Tuberculum olfactoriura Trigonum olfactorium Optic chiasm, " partly cut away Marnmillarv body in interpeduncular space Oculomotor nerve Cerebral peduncle Pulvinar Median geniculate body Sylvian aqueduct ~ Anterior part of inferior surface of brain, showing parts of olfactory lobe and structures within interpedun- cular space ; tip of right temporal lobe has been removed. as the granular layer and consists of many small nerve-cells intermingled with the fibres. The deeper part of the stratum of nerve-fibres encloses some larger nerve-cells of stellate or elongated form. The central part of the bulb, which represents the obliterated ventricular space, is filled by a gelatinous substance resembling modified neuroglia. The olfactory tract (tractus olfactorius) is a narrow band of light color, which extends from the olfactory bulb in front to the olfactory trigone behind (Fig. 993). It measures about 2 cm. in length and 2.5 mm. in width, but is broader at its pos- terior extremity, from which the olfactory stria, as its roots are called, diverge. Its ventral surface is flat and its narrow dorsal one ridged, the tract appearing in transverse section more or less triangular in outline. The structure of the olfactory tract further emphasi/es the rudimentary condition of the part in man. The ventral aspect and the rounded adjoining borders consist of: (i) a stratum of nerve-fibres, longitudinally coursing and therefore transversely cut in cross-sections, which Covers tin- sides and dorsal surface of the tract and is reduced to an extremely thin and rudimen- tary slii-rt. NVxt follows (2) i\£f/tifin>ins stratum, which represents the obliterated ventricular cavity si -en in many lower animals. Succeeding this and forming the thickest layer of the tract :ies i .; . the dorsal stratum o/'^nir inaftn; which still retains its importance as a tract of cortical ulistance from which libtvs pass toother parts of the hemisphere (page 1222). THE TELENCEPHALON. "53 994- The olfactory striae, the so-called roots of the olfactory tract (Fig. 993), are usually two, the mesial and the lateral, an additional intermediate root being some- times represented by faint strands. The mesial stria bends sharply inward', passes along the inner margin of the olfactory trigone and disappears on the mesial surface of the hemisphere by joining probably partly the callosal and partly the subcallosal gyri (Fig. 994). The diverging lateral stria obliquely courses along the antero- lateral margin of the perforated space, but usually disappears as a distinct tract before it can be traced to the uncus, its probable destination (page 1222). Occasionally the lateral root is represented by two strands, an outer and an inner, the last one fading away in the substance of the anterior perforated space. An additional intermedia^- stria is sometimes recognizable for a short time before it too sinks into the anterior perforated space. The olfactory trigone (trigonum olfactorium) is the three-sided slightly convex area embraced by the two roots of the olfactory tract at the sides, and behind sepa- rated from the anterior perforated space by a groove (sulcus parolfactorius posterior). The triangular area seen on the inferior surface of the hemisphere (Fig. 993) is really the under aspect of a more extensive pyramidal elevation, the tuberculum olfactorium, which, however, lies in large part within the olfactory sulcus and is therefore superficially not visible except at its base, the trigone. Retzius regards this part of the hemisphere as a constant deep convo- FIG. lution, gyrus tuberis olfac- torius, from which proceed two ridges, gyrus olfacto- rins medialis and later alis. These bend respectively inward and outward and support the white strands of nerve-fibres, the striae olfac- torii, which are usually de- scribed as the roots of the olfactory tract. The tuber- culum olfactorium contains a considerable amount of gray matter, which is a part of the peripheral olfactory cortex and, with other por- tions of this sheet, shares in the reception of axones from the mitral cells and in the origin of fibres passing to other parts of the rhinencephalon. The parolfactory area, or field of Broca, lies as a small curved tract upon the mesial surface of the hemisphere, just in front of and below the gyrus subcallosus which extends from the rostrum to the corpus callosum (Fig. 994). The area parolfactoria is bounded in front by the sulcus parolfactorius anterior and behind by the sulcus parolfactorius posterior, and is connected in front with the superior frontal gyrus, above with the callosal gyrus and below with the inner part of the trigonum olfactorium, the mesial olfactory gyrus above mentioned. The anterior perforated space (substantia perforata anterior) is an irregularly triangular area (Fig. 993) lying behind the trigonum olfactorium, from which it is separated by the obliquely coursing sulcus parolfactorius posterior, and in front of the optic commissure. Its inner part is narrow and extends as a point between the mesial root of the olfactory tract and the lower end of the subcallosal gyrus. Its broader outer part extends into the floor of the stem of the Sylvian tissure and behind reaches the deeper part of the uncus and, more medially, the optic tract. Its designation as perforated is justified by the large number of small oval apertures for the transmission of perforating branches from the antero-mesial and antero- lateral groups of the basal arteries. These openings, most numerous along the front margin of the space, are disposed with some regularity in parallel tows and 73 Rostrum of corpus calloi Septum lucidum Sulcus parolfactorius anterior Foramen of Monro Anterior pillar of fornix Anterior commissure Lamina cinerea parolfactc ; posterior Gyrus subcallosus Portion 01 mesial surface of right hemisphere, showing gyrus subcallosus and parolfactory area. "54 HI/MAN ANATOMY. decrease in size as they approach the inner border (Foville). The substance of the space proper consists of a thin sheet of gray matter containing groups of nerve-cells, some of which constitute the nuclei of primary centres interposed in the paths connecting the olfactory lobe with the secondary (cortical) olfactory centres (page 1222). In addition to the white strands of nerve-fibres composing the olfactory strue which after a longer or shorter superficial course sink into the substance of the perforated space, an obliquely directed narrow ribbon-like tract, the diagonal band of Broca, may be sometimes made out along the inner margin of the area perforata. In front it is continuous with the subcallosal gyrus and behind passes along the optic tract towards the anterior end of the hippocampal convolution. The band is of interest as being probably the beginning, on the basal surface of the brain, of at least FIG. 995. a part of the fibre-tracts contained within the rudimentary supracallosal gyrus (page 1157) that, in turn, is prolonged into the gyrus dentatus. The uncus is the thickened anterior extremity of the gyrus hippocampi, recurved around the front end of the hippocampal fissure (Fig. 992). Antero-inferiorly it is separated from the adjacent part of the temporal lobe by the inconspicu- ous incisura temporalis or rhinal sulcus, which in animals possessing a well developed rliinencephalon constitutes a definite boundary be- tween this part of the hemisphere and the pallium. With its deeper surface the uncus is in close relation with the anterior perforated space, whilst postero-mesially it is connected with the fimbria (page 1165) and the gyrus dentatus (page 1166). Although seemingly a part of the limbic lobe, the comparative studies of Turner and of Elliot Smith have established its morphological inde- pendence from the last-named lobe and emphasized its relation with the rhinencephalon. With the lateral olfactory stria, the uncus constitutes in man the feeble representation of the large and conspicuous pyramidal lobe, which in many animals forms the most massive part of the olfactory brain. The accessory parts of the rhinencephalon include structures which, for the most part, constitute collectively an elaborate path by which the olfactory cortical centres are connected with each other, on the one hand, and with the optic thalamus and lower levels on the other. Since these structures are by position closely asso- ciated with parts of the brain still to be described, with the exception of the anterior perforated space already noted (page 1153), they will be merely mentioned here, as components of the rhinencephalon, their details being deferred until the related parts are considered. The fornix (page 1158), the fimbria (page 1165) and the hippocampus i l>at,r<- 1165), all seen within the lateral ventricle (page 1164), constitute important paths bv which fibres pass to and from the olfactory cortical cent re. The gyrus subcallosus (page 1153), the gyrus supracallosus (page 1157) and the gyrus dentatus (page 1166) together form an additional arched tract, which, beginning at the base of the brain, follows closely the convex surface of the corpus callosum as f; Atrophic ventricu- lar area Nerve-fibre layer Granular layer. Layer of mitral cells Molecular layer Olfactory glomeruli Blood-vessel Olfactory fibre EBBS layer «^s?^LJtii':-- .... - , ; — ---- -"-"A'^-""" Transverse section of olfactory bulb ; drawing includes part of bulb lying ventral to atrophic ventricular area. X 90. OLFACTORY LOBE II. Central Portion THE TELENCEPHALON. n55 as its hind-end and then, as the dentate gyrus, extends forward along the inner sur- face of the hippocampus to the uncus. The septum lucidum (page 1 159), a sickle- shaped partition which lies between the lateral ventricles, the corpus callosum and the fornix, is also a constituent of the olfactory path, as are also, probably, the tsenia semicircularis (page 1162) and the nucleus amygdalae (page 1172). In the foregoing description of the rhinencephalon only such parts have been included as seem warranted on morphological grounds (Turner, Elliot Smith and Cunningham ). It should be pointed out, however, that the German and French anatomists include also the limbic lobe, the division and constitution of the rhinencephalon accordingly being as follows : RHINENCEPHALON. I. Peripheral Portion A. Anterior part : 1. Bulbus olfactorius 2. Tractus olfactorius 3. Tuberculum olfactorium 4. Area parolfactoria B, Posterior part : 5. Substantia perforata anterior 6. Gyrus subcallosus 1. Gyrus callosus 2. Gyrus hippocampi 3. Gyrus uncinatus 4. Hippocampus 5. Gyrus dentatus 6. Gyrus supracallosus ARCHITECTURE OF THE CEREBRAL HEMISPHERES. On drawing apart the walls of the great longitudinal fissure, it will be seen that, while in front and behind this cleft completely separates the hemispheres, the latter are connected in the intervening part of their length by a robust commissure, the corpus callosum, which floors the fissure along the middle part of its course. On making sections of the hemisphere above the level of this bridge, either in the frontal or transverse plane, the hemibrain is found to be composed of the thin reddish brown sheet of cortical gray matter (substantia corticalis), which everywhere constitutes an unbroken stratum, and the enclosed large tract of white matter, the centrum ovale. Beneath the corpus callosum lies the lateral ventricle, the cavity enclosed within the hemisphere, in whose lateral wall and floor appears the mesial division of the corpus striatum, the caudate nucleus, whilst further outward is lodged the lateral division of the nuclear mass of the end-brain, the lenticular nucleus. Attached to the under surface of the posterior half of the corpus callosum is the arched layer of fibres known as theforni.v, and below the latter, covering to a large extent the upper surface of the thalamus which forms a part of the floor of the lateral ventricle, lies the thin highly vascular sheet, the velum interpositum. These and the other structures more or less closely related to the lateral ventricle claim fuller description, which may now be undertaken. The Corpus Callosum. — This structure is the great commissure which con- nects the hemispheres and, in addition, affords passage to fibres that arise from the thalamus and, probably, other nuclei outside the hemisphere and proceed to the cerebral cortex. It lies considerably nearer the anterior than the posterior end of the hemisphere and occupies approximately one half of the latter' s length. Seen in mesial sagittal section (Fig. 996), the corpus callosum appears as a robust arched structure, white in color and composed of nerve-fibres transversely cut, whose ends are considerably thicker than the intermediate portion, the body (truncus corporis callosi). Its upper surface is convex, partly free and partly covered by the overlying hemisphere, and its lower one is concave and, where not attached to the fornix and the septum lucidum, clothed by the ependyma lining the ventricle. Its length is about 7 cm. (2^ in.) and its greatest thickness, at its posterior extremity, is about 8 mm. It is widest behind, where it measures about 20 mm., and somewhat narrower in front. The thickened front end, the genu, bends backward and is 1156 HUMAN ANATOMY. prolonged into the sharply recurved and tapering rostrum, whose thin edge is continued backward and downward into the lamina cinerea, the attenuated anterior wall of the third ventricle (page 1132). The rounded and massive posterior end of the corpus callosum, known as the splenium, overlies the pineal body and the superior colliculi, and above bounds the cleft through which the pia mater gains the velum interpositum (page 1162). The convex upper surface of the corpus callosum, where it forms the bottom of the longitudinal fissure, is free, except behind where in contact with the posterior part of the falx cerebri ; laterally it is partially overlaid by the callosal gyrus, which, FIG. 996. Opening of cere- bral vein Mesial surface of cere bral hemisphere Lamina cine Optic chiasm, cut Inftmdlbutan. Pituitary body Tulier ' Interpedunc Mammillary body Oculomo Sphenoi Cerebral Pi Basilar artery Corpora quadrigemina Pon Aqueduct of Syl Superior nn ' ' ueduct of Sylvius S SS , i medullary velum '/S Fourth ventricle'/ / Choroida! plexus' / Right vertebral artery Mesial section of brain in showing relations to skull and dura ; cerebral falx has been partly removed, arachnoid and pia arc still in place. however, is separated from it by tin- intervening ra/Av^ / ,v/^vo (sulcus o>rpui is callosi ). Although consisting practically exclusively of transversely coursing nerve-til >res, which produce a corresponding cros-, striation, the upper surface' of the corpus callosum (Fig. 997; is covered by a thin atrophic layer of gray matter iiidusetim Uii-ciim i which laterally is continuous with the cortical substance of the callosal gyrus and contains rudimentary strands of longitudinal nerve-fibres. These are arranged on each side of the slight groove marking the mid-line in two strands ; tin- one, the stria medialis, is placed close to the strand of the opposite side and with it constitutes the so-called nerves of Lonrisi. The other strand, the stria lateralis, or tn-tiia l<-ct(t, lies farther outward and is covered by the overhanging callosal gyrus. These rudimentary structures, including the thin sheet of gray matter and the two THE TELENCEPHALON. "57 striae, represent an atrophic convolution, the gyrus supracallosus. Traced forward and around the recurved genu and rostrum, the mesial stria is prolonged into the gyrus subcallosus, a small crescentic cortical tract on the mesial surface of the hemisphere immediately below the rostrum (Fig. 994); while the lateral stria is continued into the area parolfactoria (page 1153) and into the anterior perforated space. When followed backward and around the splenium, the striae and gray matter of the corpus callosum become continuous with the gyrus dentatus and, by way of the latter, with the uncus. The under surface of the corpus callosum (Fig. 998) exhibits a very evident transverse striation and forms the roof of the anterior cornu and body of both lateral ventricles. With the exception of a strip of varying width along the mesial plane, where attached to the septum lucidum in front and to the triangular FIG. 997. Frontal pole Gen Mesial longitudinal strise Upper surface of corpus callosum Lateral longitudinal stria Forceps anterior Transverse fibres Tapetum Forceps posterior Splenium Occipital pole ; on left Cerebral hemispheres from which upper and median parts have been removed to expose corpus callosum ; side longitudinal stria and thin layer of gray matter cover upper surface of corpus callosum ; 01 have been scraped away to expose transverse fibres and anterior and posterior forceps. body of the fornix behind, the corpus callosum is free and covered with the ependyma which lines the ventricular spaces. In consequence of the bridge I shorter than the length of the hemispheres, from most parts of which it receives fibres, the latter are consolidated at the ends of the corpus callosum and give ns< the genu and the splenium. On gaining the lateral margins of the corpus callosum, its fibres are no longer restrained but radiate in all directions (radiatio corpons cal- losi) towards the cortex and intersect the fibres of the corona radiate (page 1186). Those traversing the thinner body and upper part of the splenium of the coi missure pass laterally and in each hemisphere from a thin but definite fibre-J known as the tapetum, which extends over the lateral ventricle, especially i posterior horn, and constitutes the lateral wall of its posterior cornu and adjacent part of the descending horn. The fibres composing the fore-part of genu turn forward as a distinct band, the forceps anterior, towards the 1 158 HUMAN ANATOMY. pole of the hemisphere, whilst those constituting the greater part of the splenium are consolidated into a robust strand, the forceps posterior, which sweeps abruptly backward into the occipital lobe and in its course produces a curved ridge on the fore-part of the inner wall of the posterior horn of the lateral ventricle. The Fornix. — The fornix is an arched structure, white in color, and composed, for the most part, of two crescentic tracts of longitudinally coursing nerve-fibres. The two ends of these narrow crescents are free for some distance, but along their medial borders the intervening parts are connected with the under surface of the cor- pus callosum and with each other (Fig. 998), thus producing a triangular field, the body (corpus fornicis), whose apex is directed forward and is prolonged into two slender diverging stalks, the anterior pillars, and whose lateral angles are con- tinued into the downwardly arching posterior pillars. The upper surface of the body is subdivided into an attached and an unattached area. The former is a small FIG. 998. Body of fornix Mamraillary bodi Splenium of corpus callosum Lvra Free margin of fornix t'nder surface of corpus callosum Cut surfaces of hemisphere Septum lucidum 'Anterior pillar of fornix T'tider surface of genu of corpus callosum Dissection of brain, showing under surface of fornix and corpus callosum. narrow triangle, the posterior and broader part of which corresponds with the attach- ment of the fornix to the under surface of the corpus callosum ; whilst the anterior part is a mere mesial strip denoting the line along which the arching fornix is blended with the septum lucidum, the sickle-shaped partition that fills the interval between the corpus callosum and the fornix and separates the anterior horns of the lateral ven- tricles. On either side of the attached field, the fornix presents a smooth and some- what thicker marginal zone, which forms part of the floor of the lateral ventricle and, depending upon the size and distention of the ventricular space, either extends later- ally as a horizontally directed wing that overlies a part of the thalamus, or descends obliquely towards the thalamus upon whose upper surface the margin of the fornix indirectly rests. The triangular central sheet of the fornix, bounded by its unattached margins laterally and the splenium behind, exhibits transverse striation due to the nee of bundles of commissural fibres connecting the hippocampi of the two sides. This part of the fornix constitutes the commissura hippocampi, also known as the psaltcn'um or lyra. A narrow horizontal cleft, the so-called ventricle of Verga (cavum THE TELENCEPHALON. 1159 psalterii), sometimes intervenes as the result of imperfect union, between the under surface of the corpus callosum and the middle part of the body of the fornix. It should be understood, however, that this cleft is not a part of the series of true ven- tricular spaces. The under surface of the fornix rests upon the velum interpositum, which thus separates it from the third ventricle and the upper surfaces of the two thalami which it overlies. The anterior pillars of the fornix (columnae fornicis) are two slender cylin- drical strands, which, slightly diverging as they leave the anterior angle of the body, arch downward and forward, then somewhat backward, and descend to the basal surface of the brain, where they end in the mammillary bodies. In their descent they lie in the extreme front part of the lateral walls of the third ventricle, where they show as ridges (Fig. 976), and form on each side, the upper and anterior boundary of the foramen of Monro. A short distance below the latter opening, the pillar disappears from the ventricular wall in consequence of the increasing divergence from the mesial plane. On reaching the mammillary body on the basal surface of the brain, the fibres composing the anterior pillar are interrupted to a large extent in the mammillary nuclei (Fig. 967). The connections of these stations are described elsewhere (page 1129), suffice it here to recall that while a part of their fibres are continued to lower levels, a very considerable strand, known as the bundle of Vicq d' Azyr, arches upward and completes the connection between the fornix and the thalamus, in the anterior part of which these mammillo-thalamic fibres end. The relations of the anterior pillars to the olfactory paths are noted in connection with the olfactory nerve (page 1222). The posterior pillars of the fornix (crura fornicis), the widely diverging backward prolongations from the lateral angles of its body, are at first attached to the under surface of the corpus callosum. They then turn outward, and, sweeping around the posterior ends of the optic thalami, enter the descending horns of the lateral ventricles and arch downward along the dorso-mesial border of the conspicu- ous hippocampi, the elevations which mark the inferior horns of the lateral ventricles. On reaching this situation, however, the posterior pillar no longer retains its previous form, but now appears much reduced in size, as a white flattened band, known as the fimbria, which, broadest in the middle of its course, narrows as it descends, and ends by joining the uncus at the lower extremity of the ventricle. The progressive diminution of the fimbria during its descent is due to the contribution of many of its fibres to the sheet of white matter, the alveus, which covers the hippocampus. It is evident that the fornix constitutes, by means of its several parts, a continuous tract of longitudinally coursing fibres, which convey impulses from the chief cortical olfac- tory centre, the uncus and the hippocampus, to the mammillary nuclei and thence, in great part, by the bundle of Vicq d' Azyr to the thalamus. The fornix may be considered, in a sense, as a tract of white matter representing the lower edge of the hemisphere ; in front and behind these edges remain ununited and more or less widely apart. Beneath the corpus callosum they become attached not only to the under surface of this bridge, but also to each other by the commissural fibres of the psalterium. The peculiar course of the fornix is referable to the backward and downward expansion of the developing hemispheres, as the result of which the posterior end of the fornix follows the hippocampus in its litgration into the descending horn of the lateral ventricle as the temporal lobe is devel- oped. Further consideration of these changes, however, may be deferred (page 1167) until the associated structures have been described in connection with the lateral ventricle. The Septum Lucidum. — The septum lucidum (septum pelluddum) is the thin median vertical partition which fills the interval between the corpus callosum above and in front and the fornix behind (Fig. 996), with which structures its margins are firmly attached. It separates the anterior horns and adjoining parts of the lateral ventricles and is, in a modified form, triangular in shape when viewed laterally. sides of the triangle are all curved and its anterior angle, received within the bend of the genu is blunt and rounded. Its posterior angle is narrow and extends for a variable distance between the under surface of the body of the corpus callosum and the upper arched surface of the body of the fornix. The lower an£ occupies the interval between the thin edge of the rostrum and the anterior pil n6o HUMAN ANATOMY. of the fornix. The septum consists of two thin layers (laminae septi pellucidi), between which lies a narrow cleft (cavum septi pellucidi) to which the misleading name, fifth ventricle, has long been applied. This space, very variable in extent and width, is usually so narrow and contains such a small quantity of modified lymph, that the laminae forming its walls are in apposition. It is entirely closed and, therefore, cut off from the true ventricular system ; neither is it lined with ependyma. The septum lucidum in man is the rudimentary representation of what in many of the lower (macrosmatic) animals is a much more important tract of cortical substance. In some animals, as for example, the rabbit, cat and dog, the septum is solid, a cleft never appearing within it. Notwithstanding the reduction which it has suffered in man, the septum exhibits in its structure its relation to the cortex, comprising, from its cleft outward : (i) a thin layer of nerve-fibres, (2) an uncertain layer of gray matter containing numerous nerve-cells of pyramidal form, and, next to the lateral ventricle, (3) a layer of nerve-fibres, the ventricular surface of which is clothed with FIG. 999. Body of fornix Corpus callosmn. upper surface / Anterior — pillar of fornix Mammil- la ry body the usual ependyma. It is probable that axones proceeding from the cells within the septum lucidum are constituents of the olfactory strands within the fornix, which pass to the hippocampus and the uncus, and of the tenia semicircularis (page 1162), terminating in the amygdaloid nucleus (page 1172). The Lateral Ven- tricles.— The lateral ventricles (ventricula late- rales) are a pair of irreg- ular cavities contained within the cerebral hemi- spheres. They are devel- oped as outpouchings from the original cavity of the end-brain and for a time communicate with this space by wide openings. The latter, however, fail to keep pace in their growth with the expansion of the hemispheres, and in the fully developed brain are represented by the small apertures of the foramen of Monro, which maintains communication between the lateral and third ventricles, the last- named space representing the primary, cavity of the fore-brain. When viewed from above, after removal of its roof, the corpus callosum and its lateral extensions, each lateral ventricle appears as an elongated, irregularly curved cavity (Fig. 1000), which extends for about two-thirds of the entire length of the hemisphere and, in addition, penetrates the temporal lobe almost to its pole. It is lined, as are all the other true ventricles, with a delicate epithelial layer, the '•/><• intv nut, which likewise clothes tin- structures which encroach upon its lumen, as the caudate nucleus and the thalamus, as well as those which seemingly hang free within it, as the choroid plexus and the fornix. It is usual to describe the ventricle nsisting of four parts, the body, and the anterior, posterior and inferior horus. The anterior horn and the body are practically one and separated by only an arbi- trary division ; the posterior and the inferior horn extend into the occipital and the temporal lobe respectively, whilst the anterior horn enters the frontal lobe. The anterior horn (cornu antcrius ) includes from the tip of the ventricle to the foramen of Moiiro, the latter corresponding with the anterior limit of the con- spicuous choroid plexus, curves forward and outward around the head of the caudate nucleus into the white substance of the frontal lobe and in frontal sections (Fig. rneus, partially cut away Dissection showing fornix in front and above ; drawn from preparation and Sieger model. THE TELENCEPHALON. 1161 1007) appears triangular in outline. The upper side or base of the triangle, slightly curved towards the ventricle, is the lower surface of the arched corpus callosum and its antero-lateral radiations ; the mesial side is approximately vertical and formed by the septum lucidum ; the lateral side bulges strongly towards the ventricle in correspondence with the convexity of the massive head of the caudate nucleus. The floor of this part of the ventricle is narrow, often a mere groove along the junction of the sloping lateral and vertical mesial wall, and in front passes insensibly into the concave anterior wall, formed by the lateral part of the hind surface of the genu of the corpus callosum. The body (pars centralis) of the lateral ventricle includes that part of the space which extends from the foramen of Monro to the bifurcation of the ventricle into its FIG. 1000. Corpus callosum Anterior horn of lateral ventricle Caudate nucleus, head Foramen of Monro Lenticula nucleus, sectioned Septum lucidum Cavity within septun ^ - — ' Fornix , anterior pillar 1 Choroid plexus, 4 In body of Hippocampus Collateral eminence Fimbria Posterior pillar of fimbria Collateral prorruber- ance in trigonum ventriculi Bulb of forceps posterior Calcar Posterior horn of lateral ventricle Posterior horn of lateral ventricle Lateral ventricles seen from above after partial removal of corpus callosum and cerebral hemispheres. posterior and inferior horns, opposite the splenium of the corpus callosum. When viewed in frontal sections (Fig. 1010), it appears as a narrow, obliquely horizontal cleft, directed somewhat upward, roofed in by the corpus callosum. Its mesial wall is formed in front by the hind part of the septum lucidum and behind the latter by the fornix where it is attached to the under surface of the corpus callosum. A distinct lateral wall is wanting, the ventricle being here closed by the meeting of the floor and roof. Its floor is constituted by several structures of importance which, named from without inward, are: (i) the caudate nucleus ; (2) an oblique groove (sulcus intermedius), which extends from before backward and outward, between the caudate nucleus and the thalamus, and lodges, in addition to the vein of the corpus striatum, a white band of nerve-fibres known as the itznia semicircularis ; (3) a narrow portion of the upper surface of the thalamus, which is Il62 HUMAN ANATOMY. nterior horn almost completely masked by the overlying choroid plexus; (4) the choroid plexus of the lateral ventricle ; and (5) the lateral edge of \hzfornix. The caudate nucleus will be subsequently described (page 1169), suffice it to note its rapid diminution in size, as it curves backward and downward on the roof of the inferior horn. The tasnia semicircularis is more or less hidden by the superficially placed vein of the corpus striatum (vena terminalis), which lies immediately beneath the epen- dyma and shows as a distinct sinuous ridge. Receiving tributaries from the adjacent parts of the thalamus, the caudate nucleus and the walls of the anterior horn, includ- ing the septum lucidum, the vein passes to the foramen of Monro, where, meeting with the choroid vein at the apex of the velum interpositum, it forms with the last- named vessel the vein of Galen. The taenia semicircularis, or stria terminalis, the band-like tract of nerve- fibres occupying the sulcus intermedius, is a part of the complex pathway by which the primary and secondary olfactory centres are united. Its component fibres arise partly in the anterior perforated space and partly in the septum lucidum from which centres, reinforced by fibres from the anterior commissure, they converge towards the sulcus intermedius which they FIG. 1001. then follow. After leaving the body of the lateral ventricle they descend with- in the roof of the inferior horn, in close relation to the recurved tail of the caudate nucleus, to end within the amygdaloid nucleus (page 1172). The choroid plexus ( plexus chorioideus ventriculi lateralis) is a convoluted vascular complex which occupies the lateral margin of the pial sheet, the velum interpositum, within the body of the lateral ventricle, and, in addition, descends along the inferior horn of the lateral ventricle to its tip. In order to understand the relations of the choroid plexus, those of the larger sheet, of which it is part, must be described. The velum interpositum (tela chnrioidca ventriculi tertii) is a delicate sheet of pia mater whose upper surface is exposed after removal of the corpus callosuin and the body of the fornix. When viewed from above (Fig. 1002) it is triangular in outline, its apex lying at the foramen of Monro and its lateral basal angles extending into the descending horns of the lateral ventricles. Its inferior surface forms the roof of the third ventricle, beyond which on each side it covers the greater part of the upper surface of the thalamus and, in turn, is overlaid by the fornix. Behind, the velum interpositum is continuous beneath the splenium of the corpus callosum with the pia mater investing the external surface of the hemisphere. This relation readily gives rise to the impression that the- pial tissue has gained entrance to the ventricles by growing forward through the cleft beneath the splenium and the fornix. That such, however, is not the case will be pointed out later, when the development of this sheet is considered (page 1194). The relation of the velum interpositum to the ventricular ravities should be carefully noted by tracing the ependyma from the caudate nucleus inward. Leaving the convex surface of this structure, the ventricular lining covers the sulcus terminalis with its vein, and passes for a short distance over the adjoining outer part of the upper surface of the thalamus. This zone (lamina affixa") narrows in front and behind, and where broadest measures from 5-7 mm. Along the Lateral recess Cast of ventricles, viewed from above. X Posterior horn (ftetzius.) THE TELENCEPHALON. 1163 inner margin of this zone the ependyma leaves the surface of the thalarnus and passes onto the villous projections (Fig. 1003) of pia mater containing the convolu- tions of blood-vessels of which the choroid plexus is composed. Each projection, (glomus chorioideum) consists of: (i) a capillary complex formed by the terminal twigs of the anterior and posterior choroidal arteries, the former of which gains the interior through the choroidal fissure in the inferior horn of the lateral ventricle ; (2) the connective tissue of the pia ; and (3) the ependymal layer (lamina chorioidea epithelialis), which everywhere invests the pial plications and, therefore, excludes the vascular tissue from actual entrance into the ventricular cavity. While inconspicuous and often overlooked, this ependymal layer is of much morphological significance, since it represents all that persists in certain localities of the true wall of the hemi- sphere. After leaving the surface of the thalarnus and investing the vascular pro- FIG. 1002. Septum lucidum Anterior end of fornix. cut Hippocampus Velum interpositum Choroid plexus in inferior horn of lateral ventricle Splenium, under surface Posterior horn of lateral •ventricle Lateral parts of fornix, under surface Corpus callosum Caudate nucleus Choroid plexus, overlying foramen of Monro Vein of corpus striatum Choroid vein in plexus Veins of Galen Crus of fornix and posterior forceps of corpus callosum, cut Under surface of fornix, Lyra Cut anterior end of fornix Dissection of brain, showing velum interpositum and choroid plexuses of lateral ventricles; seen from above after removal of corpus callosum and fornix; latter has been cut through in front and behind and turned back, exposing its under surface. jections constituting the choroidal plexus, the ependyma becomes attached along the taenia fornicis to the thin lateral margin of the fornix, beneath which the velum interpositum protrudes to expand into the choroid plexus within the body of the ventricle. The plexus is not confined to this part of the space, but follows the hippocampus to the lower end of the inferior horn. The relation of the vascular pial tissue to this extension of the ventricle is, however, the same as within the body, since the glomeruli here, as there, are completely invested by the ependyma, which they invaginate along a groove, the choroidal fissure, above the hippocampus, in the same manner as they do higher in the ventricle. The line of attachment of the ependyma to the wall of the horn, taenia fimbriae, follows the recurved tail of the caudate nucleus, just beneath which it lies, on the one hand, and the thin mesial edge of the fimbria (the continuation of the fornix) on the other. On pulling out 1164 HUMAN ANATOMY. the entire choroid plexus of the lateral ventricle, the ependyma is torn away and an artificial opening is produced, which may be followed, as a curved narrow cleft, from the lower end of the inferior horn upward above the hippocampus and over the dorsal surface of the thalamus, beneath the fornix and the splenium, to the exterior of the hemisphere. When traced forward from its attachment along the upper surface of the thalamus, the line of the reflection of the ependyma, taenia chorioidea, leads to just above the foramen of Monro (Fig. 966), where it is joined by the similar line of the opposite ventricle. From this point the choroidal line of ependymal reflection is continuous with the taenia thalami, the sharp ridge which marks the junction of the superior and mesial surface of the thalamus (page 1119). Leaving the surface of the latter along this ridge, the ependymal layer covers the under side of the velum interpositum, as well as the double row of vascular villous projections, which, one on each side of the mid-line of the roof, constitute the chproid plexus of the third ventricle (Fig. 974). Although similar in its general structure, this vascular fringe is much smaller and less conspicuous than that within the lateral ventricle. It is evident from the foregoing description, that communication between the third and lateral ventricles is completely interrupted by the attachment of the ependymal layer and that at only one place, the foramen of Monro (page 1060), does such communication exist. It is of interest to note that these several lines of ependymal reflection — the taenia chorioidea, the ta^nia thalami and the taenia fornicis and its prolongation, the taenia fimbme — form a contin- uous line which morphologically marks the transition of the thicker nervous part of the wall of the hemisphere into the thin and atrophic area, which early undergoes an invagination leading to the production of voluminous vascular structures later seen in the definite choroid plexuses of the lateral and third ventricles. Along the margin of the choroidal fissure, at which such invagination primarily occurs, the white matter of the hemisphere becomes condensed into the tract of the fornix and its downward prolongation, the fimbria. These structures, together with the reflected ependyma and the septum lucidum, are regarded, therefore, as modified parts of the mesial surface of the hemisphere. The inferior horn (cornu inferius), also called the descending horn, begins above at the hind-end of the body of the ventricle, thence curves backward and outward around the thalamus, and sweeps downward and forward and a little inward ( Fig. 1000) into the temporal lobe well towards its tip, which, however, it fails to reach by about 2 cm. Its descent is not FIG. 1003. on^y verv abrupt, but limited . _______^ for the most part to almost a C C vertical plane ; hence this part of the ventricle does not diverge to any considerable extent be- yond the plane of the gyrus hip- pocampi, just to the outer side of which the lower end of the inferior horn lies. The roof of this cornu is formed chiefly by the tapetum of the corpus cal- losum, and within it descend the recurved attenuated tail of the caudate nucleus and the tn-nia semicircularis to join a rounded mass of gray matter, the amyg- daloid nucleus (page 1172), which lies embedded within the temporal lobe, slightly above and in front of the lower end of the inferior horn (Fig. 967). The floor of the inferior horn begins above in the triangular area, the trigonum ventriculi, between the diverging inferior and posterior horns. The greater part of this field is occupied by a low convexity, the collateral protuberance (trinonum collaterale), which is continued into a rounded ridge, the collateral eminence (eminentia Taenia fornicis Choroid plexus of III ventricle Diagram showing relation of pial tissue in velum interpositum to ependyraa in l.itnal and third ventricle; epi-ndyma is represented by red line; c, c, corpus callosum; /•", fornix; vv, so-called ventricle of Verga ; C, T, caudate nucleus and thalamus. THE TELENCEPHALON. 1165 collateralis), that extends for a variable distance along the outer part of the floor of the inferior horn. This elevation is uncertain as to prominence and length, but even when well developed does not reach the lower extremity of the ventricle. It results from the invagination of the wall of the early hemisphere by the anterior part of the collateral fissure. A second longitudinal elevation, constant and much more conspicuous than the collateral eminence and separated from the latter by a groove, forms the inner part of the floor and the adjoining mesial wall of the inferior horn of the lateral ventricle. This elevation, known as the hippocampus, is the most prominent feature of the horn and curves downward and inward to the extreme lower limit of this part of the ventricle. It is due to the early invagination of the hemisphere by the hippocampal fissure. The lower end of the hippocampus is distinctly broader and somewhat flattened and marked by a number of oblique shallow furrows and intervening low radiating ridges (digitationes hippocampi). These confer on the upper surface and especially on the outer rounded border of the elevation, a corrugated and notched appearance, (Fig. 1004) which suggests a fancied resemblance to a paw, the lower end of the projection being known as the pes hippo- FIG. 1004. campi. The upper surface and the anterior and lateral border of the pes are free and well defined, but its deeper surface and inner border, to a large extent, are blended with the surround- ing parts of the hemisphere. The minute structure of the hippocampus is described with that of the cerebral cortex ( page 1 1 8 1 ) . The dorso-mesial aspect of the hippocampus is over- laid by a white flattened band, the fimbria (fimbria hippocampi), which, although bearing a special name, is the direct prolongation of the posterior crus of the fornix, continued from the lateral angle of the corpus fornicis into the inferior horn. Its mesial Pes hippocampi Cyrus hippocampi Fimbria ps posterior concave margin is Trijjonum Calcar collaterale avis Inferior horn of left lateral ventricle, viewed from above. smooth, rounded and free, whilst its sinuous lateral border is thin and sharp and gives attachment through- out its entire length to the delicate ependymal layer which completes the mesial wall and thus closes in the descending horn (Fig. 1005). Above narrow and then broader, on reaching the pes the fimbria becomes abruptly reduced to a narrow strand, which may be followed along the inner margin of the pes to the uncus where it ends. Traced upward the fimbria passes without interruption into the posterior limb of the fornix, of which, as already noted, it is the direct downward prolongation. Beginning in the uncus, the fimbria continually receives accessions of fibres from the underlying hippocampus, with which it is closely united along its deep surface, and therefore increases in bulk as it ascends towards the body of the fornix. When the structures within the inferior horn of the lateral ventricle are viewed in their undisturbed relations (Fig. 1002), little of the hippocampus and nothing of the fimbria are seen, as these parts are hidden by the overlying mass of vascular tissue constituting the choroid plexus, which is not confined to the body of the ventricle,' where its connections have been already described, but follows the descending ii66 HUMAN ANATOMY. horn to its lower end. On turning aside the vascular fringe, its relations to this part of the ventricle will be found to be identical with those exhibited in the body of the ventricle, since here, as there, the vascular complex is everywhere covered by the thin layer of reflected ependyma and, therefore, excluded from actual entrance into the ventricular space. Tracing the line of attachment of the reflected ependyma, which alone represents the true ventricular wall closing the crescentic choroidal fissure along the dorso-mesial aspect of the inferior horn, it will be found to be continuous with the thin lateral edge of the fimbria throughout the entire length of this attenuated margin, just as it is connected with the fornix within the body of the ventricle. Passing from this line of attachment ftaenia fimbrise) over all the villous projections of the choroid plexus, the reflected ependyma returns to the thicker ventricular wall, which it joins along the mesial border of the roof. Thence the ependyma remains in close contact with the remaining parts of the walls of the inferior horn, all the surfaces of which, including those formed by the hippocampus and the collateral eminence, it covers. From these relations (Fig. 1005) it follows that the fimbria in large part is excluded, as are some other parts of the fornix, from the ventricle, only that portion of its surface which extends from its sharp lateral border to the underlying hippocampus forming, strictly regarded, a part of the ventricular wall. The rounded mesial border and the dorsal surface of the fimbria belong to the free mesial surface of the hemisphere. The dentate gyrus (fascia dentata) is part of an atrophic convolution belong- ing to the rhinencephalon (page 1151), and as such belongs systematically to that division of the hemisphere. FIG. 1005. Choroid plexus Caudate nucleus, tail Taenia semicircularis Ependyma Cavity of inferio: horn of lateral v ventricle _ Entrance to . choroidal fissure \ Since, however, it is closely associated with the struc- tures found within the inferior horn of the lateral ventricle, its description has been de- ferred until this place. The dentate gyrus lies on the mesial surface of the hemi- sphere, but is so hidden be- hind the hippocampal gyrus that it is satisfactorily dis- played only after the over- hanging parts of the thala- mus and cerebral crura are removed. On cutting away these structures and drawing downward the hippocampal gyrus, a narrow band of gray matter, notched and corru- gated by numerous minute transverse furrows, is seen protruding between the free rounded mrsial border of the fimbria above and the hippocampal fissure below ( Fig. 992). This band is the gyrus dentatus. On examining frontal sections passing through the inferior horn of the lateral ventricle (Fig. 1005), the relations of the dentate gyrus will be appreciated. In such preparations the gyrus appears as the free, somewhat thinned off edge of cortical gray matter, which is pushed to the surface just below the choroidal fissure through which the- pial tissue invaginates the ventricular wall to gain a seeming entrance to the inferior horn. Between the fimbria, which lies immediately above and parallel with it, and the gyrus a shallow groove, the sn/i'i/s finthrio-dcntatns, intervenes, whilst below it is bounded by the remains of the kippocampal or utatr fissure. The latter is no longer an evident furrow, as it was when producing the hippocampus, since it has become closed and almost com- pletely obliterated 1>\- the apposition of the bordering cortex. Traced forward, the gyrus dentatus gradually leaves the fimbria and passes deeply along the inner side of the uncus in connection with which it ends. The terminal part of tin gyrus, somewhat reduced in size, at first bends sharply medially along imbria . __ Fimbrio-dentate ^ fissure Gyrus dentatus ^P^\. \ Hippocampal fissure ^^f^ \ \ Alveus *• ^_^/^'V \ \ \ Hippocampus Gyrus hippocampi Collateral fissure Frontal section of part of left hemisphere passing through lower end of inferior horn of lateral ventricle. X 2. THE TELENCEPHALON. n67 the under surface of the uncus and then winds over the inner aspect of the latter, from within outwards, as a narrow grayish band, the frenulum of Giacomini, which continuing upon the upper surface of the uncus, for a short distance passes slightly- backward and disappears (Fig. 1006). Followed backward, the gyrus dentatus accompanies the fimbria towards the splenium, at the lower border of which the two structures part company, the fimbria passing to the under side of the corpus callosum, whilst the gyrus dentatus, losing its corrugations and becoming a smooth band, known as the fasciola cinerea, bends backward and curves around the splenium (Fig. 992) to spread out over the upper surface of the corpus callosum as the thin atrophic sheet of gray matter, the induseum griseum in which are embedded the fibre-strands of the longitudinal striae (page 1156). The structure of the gyrus dentatus is described with that of other parts of the cerebral cortex (page 1182). FIG. 1006. -Splenium of corpus callosum -Uncus Frenr1nm of Giacomini Fasciola cinerea Gyrus hippocampi \ Collateral fissure Gyrus dentatus Part of left gyrus hippocampi has been cut away to expose gyrus dentatus, which is seen continuing as frenulum of Giacomini over uncus. The fornix is to be regarded as the chief fibre-tract connecting the olfactory cortex, situated within the uncus and the hippocampus, with the thalamus. An explanation of its remarkable course as seen in the adult brain, is found in the changes which affect the position of the hippo- campus during development. Reference to Figs. 1030, 1032, will recall the origin of the hemi- sphere (pallium) as an outgrowth from the end-brain, and, further, that the hemisphere in man early covers in the thalamus and other parts of the diencephalon and the mid-brain. For a time the thalamus is connected with the hemisphere by means of only the thin recurved under and inner wall of the pallium, the bulky tracts of white matter in which it is later embedded being for a time wanting. This same independence is retained by the thalamus, even in the adult condition, on its upper and posterior aspects, where the excessively thinned out ventricular wall alone forms the partition between the ventricle and the exterior, and where the thalamus is over- laid by, but not in contact with, the hemisphere. On breaking through this partition, as after • removal of the velum interpositum, the thalamus may be directly reached by passing beneath the splenium. When a definite mesial surface of the hemisphere becomes developed, an area along the inferior margin of this aspect becomes marked off by two primary grooves, which are the early choroidal fissure below and the hippocampal fissure above. The area so defined is the primary gyrus dentatus. This tract of gray matter is connected with the thalamus by the fornix, which reaches the thalamus around the front end of the choroidal fissure. In many animals, as in the rabbit, a similar relation is permanently retained, the dentate gyrus, or its equivalent, the hippocampus, being united with the thalamus by a fornix-tract which sweeps from the lower and posterior part of the pallium (hippocampus) over the roof of the third ventricle forward and downward to the basal surface of the brain (mammillary body) and thence by the bundle of Vicq cT Azyr to the thalamus. These primary relations are changed by the future expansion of the hemisphere, which grows not only upward and backward, but also downward to form the temporal lobe, in consequence of which the dentate gyrus and the fornix, and likewise the choroid plexus and its fissure, are carried backward, downward and forward around the thalamus into the temporal lobe, where they lie on the mesial wall of the descending horn of the lateral ventricle which has coincidently been formed. Whilst in this manner the chief mass of the primary gyrus dentatus is carried into the temporal lobe, where it becomes the hippocampus and n68 Iir.MAX ANATOMY. the definite dentate gyms, a part of it, greatly attenuated and reduced, retains its connection with tin- anterior basal surface of tin.- brain (later the anterior perforated substance) and follows the upper surface of the corpus callosum, which likewise has extended backward, into the descend- ing horn of the lateral ventricle. These parts— the gyrus subcallosus, the longitudinal stria, the fasciola cinerea and the gyrus dentatus of the adult brain— constitute the supracallosal gyrus, whose gray matter is an atrophic outlying part of the primary gyrus dentatus and whose con- nections with the basal olfactory centres are retained by the fibres of the longitudinal striae. The fornix shares the displacement of its cortical area, the hippocampus, and is consequently carried with the latter into the descending horn of the lateral ventricle. In this manner parts which at first lay in proximity and were connected by short paths, become widely separated .with corresponding lengthening of the fibre-tracts uniting them, as illustrated in the long course of the fornix in the adult brain. Further, since the path of migration of the fornix and associated structures of the inferior horn of the lateral ventricle describes a curve, it follows that the relations of these parts become reversed, those originally lying above, in regard to adjacent structures, within the descending horn being below and vice versa. ^ The posterior horn of the lateral ventricle (cornu posterius), much smaller than either of the others, is an elongated diverticulum which curves backward from FIG. 1007. Superior frontal gyrus Middle frontal gyrus Longitudinal fissure— Genu of corpus callosum. Lateral ventricle, anterior horn- Inferior frontal gyrus Caudate nucleus, head Orbital gyri Frontal section of brain passing through genu of corpus callosum. the body of the ventricle into the occipital lobe. In frontal sections ( Fi^- 1034) its form is irregularly crescentic, 'the convexity of its outline including the roof and tin- Liter, il wall and the concavity corresponding with the nu-sial wall and narrow floor. Above and to the outer side, the horn is bounded by the arching fibres of the tape- tinn of the corpus callosum, lateral to which lies the important thalamo-orcipital Of optic radiation ( pa^e 112$). The lower part of the mesial wall is modelled t Fi.y. 1000) by a narrow but well marked crescentic elevation, the calcar avis, also railed the hippocampus minor, \vhirh is produced by the early imagination of the wall of the hemisphere by the anterior part of the calrarine fissure. On the same wall and just aoove the calcar avis, a second and broader, but less sharply defined, elevation (bullms conni posteriori* ), marks tin- course of the fibres of the fo|, .-ps posterior as they eneirrle the paricto-oeripital fissure in their journey to the occipital lobe. THE TELENCEPHALON. 1169 THE INTERNAL NUCLEI OF THE HEMISPHERE. Embedded within the white matter of each hemisphere and, for the most part, completely separated from the cerebral cortex, lie certain masses of gray matter to which the name basal ganglia is often applied. These include : (i) the caudate nucleus, (2) the lenticular nucleus, (3) the daustrum and (4) the amygdaloid nucleus. The first two, the caudate and lenticular nuclei, are parts of the corpus striatum, one of the three fundamental divisions of the end-brain or telencephalon. Although almost completely separated by the intervening tract of white matter, the internal capsule, the caudate and lenticular nuclei are continuous for a limited distance below and in front (Fig. 1008), and together constitute a large mass composed chiefly of gray matter, that extends from the lateral ventricle almost to the cortex of the insula. Between the latter and the lenticular nucleus lies a thin tract of gray matter, the daustrum, whilst within the temporal lobe, above and in front of the anterior extremity of the inferior horn of the lateral ventricle, is situated the amygdaloid mideus. The Caudate Nucleus. — This mass (nucleus caudatus), the inner division of the corpus striatum, is well seen from the lateral ventricle, where it appears as the large and conspicuous elevation which contributes the infero-lateral wall of the anterior horn, and the outer part of the floor of the body of the ventricle. The caudate nucleus is an elongated pyriform or comet-shaped mass of gray matter, whose bulky rounded anterior end or head (caput nuclei caudati) rapidly diminishes into the attenuated and recurved tail (cauda nuclei caudati), which sweeps backward and then downward and forward within the roof of the inferior horn to the tip of the temporal lobe, where it ends in relation with the lower part of the amygdaloid nucleus. The relations of its two chief surfaces, the mesial and lateral, are best seen in frontal sections. When sectioned through its head near the anterior pole (Fig. 1007), the caudate nucleus appears as an ovoid area of gray matter which mesially bulges strongly into the lateral ventricle, but from which it is separated by the ependyma, . and laterally is embedded within the white matter of the hemisphere. In sections passing a few millimeters farther back (Fig. 1009), the form of the nucleus has become somewhat changed, its inner convex surface being more extensive and its outer one, now somewhat concave, being serrated by the invasion of obliquely hori- zontal stripes of white matter due to the appearance of the anterior strands of the internal capsule. In the plane under consideration, these strands are not continuous but interspersed with stripes of gray matter, which below still connect the caudate with the laterally situated lenticular nucleus and produce the coarse striation from which the entire mass, the corpus striatum, derives its name. In sections passing through the body. of the ventricle (Figs. 1010, 1025), from the plane of the foramina of Monro backward, the caudate nucleus is much reduced in size, whilst, on the contrary, the lenticular nucleus, as well as the thalamus, become more conspicuous. The internal capsule, being now well established, appears as a large oblique tract of white matter, which completely separates the two parts of the corpus striatum and lies to the outer side of the thalamus (Fig. 1008). By reason of the recurved course of its attenuated tail, in horizontal sections, as well as in frontal ones passing in front of the splenium, the caudate nucleus is twice cut, one cross- section of the nucleus appearing above in the lateral wall of the body of the ventricle and the other in the roof of the inferior horn (Fig. 967). The Lenticular Nucleus. — This division of the corpus striatum (nucleus len- tiformis) is a wedge-shaped mass of gray matter, broken by laminae of white, that lies bordered by the internal capsule mesially, and laterally is separated from the cortex of the insula by a narrow tract of white matter containing a thin stratum of gray sub- stance, the claustrum. The lenticular nucleus reaches neither as far forward nor as high as the caudate nucleus, and lies lateral to both the latter and the thalamus, separated from them respectively by the anterior and posterior limbs of the internal capsule. Its dorso-mesial surface, when seen in frontal sections, is directed from above downward and inward ; in transverse sections (Fig. ion) this surface is replaced by an antero-mesial and a postero-mesial face in correspondence with the limbs of the internal capsule. Its slightly convex lateral surface is approximately 74 1 1 jo HIM AN ANATOMY. FIG. 1008. Thalamus Caudate nucleus vertical and in immediate contact with a thin sheet of white matter, the external capsule, which separates the nucleus from the claustrum. Its ventral surface is hori- zontal and only feebly curved and is continuous in front with the caudate nucleus and farther backward, about its middle, with the anterior perforated substance on the basal surface of the brain. The lenticular nucleus is unequally subdivided by two thin concentric sheets of white matter, the ex- ternal and internal medullary laminae, into three segments. The outer of these, the putamen, is much the largest and occupies the base of the nucleus, being bounded by the external capsule laterally and by the external medullary lamina mesially. Of its two somewhat rounded ends, the anterior is the broader and extends farther forward and alone joins the caudate nucleus of which it morphologically is a part (page 1169). The putamen is the most conspicuous part of the lenticular nucleus, not only on account of its size but also by reason of its darker color, in which respect it corresponds with the caudate nucleus. This contrast depends less upon the actual pigmentation of the cells of the putamen than upon the lighter color of the other zones of the nucleus. In consequence of the small number of fibres entering the external capsule from the putamen, the attachment between the latter and the capsule is relatively loose and the two structures may be Lenticular f Tail of caudate nucleus nucleus Reconstruction of corpus striatum and thala- mus ; lateral aspect ; probe lies in space occupied by internal capsule. Drawn from Steger model. FIG. 1009. Corpus callosum Septum lucidum Right lateral ventricle, anterior horn Internal orbital gyms Superior frontal gyrus Middle frontal gyrus Inferior frontal gyrus Caudate nucleus Internal capsule Lenticular nucleus Temporal lobe Continuity of caudate and lenticular nuclei Frontal section of brain passing through anterior end of corpus striatum where caudate and lenticular nuclei are continuous below. readily separated. This condition influences the course taken by extravasations of blood, which an frequent in this locality and may occupy a large part of the lateral surface of the putamen. The remaining divisions of the lenticular nucleus are much lighter in tint and together constitute the globus pallidus. They are subdivided THE TELENCEPHALON. 171 by the internal medullary lamina and form the edge of the wedge, lying in contact with the internal capsule. Although composed chiefly of gray matter, all these segments of the nucleus, but particulary the inner two, are traversed by numerous strands of nerve-fibres which break the continuity of the gray substance and produce an appearance of radial striation. The structure of the corpus striatum varies in its several parts, that of the caudate nucleus and the putamen being almost identical, whilst that of the globus pallidus, although similar in both zones, differs from the histological make up of the other parts. The close resemblance of the caudate nucleus and the putamen corre- sponds to their early common origin, since at first they constitute a single mass and become partially separated by the ingrowth of the fibres forming the anterior part of the internal capsule. The caudate nucleus is invested throughout the greater part of its periphery by a dense layer of fibres, the stratum zonale, which includes fibres passing both to FIG. 1010. Corpus callosui Choroid plexus Fornix Thalamus, mesial nucleus Thalamus, lateral nucleus Mammillo- thalamic tract Third ventricle Anterior pillar of fornix Optic tract Caudate nucleus Internal capsule Lenticular nucleus, putameu Insula Globus pallidus Claustrum Amygdaloid nucleus Pituitary body Optic nerve Frontal section of brain passing through caudate and lenticular nuclei and thalamus, showing relation of internal capsule to internal nuclei. and from the nucleus. The nerve-cells are, for the most part, rather small in size and stellate or fusiform in shape and provided with numerous dendrites beset with minute irregularities. They are chiefly cells of type I, although many of the second type are encountered, whose axones are limited to the gray matter and are not prolonged as nerve-fibres (Kolliker). The putamen is invested on its two sides, particularly on the mesial one, with a fibre-layer derived from the external medullary lamina and the external capsule, the fibres being chiefly such as enter the nucleus from other centres by way of the med- ullary layer. In addition to nerve-cells of round or stellate form, Kolliker describes those of distinctive appearance possessing a slender fusiform body and dendrites few in number but of unusual length. The globus pallidus owes its characteristic color to the light yellowish tint of the pigment within its cells and to the large number of medullated nerve-fibres which traverse its substance, especially its inner zone. The nerve-cells are mostly small and stellate, possessing numerous short but richly branched dendrites. 1 1 72 HUMAN ANATOMY. The Connections of the Corpus Striatum. — Much uncertainty prevails as to the details of the connections of the several parts of the corpus striatum and little is known regarding the function of these nuclei, notwithstanding their size ; certain general principles, however, may be accepted as established. The comparative studies of Gehuchten, Sala and others, and especially of Edinger, emphasize that the corpus striatum is to be considered as supplemental to the cortical substance, in the lower vertebrates in which the cortex of the cerebral mantle is feebly developed constituting the chief mass of cortical gray matter, and in the mammals and man being subservient to the overshadowing cortex of the hemisphere. Such being the warranted presumption, it is to be anticipated that the striate body both receives fibres conveying sensory impulses and gives off fibres (perhaps motor in function) originating from its cells, these latter tracts constituting the strio-thalainic radiation. The centripetal or afferent paths probably include : (i) the tegmento-striate fibres, which are continued chiefly from the mesial fillet, and perhaps also from the red nucleus and subthal- amic region, by way of the internal capsule, to end around the cells of the putamen and head of the caudate nucleus ; (2) the thalatno-striaic fibres, already mentioned in connection with the thalamus (page 1 123), which pass from the thalamus either by way of the internal capsule directly to the caudate nucleus, or by way of the ansa lenticularis to the putamen or, traversing the medullary laminae, to the caudate nucleus. No doubt many of the fibres which enter the lenticular nucleus do not end within the latter, but traverse its substance as part of their path to the cerebral cortex. The centrifugal, or efferent fibres, which arise from the cells of the corpus striatum include : (i) the strio-thalamic fibres, passing from the major divisions of the striate body, which comprise (a) those from the caudate nucleus to the thalamus direct ; (£) those which traverse the internal capsule and the medullary laminae and, joining fibres from the putamen, pass by way of the ansa lenticularis to the thalamus ; (c) those from the putamen which reach the thalamus by passing partly by way of the globus pallidus and partly, in greater numbers, by means of the ansa lenticularis. (2) Strio-peduncular fibres, well represented in the brains of the lower animals as the continuation of the basal tract of the fore-brain (Edinger), which pass from the caudate nucleus, and probably from the lenticular nucleus also, into the sub-thalamic region and the cerebral peduncle, within the latter forming the stratum inter- medium closely related to the substantia nigra. Whether cortico-striate fibres, extending from the cerebral cortex to the corpus striatum, exist in man is uncertain, Dejerine denying their presence, whilst Edinger regards the presence of a meagre number of such bundles as established. The Claustrum. — The claustrum is a thin lamina of gray substance embedded within the white matter intervening between the lateral surface of the putamen and the cortex of the island of Reil. Its mesial surface is smooth and parallel with the outer aspect of the putamen, from which it is separated by the thin tract of white matter constituting the external capsule. Its lateral surface presents a series of elevations and depressions which in a general way repeat the contour of the gray cortical lamina of the insula, the intervening layer of white matter being sometimes called the capsula extrema. Seen in horizontal sections (Fig. ion), the claustrum fades away both in front and behind ; in frontal sections (Fig. 1010), however, whilst it gradually disappears above, below the claustrum materially thickens and mesially becomes continuous with the anterior perforated substance. Upon comparative and developmental grounds, the claustrum must be regarded as a separated portion of the corpus striatum. Its nerve-cells are, for the most part, small and either stellate or fusiform in outline. Nothing is known with certainty as to the course or connection of its fibres. The Amygdaloid Nucleus. — This structure (nucleus aniyudalae) comprises a considerable rounded mass of gray substance (Fig. IOIO) which occupies the fore-part of the temporal lobe and lies in close proximity with the unrus, overlying the extremity of the inferior horn of the lateral ventricle. Anteriorly it is continuous with the cortical gray matter of the temporal lobe as a thickened portion of which it may be regarded. Its lower ji.m n-ci-ives the tail of the caudate nucleus and close to this, the ta nia s«-micircularis (page 1162), which accompanies the recurved nuclear tail in its descent within the roof of the inferior horn. The nucleus approaches, if indeed it docs not touch, tin- anterior perforated substance, and above comes into intimate relations with the lenticular nucleus. It is highly probable that the nucleus amygdala- forms, along with the uncus and the hippocampus, a part of the olfactory cortex (Dejerine \. THE TELENCEPHALON. "73 The Internal Capsule. — Repeated mention has been made of the important tract of white matter bearing the name of internal capsule (capsula interna) ; its description, therefore, may be appropriately undertaken at this place. It is a broad, compact band of nerve-fibres which passes between the three large basal ganglia, namely, the caudate and the lenticular nuclei and the thalamus. Although the details of the internal capsule vary with differences both of direction and of position of the FIG. ion. Corpus callosum Splenium of corpus callosi Choroid plexus in inferior horn of lateral ventricle Horizontal sections of brain, A at higher level than B, which passes through lower part of corpus striatum where caudate and lenticular nuclei are continuous; relations of limbs of internal capsule to internal nuclei seen on right side. planes of section, its general relation to these three masses of gray matter is con- stant, the caudate nucleus and the thalamus always lying to its inner side and the lenticular nucleus to its outer aspect. When exposed by frontal sections passing through the anterior part of the lateral ventricles (Fig. 1010), the internal capsule appears as a broad, oblique stripe, extending from above downward and inward, bounded by the large caudate nucleus mesially, the lenticular nucleus laterally, and below by the gray substance establishing continuity between the two nuclei. H74 HUMAN ANATOMY. FIG. 1012. Seen in frontal sections passing some distance behind the preceding section, whilst the capsule is limited laterally by the lenticular nucleus, its mesial boundary now includes the caudate nucleus, the taenia semicircularis and the thalamus. Still farther back (Fig. 968), the internal capsule is bounded internally in addition by the subthalamic structures and becomes continuous below with the crusta of the cere- bral peduncle. An upper and a lower part of the capsule are therefore recognized, the former — between the lenticular nucleus on the one side, and the caudate nucleus on the other — is known as the thalamic region (regie thalamica capsulae internae), whilst that between the lenticular nucleus and the subthalamic structures is termed the subthalamic region (regio subthalamica). Viewed in horizontal sections (Fig. ion, A), the capsule appears not only much more extensive, but is seen to consist of two mesially converging parts, a shorter anterior limb (pars frontalis) and a longer posterior limb (pars occipitalis). The two limbs form an angle which opens outward and encloses on two sides the gray triangle of the lenticular nucleus. The junction of the two mesially converging limbs forms the knee, or genu, of the internal capsule which points inward and lies opposite the taenia semicircularis, between the caudate nucleus and the thalamus. At deeper planes (Fig. ion, .#), passing through the level of the continuity between the two parts of the corpus striatum, the anterior limb is greatly reduced in length or entirely disappears, the posterior one being prolonged into the cerebral peduncle. The importance of the internal capsule will be appreciated when its function as the great pathway connecting the cerebral cortex with the lower lying centres is recalled. Its fibres, both corticipetal and corticifugal, after passing beyond, or before coming under the restraint of the boundaries of the capsule, as the case may be, radiate to and from all parts of the hemisphere, and in this manner form the striking fan-shaped fibre-mass known as the corona radiata, which continues the internal capsule upward to the cerebral cortex. The radiating strands of this great tract interlace with the radiation of the corpus callosum and thereby contribute a large part of the fibres composing the oval centre of white matter within the hemisphere. The anterior limb of the internal capsule (pars lenticulocau- data) includes the front third of the tract and extends from the genu forward and outward. It contains fibres passing both toward and away from the cortex. Its corticipetal fibres are : 1 i ) the thalaiiio-frontal, which pass from the thalamus by way of its frontal stalk through the anterior limb of the internal cap- sule and the corona radiata to the cortex of the frontal lobe ; (2) the thalamo-striate, which also pass from the thalamus into the internal capsule and proceed to the caudate and lenticular nuclei. The corticifugal fibres include : (i) tt\?.fronto-pontile> which arise in the cortex of the frontal lobe and descend by way of the corona radiata, the anterior limb of the internal capsule, the crusta of the cerebral peduncle and the ventral tracts of the pons to end around the cells of the pontile nucleus as links in the connection between the cerebral and tin- cere- bellar cortex (page 1094) ; (2) the fronto-f/iatainic, which extend from the cortex of the frontal lobe to the thalamus; and (3) the sfrio-thalamic, which proceed from the caudate and lenticular nuck-i to the thalamus. The posterior limb of the internal capsule (pars Icmicnlo- tlialamieal extends backward, outward and downward from the genu, and includes the remaining two-thirds of the tract Its hind part extends beyond the posterior limit of the lenticular nucleus, hence the posterior limb is subdivided into a lottifuliir and a n-{rolfnticnl«r portion. As does the anterior limb, so also does the posterior limb of the capsule contain both rorticipeta! and corticifugal fibres. The lenticular portion includes corticipetal fibres: ( i ) the fhalamo-cortical, which issue from the lateral and lower aspect of the thalamus, traverse the internal capsule and to a considerable Diagram showing relative posi- tions of chief tiai Is in internal cap- sule l.li .mil in i i list. i of cerebral peduncle (/>'); f-T, fronto-thal.i- tnic; F-P, frqnto-pontile ; T-O-P, temporo-occipito-pontile ; C-ff, cor- tico-lnilliar ; C-S, cortico-spinal ; S, tegmental sensory ; ('A', optic ra- diation. THE TELENCEPHALON. number, the lenticular nucleus and the external capsule and proceed to the cortex of the hind part of the frontal and of the parietal lobe; and (2) probably some thalamo-lcnticnlar fibres which pass from the thalamus to the lenticular and, perhaps, the caudate nucleus. The corticifugal fibres include : ( i ) the important motor cortico-bulbar and cortico- spinal tracts, collectively often called the pyramidal tracts, which descend from the precentral (Rolandic) cortical region through the corona radiata and the fore-part of the posterior limb of the internal capsule into the crusta of the cerebral peduncle and thence to the appro- priate levels of the brain-stem or of the spinal cord. A tract supplementary to the pyramidal motor paths, the cortico-rubral fibres, must be mentioned. These arise from the cortex (perhaps of the parietal lobe) and descend through the lenticular portion of the posterior limb to the mid-brain where they end in relation with the red nucleus. (2) The cortico- thalainic fibres, which converge from the cerebral cortex to the thalamus. The retro- lenticular portion of the posterior limb is traversed by important corticipetal fibres con- cerned in conveying impressions of special sense, as ( i ) those of the optic radiation, which, issuing as the occipital stalk, connect the thalamus and the lateral geniculate and the superior quadrigeminal body with the occipital cortex ; and (2) those of the auditory radiation, which link together the mesial geniculate and the inferior quadrigeminal body with the auditory cortical area in the temporal lobe. The corticifugal fibres are represented by (i) the temporo-occipito-pontile tracts, which pass from the cerebral cortex through the retrolenticular portion of the capsule into the crusta of the cerebral peduncle and thence to the pontile nucleus within the ventral part of the pons ; and (2) cortico-thalamic fibres, which course in reverse order through the optic radiation to end within the thalamus and lateral geniculate body. The relative positions of the longer tracts composing the internal capsule, as seen in hori- zontal sections, are, in a general way, indicated schematically in Fig. 1012. The anterior limb is shared, from before backward, by the fronto-thalamic and the fronto-pontile tracts in the order named. The genu is appropriated by the cortico-bulbar tracts, the facial fibres lying immediately in advance of the hypoglossal. The succeeding part of the posterior limb, approximately one-third, affords passage to the cortico-spinal or pyramidal tracts. Next follows a narrow segment devoted to the tegmental sensory tracts, behind which the occipito-temporo- pontile tract occupies a small area, the last part of the retrolenticular field being taken up by the optic radiation. STRUCTURE OF THE CEREBRAL CORTEX. FIG. 1013. The surface of the hemispheres is everywhere clothed with a thin continuous stratum of cortical gray matter, which encloses the white medullary substance com- posed of the interlacing tracts of nerve-fibres. This cortical sheet varies in thick- ness not only in the same area, being thicker over the summit than at the sides of the convolutions or at the bottom of the bounding fissures, but in different regions of the hemisphere. Its average thickness is about 3 mm., but where it borders the upper end of the Rolandic fissure, particularly in the paracentral lobule, this increases to over 5 mm., whilst over the frontal and occipital poles the thickness of the cortex is reduced to almost 2 mm. The entire superficial extent of the cortex of the two hemi- spheres has been estimated to be about 2000 sq. cm. , of which scarcely one- third is exposed surface, the remainder calcarine fissure Stratum zonale- Externalgray stratum- Outer stripe of Baillarger, (Stripe of Gcnnari) Internal gray stratum Medullary fibres.'^ Frontal section of hemisphere including cortex sur- frp^h rounding calcarine fissure; stripe of Gennari (outer stripe of Baillarger) is here unusually distinct. X 3- being sunken. On examining sections of brain, the cortex does not appear uniformly tinted, but exhibits, even to the unaided eye, an indistinct division into alternate light and dark layers. From without in these are: (i) a thin peripheral layer of whitish color, the stratum zonale; (2) a thicker layer of grayish hue, the external gray stratum ; (3) a thin lighter band, the outer stripe of Baillarger ; and (4) a somewhat broader, yellowish-red zone, the internal gray 1176 HUMAN ANATOMY. FIG. 1014. stratum — four layers being more or less clearly recognizable. In certain localities, as in the precentral convolution, the inner gray lamina is subdivided by an additional white line, the inner stripe of Baillarger. In the vicinity of the calcarine fissure, particularly in the adjacent part of the cuneus, the outer stripe of Baillarger, whilst narrow, is unusually distinct and confers, therefore, a character- istic appearance upon the cortex of this region (Fig. 1013). The band in this location receives the name of the stripe of Gennari, or the stripe of Vicq (f Azyr. In recognition of the priority of description, Gennari' s name is sometimes applied to the external stripe of Baillarger wherever found. The significance of these light colored strata will be pointed out in connection with the intimate structure of the cortex, suffice it here to note that the stripes of Baillarger correspond to zones in which the felt-work of horizontal cell-processes is unusually dense, the stratum zonale corresponding to a compact layer of fibres running parallel with the surface. Occasionally a condensation of tangential fibres immediately beneath the stratum zonale produces the appearance of an additional light line, which in honor of its discoverer, is known as the stripe of Bechterew. The essential histological elements of the cerebral cortex are the nerve-cells and the nerve-fibres. The importance of the former is evident when their three-fold activity is recalled — ( i ) as receptors of corticipetal impulses, (2) as distributors of the impressions so received to other parts of the brain, and (3) as originators of corticifugal impulses which control the nuclei from which immediately arise the motor nerves. No single method of preparation suffices to display satis- factorily both groups of structural elements, for when stains are employed which best bring out the cells, the fibres are inadequately shown ; and, conversely, when methods adapted for the demonstration of the fibres are followed, the cells are but imperfectly displayed. It is advantageous, there- fore, to study the histological details of the brain by more than a single method, combining the results ob- tained by the use of cellular stains with those yielded by procedures ex- hibiting the fibres. Among the latter, the well known method of Weigert, or its modifications, has been of great service in extending our knowledge concerning the various fibre-tracts. The methods of silver impregnation introduced by Golgi, although not producing true staining but only in- crustations on the cell and its pro- cesses, have materially advanced our knowledge concerning the form of the cell-bodies and the number and extent iii i k,v .1 1 n i .-Mil. ill I i \ I ,i 1 1 1 n 1.1 i 1*11-1, i , j " u > i ii' >i i ii i n i i- 1 in , _ - - i of Martinotii; A. cell of type II; F, association Of the prOCCSSCS of the neurones. ci II; /, /, corticipetal fibres; f, 2, corticifugal fibres WU.'lof mt-iri'nn- oc rr» rWaik in ( axones of pyramidal cells) ; N, N, neuroglia cells. Whilst varying 3S t different regions, the cerebral cortex presents a general plan of structure which may be considered: («) in relation to the nerve-cells and ( />} in relation to the nerve-fibres. The Nerve-Cells of the Cortex. — When sections cut perpendicular to the surface of the convolution are stained with basic stains (Fig. 1015) or prepared after silver impregnation (Fig. 1016), the cerebral cortex exhibits four layers, Subpial layer Tangential fibres Stratum zonale Layer of small pyramidal cells Outer stripe of Baillarger Layer of large pyramidal cells Layer of poly- morphic cells Medullary fibres i. im showing constituents of cerebral cortex; cells in the right half, lihres in left half of figure \ A, ft, large and small pvi.miidal cells; C polymorphic cells; THE TELENCEPHALON. 1177 FIG. 1015. which, from without inward, are : (i) the stratum zonale, (2) the layer of small pyramidal cells, (3) the layer of large pyramidal cells, and (4) the layer of poly- morphic cells. Although each presents characteristics which are distinctive, with the exception of the junction between the first and second layers where the change is well defined, no sharp demarcation separates the strata, each passing insensibly into the adjoining layer. Neither are the modifications which distinguish the cortex of certain regions abruptly assumed, one type of cortical structure being gradually replaced by another without sudden transition. The stratum zonale, also known as the molecular stratum, underlies the pia and measures about .25 mm. in thickness. The layer contains few nerve-cells and appears subdivided into (a) a narrow peripheral zone, from .010 — .030 mm. in width> composed of a subpial condensation of neuroglia and (^) a deeper zone characterized by numer- ous fibres or processes, which course parallel to the surface, and a meagre number of nerve-cells whose most distinctive representatives are small fusiform elements (Cajat's cells") provided with long tangentially directed processes. The latter give off short collaterals, which ascend towards the surface, and intermingle with the number- less terminal filaments derived from the periph- erally coursing processes of the pyramidal and other cells lying at deeper levels and from the corticipetal fibres which continue from the white core of the gyrus into the outermost layer of the cortex. The layer of small pyramidal cells is marked off from the stratum zonale, which it about equals in thickness, with some distinctness since, in contrast to the last-mentioned zone, it contains very many cells. These, as indicated by the name of the stratum, are of small size (.007 — .010 mm.) and pyramidal form, at least in the deepest part of the layer. In the superficial part the cells are rounded or irregu- larly triangular, but they assume the distinctive pyramidal outline as they approach the sub- jacent layer, whose elements they resemble in possessing apical and lateral processes. The layer of large pyramidal cells con- tains the most distinctive neurones of the cere- bral cortex. It measures usually about 1.25 mm. in thickness, but in some localities much more, and blends with the adjoining layers without sharp boundaries. The cells in- crease in size but diminish in numbers as they are traced from the second layer inward, the largest (from .020 — .040 mm. in width) and most characteristic lying in the deepest part of the stratum. The typical pyramidal cell possesses a conical body, triangular in section, the apex of which is continued into a long tapering dendrite, the apical process, -which extends toward the periphery for a variable but usually considerable distance, depending upon the position of the cell. Upon gaining the stratum zonale, towards which the apical dendrite is always directed, the process breaks up into a number of end-branches that run parallel with the surface and contribute to the fibre-complex of the outer layer. During its journey to the surface, the apical dendrite gives off an uncertain number of branches that continue horizontally and, Section of cerebral cortex. X 90. 1178 HI. MAN ANATOMY. FIG. 1016. with the collaterals and similarly directed processes from other cells, take part in producing the felt-work giving rise to the outer stripe of Baillarger. From the deeper or basal surface of the cell arises the delicate centrally directed axone, which, penetrating the intervening fourth layer, acquires a medullary coat and enters the white core of the convolution as one of the component nerve-fibres. The axone gives off one or more collaterals which, after a shorter or longer course, establish relations with other and often remote cells. In addition to the two chief processes, the peripherally directed apical dendrite and the centrally coursing axones, a variable number — from four to twelve — of secondary lateral dendrites spring from the basal angles of the cell. These processes usually divide dichotomously, each succeeding pair of branches in turn splitting into twigs, until the den- drite is resolved into an end-brush of fibrillse which aid in producing an intricate felt-work of finest threads. Each pyramidal cell con- tains a conspicuous spherical or ellipsoidal nucleolus, within which a distinct nucleus is usually distin- guishable. The cytoplasm exhibits a striation and, in addition to the masses of tigroid substance, the Nissl bodies, a mass of brownish pigment granules. The larger pyramidal cells are surrounded by an evident pericellular lymph- space. The layer of polymorphic cells includes a large number of small nerve-cells, from .008 — .010 mm. in diameter, whose forms vary greatly, irregular, spherical, triangular, stellate and fusiform elements being present. Small pyramidal cells are also often seen within this layer. In contrast to dendrites of the typical pyramidal cells, those of the polymorphic elements, although peripherally directed, do not reach the stratum zonale but end before gaining the outermost layer. Their axones puss into the subjacent fibre- layer. The radial disposition of the groups of til IKS within Nerve cells of cerebral cortex as sern after silver im- tne deepest Stratum OI the pregnation. X 90. Drawn from pivpaiaii..n made by Pro- COltical Substance limits the lessor T. G. Lee. . . . .. . . ,, .. polymorphic cells chiefly to the interfascicular areas, within which the cells consequently appear arranged in a sonit-wliut cnliimiiur order. Within the deeper layers of the cortex, therefore umoiig the polymorphic and the pyramidal elements, two additional varieties of nerve-cells are encountered. These are the cells of Murtinotti and the cells of Gol^i. The cells of Martinotti are of small si/c uiul triangular or spindle-form in outline and particularly distinguished by the unusual direction of their axones. These processes puss towards the surface and within the stratum xonale divide into branches, which an- continued horizontally in the felt-w. n k of tangential fibres. As Large pyra- midal cells Polymorphic Pols cellV THE TELENCEPHALON. 1179 in other parts of the central nervous system, so too in the cerebral cortex there is found a sprinkling of Golgi' s cells of type II. Although both dendrites and axones of these cells undergo elaborate arborization, the axone is confined to a limited territory in the vicinity of the cell and, therefore, never reaches the stratum zonale. Neuroglia cells are present in all parts of the cerebral cortex and, whilst in a general way they send fibrils in all directions between the nervous elements, which they then support, the arrangement of the fibrillse is fairly definite in certain strata. Thus within the subpial condensation of the neuroglia, the glia cells send most of their processes as inwardly directed brushes. The cells within the deeper part of the cortex give off their processes in two chief groups, one extending towards the periphery and the other towards the white core. The Nerve-Fibres of the Cortex. — When viewed in suitably stained sections cut parallel with their general course, the cortical nerve-fibres do not appear as a uni- form layer, but as radially disposed bundles which gradually become less distinct as FIG. 1017. Tangential fibre-layer Outer stripe of Baillarger they traverse the cortex and finally disappear at about the level of the outer border of the layer of large pyramidal cells. The radial fibres are partly afferent and partly efferent. The corticifugal components, which predomi- nate, are largely the centrally directed axones of the pyramidal and the polymorphic cells which are continued as the axis-cylinders of the fibres composing the subcortical white matter. The peripherally coursing axones of the cells of Martinotti also contribute to the production of the fibre- radii. The cot ticipetal constituents of these tracts include the nerve- fibres which are derived from cells situated more or less remote from the convolution in which the fibres (their axones) end. Such, for example, are the thalamo-cortical and the tegmento-cortical fibres, as well as the many commissural fibres that arise in the opposite hemisphere and cross by way of the corpus callosum. Although for the most part the corticipetal fibres end at various levels in arborizations around the pyramidal cells, some are continued into the stratum zonale where, breaking up into horizontal fibrillae, they assist in producing the tangential zone. The spaces between these radial bundles are occupied by a delicate interlacement, the interradial felt-work, which is composed in large part of the lateral and collateral processes of the cells. Within the third layer, the horizontally coursing collaterals and processes of the large pyramidal cells form a complex of unusual intricacy, which condensation gives rise to the outer stripe of Baillarger. Beyond the outer ends of the radial fibre-bundles, the intercel- lular ground-work is occupied by a second delicate interlacement of processes and collaterals, the supraradial felt-work of Edinger ; whilst immediately beneath the narrow subpial neurogliar zone innumerable delicate terminal fibrillae course horizontally and parallel with the surface and constitute the tangential fibre-layer. The components of this layer are the terminal branches of the dendrites of the pyramidal and polymorphic cells and the axones of the cells of Martinotti, as well as the main and secondary processes of the fusiform elements of the stratum zonale. Radial fibres Section of cerebral cortex stained to show fibres. X 21. u8o HUMAN ANATOMY. The evident purpose of the horizontally directed processes and collaterals being to bring into relation different cortical cells, such association tracts become evident only after the neces- sity for the exercise of the corresponding psychic functions has arisen. Hence in the cortex of young children the strata of horizontal fibres are very feebly developed. With the progressive advance of intellectual capacity, the association paths become correspondingly more marked, according to the suggestive observations of Kaes, the increase continuing beyond even middle life. Whether this augmentation is due to actual increase in the number of association fibres, or, as suggested by Edinger, is dependent upon the further growth and myelination of collaterals already present in an immature condition, is uncertain. Local Variations in the Cerebral Cortex. — It has been pointed out, in prefacing the foregoing description of the structure of the cerebral cortex, that, whilst in the main certain features are common to the cortex wherever well devel- oped, more or less evident variations occur in different localities. Such variations are, for the most part, slight and depend upon the size and number of the nerve-cells and the richness and direction of the nerve-fibres — changes which produce alterations in the relative proportions of the strata. The width of the stratum zonale is almost constant and subject to little modification, being usually well defined from the layer of small pyramidal cells. The layer of the large pyramidal cells, on the contrary, exhibits considerable variation, either in increased thickness, as in the precentral gyrus, or in diminished breadth, as in the occipital lobe. The layer of poly- morphic cells is fairly uniform, but within the precentral convolutions is reduced almost to disappearance, although ^**~ — Gyms dentatus the pyramidal cells of the superimposed (third) layer are here of unusual size. Such variations in the histological features of the cortex are prob- ably correlated with dif- ferences in the function of its various regions, FIG. 1018. Choroid plexus — Lateral __/ ventricle Fimbria Al veus covering hippocampus Frontal section across left hippocampus and gyrus dentatus. X in various although the exact relations between such differences are in many cases still obscure. Disregarding the cortical regions which are profoundly modified by their rudi- mentary character, such as the olfactory lobe (page 1152), apart from minor varia- tions in details, the cortex of the greater part of the frontal, parietal, occipital, temporal and limbic lobes and of the insula closely corresponds in its structure. That of the motor (Rolandic) region, of the calcarine (visual) area of the occipital lobe, and of the hippocampus, dentate gyrus and adjacent part of the hippocampal gyrus, however, presents modifications which call for brief description. The Rolandic cortex of the precentral gyrus, particularly towards the upper margin of the hemisphere, of tin- pararcntr.il lobule and of the adjoining part of the postcentral gyrus — the great cortical motor area of the hemisphere — is distinguished by the great breadth of the layer of large pyramidal cells, the unusual size of the last-named elements and the feeble development of the layer of polymorphic cells. The pyramidal cells collectively tend to larger size as the upp<-r end of the precentral convolution is approached and, in addition, cells of extraordinary dimensions appear. These elements, known as the giant pyramidal cells of Betz, reach their maximum si/e within the paracentral lobule, where some attain a breadth of .065 mm. or almost double that of the pyramidal elements in other regions. The giant cells are further distinguished by tlieir robust and rounded form, their distribution in small groups of from three to five in the deeper layers of the cortex, and the exceptional thickness of their axones. The occipital cortex in the vicinity of the calcarine fissure (Fig. 1013) is distinguished even macroscopic-ally by the clearness of the outer stripe of Baillarger, here called the stripe of Crnmiri or of \'i,~tj --v'»*J%tf *-s of concussion before that of compression appears ; and it is this recovery of intelligence which is most characteristic of the condition. There will often be locali/ing symptoms indicating the part of the brain cortex which is irritated or compressed. Sithdural hi'morrhagc may follow the rupture of a number of small vessels, either of the pia or dura under a depressed fracture ; or it may come from a large vessel, particularly the middle cerebral. The symptoms and treatment are very much the same as in the extradural variety. In children extradural hemorrhage is very rare, because of the relatively tinner attachment of the dura during the period of growth. The blood may escape under tin- scalp through a Hue of fracture in the skull ; or, what is more likely, it may pass through a tear iu the dura into the subdural space. In fractures of the base of the skull, at any age, owing to the adhesion of the dura, the latter is likely to be torn ; rerebro-spinal fluid may escape into the adjacent air cavities, as into the nose, pharvnx or middle ear. A close adhesion of the dura to the bone, as sometimes found at PRACTICAL CONSIDERATIONS : THE BRAIN. 1209 operation, indicates a previous inflammation, as does any tendency of the arachnoid to adhere to the dura, since these two are normally not adherent. The arachnoid, however, is normally closely attached to the pia, and for practical purposes they are usually considered as one layer, the lepto-meninx. Inflammation of this layer — lepto-meningitis — may attack the convexity or the base of the brain, and may be primary or may be secondary to other diseases, usually purulent infections. It is asserted that the primary disease attacks, as a rule, the base, the secondary, the convexity of the brain ; but this is not beyond dispute. Tuberculous meningitis is frequently found at the base, but miliary tubercles are not uncommon on the convexity of the brain. The exudate which is deposited at the base frequently leads to irritation or paralysis from pressure on the cranial nerves in close relation to the under surface of the brain. Tumors growing at the base of the brain produce localizing symptoms early by pressing on the adjacent cranial nerves. A single nerve may be involved, but more commonly a combined paralysis from involvement of several nerves results. The cerebro-spinal fluid is found in the subdural and subarachnoid spaces, and in the ventricles. Over the vault it is comparatively scanty in both spaces. At the base, however, in the subarachnoid space of the middle and posterior fossae, it is abundant, forming an excellent support and protection to the most delicate part of the brain, that containing the vital centres. The frontal lobes, of much less impor- tance as to vital function, rest directly on the bone in the anterior fossa ; and are there- fore more subject to direct traumatic influences. The fact that the subarachnoid space is continuous with the ventricles through the foramina of Magendie and of Luschka, and communicates freely at the foramen magnum with the subarachnoid space of the cord, explains how excess of pressure within the cranium at one part may be relieved by escape of fluid to other parts. It explains also why pressure on a spina bifida will sometimes produce symptoms of cerebral compression ; and vice versa, why the increased congestion of the cerebral vessels from expiratory efforts, as in coughing, will increase the tension in the spinal tumor. Occlusion of the foramen of Magendie, by the products of inflammation, may cause increase of fluid from retention in the ventricles, with the development of hydro cephalus, and it is in this way that internal hydrocephalus occasionally follows meningitis. For the purpose of determining the cause of this condition, subarach- noid fluid is sometimes withdrawn through a hollow needle. The lateral ventricles can be tapped through a trephine opening 3 cm. (i^ in.) behind the external auditory meatus, and the same distance above Reid's base line — drawn from the lower margin of the orbit through the middle of the external auditory meatus. The needle is passed towards a point on the opposite side of the skull, 6.5-7.5 cm. (2^-3 in.) vertically above the external auditory meatus. Under normal circumstances the ventricle is from 5-5.6 cm. (2-2^ in.) from the surface, but if the ventricle is distended the distance is shorter. By a trephine opening in the occipital bone in the subcerebellar region, the subarachnoid fluid has been reached at the base of the brain where it is most abundant. Lumbar puncture for withdrawing cerebro-spinal fluid for diagnostic and thera- peutic purposes is sometimes employed. The needle should be introduced between the third and fourth, or between the fourth and fifth lumbar vertebrae, at the level of the lower border of the spinous process, or opposite its lower third, and about I cm. from the median line. It should be passed somewhat upward between the sloping laminae, and should be continued inward toward the canal until, by the diminished resistance, it is recognized that the point of the needle has entered the subarachnoid space. The Brain. — Of all the affections of the brain, hemorrhage is the most frequent and most important, whilst in the spinal cord it is comparatively rare unless as a result of trauma. Hemorrhage from the meningeal vessels is most commonly due to trauma, but within the brain substance the usual cause is atheroma, sometimes with the production of miliary aneurisms. A sudden strain increases the intravascular tension and ruptures one of these diseased vessels, giving rise to pressure symptoms, depending on the seat and extent of the hemorrhage. i2io HUMAN ANATOMY. The cortex is supplied by pial vessels distinct from those supplying the basal ganglia and adjoining regions. The latter come directly from the branches of the circle of Willis at the base. The cortical vessels anastomose ; those in the region of the basal ganglia do not. The latter are ' ' end arteries, ' ' so that when one is plugged by an embolus the part supplied is deprived of blood and undergoes necrosis (softening of the brain). In such a case the cortical supply would not be permanently interfered with. When a cortical arteriole is blocked, the anastomosis may furnish a sufficient collateral circulation to prevent necrosis in the affected part, but cortical softening is exceedingly common. When one of the arteries forming the circle of Willis is occluded, as an internal carotid by ligation of the common carotid, the anastomosis in the circle is so free that, in most cases, no marked effect is apparent. Cerebral disturbances, as delirium or convulsions, do occur in some cases, and in some are fatal. Even when both carotids are ligated, with an in- terval of some days or weeks, the operation is not more frequently followed by cere- bral disturbances than when only one is tied (Pilz). A case in which the patient lived after one carotid and one vertebral had been obliterated by disease, and the other carotid ligatured, has been reported (Rossi). In another case, although both carotids and both vertebrals had been occluded, the patient lived a considerable time afterward, the cerebral circulation being maintained through the medium of anas- tomosis of the inferior with the superior thyroids, and the deep cervical with the occipital artery (Davy). Occasionally ligation of the carotid has been followed by hemiplegia. The most common seat of intracerebral hemorrhage is near the basal ganglia in the region of the internal capsule. The artery most frequently at fault is a branch of the middle cerebral, the lenticulo-striate \ or artery of Charcot (page 1207). Hemor- rhages occur with less frequency in other portions of the cerebrum, and much more rarely in the pons, medulla oblongata, and cerebellum. The symptoms produced by the hemorrhage are the result of destruction of tissue and of pressure upon adjacent parts, and will vary according to the seat of the lesion. Tumors or inflammatory products will produce essentially the same symptoms. Cerebral Localization. — In order to understand the nature of the symptoms produced by brain lesions it will be necessary to study at least some of the functional areas of the cortex and their paths of conduction through the brain substance. Taylor has summarized as follows the researches of His and of Flechsig, which are of comparatively recent date and have thrown new and valuable light upon the functions possessed by the cortical regions of the brain, by the study of their mode of development. Flechsig succeeded in following the various tracts through their myelination. The tracts which are functional earliest receive their myelin before the others. He has shown that the fibres in the spinal cord, medulla, pons and corpora quadrigemina are almost entirely medullated when the higher parts show little or no myelin. In the new-born child the cerebrum is almost entirely immature, and proportionately few of its fibres are medullated. According to Flechsig, the sensory paths in the brain first become medullated, and may be observed developing one after another, beginning with that of smell and ending with that for auditory impulses from the periphery to the cortex. In this way it has been ascertained that the individual sensory paths terminate in tolerably sharply circumscribed cortical regions, for the most part widely removed from one another, being separated by masses of cortical substance which remain for a consid- erable period immature or undeveloped. The cortical sense areas thus mapped out correspond entirely to those regions of the surface of the brain which pathological observation has shown to stand in relation to the different qualities of sensation. Olfactory fibres are found to end mainly in the uncinate gyrus. Visual fibres have been traced to the occipital lobe in the neighborhood of the calcarine fissure, and auditory fibres to the temporal lobe. Flechsig has further observed that new paths begin to develop from the points where certain of the sense fibres terminate and pur- sue a downward course. They can be followed from the cortex to the medulla and to the motor nuclei of the cord. These descending paths are mainly those known as the pyramidal or motor tracts, and the area from which they proceed, commonly railed the Rolandic region, is, according to Flechsig, concerned also in the sensation PRACTICAL CONSIDERATIONS : THE BRAIN. I2II of touch ; he calls it the somcesthetic area. It includes the precentral and postcentral convolutions, the paracentral lobule. The sensory fibres passing from the periph- ery to this area would appear to excite sensations of touch, pain, temperature, muscle- and tendon-sense, equilibrium, etc. This cortical region probably repre- sents a complex mass of sense centres rather than a single sensory area, and in addition to being a sensory field, the somaesthetic area is the great motor region of the brain. When this sensory-motor area and the various sensory areas are fully taken into account, there still remain about two-thirds of the cortex which appear to have noth- ing to do with the periphery. Flechsig calls these regions of the cortex ' ' associa- tion centres, " as he believes they furnish arrangements for uniting the various central sense areas. The best known cortical areas are the motor, speech, visual, and auditory, al- though new contributions to our knowledge are being made from time to time. Re- cently Griinbaum and Sherrington have demonstrated in the cortex of the higher apes, including the orang and several species of the chimpanzee and gorilla, that the motor area was found in the whole length of the precentral convolution and the en- FIG. 1041. Left cerebral hemisphere illustrating diagrammatically motor zone and its subdivisions. (Mills.) tire length of the central fissure. It did not at any point extend behind the central fissure. They demonstrated other important facts in connection with this and other areas. These results have been in part at least confirmed by recent histological re- searches, and by faradization of the human brain during operation for the purpose of more accurately identifying the relations of the opening to the area to be exposed. The most important, because the best known, area of the cortex, is that asso- ciated with the fissure of Rolando and the fissure of Sylvius. Before the publication of the experiments and observations just alluded to, the motor zone was regarded as extending over both central convolutions which lie one anterior and the other posterior to the central fissure or fissure of Rolando, also over the paracentral lobule on the median aspect of the hemisphere, and to some extent into the posterior extremities of the first and second convolutions. The trend of opinion is now in favor of the view that the motor region is entirely or almost en- tirely in front of the central fissure (Monakow, Mills). This is, of course, a matter of considerable importance in trephining for a tumor or hemorrhage supposed to be situated in this area, as instead of making the opening directly astride of the fissure of Rolando it would be better, if these views are correct, to operate with the idea of exposing a region two-thirds or three-fourths in front and one-third or one-fourth behind the central fissure. 1212 HUMAN ANATOMY. In the lower one-third or fourth of the motor zone are found the motor centres for the face and tongue, that is, for the facial and hypoglossal nerves. In the middle third or half are the arm centres. In the upper part of the region and paracentral lobe, are the centres for the lower extremity. Localized lesions of the motor zone may therefore produce a paralysis limited to one part controlled by the affected por- tion of the cortex, as of the face, arm or leg (monoplegia). The lesion is much more likely to involve two adjacent areas, as of the face and arm, or of the arm and leg, giving rise to a combined paralysis ; but no single lesion, unless it were crescentic in form, could involve at the same time the leg and face areas without including the intervening arm area. Within each of the larger areas a more specialized differentiation is possible, although none of them can be sharply defined, not even the larger. That the facial centre lies in the lower part of the anterior central convolution is certain, and it is believed that the upper and lower muscles of the face are each represented by a sepa- rate centre. In the upper and forward part of the face-area are represented the movements of the cheek and eye-lids ; in the posterior part the movements of the pharynx, platysma and jaws. FIG. 1042. Diagram illustrating probable relations of physiological areas and centres of lateral aspect of left cerebral hemisphere. (Mills.) In the arm-area it is considered as certain that the centre for the movements of the thumb and index finger is below; above is that for the finger and hands; and in the highest part is that for the shoulder. In the posterior parts of the second frontal convolution and in a portion of the third frontal convolution are the centres for the associated lateral movements of the eyes and lateral movement of the head (Beevor and Horsley). Our knowledge of the more special localization within the leg centre is not at all exact, and the many views held are very contradictory. It is believed that the centres for the movements of the thigh, knee, foot, and toes, are arranged in the order named, from before backward on the lateral border of the hemisphere and in the paracentral lobe. A narrow zone for the movements of the trunk, as shown by (iriinbaum and Sherrington, is located between the upper border of the arm-area and the lower bonier of the leg-area. It is now considered probable, however, that the cutaneous sensory centres are posterior to and in close contact with the motor centres in the postceniral convolution, while other centres for stereognostic perception and the muscular sense are located in the superior and inferior parietal convolutions. The speech centres are in the posterior part of the third left frontal convolution (Brora's convolution), in right-handed people in the first left temporal convolution, and perhaps in the left angular gyrus. PRACTICAL CONSIDERATIONS : THE BRAIN. 1213 In Broca's convolution is probably the centre for motor speech, and a lesion here gives motor aphasia, an inability to transform concepts into words, although the patient is conscious and the tongue can be moved. A minor part in speech is played by the posterior part of the right third frontal convolution, but in the left- handed it is probably the chief centre. In the first left temporal convolution is the auditory centre for speech, a lesion of which leads to a loss of memory for word-sounds, though the hearing may be undisturbed. The centre for memory of printed words is probably in the left angular gyrus ; and a lesion there probably causes a loss of the ability to read or to understand written language, though ordinary sight is undisturbed. The existence of a motor writing centre is doubtful (Oppenheim). If it exists, it is probably located in the posterior portion of the left second frontal convolution. We have no definite knowledge of the location of centres for smell and taste. That for smell is thought to lie in the uncinate gyrus. The centre for taste has been supposed to be in the anterior portion of the gyrus fornicatus, but it is not decided, although it is probably near the centre for smell. FIG. 1043. Diagram illustrating probable relations of physiological areas and centres of mesial aspect of right cerebral hemisphere. (Mills.) The auditory centre, as indicated, is in the upper temporal convolution. It is very likely that the centre of each side is connected with both auditory nerves, so that a paralysis of one side by a unilateral lesion of one side m'ay be compensated for by the centre of the opposite side. It is probable that no part of the cerebral cortex is absolutely without function, although the functions of some areas are very little known. Unilateral disease of the anterior portion of the frontal lobe may be extensive without notable symptoms of any kind. The atrophy is often most marked here in general paralysis of the insane, and in other forms of dementia. It is generally agreed that the seat of " the higher psychical functions ' ' is located in the prefrontal lobes, the left side being perhaps more active than in the right. Reference has already been made to the relation of the occipital cortex to sight, and of the temporal to hearing. The cuneus and calcarine fissure together constitute a primary or lower cortical or visuo-sensory centre, while the lateral aspect of the occipital lobe is a visuo-psychic area, containing sub-areas or centres concerned with higher visual processes. Mind blindness, for instance, results from destructive lesion of the lateral occipital lobe, particularly if the lesion is a large one, in the left hemi- sphere, or if lesions of both occipital lobes are present. A lesion of the cuneo- calcarine cortex causes lateral homonymous hemianopsia. This may be produced 12 14 HUMAN ANATOMY. also by a lesion in the lateral portion of the occipital lobe, if it extends inwards sufficiently to interrupt the optic radiations. In spite of extensive researches the functions of the central ganglia are very little known. Lesions of the cerebellar hemispheres may not produce distinct phenomena until the median lobe or vermiform process is involved, when two especially charac- teristic symptoms will almost certainly develop. These are a peculiar disturbance of equilibrium with a staggering gait (cerebellar ataxia), and a troublesome vertigo. Although the patient can scarcely stand alone he may possibly be able to perform the most delicate movements with his upper extremities. The vertigo occurs only in standing or walking, and is then almost always present. Nystagmus is also a frequent symptom. Vomiting is very often present, but is not characteristic, since it is equally frequent in other brain diseases. Extending along the floor of the aqueduct of Sylvius and of the fourth ventricle, that is, along the cerebral peduncles, pons and medulla, we find the nuclei of origin of the motor fibres of the cranial nerves. It should be borne in mind that the con- trolling centres of these nerves are in the cerebral cortex. Many automatic centres, as of circulation, respiration, sweating, and regulation of heat, as well as the motor and sensory tracts are found in the medulla. Cranio-Cerebral Topography. — In order that the surgeon may expose and recognize certain areas of the cortex, it becomes very important that the relations between these areas and the corresponding external surface be well understood. For this purpose advantage is taken of the landmarks of the skull (page 241). From these bony points, ridges and depressions, by means of lines and measurements, the known cortical areas may be accurately mapped out. The upper limit of each cerebral hemisphere is indicated, approximately, by the median line at the top of the skull from the glabella to the external occipital protu- berance, due allowance being made for the superior longitudinal sinus, which lies under the skull, in the longitudinal fissure, between the two hemispheres. The lower limit is represented by a transverse line, in front, just above the upper margin of the orbit. At the side of the skull the line passes from about a half inch above the external angular process of the frontal bone to just above the external auditory meatus. From here it passes to the external occipital protuberance ; this part of the line corresponding, approximately, to the lateral sinus. The cerebellum lies immediately below this line. Of the brain fissures, those of greatest importance in cerebral localization are the Rolandic and Sylvian, since by means of these all the best known cortical centres can be located. Of the two, the fissure of Rolando is much the more important, because the motor, the most definitely known cortical area, is associated with it. Its upper limit is at a point about 12 mm. (one-half inch) behind the mid-point between the glabella and the inion, and about one-half inch from the median line. It passes outward, downward, and forward, approximately, at an angle of 71° with the median sagittal line of the skull. It is 8.5 cm. (3^5 in.) long (Thane), and ends below just above the fissure of Sylvius. Near its lower end it turns rather suddenly downward, so that, in this part, it is not in the line of the angle of 71°. Many methods have been devised for the purpose of making the line of the fissure on the scalp. ( '/i ifn< •' . v method consists of folding an ordinary square sheet of paper on the diagonal line, thus dividing an angle of 90° in half, making two of 45°. One of these angles of 45° is again halved in a similar manner, making two new angles each of 22j/£°. The paper is then so unfolded that one of the angles of 22l/2° is added to that of 45°, making a new angle of 67^° ; this will be sufficiently near that of the fissure of Rolando for all practical purposes. Horsley s cyrtomctcr consists of two strips, either of thin, flexible metal or of parchment paper, each graduated in inches. The lateral arm is placed at an angle of 67° with the long arm, tin- apex of the angle being at a point 12 mm. or one-half inch behind the mid-point of the long arm. Le Fort simply dn-w a line from the beginning of the fissure, above, to the mid- dle of the zygoma, below, and marked off on this line the proper length of the fissure. PRACTICAL CONSIDERATIONS : THE BRAIN. 1215 Anderson and Mackins suggest : (i) a median sagittal line from the glabella to the inion ; (2) a frontal line from the mid-sagittal point to the depression just in front of the ear at the level of the upper border of the meatus ; (3) a squamosal line from the most external point of the external angular process, at the level of the superior border of the orbit to the junction of the middle and lower thirds of the frontal line, and prolonged for about 3. 7 cm. ( i ^ in. ) behind the frontal line. The upper ex- tremity of the central fissure was found by them to lie between the mid-sagittal point and a point 18 mm. (^ in.) behind it, and the lower extremity of this fissure they located near the squamosal line, about 18 mm. (^ in.) in front of its junction with the frontal line. The commencement of the lateral portion of the Sylvian fissure is not at a definite fixed point, but will usually be hit at a point from 3. 7-5 cm. ( i ^—2 in. ) behind the angular process, the course of the horizontal portion of this fissure corresponding closely to the squamosal line (Mills). Fissure of Rolando FIG. 1044. Bregma Line for Rolandic fissure Interparietal fissure External parieto- occipital fissure Parietal eminence Inion Lateral sinu Posterior limb of Sylvian fissure Line for Sylvian fissure Vertical limb of Sylvian fissure Horizontal limb of Sylvian fissure Glabella Nasion Semidiagrammatic view of head, showing relation of Rolandic and Sylvian fissures and lines. The fissure of Sylvius begins anteriorly, approximately, at a point 3 cm. ( I % in. ) behind the external angular process of the frontal bone ; and ends posteriorly at a point 1 8 mm. (^ in.) below the parietal eminence. A straight line between these two points will represent the fissure, which is about 10 cm. (4 in. ) long. The an- terior 1 8 mm. (34 in.) of this line will correspond to the main portion of the fissure and the remainder to the horizontal limb. The vertical limb ascends for about 2.5 cm. (i in.) from the posterior end of the main fissure. Around the posterior end of the horizontal limb, and approximately under the parietal eminence lies the supramarginal convolution. It is continuous in front with the ascending parietal convolution, and behind with the angular gyrus. The parieto-occipital fissure is most marked on the mesial surface of the brain. The external limb passes outwards, almost at right angles to the longitudinal fissure on the external surface for about 2.5 cm. and lies from 2-3 mm. in front of the lambda. The frontal lobe is divided into three main convolutions by the superior and in- ferior frontal sulci. The line for the superior frontal sulcus passes directly backward I2l6 HUMAN ANATOMY. from the supraorbital notch, and parallel to the longitudinal fissure to within 18 mm. ( y^ in. ) of the fissure of Rolando. The inferior frontal sulcus is represented, approximately, by the anterior end of the temporal ridge. In the parietal lobe the most important sulcus is the intraparietal. It begins near the horizontal limb of the fissure of Sylvius, and passes upward and backward about midway between the fissure of Rolando and the parietal eminence. It then turns backward, running about midway to the longitudinal fissure and the centre of the parietal eminence. Above the sulcus, in front, lies the ascending parietal convolution, just posterior to the fissure of Rolando and behind the superior pari- etal lobule. Below the sulcus, anteriorly, is the supramarginal convolution, and posteriorly, the angular gyrus. FIG. 1045. Eregma Lateral ventricle Middle meningeal rtery, anterior branch Posterior horn of lateral ventricle (I) Inion Lateral sinus Middle meningeal artery, posterior branch ; inferior horn or lateral ventricle seen beneath Semidiagrammatic view of head, showing position of ventricles, lateral sinus and middle meningeal arteries as projected on skull. The temporal lobe lies below the fissure of Sylvius and extends forward as far as the edge of the malar bone. The first temporal sulcus lies about one inch below and parallel with the fissure of Sylvius, and the second about 18 mm. (3/£ in.) lower. The occipital lobe lies posterior to the parieto-occipital fissure and the tem- poral lobe. The motor tracts are made up of the fibres passing from the motor portion of the cortex in the Rolandic region to the motor nuclei from which arise the nerves supplying the muscles which the cortical areas control. After leaving the cortex the fibres pass downward in the corona radiata, and converge to the posterior limb of the internal capsule. The motor fibres of the cortico-bulbar and cortico-spinal tracts, occupy the genu and adjacent third of the internal capsule (page 1188), although Drjrrine holds that the whole posterior limb is motor. They continue their course downward through the crura cerebri, pons, and medulla ; in the lower part of the latter the greater number cross to the opposite side and pass down in the cord as the lateral or crossed pyramidal tract. A small number, sometimes absent, pass down PRACTICAL CONSIDERATIONS : THE BRAIN. 1217 on the same side. We have already seen that lesions of the cortex produce mono- plegia, unless large enough to involve the whole motor zone, but cortical hemiplegia is much more common than cortical monoplegia. In the internal capsule the motor fibres are gathered together so compactly that a small lesion, as an apoplectic hemor- rhage, will frequently interrupt the whole tract and give a hemiplegia of the opposite side of the body. In the medulla and cord the tracts of both sides are so close together that a lesion may easily paralyze both sides (paraplegia) ; indeed, diseases of the cord fre- quently involve the whole transverse section, paralyzing sensation as well as motion. Hemiplegia is, therefore, the common form of cerebral paralysis ; paraplegia the common form of spinal paralysis ; while monoplegia occasionally results from lesions of the brain cortex, but more commonly from lesions of peripheral nerves. The sides and convexity of the brain can be exposed for operation, so that lesions of the cortex can be attacked and often removed ; but the region of the internal capsule, which is near the basal ganglia, cannot be reached. The soft brain may be injured by contact with its bony walls when the head is violently shaken, the spaces surrounding the brain and filled with fluid permitting considerable movement of the brain. The injury in cerebral contusion occurs more frequently on the under surface, both as regards the cerebrum and cerebellum, than on any other part (Prescott Hewett). That portion, however, which includes the medulla, pons, and interpeduncular space, rests on a large collection of cerebro- spinal fluid, and is least frequently injured. THE PERIPHERAL NERVOUS SYSTEM. IN a broad sense and as contrasted with the cerebro-spinal axis, the peripheral nervous system includes all the nerve-paths by which the various parts of the body are brought into relation with the brain and spinal cord. These paths embrace, in a general way, two groups. One group, the somatic nerves, includes the nerves FIG. 1046. Olfactory bulD Orbital surface of frontal lobe Temporal lobe Anterior perforated space Mammillary bodies Cerebral peduncle Pon Medulla, pyramid Hypoglossal nerve Cerebellum Anterior roots of spinal nerves Olfactory tract Optic nerve, cut Optic commissure Optic tract Oculomotor nerve •Trochlear nerve Trigeminal nerve Abducent nerve Facial nerve Auditory nerve Glosso-pliaryn- geal nerve r'neumogastric nerve Spinal accessory nerve spinal portion Pyramidal decussation Spinal part of XL nerve Occipital lobe Spinal cord Inferior aspect of brain, denuded of its membranes, showing superficial origins of cranial nerves ; origin of irochlc nerve is on dorsal surface and therefore not seen. \ • supplying the voluntary muscles, integument and organs of special sense ; the sec- ond group, the visceral nerves, includes those supplying the involuntary muscle throughout the body and the thoracic and abdominal viscera. The somatic nerves are subdivided into ( ) the spinal ticnrs, which are attached to the spinal cord and traverse the intervertebral foramina. The visceral, or splanchnic I2IS THE CRANIAL NERVES. 1219 nerves, although directly or indirectly connected with the cerebro-spinal axis, pre- sent peculiarities and, as the system of sympathetic nerves, are accorded, at least for convenience of description, a certain degree of independence. While by no means all of the spinal nerves contribute splanchnic branches — such branches being given off especially by the thoracic and upper lumbar nerves — they all receive sympathetic filaments, which form, therefore, integral parts of the somatic nerves. From the sympathetic neurones of the gangliated cords axones pass, by way of the gray rami communicantes (page 1357), to the trunks of the spinal nerves and thence by these are carried to all parts of the body for the supply of the involuntary muscle occur- ring within the blood-vessels and the integument and for the cutaneous glands. .Fur- thermore, it must be remembered, that although the predominating constituents of a spinal nerve may be axones derived from anterior horn root-cells and destined for voluntary muscle, such trunk also contains a number of afferent fibres which convey impulses received from the neuromuscular and neurotendinous sensory endings, the nerve-trunks reckoned as ' ' motor ' ' in all cases, when analyzed, being found to con- tain sensory and sympathetic fibres as well as efferent ones. THE CRANIAL NERVES. The cranial nerves (nervi cerebrales) include twelve pairs of symmetrically arranged nerve-trunks, which are attached to the. brain and, traced peripherally, escape from the skull by passing through various foramina at its base to be distrib- uted for the most part to the structures of the head. The point at which a cranial nerve is attached to the surface of the brain is designated its superficial origin ; the group of more or less deeply situated nerve- cells with which its fibres are directly related is often spoken of as its deep origin. From what has been said (page 1278) concerning the position of the cell-bodies of motor and sensory neurones, it is evident that only the motor fibres of the cranial nerves spring from nerve-cells within the cerebro-spinal axis, while the fibres con- ducting sensory impulses arise from nerve-cells situated within ganglia lying outside the central nervous axis and somewhere along the course of the nerve-trunks. It follows, therefore, that the term ' ' deep origin, ' ' as applied to the cell-groups within the brain, can properly relate only to the origin of motor fibres ; the cell-groups with which the sensory fibres come into relation after entering the brain-substance are in reality nuclei of reception, or of termination, and not of origin. The sensory impulses so received are transmitted to various parts of the brain by the more or less complex paths afforded by the neurones of the second, third, or even higher order. In addition to their relation to the deep nuclei, whether of origin or of reception, the fibres of every cerebro-spinal nerve are directly or indirectly influenced by neurones situated within the shell of gray matter that covers the cerebrum. The position of these higher cortical centers, as they are termed, is known with considerable accuracy for many groups of nerves, but regarding others more definite data con- cerning cerebral localization must be awaited. Bearing in mind the foregoing distinctions, for convenience we may follow the conventional description in which all the nerves are regarded as passing away from the brain, the direction in which they convey impulses, centripetally or centrifugally, being for the time disregarded. On leaving the surface of the brain at its superficial origin, each cranial nerve, invested by a sheath of pia mater, traverses for a longer or shorter distance the sub- arachnoid space, pierces the arachnoid and from the latter acquires an additional, but usually not extensive, sheath. It then enters a canal in the dura mater that leads to the foramen in the skull, through which the nerve escapes from the cranium, invested by a sheath prolonged from the dura which is continuous with the epi- neurium covering the nerve-trunk. The position of the dural aperture and that of the foramen by no means always correspond, some of the nerves, notably the fourth and sixth, pursuing an intradural course of some length before gaining their osseous exit. According to the order in which they pass through the dura lining the cranium, the pairs of cranial nerves are designated numerically from the first to the twelfth. They are further distinguished by names based upon their distribution or functions. 1220 HIM AN ANATOMY. Certain of the cranial nerves are entirely motor ; some convey the impulses of special sense ; while others transmit impulses of both common sensation and motion. A general comparison of these relations, as now usually accepted, is afforded by the following summary : THE CRANIAL NERVES. Number. Name. I. OLFACTORY : II. OPTIC : III. OCULOMOTOR IV. V. VI. VII. VIII. IX. X. XI. XII. TROCHLEAR : TRIGEMINAL ABDUCENT : FACIAL : AUDITORY, (a) Cochlear division : (b) Vestibular division GLOSSO-PHARYNGEAL : PNEUMOGASTRIC OR VAGUS; SPINAL ACCESSORY : HYPOGLOSSAL : Function. Special sense of smell. Special sense of sight. Motor to eye-muscles and levator pal- pebrae superioris. Motor to superior oblique muscle. Common sensation to structures of head. Motor to muscles of mastication. Motor to external rectus muscle. Motor to muscles of head (scalp and face) and neck (platysma). Probably secretory to submaxillary and sublingual glands. Sensory ( taste ) to anterior two-thirds of tongue. Hearing. Equilibration. Special sense of taste. Common sensation to part of tongue and to pharynx and middle ear. Motor to some muscles of pharynx. Common sensation to part of tongue, pharynx, oesophagus, stomach and respiratory organs. Motor (in conjunction with bulbar part of spinal accessory) to muscles of pharynx, cesophagus, stomach and intestine, and respiratory organs ; inhibitory impulses to heart. Spinal Part : Motor to sterno-mastoid and trapezius muscles. Motor to muscles of tongue. Practical Considerations. — Lesions may affect a cranial nerve within the brain or in its peripheral portion. A central lesion clinically is one above the nucleus of the nerve, and may be cortical or may encroach upon its intracerebral connections. It may merely irritate the nerve or may paralyze it. By a peripheral lesion is meant one involving the nucleus or the fibres of the nerve below the nucleus. THE OLFACTORY NERVE. The olfactory nerve (n. olfactorius), the first in the series of cranial nerves, presents some confusion in consequence of the name, as formerly employed, being applied to the olfactory bulb and tract as well as to the olfactory filaments — struc- tures of widely diverse morphological values. As already pointed out (page 1151), the olfactory bulb and tract (Fig. 993), with its roots, represent, as rudimentary structures, the olfactory lobe possessed by animals in which the sense of smell is highly developed. It is evident that these structures, formerly regarded as parts of the first cranial nerve, are not morphological equivalents of simple paths of conduc- tion. On the other hand such paths are represented by a series of minute filaments, the true olfactory nerves, that connect the perceptive elements within the nasal mucous membrane with the rudimentary olfactory lobe. The olfactory nerves proper, some twenty in number, are the axones of the peripherally situated neurones, the olfactory cells (page 1414), which lie within the limited olfactory area. The latter embraces in extent on the outer nasal wall chiefly THE OLFACTORY NERVE. 1221 the mesial surface of the superior turbinate bone and a somewhat larger field on the adjacent upper part of the nasal septum. The olfactory nerves (Fig. 1048), FIG. 1047. Olfactory bulb Nasal nerve, ext. br, Exit ext. br. nasal nerve Olfactory nerve-fibres An upper ant. nasal br. 'Meckel's ganglion ) Upper post, nasal brs. j Meckel's ganglion Naso-palatine nerve (Sup. ant. nasal br. of Meckel's gangl. and inf. ant. nasal br. of ant. descending palatine nerve A posterior nasal br. Meckel's ganglion Ant. descending palatine nerve, the middle palatine appearing posteriorly Right nasal fossa showing distribution of olfactory and nasal nerves on lateral wall ; mucous membrane has been partly removed to expose nerves. whose fibres are nonmedullated, exhibit a plexiform arrangement within the deeper part of the nasal mucous membrane, pass upward through the cribriform plate of FIG. 1048. Crista galli Nasal nerve Ext. br. nasal nerve, cut — J Int. (septal) br. of nasal nerve Olfactory bulb Naso-palatine nerve Olfactory nerve-fibres Sphenoidal sinus An upper ant. nasal br. of Meckel's ganglion •Naso-palatine nerve An upper ant. nasal br. 'of Meckel's ganglion Eustachian orifice Vomer, posterior border Soft palate, cut mesially Right nasal fossa showing distribution of olfactory and nasal nerves on septal wall ; mucous membrane has been partly removed to expose nerves. the ethmoid bone and enter the under surface of the olfactory bulb. Within the latter the nerve-fibres end in terminal arborizations in relation with the dendritic processes of the mitral cells (Fig. 995), sharing in the production of the peculiar olfactory glomemli. 1222 IIl'MAN ANATOMY. Central and Cortical Connections.— The impulses conveyed by the olfactory- nerves and received by the mitral cells of the olfactory bulb, which cells may be regarded as constituting the end-station or reception-nucleus of the peripheral path, are carried to neurones situated either within the gray matter of the olfactory tract, the anterior perforated space or the adjacent part of the septum lucidum (Fig. 1049). Fibres connecting the olfactory centres of the two sides pro- ceed from the cortex of the tract by way of the anterior commissure, forming the pars olfactoria of the latter, to end in relation with the cells within the opposite tract or bulb. From these primary centres the impulses are transmitted by different paths to the secondary or cortical centres situated in the anterior part of the hippocampal convolution in the vicinity of its uncus, including the hippocampus major and the nucleus amygdalae. i. 'The most direct path is by way of the lateral root of the olfactory tract (page 1153), by which fibres from cells within the trigonum olfactorium pass, skirting the Sylvian fissure, to the anterior part of the gyrus hippocampi to terminate in relation with the cortical cells of that convolution. FIG. 1049. *_ J [ — 1 1 1 1 0 Mm ' i i : Diagram showing most important connections of olfactory tracts; LC lamina cribrosa ; B, olfactory bulbs : Tr, olfactory tract ; Tg, olfactory trigone ; Ls,Ms, lateral and ; mesial striae ; X, anterior commissure : CC, corpus callo- sum; SL, septum lucidum; Fx, anterior pillar of fornix; M, mammillary body; tn-t, manimillo-thalamic tract ; A P, anterior perforated space ; Tsem, tseniasemicircularis; T, thalamus; Fm, fimbria descending on hippocampus; £/, uncus; AN, amygdaloid nucleus; TL, temporal lobe. 2. Fibres from the cells within the olfactory trigone (page 1153) and the anterior perfo- rated space (page 1 153) pass into the septum lucidum and, reinforced by others from cells of the septum, enter the fornix ; thence continuing backward and downward by way of the fimbria they reach the hippocampus major. 3. Fibres from cells within the olfactory trigone turn inward and by way of the medial root of the olfactory tract gain tin- gyrus subcallosus ; thence they pass along the upper surface of the corpus callosum within its longitudinal striae and descend by way of the dentate gyrus to reach the anterior end of the hippocampus major. 4. Fibres from cells within the antrrior perforated space and septum lucidum, joined by accessions from the opposite olfactory tract by way of the anterior commissure, converge to the ta-nia semicircularis (page 1162) and, passing along the floor of the lateral ventricle, descend within the roof of the descending horn to end in the amygdaloid nucleus (Dejerine). During their ascent from the anterior perforated space, some fibres diverge almost at right angles and pass backward directly to the optic thalamus. The connections between the cortical centres of olfaction and the optic thalamus, as well as those between the olfactory centres of the two sides, by way of the fornix, are described on page 1 167. Practical Considerations. — Lesions of the uncinate gyrus may cause loss of the .wv/.w of snifll on one «»r both sides. Paralysis of the olfactory nrrrr with loss of smell may also occur in fractures of the base of the skull in the anterior fossa, involving the cribriform plate. THE OPTIC NERVE. 1223 THE OPTIC NERVE. The optic nerve (n. options) is, as conventionally described, part of the pathway which includes additionally the optic commissure and the optic tract and transmits the visual impulses received by the retina to the primary centres within the pulvinar of the optic thalamus and the external geniculate and superior quadrigeminal bodies. The retina, the nervous tunic of the eye (page 1462), comprises three fundamental layers — (a) the percipient visual cells, (o) the receptive ganglion refines and (V) the cerebral layer. The latter contains the neurones, the axones of which constitute the nerve-fibres that converge towards the optic disc and, piercing the vascular and fibrous coats, form the greater part of the optic nerve, commissure and tract. In addition to the fibres of retinal origin, which alone carry visual impulses, the optic nerve contains a considerable number of supplementary fibres, which are only indirectly con- cerned in sight. Some of these fibres, distinguished by their small diameter, pass towards the retina, originating within the brain from the cells of the primary visual centres or from sympa- thetic neurones, and probably transmit vasomotor impulses controlling the retinal blood- vessels. Other supplementary fibres, perhaps by way of a centre situated within the medulla, pass from the retina and are regarded as conveying indirectly to the oculomotor nucleus the impulses resulting in reflex pupillary movements. The optic nerve (Fig. 1198) extends from the eyeball, which it leaves about 3 mm. to the medial side of the posterior pole, to the optic commissure. Leaving the eyeball, the nerve pursues a slightly sinuous course backward, inward and up- ward towards the apex of the orbit, where, surrounded by the origins of the recti muscles, it traverses the optic foramen in the sphenoid bone in company with the ophthalmic artery, which lies to its outer and lower side. On gaining the interior of the cranium, it converges towards the nerve of the opposite side with which it joins to form the major part of the optic commissure in the vicinity of the olivary eminence, medial to the internal carotid artery. The entire length of the optic nerve is from 30—40 mm. , of which the intraorbital part includes from 20-30 mm. , thus allowing for changes in the position of the eyeball without undue stretching of the nerve. Its diameter is from 3-4 mm. Within the orbit the nerve is embedded in the orbital fat and surrounded by the ocular muscles and, near the eyeball, by the ciliary vessels and nerves. It is crossed above and from without inward by the ophthalmic artery and the nasal nerve, and, about 10 mm. from the eyeball, is penetrated by the central artery of the retina, which, with its companion vein, continues its intra- neural course as far as the optic disc. In addition to a sheath from the pia mater and a delicate one from the arachnoid, the optic nerve receives a robust tubular pro- longation from the dura at the optic foramen. These sheaths, with the intervening subarachnoidal and subdural lymph-spaces, are continued on the nerve as far as the eyeball, where they blend with the sclerotic coat. The optic commissure (Fig. 1046), formed by the meeting of the converging optic nerves in front and the diverging optic tracts behind, is somewhat flattened and transversely oblong and measures about 12 mm. where broadest. It rests upon the olivary eminence, is embraced at the sides by the internal carotid arteries, and lies beneath the floor of the third ventricle in advance of the tuber cinereum in close rela- tion with the inferior surface of the brain. It divides posteriorly into the two optic tracts. On reaching the commissure, or chiasm, as it is sometimes called, the optic fibres, estimated at upwards of half a million (Salzer), undergo partial decussation, those from the nasal or inner half of each retina crossing to the mesial part of the opposite optic tract, while those from the temporal or outer half continue into the lateral part of the tract of the same side. The existence of a commissural loop con- necting the two optic nerves has not been established, although formerly accepted. Occasional instances have been encountered in which the decussation of the optic fibres was complete, thus repeating in man the condition that normally obtains in all nonmammalian vertebrates, as well as in a few rodents (mouse, guinea-pig). Rarely the optic commissure has been absent, the optic fibres passing directly into the tract of the same side. 1224 HUMAN ANATOMY. The entire commissure, however, is not composed of optic fibres, since its posterior part is formed by a bundle, known as Gudden's commissure (commissura inferior) (page mo), which passes forward along the mesial side of the optic tract, loops around the posterior angle of the commissure and enters the opposite tract. These fibres have no connection with the path of sight-impulses, but are probably chiefly related with the median or internal geniculate bodies and the inferior corpora quadrigemina ( page 1 1 10) . The optic commissure also contains fibre-strands that arch around its posterior angle, par- allel with, but separated by a thin layer of gray matter from Gudden's tract. Concerning the origin and destination of these fibres, termed Meynert's commissure (commissura superior), little is known. By some they are regarded as continuations of the mesial fillet that, after decussa- FIG. 1050. Diagram showing course of retinal fibres fn optic pathway and their connection with basal ganglia and primary cortical centres ; smaller figure illustrates path of light-ray and resulting impulse through retina : K, retina : ON, OC, OT, OR, optic nerve, chiasm, tract and radiation . f, pulvinar ; Eg, SQ, lateral geniculate and superior quadrigem- inal bodies ; Oc Or, occipital cortex ; ///, /K, VI, nuclei of eye-muscle nerves. tion, pass to the globus pallidus of the lenticular nucleus of the opposite side. Others deny such relations, while Kolliker describes them as bending upward, traversing the ventral part of the cerebral peduncle, to end within the corpus subthalamicum (page 1128). Additional commissural fibres (commissura ansata) descend from the floor of the third ventricle and from the peduncle of the septum lucidum, by way of the lamina terminalis, to the front and upper part of the optic chiasm ; other fibres pass from the ventricular floor to the bark of the chiasm. For the most part these fibres cross to the opposite side to be lost in the sub- stance of the optic commissure. Although regarded as in a way constituting a -central optic roof, their connections and significance are not understood. The optic tract (Fig. 993) is the continuation of the optic nerve, its chief constituents being the crossed and uncrossed retinal and the supplementary fibres. On leaving the commissure, the tract diverges in front of the interpeduncular space, mesial to tlie .-interior perforated spare and tin- termination of the internal carotid artery, and sweeps outward and backward from the base of the brain around and close to the cerebral peduncle, becoming flatter and broader as it proceeds. Near THE OCULOMOTOR NERVE. 1225 its posterior end the tract exhibits a furrow that indicates a subdivision into a mesial and a lateral root (Fig. 915). The latter, the visual portion of the optic tract, is traceable into the prominent overhanging pulvinar of the optic thalamus, the ill- defined lateral geniculate body and, by means of the superior brachium, into the supe- rior quadrigeminal body. The mesial root, on the other hand, contains the fibres forming Gudden's commissure (page mo) and is related to the distinct median geniculate body and, by the inferior brachium, to the inferior quadrigeminal body. Central and Cortical Connections. — Arising as axones of the retinal neurones, the optic nerve-fibres are continued backward through the commissure and tract and end in relation with the neurones of the primary centres situated in the pulvinar, the lateral geniculate and the superior quadrigeminal body. It is, however, within the lateral geniculate body that the greater number (80 per cent, according to Monakow) of the visual fibres terminate, relatively few pass- ing to the pulvinar and the superior quadrigeminal body (Spiller). The cortical connections are established by fibres which pass from the cells of these primary centres and, as the optic radia- tion (page 1123), sweep outward and backward into the occipital lobe to end in the cortex of the cuneus in the vicinity of the calcarine fissure. It is probable that a limited number of retinal fibres pass directly to the cerebral cortex without interruption in the primary centres. In addi- tion to the centripetal paths just mentioned, fibres arise from the cortical cells of the cuneus and, sharing the optic radiation, pass as efferent tracts which not only terminate in the lateral geniculate and quadrigeminal bodies, but also establish indirect relations with the nucleus of the oculomotor nerve. The ultimate distribution and influence of the impressions of sight are very complex and far reaching, such impressions being capable of affecting numerous motor and sensory centres. The exact path by which pupillary impulses reach the oculomotor nucleus is uncertain and perhaps two-fold. It may be assumed, however, that if they proceed by way of the superior quadrigeminal body, the optic fibres are not directly continued to the nucleus of the third nerve, but end within the superior colliculus, from whose neurones the immediate connecting links pro- ceed to the oculomotor nucleus. Accumulating evidence points to the existence of a more remote special centre for pupillary reflexes within the lower part of the medulla ; in such case the oculomotor nucleus is, perhaps, influenced by impulses which pass from the medullary centre upward by way of the posterior longitudinal fasciculus (Bach). Practical Considerations. — The cranial nerves of the eye will be discussed in connection with that organ. THE OCULOMOTOR NERVE. The third or oculomotor nerve (n. oculomotorius), the chief motor nerve of the intrinsic and extrinsic muscles of the eyeball, supplies branches to all the extraocular muscles, with the exception of the external rectus and superior oblique, as well as fibres to the sphincter pupillae and the ciliary muscle within the eyeball. Its deep origin is from the ocidomotor nucleus situated medially and deeply within the gray matter of the floor of the Sylvian aqueduct, in close relation with the dorsal surface of the posterior longitudinal fasciculus (Fig. 963). The nucleus is from 6-8 mm. in length and extends from opposite the upper end to the caudal pole of the superior quadrigeminal bodies. Below, its posterior end comes almost into contact with the nucleus of the fourth nerve, but is separated from it by a narrow interval. In its entirety the oculomotor nucleus includes a number of more or less distinct cell-groups, which vary in importance as well as in their individual prominence. Of these the most impor- tant and constant are two long columns of cells, the chief nuclei, that extend, one on each side, along the dorsal surface of the posterior longitudinal fasciculi. Each nucleus tapers slightly towards either end and consists of two fairly distinct subdivisions which, from their relative positions, are termed the dorsal and the ventral cell-group. The component nerve-cells include those of large, medium and small size, the large multipolar ones (from .04O-.045 mm. in diam- eter) probably being the elements from which the root-fibres of the third nerve arise. Dislo- cated portions of the chief nucleus are seen as small groups of nerve-cells that lie scattered among or even beneath the fibres of the posterior longitudinal bundle. Dorsal to the chief nucleus and partially overlying its postero-median surface is the taper- ing column of small nerve-cells known as the Edinger-Westphal nucleus. This tract, much more bulky above than below (Tsuchida), exhibits a subdivision into a dorso-lateral and a ventro-median portion, which, however, are fused in the superior pole of the nucleus. The 1226 HUMAN ANATOMY. exact relations of the Edinger-Westphal nucleus to the fibres of the third nerve are still unde- termined, and, indeed, even its close association with these has been questioned. The assumed importance of the nucleus as a centre for pupillary reflexes (Bernheimer) has been seriously shaken by the recent observations of Tsuchida.1 This investigator also denies the existence of a well marked and constant unpaired median nucleus as described by Perlia. but admits the presence of broken groups of medially placed cells, especially in the upper and lower thirds o* the nucleus. The lateral group of cells, beginning in the floor of the third ventricle and extend- ing caudally as far as the upper third of the chief nucleus, constitutes the nucleus of Darksche- witsch. Notwithstanding its proximity to the origin of the third nerve, this nucleus is now regarded as having no direct relation with that of the oculomotor, but as standing in intimate asso- ciation with the posterior longitudinal bundle, among whose fibres the cells to a large extent lie ; it is, therefore, now often referred to as the nucleus fasciculi longitudinalis posterioris. FIG. 1051. &1 "3 i 8. C fc « CS — U u a •?. £ •" 5 s « ft Ml I 0*5 -•- 5 "" * i. " '" ** S 2 o I 1 Ssi s asw a « -C.2 °I|I U x 3 Lachrymal gland Levator palpe- brae superioris Superior rectus muscle External rectus, insertion Inferior oblique muscle o~a $1 *z ""12 1 51 -H • -\ \ I.; ai tg.3g s|i y. «f Sa -2=^2 i: 92 Cut surface of malar bone H £3 I 35- B t-o* "H &£ . V 4; 3 -"OS ii 'C 2* 'S-i2 B' o Dissection of right orbit, showing oculomotor and abducent nerves. Although it may be assumed with much probability that the fibres destined for the different eye-muscles originate from definite groups of nerve-cells, all attempts to locate with accuracy the position of such centres within the oculomotor nucleus have met with only partial success. Tsuchida's conclusions, based upon histological, embryological, comparative and clinical data, point to an unexpected diffuseness in the origin of the oculomotor fibres with only a limited relation to distinct groups. Concerning the mooted question as to the extent of decussation of the oculomotor fibres ins probable that such crossing occurs principally within the caudal portion of the chief nuclei, although, according to Tsuchida and others, some decussating fibres are found throughout the greater part of the nuclei. The fibres of the third nerve originate principally as the axones of the cells on the same side, although a small number arc derived from the neurones lying on the opposite side of the mid-line. Some of these decussating fibres supply tin-' internal rectus and an- related with the nucleus of the sixth nerve, which sends "fibres by way of the posterior longitudinal bundle into the oculomotor nucleus. Whether these 'Arbeiten a. d. Hirnanatom. Institut in Zurich, Heft ii., 1906. 1 i THE OCULOMOTOR NERVE. 1227 fibres end within the latter nucleus around the cells from which the decussating fibres proceed, or are actually prolonged as certain of the decussating fibres is uncertain ; their purpose is to bring into coordinated action the internal rectus of one side with the opposite external rectus when the two eyes are directed laterally, as in conjugate deviation. Cortical and Central Connections. — As in the case of all other motor cranial nerves, the nucleus of the third nerve stands in direct relation to the cerebral cortex. Fibres from the cells of the cortical centre — axones from the neurones within the posterior part Olfactory bulbs Olfactory tract Optic nerve Optic chiasm Optic tract III. nerve- VII. nerve VIII. nerve IX. nerve X. nerve XL nerve XI. nerve spinal portion Part of XII. nerve Superior medullary velum FIG. 1052. Branch of supraorbital nerve Supratrochlear branch of frontal Supraorbital branch of frontal Lachrymal gland Lachrymal nerve Ophthalmicdivisipnor V. nerve — its division into frontal, lachrymal III. nerve [and nasal Maxillary division of V. nerve IV. nerve, to inner side of which is VI. nerve Mandibular division of V. nerve Gasserian ganglion Sensory root of V. VII. nerve [iierve VIII. nerve Middle cerebellar peduncle IX. nerve X. nerve XII. nerve IV. ventricle Medulla, closed part Base of skull, viewed from above, showing cranial nerves passing through dura; roof of right orbit has been removed to expose the ophthalmic nerve. of the inferior frontal convolution, slightly in front of the precentral fissure (Mills)— proceed by way of the corona radiata, the internal capsule and the cerebral peduncle to the oculo- motor nucleus, around whose cells, chiefly but not exclusively on the opposite side, they end. Other connections of the nucleus of the third nerve include : (i) indirectly with the cor- tical visual area by fibres that pass from the occipital cortex through the optic radiation and superior brachium to the superior corpora quadrigemina ; (2) indirectly with the visual centres by fibres that descend from the cells within the superior corpora quadrigemina ; (3) by means of the posterior longitudinal bundle with the nuclei of the other ocular nerves (the fourth and the sixth) and also with the vestibular (Deiters') nucleus of the eighth; (4) with the facial nucleus by fibres that descend from the oculomotor nucleus along the posterior longitudinal bundle to the cells from which proceed the fibres supplying the orbicularis palpebrarum and the corrugator supercilii muscles, which are thus brought into coordinated action with the levator palpebrarum. 1228 HUMAN ANATOMY. Intracranial Course. — Leaving their deep origin as the axones of the nuclear cells, the oculomotor fibres sweep in ventrally directed curves (Fig. 963) through the posterior longitudinal bundle, tegmentum, red nucleus and inner margin of the substantia nigra and, collected into about a dozen root-bundles, have their super- ficial origin along a shallow groove, the oculomotor sulcus (Fig. 974), on the medial surface of the cerebral peduncle, just in front of the pons and at the side of the interpeduncular space. Beyond this superficial origin, the linear group of root-fibres soon becomes consolidated into the large and conspicuous trunk of the third nerve, although not infrequently one root-bundle emerges more laterally from the ventral surface of the cerebral peduncle and for a short distance remains separated from the other constit- uents. The nerve courses forward and outward from the posterior perforated space, between the posterior cerebral and superior cerebellar arteries, to the outer side of the posterior clinoid process, where, in the triangular interval between the free and attached borders of the tentorium, it enters the dura (Fig. 1033). Embedded within this membrane, the nerve follows the upper portion of the outer wall of the cavernous sinus and leaves the cranium by entering the orbit through the sphenoidal fissure. On gaining the median end of the fissure the nerve divides into a superior and an inferior branch, which enter the orbit by passing between the two heads of the external rectus muscle, in company with, but separated by, the nasal branch of the trigeminal nerve, the sixth nerve lying below. Branches and Distribution. — The superior branch (ramus superior) (Fig. 1051), the smaller of the two, passes upward, over the optic nerve, to the superior rectus muscle, which, together with the levator palpebrse superioris, it supplies. In both cases the nerve enters the ocular surface of the muscle. The inferior branch (ramus inferior) (Fig. 1051) is directed forward and, after giving off twigs to the ocular surface of the internal and inferior recti, is continued below the eyeball, between the inferior and external straight muscles, to supply the inferior oblique, whose posterior border it enters. This, the longest branch of the oculomotor nerve, in addition to sending one or two fine twigs to the inferior rectus, contributes a short thick ganglwnic branch (Fig. 1051), which joins the postero-inferior part of the ciliary ganglion (page 1236) as its short or motor root and conveys fibres destined for the sphincter pupillae and ciliary muscles. Sensory fibres from the ophthalmic division of the fifth nerve are distributed to the muscles along with the fibres of the third, having joined the latter before it entered the orbit. Similarly in the wall of the cavernous sinus, the nerve is joined by sympathetic fibres from the cavernous plexus on the internal carotid artery. Variations.— These consist, for the most part, of unusual branches which at times seemingly replace one of the other motor orbital nerves. Thus, the third nerve may give a branch to the external rectus, either in addition to, or to the exclusion of the sixth, which may be absent ; or it may give a filament to the superior oblique. Minor deviations in the course of its branches, such as piercing the inferior rectus or the ciliary ganglion, have also been recorded. THE TROCHLEAR NERVE. The fourth or trochlear nerve (n. trochlearis), also called the pathetic, is the smallest of the cranial series and supplies the superior oblique muscle of the eyeball. The deep origin of the nerve is from the trochlear nucleus, a small oval collection of cells situated in the ventral part of the gray matter surrounding the Sylvian aque- duct, that extends from opposite the upper part of the inferior quadrigeminal body to the lower pole of the superior colliculus. This nucleus, about 2 mm. in length, lii-^ near the mid-line and immediately below (caudal to) that of the third nerve, from which, however, it is distinct, being separated by a narrow interval from the ventral part of the oculomotor nucleus. It lies in intimate relation with the pos- terior longitudinal fasciculus in a distinct depression on the dorsal surface of that l.imdle (Fig. 960). In structure the trochlear nucleus resembles that of the oculo- motor, its ucr\ -r -cells including those of large, medium and small size. Arising from the nucleus, the root-fibres of the fourth nerve pursue a course of considerable length within the mid-brain before gaining their superficial origin. THE TROCHLEAR NERVE. 1229 Leaving the upper and lateral part of the nucleus as axones of the trochlear neurones, the strands of fibres pass outward and backward within the gray matter of the floor of the aqueduct until they near the inner concave surface of the mesencephalic root of the fifth nerve, which, after being condensed into one or two bundles, they follow downward as far as the superior extremity of the fourth ventricle. Then bending sharply medially, the fourth nerve, so far as the great majority of its fibres are concerned, enters the superior medullary velum, in which it decussates with its fellow of the opposite side and crosses the mid-line to emerge at its superficial origin on the dorsal surface of the brain-stem (Fig. 957) just below the inferior corpora quad- rigemina, between the frenum of the velum and the mesial border of the superior cerebellar peduncle. Cortical and Central Connections. — The trochlear nucleus is directly connected with the cerebral cortex by fibres which descend from the inferior frontal convolution through the corona radiata, the internal capsule and the cerebral peduncle and cross to the nuc'eus of the opposite FIG. 1053. Olfactory tracts IV. nerve Optic chias Gasserian ganglion Int. carotid artery III. nerve. \_^\~ ^^^^MT- - ST ^V\X -Supratrochlear nerve .Supraorbital Levator pal- 'pebrae superioris Rectussuperior Lachrymal nerve Rectusexternus Br. of communi- cation bet.lachry. mal and temporo ir.alar br. maxil- lary nerve 'Malar br. tempo- ro-malar nerve Temporal br. temporo-malar ^p^^s&gssmrziit*®* Middle peduncle of cerebellum Medulla oblongata X. nerve IX. nerve \ \ VII. nerve VIII. nerve Temporal bone, cut Ophthalmic div. V. nerve Maxillary div. V. nerve Mandibular div. V. nerve Geniculate ganglion of VII. nerve (a part of great superficial petrosal nerve is seen passing beneath Gasserian ganglion) Dissection showing right trochlear nerve throughout its length, also oculomotor and frontal and lachrymal branches of trigeminal nerve ; roof and outer wall of orbit have been removed. side. By means of the posterior longitudinal bundle it is brought into relation with the nucleus of the third and of the sixth nerve, thus insuring harmonious action of the eye muscles; further, by means of the same path, it is probably connected with the auditory nuclei by way of the superior olive and its peduncle. Course and Distribution. — Emerging at its superficial origin, the nerve is directed outward over the superior cerebellar peduncle, then winds forward around the outer surface of the cerebral peduncle, parallel to and between the posterior cerebral and superior cerebellar arteries, and appears at the base of the brain (Fig. 1053). Proceeding forward to the floor of the cranium, the nerve enters the dura immediately beneath the free border of the tentorium, slightly behind and external to the posterior clinoid process and the third nerve, and continues in the outer wall of the cavernous sinus, at first having the third nerve above it and the ophthalmic division of the fifth below, and then crossing above the third from below inward, to gain the medial end of the sphenoidal fissure. It enters the orbit above the heads of 1230 HUMAN ANATOMY. the external rectus muscle and, directed medially, crosses above the levator palpebrse superioris and superior rectus and reaches the superior oblique, which it enters on the upper surface close to the external border (Fig. 1056). The communications of the trochlear nerve, as it courses in the wall of the cavernous sinus are: (i) filaments from the carotid sympathetic plexus; (2) fibres of common sensation from the ophthalmic division of the fifth. Variations. — The course of the trochlear nerve is sometimes through instead of over the levator palpebne superioris. Unusual branches to sensory nerves, as the frontal, supratroch- lear, the infratrochlear and the nasal, are probably due to the aberrant course of sensory fibres from the trifacial. The fourth nerve occasionally sends a branch to the orbicularis palpebrarum. THE TRIGEMINAL NERVE. The fifth, trigeminal or trifacial nerve (n. trigeminus), the largest of the cranial series, is a mixed nerve and consists of a large sensory part (portio major) and a much smaller motor portion (portio minor). The former supplies fibres of common sensation to the front part of the head, the face, a portion of the external ear, the eye, the nose, the palate, the naso-pharynx in part, the tonsil, the mouth and the tongue. The motor portion is distributed to the muscles of mastication, the mylo- hyoid and the anterior belly of the digastric. The relation of the fibres composing these two parts to the cells within the brain-stem is, therefore, very different, in the case of the motor fibres the cells being a nucleus of origin and in that of the sensory fibres one of reception. The Sensory Part. — The fibres comprising the sensory part of the trigeminal nerve, which convey sensory impulses from the various head-structures, are the pro- cesses of cells lying outside the central axis in the Gasserian ganglion on the sensory root. The portions of the fibres between the periphery and the ganglion correspond to elongated dendrites, while the much shorter centrally directed constituents of the sensory root, connecting the ganglion with the brain-stem, are the axones of the Gasserian neurones. The general resemblance between the fifth cranial nerve and a typical spinal nerve is striking, in each case the sensory root bearing a ganglion and the motor root proceeding from cells within the central nervous axis. Proceeding brainward as axones of the Gasserian cells, the sensory fibres of the trigeminal nerve become consolidated into the large sensory root, which passes through an opening in the dura mater (Fig. 1033) situated beneath the attachment of the tentorium cerebelli to the posterior clinoid process. Coursing backward through the posterior fossa of the cranium it enters, the brain-stem on the lateral sur- face of the pons, slightly behind the superior border, as the conspicuous group of robust bundles that mark the superficial origin of the nerve (Fig. 1046). Just above it is the superficial origin of the motor root, from which it is separated by a small bundle of pontine fibres which belong to the middle cerebellar peduncle. Below and in line with it are the superficial origins of the facial and auditory nerves. Entering the tegmental portion of the pons, close to the overlying superior cerebdlar peduncle, the sensory fibres soon come into relation with the extensive trigeminal reception- nucleus, a columnar mass of gray matter within the lateral part of the tegmentum (Fig. 935). This nucleus extends from the middle of the pons through the entire length of the medulla and into the spinal cord as far down as the level of the second cervical segment, where it becomes continuous with the substantia gelatinosa of the cord. The rounded and enlarged upper end of this tapering column is described as the sensory nucleus of the fifth nerve, although it com- prises only a small part of the reception-nucleus. The latter, in turn, is the upward prolongation of the substantia gelatinosa Rolandi, conspicuous in all cross-sections of the lower pons and nirdulla as an oval field of gray matter (Fig. 930). On Hearing this column the sensory fibres divide into ascending and descending branches, much in the same way as the posterior root-fibres bifurcate within the posterior columns of the rord. The ascending fibres, distinctly liner than the descending, soon penetrate the sensory- nucleus and tin- snbstantia gelatinosa and end in arborizations around the neurones of the reception nucleus. The coarser descending fibres become collected into a compact bundle, the descending or spinal root (n.ictns spin.ilis n. iriyemini), whose medially directed concavity closely embraces the lateral surface of the column of gray substance. Beginning with its descent, the THE TRIGEMINAL NERVE. 1231 spinal root gives off collaterals and fibres that bend medially, enter the adjacent substantia gel- atinosa and end in arborizations around the reception cells of that nucleus. Since the number of fibres is thus progressively reduced during the descent of the spinal root, the tract is tapering, becoming smaller and smaller as it approaches the spinal cord until within the upper part of the latter, at about the level of the second cervical nerve, it finally disappears. In its descent through the brain-stem the spinal tract becomes more and more superficially placed, in the lower part of the pons lying to the inner side of the restiform body, separated from it by the vestibular division of the auditory nerve, and lower, in the lateral area of the medulla, occupying a position close to the surface as it rests upon the expanded gelatinous substance of the tuberculum Rolandi. The central connections of the sensory part of the trigeminus (Fig. 1054), by way either of the collaterals of the fibres of the spinal root or of the axones and collaterals of the axones of the reception neurones, are undoubtedly very extensive, since the impulses collected by this important nerve are widely dispersed. The most important paths for such distributions are : 1. By axones that pass, as arcuate fibres, from the cells of the reception-nucleus across the raphe to join the opposite mesial fillet and ascend to the optic thalamus and FIG. 1054. thence, after interruption in the cells of the latter, by axones of thalamic neu- rones to the cerebral cortex. It is prob- able that some of the arcuate fibres do not cross the mid-line, but ascend within the mesial fillet of the same side. It is also probable that collaterals of the arcuate fibres pass to the trigeminal, facial and glosso-pharyngeo-vagal motor nuclei. 2. By axones from the cells of the reception nucleus that enter the infe- rior cerebellar peduncle of the same side and pass to the cerebellar cortex as con- stituents of the nucleo-cerebellar tract. 3. By collaterals that are distrib- uted to the nuclei of origin of the hypo- glossal and of the motor part of the tri- geminus and facial nerves, whereby these important motor nerves are brought directly under the influence of the sensory part of the fifth. The Motor Part. — In con- trast to the median position of the nuclei of origin of the oculomotor, trochlear, abducent and hypoglos- sal nerves, the deep origin of the motor part of the trigeminus in- cludes groups of cells that lie at some distance from the raphe and fall into series with the laterally placed nuclei of the motor parts of the other mixed cranial nerves — the facial, the glosso-pharyngeal and the vagus. 1. The largest contingent of the motor fibres of the trifacial nerve arise as axones from the neurones within the chief motor nucleus (nucleus masticatorius) (Fig. 935). This nucleus con- sists of a short columnar collection of gray matter, oval on cross-section, which lies in the upper part of the pons, close to the median side of the sensory nucleus. It is composed of large stel- late cells from which, as their axones, the motor fibres proceed outward through the tegmentum to their superficial origin on the pons. A small number of fibres, from the more medially situ- ated cells of the nucleus, pursue a dorsally convex course toward the raphe, which they cross close beneath the floor of the fourth ventricle to join the motor nucleus of the opposite side and become incorporated in the opposite trigeminal motor root. 2. A second and smaller constituent of the motor root, the descending mesencephalic root (radix descendens n. trigemini) includes fibres that arise from cells lying within the lateral part of the gray matter surrounding the Sylvian aqueduct. In cross-sections (Fig. 936) this root appears as a delicate crescentic bundle that descends from the mid-brain to join the larger tract Diagram showing relations of trigeminal root-fibres to nuclei within brain-stem; GG, Gasserian ganglion with divisions (/, //, ///) of sensory part of nerve ; SK, MR, sensory and motor roots : S, sensory nucleus ; SG, substantia gelatinosa : Sfi.R, spi- nal or descending root ; f, mesial fillet ; Ci>, nucleo-cerebellar fibre : Af, motor nucleus ; MsR, mesencephalic root ; Sf, sub- stantia ferruginea; CB, cortico-bulbar fibres. 1232 IITMAN ANATOMY. FIG. 1055. Optic nerve of fibres from the chief motor nucleus. In its downward course the mesencephalic root is joined by numerous fibres which have their origin in the pigmented cells of the substantia ferru- ginea (page 1081) of the same and, possibly, of the opposite side. The fibres from these various sources — the mesencephalic nucleus, the substan- tia ferruginea and the motor nucleus — become consolidated into the motor root of the trigeminal nerve, whose superficial origin (Fig. 1046) is just above that of the sensory root, from which it is separated by some of the superficial transverse fibres of the pons. Leaving the side of the pons, the motor root follows the same course to and through the dura mater as does the sensory, to the inner side of which it lies. It eventually passes beneath the Gasserian ganglion to become exclusively an integral portion of the mandibular division of the trigeminal. The cortical connections of the motor root are established by fibres that arise from cells within the cortical gray matter of the lower third of the precentral convolution. Thence, as constituents of the pyramidal tracts, they descend through the corona radiata, the internal cap- sule and the cerebral peduncle into the pons, where, for the most part after decussation, they terminate in end-arborizations around the radicular cells of the motor trigeminal nuclei. The Gasserian Ganglion.— The Gasserian ganglion (ganglion serailunare [Gasseri]) (Fig. 1055) is an important complex of nerve-fibres and cells, which lies in a slight depression on the apex of the petrous portion of the temporal bone. In shape it is a flattened crescent with its convexity forward, measuring from 1.5—2 cm. in width and about i cm. in length. The sur- face of the ganglion presents an irregular longitudinal or reticu- lar striation. From the anterior expanded convex border of the ganglion arise the ophthalmic and maxillary nerves and the sensory portion of the mandib- ular nerve, while its narrow concave posterior margin is con- tinued into the sensory root of the fifth nerve. The ganglion lies in Meeker s space (cavum Meckelii), a cleft produced by a delamination of the dura mater, and comes in relation internally with the cavernous sinus and the internal carotid artery. Be- neath, but unconnected with it, are the motor root of the trifacial and the great superficial petrosal nerve. In struc- ture it resembles a spinal ganglion, being composed of the characteristically modified neurones, from whose single processes proceed the peripherally directed dendrites and the centrally coursing axones. In addition to the three large trunks given off from the anterior margin, the branches of the Gasserian ganglion include some fine meningeal filaments which arise from the posterior end of the ganglion and are distributed to the adja- cent dura mater. Communications. — At its inner side the < '.usseri, m ganglion receives filaments from the fidjacent carotid plexus of the sympathetic, which end in relation with the cells of the ganglion. Divisions of the Trigeminal Nerve. — These are three in number, the oph- thalmic, the maxillary and the imindibular nerves. They arise from the anterior Gasserian ganglion Gasserian ganglion of left side viewed from above; sensory and motor roots and three divisions of trigeminal nerve are seen. THE TRIGEMINAL NERVE. 1233 margin of the Gasserian ganglion, the formation of the mandibular nerve being com- pleted by the accession of the motor root of the trigeminal. I. The Ophthalmic Nerve. — The ophthalmic nerve (n. ophthalmicus) (Fig. 1056), the smallest of the three divisions, is purely sensory and supplies the upper eyelid, the conjunctiva, the eyeball, the lachrymal gland, caruncle and sac, the fore- head and anterior part of the scalp, the frontal sinus and the root and anterior por- tion of the nose. It arises from the anterior margin of the Gasserian ganglion and passes upward and forward for about 25 mm. in the external wall of the cavernous sinus, lying below the fourth nerve. Reaching the sphenoidal fissure it breaks up into its terminal branches, which pass through the fissure into the orbit. Branches and Distribution. — The branches of the ophthalmic nerve are: (i) the reciirrent, (2) the communicating, (3) the lachrymal, (4) the frontal, and (5) the nasal, of which the last three are terminal branches. FIG. 1056. Supra tochlear nerve Nasal nerve Olfactory bulbs superior oblique muscle IV. nerve Cut edge of bone- Optic nerve Optic chiasm — -H internal carotid artery Optic tract VI. nerve III. nerves Cerebral peduncles Supraorbital nerve Lachrymal gland Levator palpebrse superioris Superior rectus Frontal nerve External rectus Lachrymal nerve Ophthalmic division of V. nerve Maxillary division of V. nerve — Mandibular division of V. nerve Gasserian ganglion Meatus auditorius internus VII. nerve, motor part Pars intermedia VIII. nerve Roof of right orbit has been removed to expose branches of ophthalmic division of trigeminal nerve; Gasserian ganglion, and third, fourth, sixth, seventh and eighth nerves also seen. 1. The recurrent branch (n. tentorii) arises shortly after the nerve leaves the ganglion. It passes across and is adherent to the trochlear nerve and is distributed between the layers of the tentorium cerebelli. 2. The communicating branches are three slender filaments which are given off before the nerve breaks up into its terminal branches ; they join the trunks of the third, fourth and sixth nerves, to whose muscles they supply sensory fibres. During its passage through the cavernous sinus, the ophthalmic nerve receives some tiny filaments from the cavernous sympathetic plexus. 3. The lachrymal nerve (a. lacrimalis) (Fig. 1053) is the smallest of the terminal branches. It lies to the outer side of the frontal nerve and traverses the outer angle of the sphenoidal fissure in its own sheath of dura mater. It passes above the origin of the orbital muscles and courses along the lateral wall of the orbit, above the external rectus, to the upper outer angle of the orbit, where it pierces the palpebral fascia near the external canthus to terminate in the upper eyelid. It sup- plies the lachrymal gland, the upper eyelid and the skin around the external canthus. 78 1234 HUMAN ANATOMY. Within the orbit the lachrymal nerve communicates with the temporal branch of the temporo-malar nerve and on the face with the temporal branch of the facial. The latter is one of the numerous sensory-motor communications between the terminal fibres of the fifth and seventh nerves. Variations. — Occasionally the lachrymal nerve seems to be partly derived from the troch- lear ; the true source of such fibres, however, is probably the ophthalmic nerve, by way of its communicating branch to the fourth. Considerable variation is found in connection with the temporal branch of the temporo-malar nerve. The lachrymal nerve or the temporal branch of the temporo-malar may be absent, the place of either being taken by the other, or the lachrymal may be small at its origin and later increased to normal size by accessions from the temporal branch of the temporo-malar. 4. The frontal nerve (n. frontalis) (Fig. 1053) is the largest branch of the ophthalmic. It enters the orbit, invested by its own dural sheath, through the sphenoidal fissure and above the orbital muscles and passes directly forward between the periosteum and the levator palpebrae superioris. At a variable point, usually about the middle of the orbit, it divides into its terminal branches, the (a) supra- trochlear and (6) the supraorbital. a. The supratrochlear nerve (n. supratrochlearis) is the smaller of the two terminal branches. It passes inward and forward over the pulley of the superior oblique and thence between the orbicularis palpebrarum and the frontal bone, leaving the orbit at its upper inner angle. Near the pulley it gives off a branch which joins the infratrochlear (Fig. 1057) and at the edge of the orbit supplies filaments (nn. palpebrales superiores) to the skin and conjunctiva of the upper eyelid. It then turns upward and subdivides into a number of small branches which pierce the substance of the frontalis and orbicularis palpebrarum muscles to supply the inner and lower part of the forehead. b. The supraorbital nerve (n. supraorbitalis) (Fig. 1056) continues directly the course of the frontal nerve. It lies close to the periosteum throughout its entire orbital course and leaves the orbit through the supraorbital notch or foramen. In this situation it sends a small filament to the frontal sinus to supply its diploe and mucous membrane. As it leaves the orbit it sup- plies some fine twigs to the upper eyelid and then divides into a larger outer and smaller inner branch. These pass upward on the forehead beneath the frontalis muscle, occasionally occupy- ing quite deep grooves in the frontal bone, and terminate by being distributed to the scalp and pericranium. The outer branch extends back nearly to the occipital bone, while the inner passes only a short distance posterior to the coronal suture. Both branches of the frontal, the supratrochlear and the supraorbital, communicate with branches of the facial nerve and thereby supply sensory filaments to muscles supplied by the seventh. •*•$ Variations. — The nerve may divide before leaving the orbit*and in that event only the outer branch passes through the normal osseous channel. The inner sometimes has a special groove, named by Henle the frontal notch. 5. The nasal nerve (n. nasociliaris) (Fig. 1057) is intermediate in size between the lachrymal and the frontal. It enters the orbit, clothed in dura mater, through the sphenoidal fissure, between the heads of the external rectus and betwe« the superior and inferior divisions of the oculomotor nerve. Turning obliquely ii ward, it crosses the optic nerve and passes beneath the superior oblique and superic rectus muscles and above the internal rectus. Thence it traverses the anterior eth- moidal foramen to enter the cranial cavity, where it passes forward in a groove in the lateral part of the cribriform plate of the ethmoid bone. Leaving the cranium through the nasal fissure, the nerve enters the nasal fossa, where it breaks up into its three terminal branches. Branches. — These are : (a) the ganglionic, (b} the long ciliary, (r) the infr trochlcar, (d) the internal nasal, (e) the external nasal and (/) the anterior nasal, which the last three arc terminal branches. a. The gaiix/ionic branch (radix lonna) (Fig. 1057) usually leaves the nerve between the heads df tlu- external rectus and passes forward along the outer side of the optic nerve to enter the upper posterior portion of the ciliary ganglion, of which it forms the sensory or long root />. The /<>;/ Internal rectus muscle^ ( Infratrochlear br. of nasal Nasal nerve Olfactory bul Leva tor palpebne superi- oris, inverted III. nerve, superior division' Frontal nerv Optic nerve Internal carotid artery 5 III. nerv Pons, displaced backward Cerebral peduncle- Levator palpebrse superiorts Superior rectus Lachrymal gland Nerve to inferior oblique External rectus muscle Ciliary ganglion Nasal nerve Lachrymal nerve Maxillary division of V. Ophthalmic division of V Mandibular division of V •asserian ganglion VI. nerve IV. nerve VII. nerve VIII. nerve Deeper dissection of right orbit, viewed from above ; branches of nasal nerve shown. upper lateral cartilage of the nose, finally emerging from under cover of the compressor naris muscle. It supplies the skin of the fore-part and tip of the nose. Variations. — The nasal nerve may send branches to the superior and internal recti and levator palpebrae superioris muscles. In one case a small ganglion connected with the nasal nerve sent fibres to the third and sixth nerves. Instances are recorded of absence of the infratrochlear branch, the deficiency being supplied by the supratrochlear. Branches to the frontal and ethmoidal sinuses are described as being given off in the anterior ethmoidal fora- men, and a branch has been found which passes through the posterior ethmoidal foramen to supply the sphenoidal and posterior ethmoidal sinuses. The latter has been called by Luschka the spheno- ethmoidal and by Krause the posterior ethmoidal branch. The Ganglia associated with the Trigeminal Nerve. — Four small ganglia are connected with the extracranial portion of the fifth nerve. They are the ciliary, the spheno-palatine, the otic and the submaxillary. The ciliary ganglion is associated 1236 HIM AN ANATOMY. with the ophthalmic nerve, the spheno-palatine with the maxillary and the otic and submaxillary with the mandibular. Each is the recipient of three roots — a motor, a sensory and a sympathetic — and from each ganglion branches are given off to more or less contiguous structures. The significance of these bodies — whether of the nature of spinal or sympathetic ganglia — has long been a subject of discussion. The close resemblance of their nerve-cells to the stellate neurones of undoubted sympathetic ganglia, as shown by the investigations of Retzius, Kolliker and others, as well as the results of experimental studies (Apolant), justifies the conclusion that these ganglia are properly regarded as belonging to the sympathetic group. They are, therefore, probably stations in which certain motor and secretory fibres contributed by various nerves end in arborizations around sympathetic neurones, from which axones pass for the immedi- ate supply of involuntary muscle and glandular tissue. The fact that these small ganglia are derivations of the early Gasserian ganglion is in accord with the mode of origin of the sympathetic ganglia elsewhere (page 1013). FIG. 1058. Internal carotid artery IV. nerve \ Cerebral peduncle \ V Levator palpebrae supcriorls Superior oblique muscle Lachrymal gland Superior rectus muscle Long ciliary branches of nasal nerve Ext. rectus, insertion Inferior oblique muscle Middle cerebellar peduncle Casserian ganglion / Ext. rectus muscle VI. ner Ganglionic branch of nasal Ciliary ganglion Short ciliary nerves Branch to inf. oblique Inferior rectus muscle Dissection of right orbit after removal of its lateral wall ; external and superior eye-muscles have been cut and displaced to expose ciliary ganglion and nerves. The Ciliary Ganglion. — The ciliary, ophthalmic or Icnticnlai ganglion (g. ciliare) (Fig. 1058), as it is varyingly called, is a small reddish mass, about 2 mm. long in the antero-posterior direction, and approximately quadrilateral in out- line. It is compressed laterally and to each angle is attached one or more bundles of nerve-fibres. It lies near the apex of the orbit on the outer side of the optic nerve, between the latter and the external rectus muscle and anterior to the ophthalmic artery. The nerve-cells within the ganglion are chiefly multipolar elements, which closely resemble sympathetic neurones ( Retzius) and send their axones towards the eye by way of the short ciliary nerves. Roots. — All of these enter the posterior margin of the ganglion. The motor or short root (radix brcvis), the thickest of the roots and sometimes double, is an off- shoot from tin- branch of the oculomotor nerve which supplies the inferior oblique muscle. It is short and comparatively robust and joins the postero-inferior portion of the ganglion. _ The snisory or long root (radix lonija) arises from the nasal branch of the- ophthalmic, leaving the latter between the heads of the external rectus. It is 'ong- and slender and passes forward to enter the upper posterior angle of the gang- lion, occasionally being fused with the sympathetic root. The sympathetic root (radix THE TRIGEMINAL NERVE 1237 media) is a tiny filament which arises from the cavernous plexus and runs forward to enter, either alone or with the sensory root, the upper posterior angle of the ganglion. Branches. — -These are the short ciliary nerves (tin. ciliares breves). They number from four to six and by division are increased to twelve or twenty before reaching the eyeball (Fig. 1058). They arise as two fasciculi from the upper and lower anterior angles of the ganglion and pass forward above and below the optic nerve. The lower set is the more numerous and on its way forward is joined by the long ciliary nerves from the nasal, with which one or more of its constituent branches usually fuse. After piercing the sclerotic coat in two groups, one below and the other above the entrance of the optic nerve, they pass forward in grooves on the inner surface of the sclerotic to supply the choroid, iris, ciliary muscle and cornea. The short ciliary nerves include three sets of fibres : ( i ) Sympathetic fibres destined for the walls of the blood-vessels and the radial (dilator) muscle of the iris ; these are links in the chain made up of (a) white rami communicantes from the upper thoracic spinal nerves to the cervical gangliated cord, and (b) the axones of neurones within the sympathetic ganglia. (2) Fibres supplying the ciliary muscle and the circular (sphincter) muscle of the iris, which, while in a sense the continuations of the oculomotor nerves, are immediately the axones of the stellate sympathetic neurones within the ciliary ganglion. (3) Trigeminal fibres which transmit sensory impulses from the interior of the eyeball, in conjunction with the long ciliary nerves. Variations. — The motor root occasionally bifurcates before it reaches the ganglion. As noted above, the sensory and sympathetic roots frequently form a common trunk of entrance into the ganglion. Occasionally the ganglion is very small, due possibly to the scattering of its constituent neurones among the nerves connected with it (Quain). Additional roots have been described as corning from the superior division of the oculomotor, from the trochlear, from the lachrymal, from the abducent and from the spheno-palatine ganglion. Absence of the sensory root has been noted, the deficiency possibly being corrected by the long ciliary nerves convey- ing sensory fibres directly from the nasal to their destination, instead of these fibres passing through the ganglion. The sympathetic root may be multiple, a condition held by some to be normal, some of the fibres accompanying the oculomotor nerve. II. The Maxillary Nerve or superior maxillary nerve (n. maxillaris) is purely sensory and is intermediate in size between the ophthalmic and mandibular divisions of the trigeminus. It supplies the cheek, the anterior portion of the temporal region, the lower eyelid, the side of the nose, the. upper lip, the upper teeth, and the mucous membrane of the nose, naso-pharynx, maxillary antrum, posterior ethmoidal cells, soft palate, tonsil and roof of the mouth. Arising from the middle of the anterior convex border of the Gasserian ganglion, it passes forward beneath the dura mater in the middle cranial fossa, lying below the cavernous sinus (Fig. 1053). The nerve leaves the cranium through the foramen rotundum, traverses the spheno-maxillary fossa and enters the orbital cavity by means of the spheno-maxillary fissure. It occupies and then parallels the floor of the orbit in the infraorbital groove and canal, finally emerging on the face by passing through the infraorbital foramen. Here it breaks up fanlike into three terminal groups of branches (Fig. 1060). Branches and Distribution. — Branches are given off from the maxillary nerve in the cranium, in the spheno-maxillary fossa, in the infraorbital canal and on the face. These are : within the cranium, ( i ) the recurrent ; within the spheno-maxillary fossa, (2) the spheno-palatine, (3) the posterior superior dental and (4) the temporo- malar ; in the infraorbital canal, (5) the middle superior dental and (6) the anterior superior dental; on the face (7) the inferior palpebral, ( 8 ) the lateral nasal and ( 9 ) the superior labial. The last three are terminal branches. 1. The recurrent branch (n. meningeus) is given off before the maxillary nerve passes through the foramen rotundum. It supplies the dura mater in the middle cranial fossa. 2. The two or three spheno-palatine branches (nn. sphenopalatini) (Fig. 1061) arise in the spheno-maxillary fossa. They are short and thick and pass directly downward to the upper margin of the spheno-palatine ganglion, whose sensory root they supply. Only a small part of their fibres actually traverse the ganglion, the much larger part passing lateral to or in front of the ganglion, to be continued 1238 HUMAN ANATOMY. into the orbital, posterior nasal and palatine branches. While in neither case are the trigeminal fibres interrupted in the ganglion, in both instances they receive sympa- thetic fibres from the ganglion, which accompany the trigeminal ones. 3. The posterior superior dental nerve (r. alveolaris superior posterior) (Fig. 1060) is frequently double. It passes downward and forward with the posterior dental artery through the pterygo-maxillary fissure to reach the zygomatic surface of the maxilla. It supplies tiny filaments to the gum and adjacent mucous membrane of the cheek and enters the posterior dental canals to supply the molar teeth. It forms a fine plexus (plexus deotalis superior) (Fig. 1059) with the middle and anterior superior dental nerves. Variation. — In the absence of the buccal branch of the fifth, the posterior superior dental has been observed to be of large size and to assume the distribution of the buccal. 4. The temporo-malar or orbital nerve (n. 23 gomaticus) (Fig. 1053) after arising from the maxillary passes from the spheno-maxillary fossa into the orbit FIG. 1059. INT PAIPCBKAL Diagram showing plan and connections of second and third divisions of trigeminus and their ganglia. through the spheno-maxillary fissure. It courses along the external orbital wall and divides into a temporal and a malar branch. The temporal branch (n. z) gomaticoteni' poralis) after inosculating with the lachrymal nerve passes through the spheno-malar foramen to enter the temporal fossa. It then runs between the bone and the temporal muscle and pierces the temporal fascia to be distributed to the skin of the anterior temporal region. It communicates with the temporal branch of the facial nerve. The malar 'hrauch (n. /vnomaticofadalis) traverses the malar foramen to supply the skin of the malar region. It joins with filaments from the malar branch of the seventh. Variations.— The nerve may pass through the malar bone before it divides, both branches in. iv pass separately through canals confined to the malar bone, or the temporal branch may THE TRIGEMINAL NERVE. 1239 through the spheno-maxillary fissure. Either branch may be absent or smaller than normal, the other branch supplying the deficiency. The malar may be replaced in its distribution by the infraorbital and the temporal may be substituted or augmented by the lachrymal. 5. The middle superior dental nerve (r. alveolaris superior medius) leaves the maxillary in the posterior part of the infraorbital canal. It occasionally arises from the anterior superior dental. It passes down in a canal in the outer wall of the maxillary antrum and after forming a plexus with the other two dental nerves supplies the premolar teeth. 6. The anterior superior dental nerve (r. alveolaris superior anterior) is the largest of the three superior dental nerves. It arises from the maxillary just before the exit of the latter at the infraorbital foramen and descends in a canal in the anterior wall of the antrum. It gives off a nasal branch, which enters the nose FIG. 1060. Middle superior dental ner Maxillary ner Posterior superior dental ner Buccal ner Sensory division of mandibular ner' Middle menlngeal artery Auriculo-temporal nerv Ext. pterygoid muscle Lingual nerve Inferior dental ne Superficial temporal artery Internal maxillary artery Int. pterygoid muscle Part of mandible Ext. carotid artery Parotid glan Mylo-hyoid branch of inferior dental Submaxillary ganglior Submaxillary gland Inferior palpebra- Lateral nasal and superior laoiai branches of infraorbital nerve Anterior superioi dental nerve Nasal branch of anterior superior dental Ktional surface of andible Digastric muscle, anterior belly Myo-hyloid muscle, cut to show lingual nerve Dissection showing maxillary and mandibular nerves and their branches; outer wall of orbit, part of facial wall of maxillary sinus and part of mandible have been removed. through a tiny canal in the outer wall of the inferior meatus of the nose and supplies the mucous membrane of the anterior part of the inferior nasal meatus and floor of the nose. After helping to form the superior dental plexus, the anterior superior dental supplies the canine and incisor teeth. Two thickenings are sometimes found in the superior dental plexus. One of these, known as the ganglion of Valentin, lies above the tip of the root of the second premolar tooth, at the junction of the middle and posterior superior dental nerves; and the other, sometimes called the ganglion of Bochdalek, is situated more anteriorly, at the junction of the middle and anterior dental nerves. Neither of these enlargements is a true ganglion, being without nerve- cells and consisting of interlacing bundles of nerve-fibres. 1240 HUMAN ANATOMY. 7. The inferior palpebral branches (rr. palpebrales inferiores) (Fig. 1060) usually two in number, are the smallest of the terminal branches. They pass upward from the infraorbital foramen, pierce the origin of the levator labii superioris, pass around the lower margin of the orbicularis palpebrarum and supply the conjunctiva and skin of the lower eyelid. 8. The lateral nasal branches (rr. nasales externi) (Fig. 1060), from two to four in number, pass inward under the levator labii superioris alaeque nasi and supply the skin of the side of the nose. 9. The superior labial branches (rr. labiales superiores) (Fig. 1060), two to four in number, are the largest of the terminal branches. They pass downward under the levator labii superioris and, after supplying the anterior portion of the skin of the cheek, terminate in the mucous membrane and skin of the upper lip. The last three branches inosculate freely under the levator labii superioris with the infraorbital branch of the facial, forming the infraorbital plexus (Fig. 1068). The Spheno-Palatine Ganglion. — The spheno-palatine ganglion (g. spheno- palatinum), also known as Meckel's, the spheno-maxillary or the nasal ganglion, is a small triangular reddish-gray body, with the apex directed posteriorly, situated in the upper portion of the spheno-maxillary fossa. It is flat on its mesial surface, and convex on its lateral, and measures about 5 mm. in length. It lies in close proximity to the spheno-palatine foramen and just beneath the maxillary branch of the trigeminal nerve (Fig. 1061). The ganglion is regarded as belonging to the series of sympathetic nodes, and consists of an interlacement of nerve-fibres in which are embedded numerous stellate sympathetic neurones. Roots.— The sensory root consists of two, sometimes three, short stout filaments, the spheno-palatine nerves (nn. sphenopalatini), which pass directly downward from the lower margin of the maxillary nerve to the upper border of the ganglion. While some few of the fibres of this root are axones of the sympathetic ganglion-cells, the great majority are dendrites of the cells of the Gasserian ganglion which pass to a limited extent through, but mostly around, the spheno-palatine ganglion independently of its cellular elements. They are continued entirely into the various trunks that are usually described as branches of distribution of the ganglion (see below). The motor root is the great superficial petrosal nerve (n. petrosus superficialis major) which, in all probability, carries sensory as well as motor fibres. It arises from the facial nerve in the facial canal, passes through the hiatus Fallopii and a groove in the petrous portion of the temporal bone and then under the Gasserian ganglion to reach the cartilage occupying the middle lacerated foramen. Here the great super- ficial petrosal nerve is joined by the sympathetic root, the great deep petrosal, (n. petrosus profundus), which is a branch from the carotid plexus. The two great petrosal nerves fuse over the cartilage at the middle lacerated foramen to form the Vidian nerve (n. canalis pterygoidei [Vidii] ) (Fig. 1061), which traverses the canal of the same name and enters the spheno-maxillary fossa to join the spheno-palatine ganglion. In its course through the canal the Vidian nerve gives off a few tiny nasal branches, which, composed of trigeminal and sympathetic fibres, supply the pharyngeal ostium of the Eustachian tube and the posterior part of the roof of the nose and the nasal septum. While in the canal, the Vidian nerve receives a filament from the otic ganglion. In addition to supplying (according to many anatomists) motor fibres to the levator palati and a/ygos uvulae muscles, some of tin.- farial fibres are especially destined for glandular struc- tures. Such fibres are probably interrupted around the stellate cells of the spheno-palatine ganglion, the axones of which then complete the paths for the secretory impulses. The sensory constituents of tin- great superficial petrosal nerve are, perhaps, of two kinds: (a) fibres from the cells of the geniculatr ganglion of the facial to the palatine taste-buds, and (b} recurrent trigeminal fibres, that, by way of the maxillary, spheno-palatine and great superficial petrosal nerves, are distributed with the peripheral branches of the Vidian or of the facial nerve. The great deep petrosal nerve represents the association cord between the superior cervical sympathetic and the spheno-palatine ganglion. Many of its fibres end in arborizations around the stellate spheno-palatine cells, from which, in turn, axones pass to blood-vessels and glands by way of the ganglionic branches of distribution. THE TRIGEMINAL NERVE. 1241 Branches. — The branches of distribution of the spheno-palatine ganglion are conveniently grouped into four sets : (i) the ascending, (2) the descending, (3) the internal and (4) the posterior. 1. The ascending or orbital branches (rr. orbitales) (Fig. 1059) are two or three tiny filaments, which pass into the orbit through the spheno-maxillary fissure and, after traversing the posterior ethmoidal canal or a small special aperture, are distrib- uted to the sphenoidal and posterior ethmoidal air-cells and the periosteum of the orbit. 2. The descending branches (nn. palatial) (Fig. 1059) are three : (a) the large posterior palatine, (6) the small posterior palatine, and (c) the accessory pos- terior palatine nerves. a. The large posterior palatine nerve (n. palatinus anterior) leaves the spheno-maxillary fossa by means of the large posterior palatine canal, through which it descends to the inferior surface of the hard palate. While in the canal it gives )ff one or two posterior inferior nasal branches FIG. 1061. Cavernous plexus and 'nt. carotid artery .Great superficial ^-'petrosal nerve Great deep petrosal nerve from carotid plexus Post. inf. nasal brs. of larg posterior palatin Large, small and accessory poste- rior palatine nerves Naso-palatine nerve, termination Otic ganglion Cartilage of Eustachinn tube, cut Int.br. of ascending ramus sup. cerv. gangl Ext. br. of ascending ramus of sup. cerv ganglion Sup. cerv. gangliok of sympathetic Int. carotid artery tor palati Tensor palati, cut above Dissection showing spheno-palatine and otic ganglia viewed from within. (rr. nasales posteriores inferiores), which, escaping through small apertures in the perpendicular plate of the palate bone, enter the nasal fossa and supply the mucous membrane of all but the anterior portion of the inferior turbinate bone and the adjoining portions of the middle and infe- rior nasal meatuses. Emerging from its canal the main nerve passes forward in a groove on the inferior aspect of the hard palate and inosculates with the terminal filaments of the naso-palatine nerve. It supplies the hard palate and its mucous membrane, as well as the inner side of the gum. b. The small posterior palatine nerve (n. palatinus posterior) descends in the small pos- terior palatine canal. It supplies sensory filaments to the mucous membrane of the soft palate and the tonsil and motor ones to the levator palati and azygos uvulae muscles. c. The accessory posterior palatine nerves (nn. palatinus medius) are one or more small filaments which pass through the accessory posterior palatine canals and supply the mucous membrane of the soft palate and tonsil. 3. The internal branches (rr. nasales posteriores superiores) (Fig. 1059) pass from the spheno-maxillary into the nasal fossa through the spheno-palatine foramen. They are : (#) the posterior superior nasal and (b~) the naso-palatine nerve. 1242 HUMAN ANATOMY. a. The posterior superior nasal nerve (rr. laterales) supplies the mucous membrane of the posterior superior portion of the outer wall of the nasal fossa. b. The naso-palatine nerve (n. nasopalatinus) (Fig. 1059) crosses the roof of the nasal chamber and passes downward and forward in a groove in the vomer and septal cartilage to reach the anterior palatine canal. It then passes through the foramen of Scarpa, the left inrrvt: through the anterior and the right one through the posterior canal, the two nerves forming in this situation a fine plexus. Having reached the inferior surface of the hard palate, the naso- palatine inosculates with the large posterior palatine nerve. It supplies the roof and septum of the nose and that portion of the hard palate which lies posterior to the incisor teeth. 4. The posterior branch (Fig. 1059) also known as the pharyngeal or pterygo-palatine, leaves the spheno-maxillary fossa through the pterygo-palatine canal and supplies the mucous membrane of the naso-pharynx in the region of the fossa of Rosenmiiller. Variations. — Branches of the ganglion have been described as passing to the abducent nerve, to the ciliary ganglion and to the optic nerve or its sheath. The accessory posterior palatine nerve is sometimes absent. Quite frequently the left naso-palatine nerve passes through the posterior foramen of Scarpa and the right nerve through the anterior. III. The Mandibular Nerve. — The mandibular or inferior maxillary branch (n. mandibularis) of the trigeminal nerve is the largest of its three divisions and, being a mixed nerve, consists of two portions, one sensory and the other motor. The sensory part is the larger and arises from the lower anterior portion of the Gasserian ganglion. The smaller motor part is the motor root of the trigeminal nerve, which contributes exclusively to this division of the fifth nerve. Although these two portions are inti- mately associated in their passage through the foramen ovale, the motor bundle lying to the median side of the sensory, it is not until they emerge from the skull that they unite, immediately below the lower margin of the foramen ovale, to form the mandibular nerve. The sensory portion supplies the skin of the side of the head, the auricle of the ear, the external auditory meatus, the lower portion of the face and the lower lip, the mucous membrane of the mouth, tongue and mastoid cells, and the lower teeth and gums, the salivary glands, the temporo-mandibular articulation, the dura mater and the skull. The motor portion supplies the muscles of mastication (the temporal, the masseter and the external and internal pterygoids), the anterior belly of the digastric, the mylo-hyoid, the tensor palati and the tensor tympani muscles. By union of the two constituents, a thick common trunk is formed, which, after a course of from 2-3 mm. , separates under cover of the external ptery- goid muscle into an anterior and a posterior division (Fig. 1063). Branches and Distribution. — The branches from the main trunk of the mandibular nerve are : ( i ) the recurrent branch and (2) the internal pterygoid nerve. 1. The recurrent branch (n. spinosus) arises just beneath the foramen ovale and accompanies the middle meningeal artery into the cranium through the foramen spinosum. It then divides into two branches, the anterior of which supplies the greater wing of the sphenoid and the adjacent dura mater, while the posterior passes through the petro-squamous suture and supplies the mucous membrane of the mastoid air-cells. 2. The internal pterygoid nerve (n. pterygoideus interims) (Fig. 1059) passes downward on the mesial side of its muscle and, in addition to supplying the pterygoid muscle, gives off the motor root of the otic ganglion and filaments to the tensor tympani and tensor pal. it i muscles. The Anterior Division of the mandibular nerve (n. masticatorius) is motor, with the exception of its buccal branch, and receives almost the entire motor constit- uent of the trigeminal. It | MS-US downward and forward for a short distance under the external pterygoid muscle and then breaks up into its branches. Branches. — These are : ( i ) the massc-lcric > (2) the external ptcrvgoid , (3) the deep temporal and ( 4) the buccal ncrrc. i. The masseteric nerve (n. niasscterictis) (Fig. 1063) passes over the upper border of tin- external pteiy^oid and behind the posterior margin of the temporal muscle. It takes a course horixontally outward and traverses the sigmoid THE TRIGEMINAL NERVE. 1243 notch of the mandible to enter the posterior portion of the mesial surface of the masseter. It supplies one or two filaments to the temporo-mandibular articulation. 2. The external pterygoid nerve (n. pterygoideus externus) (Fig. 1063), usually takes its origin as a common trunk with the buccal nerve. It enters the deep surface of the external pterygoid. 3. The deep temporal nerves (nn. temporales profundi anterior et posterior) (Fig. 1063), are usually three or two in number. The anterior accompanies the buccal nerve between the heads of the external pterygoid, after which it passes upward to supply the anterior portion of the temporal muscle. The middle passes outward across the upper margin of the external pterygoid and then upward close to the bone to enter the deep surface of the temporal muscle. It often fuses with either the anterior or posterior deep temporal, thus reducing the number of temporal FIG. 1062. Gasserian ganglion V. nerve, sensory root Maxillary division V. nerve Mandihular division V.n< Auriculo- *emporal n< Mandib. div. V. nerve, partly cut' Chorda tympani Otic ganglion ; its br. to tensor ty pani is seen above the leadei Middl meningeal artery Part parotid gland 'Spheno-palatine Posterior superior dental Br. from otic ganglion to auriculo-temp. nerve' Br. from otic gang, to chorda tympani Int. pterygoid nerve Int. pterygoid muscle Buccinator muscle Inferior dental ne Lingual br. V. nerve Dissection showing lateral view of spheno-palatine and otic ganglia. nerves to two. The posterior frequently accompanies the nerve to the masseter for a variable distance, after which it turns upward along the bone to enter the deep surface of the posterior portion of the muscle. 4. The buccal nerve (n. buccinatorius) (Fig. 1062) is purely sensory. It arises in common with the external pterygoid and anterior deep temporal nerves and is accompanied by the latter between the heads of the external pterygoid. Passing downward on the inner side of the temporal muscle it reaches the outer surface of the buccinator, where it breaks up into several branches which form a plexus around the facial vein, with the buccal branch of the facial nerve. Some of its branches pierce the buccinator muscle to supply the mucous membrane of the cheek as far forward as the angle of the mouth, while the others supply the skin of the cheek. Variations.— Instead of lying to the inner side, the nerve may pierce the temporal muscle. It may be derived from the posterior superior dental nerve or from the inferior dental in the latter instance emerging from the inferior dental canal by a small foramen m the alveolar border I244 HUMAN ANATOMY. of the mandible, just anterior to the ramus. It has been seen in one case to arise directly from fe Gasseriaa ganglion and emerge from the cranium through a special foramen s.tuated between the foramina rotundum and ovale. The Posterior Division of the mandibular nerve is sensory, with the exception of the mylo-hyoicl nerve. It passes downward beneath the external pterygoid and, after giving off the two roots of the auricula-temporal nerve, terminates by dividing into the lingual and the inferior dental nerve. Branches.— These are: (i) the auriculo-temporal, (2) the lingual and (3) the inferior denial. i. The auriculo-temporal nerve (n. auriculotemporalis) (Fig. 1063) arises just below the foramen ovale by two roots which enclose between them the middle meningeal artery. It passes backward beneath the external pterygoid muscle and between the spheno-mandibular ligament and the neck of the mandible, and then turns upward through the parotid gland between the temporo-mandibular articulation and the external ear. Emerging from the upper margin of the gland, the nerve passes over the root of the zygoma and ascends to the temporal region behind and in company with the superficial temporal artery. Branches.— These are : (a} the articular, (£) the parotid, (c) the meatal, (a) the anterior auricular and (*) the superficial temporal. The last three are terminal branches. a. The articular branches (rr. ariiculares) are one or two delicate filaments which enter the posterior portion of the temporo-mandibular articulation. b. The parotid branches (rr. parotidei) pass to the gland; they arise either from the auriculo-temporal or from its communicating filaments with the facial nerve. c. The meatal branches (nn. meatus auditorii externi) are two in number, an upper and a lower. They enter the external auditory canal between the bone and the cartilage and supply the skin covering the corresponding parts of the meatus, the upper branch in addition sending a twig (r. membranae tympani) to the tympanic membrane. d. The anterior auricular nerves (nn. auriculares anteriores), usually two in number, supply skin of the tragus and of the upper anterior portion of the auricle. e. The superficial temporal nerve (rr. temporales superficiales) (Fig. 1068) breaks up into a number of fine twigs which supply the skin of the temporal region and of the scalp almost to the sagittal suture. The auriculo-temporal communicates by its roots, close to their origin, with branches from the otic ganglion, and by its parotid and superficial temporal branches with the facial nerve. By the first of these communications secretory fibres of the glosso-pharyngeal and sympathetic fibres are carried to the parotid gland ; by means of the second junction sensory trigeminal fibres accompany the peripheral motor filaments of the facial. Variations. — In a specimen found in the anatomical laboratory of the University of Pennsyl- vania, the middle meningeal artery, instead of passing between the two roots of the nerve, pierced the anterior one. 2. The lingual nerve (n. lingualis) (Fig. 1079) is the smaller of th terminal branches of the mandibular nerve. Lying internal and anterior to th inferior dental nerve, it passes downward beneath the external pterygoid as far as the lower border of that muscle. It is usually connected with the inferior dental nerve by an oblique strand of fibres, which occasionally crosses the internal maxillary artery ami, close to its origin, it is additionally joined at an acute angle by the chorda tympani nerve. Alter emerging from under cover of the external pterygoid, it passes between the internal pterygoid and the ramus of the mandible. It then turns inward, forward and downward under the mucous membrane of the floor of the mouth, cross- ing over the superior border of the superior constrictor of the pharynx and the dee portion of the; submaxillary ^land, and passes under the submaxillary duct bet wee the inylo -hyoid and hyo-glossus niusrles. Reaching the side of the tongue the nerv continues forward to the apex, lying just beneath the mucous membrane. Branches. — The lingual nerve supplies small filaments to the sublingual gland the floor and side of the mouth, the side of the tongue and the lower gum. It i^ives oft the sensory root of the snlmiaxillary ganglion and its terminal filaments (rr. linnualcs) pass upward through the muscles of the tongue to supply the mucoi THE TRIGEMINAL NERVE. 1245 membrane of the anterior two-thirds of the dorsum. Its fibres have their main termination in the filiform and fungiform papillae. The lingual nerve communicates with the chorda tympani and the inferior dental and in its anterior portion forms loops with the hypoglossal. 3. The inferior dental nerve (o. alveolaris inferior) (Fig. 1063) is the larger of the terminal branches of the mandibular. Lying posterior and external to the lingual, to which it is connected by a small nerve strand, it passes downward and forward under cover of the external pterygoid. Leaving the lower margin of that muscle, it runs between the ramus of the mandible and the spheno-mandibular ligament and enters the inferior dental canal, along which it courses in company FIG. 1063. Motor division of mandibular ne Deep temporal branch Sensory division of mandibular nerve Internal pterygoid nerve Chorda tympani nerve Middle meningeal artery Auriculo- temporal nerve Superficial temporal artery Mylo-hyoid nerve Internal maxillary artery — Connection between auriculo-temporal and facial nerves Facial nerve Inferior dental nerve Part of mandible Parotid gland External carotid artery Mylo-hyoid nerve Ext. pterygoid muscle, cut, and its nerve Masseteric branch giving off a temporal branch Cut edge ot buccinator Int. pterygoid muscle Lingual nerve Mylo-hyoid muscle. cut to show lingual nerve Submaxillary ganglion Digastric muscle, anterior belly Submaxillary gland Dissection showing mandibular nerve and its branches : mandible has been partially removed, exposing inferior dental nerve in its canal. with the inferior dental artery, and supplies filaments to the teeth, as far as the mental foramen. Here the nerve breaks up into its terminal branches, one of which, the incisor, continues within the mandible to the mid-line, while the other and larger, the mental, emerges at the mental foramen. Branches. — These are : («) the mylo-hyoid, (b) the dental, (c} the incisor and (v way of its tympanic branch (page 1075) on the one * y means of communicating filaments, between the otic and the geniculate ,o- a, on of the- facial nerve on the other. As the continuation of the tympanic nerve, after union with the filaments from the geniculate ganglion, the small superficial petrosal leaves the upper ami fore part of the tympanic cavity, traverses a small canal in the temporal bone, and emerges on the upper surface of the latter, to the outer side of the hiatus Fallopii. It then turns downward, passes through the petro-sphenoidal figure or through a special canal in the sphenoid bone, and joins the otic ganglion. THE TRIGEMINAL NERVE. 1247 By means of these connections and the branches of distribution from the otic gang- lion, secretory fibres are carried along with those of the auriculo-temporal (page 1244) to the parotid gland. The small superficial petrosal nerve also contains taste- fibres, which pass either to the petrous ganglion of the ninth or to the geniculate ganglion of the seventh, and thence centralward to the reception-nuclei in the medulla. The motor root is a branch from the internal pterygoid nerve. The sympathetic root is represented by one or two nerve-filaments from the plexus on the middle meningeal artery. The ganglion also receives the sphenoidal branch from the Vidian nerve. Branches. — A number of delicate strands pass from the otic ganglion to adja- cent nerves. These so-called branches of distribution include : («) two or more fila- ments which join the roots of the auriculo-temporal nerve and so convey secretory fibres from the glosso-pharyngeal to the parotid gland, (<5) a communicating branch FIG. 1065. Ophthalmic Ophthaln: Maxillary Diagrams showing distribution of cutaneous branches of trigeminal and cervical spinal nerves. to the chorda tympani and (» another to the buccal nerve, (d*) a branch to the internal pterygoid nerve, and (V) and (/) branches to the nerves supplying the tensor palati and tensor tympani muscles. The Submaxillary Ganglion. — The submaxillary ganglion (g. submaxillare) (Fig. 1063) is a reddish triangular or fusiform body, measuring from 2-3 mm. in its greatest length, and is the smallest of the sympathetic ganglia connected with the fifth nerve. It is situated above the deep portion of the submaxillary gland and upon the hyo-glossus muscle and lies between the submaxillary duct and the lingual nerve, apparently suspended from the latter by two short slender filaments. The anterior of these transmits chiefly sympathetic fibres that pass from the ganglion to the lingual nerve, the posterior fibres going from the lingual to the ganglion as its sensory and motor roots. Roots. — The sensory root is contributed by the lingual nerve ; the motor root proceeds from the facial by way of the chorda tympani and contains secretory fibres ; and the sympathetic root is derived from the adjoining plexus on the facial artery. Branches. — The branches of distribution include: (#) a number of fibres which pass to the submaxillary gland, (£) others which are distributed to the submaxillary duct and the mucous membrane of the floor of the mouth and (^ FIG. 1066. Brain-stem with nuclei of cranial nerves shown diagrammatically ; motor nuclei and fibres are blue: sensory nuclei and fibres are red. a. oculomotor nerve ; f>, trochlear nerve; c, motor part of trigeminal nerve; rf, sensor part of trigeminal nerve ; f, spinal root of sensory part of trigeminal nerve;/; facial nerve; j?, abduct-ns nerve; i vestibular portion of auditory nerve; »', cochlear portion of auditory nerve; j, glosso-pharyngeal nerve; k, vagu nerve, showing also the nucleus ambiguus in black; /, hypoglossal nerve; m. vagus portion of spinal accessor nerve. (Posty and Spiller.) enlargement on the facial nerve known as the geniculate ganglion, which is situate within the facial canal at the so-called knee. Passing through the proximal part of the facial canal, the axones of the geniculate ganglion cells enter the cranium through the internal auditory meatus, lying above the auditory nerve and below the motor root of the seventh, with both of which they communicate. Leaving the meatus, they pass inward and enter the brain-stem at the superficial origin, (Fig. 1046), which is located at the lower border of the pons, between the moto root of the seventh and the auditory nerve. Entering tin- substance of tin- medulla, the sensory fibres pass either through ordorsally to the spinal root of the trigeminal nerve to reach the superior part of tin- nucleus of reception, which it shares with the glosSO-pharyngeal and vagus nerves (page 1262). On gaining this nucleus, the sensory tilm-s divide into short ascending and much longer descending branch'- thus behaving in a manner identical with that of the corresponding fibres of the trigeminus an other mixed cranial nerves. The termination of the sensory fibres is around the neuron of tin- reception-nucleus, from which axones pass to the mesial fillet of the opposite side, at* eventually, to the cep-bral cortex. n THE FACIAL NERVE. 1251 The motor part is by far the larger of the two and constitutes both anatom- ically and functionally the more important portion of the nerve. The deep origin of the motor root is from the facial nucleus (Fig. 933), an oval collection of some half dozen groups of large multipolar neurones, which measures about 5 mm. in length, and is situated in the posterior portion of the tegmentum of the pons. It lies within the formatio reticularis medial to the spinal root of the trigeminal nerve and, in its lower part, close to the fibres of the corpus trapezoides ; higher up it is tilted dorsally and separated from these fibres by the superior olive, to the upper and outer side of which it lies. Although the facial nucleus is situated close to the superficial origin of the seventh nerve, the root-fibres instead of taking a direct route to the ventral surface of the brain-stem follow a devious course. The intra- cerebral part of the nerve has been divided for convenience of description into a radicular, an ascending and an emergent portion. The radicular portion consists of numerous loose fasciculi of root-fibres which arise from the dorso-lateral aspect of the nucleus of origin and pass backward and slightly inward. The upper fibres stream over the dorso-lateral surface of the nucleus of the abducent nerve and then, with the other fibres of the motor root, bend mesially along the floor of the fourth ventricle. As they near the mid-line they turn sharply upward and assemble to form a solid strand, the ascending portion of the seventh nerve. This upward course continues for about 5 mm. and in this situation the nerve is separated from the floor of the fourth ventricle, beneath which it runs within the funiculus teres, only by the lining ependyma and lies immediately dorsal to the pos- terior longitudinal bundle and mesial to the abducent nucleus. The nerve now bends abruptly outward at a right angle and enters upon the emergent portion of its course, during which it crosses the dorsal aspect of the abducent nucleus and passes backward and ventro-laterally, be- tween its own nucleus of origin and the spinal root of the trigeminal nerve, to gain the exterior of the brain-stem (Fig. 1066). The central and cortical connections of the motor part of the facial nerve include paths whereby the nucleus is brought under the influence of the reflex and the cortical centres. (a) While not beyond dispute, it is probable that a limited number of root-fibres are connected with the facial nucleus of the opposite side. (£) The evidence adduced from clinical observa- tions and pathological findings points to the existence of a special group of cells from which arise the fibres supplying the orbicularis palpebrarum and frontalis muscles. These fibres, sometimes called the superior facial nerve, may retain their functional integrity notwithstanding the occurrence of paralysis of the other muscles supplied by the seventh nerve, (c) The latter, morever, is brought into association with the visual and auditory centres by paths, probably within the posterior longitudinal fasciculus, by which the facial cells respond to the impulses of sight and hearing, as shown by the automatic closure of the eyelids, (d) Connection with the hypoglossal nerve has been assumed in explanation of the coordinated action of the muscles of the lips with those of the tongue, (e) The motor facial nucleus is brought under the influence of the cortical area by the cortico-bulbar fibres which proceed as axones from the motor neurones lying within the lower part of the precentral convolution. These fibres descend in company with the cortico-spinal tracts to appropriate levels and end around the radicular cells of the facial nucleus of the opposite side, a few fibres, however, probably terminating in the nucleus of the same side. The superficial origin of the motor root is at the lower border of the pons, to which it may be adherent, in a groove between the inferior olive and the inferior cerebellar peduncle (Fig. 1046). Just above the facial as it escapes, often as several strands of root-fibres, lies the fifth nerve and to its outer side is the auditory, from which it is separated by the sensory root of the seventh. Emerging from the surface of the brain-stem, the nerve passes outward, its motor and sensory roots ununited, to the internal auditory meatus, through which it passes above and anterior to the auditory. At the bottom of the meatus the seventh and eighth nerves part company, the facial entering the facial canal, whose course it follows throughout. At first the canal is directed horizontally outward, between the cochlea and the vestibule, until it reaches the mesial tympanic wall. It then bends abruptly backward, passes above the fenestra ovalis and turns down- ward, behind the pyramid, in the posterior wall of the tympanic cavity, to end at the stylo-mastoid foramen. The point where the canal turns backward marks a corre- sponding bend, the genu, of the facial nerve. In this situation is found the geniculate ganglion and here the two roots fuse to form a single trunk. After emerging from 1252 HUMAN ANATOMY. the stylo-mastoid foramen the nerve passes downward, outward and forward through the parotid gland, and divides, just posterior to the ramus of the mandible, into its terminal branches, the temporo -facial and the cervico-facial. The filaments of these branches freely join with one another and form the fan-like parotid plexus (plexus parotideus), also called PCS anscriniis. The geniculate ganglion (g. geniculi) is a small oval or fusiform thickening on the facial nerve, at the point where it turns backward (geniculum n. facialis), and contains unipolar neurones, whose axones form the sensory root of the facial nerve and whose dendrites form the sensory fibres of distribution of the seventh. The so-called branches of the geniculate ganglion— the great and external superficial petrosal nerves and the branches to the tympanic plexus— are only in part composed of fibres connected with the ganglion cells ; they are, therefore, more appropriately regarded as branches of the facial nerve. Branches and Distribution. — Within the facial canal, the facial nerve gives off: (i) the great superficial petrosal, (2) the branch to the tympanic plexus, (3) the external superficial petrosal, (4) the stapedial, (5) the chorda iympani and FIG. 1067. Diagram showing1 branches and connections of facial nerve within facial canal. (6) the communicating branch to the vagus. The first three are closely connected with the geniculate ganglion. Outside the facial canal arise: (7) \he posterior auric- ular, (8) the digastric, (9) the stylo-hyoid, (10) the temporo-facial and (u) the cervico-facial -nerve. The last two nerves arise in an uncertain manner from that irregular plexiform expansion, known as the pes anserinus, into which the facial broadens within the substance of the parotid gland after emerging from the stylo- mastoid foramen. 1. The great superficial petrosal nerve (n. pctrosus supcrficialis major) (Fig. 1061), while issuing directly from the ganglion, contains motor fibres in addition t<> the sensory. It leaves the facial canal through the hiatus Fallopii, enters the middle cranial fossa and passes forward under the Gasserian ganglion and over the cartilage of the middle lacerated foramen. The nerve then crosses the outer side of the internal carotid artery to reach the posterior opening of the Yidian canal, where it is joinec by the great deep petrosal nerve (page 1360) from the carotid sympathetic plexus, with which it unites to form the I'nliun >i]theno-]>alatinc ganglion, who-,(- motor and sympathetic roots it contributes. The probable relations and destination of these fibres have been considered in connection with the spheno-palatine ganglion (page 1240). 2. The communicating branch to the tympanic plexus (r. anastomoticus cum plexu tympanico) traverses a tiny canal in the temporal bone to reach the tympanic cavity, where it joins the main continuation of the tympanic plexus of th( it ; ie al ie al J THE FACIAL NERVE. 1253 glosso-pharyngeal to form the small superficial petrosal and proceeds to the otic ganglion, which it enters as the sensory root (page 1246). The fibres from the tym- panic plexus, probably secretory in function, are distributed from the otic ganglion to the parotid gland. 3. The external superficial petrosal nerve is very small and is not always present. It joins the sympathetic plexus on the middle meningeal artery. 4. The stapedial nerve (n. stapedius), for the supply of the stapedius muscle, is given off as the facial passes downward behind the pyramid in the posterior wall of the tympanic cavity, the nerve gaining access to the muscle by passing through a minute orifice in the base of the pyramid. FIG. 1068. Sup Temporal branch of temporo-malar— ifcLj] Mastoid branch of great auricular — Auriculo-temporal Malar branch of facial Temporal branch of facial Infraorbital branch of facial Facial nerve Great occipital nerve Buccal branch of facial Supramanclibular branch of facial Inframandibular branch of facial Small occipital nerve Cutaneous branch of III. cervical Great auricular nerve iraorbital nerve •atrochlear nerve ratrochlear nerve •Malar branch of temporo- malar Infraorbital nerve External branch of nasal nerve Superficial cervical nerve Superficial Dissection of head and neck, showing terminal branches of trigeminal, facial and great occipital nerves, as well as associated branches of cervical plexus. 5. The chorda tympani nerve (n. chorda tympani), while conveying both motor and sensory impulses, consists mainly of sensory fibres derived from the cells of the geniculate ganglion. It arises from the facial a short distance above the stylo- mastoid foramen and courses upward and forward through the iter chordae posterius to enter the tympanic cavity (Fig. 1067). Passing between the fibrous and mucous layers of the membrana tympani, over the tendon of the tensor tympani and between the long processes of the incus and malleus, it arrives at the anterior edge of the membrane. It then traverses the iter chordae anterius to reach the pterygo-maxillary region, and, after receiving a filament from the otic ganglion, takes a course down- ward and forward, after which, under cover of the external pterygoid muscle, it unites and becomes incorporated with the lingual branch of the mandibular nerve. As the latter passes above the submaxillary ganglion, the motor fibres of the chorda tympani (facial) descend to the ganglion as its motor root and probably eventually end as secretory fibres to the submaxillary and sublingual glands. The sensory I254 HUMAN ANATOMY. fibres of the chorda tympani, on the other hand, are distributed to the mucous membrane covering the anterior two-thirds of the side and dorsum of the tongue, and are probably concerned in transmitting taste-impulses. 6. The communicating branch to the auricular branch of the vagus (r. anastomoticus c. rarao auriculari n. vagi) is given off just above the stylo-mastoid foramen and joins the- auricular at the point where the latter crosses the facial canal. 7. The posterior auricular nerve (n. auricularis posterior) arises just outside the stylo-mastoid foramen. It passes backward and upward between the external ear and the mastoid process and divides into (a) an occipital branch, which supplies the occipitalis muscle and (£) an auricular branch, whicK supplies the posterior auric- ular muscle, often partially the superior, and the transversus, the obliquus and the antitragicus of the intrinsic muscles of the auricle. The posterior auricular nerve communicates with the auricular branch of the vagus, the small occipital and the great auricular nerve. 8. The digastric branch (r. digastricus) arises from the facial below the pos- terior auricular nerve and breaks up into several filaments which enter the posterior belly of the digastric. One of these filaments, after passing through or above the digastric, may join the glosso-pharyngeal nerve. 9. The stylo-hyoid branch (r. stylohyoideus) is a small twig which arises in common with the digastric branch and passes forward to enter the posterior portion of the stylo-hyoid muscle. 10. The temporo-facial division (r. teraporofacialis) (Fig. 1087) is the larger of the two terminal branches. It traverses the upper portion of the parotid gland in a forward and upward direction, lying superficial to the external carotid artery and the temporo-maxillary vein. By repeated branchings and unions the nerve forms an intricate looped plexus which breaks up into three more or less defi- nite groups. Branches. — These are : (a) the temporal, (£.) the malar and (c) the infraorbital. a. The temporal branches ( rr. temporales ) pass upward and forward over the zygomatic arch and supply the frontalis, the corrugator supercilii, the upper part of the orbicularis palpe- brarum, the auricularis superior and the auricularis anterior. The temporal branches of the facial communicate with the following branches of the trigeminal : the auriculo-temporal, the supraorbital, the lachrymal and the temporal branch of the temporo-malar. b. The malar branches (rr. zygomatici) are rather small. They extend forward over the malar bone and are sometimes incorporated with the temporal or infraorbital branches. They supply the lateral part of the orbicularis palpebrarum and sometimes the zygomatici major et minor. The malar branches communicate with the malar branch of the temporo-malar. c. The infraorbital branches (rr. buccales superiores) are comparatively large. They course horizontally forward across the masseter muscle in company with the parotid duct and supply the lower part of the orbicularis palpebrarum, a portion of the buccinator, the zygomatici major et minor and the muscles of the nose and upper lip. •I The most important of the communications is the one between the infraorbital and the terminal branches of the maxillary division of the trigeminal. This is a sensory-motor plexus which lies below the infraorbital foramen and under the levator labii superioris and is called the in fmorbital plexus (Fig. 1068). The nasal and infratrochlear nerves communicate with the infraorbital at the side of the nose. ii. The cervico-facial division (r. cervicofacialis) (Fig. 1087) is the smaller of the terminal branches of the facial and resembles in its general arrangement the temporo-facial. It passes downward, outward and forward through the parotid and finally breaks up into three branches. Branches. — These are : (a) the buccal, (6) the supramandibular and (c) the inf ra >ii ti>nfi /'it /(t>-. a. The buccal branch ( rr. Iwccales) may be single or multiple. It crosses the masseter and supplies the buccinator and orbicularis oris muscles. It communicates on the outer surface of the buccinator muscle with the sensory buccal branch of the maiulibular division of the trigeminal nerve. THE FACIAL NERVE. 1255 b. The supramandibular branch (r. marginalia mandibularis) passes forward between the lower lip and the chin and supplies the muscles of the lower lip. Its filaments communicate with those from the mental branch of the inferior dental. c. The inframandibular branch (r. colli) emerges from the lower margin of the parotid gland and takes a downward course behind the angle of the jaw. Piercing the deep cervical fascia, it passes forward in the neck and forms a series of loops beneath. the platysma myoides as far down as the hyoid bone. It supplies the platysma myoides. The nerve communicates with the superficial cervical branch of the cervical plexus. FIG. 1069. e facial artery inches of the facial nerve Superficial temporal artery Practical Considerations. — The facial nerve may be the seat of spasm (tic convulsif) or of paralysis. The lesion may be central or peripheral, the latter being more common. When the spasm is confined to certain branches it usually involves the muscles about the eyes. If only the orbicularis is involved it is called blepharo- spasm ; if the adjacent muscles also are involved, spasmus nictitans. The facial nerve is more frequently associated with spasm than any other in- the body, except the spinal accessory. Facial paralysis is relatively common. If the central lesion — as a tumor, abscess or hemorrhage — is limited to the facial centre in the cortex, a monoplegia of the facial nerve will result, and the paralysis will usually be confined to the lower branches of the nerve in the face and neck, the upper branches escaping probably because of bi- lateral innervation of the upper muscles of the face. A cortical isolated paralysis of this type is exceedingly uncommon. If the lesion, as an apo- plectic hemorrhage, is in the internal capsule, a hemiplegia on the same side as the facial paral- ysis will be associated with it, and this also usually occurs when the lesion is cortical. A lesion in the upper part nf r1-iF» nrmc will crivp ri^P Dissection showing relations of facial nerve branches as b win give i is they cross masseter muscle. to a similar condition, but if it is in the middle or lower part of the pons the facial nerve will be paralyzed on the side of the lesion, the hemiplegia being on the opposite side (crossed paral- ysis). This is explained by the fact that the facial fibres cross to the opposite side in the pons, while the motor fibres to the extremities and trunk cross in the medulla. A lesion in the middle or lower part of the pons on one side, therefore, will involve the facial fibres after they have crossed, and the motor fibres to the extremities before they have crossed. Thus the facial nerve will be paralyzed on the side of the lesion, and there will be a hemiplegia of the opposite side. The peripheral portion of the facial extends from its exit at the pons to its terminal filaments on the face, but a lesion of the facial nucleus in the pons gives rise to much the same symptoms as one of the nerve at its exit from the pons. Its intra- cranial portion may be involved by tuberculous deposits, tumors, etc. In its long course through the Fallopian canal it may be affected by swelling of the soft tissues, by middle ear disease, or by fractures of the base of the skull in the middle fossa. After it leaves the stylo-mastoid foramen it is in greatest danger, as from exposure to atmospheric influences, and to accidental and operative wounds. It is especially apf to be wounded in that portion which lies within the parotid gland. Parotid gland rotid duct eter muscle 1256 HUMAN ANATOMY. When all branches of the facial are paralyzed the symptoms are characteristic. Only one side of the forehead wrinkles ; the tears fail to enter the canaliculi, and flow over the cheek ; the eye cannot be closed ; foreign bodies on its surface are not removed by the lid, and conjunctivitis from irritation results. The affected half of the face is expressionless, and the corner of the mouth on that side remains partly open and hangs down, so that the saliva tends to run out. The mouth is drawn to the opposite side ; the upper lid cannot be elevated ; whistling is impossible be- cause the orbicularis cannot now pucker the lips ; food lodges in the affected side of the mouth, because the buccinator muscle is paralyzed, and, for the same reason, the mucous membrane often gets caught between the teeth. In those cases of facial paralysis in which the lesion of the nerve is posterior to the stylo-mastoid foramen, attempts have been made recently to restore function to the peripheral portion by dividing the trunk posterior to the parotid gland, and anastomosing the peripheral end to a neighboring cranial nerve, as the spinal accessory or the hypoglossal. The results have not been entirely satisfactory. The line of the main trunk of the nerve is from the slight depression between the back of the ear and the mastoid process, forward and slightly downward. It passes through the deeper portion of the parotid gland. THE AUDITORY NERVE. The eighth or auditory nerve (n. acusticus) is not only, as its name implies, the nerve by which sound impulses are transmitted to the brain, but also the nerve of equilibration. It consists of two portions, the cochlear } the true nerve of hearing, and the vestibular, which is concerned with equilibration. Traced from the brain toward the ear, the auditory nerve arises at its super- ficial origin by two roots, a mesial (radix vestibularis) and a lateral (radix coch- learis), which embrace the inferior cerebellar peduncle, the mesial passing to the inner and the lateral to the outer side of the peduncle. The nerve thus formed by the union of these two roots, leaves the surface of the brain-stem at the posterior border of the pons, where it is adherent to the middle cerebellar peduncle. To its inner side and closely associated with it are the motor and sensory roots of the facial nerve (Fig. 1046), which lie within a groove on the mesial surface of the auditory and with it enter and traverse the internal auditory canal. Within the latter, the auditory nerve separates into two divisions, of which the superior and larger is the vestibular nerve (o. vestibuli) and the inferior and smaller is the cochlear nerve (n. cochleae). Although in a general way these divisions continue the corresponding roots, this agreement, as to the source of their fibres, is not complete, since, as will be more fully noted, strands of vestibular fibres are incorporated with the cochlear nerve. On reaching the bottom of the internal auditory canal, the facial nerve leaves the meatus and enters the facial canal, while the fibres of the auditory nerve dis- appear through apertures in the lamina cribrosa (Fig. 201) to gain the several parts of the membranous labyrinth of the internal ear. During their journey through the meatus, the vestibular and facial trunks are connected (fila anastomica) by a branch which passes from the pars intermedia to the vestibular nerve, and by one from the latter to the geniculate ganglion. These apparent communications between the seventh and eighth nerves are, in fact, only aberrant strands of facial fibres that return to the seventh after temporary association with the auditory. The vestibular nerve divides into three terminal branches which pass through apertures in the cribriform plate above the falciform crest and supply: (i) the utricle, (2) the superior vu& (3) the external semicircular canal. Not all the fibres of the vestibular root, however, are included in these branches ; of the three branches given off by the cochlear nerve two, (4) those to the saccn/e and (5) to tin- posterior semi- circular f! the lateral fillet (tf) t<> the cells within the inferior colliculus, or (fi) without interruption through the inferior brachium to the cells within the median geniculate body. THE AUDITORY NERVE. 1259 FIG. 1072. Floor of IV. ventricle Superior cerebellar peduncle Inferior cerebellar peduncle Lateral vestibular (Deiters') nucleus Median vestibular nucleus Cochlear fibres Dorsal cochlear nucleus Descending vestibular root 4. Neurones of the inferior colliculus and of the median geniculate body, whose axones pass, as the auditory radiation, to the auditory cortical area within the temporal lobe of the cerebrum. Although the exact extent of the auditory area is still uncertain, the most important part of this centre includes the superior temporal and the subjacent part of the middle temporal convolution. The cochlear fibres that do not undergo decussation ascend through the lateral fillet of the same side and eventually establish cortical relations with the corresponding hemisphere ; from th.e preceding account, however, it is manifest that the auditory area is connected chiefly with the cochlea of the opposite side. Peripheral, Central and Cortical Connections of the Vestibular Nerve. — The fibres of the vestibular portion of the auditory nerve are the axones of the bipolar nerve-cells situated within the small vestibular ganglion (g. vestibulare) or Scarpa's gang/ion, which lies at the bottom of the internal auditory canal. The dendrites of these cells constitute the five branches of dis- tribution of the vestibular nerve and pass through the various openings in the inner wall of the bony labyrinth, in the manner above described (page 1256), to reach the specialized areas, the macula acusticce, within the saccule, the utricle and the ampullae of the semicircular canals, where the nerve-filaments end, really begin, in intimate relation with the neuro- epithelium. While the centrally directed axones of the neurones supplying the utricle and the superior and external semicircular canals become consolidated to form the vestibular nerve of descriptive anatomy, those from the saccule and the posterior semicircular canal join the coch- lear fibres and with these course within the cochlear nerve until the latter and the vestibular nerve unite to form the common auditory trunk. Where the common trunk separates into the two roots, the vestibular fibres leave the cochlear and permanently assume their natural companionship with the remain- ing fibres of the vestibular root. The vestibular fibres enter the brain-stem at a slightly higher level than does the cochlear root, lying mesial to the latter and the ventral cochlear nucleus, and pass dorsally within the pons between the inferior cerebellar peduncle and the spinal trigeminal root. On reaching a level dorsal to the latter, the vestibular fibres divide into short upward and longer downward coursing branches, which, after condensing into an ascending and a descending root respectively, end in arborizations around the cells of the vestibular nucleus of reception. The exact extent and constitution of this nucleus, which under- lies the area acustica in the floor of the fourth ventricle (page 1097), are uncertain, since the neurones directly related to the vestibular fibres contribute only a part of those contained within a large diffuse complex of cells and fibres, many of whose constituents probably have only an indirect connection with the vestibular nerve. When reconstructed, as has been successfully done by Sabin, this complex has the form shown in Fig. 1072 and comprises two general parts, (a) an extended irregularly triangular mass of cells lying for the most part mesial to the tract formed by the ascending and descending branches of the vestibular fibres, and (d) a smaller mass of cells which lies above the larger one and partly to the inner and partly to the outer side of the tract of the vestibular fibres. The apex of the large triangular mass approaches the mid-line and its superior and inferior basal angles are prolonged upward and downward along the vestibular tract. When examined microscopically the large mass is found to include three subdivisions : (a) a tapering caudally directed nucleus which continues the inferior angle along the descending vestibular root, (k) an extended triangular nucleus that includes the greater part of the large mass and (c] an irregular pyramidal nucleus that prolongs upward the superior angle. The first of these subdivisions (a) is known as the spinal vestibular nucleus (nuc. spinalis n. vestibularis), the second (£) as the median vestibular nucleus (nuc. mediali.-s n. vestibularis), also as the chief nucleus or the triangular nucleus and the third (r) as the superior vestibular nucleus or the Nucleus cuneatus Closed part of medulla Vestibular nuclei as shown in reconstruction by Dr. Florence R. Sabin. i26o HUMAN ANATOMY. nucleus of Bechterew. The small mass corresponds with the lateral vestibular nucleus (nuc. lateral!* n. vestibularis) or nucleus of Deiters. The fibres of the descending root end around the neurones within the spinal nucleus in a manner similar to that in which the constituents of the spinal root of the trigeminus terminate in relation with the neurones within the substantia gelatinosa, whilst those of the ascending vestibular root end around the cells within the remain- ing vestibular nuclei. Although much uncertainty and conflict of opinion exist as to the details of the secondary paths by which the impulses carried by the vestibular fibres are distributed, it may be accepted as established that fibres pass from the nuclei of reception : (a) to the cerebellum (chiefly to the roof nucleus of the opposite side and, possibly, aJso to the nuclei globosus and emboliformis) as constituents of the nucleo-cerebellar tract, by which the impulses of equilibration are carried to the great coordinating centres, (t>) as arcuate fibres ventro-medially into the tegmentum of the pons, cross the mid-line and bend upward or downward to pass to other levels, some fibres, however, remaining on the same side. From the character of the impulses it is probable that only relatively few vestibular fibres join the median fillet to ascend to the optic thalamus. Other connections of the nuclei include : (r) commissural fibres between Bechterew's nucleus of the two sides, (d ) fibres to the abducent nucleus, (e) crossed and uncrossed fibres from Deiters' nucleus to the posterior longitudinal fasciculus and (/) fibres from the same nucleus to the spinal cord. It must be understood that by no means all of the neurones of Deiters' nucleus are con- cerned in transmitting afferent impulses to the cerebellum, for, as a matter of fact, many are links in the path by which the cerebellar cells exercise coordinating influences over the root- cells of the spinal nerves. Starting in the cerebellum, such efferent impulses are carried by efferent fibres which descend through the median part of the inferior cerebellar peduncle and probably end around certain of the cells within Deiters' nucleus. From these cells, in turn, originate the fibres of the vestibulo-spinal tract, which, after traversing the medulla, enter the antero-lateral column of the cord and end in relation with the motor root-cells. A shorter and more direct path for vestibular reflexes is probably formed by the collaterals of the vestibular fibres that end around the spinal neurones of Deiters' nucleus. It must not be forgotten that Deiters' nucleus is the origin for important contributions to the posterior longitudinal fasciculus (page 1117), by which the vestibular impulses impress the nuclei of the motor and, perhaps to a limited degree, also those of the sensory nerves. Practical Considerations. — The auditory nerve is rarely the seat of primary disease. It is most frequently affected consecutively to disease of the middle and in- ternal ears. It is sometimes, though seldom, paralyzed in fractures of the base of the skull. Operations on this nerve have been performed for relief from persistent and annoying tinnitus. THE GLOSSO-PHARYNGEAL NERVE. The ninth or glosso-pharyngeal nerve (n. glossopharyngeus) is a mixed nerve, containing motor and sensory fibres, the latter including those transmitting tin- impulses of the special sense of taste. The motor element is quite small and sup- plies only the stylo-pharyngeus muscle and secretory fibres to the parotid gland, while the sensory fibres are distributed to the mucous membrane of the middle ear fauces, tongue and pharynx. The Nuclei of the Glosso-Pharyngeal, Vagus and Accessory Nerves. In the description of the medulla (page 1073) attention was called to the presence nuclei common to a greater or less extent to the series of lower lateral nerves includin the seventh, ninth, tenth and vagal part of the eleventh, which, with the exception of the last named, are mixed nerves. The motor fibres of these nerves differ from those of the series of median motor nerves — the third, fourth, sixth and twelfth — (a} in the more lateral situation and less compact grouping of their cells of origin and (/>) in the less direct course they follow to reach the surface of the brain. To avoid repeti- tion, the general arrangement and characteristics of the nuclei related to the glosso pharyngeal. vagus, and accessory part of the eleventh nerve will he here described The Motor Nuclei. — These include the root-cells within the tforsa/ nuclei* and those constituting the nucleus ainliignns. The dorsal nucleus (nucleus dorsalis | a nucleus both of origin and of reception for the fibres of the ninth and tenth nerves i~, a DaiTOW elongated tract of nerve-cells, whose upper three-fourths underlies the fl""i' of the fourth ventricle, stretching from the stria- acusticae above to the tip of the ventricle below, and whose lower fourth extends into the closed part of th in i THE GLOSSO-PHARYNGEAL NERVE. 1261 I073- medulla to the level of the nucleus gracilis. It lies immediately lateral to the lower part of the median vestibular nucleus and the upper part of the hypoglossal nucleus, its upper third being covered by the spinal vestibular nucleus and its lower third overlying the hypoglossal nucleus. Its middle third corresponds to thefovea vagi (Fig. 949) and comes into intimate relation with the ventricular floor. When examined in cross-sections (Fig. 928) the nucleus appears prismatic in outline and is seen to consist of subgroups of cells, of which the median contains the larger and more conspicuous elements and corresponds to the dorsal motor nucleus. The remaining groups, the dorsal sensory nucleus, are composed for the most part of small irregular and often spindle cells, that receive end arborizations of afferent fibres. The nucleus ambiguus (nucleus ventralis) consists of an ill-defined slender column of large multipolar cells, which extends from the level of the entrance of the cochlear nerve at the upper border of the medulla to about the level of the beginning of the pyramidal decussation, and is best developed in its upper part. In transverse sections of the medulla (Fig. 927), the tract is distinguishable within the formatio reticularis grisea, midway between the dorsal accessory olivary nucleus and the substantia gelati- nosa, as a small and inconspicuous group of cells. Arising as axones of the latter, the loosely grouped motor fibres at first pass dorsally to the vicinity of the ventricular floor, then bend sharply outward, and, as in the case of the vagus, join with the similar fibres proceed- ing from the dorsal motor nucleus to form the emergent root strands. The Sensory Nuclei. — The nuclei receiving the afferent fibres of the lateral mixed nerves in question include the sensory part of the dorsal nucleus (nu- cleus alae cinereae), above de- scribed, and a tapering column of gray matter, the spinal nucleus (nucleus tractus solitarii), which resembles the corresponding nucleus of the trigeminus. The spinal nucleus is closely associated with a conspicuous longitudinal tract of caudally directed fibres, the fasciculus solitarius (tractus solitarius), so called on account of the apparent isolation of the bundle when viewed in transverse sections (Fig. 927). That such, however, is not the case is evident when the fact is recalled that the fibres which turn downward to form the tract are accompanied by the spinal nucleus of reception, around whose cells they end. The fasciculus solitarius extends from the upper border of the medulla to the level of the lower limit of the decussation of the fillet and is related to the sensory fibres of three nerves. The first of these, the facial, contributes only a limited number of fibres that occupy the uppermost part of the bundle ; the second, the glosso-pharyngeal, forms by far the largest constituent of the fasciculus ; whilst the third, the vagus, adds fibres that course within the lowest segment of the tract. Diagram showing connections of root-fibres of glosso-pharyngeal and pneumogastric nerves and of sensory fibres of facial ; sensory fibres are black, motor ones red; VII, geniculate ganglion; IX, A", ganglia of ninth and tenth nerves; DN, dorsal nucleus; FS, fasciculus solitarius, accompanied by column of gray matter; NA, nucleus ambiguus; AcV, accessory vagus (bulbar portion of AY); MF, median fillet. Central and Cortical Connections of the Motor Part of the Glosso-Pharyngeal Nerve. — The motor fibres of the glosso-pharyngeal nerve are the axones of the motor neurones situated 1262 HUMAN ANATOMY. within the dorsal nucleus and the nucleus ambiguus. The fibres proceeding from the dorsal nu- cleus are distributed to involuntary muscle ; hence the nucleus is sometimes called the sympathetic motor. Those taking origin from the nucleus ambiguus supply voluntary muscle and pass at first toward the floor of the fourth ventricle ; they then abruptly change their direction by bending out- ward and, joining the fibres arising from the dorsal motor nucleus, proceed ventro-laterally through the gray reticular formation, just ventral to or across the spinal root of the trigeminus, to emerge at their superficial origin along the bottom of the postolivary sulcus, incorporated with the afferent fibres in ilu- five or six root-fasciculi forming the entire ninth nerve. The cortical con- nections of the motor fibres are established by cortico-bulbar fibres that arise from cells situated within the gray matter of probably the lower part of the precentral gyrus. After traversing the motor path through the corona radiata, internal capsule, cerebral peduncle and pons, the cortical fibres end, on reaching the upper level of the medulla, in arborizations around the motor root-cells chiefly of the opposite side. FIG. 1074. Olfactory bulbs Optic nerve Internal carotid artery Optic chlasin _^J Branch of supraorbital nerve Supraorbital nerve Lachrymal gland Supratrochlear nerve Superior rectus muscle Levator palpehrne sujierioris Lachrymal nerve IV. nerve Ophthalmic nerve at point of division V. nerve, iensory 1001 v i Cerebral peduncle^-^K Middle peduncle of cerebellum IX.,X.andXI. nerves XII. nerve Mandibular nerve VII. nerve, motor part Pars intermedia II 1 1. nerve Superior peduncle of cerebellum, cut IX., X. and XI. nerves Floor of IV. ventricle •Spinal portions of XI. nerves Interior aspect of base of skull, viewed from above and behind, showing particularly posterior group of cranis nerves passing from brain-stem to points of emergence through dura ; posterior part of skull has been removed. Central Connections of the Sensory Part of the Glosso-Pharyngeal Nerve. — The afferer or sensory fibres of the glosso-pharyngeal nerve are the axones of cells within the jugular anc petrous ganglia situated along the upper part of the nerve-trunk. Entering the skull through the jugular foramen, the sensory fibres approach the brain-stem in the five or six delicate root- bundles that reach the medulla along the groove between the olivary eminence and the inierior cerebellar peduncle. Passing to the ventral side of the spinal root of the trigeminus, or traversing this field, in company with the motor fibres, the afferent fibres continue dorso- mesially through the formatio reticularis grisea towards the dorsal nucleus. Just before rvach- ing tin latter, however, the sensory fibres separate into two groups, a medial and a /a/era/. The fust and smaller of these continues its course to the dorsal sensory nucleus, around the cells of which its fibres end. It is possible that the cells constituting the upper groups of the dorsal sensory nucleus are particularly concerned in receiving the impulses giving rise to gustatory impressions, since the glnsso-pharyngeal is recogni/ed as the nerve of taste. Considering the fact that the afferent fibres of the facial nerve, which constitute the pars intermedia of \Vrisberg, are distributed peripherally chiefly by the chorda tympani, are also concerned in convex ing taste-impulses and end, in part at least, in the same nucleus as does the ninth, the sensory portion of the seventh nerve may be regarded, at least functionally, if not from a morphological standpoint, .is an aberrant strand of the glosso-pharyngeal. THE GLOSSO-PHARYNGEAL NERVE. 1263 The second and much larger group turns outward and abruptly downward to form the chief constituent of the spinal tract, the fasciculus solitarius. In transverse sections ( Fig. 927) the latter appears as a conspicuous, compact, rounded bundle, that lies lateral to the dorsal nucleus and behind the strands of root-fibres. The solitary fasciculus is accompanied through- out its course by a slender column of gray matter, which lies partly on the surface of the bundle and partly amongst its fibres and contains numerous nerve-cells of small size which constitute the reception-station for the greater number of the afferent fibres of the ninth nerve. Since these fibres are continually ending at different levels in their descent, it follows that both the fascic- ulus and its nucleus gradually diminish in size, until, at about the level of the sensory decussa- tion, they are no longer distinguishable. Course and Distribution. — Leaving the superficial origin along the groove separating the olivary eminence from the inferior cerebellar peduncle, the isolated root-fasciculi, about half a dozen in number and in series with those of the vagus, assemble to form a single trunk, which passes outward in front of the flocculus of the cerebellum to the jugular foramen. As it traverses this foramen, the glosso-pharyn- FIG. 1075. Diagram showing tympanic plexus and connections of glosso-pharyngeal nerve. geal lies external and anterior to the tenth and eleventh nerves and in its own separate dural sheath. It occupies a groove, or sometimes a bony canal, in the fora- men and in this situation presents two thickenings, ti\e jugular &[\& petrous ganglia. Emerging from the foramen, the nerve passes between the internal carotid artery and the internal jugular vein and, dipping beneath the styloid process, follows a downward course along the posterior border of the stylo-pharyngeus muscle, with which it passes between the internal and external carotid arteries. Turning gradually forward, it reaches the outer side of the stylo-pharyngeus muscle and stylo-hyoid ligament and disappears beneath the hyo-glossus muscle to break up into its terminal branches to the tongue (Fig. 1079). Ganglia of the Glosso-Pharyngeal Nerve. — In the course of the nerve two ganglia are found, the jugular and the petrous. They contain aggregations of neurones whose dendrites constitute the peripheral sensory fibres and whose centrally directed axones form the sensory root-fibres of the nerve. The jugular ganglion (g. superius) which may be regarded as a detached portion of the petrous ganglion, lies in the upper part of the groove occupied by the glosso-pharyngeal nerve in its transit through the jugular foramen. It is variable in size and not always present and measures only from 1-2 mm. in length. The ganglion does not include the entire thickness of the nerve but only the inferior portion, the fibres of the superior portion passing uninterruptedly over it. 1264 HUMAN ANATOMY. The petrous ganglion (g. petrosum) is larger than the jugular and involves the entire nerve. It is oval or fusiform in shape, measures from 4-5 mm. in length, and is lodged within a slight depression in the lower part of the groove for the nerve in the jugular foramen. The communications of the petrous ganglion include filaments (a) from the superior cervical ganglion of the sympathetic, (b) to the auricular branch of the vagus and sometimes (c) to the ganglion of the root of the vagus. Branches. — The branches of the glosso-pharyngeal nerve are: (i) the tym- panic, (2) the pharvngeal, (3) the muscular, (4) the tonsillar and (5) the lingual. 1. The tympanic nerve (n. tympanicus) or Jacobsori s nerve, arises from the petrous ganglion as its most important branch and traverses a tiny canal in the osseous bridge between the jugular fossa and the carotid canal. Entering the tym- panic cavity and receiving fibres from the carotid plexus of the sympathetic by way of the small deep petrosal (n. caroticotympanicus), the tympanic nerve passes upward and forward in a groove on the promontory and breaks up in this situation to form the tympanic plexus (plexus tympanicus pacobsoni]). After distributing filaments to the mucous membrane lining the tympanic cavity and the associated air-spaces (mastoid cells and Eustachian tube), its fibres reassemble and join with a filament from the geniculate ganglion to continue as the small superficial petrosal nerve to the otic ganglion (Fig. 1075). Branches. — These are: (a) the small superficial petrosal nerve, (b) the branch to the fenestra ovalis, (r) the branch to the fenestra rotunda, (*/) the branch to the Eustachian tube, (e~) the branch to the mastoid cells and (/") the branch to the great superficial petrosal nerve. a. The small superficial petrosal nerve (n. petrosus superficialis minor) (Fig. 1075) is the continuation of the tympanic nerve, formed by a reassembling of the fibres of the plexus, sup- plemented by a filament from the geniculate ganglion of the facial. It traverses a canal which begins at the anterior superior portion of the tympanic cavity, passes beneath the upper end of the canal for the tensor tympani and appears on the superior surface of the petrous portion of the temporal bone, to the outer side of the cranial opening of the hiatus Fallopii. While in the canal it sometimes receives a communicating branch from the great superficial petrosal nerve. It leaves the cranium through a canal in the greater wing of the sphenoid, or through the fissure between the greater wing and the petrous portion of the temporal bone, and on reaching the base of the skull, joins the otic ganglion as its sensory root ( Fig. 1075). b. The branch to the fenestra ovalis supplies the mucous membrane in the neighborhood of the oval window. c. The branch to the fenestra rotunda is distributed to the mucous membrane over and around the fenestra. d. The branch to the Eustachian tube supplies the mucous membrane lining the osseous portion of that canal. e. The branch to the mastoid cells supplies the mucous lining of these cells. f. The branch to the great superficial petrosal nerve joins the latter in the hiatus Fallopii. 2. The pharyngeal branches (rr. pharyngei) number two or more, of which the largest descends along the course of the internal carotid artery and joins the pharyngeal branches of the vagus and sympathetic to form the pharyngeal plexus, which supplies the mucous membrane and muscles of the pharynx. The smaller pharyngeal branches pierce the superior constrictor and are distributed to the mucous membrane lining the upper portion of the pharynx. 3. The muscular branch (r. stylophan ngcus) enters the stylo-pharynegus, and, after giving off fibres for the supply of that muscle, passes through it to be distributed to the mucous membrane of the pharynx. 4. The tonsillar branches (rr. tonsillares^ are given off near the base of the tongue. They are slrndi T filaments which form a plexiform ramification, the circulus tousillaris, around the tonsil. From this plexus filaments are distributed to tin- tonsil, the soft palate and the faucial pillars. 5. The lingual branches (rr. linguales) are the two terminal filaments of the nerve. The larger posterior brunch passes upward and separates into a number of filaments which supply the circumvallate papilla- and the mucous membrane covering THE VAGUS NERVE. 1265 the posterior part of the dorsum of the tongue, the glosso-epiglottic and pharyngo- epiglottic folds and the lingual surface of the epiglottis. The smaller anterior branch supplies the mucous membrane of the side of the tongue half way to the tip. FIG. 1076. ro-hyoidbr. XII. nerve Superior laryngeal nerve Digastric, post. belly in part I. cervical ne Spinal accessory nerve — Occipitalis minor nerve — II. ce Hypoglossal (XII.) nerve Sup. cerv. gangl. of sympathetic Br. from II. cerv. nerve to sp. access/ 1 1 1 1. cervical nerve —I Communicans hypoglossi Greatauric. and superf. cerv. nerves IV. cervical nerve V. cervical nerve Br. to rhomboidei VI. cervical ner Communicating br. to sp. accessory A cutaneous b: VI I. cervi- cal nerve Nerve to subclavius VI II. cerv. nerve Post. thor. nerve Supra- scapular nerve jt External pterygoid -Ling. br. V. nerva \ Chorda j tympani nerve Int. pterygoid Edge of oral mu- cous membrane pGIosso-pharyngeal ve ntal nerve f. dental nerve, stal portion iblingual gland Submaxillary gangl. Stylo-pharyngeus Middle cervical ganglion of sympathetic (the cord connecting it with superior gangl. is also seen) Scalenus anticus Subclavian artery Deep dissection of neck showing ninth, tenth, eleventh and twelfth cranial nerves and their branches. Variation. — Instances are recorded in which the mylo-hyoid nerve was absent and a branch^of the glosso-pharyngeal supplied the mylo-hyoid muscle and the anterior belly of the digastric, the innervating fibres being, probably, aberrant filaments of the trigeminus. THE VAGUS NERVE. The tenth, vagus or pneumogastric nerve (n. vagus) is the longest and most widely distributed of the cranial series. Starting in the cranium, it passes through the neck, thorax and upper part of the abdomen before breaking up into its terminal branches. In addition to certain filaments concerned with special functions, distrib- uted to the heart and abdominal viscera, it contains both motor and sensory fibres. Some of the motor constituents of the nerve arise from its own origin, but the major- ity perhaps are contributions of the accessorius vagi, the so-called accessory part of the spinal accessory nerve. The vagus supplies motor fibres to the muscles of the soft palate (with the exception of the tensor palati and, probably, partly the levator palati and azygos uvulae), pharynx, oesophagus, stomach, and intestine (with the exception of the rectum), and to those of the larynx, trachea, and bronchi and their subdivisions. It distributes sensory fibres to the dura mater, external ear, pharynx, oesophagus, stomach, larynx, trachea, bronchi and subdivisions and pericardium. 80 1266 HUMAN ANATOMY. Special fibres are furnished to the heart, liver, spleen, pancreas, kidneys, suprarenal bodies and intestinal blood-vessels. It is generally admitted that the bulbar or accessory portion of the eleventh nerve forms an integral part of the motor division of the vagus, and, hence, should be included with the efferent fibres of the tenth. As to the ultimate distribution of these accessory fibres, and conversely of the vagus motor fibres proper, much discussion and many conflicting views have existed and, even at present, a consensus of opinion can scarcely be said to have been reached. After reviewing the evidence, both anatomical and experimental, Van Gehuchten ' concludes that the accessory fibres are distributed chiefly, if not indeed exclusively, to the larynx through the infe- rior laryngeal branch of the vagus, and are continued neither to the heart nor to the stomach. The efferent vagus fibres proceeding to the heart are inhibitory in function ; whether they directly reach the cardiac muscle is doubtful, since, reasoning from analogy, it is probable that the vagus fibres end around sympathetic neurones whose axones are the filaments coming into immediate relations with the muscle-fibres. Of the efferent fibres of the vagus distributed to the stomach and other parts of the digestive tract, some are secretory, while others, possibly, influence the caliber of the blood-vessels, in both cases being interrupted in sympathetic ganglia before gain- ing their destination. Deep Origin of the Motor Portion. — As stated above, the efferent fibres of the vagus consist of two sets, vagus fibres proper and those derived from the acces- sory portion of the spinal accessory. The former have their deep origin in the nu- cleus ambiguus and the dorsal motor nucleus, in series with the motor fibres of the ninth nerve ; the accessory fibres arise from the nucleus ambiguus only. The detailed description of these nuclei has been given (page 1260). The fibres arising from the nucleus ambiguus at first pass backward toward the floor of the fourth ventricle, then bend sharply outward and, condensed into compact strands that receive the fibres originating from the motor cells of the dorsal nucleus, proceed, ventro-laterally in company with the sensory fibres, to their superficial origin along the postero-lateral groove behind the olivary eminence. Central Connections of the Sensory Portion. — The afferent root-fibres of the vagus are the axones of the neurones lying within the ganglia of the root and of the trunk situated on the upper part of the nerve. The centrally directed processes pass into the medulla, in company with the motor strands, and divide into two sets. Those forming the larger of these end in arborizations around the cells within the lower portion of the dorsal sensory nucleus ; those of the smaller set bend downward and enter the fasciculus solitarius to terminate in arborizations around the cells of the spinal nucleus of reception. (For details of these nuclei see page 1260). As in the case of the other mixed nerves — the fifth, seventh and ninth — the secondary paths distributing the sensory impulses include (a) fibres that pass from the recep- tion-nuclei to the tract of the mesial fillet, and so on to the great brain, and (b) those that pass to the cerebellum. Course and Distribution. — The vagus, disregarding its accessory fibres which at first are incorporated in a common trunk with the eleventh nerve, arises from its superficial origin by a row of twelve or fifteen filaments which emerge from the surface of the medulla along the postero-lateral sulcus between the olivary eminence and the inferior cerebellar peduncle. These fasciculi lie in series with those of the ninth nerve above and of the eleventh below (Fig. 1046). After leaving the surface of the brain-stem, the converging rootlets of the vagus fuse to form a single flattened trunk, which passes outward beneath the flocculus of the cerebellum to the jugular foramen (Fig. 1074). The trunk leaves the cranium through the rear division of the middle compartment of this foramen, invested by a dural sheath shared by the spinal accessory IHTYC. In this situation it presents a ganglionic enlargement called fat ganglion of the root. Emerging from the jugular foramen, the vagus bears a second thickening, the ganglion of ///<" trunk, and niters the carotid sheath, through which it passes downward the entire length of the neck. Within the carotid sheath the nerve lies at first between the internal carotid artery and the internal jugular vein, and then between the common carotid artery and the vein, occupying the posterior groove between these vessels. At the root of the 1 Anatnmir ) spinal accessory nerves, (c) the loop between the first and second cervical nerves and (d) the superior cervical ganglion of the sympathetic. Branches. — The vagus nerve gives off the following branches: from the ganglion of the root, (i) the meningeal and (2) the auricular ; from the ganglion FIG. 1078. ICERV.N. n.y / A r \ «' HCCRV. Diagram of upper part of right vagus nerve, showing its pharyngeal and laryngeal branches with connections. of the trunk, (3) the pharyngeal and (4) the superior laryngeal ; in the neck, (5) the superior cervical cardiac, and (6) the inferior cervical cardiac ; in the thorax, (7) the inferior laryngeal, (8) the thoracic cardiac, (9) the anterior pulmonary, (10) the posterior pulmonary, (n) the cesophageal and (12) the pericardia/ ; and in the abdomen, (13) the abdominal. 1. The meningeal branch (r. meninijeus) arises from the ganglion of the root and follows a recurrent course upward through the jugular foramen to supply the dura mater of the posterior fossa of the cranium, especially in the vicinity of the lateral and occipital sinuses. 2. The auricular branch (r. auricularis) is given off from the ganglion of the root. It receives a filament of communication from the petrous ganglion of the ninth nerve and follows the outer margin of the jugular foramen to an opening between the stylo-mastoid and jugular foramina. Entering this foramen it traverses a (anal in the temporal bone which crosses the inner side of the facial canal and terminates between the mastoid process and the external auditory meatus. THE VAGUS NERVE. 1269 Leaving the canal the nerve supplies the skin of the posterior part of the auricle and of the posterior inferior portion of the external auditory meatus. While traversing the temporal bone the auricular nerve communicates with the facial and, after reaching its area of distribution, with the posterior auricular nerve. Variations.— The auricular nerve may be absent or may fuse with the main trunk of the facial, its fibres under these circumstances probably reaching their destination through the pos- terior auricular nerve. Its branch of communication with the facial may be absent. 3. The pharyngeal branches (rr. pharyngei), usually an upper and a lower but sometimes more or only one, are given off from the upper portion of the gang- FIG. 1079. Lower head of external pterygoid muscle Internal pterygoid muscl Auriculo-temporal nerve Internal carotid artery Pneumogastric nerve Inferior dental nerve Spinal accessory nerve - Part of facial nei Hypoglossal nerve Stylo-pharyngeus muscle Glosso-pharyngeal nerve I. cervical nerv Pneumogastric nerve Superior cervical ganglion of sympathetic Superior laryngeal nerve Descendens hypoglossi 1 1. cervical nerve III. cervical ner IV. cervical nerve T Association cord of sympathetic Middle cervical ganglion^ Inferior cervical gang! Phrenic nerve Branches from inf. cervical ganglion Inferior cervical cardia of sympathet: Recurrent laryngeal nerve Internal mammary artery Cartilage of I. rib' Clavicular facet of sternu Lingual nerve External laryngeal branch uperior cervical cardiac of sympathetic Middle cervical cardiac of sympathetic "Recurrent laryngeal nerve Middle cervical cardiac of Common [pneumogastric carotid artery Inferior cervical cardiac ot pneumogastric Deep dissection of right side of head and neck, showing lingual, glosso-pharyngeal, pneumogastic, hypoglossal and sympathetic nerves. lion of the trunk and include to a considerable extent fibres brought to the vagus by its accessory portion. They pass downward and inward, between the external and internal carotid arteries, and join the pharyngeal branches from the glosso-pharyn- geal nerve and from the superior cervical ganglion of the sympathetic to form the pharyngeal plexiis (plexus pharyngeus) (Fig. 1078). This plexus contains one or I27o HUMAN ANATOMY. more minute sympathetic ganglia and ramifies over the middle constrictor of the pharynx. It supplies motor fibres to the muscles of the pharynx and of the soft palate, with the exception of the stylo-pharyngeus and the tensor palati. From the plexus proceed sensory filaments to the mucous membrane of the pharynx. A filament from this -plexus, the lingual branch of the vagus (r. lingualis vagi), com- posed of fibres from both the ninth and tenth nerves, joins the hypoglossal as it hooks around the occipital artery. Variation. — A slender branch, the middle laryngeal nerve, is described as arising from the pharyngeal plexus and supplying the crico-thyroid muscle, after which it pierces the crico- thyroid membrane and supplies the mucous membrane of the lower part of the larynx. 4. The superior laryngeal nerve (n. laryngeus superior) (Fig. 1079) arises from the middle of the ganglion of the trunk and takes a downward and inward course beneath the external and internal carotid arteries toward the superior cornu of the thyroid cartilage. It divides terminally into (a) the external and (£) internal laryngeal branches. Communications. — Before dividing, the superior laryngeal nerve receives filaments from the superior cervical sympathetic cardiac and from the pharyngeal plexus. The cardiac twig given off by the external laryngeal nerve joins with the superior cervical cardiac branch of the sympathetic. In the lower part of the larynx the external laryngeal nerve inosculates with the terminal fibres of the internal laryngeal. At the inferior portion of the larynx, the internal laryngeal nerve communicates with the terminal filaments of the external laryngeal, and in this way supplies sensory fibres to the mucous membrane lining the lower part of the larynx and to the muscles. Variation. — Instead of passing to the inner side of the internal carotid artery the nerve may lie external to it. a. The external laryngeal branch (r. externus), much smaller than the in- ternal, passes downward upon the inferior constrictor of the pharynx and beneath the infrahyoid muscles to the crico-thyroid muscle, which it supplies. It sends filaments also to the inferior pharyngeal constrictor and gives off a cardiac twig which joins the superior cervical cardiac branch of the sympathetic. Variations. — The external laryngeal has been seen to send filaments to the thyroid gland, the pharyngeal plexus, the sterno-hyoid, sterno-thyroid, thyro-hyoid and crico-arytenoideus lat- erahs muscles and to the mucous membrane of the vocal cord and lower portion of the larynx. b. The internal laryngeal branch (r. internus), larger than the external, passes downward and inward between the middle and inferior constrictors of the pharynx and enters the larynx by piercing the thyro-hyoid membrane. By means of its cpiglottic, pharyngeal, descending and communicating branches, it supplies the mucous membrane covering the internal and pharyngeal surfaces of the larynx and the mucous membrane of the base of the tongue. Variation. — Instead of piercing the thyro-hyoid membrane the nerve may obtain entrance to the larynx through a small foramen in the thyroid cartilage. 5. The superior cervical cardiac branch (rr. cardiac! supcriorcs — both cervi- cal cardiacs) arises from the vagus in the upper part of the neck. It either joins a cardiac branch of the vagus or passes independently down the neck and along tin- side of the trachea to end in the deep cardiac plexus (Fig. 1132). 6. The inferior cervical cardiac branch leaves the vagus at the root of the neck. On the right side it courses along the side of the innominate artery and either independently, or after joining one of the other cardiac nerves, enters the deep car- diac plexus. The left passes in front of the arch of the aorta and joins the superior cervical cardiac branch of the left sympathetic to form the superficial cardiac plexus (Fig. 1132). 7. The inferior or recurrent laryngeal nerve (n. rccurrens) (Fig. 1080) differs on the two sides in the early part of its course. The right »(•>:'<• is given off at THE VAGUS NERVE. 1271 the root of the neck as the vagus crosses the anterior surface of the subclavian artery, from which point it passes under and behind the artery and ascends. The left nerve takes its origin as the vagus crosses the anterior aspect of the aortic arch, and after passing below and behind the arch, lateral to the obliterated ductus arteriosus, ascends in the superior mediastinum to enter the neck. After entering the neck the further course of the nerve is the same on both sides. It passes upward posterior to the carotid sheath, either anterior or posterior to the inferior thyroid artery, occupies the FIG. 1080. Superior cervical cardiac branch of sympatheti Middle cervical ganglion Inferior cervical ganglion Superior cervical cardiac of vagus Middle and inf. cervical cardiac branches of sympathetic Recurrent laryngeal nerve Pulmonary branch of vagus Vena azygos major Phrenic nerve Right pulmonary artery Pulmonary vein Aorta Right auricular appendix Pcricardiu Superior cervical cardiac branch of sympathetic Superior cervical cardiac branch of vagus Middle cervical ganglion Middle cervical cardiac branch of sympathetic (of sympathetic Inf. cervical cardiac branch Inf. cervical ganglion Middle cervical cardiac branch of vagus Inf cervical cardiac branch of vagus Phrenic nerve Dissection showing cardiac branches of pneumogastric nerves and of sympathetic cords ; aortic arch and branches and pulmonary artery partially removed ; pericardium laid open. groove between the oesophagus and the trachea, and, dipping beneath the lower edge of the inferior constrictor of the pharynx, enters the larynx at the inferior margin of the cricoid cartilage. The asymmetry observed in the first part of the course of the nerves of the two sides is secondary and referable to the changes incident to the development of the large arterial trunks. In the foetus both nerves hook around the fourth aortic arch of the corresponding sides and are, therefore, for a time symmetrically disposed. Since, however, on the left side this arch becomes the arch of the aorta, and on the right the innominate and subclavian arteries (page 726), it is evident that the vagi, although retaining their primary associations, later alter their actual position and relations in consequence of the unequal growth and downward displacement which these blood-vessels undergo. Branches. — During its course the inferior laryngeal nerve gives off : (a) the cardiac, (3) the tracheal, (Y) the cesophageal, (d) the -muscular and (e) the terminal branches. i272 HUMAN ANATOMY. a. The cardiac branches (rr. cardiad inferiores) are given off in the superior mediastinum and enter the deep cardiac plexus. b and c. Tracheal and cesophageal branches (rr. tracheales et oesophagei) are given off as the nerve ascends in the neck between the trachea and oesophagus. d. Muscular branches enter the inferior constrictor of the pharynx. e. The terminal branches (n. laryngeus inferior) are formed at the point where the nerve breaks up on the inner side of the thyroid cartilage. They supply the intrinsic muscles of the larynx, with the exception of the crico-thyroid. As it turns to ascend, the inferior laryngeal nerve communicates with the inferior cervical ganglion of the sympathetic, its terminal filaments joining with those of the internal laryngeal. Variations. — The inferior laryngeal nerve has been seen to supply twigs to the crico-thyroid muscle. In cases in which the subclavian artery arises dorsally, the right recurrent laryngeal passes directly downward and inward from the vagus to the larynx. 8. The thoracic cardiac nerves (rr. cardiac! inferiores) of the right side are derived both from the vagus as it lies beside the trachea and from the inferior laryn- geal. Those of the left side arise exclusively from the inferior laryngeal. They help to form the deep cardiac plexus. 9. The anterior pulmonary branches (rr. bronchiales anteriores) are two or three small filaments which, on the right side, receive communicating fibres from the deep cardiac plexus and, on the left side, are joined by filaments from both car- diac plexuses. These unite to form the anterior pulmonary plexuses (plexus pulmonales anteriores) (Fig. 1080), which communicate with each other and with the posterior plexuses, and ramify over and supply the anterior aspect of the bronchus and root of the lung. 10. The posterior pulmonary branches (rr. bronchiales posteriores) are several large twigs which join with filaments from the second, third and fourth tho- racic ganglia of the sympathetic to form the posterior pulmonary plexus (plexus pulmonalis posterior). Fibres from this plexus communicate with the corresponding structure of the opposite side and with the anterior pulmonary plexuses, in this way each vagus sending fibres to both lungs. Branches from the plexus, bearing tiny ganglia, follow the subdivisions of the bronchi to supply the ultimate units of the lung. n. The cesophageal branches (rr. oesophagei) are given off in two situa- tions : in the superior mediastinum, where the right vagus and the left inferior laryngeal distribute cesophageal branches, and in the posterior mediastinum, where the oesophagus is surrounded by branches from the cesophageal plexus or plexus gul- dural space, between the ligamentum denticulatum and the posterior nerve-roots (Fig. 879), to the foramen magnum, through which it enters the cranium. Upon reaching the side of the medulla, the spinal accessory nerve turns outward to enter the middle compartment of the jugular foramen and to unite temporarily with the accessory vagus. It occupies the posterior part of the middle compartment of the jugular foramen, lying within a dural sheath which contains also the vagus. On reaching the lower margin of the foramen, the fibres accessory to the vagus perma- nently leave the eleventh nerve. The latter, often described as the spinal part, courses downward for a short distance in the interval between the internal carotid artery and the internal jugular vein and then passes backward, either anterior or pos- terior to the vein, until it reaches the deep surface of the sterno-mastoid muscle, which it usually enters. While within the substance of the muscle, the spinal accessory gives off filaments which unite with a branch from the second cervical nerve to form the sterno-mastoid plexus (Fig. 1082) for the supply of that muscle. Emerging from beneath the posterior edge of the sterno-mastoid, the eleventh nerve crosses the occipital triangle and dips under the anterior margin of the trapr/ius along the deep surface of which it descends almost to the lower margin of the muscle. Under the trapezius the nerve forms a plexus of varying degrees of intricacy with the third and fourth cervical nerves. This is called the snbtrapi-zial plexus ( Fig. 1082 ), its fibres of distribution supplying solely the trapr/ius muscle. THE HYPOGLOSSAL NERVE. 1275 Variations. — Considerable deviation from the normal has been described with regard to the spinal portion. The lower limit of its origin has been observed as high as the third cervical nerve and from that level as far down as the first thoracic. In one instance the nerve left the subdural space below the first cervical nerve and re-entered at a higher level. Quite fre- quently it fails to pierce the sterno-mastoid muscle. In one reported case the nerve ended in the sterno-mastoid, the trapezius being supplied only by the third and fourth cervical nerves. Two similar cases have been observed in the dissecting room of the University of Pennsylvania. Rarely it gives off a filament which joins the n. descendens cervicalis. Practical Considerations. — The spinal accessory nerve supplies the sterno- cleido-mastoid and trapezius muscles. A few fibres of the second and third cervical nerves enter into the supply of the sterno-mastoid, but the muscle is almost com- pletely under the control of the spinal accessory. The cervical nerves take a greater part in the supply of the trapezius, so that paralysis of the spinal accessory does not always paralyze this muscle. Spasm of the trapezius will draw the head backward and toward the affected side and will pull the scapula toward the spine. In spasm of the sterno-mastoid, as in " wry neck," the chin will be turned to the opposite side and elevated, while the ear will look forward. If both sterno-mastoids are in contraction the chin will be in the median line and will be drawn toward the sternum. Paralysis of one muscle will produce a condition somewhat similar to that produced by a spasm of the opposite one. The spinal accessory nerve enters the under surface of the sterno-mastoid muscle near the junction of its upper and middle thirds, where it may be reached by an incision along the anterior border of the muscle. The nerve emerges from the muscle near the middle of its posterior border. THE HYPOGLOSSAL NERVE. The twelfth or hypoglossal nerve (n. hypoglossus) is a purely motor nerve and supplies the musculature of the tongue, intrinsic as well as extrinsic, with the excep- tion of the palato-glossus. Central and Cortical Connections. — The hypoglossal nerve takes its deep origin from several associated groups of neurones called the hypoglossal nucleus (nucleus n. hypoglossi) (Fig. 949), which underlies the floor of the fourth ventricle. This nucleus is a narrow elongated collection of large multipolar cells, measuring about 18 mm. in length by 2 mm. in width, that partly corresponds in position to the trigonum hypoglossi in the floor of the fourth ventricle. The entire nucleus, however, is more extensive than the trigonum and extends from the level of the striae acusticse above into the closed part of the medulla as far down as the decussation of the pyramids (Fig. 927). It lies ventral and very slightly lateral to the central canal of the medulla and the median groove in the floor of the fourth ventricle, close to the mid-line and its fellow of the opposite side. The large size and branched form of the nerve-cells composing the nucleus, as well as their ventral position in relation to the central canal, emphasize the close correspondence of these elements with the cells of the motor roots of the spinal nerves. Indeed, as noted later (page 1380), the gray matter enclosing the hypoglossal nucleus is the morphological equivalent of the bases of the anterior cornua. Immediately after arising and before leaving the nucleus, the axones converge into a number of fasciculi which, emerging from the ventral aspect of the nucleus, take a ventro-lateral course and traverse the interval between the gray and white reticular formations. From this situation the hypoglossal fibres continue their course to the anterior surface of the medulla by passing, for the most part, between the nucleus of the inferior olive and the mesial accessory olivary nucleus, although quite a number of the strands penetrate the ventral portion of the olivary nucleus (Fig. 927). The central connections of the hypoglossal nucleus include: (a] crossed fibres from the nucleus of the opposite side ; (£) fibres from, and probably also to, the posterior longitudinal fasciculus, by means of which the nucleus of the twelfth is brought into relation with the nuclei of other motor nerves; and (c) fibres which join the dorsal bundle of Schiitz, a system of longitudinal fibres underlying the floor of the fourth ventricle and traceable upward beneath the Sylvian aqueduct, by which the nuclei of the sensory cranial nerves are connected. The cortical centre of the hypoglossal nerve probably lies within the lower or opercular extremity of the precentral convolution. The fibres arising as the axones of the cells within this area pass over the upper border of the lenticular nucleus and through the internal capsule and descend in the brain-stem within the median part of the pyramidal tract as far as the 1276 HUMAN ANATOMY. medulla. The cortico-nuclear fibres then bend dorso-medially and, for the most part but not entirely, cross the raphe to enter the ventro-lateral surface of the hypoglossal nucleus of the opposite side and end in arborizations around the root-cells. Course and Distribution. — The hypoglossal takes its superficial origin from the surface of the brain-stem in the form of from ten to fifteen slender fasciculi, which emerge from the ventral surface of the medulla in the groove between the olivary eminence and the pyramid (Fig. 1046). FIG. 1082. Digastric muscle, I. cervical nerve Spinal accessory nerve — Small occipital nerve II. cervical nerve Hypoglossal nerve Superior cervical gang! Branch of II. cervical to spinal accessory III. cervical nerve Communicans hypoglossi Stumps of great auricular and superficial cervical nerves) ,v_ CCTvicai nerve VI. cervical nerve Branch of communication to spinal accessory Cutaneous branch VII. cervical nerve Nerve to subcla' VIII. cervical nerve Posterior thoracic ner Supra scapula nerve I. thoracic nerve External pterygoid muscle Lingual branch of V. nerve f Chorda tympani nerve Internal pterygoid muscle Edge of oral mucous membrane losso-pharyngeal nerve -Mental nerve Inferior dental nerve, cut Sublingual gland Submaxillary ganglion 'Stylo-hyoid muscle Thyro-hyoidbranchofXII. nerve Superior laryngeal nerve Descendens hypoglossi ; sympathetic cord is toils outer side Vagus nerve External laryngeal nerve Omo-hyoid muscle, cut Phrenic nerve Middle cervical ganglion of sympathetic Scalenusanticus muscle lavian artery Deep dissection of neck showing branches of vagus, spinal accessory and hypoglossal nerves. These root-bundles pass outward, dorsal to the vertebral artery, and assemble into two groups, which pierce the dura mater separately at a point opposite the anterior condyloid foramen. Either within this canal or as they leave the cranium through its external opening they unite into a single trunk. Arriving at the inferior aspect of the base of the skull, the deeply placed hypoglossal nerve descends and hooks around the ganglion of the trunk of the vagus, to which it is closely attached by connective tissue. It then takes a downward and forward course between the internal carotid artery and the internal jugular vein. Arriving at the inferior margin of the 'posterior belly of the digastric, the nerve winds around the occipital artery and courses downward and forward to the outer side of .the external and internal carotid arteries. It then continues forward above the hyoid bone to the under surface of the tongue, passing beneath the tendon of the digastric, THE HYPOGLOSSAL NERVE. 1277 under the stylo-hyoid and mylo-hyoid muscles and over the hyo-glossus (Fig. 1082). It terminates by piercing the genio-hyo-glossus and breaking up into a number of fibres for the supply of the lingual muscles. Communications. — Immediately after emerging from the anterior condyloid foramen, (a] a tiny branch connects with the superior cervical ganglion of the sympathetic, (I)) one or two filaments pass to the loop between the first and second cervical nerves and (c) several fibres associate the nerve with the ganglion of the trunk of the vagus. At the point where the hypo- glossal nerve and the occipital artery cross, (d) the lingual branch of the vagus joins the twelfth ; and as the nerve lies beneath the mylo-hyoid and upon the hyo-glossus muscle, it communi- cates with (e) the lingual branch of the mandibular nerve. Branches. — The branches of the hypoglossal nerve are : (i) the meningeal, (2) the descending, (3) the thyro-hyoid and (4) the lingual. 1. The meningeal branch (r. meningeus) consists of one or two minute filaments which supply the dura mater of the posterior cranial fossa and the diploe of the occipital bone. As the hypoglossal is motor in function, it is likely that these twigs are contributed to the nerve by the loop between the first and second cervical nerves. 2. The descending branch (r. descendens), or r. descendens hypoglossi, is in reality only to a limited extent a branch of the twelfth, since the greater number of its fibres are accessions to the hypoglossal from the first and second cervical nerves. There is reason, however, to believe that these cervical nerves are not the exclusive source of the fibres of the descendens hypoglossi, but that some arise from the cells of the hypoglossal nucleus. The descending branch arises near the point where the hypoglossal nerve hooks around the occipital artery and runs downward and inward in front of or within the carotid sheath. It gives off a branch to the an- terior belly of the omo-hyoid and, about the middle of the neck, joins the descend- ing cervical nerve, or n. communicans hypoglossi, from the second and third cervical nerves. A loop or plexus, termed the ansa hypoglossi, is thus formed and from it filaments are supplied to the sterno-hyoid and sterno-thyroid muscles and to the posterior belly of the omo-hyoid (Fig. 1082). 3. The thyro-hyoid nerve (r. thyreohyoideus) is also only an apparent branch of the hypoglossal, as its fibres can be traced back to the cervical plexus. It is given off before the nerve dips beneath the stylo-hyoicl muscle and passes down behind the greater cornu of the hyoid bone to reach its distribution to the thyro-hyoid muscle. 4. The lingual branches (rr. linguales) with one exception, comprise the real distribution of the hypoglossal. As the nerve lies beneath the mylo-hyoid muscle filaments are given off to the hyo-glossus, the stylo-glossus and the genio-hyoideus. The fibres going to the genio-hyoid are in all probability de- rived from the cervical plexus and are not of true hypoglossal origin. After giving off the above-named branches, the hypoglossal nerve breaks up into the terminal filaments which pierce the genio-hyo-glossus to supply it and the lingualis muscle. Variations. — Occasionally the hypoglossal has been found to possess a posterior root bear- ing a ganglion. This condition is to be regarded as a persistence of the temporary embryonal stage during which the nerve is provided with a posterior root and a ganglion of Froriep (page 1380). In one case the superficial origin was located at the posterior aspect of the me- dulla. Quite frequently the vertebral artery passes between the rootlets of origin and in rare instances behind them. Sometimes a cross filament, situated either between the genio-hyo- glossus and genio-hyoid muscles or in the substance of the latter connects the two hypoglossal nerves. Rarely the hypoglossal has been seen to send a filament to the mylo-hyoid, the .digas- tric or the stylo-hyoid muscle. Occasionally the r. descendens hypoglossi seems to be derived, either entirely or in part, from the vagus, but in these instances the fibres can be traced back to their true origin from the cervical nerves. A filament from the Descending nerve sometimes passes into the thorax, where it joins the vagus or the sympathetic ; in such cases the aberrant branch is probably derived originally from either the sympathetic or the vagus. The r. descen- dens hypoglossi may send a branch to the sterno-mastoid muscle. Practical Considerations. — Involvement of the hypoglossal nerve, usually together with other cranial nerves is frequent in bulbar disease. The most character- istic symptom is a deviation of the tongue, when protruded to the affected side, caused 1278 HUMAN ANATOMY. by the unopposed action of the muscles of the opposite side. The nerve may be injured by operative or other wounds in the submaxillary region or in the mouth, as in gun-shot wounds. It can be easily reached in the submaxillary region by the same incision as that used for ligating the lingual artery (page 736). It passes for- ward to the tongue, just above the hyoid bone, and forms the upper boundary of the smaU ' ' lingual triangle, ' ' which is exposed when the submaxillary gland is elevated. THE SPINAL NERVES. The cranial or cerebral division of the nerves having been considered, the spinal group next claims attention, the visceral or splanchnic (sympathetic) nerves being reserved for a final and separate description. The spinal nerves (nn. spinales) include a series of usually thirty-one pairs of symmetrically disposed trunks which pass laterally from the spinal cord and emerge from the vertebral canal through the intervertebral foramina (Fig. 880). Each nerve arises from the cord by a dorsal sensory and a ventral motor root, which sepa- rately traverse the subarachnoid and subdural spaces and evaginate or pierce the pia mater, arachnoid and dura mater. Within the intervertebral foramina the roots unite to form a common trunk, which carries with it a sheath composed of the three membranes, the contribution of the arachnoid and pia, however, soon ending, whilst the dural covering is prolonged to become continuous with the epineural sheath of the nerve. Nomenclature. — The spinal nerves are designated not relative to the position at which they arise from the cord, but according to their point of emergence from the vertebral canal. They are divided, therefore, into the cervical, thoracic, lumbar, sacral and coccygeal groups. With the exception of those in the cervical region, the individual nerves are named according to the vertebra below which they emerge from the vertebral canal. On account of the disproportion between the eight cervi- cal nerves and the seven cervical vertebrae, this arrangement necessarily can not prevail in the neck. The first cervical nerve, often called the suboccipital nerve, emerges between the occipital bone and the atlas ; the second emerges below the first vertebra, the third below the second and so on down to the eighth, which traverses the foramen between the seventh cervical and first thoracic vertebral segments. Constitution. — Every spinal nerve arises by two roots, a posterior sensory and an anterior motor, the latter being composed of the axones proceeding from the motor neurones situated within the gray matter of the anterior cornu of the spinal cord, whilst the fibres composing the posterior or sensory root are the axones of the neurones within the ganglia which are invariably present on these roots. The formation of the common trunk, by the union of the two roots, affords opportunity for the two varie- ties of fibres to intermingle, so that the anterior and posterior primary divisions into which the common trunk divides contain both sensory and motor fibres. In addition to these fibres, which are destined for the somatic muscles and the integument, others are added from the sympathetic neurones for the supply of the outlying involuntary muscle and glandular tissue occurring in the regions to which the spinal nerves are distributed. It is evident, therefore, that the terms "motor" and "sensory," as applied to the somatic branches of the spinal nerves, are relative and not absolute, since in all cases the nerves passing to the muscles contain sensory and sympathetic fibres in addition to those ending as motor filaments in relation with the striated muscle fibres. Likewise, in the case of the sensory branches distributed to the integ- ument, sympathetic filaments (motor to the involuntary muscle of the blood-vessels and secretory to the glands) accompany those concerned in collecting sensory impulses. On the other hand, where they retain their typical plan, as in the case of the thoracic nerves, the spinal nerves contribute motor fibres which end around the sympathetic neurones to supply motor impulses either to the involuntary muscle of the organs, by way of the splanchnic efferents, or to the outlying involuntary muscle- along the somatic IHTVCS in the manner above described. The sensory, posterior or dorsal roots i radices posteriores) of the spinal nerves are usually larger than the motor, a condition due to tin- increased number of their filaments and the greater size of those filaments ( lila ladicularin). The fas- ciculi which form the sensory root are attached to the cord along the postero-lateral 1279 groove as a continuous series, called the posterior root zone (Fig. 884). These rootlets are sometimes so numerous and so crowded, that those of adjacent nerves overlap and adhere to one another. Where more typically disposed, as in the thoracic region, the cord-segments (page 1024) are distinct. The fasciculi for any one nerve usually collect into two bundles which pass to the proximal aspect of the spinal ganglion. The spinal ganglia (gg. spinalia) are aggregations of nerve-cells found on the posterior roots of all the spinal nerves (Fig. 852). They are usually ovoid in shape, from 4-6 mm. in length, and are occasionally bifid at their proximal ends. They consist of a cluster of unipolar neurones, whose centrally directed axones form the sensory root of the spinal nerve and whose dendrites extend peripherally as the sensory distribution. The ganglia are usually situated in the intervertebral foramina, but exceptions to this rule are presented by the ganglia of the first and second cervi- cal nerves, which lie upon the neural arches of the atlas and axis respectively, and by those of the sacral and coccygeal nerves, which are lodged within the vertebral canal. Although situated beyond the dural sheath of the cord, with the exception of the ganglion of the coccygeal nerve, they are invested by a prolongation of it. Variations. — The first cervical nerve may either have no posterior root or may derive it" from or share it with the eleventh cranial nerve. Its ganglion may be very rudimentary or entirely absent. Considerable variation is found in the thoracic region, where either the anterior or pos- terior or both roots of one of the nerves may seemingly be absent. In the lumbar and upper sacral nerves the ganglion may be double, each bundle of the posterior root having its own. Ganglia aberrantia are small detached portions of the spinal ganglia occasionally found along the posterior roots of the upper cervical, the lumbar and the sacral nerves. The motor, anterior or ventral roots (radices anteriores) are smaller than the posterior and have no ganglia. They emerge from the anterior surface of the cord in a series of fasciculi (fila radicularia), the anterior root-zone, with a tendency to form two groups which unite in the completed root (Fig. 878). As in the pos- terior roots, the fasciculi of origin may overlap one another or fuse with those of adjoining nerves. Number. — As usually found the thirty-one pairs are grouped as follows: — eight cervical, twelve thoracic, five lumbar, four sacral and one coccygeal. Variations. — Should there be any anomaly in the number or arrangement of the vertebrae, there is a corresponding modification of the nerves. The greatest variation occurs in the coccygeal region. There may be none at all in this situation, or one or two additional ones may be found. Traces of two extra ones, which are rudimentary caudal nerves, may be found in the filum terminale. Size. — The largest spinal nerves are those which are concerned in the forma- tion of the limb plexuses — brachial, lumbar and sacral — and are, therefore, the lower cervical, the first thoracic, the lower lumbar and the upper sacral. The largest nerves in the entire series are the lower lumbar and upper sacral. The smallest are the lower sacral and the coccygeal. Those of the upper cervical region are smaller than those of the lower, the sixth being the largest of those in the neck. With the exception of the first, the thoracic nerves are comparatively small. Divisions. — The common trunk formed by the union of the two roots emerges from its intervertebral foramen and almost immediately gives off a meningeal or recurrent branch (r. meningeus). This tiny nerve is joined by a filament from a gray ramus communicans and enters the vertebral canal through the foramen to be distributed to the vertebrae and their ligaments, and to the blood-vessels of the vertebral canal and of the spinal cord and its membranes. After giving off the recurrent twig, each trunk soon splits into two branches, called the anterior and posterior primary divisions (rr. anterior et posterior), each of which is composed of fibres from both roots (Fig. 1085), as well as of sympathetic filaments. THE POSTERIOR PRIMARY DIVISIONS OF THE SPINAL NERVES. The posterior primary divisions (rr. posteriores) of the spinal nerves are as a rule smaller than the anterior (rr. anteriores). They arise either as a single cord from the trunk formed by the union of the two roots, or as two separate strands 1280 HUMAN ANATOMY. from the roots themselves. They turn dorsally almost immediately and divide into an internal (r. medialis) and an external branch (r. lateralis), which supply the FIG. 1083. Great occipital nerve Cutaneous brs. of III. cervical, dorsal div. Cutaneous brs. of IV. and V. f cervical, dorsal division | VII. cervical spinous process Cutaneous brs. of dorsal divisions of : — I. thoracic II. thoracic III. thoracic IV. thoracic V. thoracic VI. thoracic VII. thoracic VIII. thoracic IX. thoracic X. thoracic Spinous process of XII. thoracic vertebra Cutaneous brs. of dorsal divisions of : — XI. thoracic XII. thoraci I., II. and III. lumbar ( nerves, ext. brs. of 4 dorsal divisions J Cutaneous brs. of dorsal divisions of sacral nerves Occipitalis major nerve III., IV., V., VI., VII., VIII., IX. ,X. and XI. thoracic nerves, lateral cutaneous branches XII. thoracic, lateral cu- neous br. Iliac br. ilio-hypojjastric nerve XII. thoracic, lateral cu- taneous br. . Superficial dissection, showing cutaneous branches of posterior divisions and lateral cutaneous branches of anterior divisions of spinal nerves. dorsal muscles and integument. At the two extremities of the spinal series the division into internal and external branches does not prevail, the first cervical, THE CERVICAL NERVES. 1281 the fourth and fifth sacral and the coccygeal nerve failing in this respect. Down to and including the sixth thoracic nerve, the internal branches are mainly cutaneous and the external entirely muscular. From the seventh thoracic down, the reverse con- dition exists. In the former region the internal branches become cutaneous near the spine, whilst in the latter the sensory filaments pass laterally for some distance through the muscles before reaching their cutaneous distribution. THE CERVICAL NERVES. The first cervical nerve (n. suboccipitalis), the first of the spinal secies, is atypical in several respects. Its posterior root is either insignificant or entirely absent, and its posterior division, which does not divide into internal and external branches, is larger than the anterior and usually does not send off any direct cutaneous branch. The nerve passes dorsally between the occipital bone and the posterior arch of the atlas and traverses the suboccipital triangle, occupying a position below and posterior to the vertebral artery. Superficial to it is the complexus muscle. Branches. — These are : (i) the muscular, (2) the communicating and (3) the cutaneous. 1. The muscular branches supply the superior and inferior oblique, the corn- plexus and the rectus capitis posticus major and minor muscles. 2. The communicating branch forms a loop with the second cervical nerve. It usually arises in common with the twig to the inferior oblique muscle, through or over which muscle it passes to reach its destination. It may arise with the nerve to the complexus, after piercing which muscle it communicates with the great occipital nerve. In the neck and close to the vertebrae is a series of loops between the posterior divisions of the first, second, third and sometimes the fourth cervical nerves. This is called fae posterior cervical plexus and from it filaments are distributed to the neigh- boring muscles. 3. The cutaneous branch is not always present. It accompanies the occipital artery, inosculates with the small and great occipital nerves and supplies the occipital region. The second cervical nerve is distinguished by the size of its posterior division, (r. posterior) which is larger than the anterior (r. anterior). Its posterior division takes a dorsal course between the atlas and the axis, and then between the inferior oblique and semispinalis colli muscles. Reaching the deep surface of the complexus it breaks up into its external portion (r. lateralis), which supplies the complexus, obliquus inferior, semispinalis colli and multifidus spinse muscles, and its internal portion (r. medialis). The latter is called the great occipital nerve (n. occipitalis major). This nerve (Fig. 1087) passes upward over the inferior oblique, pierces the complexus and trapezius, and accompanies the occipital artery to the scalp, to the posterior half of which it is the main sensory nerve. It becomes superficial at the superior nuchal line, at a point from 2-3 cm. lateral to the external' oc- cipital protuberance, and spreads out into numerous branches which supply the scalp as far forward as the vertex. The great occipital nerve communicates with the small and least occipital and the posterior and great auricular nerves. Variations. — An approximate balance is maintained between the great and small occipital nerves, any deficiency in the distribution of either usually being equalized by a compensatory enlargement of the other. Sometimes the great occipital sends a branch to the auricle. The external branch may give off a cutaneous filament or may furnish a twig to the superior oblique. The third cervical nerve has a smaller posterior division than has the second. Passing backward, the former helps to form the posterior cervical plexus and divides into external and internal branches. The external branch (r. lateralis) supplies adjacent muscles and the internal branch (r. medialis), known as the least or third occipital nerve (n. occipitalis tertius), pierces the complexus, splenius and trapezius to supply the skin of the occipital and posterior cervical regions (Fig. 1083). 81 1282 HUMAN ANATOMY. In addition to assisting in the formation of the posterior cervical plexus it communicates with the great occipital nerve. The fourth, fifth, sixth, seventh and eighth cervical nerves have quite small posterior primary divisions (rr. posteriores). The fourth, fifth and sixth divide into the usual external and internal branches (rr. laterales et mediales), which supply respectively the adjacent muscles and the dorsal integument. The seventh and eighth usually have no cutaneous branches and are distributed solely to the deeper muscles of the back. A communicating filament from the fourth may aid in the formation of the posterior cervical plexus. Variations. — The cutaneous branches of the fifth and sixth may be very small or absent entirely. THE THORACIC NERVES. The posterior primary divisions (rr. posteriores) of the thoracic or dorsal nerves (nn. thoracales) follow the general arrangement of dividing into external and internal branches. Of these the internal branches of the upper six are mainly cutaneous and the external entirely muscular. In the lower six, on the contrary, the external branches are principally cutaneous and the internal entirely muscular. The external branches (rr. laterales) gradually increase in size from above downward. They pierce or pass under the longissimus dorsi to reach the interval between that muscle and the ilio-costalis, eventually reaching and supplying the erector spinae. Those from the lower half of the thoracic nerves distribute sensory fibres for the supply of the skin overlying the angles of the ribs (Fig. 1083). The internal branches (rr. mediales) of the upper six or seven pass dorsally between the multifidus spinae and semispinalis muscles. After innervating the trans- verso-spinales they become superficial close to the median dorsal line and supply the skin of the back, sometimes extending laterally beyond the vertebral border of the scapula. The internal branches of the lower nerves traverse the interval between the longissimus dorsi and the multifidus spinae and supply the latter muscle. Variations. — The sixth, seventh and eighth thoracic nerves may give off cutaneous twigs from both external and internal branches. The first thoracic nerve may have no cutaneous branch. THE LUMBAR NERVES. The posterior primary divisions (rr. posteriores) of the lumbar nerves (nn. him- bales) divide into the usual external and internal branches. The external branches (rr. laterales) of all five lumbar nerves enter and sup- ply the erector spinae, those of the lower two terminating there. From the external branches of the first, second and third arise cutaneous offshoots (nn. clunium supc- riores) of considerable size (Fig. 1083). These pierce the ilio-costalis and the aponeurosis of the latissimus dorsi above the crest of the ilium and supply tin- skin of the gluteal region as far forward as the great trochanter. From the fifth a branch passes downward to inosculate with a similar branch of the first sacral nerve to aid in the formation of the posterior sacral plexus. The internal branches (rr. mediales) turn directly backward and supply the multifidus spinae muscle. THE SACRAL NERVES. The posterior primary divisions (rr. posteriores) of the sacral nerves (nn. ^•ici.ik'si, with the exception of that of the fifth, emerge from the- vertebral canal through the posterior sacral foramina. The first, second and third pass outward under cover of the multifidus spina- and divide into external and internal branches. • The external branches ( rr. laterales ) of the first , second and third sacral nerves unite over the upper part of the sacrum with a similar branch of the fifth lumbar and with the fourth sacral nerve to form a series of loops, the posterior sacral plexus THE SACRAL NERVES. 1283 (Fig. 1084). From this structure branches pass laterally till they reach the inter- val between the great sacro-sciatic ligament, which they pierce, and the deep surface of the glutens maximus, where they form a second series of loops. From the pri- mary loops branches are supplied to the multifidus spinae and from the secondary loops proceed two or more filaments, usually two (nn. clunium medii), which pierce the gluteus maximus on a line connecting the posterior superior spine of the ilium FIG. K Loops of communication between V. lumbar, and I., II. and III. posterior sacral nerves Cutaneous branches from last lumbar and first three sacral nerves, post, divisions From XII thoracic nerve Part of multifidus spinae muscle V. lumbar nerve, posterior division I. sacral nerve, posterior division II. sacral nerve, posterior division HI. sacral nerve, posterior division IV. sacral nerve, posterior division xygeal nerve, anterior division V. sacral nerve, posterior division Coccygeal nerve, posterior division IV. sacral nerve, anterior division, the coccygeus :le being partly cut away V. sacral nerve, anterior division Coccygeus muscle, part of its ccygeal attachment cut Cutaneous branch of IV. anterior sacral nerve, giving off here also muscular branches to levator ani Dissection showing left posterior sacral plexus. and the tip of the coccyx. One is usually situated near the lower portion of the sacrum and the other at the side of the coccyx. They pass laterally and supply the skin of the buttock (Fig. 1083). The internal branches (rr. mediates) of the first, second and third sacral nerves are small in size and are distributed to the multifidus spinse. The posterior primary divisions of the fourth and fifth sacral nerves are of small size. They pass below the multifidus spinae and continue as single trunks, not breaking up as do the others, into two branches. They are connected with each other and with the coccygeal nerve by loops which form the posterior sacro- coccygeal nerve. From this structure fibres which pierce the great sacro-sciatic ligament are given off to be distributed to the integument in the coccygeal region (Fig. 1084). 1284 HUMAN ANATOMY. THE COCCYGEAL NERVE. The posterior primary division (r. posterior) of the coccygeal nerve (n. coccy- geus) does not divide into internal and external branches. It unites with the fourth and fifth sacral to form the posterior sacro-coccygeal nerve, whose course and distri- bution are described above. THE ANTERIOR PRIMARY DIVISIONS OF THE SPINAL NERVES. The anterior primary divisions (rr. anteriores) of the spinal nerves, like the posterior (rr. posteriores), contain fibres from both the anterior and posterior roots and, with the exception of those of the first and second cervical nerves, are larger than the posterior. After liberation from the main trunk at the intervertebral foramina, they pass ventrally and supply the lateral and anterior portions of the neck and trunk, as well as the limbs. Shortly after leaving its foramen, each anterior division is joined by a slender fasciculus from the gangliated cord of the sympathetic, called the gray ranms communicans (page 1357). Branches to the sympathetic system are given off from some of the thoracic, lum- FIG. 1085. kar ancj sacrai nerves, in the shape of small fasciculi of medullated fibres, called the ichitc rami communicantcs. These are destined for the various structures of the splanchnic area and consti- tute the visceral or splanch- nic distribution of the spinal nerves. The remainder of the fibres are supplied to the body wall and ex- tremities and constitute the somatic distribution of the nerves. In the case of the cervical, first and some- times second thoracic, lum- bar, sacral and coccygeal nerves, plexuses of a greater or less degree of intricacy are interposed between the origin and distribution of the nerves. This renders the tracing of any set of fibres a matter of extreme difficulty, but in the greater portion of the thoracic region the original segmental and less complex arrangement persists. A typical spinal nerve (Fig. 1085), such as one of those in the mid-thoracic region, is arranged as follows. The constitution of the main trunk (page 1278) and the distribution of its posterior branch (page 1279) have already been described. The anterior primary division (r. anterior) leaves the intervertebral foramen and almost immediately is connected with the gangliated cord by gray and white rami communicantes. It then enters an intercostal space through which it courses between the external and internal intercostal muscles, both of which it supplies. At the side of the chest it gives of? a lateral cutaneous branch (r. cntancus lateralis), which distributes a few tiny motor twigs and then pierces the external intercostal muscle to supply the skin over the lateral portion of the trunk. On reaching the superficial fascia it usually breaks up into two branches, a larger anterior ( r. anterior) and a smaller posterior (r. posterior). Having given off the lateral « ui ineous branch, the main anterior primary division continues its forward course nearly to the mid-line, where it pierces the muscle and becomes superficial as the anterior terminal cutaneous branch (r. cutaneus anterior). Diagram illustrating constitution and division of typical spinal nerve; SC, spinal cord ; AR, PR, anterior and posterior roots; SG, spinal gang- lion; CT, common trunk; AD, PD, anterior and posterior primary divis- ions; PC, LC, AC, posterior, lateral and anterior cutaneous branches; RC, ramus communicans ; Sy, sympathetic ganglion and cord. THE CERVICAL PLEXUS. 1285 FIG. 1086. vertebral artery emerge, the first 1C. The integument is therefore supplied, from dorsal to ventral mid- line, by the posterior primary division, the posterior and anterior divisions of the lateral cutaneous branch and the anterior cutaneous branch of the anterior primary division. The muscles derive their nerve-supply from both the anterior and the posterior primary divisions. THE CERVICAL NERVES. The anterior primary divisions (rr. anteriores) of the eight cervical nerves (nn. cervicales), assisted by the first and second thoracic, supply the head, neck, upper extremity, thoracic integument and diaphragm. The first, second, third and fourth communicate freely and form the cervical plexus for the supply of the head and neck and the skin of the upper pectoral and shoulder regions, whilst the fifth, sixth, seventh, and eighth, aided by the first and sometimes by the second thoracic, form the brachial plexus, which supplies the upper extremity and the lateral thoracic wall. THE CERVICAL PLEXUS. The cervical plexus (plexus cervicalis ) is formed by the union of the anterior primary divisions (rr. anteriores) of the upper four cervical nerves (Fig. 1086). After traversing the intervertebral foramina, they pass behind the and be- tween the rectus capitis lateralis and the rectus capitis anticus minor muscles, and the others first between the inter- transversales muscles and then between the rectus capitis anticus major and scalenus me- dius muscles. Each is joined by a gray ramus communicans, derived either from the superior cervical ganglion of the sympathetic or from the association cord be- tween the superior and middle cervical ganglia. Under cover of the sterno-mastoid the four nerves are connected to form the cervical plexus. The second, third and fourth each divide into an ascending and a descending branch ; the first does not divide. These branches are connected in an irregular series of loops that constitute the cervical plexus, which lies opposite the first four cervical vertebrae and upon the sca- lenus medius and levator anguli scapulae muscles, and is covered by the sterno-mastoid. Branches. — The branches of the plexus may be divided into a superficial and a deep set. The former reach the under surface of the deep fascia at about the middle of the posterior margin of the sterno-mastoid and are distributed to the integument of the head, neck, shoulder and upper pectoral region. The latter are divided into an internal and an external group, some of which supply the muscles of the neck Diagram illustrating plan of cervical plexus. 1286 HUMAN ANATOMY. and the diaphragm, whilst others communicate with the ninth, eleventh and twelfth cranial and the sympathetic nerves. THE CERVICAL PLEXUS. I. Superficial Branches. II. Deep Branches. A. Ascending branches : D. External branches : 1. Small occipital 7- Muscular 2. Great auricular 8. Communicating B. Transverse branch : E. Internal branches : 3. Superficial cervical 9. Muscular C. Descending branches : 10. Phrenic 4. Suprasternal n. Communicating 5. Supraclavicular 6. Supraacromial I. The superficial branches are purely sensory. They become superficial at the posterior border of the sterno-mastoid, slightly above its middle, and from that point radiate in all directions to reach their cutaneous destinations (Fig. 1087). 1. The small occipital nerve (n. occipitalis minor) (Fig. 1087) may be either single or double. It originates from the second and third cervical nerves, or from the second only, and passes backward and upward beneath the deep fascia along or overlapping the posterior border of the sterno-mastoid muscle, where it gives off (a) the cervical branches. It pierces the deep fascia at the upper angle of the occipital triangle and breaks up into its terminal branches : (6) the auricular, (<:) the mastoid and (d) the occipital. a. The cervical branches are tiny twigs which supply the skin over the upper part of the occipital triangle. b. The auricular branch supplies the integument over the cranial aspect of the posterior part of the pinna. c. The mastoid branch supplies the scalp overlying and above the mastoid process. d. The occipital branch is distributed to the area of scalp of the occiput lying between the mastoid process and the distribution of the great occipital nerve. The small occipital communicates with the posterior and great auricular nerves and with the great occipital. Variations. — The small occipital varies in size and may be so small as to be distributed only to the integument in the neck. In such an event, and usually in case of any deficiency, the unsupplied area receives fibres from the great occipital. It sometimes passes backward instead of upward and pierces the trapezius near the upper border before reaching the scalp. 2. The great auricular nerve (n. auricularis magnus) (Fig. 1087) is the larg- est of the superficial set and arises, usually with the superficial cervical nerve, from the second and third, from the third alone, or from the third and fourth cervical nerves. Turning over the posterior margin of the sterno-mastoid it ascends toward the ear between the platysma and the deep fascia. Below the ear it gives off a few (a} facial twigs and then terminates by dividing into (£) auricular and (r) wastoid branches. a. The facial twigs pass through the parotid gland and over the angle of the mandible, supplying the integument over the parotid gland and masseter muscle and communicating with the cervico-facial division of the seventh cranial nerve. b. The auricular branches (r. anterior) supply mainly the cranial surface of the posterior part of the pinna. One filament passes through the cartilage by means of a cleft between the concha and the antihelix and supplies the outer surface, while a few twigs are distributed to the outer surface of the lobule. The auricular branches inosculate with the small occipital and pos- terior auricular nerves. c. The mastoid branch ( r. posterior) is distributed to the skin overlying the mastoid process and the upper part of the sterno-mastoid muscle. It inosculates as does the auricular branch. Variation. — The mastoid branch may arise independently from the plexus and pass upward to its destination between the small occipital and great auricular nerves. THE CERVICAL PLEXUS. 1287 3. The superficial cervical nerve (n. cutaneus colli) usually arises in com- mon with the great auricular from the second and third, the third only, or from the third and fourth cervical nerves (Fig. 1087). From the posterior margin of the sterno- mastoid it passes almost directly forward over the middle of that muscle and under FIG. 1087. Supraorbital nerve Supratrochlear nerve Temporal branch of facial Occipital branch of great auricular Posterior auricular nerve Small occipital nerve Branch of communication with facial Cutaneous branch of III. cervical- Great auricular nerve Communication between cervical nerves and spinal accessory Supraacromial branch Supraclavicular branch Infraorbital branch of facial Buccal branch of facial Communication with buccal branch of mandibular Supramandibular branch of facial Inframandibular branch of facial Superficial cervical nerve 'Superficial descending branch iprasternal branch Dissection showing superficial branches of cervical plexus, as well as parts of trigeminal, facial, spinal accessory and great occipital nerves ; ear has been drawn forward. the platysma myoides and the external jugular vein. It perforates the deep cervical fascia near the anterior border of the sterno-mastoid and divides into (a) an upper and (^) a lower set of branches. a. The upper branches (rr. superiores) form an extensive inosculation with the inframandib- ular branch of the facial nerve, after which they pierce the platysma and supply the integument of the neck as far forward as the median line and as far up as the inferior margin of the mandible. b. The lower branches ( rr. inferiors ) after piercing the platysma are distributed to the skin of the lower part of the neck to the mid-line as far down as the sternum. 1288 HUMAN ANATOMY. Variation. — The superficial cervical, instead of a single nerve, may arise as two or more filaments from the cervical plexus. The descending branches (nn. supraclaviculares) (Fig. 1089) arise from the third and fourth cervical nerves and pass downward in the anterior margin of the occipital triangle along the posterior edge of the sterno-mastoid. On nearing the clavicle they break up into three distinct sets : (4) the suprasternal , (5) the supra- clavicular and (6) the supraacromial. FIG. 1088. Third occipital nerve Great occipital nerve Branch from III. cervical, dorsal division Branches from IV. cer- J vical, dorsal division J Inosculation between facial nerve and small occipital and great auricular nerves Sterno-cleido-mastoid muscle Great auricular nerve Small occipital nerve „ Superficial cervical nerve Superficial descending branch of cervical plexus; the leader crosses the suprasternal branch Spinal accessory nerve Muscular branch to trapezius raclavicular branches Supraacromial branches Dissection showing superficial branches of cervical plexus and posterior cutaneous branches. 4. The suprasternal branches (rr. supraclaviculares anteriores) are the smallest. They pass over the lower end of the sterno-mastoid and the inner end of the clavicle and supply the skin of the chest as far down as the angulus Ludovici. One or two filaments terminate in the sterno-clavicular articulation. 5. The supraclavicular branches ( rr. supraclaviculares medii) pass across the middle of the clavicle and supply the integument of the chest as far down as the third or fourth rib, inosculating with twigs from the anterior cutaneous branches of the upper thoracic nerves. Variation. — A twig may perforate the clavicle. THE CERVICAL PLEXUS. 1289 6. The supraacromial branches (rr. supraclaviculares posteriores) cross the cla- vicular insertion of the trapezius and are distributed to the skin over the anterior, external and posterior aspects of the shoulder as far down as the lower portion of the deltoid. II. The deep branches are divided into two sets, an external and an internal. Both arising- beneath the sterno-mastoid, the former pass away from and the latter toward the median line of the neck. 7. The external muscular branches are distributed as follows: — a. The sterno-mastoid receives a branch from the second cervical which enters the deep surface of the muscle and interlaces with a branch of the spinal accessory nerve to form the sterno-mastoid plexus. f FIG. 1089. - \ / V Muscular brs. to compfexus biventer from occip. m Third occipital nerve -iVK'' Fascial septum from ligamentum nuchae ™™ Great occipital nerve Rectus capitis posticus major Branch to obliquus inferior Spine of II. cervical vertebra Cutaneous br. from III. cervical Part of complexus and biventer Third occipital nerve Branch to complexus from II. cervical Branch to complexus from III. cervical Internal br. dorsal division of VI. cervical nerve VII. cervical, dorsal di VIII. cervical, dorsal division Internal br. of post. div. of V. cervical nerve Spinous process of VII. cervical vertebra yg^fcjy Ant. division I. cervi- dorsal division passing II. cervical nerve, . orsal division backward Levator anguli scapulae Branch to trachelo-mastoid III. cervical nerve, dorsal division Communication between II. and III. dorsal division External brs. of III. cervical, dorsal division IV. cervical rerve, dorsal division Ext. branch of dorsal division V. cervical nerve V. cervical nerve, dorsal division Ext. brs. dorsal division VI. cervical nerve VI. cervical nerve, dorsal division Transverse process I. thoracic vertebra Transverse process II. thoracic vertebra Levator anguli scapulas Trapezius Dissection of right side of neck, showing deeper relations of cervical nerves. b. The trapezius receives fibres from the third and fourth cervical nerves which arise with and accompany the descending branches of the superficial set through the occipital triangle. They dip under the anterior margin of the trapezius, before and after which they form a more or less complex inosculation with the spinal accessory, called the subtrapezial plexus, from which filaments are distributed to the trapezius muscle (Fig. 1088). c. The levator anguli scapulae receives two branches which take their origin from the third and fourth nerves. d. The scalenus medius and ( e ) scalenus posticus also receive fibres from the third and fourth. 8. The communicating branches form points of contact and union with 'the spinal accessory nerve (a) under the sterno-mastoid and (d~) in the occipital triangle and under the trapezius. By means of these inosculations are formed the sterno-mastoid and subtrapezial plexuses. I2QO HUMAN ANATOMY. 9. The muscular branches are distributed to (a) certain prevertebral muscles and to (6) the genio-hyoid and the infrahyoid muscles. a. The rectus capitis anticus major and minor and the rectus capitis lateralis are supplied by a filament arising from the loop between the first and second cervical nerves. The intertrans- versales, the longus colli and a portion of the rectus capitis anticus major receive their supply from the second, third and fourth, and the upper part of the scalenus anticus receives a twig from the fourth cervical nerve. b. The genio-hyoid and the four muscles of the infrahyoid group are innervated by the cervical plexus in a rather roundabout manner. From the first and second cervical nerves are given off one or more branches which join the hypoglossal nerve shortly after its appearance in the neck. These fibres for a time form an integral portion of the hypoglossal and eventually escape from it as the nerve to the genio-hyoid, the nerve to the thyro-hyoid and the n. descen- dens hypoglossi (Fig. 1082). The last-mentioned nerve leaves the hypoglossal at the point where the latter crosses the internal carotid artery and then descends in the anterior cervical triangle. In front of, or sometimes within, the carotid sheath it forms a loop of communication, called the hypoglossal loop or ansa cervicalis (ansa hypoglossi) by inosculation with the descending cervical nerve (n. descendens cervicalis) (Fig. 1082). This descending cervical nerve is derived from the second and third cervical nerves and at first consists of two twigs which later unite in front of the internal jugular vein. From this point it passes downward and inward as a single trunk to reach its point of entrance into the ansa hypoglossi. The ansa may be either a simple loop or a plexus and is situated anterior to the carotid sheath at a variable point in the neck. From it branches are given off to the sterno-hyoid, the sterno-thyroid and the posterior belly of the omo-hyoid (Fig. 1076). 10. The phrenic nerve (n. phrenicus), although an internal muscular branch of the cervical plexus, is of such importance as to merit a separate description. Whilst mainly the motor nerve to the diaphragm, it contains some sensory fibres ; in this connection it may be pointed out that the phrenic is not the only motor nerve to the diaphragm, the lower thoracic nerves aiding in its innervation. The phrenic arises mainly from the fourth cervical nerve but receives additional fibres from the third and fifth (Fig. 1090). It passes down the neck on the scalenus anticus, which it crosses from without inward, and at the base of the neck accompanies that muscle between the subclavian artery and vein. At the entrance to the thorax it passes over the root of the internal mammary artery from without inward and backward, occupying a position behind the sterno-clavicular articulation and the point of junc- tion of the subclavian and internal jugular veins. It then follows a course almost vertically downward, over the apex of the pleura and through the superior and middle mediastina, to the upper surface of the diaphragm. The right phrenic (Fig. 1090) is shorter than the left on account of its more direct downward course and the greater elevation of the diaphragm on that side. It crosses the second part of the subclavian artery and accompanies the right innominate vein and the superior vena cava on their lateral aspect. It then passes in front of the root of the lung and finishes its course by de- scending between the lateral aspect of the pericardium and the mediastinal pleura. Nearing the diaphragm it breaks up at the antero-lateral aspect of the quadrate foramen into its terminal branches, a few of which enter the abdomen through this opening. The left phrenic (Fig. 1090), having to wind around the left side of the heart and reach the more inferior half of the diaphragm, is longer than its fellow, about one-seventh longer (Luschka). Entering the thorax between the subclavian artery and the left innominate vein it crosses the anterior face of the left vagus nerve and continues its downward course by passing over the left side of the aortic arch. Reaching the middle mediastinum it courses in front of the root of the lung, behind the lower left angle of the pericardium, and descends to the diaphragm between the pericardium and the mediastinal pleura. It breaks up into its terminal branches before arriving at the thoracic surface of the diaphragm, which it enters at a point further from the median line and more anterior than does the ri^lit. Branches of the phrenic nerve are : (a) the pleura/, {b} the pcricardiac and f the terminal. THE CERVICAL PLEXUS. 1291 a. The pleural branches, two in number, are almost microscopic in size, and are given off as the nerve crosses the apex of the pleura. One supplies the costal pleura and the other, which sometimes accompanies the internal mammary artery, is distributed to the medias- tinal pleura. b. The pericardiac branch (r. pericardiacus) is a tiny filament which is usually given off opposite the lower margin of the third costal cartilage. It is sometimes absent on the left side. c. The teiminal branches arise under cover of the pleura and differ to some extent on the two sides. The right phrenic divides antero-lateral to the opening for the inferior vena cava into (aa) an anterior and (bb) a posterior branch. aa. The anterior branch breaks up under the pleura into five or six fine twigs, which spread out antero-laterally in the sternal portion and the anterior part of the right costal portion of the FIG. 1090. Scalenus medius muscle Vagus nerv V. cervical nerve Scalenus atiticus muscle Upper trunk of brachial plexus VII. cervical nerve Superior intercostal artery VIII. cervical nerve - I. thoracic nerve Clavicle Phrenic nerve Internal mam- mary artery Innominate veins " Vena cava superior - I.ung, mesial surface Pericardium Sterno-cleido-mastoid .Vagus nerve Internal jugular vein Subclavian artery Omo-hyoid muscle Subclavian vein Clavicle Subclavius muscle I. rib " Manubrium sterni — Vagus nerve Diaphragm, up- per surface VII. rib Dissection showing phrenic nerves; parts of sternum and ribs have been removed ; lungs are pulled aside; pericardium is undisturbed. diaphragmatic musculature. Tiny filaments traverse the interval between the sternal and costal portions and enter the abdomen, where they are distributed to the peritoneal covering of the diaphragm and to the falciform ligament of the liver in the direction of the umbilicus. bb. The posterior branch pierces the central tendon at the outer margin of the quadrate opening and divides into a muscular branch and the right phrenic o-abdominal branch (r. phrenico- abdominalis dexter). The former supplies the lumbar portion of the musculature of the diaphragm. The latter traverses the quadrate foramen and first gives off a recurrent branch which accompanies the inferior vena cava back to the right auricle. After giving off this branch, under cover of the peritoneum some of its fibres enter the diaphragmatic ganglion and others unite with filaments from the coeliac plexus to form at the inferior surface of the diaghragm the diaphragmatic plexus, which is joined by twigs from the diaphragmatic ganglion. From this plexus fibres are distributed to the coronary ligament and peritoneum of the liver and to the right supra- renal body. The left phrenic pursues a general antero-lateral course and pierces the diaphragm at the junction between the musculature and the central tendon. Under cover of the peritoneum it splits up into an anterior, a lateral and a posterioi branch. The anterior branch supplies the muscle of the left sternal portion and the antero-late^al part of the left costal portion. The 1292 HUMAN ANATOMY. lateral branch supplies the corresponding part of the left costal portion. The posterior branch (r. phrenicoabdominalis sinister) is distributed to the left lumbar portion of the muscle of the diaphragm and usually either a filament passes to the left semilunar ganglion or several small threads to the cceliac plexus, one of which can be traced to the left suprarenal body. The phrenic nerve communicates in the lower part of the neck with the middle or inferior cervical ganglion of the sympathetic. At the inferior aspect of the diaphragm it communicates, on the right side, with the diaphragmatic plexus of the sympathetic and, on the left side, with the semilunar ganglion or the cceliac plexus. Variations. — The phrenic may receive additional roots from the nerve to the subclavius, the nerve to the sterno-hyoid, the second or the sixth cervical nerve, the n. descendens cervi- calis or the ansa hypoglossi. It may arise exclusively from the nerve to the subclavius or, aris- ing normally, may give a branch to that muscle. It sometimes passes along the lateral border of or pierces the scalenus anticus muscle. Instead of descending behind the subclavian vein it may pass anterior to it or even through a foramen in it. The accessory phrenic nerve arises either from the fifth alone or from the fifth and sixth cervical nerves and, entering the thorax either anterior or posterior to the subclavian vein, joins the phrenic at the base of the neck or in the thorax. II. The communicating branches of the internal set effect unions with (a) the sympathetic, (d) the vagus and (c) the hypoglossal. a. The superior cervical ganglion of the sympathetic or the association cord connecting the superior and middle ganglia sends gray rami communicates to the first, second, third and fourth cervical nerves. b. The ganglion of the trunk of the vagus is sometimes connected by means of a tiny nerve with the loop between the first and second cervical nerves. c. The hypoglossal nerve receives, just below the anterior condyloid foramen, a good sized branch from the loop between the first and second cervical nerves. This communication furnishes sensory fibres to the hypoglossal nerve which subsequently leaves the latter as its men- ingeal branch ; other spinal fibres leave the twelfth as the n. descendens hypoglossi and as the nerves to the genio-hyoid and thyro-hyoid muscles. Practical Considerations. — Of the motor nerves of the cervical plexus the phrenic is most commonly the seat of trouble and this may result in or be associated with spasm or paralysis of the diaphragm. The involvement of the diaphragm may be part of a progressive muscular paralysis, as from lead poisoning, or from injuries or diseases of the spine. The nerve may be compressed by tumors or abscesses of the neck, or be injured in wounds of the neck. It passes downward under the sterno- mastoid muscle and on the scalenus anticus, from about the level of the hyoid bone. It is covered and somewhat fixed by the layer of deep fascia covering the scalenus anticus muscle. The clonic variety of spasm, singultus or hiccough, is very common, and is occasionally though rarely dangerous by preventing rest and sleep ; it may complicate apoplexy, peritonitis or chronic gastric catarrh. If only one phrenic is paralyzed the disturbance of function is slight and not easily recognized. In a bilateral paralysis, as from alcoholic neuritis, respiration depends almost entirely on the intercostal muscles, since the diaphragm is completely paralyzed. Dyspnoea, therefore, occurs on slight exertion. The epigastrium is depressed rather than prominent and the lower border of the liver is drawn upward. The superficial branches of the cervical plexus emerge together through the deep fascia near the middle of the posterior border of the sterno-mastoid muscle, and from this point pass in various directions. The auricularis magnus passes upward and forward over the sterno-mastoid to the ear and parotid gland, the occipitalis minor along the posterior margin of the same muscle to the scalp, and the superficial cervical branch obliquely forward and upward to the submaxillary region. The descending branches are three in number and pass respectively in the direction of the sternum, clavicle and acromion. They give rise to little or no disturbance when wounded. THE BRACHIAL PLEXUS. The brachial plexus (plexus hrachialis) is a somewhat intricate interlacement of the anterior primary divisions of usually the lower four cervical and first thoracic nerves. To these are sometimes added a branch from the fourth cervical, a branch from the second thoracic, or branches from both of these nerves. The fasciculi form- THE BRACHIAL PLEXUS. 1293 ing this plexus emerge in the interval between the scalenus anticus and medius and from the side of the neck pass beneath the clavicle and into the axilla through its apex. The plexus is divided, therefore, into two portions, a cervical or supradavicular part (pars supraclavicularis) and an axillary or infradavicular part (pars infraclavicularis). In the posterior cervical triangle the plexus lies first above and then to the outer side of the subclavian artery and vein, is crossed by the posterior belly of the omo-hyoid muscle and is frequently threaded by the transverse cervical or the posterior scapu- lar artery. After entering the axilla its component parts, while lying mainly to the outer side, form a close fasces around the axillary artery, whose sheath they occupy. In the upper part of the axilla the plexus is overlain by the subclavius and pectoralis major muscles and before dividing into its terminal branches it lies enclosed between the pectoralis minor and subscapularis muscles. Constitution and Plan. — In the various weavings of the component elements of the plexus five stages can be recognized : (a) anterior primary divisions of the spinal nerves, (£) trunks, (c) divisions, (#) cords and (riir/iirifn/e to the coraco-brachialis, which commonly has an independent origin, is usually double, one filament going to each portion of the muscle. The nerves to the biceps and brachialis anticus are given off while the musculo-cutaneous is in transit between those muscles. b. The humeral branch accompanies the nutrient branch of the brachial artery into the humerus. c. The articular branch aids in the supply of the elbow joint. d. The terminal part (n. cutaneus antebrachii lateralis) (Fig. 1103) of the musculo-cutaneous divides into two branches, (ad) an anterior and (bb) a posterior. aa. The anterior branch descends in the antero-lateral portion of the superficial fascia of the forearm (Fig. 1104). It inosculates above the wrist with the radial nerve and supplies the in- tegument of the antero-lateral part of the forearm. It also distributes fibres to the skin over the thenar eminence, to the wrist joint and to the radial artery. bb. The posterior branch passes downward and backward and supplies the skin of the postero-lateral portion of the forearm down to or slightly beyond the wrist joint (Fig. 1102 ). It inosculates with the radial nerve and with the inferior external cutaneous branch of the musculo- spiral. Variations.— Instead of piercing the coraco-brachialis the nerve may adhere to the median or its outer head for some distance down the arm, and then either as a single trunk or as several branches pass between the biceps and brachialis anticus muscles. Sometimes only a part of the nerve follows this course, joining the main trunk after the latter's transit througn the muscle. The muscular part only or the cutaneous part only may pierce the muscle. The nerve may be accompanied through the muscle by fibres of the median which rejoin the latter below the coraco-brachialis. The nerve may remain independent and fail to pierce the coraco-brachialis, either passing behind it or between it and the associated head of the biceps. It may perforati not only the coraco-brachialis but also the brachialis anticus or the short head of the biceps Rarely the entire outer cord, after giving off the external anterior thoracic, may traverse th coraco-brachialis. Anomalies in distribution include a branch to the pronator radii teres, th supply of the skin of the dorsum of the hand over and adjacent to the first metacarpal bon a branch to the dorsum of the thumb in the absence of the radial nerve and the giving off dorsal digital nerves to both sides of the ring finger and the adjacent side of the little finger. 8. THE MEDIAN NERVE. The median nerve (n. medianus) (Fig. 1098) consists of fibres which can traced to the sixth, seventh and eighth cervical and first thoracic nerves. It aris by two heads, an outer and an inner, which are derived respectively from the out and inner cords of the plexus, the former containing fibres from the sixth an seventh cervical and the latter fibres from the eighth cervical and first thoraci nerves. The two heads, the inner of which usually crosses the main artery the upper extremity at about the point where the axillary becomes brachi unite either in front of or to the outer side of the artery. From the point of fusio of the two heads the nerve passes down the arm in close relation with the brachia artery, usually lying lateral or antero-lateral to the artery in the upper part of the arm, and as the elbow is neared, gradually attaining the inner side by crossing obliquely the anterior surface of the artery (Fig. 1098). It passes through the cubital fossa beneath the median-basilic vein and the bicipital fascia, and enters the forearm between the heads of the pronator radii teres muscle, the deep head of THE BRACHIAL PLEXUS. 1299 which separates the nerve from the ulnar artery. It follows a straight course down the forearm, accompanied by the median artery, lying upon the flexor profundus FIG. 1095. Deltoid Sup. ext. cutaneous br. musculo-spiral Inf. ext. cutaneous br. musculo-spiral Musculo-cutaneous nerve, ant. and post, b: Musculo-spiral nerve Posterior interosseous nerve. Radial ner Supinator brev Pronator radii ter Extensor carpi rad. longic Extensor carpi rad. brevii Radial arte Brachio-radial Flexor subl. digitonu radial head" Flexor carpi radialis- Mediati nerve- Palmar cutaneous br. of median- Abductor pollicis Digital brs. of median nerve - Median nerve - Brachial artery Edge of triceps -Ulnar nerve -Inferior profunda artery ^H — Brachialis anticus Biceps tendon Pronator radii teres, humeral head Articular branches of median nerve • Flexor carpi radialis • Flexor sublimis digitorum • Ulnar nerve Ulnar artery • Flexor profundus digitorum Flexor carpi ulnaris Palmar cutaneous br. of ulnar Dorsal cutaneous br. of ulnar nerve • Flexor sublimis digitorum -Pisiform bone •Deep br. of ulnar nerve •Palmaris brevis, reflected -Abductor minimi digiti Flexor brevis minimi digiti Digital brs. of ulnar nerve Dissection of right upper extremity, showing nerves of anterior surface; anterior annular ligament has been cut away to show median nerve and flexor tendons. digitorum and covered by the flexor sublimis digitorum. Near the wrist the median becomes more superficial, with the tendons of the flexor sublimis digitorum 1300 HUMAN ANATOMY. and palmaris longus lying mesial and that of the flexor carpi radialis lateral to it (Fig. 1095). It passes into the hand beneath the anterior annular ligament, at the lower margin of which it spreads out into a reddish gangliform swelling, which lies upon the flexor tendons. Below this point it breaks up into its terminal branches. Branches. — The median, as is the case with the ulnar, gives off no branches in the arm. In the forearm the branches are : (a} the articular, (3) the muscular, (c) the anterior interosseous and (d) the palmar cutaneous, and in the hand : (e) the muscular and (_/) the digital. a. The articular branch consists of one or two tiny twigs which supply the anterior portion of the elbow joint. FIG. 1096. Brachial artery Musculo-spiral nerve Cephalic vein Posterior interosseous nerve Brachio-radialis muscle Radial nerve Radial recurrent artery Communications between deep and superficial veins Cutaneous branch of musculo- cutaneous nerve Radial vein Radial artery Median nerve ichial vein Tendon of biceps . .^Internal cutaneous nerve Bicipital fascia •Median nerve Pronator radii teres Superficial dissection of right arm, showing relations of nerves to blood-vessels on front of elbow. b. The muscular branches (rr. musculares) (Fig. 1095) consist of a fasces of nerve-bundles which arise from the median a short distance below the elbow. They are distributed to the pronator radii teres, the flexor carpi radialis, the palmaris longus and that portion of the flexor sublimis digitorum which arises from the inner condyle and from the ulna. Two additional filaments from the median supply the flexor sublimis, one entering the radial head and the other that portion which flexes the index finger. c. The anterior interosseous nerve (n. interosseus antebrachii volaris) (Fig. 1098) arises from the posterior aspect of the median a short distance below the elbow. It passes down the forearm, accompanied by the anterior interosseous artery, on the anterior surface of the interosseous membrane between the flexor longus pollicis and the flexor profundtis digitorum. At the upper margin of the pronator quadratus muscle it dips under that muscle and continues down for sonic distance, finally entering the deep surface of the pronator quadratus. THE BRACHIAL PLEXUS. 1301 It supplies the flexor longus pollicis, the radial half of the flexor profundus digitorum and the pronator quadratus. It distributes filaments to the interosseous membrane, the anterior interosseous vessels, the shafts of the radius and ulna (the twigs to these bones entering them with the nutrient arteries) , the periosteum of the radius and ulna and the radio-carpal articulation. d. The palmar cutaneous branch (r. cutaneus palmaris) (Fig. 1097) leaves the median at a varying distance above the wrist. It becomes superficial near the upper margin of the anterior annular ligament by piercing the deep fascia between the flexor carpi radialis and the palmaris longus. It supplies the skin of the palm and inosculates with the palmar cutaneous branch of the ulnar and with filaments of the radial and musculo-cutaneous nerves. e. The muscular branch in the hand (r. muscularis) (Fig. 1097) is a short nerve which arises below the anterior annular ligament and curves outward toward the base of the thumb. It breaks up into filaments which supply the abductor pollicis, the opponens pollicis and the superficial head of the flexor brevis pollicis. f. The digital branches (Fig. 1097) are five in number and, with the exception of the twigs supplying the two outer lumbricales, are purely sensory. They arise from the median, a short distance below the anterior annular ligament of the wrist ( nn. digitales volares communes) and pass distally beneath the superficial palmar arch and over the flexor tendons. As they approach the interdigital clefts they pass between the primary divisions of the median portion of the palmar fascia and become more superficial as they continue along the borders of the fingers (nn. digitales volares proprii). The first lies along the radial side of the thumb and inosculates around its radial aspect with the radial nerve. The second occupies the ulnar side of the thumb. The third gives off a branch to the first lumbricalis and supplies the radial side of the index finger. The fourth supplies the second lumbricalis and then divides into two branches which are distributed to the adjacent sides of the index and middle fingers. The fifth, after being connected with the ulnar nerve by a stout filament (r. anastomot- icus cum n. ulnare), divides for the supply of the adjoining aspects of the middle and ring fingers. In the fingers these nerves lie anterior to the vessels and in their course toward the tip of the finger they give off anterior and posterior branches, the latter supplying the skin over the middle and distal phalanges of the index, middle and ring fingers and over the distal phalanx of the thumb. Twigs are supplied to the interphalangeal articulations and near the end of the finger each of the five breaks up into two terminal branches, one of which is destined for the sensitive skin over the anterior portion of the distal phalanx and the other for the matrix of the nail. Variations. — Some of these are described on page 1298. The fibres usually contributed to the median nerve by the first thoracic may be wanting. Either the outer or the inner head may consist of two nerve-bundles. The point at which the heads unite is a very variable one and has been found as far down as the elbow. The heads may enclose the axillary vein instead of the artery. In those instances, many of which have been found in the anatomical rooms of the University of Pennsylvania, in which a single large branch of the axillary artery gives off the two circumflex arteries, the subscapular and the two profunda arteries, this trunk, instead of the axillary artery, is embraced by the heads of the median nerve. The inner head, the outer head or the median itself may pass behind the axillary artery instead of in front. The outer head has been seen to arise in the middle of the arm and pass behind the artery to join the inner head. One instance has been reported in which the median entered the forearm over the pronator radii teres instead of between the heads of that muscle. It has been seen lying on the superficial surface of the flexor sublimis digitorum. The median may be cleft for a short distance in the forearm, giving passage to the ulnar artery or one of its branches, to the superficial long head of the flexor longus pollicis or to an extra palmaris longus muscle. A communication in the arm between the median and ulnar nerves has been noted in one instance. A similar connection in the forearm, occurring in numerous ways, is found in from 20-25 per cent, of cases examined. A connection with the ulnar in the hand may pass either from the ulnar to the median or from the median to the ulnar. The anterior interosseous has been seen to receive a filament from the musculo-spiral through the interosseous membrane, and inosculation between the two interosseous nerves has been noted at the lower part of the forearm ; according to Rauber, this is the normal arrangement. One case has been described in which the abductor indicis was supplied by the median. During the exchange of position between the digital branches of the median nerve and the digital arteries the former are often pierced by the latter. The fifth digital branch may arise in the forearm and enter the hand independently. Practical Considerations. — A pure paralysis of the median nerve is rare, and is almost always traumatic in origin. The paralysis is more commonly a part of a more extended involvement of the brachial plexus. When this nerve is paralyzed above there is inability to pronate the forearm or flex the wrist properly, since the 1302 HUMAN ANATOMY. pronators and all the flexors except the flexor carpi ulnaris and the ulnar half of the flexor profundus digitorum are supplied by it. The second phalanges of the middle and index fingers cannot be flexed, although the first phalanges can be flexed and the second and third extended in all the fingers through the interossei muscles ; flexion of the third phalanges of the little and ring fingers can be accomplished by the ulnar half of the flexor profundus, which is supplied by the ulnar nerve. The FIG. 1097. Brachio-radialis tendon Branch of radial nerve Palmar cutaneous br. of median nerve Median nerve Flexor carpi radialis tendon Abductor pollicis Opponens pollicis Abductor pollicis Digital brs. of median nerve Adductor transversus pollicis Flexor sublimis digitorum Flexor carpi ulnaris Palmar cutaneous br. of ulnar nerve, lying upon ulnar artery Ulnar nerve Pisiform bone Deep branch of ulnar nerve Abductor minimi digiti Palmaris brevis, reflected Digital brs. of ulnar nerve pponens minimi digiti .Flex, brevis minimi digiti Superficial dissection of right palm, showing branches of median and ulnar nerves; part of anterior annular ligament has been removed to expose median nerve. thumb cannot be flexed or abducted, although it may be adducted. One of the most characteristic features of the hand is lost — that is, the ability to appose the thumb to any one of the fingers, as in picking up small objects. In wounds of the axilla the median is the nerve most frequently injured, the musculo-spiral least frequently, as the median lies more superficially and the musculo- spirul behind the vessels. In the arm the median can be easily found to the inner side of the biceps and coraco-brachialis muscles, where it lies on the brachial vessels. At the elbow it is found to the inner side of the brachial artery, the guide to which is the biceps tendon which in turn lies just to the outer side of the artery. At about the middle of the wrist the nerve lies under the palmaris longus tendon. THE BRACHIAL PLEXUS. 1303 9. THE INTERNAL ANTERIOR THORACIC NERVE. The internal anterior thoracic nerve (n. thoracalis anterior medialis) (Fig. 1093) arises from the inner cord and consists of fibres derived from the eighth cervical and first thoracic nerves. It passes forward between the axillary artery and vein and, after giving off a branch which forms a loop with a similar branch from the external anterior thoracic, pierces the pectoralis minor, in which some of its fibres terminate. The remainder enter the deep surface of the pectoralis major to supply the lower part of the sternal portion of that muscle. Variations. — The fibres which supply the pectoralis major may wind around the lower border of the pectoralis minor. Filaments from both of the anterior thoracic nerves may supply the integument of the axillary and mammary regions. 10. THE LESSER INTERNAL CUTANEOUS NERVE. The lesser internal cutaneous nerve (n. cutaneus brachii medialis) (Fig. 1093), also called the nerve of Wrisberg, can be traced to the first thoracic nerve. It arises from the inner cord usually in common with the internal cutaneous. After leaving its point of origin, it descends in the arm along the inner side of the axillary and basilic veins, pierces the deep fascia about the middle of the arm and supplies the integument of the inner aspect of the upper extremity as far down as the elbow. At a variable point it forms a loop with the intercosto-humeral nerve. Variations. — The lesser internal cutaneous nerve may be absent. It may receive fibres from the eighth cervical or the second thoracic nerve. There may be present a communication between the lesser internal cutaneous nerve and the lateral cutaneous branch of the third tho- racic. The inosculation with the intercosto-humeral may be either simple or plexiform and either nerve may be deficient, the other usually recompensing for the deficiency. • ii. THE INTERNAL CUTANEOUS NERVE. The internal cutaneous nerve (n. cutaneus antebrachii medialis) (Fig. 1094) comprises fibres from the eighth cervical and first thoracic nerves. It has its origin from the inner cord of the plexus usually as a common trunk with the lesser internal cutaneous nerve. After distributing some small filaments to the integument of the upper arm below the axilla, it runs down the arm between the brachial artery and the basilic vein and at about the middle of the upper arm breaks up into its terminal branches, (a) the anterior and (£) the posterior. a. The anterior branch (r. volaris) passes over, sometimes under, the median-basilic vein and supplies the skin of the ulnar half of the forearm as far down as the wrist (Fig. 1104). It inosculates with the superficial branch of the ulnar nerve. b. The posterior branch (r. ulnaris) turns obliquely around the inner side of the upper part of the forearm and supplies the integument as far around as the ulna down to the lower third or fourth of the forearm. It unites above the elbow with the lesser internal cutaneous nerve and in the forearm with the anterior branch of the internal cutaneous and sometimes with the dorsal ramus of the ulnar. 12. THE ULNAR NERVE. The ulnar nerve (n. ulnaris) (Fig. 1092) is the largest branch _ of the inner cord. Its fibres can be traced to the eighth cervical and first thoracic nerves and sometimes, by a root from the outer cord, to the seventh cervical. Arising from the inner cord between the axillary artery and vein and posterior to the internal cutaneous nerve it pursues a downward course in front of the triceps and to the inner side of the axillary and brachial arteries. Reaching the middle of the arm it follows an inward and backward direction, in which it is accompanied by the inferior profunda artery, and passing either over the inner margin of or through the internal intermuscular septum and in front of the inner head of the triceps, attains the interval between the internal condyle of the humerus and the olecranon (Fig. 1098). It becomes an occupant of the forearm by passing between the heads of the flexor carpi ulnaris muscle, a situation the nerve shares with the inferior profunda and posterior 1304 HUMAN ANATOMY. ulnar recurrent arteries. From this point the nerve follows a straight course to the wrist, lying in the forearm upon the flexor profundus digitorum and covered by the FIG. 1098. • Pectoralis minor Musculo-cutaneous nerve - Outer head of median nerve- Inner head of median nerve- Long head of biceps- Short head of biceps, everted- Coraco-brachialis- L -Internal cutaneous nerve -Ulnar nerve Musculo-cutaneous nerve • -Internal cutaneous branch of musculo-spiral nerve Branchialis anticus- -Inferior profunda artery Musculo-spiral nerve- Cutaneous branches of musculo-cutaneous » Brachio-radialis- Posterior interosseous nerve - Biceps tendon - Radial artery - Supinator brevis- Ext. carpi radialis longior- Ext. carpi radialis brevior- Radial nerve- Pronator radii teres, cut- Flexor longus pollicis- .Muscular branch of musculo-cutaneous -Superficial flexors, origin -Articular branches of median nerve -Articular branches of ulnar nerve -Flexor carpi ulnaris -Ulnar artery -Anterior interosseous nerve -Anterior interosseous artery i -Ulnar nerve -Flexor profundus digitorum, cut .Palmar cutaneous branch .Pronator quadratus, cut -Dorsal branch of ulnar nerve Palmar cutaneous branch of median Opponens pollicis Deep branch of ulnar nerve \bd\H-tnr minimi digiti Opponens minimi digiti Flexor brevis minimi digit! 3rd and 4th flexor tendons with 3rd and 4th lumbricales, turned forward Dissection of right upper extremity, showing iln )>i-i branches of nerves of anterior surface. flexor carpi ulnaris. At about the middle of its course through the lower arm it approximates the ulnar vessels, close to the inner side of which it lies. At the THE BRACHIAL PLEXUS. 1305 wrist, accompanied by the ulnar artery, it pierces the deep fascia just above the annular ligament, to the outer side of the pisiform bone, and enters the hand by passing superficial to the anterior annular ligament (Fig. 1097). After crossing the ligament it divides into its terminal branches, the siiperficial and the deep. Branches. — None are given off in the arm. In the forearm they are : (a) the articular, (£) the muscular, (V) the cutaneous and (d) the dorsal branch to the hand. The terminal branches in the hand are : ( the other cutaneous branches of the musculo-spiral and to that part of the posterior aspect the forearm between the portions supplied by the posterior branch of the internal cutaneoi and the posterior branch of the musculo-cutaneous. In the neighborhood of the wrist it inos culates with the musculo-cutaneous and sometimes with the branch to the dorsum of the hanc from the ulnar. b. The muscular branches (rr. musculare«0 are given off (aa) before the musculo-spir enters the musculo-spiral groove and (f>t>) after leaving the groove. aa. Before entering the groove branches arise for the supply of the three heads of the triceps and the anconeus. THE BRACHIAL PLEXUS. 1311 Int. cutaneous branch of mus- culo-spiral nerve Lesser int. cu- taneous nerve Inf. ext. cutaneous branch of musculo- spiral nerve Int. cutaneous nerve, post. branch Post, cutaneous br. of musculo- cutaneous nerve The branch for the long head of the triceps, before its entrance into the muscle, breaks up into four or five filaments. The nerve supply of the inner head of the triceps is usually effected by two branches, an upper and a lower. The upper is short and enters the muscle soon after leaving the musculo- spiral. The lower, called the collateral ulnar branch, is longer and extends for a considerable distance along the inner surface of the triceps in close association with the ulnar nerve. Posterior to the internal intermuscular septum it enters its muscle. Tiny FIG. 1102. filaments accompany the collateral ulnar artery to the capsular ligament of the elbow. The nerves to the outer head of the triceps and to the anconeus take their origin as a single trunk. The former passes directly to the inner surface of the outer head, while the latter leaves the musculo-spiral groove and tra- verses the outer portion of the internal head of the triceps until the anconeus is reached. bb. After leaving the groove and while lying in the cleft between the brachialis anticus and the brachio- radialis, twigs are given off for the supply of the brachio-radialis, the extensor carpi radialis longior and the brachialis anticus. The nerve to the brachio-radialis enters the mesial surface of that muscle and usually supplies a filament to the capsule of the elbow. The nerve to the extensor carpi radialis longior may arise either from the posterior interosseous or directly from the musculo-spiral. The nerve to the brachialis anti- cus, while usually present, is not con- stant. It enters and supplies the lateral portion of that muscle. c. The humeral branches com- prise one which is supplied to the periosteum of the extensor surface of the humerus and one which enters the shaft of the humerus with the nutrient artery, when the latter arises as a branch of the superior profunda. d. The articular branches are des- tined for the elbow. They arise from the musculo-spiral as it lies between the brachialis anticus and the brachio- radialis, from the ulnar collateral nerve and from the nerve to the anconeus. e. The terminal branches of the musculo-spiral arises at about the level of the external condyle and in the fis- sure between the brachialis anticus and the brachio-radialis. They com- prise (aa) the posterior interosseous and (bb) the radial. Inf. ext. cut branch muscul o-sp: Dorsal branch of ulnar nerve From ulnar nerve From median Superficial dissection of right forearm, showing cutaneous nerves of posterior surface. aa. The posterior interosseous nerve (r. profundus n. radialis) ( Fig. noo) is the larger of the terminal branches and is mainly motor in function. Its fibres can be traced back to the sixth, seventh and sometimes the eighth cervical nerve. Shortly after its origin it approaches the supinator brevis, through a fissure in whose substance it makes its way to the lateral side of the radius, in this way reach- 1312 HUMAN ANATOMY. Supraacromial brs. cervical plexus ing the posterior aspect of the forearm. Here it takes a position between the two layers of the extensor muscles and rapidly decreases in size by giving off in quick succession branches to the neighboring muscles. As a much attenuated nerve it reaches the posterior surface of the interosseous membrane at the junction of the middle and lower thirds of the forearm. From the interval between the extensores longus and brevis pol- FIG. 1103. licis it courses along the membrane, cov- ered in turn by the ex- tensor longus pollicis, the extensor indicis and the tendons of the extensor longus digitorum, finally reaching the dorsum L^> of the wrist, where it presents a small gangliform swelling. In the lower fourth of its course it is some- ; times called the ex- t ern al interosseous nerve. B ran che s of Lesser internal the posterior interOS- cutaneous nerve seous nerve comprise two sets: those given off before and after traversing the supina- tor brevis. _ Cutaneous brs. circumflex nerve Sup. ext. cutaneous br. of musculo- spiral nerve Inf. ext. cutaneous I br. of musculo-'j spiral nerve I Musculo-cutaneous nerve, post, cutaneo branch Musculo-cutaneous nerve, ant. cutaneous brancb Musculo-cutaneous, post, cutaneous br. Internal cutaneous nerve Those arising be- fore the nerve enters the muscle comprise the nerves for the extensor carpi radialis bn'vior and the supittator brevis. The latter receives two filaments, which supply the two strata of muscle consequent upon the de- lamination of the supin- ator brevis by the pos- terior interosseous nerve. Quite frequently the nerve to the exten- sor carpi radialis long- ior arises from this por- tion of the posterior interosseous. The branches giv- en off after leaving the muscle include the sup ply of the extensor car- pi nlnaris, the extensor communis digitorum, the extensor minimi digiti, the th'ree extensors of the thumb and the extensor indicis. The first three of these muscles are supplied by a branch which leaves the posterior inter- osseous soon after its emergence from the supinator brevis. This nerve divides into two branches, one of which is distributed to the extensor carpi nlnaris and the other to the remain- ing two muscles. The extensor communis digitorum receives additional innervation from a twig which arises from the posterior interosseous further down the forearm. Superficial dissection of right arm, showing cutaneous nerves of anterior surface; cephalic vein is seen passing up to delto-pectoral interval ; basilic vein pierces deep fascia at lower inner aspect of ami. THE BRACHIAL PLEXUS. 1313 Inf. ext. cutaneous br. of musculo- spiral nerves Musculo-cuta- neous nerve, ant- cutaneous br. neous nerve, post cutaneous br Radial nerve Brs. of ant. br. of. musculo-cutatieons From radial nerve Digital brs. ot median nerve Lesser internal cutaneous nerve Internal cutaneous nerve Ant. br. internal cutaneous nerve The extensor ossis metacarpi pollicis and the extensor brevis pollids are innervated by a branch arising below the preceding, which breaks up into two decurrent twigs, one of which goes to each muscle. The extensor longus pollids is the recipient of a small filament, which arises from the posterior interosseous a short distance below the preceding nerve. The extensor indicis is sup- plied by the lowermost motor FIG. 1104. filament arising from the poste- rior interosseous. Terminal twigs are distrib- uted to the dorsal portion of the wrist joint, the intercarpal and carpo-metacarpal joints, the peri- osteum of the radius and ulna and the interosseous membrane. One of the filaments supplying the last-mentioned structure fre- quently inosculates with a branch from the anterior interosseous. The filaments to the carpus Muscuio-cuta- are continued through the meta- carpal spaces and are joined by twigs from the deep branch of the ulnar (page 1305). The joint nerves thus formed break up into two branches which accompany adjoining metacarpal bones to the metacarpo-phalangeal articu- lations. The branch to the first metacarpal space breaks up into seven branches (Rauber). Palmar cuta- neous br. of ul- nar nerve Palmar cutane- ous br. of me- dian nerve Digital brs. of ulnar nerve Digital brs. of median nerve bb. The radial nerve (r. superficialis n. radialis) (Fig. 1095) is smaller than the posterior interosseous and is purely sensory in its function. Its fibres originate from the sixth cervical nerve and sometimes from the fifth or seventh. From the end of the musculo-spiral it passes down the radial side of the forearm under cover of the brachio-radialis and anterior to the supinator brevis, the pronator radii teres and the radial head of the flexor sublimis digitorum. It accompanies, for the greater part of its course, the radial artery, to the radial side of which the nerve lies. At the junction of the middle and lower thirds of the forearm it begins to turn gradually backward over the radius and under the tendon of the brachio-radialis (Fig. 1095). Reaching the extensor surface of the forearm just above the wrist it divides into two diverging branches, which supply the back of the hand and the three outer digits (Fig. 1102). Branches. — The radial nerve divides into two terminal branches, an external and an internal. 83 Superficial dissection of right forearm and hand, showing cutaneous nerves of anterior and palmar surface. I3i4 HUMAN ANATOMY. The external or radial branch inosculates with the musculo-cutaneous nerve and dis- tributes filaments to the integument of the thenar eminence and the radial side of the thumb as far out as the base of the nail. The internal or ulnar branch splits into two parts. The inner of these likewise under- goes dichotomous division and supplies the dorsal aspect of the adjacent surfaces of the thumb and the index finger. The outer divides similarly to the inner and is distributed to the adjoining sides of the index and middle fingers. It gives off a branch which inosculates with the adjacent filament from the dorsal branch of the ulnar nerve, so that the contiguous surfaces of the middle and ring fingers are the recipients of fibres from both the radial and ulnar nerves. As the ulnar side of the hand is approximated the digital area of distribution of the radial nerve gradually recedes toward the wrist. On the thumb the radial extends as far out as the base of the nail, on the index finger as far as the middle of the second phalanx and on the middle finger only over the proximal portion of the first phalanx. The deficiency in these instances is supplied by twigs from the digital branches of the median nerve. Variations. — The musculo-spiral may accompany the circumflex nerve through the quad- rilateral space. It may communicate with the ulnar nerve in the upper arm. Cases are recorded in which the dorsal digital nerves to the little and the ulnar side of the ring finger were furnished by the musculo-spiral instead of by the ulnar and in which the inferior external cutaneous branch extended to the first phalanx of the ring finger and the second phalanx of the little finger. The radial nerve may supply the entire dorsum of the hand and the dorsal aspect of all the fingers, or it may be absent, the musculo-cutaneous going to the thumb and the ulnar to the remainder of the digits. The external division may send a branch to the palm. The posterior interosseous may pass over the surface of the supinator brevis and may furnish a branch to the anconeus muscle. Two instances are reported in which the posterior interosseous supplied the opposed surfaces of the middle and index fingers. Practical Considerations. — The musculo-spiral is more frequently paralyzed than any of the other branches of the brachial plexus. Its axillary portion often suffers from crutch pressure ; and the nerve is also particularly exposed to com- pression where it passes between the triceps muscle and the humerus, as when the arm, during sleep, is used for a pillow. It has been injured by violent contraction of the triceps muscle, as in the act of throwing. It is frequently lacerated by the fragments in fractures of the middle of the shaft of the humerus . When the lesion is in the axilla the triceps will be included in the paralysis. If the portion in the arm is affected the tri- ceps and anconeus will escape, but the following muscles will be paralyzed : the supina- tors, the extensors of the hand, the extensor communis digitorum, together with the extensor indicis, the extensor minimi digiti and the extensors of the thumb. The characteristic symptom is the inability to extend the hand at the wrist (wrist drop), and this is the most common form of musculo-spiral paralysis. THE THORACIC NERVES. The thoracic nerves (nn. thoracales) (Fig. 1105) consist of twelve pairs of sym- metrical nerve-cords, the upper eleven of which, because of their position in the intercostal spaces, are called intercostal nerves, and the twelfth, which lies below the twelfth rib and is an occupant of the abdominal wall, the subcostal. Since only seven ribs reach the sternum, the upper six thoracic nerves alone are continued throughout their entire course in intercostal spaces. The lower six, with the exception of the twelfth, after traversing their respective intercostal spaces proceed within the abdom- inal wall, through which they course to within a short distance of the median line. In accordance with the direction of the ribs, the upper nerves lie more horizontally than the lower, the latter becoming more and more oblique as the lower part of the abdominal wall is reached. As they advance from the spine, they distribute motor filaments to the external and internal intercostals, the subcostals, the levatons costarum, the serrati postici superior et inferior, the triangularis stcrni, the external oblique, the internal oblique, the trausversalis, the reetus, the pyramidalis and a por- tion of the diaphragm. Their cutaneous distribution comprises the integmm-nt of the chest and abdomen anterior to the area supplied by the posterior primary divisions of the thoracic nerves. On account of the presence of the shoulder girdle, the usual nerve distribution is modified in the- upper thoracic region and the supra- davicular branches of the rervieal plexus assume a function belonging to the thoracic nerves. At the lower portion of the trunk the usual arrangement is likewise- alt en THE THORACIC NERVES. 1315 the area immediately above Poupart's ligament and the pubes being innervated, not by the thoracic, but by the lumbar nerves (Fig. 1105). The supply of the cutane- ous area is provided by two rows of sensory twigs, which become superficial by piercing the musculature and deep fascia of the trunk. Each of the thoracic nerves, with the exception of the first, sends out a lateral cutaneous branch and, with no exceptions, an anterior cutaneous branch. The upper thoracic nerves deviate variously from this typical arrangement, the first having no lateral and sometimes no anterior cutaneous branch, and a portion of the lateral cutaneous branch of the second, called the intercosto-humeral nerve, leaving the thorax to be distributed in the upper extremity. The third nerve of the series is the first to present a typical arrangement, although it, indeed, sometimes forms a loop with the lesser internal cutaneous nerve of the arm. The anterior cutaneous branches are the terminal portions of the thoracic nerves and are constant in their arrangement and distribution, with the exception of the first, which is either very small or absent and a filament from the last, which passes over the crest of the ilium to the gluteal integument. After separating from the posterior primary divisions, the anterior primary divisions of the thoracic nerves, with the exception of the twelfth, enter the inter- costal spaces by passing between the anterior costo-transverse ligaments and the external intercostal muscles. From this situation to the angles of the ribs they lie between the posterior intercostal membrane and the external intercostal muscles. Anterior to this point, they are situated between the two sets of intercostal muscles, as far forward as the termination of the external set of muscles at the. costo-chondral articulations, from which point forward their superficial covering is the anterior inter- costal membrane and the deep the internal intercostal muscles. At first they lie within the upper part of the intercostal space, but as they advance they show a tendency to occupy the middle of the space. While accompanying the intercostal vessels, they lie below the latter and at a greater distance from the rib next above. The upper two nerves extend for a portion of their course along the inner surface of the corre- sponding ribs; the twelfth passes in front of the quadratus lumborum. The upper thoracic nerves, as they approach the margin of the sternum, tra- verse the substance of the internal intercostal muscles and hold a position anterior to the internal mammary artery and the lateral portion of the triangularis sterni muscle. They terminate by piercing the anterior intercostal membrane and the pec- toralis major, and ramify in the pectoral integument as the anterior cutaneous nerves of the thorax (Fig. 1105). The lower thoracic nerves pass forward and at the anterior ends of the ribs take up a deeper position in the trunk wall by piercing the substance of the internal intercostal muscles. They then traverse the intervals between the digitations of the diaphragm and enter the abdominal wall, the seventh, eighth and ninth nerves lying behind the cartilages of the eighth, ninth and tenth ribs respectively. From this point their course is ventral, between the internal oblique and the transversalis, as far as the lateral edge of the rectus sheath, which they enter by piercing its pos- terior lamella. They ultimately turn forward and become superficial by traversing the rectus and its anterior aponeurotic covering, terminating as the anterior cuta?ieous nerves of the abdomen (Fig. 1105). Communications. — Each thoracic nerve is connected with the sympathetic gangliated cord by one or two rami communicantes (Fig. 1130). Ordinarily there is no intercommunication between the upper intercostal nerves, but in rare instances a twig passes from one nerve over the inner surface of the rib next below to the sub- jacent nerve. The lower three or four thoracic nerves, while lying between the broad abdominal muscles are occasionally united to one another, sometimes to the extent of forming a small plexus. Peculiar thoracic nerves. — The first, second, twelfth, and sometimes the third, thoracic nerves present peculiarities which differentiate them from the others. The first thoracic nerve sends a large portion of its fibres to the brach- ial plexus, thus suffering great reduction in its size. Although occasionally a very small branch to the axilla is found, a lateral cutaneous branch is rare, it being generally held that the contribution of this nerve to the brachial plexus is the 1316 HUMAN ANATOMY. Supraacromial branches ot cer- Pectoralis minor muscle vical plexus Pectoralis major muscle Lesser internal cutaneous nerve FIG. 1105. Descending branch of superficial^ colli Suprasternal and supra- clavicular branches of cervical plexus Lateral cutaneous branches of III.. IV., V. and VI. thoracic nerves Anterior cutaneous brs. ., III. ,1V., V. and VI. tho- racic nerves VII., VIII., IX. and X. thoracic. nerves Lateral cutaneous branch of. XI. thoracic nerve XI. thoracic nerve- XII. thoracic nerve- Lateral cutaneous branch of XII. thoracic nerve External oblique muscle cut and everted Internal oblique muscle, cut Ilio-hypogastric nerv Transversalis muscle Lateral cutaneous branch of XII. thoracic nerve c— JL gastric nerVe } iliac {J™£{J Internal oblique musrl< Ilio-inguinal nerve Poupart's ligament — - Anterior brs. of IX. thoracic nerve, the lowest one be. longing to the'X. • Umbilicus Rectusabdom- inis, cut Anterior branch of X. thoracic nerve Anterior branch of _ XI. thoracic Anterior branch of XII. thoracic nerve Hypogastric portion of ilio hyiH>i;astric nerve Aponenrosis of external mus- cle, cut edge Ilio-inguinal nerve Dissection showing thoracic, ilio-hypogastric and ilio-inguinal nerves. equivalent of a lateral cutaneous ttninch. In addition to the lateral cutaneous, the anterior cutaneous branch may also be wanting, the area typically supplied by the absent branch lu-in^ served by the descending branches of the cervical pk.xus. THE THORACIC NERVES. 1317 The second thoracic nerve sometimes contributes fibres to the brachial plexus. The posterior ramus of its lateral cutaneous branch is called the inter costo- humeral nerve. The intercosto-humeral nerve (n. intercostobrachialis) (Fig. 1105) is quite large and pierces the inner axillary wall between the second and third ribs. Enter- ing the axilla, it crosses that space toward the arm and communicates with the lesser internal cutaneous nerve from the brachial plexus. After piercing the deep fascia, the intercosto-humeral nerve supplies the internal and posterior portion of the integ- ument of the upper half of the arm, a few of its fibres extending slightly beyond the margin of the scapula. The third thoracic nerve may form an inosculation with the lesser internal cutaneous nerve. The twelfth thoracic or the subcostal nerve lies below the last rib and therefore does not occupy an intercostal space, but passes outward below the external arcuate ligament and anterior to the quadratus lumborum muscle. It contributes a twig to the lumbar plexus which passes down to join the first lumbar nerve. Its lateral cutaneous branch is not confined in its distribution to the abdominal wall, since, after piercing the internal oblique and sending a filament to the lower digitation of the external oblique, it penetrates the substance of the latter muscle at a point from 2—10 cm. above the crest of the ilium and supplies the integument of the gluteal region as far down as the upper margin of the great trochanter (Fig. 1083). Branches of the thoracic nerves are : (i) the muscidar and (2) the cutaneous. i. The muscular branches (rr. musculares) may be divided into two groups: (a) the thoracic and (b) the abdominal. a. The thoracic muscular branches arise from the first to the seventh inclusive and supply the external and internal intercostals, the subcostals, the levatores costarum, the serratus posticus superior, the triangularis sterni and the rectus abdominis. The branches to the intercostal and subcostal muscles are distributed throughout the course ">f each nerve. The first to be given off is the largest and courses forward for some distance along the lower part of the intercostal space. The others vary greatly in number and size. The branches to the levatores costarum consist of fine threads, one arising from each nerve beyond the anterior costo-transverse ligament. They pierce the external intercostal muscles and enter the deep surface of the muscles which they supply. The branches to the serratus posticus superior arise from the upper four nerves. After piercing the external intercostal muscles they pass along the outer margin of the ilio-costalis and supply the four digitations of their muscle. The branches to the triangularis sterni are terminal continuations of the third to the seventh intercostal nerves. After piercing the internal intercostal muscles they pass forward between the triangularis sterni and the internal intercostals or, in the case of the seventh, anterior to the transversalis muscle. In addition to supplying the triangularis sterni the seventh sends fibres to the first digitation of the transversalis. The branches to the rectus arise from the fifth, sixth and seventh and enter the deep surface of the muscle. b. The abdominal muscular branches arise from the eighth to the twelfth inclusive and are distributed to the intercostals, the subcostals, the levatores costarum, the serratus posticus inferior, the external obique, the internal oblique, the transversalis, the rectus, the pyramidalis and the diaphragm. The branches to the intercostal, subcostal and levatores costarum muscles, with the excep- tion of arising from the lower thoracic nerves, resemble in origin, course and distribution those arising from the upper nerves. The branches to the serratus posticus inferior are larger than those to the serratus posticus superior. They arise from the ninth, tenth and eleventh nerves and pass around the lateral margin of the ilio-costalis to reach their destination. ' The branches to the external oblique, the internal oblique and the transversalis comprise numerous fine twigs which supply those muscles and arise from the lower five thoracic nerves as they course forward between the transversalis and the internal oblique. The branches to the rectus arise from the eighth to the twelfth nerves inclusive after they have entered the sheath and as they pierce the rectus on their way to the surface. The branches to the pyramidalis are derived from the twelfth thoracic and first lumbar nerves. The branches to the diaphragm are supplied to its costal portion and consist of fine filaments which are given off by the lower six thoracic nerves (Luschka). I3i8 HUMAN ANATOMY. 2. The cutaneous branches are larger than the muscular and consist of two sets : (a) the lateral cutaneous and (6) the anterior cutaneous. a. The lateral cutaneous branches (rr. cutanei laterales) consist of two series, an upper and a lower, the former originating from the first to the sixth and the latter from the sixth to the twelfth thoracic nerves. Those of the upper series pierce the external intercostal muscles and those of the lower the external oblique in a line situated midway between the mammary and mid-axillary- lines. The upper seven pass between the digitations of the serratus magnus and the lower between the digitations of the latissimus dorsi and the external oblique. The one arising from the twelfth pierces the musculature of the external oblique. Each lateral cutaneous nerve divides into (aa) an anterior and (66) a posterior branch (Fig. 1083). aa. The posterior branches (rr. posteriores) are smaller than the anterior. They wind around the edge of the latissimus dorsi and supply the integument of the lateral area of the trunk as far back as the anterior margin of the region supplied by the posterior primary divi- sions of the thoracic nerves. The branches from the third to the sixth inclusive have fibres which are distributed over the lateral portion of the scapula. bb. The anterior branches (rr. anteriores [pectorales et abdominales]) are of considerably greater size than the posterior. Those from the second to the seventh pass toward the lateral margin of the pectoralis major and supply the integument of this region as far forward as the nipple. Branches (rr. mammarii laterales) from the fourth, fifth, and sixth send filaments to the skin and substance of the mammary gland. Those from the seventh to the eleventh supply the integument of the abdomen as far anterior as the lateral margin of the rectus. The anterior branch from the twelfth has a filament which passes over the iliac crest to the integument of the gluteal region, usually sending a branch as far as the great trochanter. It maintains a more or less even balance with the corresponding branch of the first lumbar nerve, each supplying any deficiency in the other. b. The anterior cutaneous branches (rr. cutanei anteriores) are the terminal fibres of the thoracic nerves. Those from the upper six (rr. cutanei pectorales anteriores) pierce the pectoralis major near the lateral margin of the sternum and supply the adjacent integument of the thorax. Filaments (rr. mammarii mediales) are distributed to the skin of the mesial portion of the mam- mary gland. The anterior cutaneous branches from the lower six (rr. cutanei alidominales ante- riores) vary in position. They consist of the terminal filaments which perforate the anterior portion of the rectus sheath at a situation anywhere between the linese alba and semilunaris. Those from the seventh become superficial near the ensiform cartilage, those from the tenth supply the region of the umbilicus and those from the twelfth are distributed to the area located midway between the umbilicus and the pubic crest (Fig. 1105). Practical Considerations. — Of the branches of the thoracic spinal nerves, the anterior or intercostals suffer most frequently from sensory disturbances, and the posterior from motor disturbances. Intercostal neuralgia may result from pressure, as from aneurism or spinal disease, or it may be due to injury. The lower intercostals enter into the supply of both the thoracic and the anterior abdominal walls, the pleura also being supplied by them. Pain referred to the abdominal wall and rigidity of the abdominal muscles may therefore be due to diseases within the chest, as pleurisy. Such diseases in the upper part of the chest may cause pain to extend down the arm along the intercosto-humeral nerve, which is the lateral cuta- neous branch of the second intercostal nerve, or sometimes of the second and third intercostals. The pain of intercostal neuralgias often becomes intense, especially after violent expiratory efforts, as in coughing and sneezing ; not infrequently after the pain ceases, herpes zoster appears in the line of the nerve affected. This may be a trophic disturbance or an extension of the inflammation along the nerve endings to the skin. Mastodynia, or the so-called "irritable breast of Cooper," is clue to intercostal neuralgia, and occurs in the female during the child-bearing period. The lower intercostal nerves, with the ilio-hypogastric and ilio-inguinal, supply the muscles of the abdominal wall, and are frequently injured by the incisions made in abdominal operations, thus leading to more or less impairment of the muscles sup- plied and favoring the later development of hernia. The incision should therefore, so far as possible, be made in the line of the fibres of the muscles ( page 535)- The intercostal nerves continue their oblique line through the abdominal mus- cles. The pain from Pott's disease is often transferred along the nerves coming from the affected segment of the cord. In this way pain in the abdominal region may THE LUMBAR PLEXUS. 1319 result from this disease, and an abdominal lesion may be suspected ; this has occurred more particularly in children. A feeling of tightness is sometimes observed about the abdomen, corresponding to the course of one or more pairs of these nerves, and may be due to impaired sensation in them. Since the abdominal muscles are supplied chiefly by the seven lower intercostal nerves, they are concerned in respiration. When they are contracted as in general peritonitis, the lower ribs become immobile, and breathing takes place chiefly in the upper portion of the chest. FIG. i i 06. THE LUMBAR PLEXUS. The lumbar plexus (plexus lumbalis) lies in the substance of the psoas magnus muscle, anterior to the transverse processes of the lumbar vertebrae, and consists of a series of loops formed by the anterior primary divisions of the first, second and third lumbar nerves, the smaller subdivision of the fourth lumbar and sometimes a branch from the twelfth thoracic nerve. The remainder and major portion of the fourth lumbar nerve unites with the entire anterior primary division of the fifth to form a conjoint trunk, the lumbo-sacral cord (truncus lumbosacralis), which passes into the pelvis to become a constituent of the sacral plexus (Fig. 1106). The lumbar nerves increase in thickness from above downward, the first being only 2.5 mm., while the fifth attains a diameter of 7 mm. The length of the nerves from their exit at the intervertebral foramina to their point of division varies considerably, in the case of the first being i mm. or less, of the second 10 mm. and of the third from 20-25 mm. Constitution and Plan. — In forming the plexus (Fig. 1106), the first lumbar nerve divides almost immediately after its exit from the vertebral column into an upper and a lower branch. The upper, which may receive a contribution from the twelfth thoracic nerve, becomes the ilio-hypogastric'd3\& ilio-inguinal nerves. The lower branch, near the body of the second lumbar vertebra joins the upper part of the second lumbar nerve, which, like the first, divides into an upper and a lower branch. The union of the lower branch of the first and the upper branch of the second results in the formation of the genito-crural nerve. Sometimes fibres from the first aid in the formation of the anterior crural and obturator nerxres. The lower branch of the second, all of the third and that part of the fourth which enters the lumbar plexus divide into smaller anterior and larger posterior trunks. From the union of the anterior branches of these three the obturator nerve is formed, and from the union of the posterior results the an- terior crural nerve. The posterior por- tions of the second and third nerves give off from their dorsal aspect small Diagram illustrating plan of lumbar plexus. branches which unite into the external cutaneous nerve. The accessory obturator nerve, when it exists, arises from the third and fourth lumbar between the roots of the anterior crural and obturator nerves. Communications. — All of the lumbar nerves receive gray rami communicantes from the gangliated cord of the sympathetic ; and from the first and second, and possibly the third and fourth, white rami communicantes pass to the lumbar portion of the gangliated cord. 1320 HUMAN ANATOMY. Variations. — That portion of the fourth lumbar nerve, or n.furcalis, which joins the lumbo- sacral cord, is usually less than half of the parent trunk, but varies from one-twentieth to nine-tenths. When large, it may be joined by a branch from the third lumbar, and when small the fifth lumbar may contribute to the lumbar plexus, the fibres going to the ante- rior crural alone or to the anterior crural and obturator nerves. The branch to the lumbo- sacral cord from the fourth lumbar may be absent and in such an event the fifth is the only furcal nerve sending fibres to both the lumbar and the sacral plexus. It is thus possible to have as furcal nerves the third and fourth, the fourth alone, the fourth and fifth or the fifth alone, and according to the high or low position of these there is found a corresponding origin of the branches of the lumbar plexus. In this manner are accounted for the high and low , or prefixed and postfixed types of plexus. Branches of the lumbar plexus are : 1. The Muscular 5. The External Cutaneous 2. The Ilio-Hypogastric 6. The Obturator 3. The Ilio-Inguinal 7. The Accessory Obturator 4. The Genito-Crural 8. The Anterior Crural i. THE MUSCULAR BRANCHES. The muscular branches (rr. musculares) supply the quadratus lumborum, the psoas magnus and the psoas parvus. The branches to the quadratus lumborum arise from the upper three or four lumbar nerves, and sometimes from the last thoracic, and pass directly into the quadratus. The branches to the psoas magnus arise mainly from the second and third lumbar nerves, there sometimes being additional ones from the first and fourth. They pass directly into the muscle. The branches to the psoas parvus consist of filaments from the first or second lumbar nerve which reach the muscle by piercing the underlying psoas magnus. 2. THE ILIO-HYPOGASTRIC NERVE. The ilio-hypogastric nerve (n. iliohypogastricus) (Fig. 1107) is the uppermost branch of the plexus and is somewhat larger than its associate, the ilio-inguinal. Whilst it derives the major portion and sometimes all of its fibres from the first lumbar nerve, it usually receives others from the twelfth and occasionally the eleventh thoracic. It emerges from the lateral margin of the upper portion of the psoas magnus and runs, below and parallel with the twelfth thoracic nerve, outward and downward, posterior to the kidney and anterior to the quadratus lumborum. Reaching the crest of the ilium, it pierces the transversalis muscle and occupies the intermuscular space between the internal oblique and the transversalis. After coursing along this interval as far as the middle of the iliac crest, it divides into its terminal branches, (#) the iliac and (£) the hypogastrif) which correspond morphologically with the lateral and anterior cutaneous branches of the thoracic nerves. There are also some (c) muscular branches. a. The iliac branch (r. cutaneus lateralis) pierces the internal and external obliques about the middle of the iliac crest and is distributed to the integument of the anterior gluteal region which covers the gluteus medius and the tensor fasci;e femoris (Fig. 1083). It forms an inosculation with the lateral cutaneous branch of the twelfth thoracic nerve and maintains an even balance with it, deficiency in the development of either being recompensed for by a com- pensating increase in size of the other. b. The hypogastric branch (r. cutaneus anterior) continues the direction and course of the main trunk between the transversalis and the internal oblique almost to the linea alba. NVar the anterior superior spine of the ilium it forms an inosculation with the ilio-inguinal nerve. As it approaches the region of the internal abdominal ring it begins to push its way gradually through the internal oblique and gain the interval between the internal and the exter- nal oblique (Fig. 1105). A short distance superior and internal to the external abdominal ring it travers.-s a tiny foramen in the aponeurosis of the external oblique and breaks up into fibres ot termination which supply the integument of the snprapubic region. c. Muscular branches (rr. musculares) arise from the hypogastric branch in its course through the abdominal wall and supply the transversalis, the internal oblique and the external oblique. THE LUMBAR PLEXUS. 1321 Variations. — The iliac branch may be absent, its place being taken by the lateral cutaneous branch of the twelfth thoracic nerve. The hypogastric branch may inosculate with the twelfth thoracic and may supply the pyramidalis muscle. 3. THE ILIO-INGUINAL NERVE. The ilio-inguinal nerve (n. ilioinguinalis) (Fig. 1107) is the second branch of the lumbar plexus and is somewhat smaller than the ilio-hypogastric. Its fibres usually arise from the first lumbar nerve, with accessions from the twelfth thoracic. FIG. 1107. XII. rib XII. thoracic nerve Quadratus lumborum Psoas magnus External oblique Lateral cutaneous branch of XII. dorsal nerve Internal oblique Transversalis Ilio-hypogastric nerve Ilio-inguinal nerve Iliac branch of ilio-hypogastric Lateral cutaneous branch of XII. dorsal nerve External cutaneous nerve Anterior crural nerve Genital branch of genito-crural nerve Crural branch of genito-crural nerve Branches of middle cutaneous nerve I. lumbar ganglion Rami communicantes Aorta IV. lumbar nerve . lumbar ganglion . lumbar nerve irt of V. lumbar ganglion jnito-erural nerve sacral ganglion sacral nerve sacral nerve sacral ganglion urator nerve essory obturator nerve ) Hypogastric branches ) of ilio-hypogastric nerve Ilio-inguinal nerve Branch of internal cutatieous nerve Deep dissection, showing nerves arising from lumbar plexus and lower part of sympathetic gangliated cord. Sometimes it arises entirely from the twelfth thoracic or from the second lumbar or from the loop between the first and second lumbar nerves. It occasionally forms a common trunk of considerable length with the ilio-hypogastric. In the early part 1 322 HUMAN ANATOMY. of its course it parallels the ilio-hypogastric, appearing at the edge of the psoas magnus, crossing the quadratus lumborum behind the kidney and piercing the trans- versalis to reach the intermuscular cleft between the transversalis and the internal oblique (Fig. 1105). While in the last situation it inosculates with the ilio-hypo- gastric and continues forward to enter the inguinal canal, from which it emerges either through the external abdominal ring or through the external pillar of the ring, infero-lateral to the spermatic cord. Some of the branches of the ilio-inguinal supply the integument of the upper inner portion of the thigh. Others (nn. scrotales anteriores) are distributed to the pubic region and the base of the penis and scrotum or, in the female (nn. labiales anteriores), the monsVeneris and labia majora. Tiny motor filaments (rr. musculares) are given off in the course of the nerve to the transversalis, the internal oblique and the external oblique. Variations. — The ilio-inguinal may be small and terminate near the iliac crest by joining the ilio-hypogastric, which then sends off an inguinal branch with the course and distribution of the absent portion of the ilio-inguinal. The nerve may be absent entirely and replaced by either branch, usually the genital, of the genito-crural. It may give off a lateral cutaneous or iliac branch for the supply of the integument in the region of the anterior superior spine of the ilium. The ilio-inguinal may partially replace the genital branch of the genito-crural or, in rare in- stances, the external cutaneous. 4. THE GENITO-CRURAL NERVE. The genito-crural nerve (n. genitofemoralis) is formed by two roots, one of which arises from the loop between the first and second lumbar nerves and the other directly from the second lumbar nerve, its fibres being derivatives of the first and second lumbar. The nerve passes obliquely forward through the musculature of the psoas magnus, near the inner border of whose anterior surface it emerges opposite the body of the third lumbar vertebra, where division into the two terminal branches, (a) the genital and (<£) the crural, takes place (Fig. 1107). Occa- sionally division occurs earlier in the course of the nerve, in the substance of the psoas, and under these circumstances the two branches emerge separately from the muscle. In addition to the terminal branches there are some (c) muscular twigs. a. The genital branch (n. spermaticus externus) obtains its fibres from the first lumbar nerve. Passing downward on the inner margin of the psoas magnus, it crosses the external iliac artery and bends forward toward the posterior wall of the inguinal canal. It then enters the canal either by piercing the infundibuliform or the transversalis fascia and, lying internal to and below the spermatic cord, traverses the canal and enters the scrotum (Fig. 1108). It sends a filament to the external iliac artery and supplies the cremaster muscle, the skin of the scrotum and the integument of the thigh immediately adjacent to the scrotum. In the female it is smaller and accompanies the round ligament of the uterus to the labium majus, to whose in- tegument it is distributed. It communicates with the ilio-inguinal nerve and with the spermatic- plexus of the sympathetic. b. The crural branch (n. lumboinguinalis) consists of fibres from the second lumbar nerve. It courses down on the anterior surface of the psoas magnus, lateral to the genital branch and to the external iliac vessels, and enters the thigh by passing beneath Poupart's ligament. One of its filaments traverses the saphenous opening, while the remainder of the nerve pierces the fascia lata to the outer side of the opening (Fig. 1107). Its branches vary considerably in size and length and are distributed to the cutaneous area of the upper anterior part of the thigh between the regions supplied by the external cutaneous and ilio-inguinal nerves, sometimes extending downward as far as the middle of the thigh. It furnishes a minute branch to the femoral artery and inosculates with the middle cutaneous nerve. c. Muscular branches to the internal oblique and transversalis are frequently given off by the genital branch. Variations. — The genital and crural branches may arise as separate offshoots of the lumbar plexus and either of them may be derived entirely from the first or the second lumbar nerve. The genital branch sometimes contains fibres from the twelfth thoracic. Absence- of the genito- crural or of either branch may occur, the fibres of the genital branch being contained in the ilio- inguinal and thos»- of the crural in the external cutaneous or the anterior crural. The genital branch may replace or reinforce the ilio-inguinal nerve; the crural branch may act similarly toward tin- external or the middle cutaneous nerve. A sjH-cimen found in the anatomical labo- ratory of the I'niversity of Pennsylvania showed unusually extensive distribution of the crural THE LUMBAR PLEXUS. 1323 branch. It was larger than normal, its size being that of the normal external cutaneous, and it emerged from the deep fascia below Poupart's ligament directly anterior to the femoral vein. It Psoas parvus Genito-crural nerve Psoas niagnus Anterior crural nerve External cutaneous nerve Genital branch of genito-crural Sartorius, stump Branch to pectineus Branch to rectus femoris Branch to vastus externus Rectus femoris Middle cutaneous nerve Rectus femoris Accessory obturatoi Crural branch of genito-crural Ilio-inguinal nerve Pectineus Adductor longus Internal saphenous nerve Internal cutaneous nerve Muscular branch of superficial division of obturator nerve i from internal saptienous to subsartorial plexus Branch to vastus internus Anterior branch of internal cutaneous Cutaneous branch of superficial division of obturator nerve Posterior branch of internal cutaneous From posterior branch of internal cutaneous Articular branch from nerve to vastus internus Cutaneous patellar branch of internal saphenous nerve Internal saphenous nerve Dissection of right thigh, showing branches of anterior crural nerve. divided into a smaller mesial and larger lateral branch and was distributed to the integument ot the thigh as far down as the junction of the middle and lower thirds. I324 HUMAN ANATOMY. 5. THE EXTERNAL CUTANEOUS NERVE. The external cutaneous nerve (n. cutaneus femoris lateralis) (Fig. 1109) arises at the posterior aspect of the lumbar plexus from the second and, to a less extent, the third lumbar nerve. It may arise from the first and second, from the second alone or may derive a majority of its constituent fibres from the third. It passes obliquely downward and outward beneath the lateral margin of the psoas magnus and over the iliacus muscle, through the iliac fossa, covered by the iliac fascia. After crossing the deep circumflex iliac artery it enters the thigh beneath Poupart's ligament, mesial to the anterior superior spine of the ilium, and passes over, sometimes through or under, the pointed tendinous origin of the sartorius. The nerve then descends in the thigh beneath the fascia lata and soon divides into (a) an anterior and (£) a posterior terminal branch (Fig. mo). a. The anterior branch (r. anterior) follows a downward course in the thigh in a tubular canal in the fascia lata, from which it emerges at a point 10-15 cm- below the anterior superior iliac spine. It continues downward anterior to the vastus externus muscle and is distributed to the integument of the antero-lateral aspect of the thigh as far as the knee. Numerous collateral branches are given off, the majority of which arise from its lateral edge and supply the skin ovei the ilio-tibial band. The main trunk may extend quite to the knee and become a participant in the formation of the patellar plexus. b. The posterior branch (r. posterior) passes obliquely backward through the fascia lata and breaks up into several branches which are distributed to the integument over the tensor fasciae femoris and the lower portion of the gluteal region. The uppermost filaments are crossed by twigs from the lateral cutaneous branch of the twelfth thoracic nerve. Variations. — The external cutaneous may be associated with the anterior crural until after Poupart's ligament has been passed. A branch of the genito-crural may replace the posterior branch. In one case a branch of the ilio-inguinal took the place of the external cutaneous.' Three specimens found in the anatomical rooms of the University of Pennsylvania showed decided anomalies. In one the nerve passed beneath Poupart's ligament at a point midway between the anterior superior spine of the ilium and the femoral artery. In another the nerve of the right side resembled in position the one just mentioned, while the left was apparently absent, its place being taken by a branch of the anterior crural. In the third the posterior branch emerged from beneath Poupart's ligament 5 cm. to the inner side of the anterior superior iliac spine. The anterior branch formed a common trunk with the external branch of the mid- dle cutaneous nerve. From the joint trunk a small branch passed to join the internal branch of the middle cutaneous after the latter had pierced the sartorius muscle. 6. THE OBTURATOR NERVE. The obturator nerve (n. obturatorius) (Fig. 1109) is composed of fibres which arise from the second, third and fourth lumbar nerves, the fourth supplying the largest and the second the smallest contribution, the latter sometimes being absent entirely. Occasionally additional roots are derived from the first and fifth lumbar nerves, and sometimes the nerve arises, in the high form of plexus, from the first, second and third lumbar nerves. The three roots having united in the substance of the psoas magnus, the nerve passes vertically downward and emerges, the only constant branch of the plexus to do so, from the mesial margin of the psoas muscle opposite the brim of the true pelvis. Lying posterior to the common and lateral to the internal iliac vessels, the obturator nerve courses along the antero-lateral wall of the pelvis below the ili<>- pectineal line, above the obturator vessels and upon the inner surface of the pelvic fascia. It escapes from the pelvis through the obturator canal in the obturator mem- brane and divides into its terminal branches, either while still within the foramen or shortly after emerging from it. These branches are separated from each other first by the anterior fibres of the obturator externus muscle and later by the adductor luvvis muscle. They supply the adductor muscles, the hip and knee joints and the integument of the mesial aspect of the thigh. Branches. — The obturator gives off: (a} a branch to the obturator c.vlcrnus muscle and then divides into its terminal branches, (to) the anterior and (c) the Posterior. THE LUMBAR PLEXUS. 1325 a. The branch to the obturator externus arises within the pelvis from the inner surface of the obturator nerve. It accompanies the parent trunk through the foramen, immediately after FIG. 1109. Ext. cutaneous nerve Ant. sup. spine of ilium Ant. crural nerve Br. to rectus Sartorius Artie, br. of accessory obturator Iliacus Br. to vastus ext. and crureus Rectus Middle cutaneous nerve Int. cutaneous, ant. branch Femoral artery Int. cutaneous, post, branch Int. sapheiious nerve Nerve to vastus interims Rectus Artie, br. from nerve to vastus int. 'Ext. iliac artery Int. iliac artery Accessory obturator nerve Obturator nerve Pectineus Obturator nerve, ant. division Adductor longus, cut Obturator nerve, post division Articular br. to hip-ioint Adductor brevis Pectineus Adductor magnus Adductor brevis Gracilis Adductor longus Terminal br. ant. division obturator nerve Cutaneous branch Br. from int. cutaneous to subsartorial plexus Artie, br. to knee-joint from obturator nr. to subsartorial plexus and femoral artery Cutaneous br. to inner surface Internal saphenous nerve Cutaneous patellar br. int. saohenous Sartorius, insertion Post. br. int. cutaneous Internal saphenous Dissection of right thign, showing branches of anterior crural and obturator nerves. escaping from which it dips down in the interval between the obturator membrane and the obtur- ator externus muscle. From this situation its fibres pass through the deep surface :ato the substance of the muscle. I326 HUMAN ANATOMY. b. The anterior branch ( r. anterior), the more superficial, descends in front of the obturator externus and adductor brevis muscles and between the pectineus and the adductor longus. Having reached the interval between the adductores brevis and longus it separates into its terminal branches. Branches of the anterior division are: (aa) the articular, (bb) the muscular, (cc) the cutaneous, (dd) the communicating and (ee) the vascular. aa. The articular branch leaves the obturator at the inferior margin of the obturator foramen and passes through the cotyloid notch to supply the hip joint. bb. The muscular branches supply the adductores brevis and longus and the gracilis. The branch to the adductor brevis enters the muscle near the upper margin of the anterior surface. The branch to the adductor longus enters the posterior surface of the muscle and some- times gives off the cutaneous branch of the obturator (see below). The branch to the gracilis passes inward behind the adductor longus and enters the deep surface of its muscle. cc. The cutaneous branch (r. cutaneus) (Fig. mo) is variable in size and maintains an approximately even balance with the internal cutaneous branch of the anterior crural. Some- times arising from the nerve to the adductor longus, it becomes superficial in the middle of the thigh by passing between the adductor longus and the gracilis. It supplies the integument of the lower inner portion of the thigh and beneath the sartorius forms an inosculation with branches of the internal cutaneous and internal saphenous nerves, called the subsartorial or obturator plexus. dd. The communicating branches consist of twigs which unite in the pelvis with the accessory obturator nerve and in the thigh anterior to the capsular ligament of the hip joint with the anterior crural. ee. The vascular branch enters Hunter's canal along the mesial edge of the adductor longus and spreads out over the lower portion of ihe superficial femoral artery. c. The posterior branch (r. posterior), the deeper, pierces the anterior fibres of the obturator externus muscle and descends in the cleft between the adductores brevis and magnus, and in the latter situation splits into its terminal twigs. Branches of the posterior division are : (aa) the muscular and (bb) the articular. aa. The muscular branches supply the obturator externus, the adductor magnus and the adductor brevis. The branch to the obturator externus is additional to the twig from the main trunk of the obturator which supplies that muscle. It arises from the posterior surface of the posterior division and enters the superficial surface of the muscle. The branch to the adductor magnus is associated with the branch to the knee and leaves the latter as the conjoint nerve passes through the substance of the adductor magnus. The branch to the adductor brevis enters the posterior surface of the muscle and is present only when the usual branch from the anterior division is absent. bb. The articular branches are destined for the supply of the hip and knee joints. The branch to the hip Joint consists of one or two fine txvigs which pass beneath the pectineus to be distributed to the antero-median portion of the capsular ligament. The branch to the knee joint or the geniculate branch continues the course of the posterior division. Associated with the nerve to the adductor magnus, it courses down the anterior sur- face to the adductor magnus, which it pierces at the lower portion of the thigh. Here its muscu- lar fibres terminate in the adductor magnus while the articular portion enters the popliteal space. The nerve continues downward on the popliteal artery, to which it distributes filaments, and finally terminates by entering the knee joint through the posterior ligament. Variations. — In rare instances the root from the second lumbar nerve is absent, branches are sometimes given off to the obturator internus and to the pectineus. Tiny brandies have been found going to the obturator artery and to the periosteum of the pelvic surface of the os pubis. In a cadaver dissected in the anatomical laboratory of the University of Pennsyl- vania the obturator of the right side divided into the usual anterior and posterior branches, but both of them passed posterior to the adductor brevis. ( )n the left side the normal arrangement was present. In another specimen in the same laboratory the branch from the main trunk to the obturator externus muscle lay to the outer instead of the inner side of the obturator nerve. 7. THE ACCESSORY OBTURATOR Ni RVI . The accessory obturator nerve is an inconstant branch of the lumbar plexus, bein^- found in 29 per cent, of the cadavers examined (Eisler). Its fibres arise from tin third and fourth lumbar nerves, with an occasional root from the fifth ; it may be derived from the third alone. The roots of origin are situated between those of the anterior crural and the obturator, and the nerve may be intimately associated with either of these two, usually the former. THE LUMBAR PLEXUS. 1327 The accessory obturator courses downward mesial to the psoas magnus and beneath the iliac fascia, and leaves the pelvis by passing over the horizontal ramus of the pubes and under the pectineus. In the latter situation it breaks up into its branches, one of which (a) supplies the pectineus, another (£) the hip joint, while the third (c) inosculates with the anterior division of the obturator nerve. Some- times it is very small and its fibres pass only to the hip joint. By means of its in- osculation with the obturator some of its fibres may reach the adductores longus and brevis and gracilis muscles, as well as the integument of the inner region of the thigh. 8. THE ANTERIOR CRURAL NERVE. The anterior crural or femoral nerve (n. fcmoralis) (Fig. 1108), the largest branch of the lumbar plexus, arises from the first, second, third and fourth lumbar nerves. It passes obliquely downward and outward, posterior to the psoas magnus, and emerges from beneath the middle of the lateral margin of that muscle. Thence it continues its course between the outer edge of the psoas and the mesial edge of the iliacus, covered by the iliac fascia, as far as Poupart's ligament, under which it passes to become an occupant of the anterior portion of the thigh. The nerve lies to the outer side of the external iliac and femoral vessels, in the abdomen being separated from them by the psoas magnus, but, as the thigh is reached, gradually nearing them until in Scarpa's triangle the nerve lies in apposition to the femoral sheath. In the immediate neighborhood of Poupart's ligament, the anterior crural nerve rapidly splits up into a number of Branches, which may be grouped into (£) a superficial division, principally sensory, and (c) a deep division, mainly motor. In addition there are (a) branches arising from the main trunk. a. The branches from the main trunk consist of (aa) the muscular branches and (bb) the nerve to the femoral artery. aa. The muscular branches supply the iliacus, the psoas magnus and the pectineus. The branches to the iliacus consist of two to four filaments which arise in the abdomen, pass outward and enter the inner margin of the iliacus muscle. The branch to the psoas magnus arises in the lower part of the iliac fossa and supplies the inferior portion of that muscle. It may originate in common with the nerve to the femoral artery. The branch to the pectineus leaves the anterior crural beneath Poupart's ligament, passes inward posterior to the femoral vessels and enters the anterior surface of its muscle. bb. The nerve to the femoral artery usually takes origin in the iliac fossa, but frequently arises higher, sometimes as a distinct branch from the third lumbar nerve. It accompanies the anterior crural as far as Poupart's ligament, leaving the parent trunk at the lateral margin of the femoral sheath. At the ligament it gives off fine twigs which ramify over the posterior part of the femoral vessels, and from them tiny filaments pass to the middle of the thigh. Other twigs are distributed to the deep femoral artery and from this group a fine terminal thread traverses the nutrient foramen of the femur, after supplying branches to the periosteum. b. The anterior or superficial division is mainly cutaneous in distribution. It supplies sensory twigs to the. anterior and mesial surfaces of the thigh and motor twigs to the sartorius. Branches of this division are : (aa) the middle cutaneous and (bb) the internal cutaneous. aa. The middle cutaneous nerve (rr. cutaneianteriores) (Fig. uio) consists of two branches, an external and an internal, both of which contain motor as well as sensory fibres. The external branch passes downward under the sartorius, to whose posterior surface are given off a row of fine twigs which enter the upper portion of the muscle. The continuation of the nerve pierces the sartorius at the junction of the upper and middle thirds, then pushes its way through the fascia lata and splits into fine filaments which supply the integument over the rectus femoris as far as the knee. The internal branch is sometimes united in the upper part of its course with the external. It supplies twigs to the sartorius but seldom pierces that muscle, usually passing internal and anterior. This branch, like the external, is distributed to the anterior integument of the thigh as far down as the knee and frequently inosculates with the crural branch of the genito-crural. Variations.— Sometimes the middle cutaneous arises from the beginning of the anterior crural or from the lumbar plexus and replaces in toto or in part the crural branch of the genito-crural. 1328 HUMAN ANATOMY. From ext. cutaneous nerve Communication tween ext. cutaneous and middle cutaneous Crural br. of genito-c rural From ext. cuta- neous nerve Middle cuta- neous nerve llio-inguinal nerve (emerging through ext. abd. ring). Upper br. of int. cutaneous nerve (or twigs from posterior branch) Internal saphenous vein bb. The internal cutaneous nerve (rr. cutanei mediates ) leaves the anterior crural in the neighborhood of Poupart's ligament and descends in Scarpa's triangle, at the apex of which it crosses obliquely the femoral vessels to attain their mesial side. It passes superficial to or through the sartorius muscle and divides, either anterior or internal to the superficial femoral artery, into its terminal branches, the anterior and the posterior (Fig. mo). Two or three branches are given off by the main trunk. One of these pierces the fascia lata immediately below the saphenous opening and accompanies the internal saphenous vein down to the middle of the thigh, supplying the integument in its immediate vicinity. Another branch pierces the fascia lata FIG. i no. at about the middle of the thigh and supplies the skin of the antero-median aspect as far down as the knee. These branches sometimes arise di- rectly from the anterior crural, and not infrequently the nerve to the pectineus gives off a branch which forms a loop at the linner side of the femoral artery with a nerve which passes anterior to that vessel. The anterior branch pierces the fascia lata in the lower third of the thigh, de- scends in the neighborhood of the tendon of the adductor magnus and eventually passes across the patella to reach the lateral region of the knee. It supplies the skin in the vicinity of the adductor magnus tendon and inosculates at the knee with a branch of the internal saphenous nerve. The posterior branch con- tinues down beneath the pos- terior edge of the sartorius and becomes superficial by perforating the fascia lata at the mesial aspect of the knee. Its ultimate filaments supply the integument of the lower part of the inner side of the thigh and the upper portion of the leg. Before becoming su- perficial it inosculates be-low the middle of the thigh with the obturator and internal saphenous nerves to form the subsartorial or obturator plexus (Fig. 1109). At the knee and in the upper part of the leg it again forms connections with the internal saphenous nerve. c. The posterior or deep division of the anterior crural nerve consists of a fasces of nerve-bundles which furnishes innervation to those muscles which comprise the quadriceps extensor femoris and terminaU-s is tlie internal saphenous nerve. Branches of this division arc: (a-> ris arises with the lower branch to the peroneus longus and enters the musculature of the pm UK-US brevis. TERMINAL BRANCHES. 1339 66. The internal terminal branch (n. cutaneus dorsalis medialis) (Fig. 1117), larger than the external, passes obliquely inward in front of the ankle and then forward over the dorsum of the foot. Cutaneous twigs are distributed FIG. 1117. From internal -cutaneous to the anterior aspect of the lower third of the leg and the dorsum of the foot. Just below the anterior annular ligament the nerve breaks up into an inner, a middle and an outer branch. The inner branch inosculates with the internal saphenous nerve, from which it receives an accession of fibres, and passes forward to supply the integument of the mesial aspect of the foot and great toe. The middle branch follows the first metatarsal space and inosculates with the inner branch of the anterior tibial nerve. The outer branch courses down the second metatarsal space and divides into the two dorsal digital nerves (nn. digitales dorsales pedis) which supply the contig- uous sides of the second and third toes. This branch is sometimes derived from the external terminal part of the musculo- cutaneous. cc. The external terminal branch (n. cutaneus dorsalis intermedius) (Fig. 1117) courses down the leg anterior to the ankle and lateral to the inner branch, giving off twigs to the antero-lateral por- tion of the integument of the lower part of the leg and dorsum of the foot. Having reached the foot it breaks up into inner and outer branches. The inner branch divides into dorsal digital branches for the supply of the adjacent sides of the third and fourth toes, and the outer branch, after receiving an accession of fibres through inoscula- tion with the external saphenous, divides similarly into twigs for the contiguous sides of the fourth and fifth toes. The dorso-lateral aspects of the terminal phalanges and the nails receive addi- tional filaments from the plantar nerves. Variations. — Deficiencies in the in- ternal branch are usually supplied by the anterior tibial nerve and in the ex- ternal by the short saphenous. In case the external branch ends at the dorsum of the foot, the external saphenous, which would fill the vacancy at the digits, has its root from the external popliteal more strongly developed than usual, and thus the toes are supplied in an unusual manner but still by fibres from the ex- ternal popliteal nerve. From middle cutaneous • nerve From . peroneal nerve Sural br peroneal nerve Musculo-cuta- . neons nerve Ext. sapl Ext. terminal br. musculo- cutaneous nerve From internal cutaneous nerve Cutaneous patellar br. int. saphenous nerve Int. saphenous nerve Crest of tibia Int. saphenous nerve Int. saphenous veiii Int. terminal br. musculo- cutaneous nerve Int. terminal br. ant. tibial nerve 6. THE INTERNAL POPLITEAL NERVE. The internal popliteal or tib- ial nerve (n. tibialis) (Fig. 1115) is of greater size than the external and corresponds in its distribution to the combined median and ulnar nerves of the arm. Arising from the anterior portion of the sacral plexus, it includes fibres derived from the fourth and fifth lumbar Superficial dissection of right leg and foot, showing cutaneous nerves of anterior surface. 1340 HUMAN ANATOMY. and first, second and third sacral nerves. Leaving the pelvis through the greater sacro-sciatic foramen below the pyriformis, and passing through the gluteal region and upper part of the thigh as the inner portion of the great sciatic nerve, it becomes an independent trunk at the point of bifurcation of the sciatic. Emerging from beneath the hamstring muscles and descending vertically through the middle of the Musculo-cutaneous nerve Fibula Extensor longus digitorum tendon Peroneus tertius tendon Anterior tibial nerve Articular branches to ankle joint Peroneus longus tendon External saphenous nerve Musculo cutaneous nerve — external division External division of anterior tibial nerve Extensor brevis digitorum Metatarsal branches of external division of anterior tibial nerve External saphenous nerve Digital branches of external division of musculo- cutaneous nerve Extensor proprius hallucis tendon Internal saphenous nerve Tibialis anticus tendon tensor brevis digitorum Internal division of niusculo- cutaneous nerve Anterior tibial nerve — internal branch Dissection of dorsuin of right foot, showing distribution of anterior tibial, musculo-cutaneous, and internal and external saphenous nerves. popliteal space, it gradually attains the inner side of the popliteal vessels, crossing them superficially from without inward. In the lower part of the space the nerve lies posterior to the popliteus muscle and anterior to the plantaris and the gastroc- nemius. At the lower border of the popliteus muscle the internal popliteal becomes the posterior tibial nerve (Fig. 1119). TERMINAL BRANCHES. Branches of the internal popliteal are : (a) the articular, (6) the muscular, (Y) the cutaneous and (d) the posterior tibial. FIG. 1119. Gracilis Semitendinosus Semimembranosus Sartori Superior internal articular ner Inner head of gastrocnemi Inferior internal articular ner 1 (internal popliteal) nerve neal (external popliteal) nerve Peroneal communicating Popliteus muscle Sural branch Articular branch Tibialis posticus tendon Inner malleolus Abductor hallucis External plantar nerve Internal plantar nerve Abductor hallucis and fascia Peroneus longus Peroneus brevis Internal calcanean branch Tendo Achillis Flexor brevis digitorum, cut Dissection of the posterior surface of right leg, showing posterior tibial nerve and its branches and part of peroneal nerve. a. The articular branches (rr. articulares) supply the hip and knee joints. The one destined for the hip has been described on page 1333. The branches to the knee are of 1342 HUMAN ANATOMY. small size and of varying number. There are usually two, an upper and a lower, and these break up into small filaments which inosculate with the lower articular fibres of the external popliteal, forming the popliteal plexus of Riidinger. The upper or azygos branch usually pierces the posterior ligament of the joint, while the lower accompanies the inferior internal articular artery. When a third is present it accompanies the superior internal articular artery. From the popliteal plexus a number of fine filaments are furnished to the posterior portion of the knee joint and an occasional twig enters the popliteus muscle by piercing its posterior surface. b. The muscular branches (rr. musculares) comprise two sets, those given off from the part above the division of the sciatic nerve and those given off below. The former have been described on page 1333. The latter consist of a series of five twigs which innervate the gastrocnemius, the soleus, the plantaris and the popliteus. The nerves to the gastrocnemius, soleus and plantaris consist of two stout nerve trunks, an upper and a lower. The upper arises in the middle of the popliteal space and enters the lateral aspect of the inner head of the gastrocnemius. The lower arises a short distance below the upper and, frequently combined with the nerve to the plantaris, divides into two branches, a shorter for the outer head of the gastrocnemius, and a longer, which enters the superior bor- der of the soleus, the upper part of which muscle it supplies. From the nerve to the plantaris is furnished a filament to the knee joint. The nerve to the popliteus is a complex structure, with a distribution much wider than is implied in its name. After reaching the lower margin of the popliteus muscle the nerve turns forward, ascends between the anterior aspect of the muscle and the tibia, and enters the anterior surface of the popliteus. A branch supplies the periosteum of the tibia and then enters the nutrient foramen of that bone. Another, the interosseous branch (n. interosseus cruris) courses first posterior to and then between the layers of the interosseous membrane almost to its lower margin. Terminal fibres are distributed to the periosteum of the tibia and to the inferior tibio-fibular articulation. Other filaments reach the tibialis posticus muscle and the superior tibio-fibular articulation. c. The cutaneous branch is the tibial communicating nerve. The tibial communicating nerve or n. tibialis communicans (n. cutaneus surae medialis) (Fig. 1119) arises in the upper portion of the popliteal space, through which it passes, posterior to the internal popliteal nerve, to the fissure between the heads of the gastrocnemius. In company with the external saphenous vein, the nerve descends in this interval to the tendo Achillis and, after piercing the deep fascia at about the middle of the leg, is joined by the peroneal communicating nerve, the fusion resulting in the external or short saphenous nerve (n. suralis). This joint nerve (Fig. 1119) courses down the postero-lateral aspect of the lower part of the leg, passes posterior to and beneath the external malleolus in company with the external saphenous vein and follows a course obliquely downward and forward along the lateral margin of the foot to the dorsal aspect of the outer side of the fifth toe, at the far end of whose distal phalanx the nerve terminates. In its course through the leg and foot it supplies sensory twigs to the postero-lateral part of the lower third of the leg, the region over the external malleolus, the lateral portion of the heel (rr. calcanei laterales), the dorso-lateral portion of the foot (n. cutaneus dorsalis lateralis) and the outer half of the dorsum of the fifth toe. Twigs are furnished to the ankle, and to the astragalo-calcanean and possibly other inter- tarsal articulations. In the foot it communicates with the anterior tibial nerve. Variations. — The point of union of the two tributaries of the external saphenous is subject to wide variations, sometimes being high in the popliteal space and sometimes there being no union at all, in the latter instance the nerve which reaches and supplies the foot usually being the n. communicans tibialis. In one specimen found in the anatomical rooms of the University of Pennsylvania the great sciatic nerve divided just below the margin of the glutens inaximiis. The n. communicans fibularis arose in the middle of the thigh and the n. communicans tibialis in the popliteal space. Union took place 3 cm. below the origin of the n. communicans tibialis, the n. communicans fibularis sending a few fibres across to the internal popliteal nerve before entering the external saphenous. In another cadaver in the same laboratory the two tributa- ries arose 3 cm. apart from each other about 10 cm. above the knee, the n. communicans tib- ialis arising the higher and piercing the inner head of the gastrocnemius before joining the n. communicans fibularis. Variations in distribution may occur, the nerve sometimes supplying the dorsal aspect of two and one-half digits, under such circumstances the n. communicans fibularis usually being of increased size. The nerve may terminate in the foot and not have any digital distribution. d. THE POSTERIOR TIBIAL NERVE. The posterior tibial nerve (n. tibinlis) (Fig. 1119) is the direct continuation of the internal popliteal and begins at the lower border of the popliteus muscle. It extends downward, in a sheath shared by the posterior tibial vessels, between the superficial and deep muscles of the posterior portion of the leg. Anterior to it are TERMINAL BRANCHES. 1343 the tibia and the deep leg muscles and posteriorly lie the soleus and gastrocnemius in the upper part of the leg. Above the ankle the nerve becomes superficial, and is covered only by integument and the fasciae. Owing to the inward inclination of the posterior tibial vessels the nerve, while pursuing a straight course, changes its rela- tive position to the vessels, in the upper part of the leg lying to the inner side, lower down behind and above the ankle attaining the outer aspect of the vessels (Fig. 1121). Passing posterior to and then below the internal malleolus, the posterior tibial nerve divides, under cover of the internal annular ligament, into its terminal branches, the internal and the external plantar. FIG i i 20. Internal calcanean branch of posterior tibial nerv Digital branches of internal plantar nerve External saphenous nerve — -External saphenous nerve Digital branches of external plantar nerve Superficial dissection of right foot, showing cutaneous nerves on plantar surface. Branches of the posterior tibial nerve are : (a#) the muscular, (bb} the internal calcanean, (cc~) the articular, (dd} the internal plantar and (rs. <>f • small sciatic nerve InlVHor piuk-nda! Superficial dissection of right buttock and adjacent regions, showing cutaneous nerves. THE PUDENDAL PLEXUS. 1347 posterior portion of the external sphincter and distributes sensory fibres to the integument over the base of the ischio- FIG. 1124. rectal fossa and the tip of the coccyx. >-"'."% \ ' Variation. — This nerve, instead of pierc- ing the coccygeus, may pass between that mus- cle and the levator ani. The nerve to the levator ani is derived usually from the third and fourth, sometimes the second and third, sacral nerves and en- ters the muscle by piercing its mesial surface. 3. The perfo- rating cutaneous nerve (Fig. 1126) is an inconstant branch, being jiound in about two thirds of the bodies examined. It springs from the dor- sal aspect of the second and third sac- ral nerves and at its point of origin may be associated with the pudic or the small sciatic. Passing downward and back- ward it pierces the great sacro-s c i a t i c ligament in company with the coccygeal branch of the sciatic artery and winds around the lower bor- der of, or in rare in- stances pierces, the gluteus m a x i m u s . Perforating the deep fascia slightly lateral to the coccyx, it be- comes superficial and is distributed to the integument over the inner and lower por- tion of the gluteus maximus. Cutaneous brs. from post, sacral nerves Inf. pudendal nerve, and a glu- teal cutaneous br. of small sciatic Small sciatic nerve From lateral cutane- ous br. of XI I. thoracic From I. lumbar nerve A gluteal cuta- neous br. of small sciatic nerve From lateral cuta- neous br. of XII. thoracic From ext. cutaneous nerve An ext. femoral br. of small sciatic From ext. cuta- neous nerve Superficial dissection of right buttock and thigh, showing cutaneous nerves of posterior surface. Variations. — I n - stead of piercing the ligament it may accompany the pudic nerve or pass between the ligament and the gluteus maximus. It may be replaced by a branch of the small sciatic or by a nerve, called by Eisler 1348 HUMAN ANATOMY. the n. perforate coccygens major, which arises from the third and fourth or fourth and fifth sacral and pierces the coccygeus muscle. FIG. 1125. From obturator nerve From internal cutaneous Internal saphenous nerve From small sciatic nerve — Small sciatic nerve Sural from peroneal nerve Peroneal communicating \— Part of sural branch Tibial communicating External saphenous nerve Inner malleolus External calcanean branches Ext. branch of musculo-cutaneous •Anterior branch of ext. saphenous Cutaneous nerves of posterior surface of right leg. 4. THK SMALL SCIATIC NERVI.. The small sciatic nerve (n. cutancus femoris posterior) (Fijj. 1114) is a purely sensory structure. It originates from the back of the first, second and third, or THE PUDENDAL PLEXUS. 1349 from only the second and third, sacral nerves, the upper root usually being associ- ated with one of the roots of the inferior gluteal nerve, and the lower root with the perforating cutaneous or the pudic nerve. Leaving the pelvis through the great sacro-sciatic foramen below the pyriformis, it descends in the gluteal region between the tuber ischii and the great trochanter, posterior to the great sciatic nerve and anterior to the gluteus maximus, accompanied by the inferior gluteal nerve and the sciatic artery. Emerging into the thigh at the lower border of the gluteus maximus it continues downward beneath the deep fascia and superficial to the hamstring muscles to a short distance above the knee, where it pierces the deep, and becomes an occupant of the superficial, fascia. Thence it passes downward through the roof of the popliteal space and through the upper part of the calf, in the latter situation accompanying the external saphenous vein and inosculating with the external saphenous nerve. It rarely extends beyond the middle of the calf, tapering off into tiny threads which are distributed to the skjn of the posterior surface of the upper half or two thirds of the leg (Fig. 1125.) Branches of the small sciatic nerve are : fa) the inferior pudc ndal, (6) the gluteal> (V) \\\e femoral and (d^) the sural. a. The inferior pudendal or perineal branch (rr. perineales) (Fig. 1126) leaves the parent nerve at the lower margin of the gluteus maximus, curves mesially below the tuberosity of the ischium and over the origin of the hamstrings and courses through the groove between the thigh and the perineum. Piercing the deep fascia lateral to the pubic ramus, it enters the perineum and supplies the integument of the scrotum and base of the penis, or of the labium majus and clitoris. Branches are distributed to the skin of the upper mesial portion of the thigh and to the perineal body and anus. This nerve communicates with the ilio-inguinal nerve and with the perineal and inferior hemorrhoidal branches of the pudic nerve. It may pierce the great sacro-sciatic ligament. b. The gluteal cutaneous branches (rr. clunium inferiores) (Fig. 1124) consist of two, three or more stout filaments which arise from the small sciatic a short distance above the inferior margin of the gluteus maximus, around which they wind. Piercing the fascia lata individually they turn upward over the lower portion of the gluteus maximus and are dis- tributed to the skin of the inferior gluteal region, as far externally as the great trochanter and internally almost to the coccyx. The outer branches overlap the terminal twigs of the posterior branch of the external cutaneous nerve and the posterior primary divisions of the first, second and third lumbar nerves. The inner branches sometimes pierce the great sacro-sciatic liga- ment ; they reinforce or may replace the perforating cutaneous nerve. c. The femoral branches (Fig. 1124) consist of two series of twigs, an internal and an external, which pierce the fascia lata of the posterior aspect of the thigh and supply the integu- ment of that region. d. The sural branches (Fig. 1125) are usually two terminal twigs which innervate ^to a varying extent the integument of the back of the leg, sometimes not extending beyond the confines of the popliteal space and sometimes continuing all the way to the ankle. They inosculate with the external saphenous nerve, and when they are lacking their place is taken by the external saphenous. Variations. — In those cases in which the internal and external popliteal nerves are separate from their incipiency, the small sciatic also is double. The ventral portion accompanies the internal popliteal and gives off the inferior pudendal and internal femoral branches, while the dorsal portion accompanies the external popliteal and gives off the gluteal and external femoral branches. Sometimes the small sciatic is joined in the thigh by a branch from the great sciatic. 5. THE PUDIC NERVE. The pudic nerve (n. pudendus) arises from the front of the second, third and fourth sacral nerves, its main root, coming from the third and there being a doubtful root from the first. Leaving the pelvis by way of the great sacro-sciatic foramen between the pyriformis and the coccygeus and below the great sciatic nerve, it passes forward, with the internal pudic artery and the nerve to the obturator internus, over the base of the lesser sacro-sciatic ligament to the spine of the ischium (Fig. 1126). Reaching the small sacro-sciatic foramen internal to the internal pudic artery, the nerve traverses this opening and enters the ischio-rectal fossa, where it gives off the inferior hemorrhoidal nerve. The main trunk courses forward in a canal (Alcock's) in the obturator fascia on the outer wall of the ischio-rectal fossa 135° HUMAN ANATOMY. (Fig. 1126), at whose anterior portion the nerve approaches the base of the tri- angular ligament and divides into its terminal branches, the perineal and the dorsal nerve of the penis or chtoris. Branches of the pudic nerve are : (a) the inferior hemorrhoidal nerve , (£) the perineal nerve and (c) the dorsal nerve of the penis or clitoris. a. The inferior hemorrhoidal nerve (nif. hemorrhoidales inferiores) (Fig. 1127) is usually given off by the pudic upon entering the ischio-rectal fossa, but it may be derived directly from the plexus, its fibres being offshoots of the third and fourth sacral nerves. In company with the inferior hemorrhoidal vessels it passes mesially across the base of the ischio-rectal fossa toward FIG. 1126. Coccygeal nerves, posterior divisions Coccyx Perineal branch of IV. ante- rior sacral Anus Vulva Coccygeal nerve, anterior division P Cutaneous branches from loops of V. lumbar and I. II. and 111. sacral nerves, posterior divisions Branch of IV. sacral nerve (perforating cutaneous) Levator ani and anal fascia Pudic nerve Cut edge of obturatoi fascia Inferior hemor- rhoidal nerve Internal pudic artery Perineal division of pudic nerve Dorsal nerve ot clitoris Inferior pudendal Superficial dissection of right side of female perineum and adjacent region, showing cutaneous nerves ; obturator fascia has been partly removed to expose pudic nerve and accompanying blood-vesseis in canal on outer wall of ischio-rectal lossa. the anus, on approximating which it splits into a number of filaments, which supply the external sphincter and the integument of the anal region, and inosculate with the small sciatic, pudic and fourth sacral nerves. b. The perineal nerve (n. perinei) (Fig. 1126) is one of the terminal branches of the pudic and arises at the bifurcation of that nerve near the posterior margin of the triangular ligament. Soon after its origin it splits into : (a a ! a superficial and (bb] a deep branch. aa. The superficial branch is entirely sensory and consists of two parts, a lateral or posterior and a mesial or anterior. These pass forward toward the base of the scrotum in company with the superficial perineal vessels. The lateral, external or posterior branch courses along the lateral margin of the perineum, distributing twigs in this region and sometimes sending brandies to the inner aspect of the thigh and a tilametit to the origin of the ischio-cavernosus muscle (Schwalbe). The mesial, internal or anterior h>-'uuh is larger than the lateral and is more deeplv placed. It pierces the posterior margin of the triangular ligament and runs forward either beneath or through the trausversus perinei muscle. It splits into two or more branches (nn. scrotales vel laM.iles posterities ) which inosculate freely with each other and supply the integument of the scrotum or labium majus. They communicate with the pudendal branch of the .small sciatic nerve and with the inferior hemorrhoidal. THE PUDENDAL PLEXUS. bb. The deep branch of the perineal nerve is mainly muscular and consists of a single trunk which breaks up into several branches, whose main destination is the muscles of the perineum. Passing forward from the ischio-rectal fossa it enters the deep perineal interspace and sends filaments to the external sphincter ani, the levator ani, the transversus perinei, the ischio-cavernosus, the bulbo-cavernosus or sphincter vaginae and the compressor urethrae. One branch, the nerve to the bulb, accompanied by the artery of the same name, enters the bulb, supplying its tissue and that of the corpus spongiosum, and innervating the urethra as far forward as the glans penis. c. The dorsal nerve of the penis (n. dorsalis penis) (Fig. 1127) a terminal branch and the most deeply situated of all the branches of the pudic, accompanies the dorsal artery of the penis through the deep perineal interspace. It lies beneath the crus penis, the ischio-cavernosus muscle and the inferior layer of the triangular ligament and over the compressor urethra? FIG. 1127. Dorsal nerve of penis Crus penis, detached Ischio-cavernosus. detached Nerve to ischio- cavernosus Nerve to bulbo- cavernosus Nerve to bulb Nerve to traus- versus perinei Muscular br. of perineal division of pudic nerve Dorsal nerve of penis Cutaneous br. of perineal division of pudic nerve Pudic nerve Inferior hemor- rl oi lal nerve Sphincter an' externus Colles' fascia, reflected Crus penis and ischio-cavernosus Anterior (internal) superficial perineal nerve Inferior pudendal nerve Transversus perinei Posterior superfi- cial perineal nerve Dorsal nerve of penis Perineal division of pudic nerve, muscular portion Pudic nerve Inferior hemor- rhoidal nerve Cluteus maximus T"^™™*- From IV. sacral nerve Perforating cutaneous nerve and a branch of IV. sacral nerve Dissection of male perineum, showing distribution of pudic nerve; on left side of body Colics' fascia has been reflected to expose superficial perineal interspace ; dorsal nerve of penis is seen in deep interspace on right side. muscle. Piercing the inferior layer of the triangular ligament and the suspensory ligament of the penis it reaches the dorsum of the penis, along which it courses as far as the glans. It gives off the nerve to the corpus cavernosum, which pierces the triangular ligament and supplies the erectile tissue of the crus penis and corpus cavernosum. The main nerve innervates the anterior two thirds of the penis, including the glans, and sends off ventral branches which pass around to the under surface of the organ. The dorsal nerve of the clitoris (n. dorsalis clitoridis)(Fig. 1128), while much smaller than the dorsal nerve of the penis, has a corresponding course and distribution. The dorsal nerve of the penis or clitoris communicates with the inferior pudendal branch of the small sciatic. Variations. — The pudic may receive a root from the fifth lumbar, in the high form of plexus. A root from the fifth sacral is described by Henle. The inferior hemorrhoidal may pierce either the great or the small sacro-sciatic ligament, and the former of these ligaments may be perforated by the lateral superficial perineal nerve. 1352 HUMAN ANATOMY. THE COCCYGEAL PLEXUS. 6. The sacro-coccygeal nerves (nn. anococcygei) are derived from a small nerve inosculation called the coccygeal plexus (plexus coccygeus), a structure formed by the fifth sacral and the coccygeal nerve, with a contribution from the fourth sacral which descends over or through the great sacro-sciatic ligament. The fifth sacral, having been joined by this twig from the fourth, descends along the margin of the coccyx and is joined by the coccygeal nerve, the resulting nerve-bundle constituting the coccygeal plexus. From it arise minute filaments which pierce the great sacro- sciatic ligament and are distributed to the integument in the immediate neighbor- hood of the coccyx (Fig. 1084). Practical Considerations. — Of the branches of the sacral plexus, the great sciatic nerve is the most important, owing to its size, its extensive distribution and its exposed position. The greater part of the sacral plexus is continued into the FIG. 1128. Clans clitoridis Deep layer of triangular ligament Dorsal nerve of clitoris Superficial perineal Bulbo-cavernosus Ischio- cavern OS us Inferior pudenda) nerve Posterior superficial perineal nerve Anterior superficial perineal nerve Transversus perinei superficial Perineal division of pudic nerve Inferior hemorrhoidal nerve Sphincter ani Anal fascia Deep fascia of buttock Coccyx From IV. sacral nerve Coccypeus Dissection of female perineum, showing nerves; anal fascia in position on right side of body, removed on left; Colics' fascia removed on right side, exposing superficial perineal interspace; superior layer of triangular ligament, denuded of muscular tissue, seen on left side. nerve. Except in complete lc. ions of the spinal cord this nerve is rarely paralyzed in all its branches. The paralysis may result from fractures of the lumbar vertebrae, of the sacrum or of the innominate bone, frym pressure of tumors in the pelvis or of the child's head in labor or from the use of forceps. It is the structure in greatest 'lander in dislocation of the hip, since the head of the femur in the most frequent varieties sweeps backward against this nerve. In the reduction of these posterior dislocations the nerve has been hooked up by the head and made to pass across the front of the neck of the bone. From its close relation to the head and neck, it may be injured in violent movements of the hip joint without dislocation. It passes out of the pelvis through the greater sacro-sciatic foramen, below the pyrifonnis muscle, and after curving outward and downward under the gluteus maxi- mus muscle it continues its course, approximately, in a line from a point midway THE SYMPATHETIC SYSTEM OF NERVES. 1353 between the greater trochanter and the tuberosity of the ischium above to the middle of the popliteal space below. At about the junction of the middle and lower thirds of the thigh it divides into the internal and external popliteal nerves. Below the gluteus maximus muscle it is comparatively superficial, so that tenderness of the nerve, as from sciatica, is easily elicited by pressure. At the point where it emerges from under the gluteus maximus it is readily reached for operation. After a vertical in- cision through the skin and fascia at this level, the biceps muscle is exposed. The lower margin of the gluteus maximus is raised and the biceps drawn inward, when the nerve can be easily hooked up with the finger. Because of the great importance of this nerve to the lower extremity it is not advisable to excise or divide it as this would paralyze its whole area below. Stretching is the only justifiable operation, although the results obtained are often disappointing, and the operation may cause acute neuritis. According to Trombetta, it will require a tension equal to the weight of 183 Ibs. to break it, and it is more likely to yield at its attachment to the spinal cord than elsewhere. It should, therefore, tolerate a stretching force of from 100 to 1 60 Ibs. (Treves). A safe working rule is to use a force sufficient to raise the affected limb from the table, the patient lying in the prone position. It has been observed that when the paralysis is due to some pressure upon the nerves of the sacral plexus within the pelvis it is often confined to the peroneal or ex- ternal popliteal nerve, or is most marked in it. This has been explained by the fact that the fibres for the peroneal nerve lie close together directly on the pelvic bones, and are, therefore, particularly exposed to pressure. They arise for the most part from the lumbo-sacral cord, formed by the fourth and fifth lumbar and first sacral nerves, which lie directly on the innominate crest, the rest of the plexus lying on the pyriformis muscle. In paralysis of the external popliteal or peroneal nerve the extensors of the foot and toes, the tibialis anticus and the peronei muscles are involved. The foot hangs down from its own weight (foot drop), and turns in from paralysis of the peronei. In some cases the anterior tibial muscle escapes. In walking the knee must be un- duly flexed to prevent the toes from dragging on the ground and the arch of the foot is flattened from the loss of the support given to the arch by the peroneus longus. If sensation is disturbed it will be only to a slight extent over the anterior part of the leg about the shin, and outward from this on the dorsum of the foot and toes, but not at the sides of the foot. The peroneal nerve may be divided accidentally in a sub- cutaneous tenotomy of the biceps tendon for contraction at the knee, the nerve lying close to the inner border of the tendon. It may be injured by external violence, as it passes around the head and neck of the fibula, where if necessary, an incision will easily expose it ; or it may be injured by pressure, as in prolonged kneeling. In paralysis of the internal popliteal nerve all the other muscles of the leg, in- cluding the superficial and deep flexors, the tibialis posticus, the plantar muscles and interossei are affected. The patient cannot extend the ankle and therefore cannot stand on his toes. The toes cannot be flexed or moved sideways. Sensation is dis- turbed on the inner and posterior surface of the leg, the outer border of the foot, the sole and the plantar surface of the toes. In paralysis of the entire sciatic nerve the flexors of the knee also are involved, so that the patient cannot bring the heel toward the buttock. If only one sciatic is involved he can still walk by fixing the knee in extension, the whole limb being brought forward by the quadriceps extensor, which is supplied by the anterior crural nerve. THE SYMPATHETIC SYSTEM OF NERVES. The sympathetic portion (systema nervorum sympatheticum) of the peripheral nervous system differs from that already described — the spinal and the cranial nerves —in being particularly concerned in carrying efferent and afferent impulses to and from the thoracic and abdominal organs (collectively termed the splanchnic area}, in contrast to the great somatic ( skeletal) masses of voluntary muscle. Whilst the paths for the afferent or sensory impulses conducted from the splanchnic area differ in no important respect from those formed by the cerebro-spinal nerves, the efferent or motor paths are peculiar (a) in supplying the involuntary and cardiac muscle and 1354 HUMAN ANATOMY. FIG. 1129. the glandular tissue and (£) in consisting of at least two, sometimes of more, links be- tween the source of the impulse (the spinal cord) and the structure upon which it is expended. It is these interposed links that constitute the sympathetic elements proper — the sympathetic iit'itronrs. The cell-bodies of these neurones exhibit a marked disposition to become aggregated into larger or smaller collections, which constitute the innumerable ganglia that form a conspicuous feature of the sympathetic system, whilst their axones serve to connect the ganglia with the terminal structures (muscles or glands) or with other neurones. It is evident, therefore, that the sympathetic system consists of a complex of spinal and sym- pathetic fibres intermingled with groups of ganglion-cells. The latter are, for the most part, stellate in form and pro- vided with axones which, while often pursuing a long course as splanchnic efferenis, acquire only partially or not at all a medullary coat and hence may be classified usually as non- medullated fibres. Since the spinal fibres are provided with this covering, the bundles of such fibres present the whitish color distinguishing medullated strands, in contrast to the gray- ish tint of the strands of the nonmedullated sympathetic fila- ments. It is upon this histolog- ical variation of their predomi- nating fibres that the difference recognized in the white and gray rami communicantes, pres- ently to be described, depends. Although the supply of the thoracic, abdominal and pelvic organs constitutes an important part1 of the duty of the sympa- thetic nerves, it is by no means their entire concern, the inner- vatic >n of the involuntary muscle of the vessels and of the skin and the glands throughout the body being likewise their task. In order to meet their obliga- tions to the structures within the body cavities, the sympa- thetic nerves naturally follow the course of the blood-ve>sels, with the result that every artery of consequence within these re- gions is surrounded by a more or less elaborate net-work, these plexuses in most l>»Mring the names of the arteries which they accompany. In order to provide for the outlying tracts of involuntary muscle contained within the blood-vessels outside the body-cavities and within the skin, as well as for the glands, the sympathetic fibres join, by way of the gray rami communicantes, the somatic spinal nerves, which they accompany to all parts of the body. For this reason the peripheral somatic nerve-trunks coiuain three varieties of fibres — afferent and efferent spinal and efferent sympathetic. Diagram showing constitution of sympathetic system ; spinal efferents are black ; sympathetic efferents are red ; i\ mpattietic (vis- cenl) afbrentt are blue; SC, spiiuil ond; -IA', /'A', anterior and posterior root of spinal nerve ; SG, spinal ganglion ; AD, /^anterior and posterior primary divisions; It 'A', (,'A\ white and gray rami communicantes. CC, gangliated cord ; SyG, sympathetic ganglia; CG, cervical sympathetic ganglion; PvG, StibG', Tr(,'. prevertehval, subsidiary and terminal ganglia; Spfy, splanchnic efferents; SoEf, somatic efferents ; V, vessels of the spinal meninges; /.intestine. THE SYMPATHETIC SYSTEM OF NERVES. 1355 Constitution and General Arrangement. — The sympathetic system serves to receive, rearrange and distribute the visceral filaments of the cerebro-spinal nerves, FIG. 1130. Common carotid artery Vagus nerve Superior cervical cardiac branch of vagui Scalenus anticus Gray rain us communicans to VI 1 1. cervical nerve VIII. cervical nerve I. thoracic nerve I. rib III. thoracic nerve IV. thoracic ganglion Kami communicantes XII. thoracic ganglion Branch to I. lumbar ganglion - Hyoid bone Interganglionic cord of sympathetic Thyroid cartilage [sympathetic Superior cervical cardiac branch of Middle cervical ganglion Crico-thyroid muscle Inferior cervical ganglion I. thoracic ganglion Right recurrent laryngeal nerve Combined cervical cardiac branches Right vagus [of sympathetic Inferior cervical cardiac branch of vagus Left middle and inferior cervical cardiacs of sympathetic Trachea Right bronchus CEsophagus Vena azygos Great splanchnic Aorta Left vagus nerve Diaphragm Dissection showing ngui ganghateci cord 01 sympathetic and its branches. and to complete, by the interposition of one or more of its especial neurones, the path for the impulses brought by such fibres to the objective organs. It comprises 1356 HUMAN ANATOMY. two principal parts, the gangliated cords and the plexuses, with their associated ganglia. The gangliated cord (truncus sympatheticus), one of a symmetrically placed pair of gangliated trunks situated anterior or lateral to the bodies of the vertebrae (Fig. 1133), begins in the head and extends through the neck, thorax and abdo- men to the lower portion of the pelvis. In the head it consists of a plexus of fibres continued up from the neck in an intricate interlacement which follows the internal carotid artery ; and in the pelvis it terminates by the two cords forming a loop or fine inosculation, situated anterior to the coccyx and containing the coccygeal ganglion or ganglion impar. The plexuses (plexus sympathetic!) are a series of more or less distinct col- lections of groups of nerve-cells (ganglia) and fibres, situated mainly in the axial line and giving off and receiving fibres connected with the various viscera of the trunk. The component elements of the plexuses and, indeed, of the entire sympathetic system, are the ganglia and the nerve-fibres. The ganglia, whilst following a general plan of arrangement as to number, size and position, are subject to wide individual variations and, moreover, where they approach a segmental type, as in the gangliated cord, there is considerable deviation from the arrangement presented by the cerebro-spinal system. A ganglion may or may not be connected with a spinal nerve, but it is always linked by association cords with other ganglia. According to their position, three varieties of ganglia are recognized. One group includes the prevertebral ganglia (g. trunci sym- pathetic!), those found as nodes in the gangliated cord ; a second variety comprises the collateral or intermediate ganglia (g. plexuum sympatheticorum), which lie either on the peripheral branches of the gangliated cord or in a prevertebral plexus ; whilst to the third set belong the innumerable minute terminal ganglia, composed of nerve-cells which lie at or near the visceral distributions of the sympa- thetic fibres. Each ganglion consists of an indefinite number of multipolar neurones, whLh possess one axone and a number of dendrites, the whole cluster of cells being enclosed in an envelope of fibrous tissue. The axone is often medullated in the immediate vicinity of its cell, but usually loses this sheath as it gets farther and farther away from its origin. The course taken by the axone of a prevertebral gang- lion-cell may be one of three : (i) it may pass by means of an association cord into an adjoining prevertebral ganglion, (2) it may proceed as a constituent of a gray ramus communicans to join a spinal nerve or (3) it may follow a splanchnic efferent toward a viscus. The nerve-fibres encountered within the sympathetic system include two sets : (a) those derived from the cerebro-spinal system, which are usually medullated, and (£) the sympathetic fibres proper, for the most part nonmedullated, although as stated above, many of the axones possess a medullary sheath for a short distance beyond their origin from the nerve-cell. This distinction between medullated and nonmedullated fibres is, however, somewhat indefinite, since the medullated spinal fibres often become nonmedullated before terminating, whilst the sympathetic fibres occasionally are medullated throughout their course. Kami Communicantes. — Where the typical segmental arrangement prevails, as in the thoracic region, each spinal nerve is connected with the adjacent gangliated cord by a pair of short nerve-trunks, known as the rami communicantes ( Fig. 1 129). These are divided into two groups, the white rami and \hz gray rami, a distinction depending primarily upon the difference in the appearance of the strands when seen in the fresh condition ; this distinction, moreover, corresponds with the histological difference above noted — white rami appearing so in consequence of the prepon- derance of opaque medullated fibres, and the gray rami possessing the darker tint on accourit of the absence of the refracting myelin coat. The rami communicantes pass directly between the spinal nerves and the gangliated cord, in relation to the latter joining either a ganglion or an association cord between nodes. The white rami communicantes are composed almost exclusivefy of the visceral branches of certain of the spinal nerves which use the sympathetic system as the pathway by which they arrive at their destination. They consist of fasciculi of THE SYMPATHETIC SYSTEM OF NERVES. 1357 medullated nerve-fibres derived from both the anterior and the posterior roots of the spinal nerves. The fibres arising from the anterior root are called the splanch- nic efferent fibres and those from the posterior root the splanchnic afferent. Not all of the spinal nerves, however, give off white rami, these strands of communication forming a thoraco-htmbar groiip, from the first or second thoracic to the second or third lumbar nerve inclusive, and a sacral group, derived from the second and third, or third and fourth sacral nerves. The cervical nerves do not give off white rami. The splanchnic efferent fibres are the axones of cells located within the lateral horn of the gray matter of the spinal cord. They furnish motor impulses to the unstriped muscle of the vessels and viscera, and secretory ones to the glands of the splanchnic area ; they also convey motor impulses to the heart. Leaving the spinal cord by way of the anterior root, they pass peripherally, enter a white ramus communicans and reach the gangliated cord. One of three courses is then pursued by these fibres : (l) they may end at once by forming arborizations around cells in the ganglion which they first enter, (2) they may pass through this ganglion, thence up or down through an association cord to end around the cells of a node of the gangliated cord above or below the level of entrance, or (3) they may course through the gangliated cord and one of its visceral branches, and terminate in arborizations around the cells of a prevertebral or of a collateral ganglion. It is possible that in some cases the spinal efferents may continue without interruption through the several divisions of its path as far as the terminal ganglia. The path connecting the spinal cord with the involuntary muscle always consists of two fibres, the preganglionic and postganglionic. The latter is the axone of the sympathetic neurone and always forms the last link of the path carrying the stimulus to the involuntary muscle. The splanchnic afferent fibres are the sensory fibres of the splanchnic area and consist of the dendrites of cells situated within the intervertebral ganglia on the pos- terior roots of the spinal nerves. Whilst the greater number of these fibres are found in the white rami, a few are thought to be constituents of the gray rami. Beginning in the viscera, they run centrally, without interruption, through the terminal and collateral ganglia, through the gangliated cord and the white (or gray) rami to the spinal nerve, and thence after coming into relation with the cells of the ganglion of the posterior root, they pass by way of the posterior roots into the spinal cord. The gray rami communicantes are bundles of axones of sympathetic neu- rones which pass from the gangliated cord to each one of the entire series of spinal nerves. The reason of this generous provision will be evident when the purpose of the communications effected by the gray rami is recalled, namely, to provide sympa- thetic filaments to the outlying muscles and glands by way of the convenient path afforded by the distribution of the somatic nerves. Mingled with the gray fibres, a few of the medullated variety are often encountered ; these are probably partly splanchnic afferent fibres and partly medullated sympathetic fibres. Variation in the origin of the gray rami from the gangliated cord is not uncommon ; they may arise either from a ganglion or from the association cord between two ganglia ; after leaving the gangliated cord, a single ramus may divide and supply two spinal nerves ; or the reverse may happen, two or more rami arising independently and either separately or after fusing, joining a single spinal nerve. The further course of the sympathetic fibres, after having joined the spinal nerves by way of the gray rami, is as follows : (i) they may course peripherally along with the anterior or posterior primary divisions of the spinal nerve and convey vasomotor, pilomotor or secretory impulses to the involuntary muscle and glands of the somatic area ; or (2) they may enter the spinal canal by way of the anterior or posterior nerve-roots and be distributed to the spinal meninges, but not to the nervous column. According to Dogiel, it is probable that a small number of axones of sympathetic t neurones enter the root-ganglia of the spinal nerves to end in arborizations around cells of the ganglia. The association cords (Fig. 1130) are the longitudinally disposed bundles of fibres comprising the interganglionic portion of the gangliated cord ; they contain both white and gray fibres. The gray ones are the axones of sympathetic neurones which are either passing between adjacent or more remote ganglia, or taking an upward or 1358 HUMAN ANATOMY. downward course before passing distally to their ultimate splanchnic distribution. The white fibres are either spinal splanchnic efferent or afferent fibres. The branches of distribution from the gangliated cord include the somatic and the visceral. The somatic branches are the gray rami communicantes ; the visceral branches comprise the splanchnic efferents, which consist of both white and gray efferent fibres, as well as the white splanchnic afferents. THE CERVICO-CEPHALIC PORTION OF THE GANGLIATED CORD. The cervico-cephalic portion of the gangliated cord (pars cephalica et cervicalis systematis sympathetic!) consists of a series of ganglia, usually three, but often only two, connected by composite association cords (Fig. 1131). It lies posterior to the FIG. 1131. Lower head of external pterygoid muscle Internal pterygoid muscle Auriculo-temporal nerve Internal carotid artery Pneumogastric nerve. Inferior dental ne Spinal accessory nerv Part of facial nerv Hypoglossal nerv Stylo-pharyngeus muscl Glosso-pharyngeal ner I. cervical nerv Pneumogastric nerv Superior cervical ganglion o sympathetic Superior laryngeal nerv Descendens hypoglossi II. cervical nerve III. cervical nerve IV. cervical nerve erganplionic associai cord of sym)>athetic Middle cervical ganglion Branch to I. thoracic gttgl Inferior cervical cardiac of sympathetic' Recurrent laryngeal nerve Internal mammary artery Cartilage of I. ri' Clavicular facet of sternum Ungual nerve External laryngeal branch Superior cervical cardiac of alietic Middle cervical cardiac of sympathetic -Pneumogastric nerve Middle cervical cardiacoi [pneumogastrlc irtery i . ical cardiac of pneumugastric Deep dissection of neck, showing cervical portion of sympathetic gangliated cord and its connections carotid sheath and anterior to the prevertebral fascia and the rectus capitis anticus major and scalenus anticus muscles. Inferiorly it is continued into the thoracic portion of the gangliated cord, and superiorly, at the base of the skull, it forms an intricate plexus around the internal carotid artery, in whose company it enters the THE SYMPATHETIC SYSTEM OF NERVES. 1359 cranium. The small ganglia connected with the trigeminal nerve — the ciliary, the spheno-palatine, the otic and the submaxillary — are regarded as outlying nodes be- longing to the cephalic continuation of the gangliated cord. The dominant characteristic of this portion is the absence of white rami, the spinal fibres present reaching the cervical region from the upper thoracic nerves by way of the association cord between the highest thoracic and lowest cervical gang- lion, around whose cells, as well as those of the higher cervical ganglia, the processes of the spinal neurones end. The distribution of the cervical portion of the cord includes pupillo-dilator fibres, cardio-accelerator fibres, vasomotor fibres to the arteries of the head, neck and upper extremities, pilomotor fibres to the integument of the head and neck, motor fibres to the involuntary muscles of the orbit and eyelids and secretory fibres to the glands. The branches consist, as elsewhere, of two groups, somatic and visceral, the former reaching their area of distribution by way of certain cranial and spinal nerves, and the latter, either alone or in conjunction with other nerves, forming plexuses which accompany blood-vessels and supply various viscera and vessels of the head, neck and thorax. The ganglia of the cervical portion include a superior, a middle and an inferior. The Superior Cervical Ganglion. — The superior cervical ganglion (g. cervi' cale superius) (Fig. 1077) is the largest of the entire sympathetic series, measuring 2-3 cm. in length and 4-6 mm. in width. It rests posteriorly on the rectus capitis anticus major muscle opposite the second and third cervical vertebrae, with the internal carotid artery anterior to it and the vagus nerve to its lateral aspect. With the typical reddish-gray hue of the sympathetic ganglia, it is fusiform in outline, although it may present constrictions, usually three, which indicate its composition of four fused ganglia. The somatic branches consist of (i) rami communicantes and (2) some of the communicating branches to the cranial nerves. 1. The rami communicantes consist of four gray rami which join the anterior primary divisions of the first four cervical nerves. 2. The communicating branches to the cranial nerves are given off from the upper portion of the ganglion, (i) one joining the petrous ganglion of the glosso- pharyngeal, (2) others entering the ganglia of the root and trunk of the vagus and (3) another joining the hypoglossal nerve. In addition to these there is frequently given off from the lower portion of the ganglion (4) a branch which joins the exter- nal laryngeal nerve. The visceral branches comprise : (i) the pharyngeal , (2) the superior cervi- cal cardiac , (3) the vascular and (4) the vertebral. 1. The pharyngeal branch or branches (rr. laryngopharyngei) arises from the antero-mesial aspect of the ganglion and courses obliquely inward and downward posterior to the carotid sheath to reach the surface of the middle constrictor of the pharynx. Here it unites with the pharyngeal branches of the glosso-pharyngeal and vagus nerves to form the pharyngeal plexus (page 1269), from which fibres are distributed to the muscles and mucous membrane of the pharynx, a few filaments pining the superior and external laryngeal nerves. 2. The superior cervical cardiac nerve (n. cardiacus superior) (Fig. 1131) arises as two or three twigs from the ganglion, with sometimes an additional filament from the association cord between the superior and middle ganglia. It courses down- ward anterior to the longus colli muscle in the posterior part of the carotid sheath, crosses the anterior or the posterior surface of the inferior thyroid artery, and then descends in front of the inferior laryngeal nerve. At the base of the neck the course of the nerve begins to differ on the two sides. The right nerve enters the thorax either anterior or posterior to the subclavian artery and accompanies the innominate artery to the aorta, where it enters the deep cardiac plexus, a few fibres passing to the anterior surface of the aorta. On the way down a few twigs join the inferior thyroid artery and with it enter and supply the substance of the thyroid body. The left nerve upon entering the thorax joins the common carotid artery, along whose lateral and anterior surfaces it courses to the aorta, upon reaching which it 1360 HUMAN ANATOMY. joins the superficial cardiac plexus. In some instances the nerve remains behind the carotid artery and joins the deep cardiac plexus. A pretracheal branch, derived from the loop between the superior cervical cardiac nerve and the inferior laryngeal, descends anterior to the trachea and is dis- tributed to the pericardium and the anterior pulmonary plexus (Drobnik. ) The superior cervical cardiac nerve communicates freely in the neck with the middle cardiac and other branches of the sympathetic, and with the external laryngeal and superior cervical cardiac branches of the vagus. In the thorax it inosculates with the inferior laryngeal nerve. Variations. — The superior, as well as the other cardiac nerves, presents a considerable degree of variation, sometimes to so grea; an extent as to show no resemblance to the accepted typical plan of arrangement. It is sometimes absent, especially on the right side, and in such event appears to be replaced by a branch from the vagus or from the external laryngeal nerve. It may have no independent course, but join one of the other sympathetic cardiac nerves and reach its destination as a part of the latter. 3. The vascular branches comprise plexiform nerve-structures which accom- pany the terminal divisions of the common carotid artery. They consist of : (a) the external carotid branch and (b) the internal carotid branch. a. The external carotid branch (n. caroticus externus) (Fig. 1061) joins the external carotid artery and furnishes subsidiary plexuses which accompany the branches of that vessel. In addition to supplying vasomotor fibres to the external carotid tree, sympathetic filaments are furnished to two of the ganglia of the trigem- inal nerve. A branch (radix g. submaxillaris) from the plexus on the facial artery (plexus maxillaris externus) joins the submaxillary ganglion as its sympathetic root, and one or more, the smallest deep petrosal nerve, from the plexus on the middle meningeal artery (plexus meningcus), forms the sympathetic root of the otic ganglion. Ganglia of microscopic size have been described on these vascular plexuses. The most important of these, the temporal ganglion, is situated on the external carotid at the point of origin of the posterior auricular artery and is said to receive a filament of communication from the stylo-hyoid branch of the facial nerve. b. The internal carotid branch (n. caroticus interims) is apparently an upward, cranial extension of the superior ganglion (Fig. 1061). Ascending beneath the internal carotid artery, it accompanies that vessel into the carotid canal, where it divides into two plexuses, the carotid and the cavernous, the former ramifying on the lateral and the latter on the mesial aspect of the artery. While the individuality of these two is distinct, there are numerous fine fibres connecting them as they pass upward into the cranium. The carotid plexus (plexus caroticus internus) is located on the lateral or outer surface of the internal carotid artery at its second bend. In addition to supplying fine plexuses which accompany the branches of the artery to their ultimate ramifica- tions, the following arise from the carotid plexus : (aa~) the carotid brandies, (bb) the communicating branch to the abducent nerve, (cc) the communicating branches to the Gasserian ganglion, {dd ) the great deep petrosal nerve and (ee} the small deep petrosal nerve. aa. The carotid branches consist of numerous fine twigs which are supplied to the internal carotid artery. fib. The communicating branch to the abducent nerve consists of one or two twigs which join the nerve as it lies in the wall of the cavernous sinus in close proximity to the internal carotid artery. cc. The communicating branches to the Gasserian ganglion comprise several small fila- ments which pass to tin- ganglion ; they usually arise from the carotid but sometimes are derived from the cavernous plexus. dd. The great deep petrosal nerve courses forward to the posterior end of the Vidian canal, where it joins the great superficial petrosal to form the Vidian nerve (page 1059), finally en- tering Meckel's ganglion as its sympathetic root. ee. The small deep petrosal nerve or n. carotico-tympanieus joins the tympanic plexus (page 1075), a structure formed by the tympanic branch of the glosso-pharyngeal, a filament from the geniculate ganglion of the facial nerve and the small deep petrosal nerve. In addition THE SYMPATHETIC SYSTEM OF NERVES. 1361 to furnishing twigs to the mucous membrane of the middle ear and vicinity, this plexus con- tributes a large part of the small superficial petrosal nerve, which joins the otic ganglion as its sensory root (page 1246). The cavernous plexus (plexus cavernosus) lies inferior and internal to the internal carotid artery and in intimate relation with the cavernous sinus. Its branches are: (aa) the carotid branches, (66) the communicating branch to the oculo- motor nerve, (cc~) the communicating branch to the trochlear nerve, (dd") the com- municating branch to the ophthalmic division of the trigeminus nerve, {ee) a branch to the ciliary ganglion and {ff) branches to the pitidtary body. FIG. 1132. Superior cervical cardiac branch ot sympathet Sympathetic association cord Right vagus m Middle cervical ganglion Inferior cervical ganglion luperior cervical cardiac of vagus fiddle and inf. cervical cardiac branches of sympathetic Re nt la al nerve Pulmonary branch of vagus Vena azygos major Phrenic nerve Right pulmonary artery Right auricular appendix Pericardium Superior cervical cardiac branch of sympathetic Superior cervical cardiac branch of vagus Middle cervical ganglion Middle cervical cardiac branch of sympathetic fof sympathetic Inf. cervical cardiac branch Inf. cervical ganglion Middle cervical cardiac branch of vagus Inf. cervical cardiac branch of vagus Phrenic nerve Left vagus nerve Recurrent laryngeal nerve Left pulmonary artery Pulmonary veins Pul .mnary orifice Mesial surface of lung Pericardium \ Dissection showing cardiac branches of pneumogastric nerves and of sympathetic cords; aortic arch and branches and pulmonary arlery partially removed ; pericardium laid open. aa. The carotid branches are distributed to the internal carotid artery. bb. The communicating branch to the oculomotor nerve joins the latter about at the point where it breaks up into its superior and inferior divisions. cc. The communicating branch to the trochlear nerve, sometimes derived from the carotid plexus, joins the trochlear in the wall of the cavernous sinus. dd. The communicating branch to the ophthalmic division of the trigeminus nerve joins the mesial surface of that nerve. ee The branch to the ciliary ganglion (radices sympatheticae g. ciliaris) arises in tl cranium and enters the orbit through the sphenoidal fissure, either as an independent structure or jointly with the nasal or with the oculomotor nerve. As the sympathetic root (radix media;, it enters the upper posterior angle of the ciliary ganglion (Fig. 1058), either alone or as a common trunk with the sensory root. 86 1362 HUMAN ANATOMY. ff. The branches to the pituitary body consist of several tiny filaments which enter the substance of that body. 4. The vertebral branches consist of two or three filaments which pass backward, pierce the prevertebral muscles and are distributed to the bony and liga- mentous structures of the upper portion of the vertebral column. The Middle Cervical Ganglion. — The middle cervical ganglion (g. cervicale medium), a structure not infrequently absent, consists of one or two collections of nerve-cells situated posterior to the carotid sheath in the neighborhood of the inferior thyroid artery (Fig. 1131). It lies about the level of the sixth cervical vertebra and represents the fusion of two primitive cervical ganglia. The somatic branches are : (i) the gray rami communicantes and (2) the subclavian loop. 1. The gray rami communicantes arise either from the ganglion or from its upper or lower association cord. They consist of two trunks which pass backward and join the anterior primary divisions of the fifth and sixth cervical nerves. 2. The subclavian loop (ansa subclavia [Yieussenii] ) is a nerve, frequently double, which passes over the subclavian artery and joins the inferior cervical gang- lion sending twigs (plexus subclavius) to the subclavian artery and its branches and to the phrenic nerve. The visceral branches are: (i) the thyroid plexus and (2) the middle cervical cardiac nerve. In case of absence of the middle cervical ganglion, these branches arise from the interganglionic association cord between the superior and inferior ganglia. 1. The thyroid plexus (plexus thyreoideus inferior) consists of several fine inosculating twigs which accompany the inferior thyroid artery into the substance of the thyroid body. 2. The middle cervical cardiac nerve (n. cardiacus medius) (Fig. 1131) differs in its course on the two sides of the body. Descending in the neck, where it inosculates with the superior cervical cardiac and inferior laryngeal nerves, it passes, on the right side, either anterior or posterior to the subclavian artery, to the front of the trachea where it receives filaments of inosculation from the inferior laryngeal nerve. On the left side it enters the thorax between the common carotid and subclavian arteries. On both right and left sides it terminates posterior to the arch of the aorta by entering corresponding sides of the deep cardiac plexus. Variations. — The gangliated cord, in the region of the middle ganglion, may lie posterior to the inferior thyroid artery or may be bifurcated, the artery lying between the two portions. The Inferior Cervical Ganglion. — The inferior cervical ganglion (g. cervicale infenus) (Fig. 1079) is situated at the root of the neck, over the first costo-central articulation, between the neck of the first rib and the transverse process of the seventh cervical vertebra. In shape it is irregular, being flat, round or cres- centic, and it is often fused with or only partially separated from the first thoracic ganglion. Situated in the external angle between the subclavian and vertebral arteries it is usually connected above with the middle ganglion by an association cord and by the subclavian loop, the former, passing posterior to the vertebral artery, but sometimes, especially on the left side, forming a nervous ring around that vessel. The somatic branches consist of: (i) the gray rami communicantes, (2) the subclavian loop and (3) a communicating branch to the inferior laryngeal ncrrc. 1. The gray rami communicantes consist of two nonmedullated trunks which join the anterior primary divisions of the seventh and eighth cervical nerves. 2. The subclavian loop (ansa subclavia [Vietissenii] ) has already been de- scribed, as a branch of the middle cervical ganglion. 3. The- communicating branch to the inferior laryngeal nerve frequently accompanies the inferior cervical cardiac nerve ; it joins the inferior laryngeal pos- trrii>r to tin- subclavian artery. The visceral branches comprise : ( i) the vertebral plexits m& (2) the inferior cervical cardiac nerve. THE SYMPATHETIC SYSTEM OF NERVES. 1363 i. The vertebral plexus (plexus vertebralis) is a closely woven net-work of fibres which follows the course and distribution of the vertebral artery in the neck and cranium. FIG. 1133. I. rib II. thoracic nerve Intercostal artery IIL thoracic nerve Intercostal artery V. thoracic nerve XI. thoracic ganglion ; immedi- ately below it is the XII. Ilio-hypogastric nerve II. and III. lumbar ganglia, fused Ilio-inguinal nerve IV. lumbar ganglion IV. lumoar nerve V. lumbar ganglion Interganglionic association cord I. sacral ganglion Anterior crural nerve II. sacral ganglion III. sacral nerve IV. sacral ganglion XII. rib Diaphragm I. lumbar ganglion I. thoracic ganglion partially blended with inferior cervical ganglion II. thoracic ganglion Aorta Great splanchnic nerve Small splanchnic nerve Least splanchnic nerve Semilunar ganglion and solar plexus Dissection showing thoracic, lumbar and sacral portions of right gangliated cord and their branches. 2. The inferior cervical cardiac nerve (n. cardiacus inferior) (Fig. 1132), sometimes arising from the first thoracic ganglion, descends in the thorax posterior to 1364 HUMAN ANATOMY. the subclavain artery, inosculates with the middle cervical cardiac and inferior laryngeal nerves and terminates in the deep cardiac plexus. THE THORACIC PORTION OF THE GANGLIATED CORD. The thoracic portion of the gangliated cord (pars thoracalis systematis sympa- thetici) consists of a series of eleven, twelve, ten or even fewer irregularly triangular, fusiform or oval ganglia (gg. thoracalia), situated lateral to the bodies of the thoracic vertebrae, covered by parietal pleura and interconnected by association cords which lie anterior to the intercostal blood-vessels (Fig. 1133). The largest of the ganglia is the first, which is situated at the mesial end of the first intercostal space and is not infrequently fused with the inferior cervical ganglion. The location of the thoracic ganglia corresponds usually to the heads of the ribs, the lowest being placed anterior to the head of the twelfth rib and at the upper margin of the twelfth thoracic vertebra. A characteristic of the thoracic ganglia is the almost unvarying presence of white rami communicantes ', all of the series, with the possible exception of the first, receiving these rami from the thoracic spinal nerves. They consist of an zipper and a lower series, the former coming from the upper five nerves and coursing head-ward to enter and be distributed mainly by way of the cervico-cephalic portion of the gangliated cord ; and the lower arising from the lower seven and being distributed to certain thoracic and abdominal structures. As elsewhere, so here from each of the ganglia is given off a gray ramus communicans to a thoracic spinal nerve. The somatic branches of the thoracic portion of the gangliated cord are chiefly the gray rami communicantes. These arise from each of the thoracic ganglia and, in close proximity to the white rami, pass backward and join the anterior pri- mary divisions of all the thoracic spinal nerves. The visceral branches arise from the ganglia and their association cords and consist of gray splanchnic efferent and white splanchnic efferent and afferent fibres. The splanchnic afferent fibres have no sympathetic connections, and consist merely of tracts which carry impulses from the splanchnic area through the thoracic and spinal ganglia to the posterior roots of the spinal thoracic nerves. The splanchnic efferent fibres, after passing through the gangliated cord or its peripheral branches, form links with the cells of the collateral or terminal ganglia, from which nonmedullated axones are derived for the supply of various visceral or vascular structures. Those of the upper series are distributed mainly as branches of the cervical ganglia; while those of the lower series, from the sixth to the twelfth thoracic nerves inclusive, in the thorax supply the aorta and lungs with vasomotor fibres. Below the thorax their distribution is quite extensive, including, in conjunction with the vagus, viscero-inhibitory fibres for the stomach and intestine, motor fibres for a portion of the circular muscle of the rectum, vasomotor fibres for the abdominal aorta and its branches and secretory and sensory fibres for the abdominal viscera. The thoracic gangliated cord is peculiar in containing, along with the visceral fibres dis- tributed by its splanchnic efferents, many efferents proceeding from the spinal cord destined for regions supplied by way of the limb nerves arising from the cervical and lumbo-sacral segments of the spinal cord. In order to provide gray rami at appro- priate levels to join the spinal nerves the spinal efferents course both up and down in the gangliated cord beyond the thoracic region. In this manner the thoracic nerves, in addition to giving off the splanchnic efferents, provide vasomotor, pilo- motor and secretory filaments for the greater part of the lower half of the body. The visceral branches comprise : (i) the pulmonary branches, (2) the aortic branches and (3) the splanchnic nerves. 1. The pulmonary branches (IT. pulmoualcs) are derived from the second, third and fourth ganglia and proceed forward to join the posterior pulmonary plexus. 2. The aortic branches arise from the upper four or five ganglia and, after furnishing a few fine tui^s to the vertebras and their ligaments, inosculate around the thoracic aorta in the form of a fine plexus (plexus anrtictis thoracalis). 3. The splanchnic nerves (nn. splanchnici) (Fig. 1133) are three trunks which arise from the lower part of the; thoracic cord and are distributed to structures situated in the abdominal cavitv. THE SYMPATHETIC SYSTEM OF NERVES. 1365 The great splanchnic nerve (n. splanchnicus major) arises by a series of roots rrom the gangliated cord from the fifth to the ninth ganglia inclusive. Descending along the antero-lateral aspect of the vertebral column, this nerve pierces the crus of the diaphragm and enters the upper end of the semilunar ganglion, some of its fibres being traceable to the suprarenal body and the renal plexus. In the thoracic portion of its course is developed \hzgreat splanchnic ganglion (g. splanchnicum) from I366 HUMAN ANATOMY. which, as well as from the nerve itself, are given off filaments for the supply of the oesophagus, the thoracic aorta and the vertebrae. Sometimes in the thorax it is divided and forms a plexus with the small splanchnic and in this event several small ganglia are present. This nerve consists mainly (four-fifths, according to Rudinger) of medullated fibres, which are direct continuations of white rami from as far up as the third thoracic nerve or even higher. The small splanchnic nerve (n. splanchnicus minor) arises from the ninth and tenth, or tenth and eleventh ganglia or from adjacent portions of interganglionic cords. Entering the abdomen by piercing the cms of the diaphragm either in association with or in close proximity to the great splanchnic, it terminates in that portion of the semi- lunar ganglion called the aortico-renal ganglion. The least splanchnic nerve (n. splanchnicus imus) arises from the lowest of the thoracic ganglia and may receive a filament from the small splanchnic, from which it occasionally takes origin. Piercing the diaphragm in company with the gangli- ated cord it terminates in the renal plexus. A fourth splanchnic nerve is rarely present. It is described by Wrisberg as having been found in eight cadavers out of a large number examined. It is formed by filaments from the cardiac nerves, aided by twigs from the lower cervical and upper thoracic ganglia. THE LUMBAR PORTION OF THE GANGLIATED CORD. The lumbar portion of the gangliated cord (pars abdominalis systematis sympa- thetic!) (Fig. 1 134) consists usually of four small oval ganglia connected by association cords. There may be a decided increase in the number of the ganglia, as many as eight having been found, and, on the other hand, occasionally there are fewer than four, there being under these circumstances a compensatory increase in the size of the ganglia present. The lumbar portion of the sympathetic lies nearer the median line than does the thoracic, the cords being placed anterior to the bodies of the lumbar vertebrae and the lumbar vessels, along the mesial border of the psoas magnus, on the left side being partially concealed by the aorta and on the right by the inferior vena cava. It is connected with the thoracic portion by a small association cord, which passes either through or posterior to the diaphragm, and with the sacral portion by a cord which descends behind the common iliac artery. White rami communi- cantes are received from the first, the second and sometimes the third lumbar nerve, additional white fibres being derived from the lower thoracic nerves by way of the gangliated cord. The somatic branches comprise the peripheral distribution of the gray rami communicantes. These are the longest to be found in the body, on account of the distance between the ganglia and the intervertebral foramina. They accompany the lumbar vessels and pass beneath the fibrous arches from which the psoas magnus takes origin. 1. The white rami communicantes are derived from the upper two or three lumbar nerves and join the upper ganglia or the adjacent portion of the inter- ganglionic cord. They contain splanchnic efferent and afferent fibres, which continue downward the distribution of the thoracic portion of the gangliated cord, including vasomotor and secretory fibres for the lower extremities, pilomotor fibres, vaso- motor fibres for the abdominal vessels, motor fibres for the circular musculature of the rectum and inhibitory fibres for the longitudinal muscle of the rectum. Fibres peculiar to the lumbar region include vasomotor nerves of the penis and motor fibres for the bladder and uterus, those to the bladder supplying the sphincter as well as the circular and longitudinal muscle-fibres, those to the last-mentioned group being inhibitory. 2. The gray rami communicantes are irregular in number and arrange- ment, sometimes a single one. dividing and joining two lumbar nerves and sometimes two to five passing to a single spinal nerve. The visceral branches vary considerably in their distribution, some joining the hypogastrir plexus ( plexus hypotfastiicus ), others the aortic plexus ( plexus aorticus ulMlomiiialis) and still others supplying the vertebrae and their ligaments. THE SYMPATHETIC SYSTEM OF NERVES. 1367 THE SACRAL PORTION OF THE GANGLIATED CORD. The sacral portion of the gangliated cord (pars pelvina systematis sympathetic!) consists of four ganglia interconnected by association cords, there being a consider- able degree of variation in both the number and the size of the ganglia (Fig. 1133). Lying anterior to the sacrum and internal to the anterior sacral foramina, it is con- nected above with the lumbar portion by a single or double association cord which lies posterior to the common iliac artery, and below it gradually approaches the median line and is united in front of the coccyx with its fellow of the opposite side by a loop or fine plexus in which is situated the single coccygeal ganglion or gang- lion impar. While this portion of the gangliated cord receives no white rami communicantes, in the sense of trunks passing from the sacral spinal nerves to the sacral ganglia, the visceral branches of the pudendal plexus pass directly to the pelvic plexus without traversing ganglia, and are considered as being homologous with white rami. In addition to these, white fibres reach the sacral from the lumbar portion of the gangliated cord. The somatic branches are the gray rami communicantes. They arise from the sacral ganglia and pass dorsally to join the anterior primary divisions of the sacral and coccygeal spinal nerves. The visceral branches are distributed through the medium of the pelvic plexus (page 1374) and furnish motor fibres to the longitudinal and inhibitory fibres to the circular musculature of the rectum, the chief motor fibres to the bladder (probably to the longitudinal muscular fibres), motor fibres to the uterus, the nervi erigentes or vaso-dilators of the penis and secretory fibres to the prostate gland. Additional strands, the parietal branches unite and ramify, anterior to the sacrum, with similar twigs from the opposite side and furnish filaments to the sacrum and coccyx and their ligaments, and to the coccygeal body. THE PLEXUSES OF THE SYMPATHETIC NERVES. The tendency of the sympathetic nerves to form intricate and elaborate plexuses (plexus sympathetici) is a marked feature of this portion of the nervous system. They lie, in the main, anterior to the plane of the gangliated cord and consist of fibres alone or of fibres and ganglia, from which smaller plexuses or branches pass to the viscera. Some of them are of sufficient importance, size and individuality to merit separate descriptions ; such are the cardiac, the pulmonary, the cesophageal, the solar and the pelvic. The pulmonary and oesophageal plexuses have been described in connection with the vagus nerve (page 1272). THE CARDIAC PLEXUS. The cardiac plexus (plexus cardiacus) consists of an interlacement of nerve-fibres, containing one well-marked ganglion, to which accessions are brought by the vagus and sympathetic nerves and from which fibres are furnished to the heart and, to a slight degree, the lungs. It comprises two portions: (i) the superficial cardiac plexus and (2) the deep cardiac plexus. 1. The superficial cardiac plexus (Fig. 1135) is much the smaller of the two and consists of a fine inosculation of nerve-fibres in the meshes of which is con- tained a small ganglion, the ganglion of Wrisberg (g. cardiacum [Wrisbergi] ). It is situated in the concavity of the arch of the aorta, between the obliterated ductus arteriosus and the right pulmonary artery. Tributary to it are the superior cervical cardiac branch of the left gangliated cord and the inferior cervical cardiac branch of the left vagus, whilst its fibres of distribution contribute to (a) the right coronary plexus, (£) the left half of the deep cardiac plexus and, along the left pulmonary artery, (c) the left anterior pulmonary plexus. 2. The deep cardiac plexus (Fig. 1135), considerably larger than the su- perficial, is located above the bifurcation of the pulmonary artery, posterior to the arch of the aorta and anterior to the lower end of the trachea. It comprises two 1 368 HUMAN ANATOMY. distinct portions, a right and a left, united by numerous fibres around the lower end of the trachea. The right portion receives as tributaries all of the cardiac branches of the sympathetic, vagus and inferior laryngeal nerves of the right side. The left portion receives all of the cardiac branches of the left vagus and sympathetic nerves, except the two which enter the superficial plexus (the superior cervical cardiac branch of the left gangliated cord and the inferior cervical branch of the left vagus), with the addition of filaments from the left inferior laryngeal nerve and from the superficial cardiac plexus. FIG. 1135. Thyroid body Superior cervical cardiac branch of sympathetic Vagu Clavicle Combined cervical • cardiac brs. of right sympathetic I. rib' Superior cervical cardiac sympathetic pathetic nerve s nerve 1 or cervical cardiac branch of vagus ddle cervical ganglion :nus anticus Middle cervical cardiac of sympathetic Brachial plexus Inferior cervical ganglion •vical cardiac br. of sympathetic, cross- : nerve I in^ vertebral artery tian artery to join middle br- rvical cardiac branch of vagus irrent laryngeal nerve enic nerve Recurrent laryngeal nerve Suiierndal cardiac plexus, showing ganglion of Wrisberg Pulmonary artery Left coronary artery Dissection showing constituents of superficial cardiac plexus, other cardiac nerves and right coronary plexus. From the right portion of the plexus arises the right or anterior coronary plexus (plexus coronarius cordis anterior), to which fibres are sent from the superficial plexus. This plexus reaches the heart by coursing along the ascending aorta and then follows the right coronary artery, in whose course it distributes fibres to adjacent portions of the heart. Other branches from the right portion join the superficial cardiac plexus and the right anterior pulmonary plexus. From the left portion originates the left or posterior coronary plexus (plexus coronarius cordis posterior) which, reinforced by fibres from the superficial plexus, follows the course and distribution of the corresponding artery. The left portion contributes filaments to the superficial cardiac and left anterior pulmonary plexuses. THE SOLAR PLEXUS. The abdominal and pelvic cavities arc innervated by the solar, hypogastric and pelvic plexuses, composed of the visceral branches of the lower thoracic, lumbar and upper sacral portions of the gangliated cord, in conjunction with the central nervous THE SYMPATHETIC SYSTEM OF NERVES. 1369 axis by means of the rami communicantes of the lower thoracic and upper lumbar nerves and the visceral branches of the pudendal plexus. The solar or epigastric plexus (Fig. 1136), the largest of the series, is situated in the upper abdominal region, posterior to the stomach, anterior to the aorta and the crura of the diaphragm, superior to the pancreas, between the suprarenal bodies and around the origins of the cceliac axis and the superior mesenteric artery. It is continuous above with the diaphragmatic plexus, laterally with the suprarenal and FIG. 1136. Left vagus nerve CEsophagus Phrenic nerve Diaphragmatic ganglion Great splanchnic nerve Right semilunar ganglion Small splanchnic nerve Spermatic artery III. lumbar ganglion Aortic plexus IV. lumbar ganglion Ureter Hypogastric plexus Disc between V. lumbar vertebra and sacrum 1. sacral ganglion Phrenic nerve Diaphragm Right vagus nerve Aorta •Coelic axis f— Left semilunar ganglion of solar plexus Superior mesenteric ganglion .Renal ganglion II. lumbar ganglion Spermatic ganglion III. lumbar ganglion IV. lumbar ganglion Spermatic artery and plexus Left common iliac vein Left pelvic plexus Dissection of abdominal sympathetic nerves, showing solar, hypogastric and secondary plexuses. renal plexuses, below with the superior mesenteric and aortic plexuses and, by means of the aortic and hypogastric plexuses, with the two pelvic plexuses. Con- tributory to it are the right vagus and the great and small splanchnic nerves. The fully formed plexus consists of two portions: (i) the semilunar ganglia and (2) the coeliac plexu s. i. The semilunar ganglia (gg. coeliaca) (Fig. 1136), the largest of the ganglionic elements in the solar plexus, are situated upon the crura of the diaphragm at the superior and lateral portions of the plexus, partly overlapped by the suprarenal bodies and separated from each other by the cceliac axis and the superior mesenteric artery ; the right one is partially covered by the superior vena cava and the two are HUMAN ANATOMY. connected by cords which pass transversely above and below the root of the coeliac axis. The upper end of each is expanded and receives the termination of the great splanchnic nerve, while the lower portion, the aortico-renal ganglion, is partially detached and receives the small splanchnic nerve. A third portion, located below and to the right of the root of the superior mesenteric artery, is called the superior mesenteric ganglion (g. mesentericum superius). From each semilunar ganglion branches emerge in all directions to join those plexuses which are continuous with the solar. 2. The coeliac plexus (plexus coeliacus) embraces the cceliac axis and consists of a dense felt-work of nerve-fibres, in which are embedded numerous small ganglia, and which is joined by branches from both semilunar ganglia and from the right FIG. 1137. Knsiform cartilage K i.;ht gastro-epl- plolc artery and plexus Liver, Spigelian lobe Oesophagus Left vagus nerve Kight vagus nerve •Aorta (iastrii artery and plexus Splenic artery and plexus Hepatic artery and plexus Left gastro-epiplolc artery Rranches of left agus Dissection showing gastric and hepatic plexuses. vagus. Inferiorly it is continued into the superior mesenteric and aortic plexuses and from it arise the coronary, hepatic and splenic plexuses. The gastric plexus (plexus gastricus superior) accompanies the gastric artery along the lesser curvature of the stomach, inosculates with both vagus nerves and distributes branches which run for a short distance beneath the peritoneum and then enter and supply the deeper coats of the stomach. The hepatic plexus (plexus hepaticus) traverses the lesser omentum in company with the bile duct, the hepatic artery and the portal vein and, after inosculating with fibres of the left vagus, enters the liver, in which it ramifies. In addition to its terminal distribution it contributes filaments to the right suprarenal plexus and furnishes offshoots which follow the collateral branches of the hepatic artery, sup- plying the areas to which these arteries are distributed. The splenic plexus (plexus lienalis), which surrounds the splenic artery, receives accessions from the left sc-milunar ganglion and the right vagus and enters the spleen. Branches of the plexus accompany the branches of the splenic artery and are distributed similarly. THE SYMPATHETIC SYSTEM OF NERVES. The diaphragmatic or phrenic plexus (plexus phrenicus) is derived from the upper portion of the semilunar ganglion and accompanies the phrenic branch of the abdominal aorta to the diaphragm, the right being larger than the left. After supplying some filaments to the suprarenal body, it enters the musculature of the diaphragm and there unites with the phrenic nerve from the cervical spinal plexus. At the point of inosculation, on the right side only, near the suprarenal body and on the under surface of the diaphragm, is a small ganglion called the phrenic ganglion (g. phrenicura). From it are given off branches to the suprarenal body, the inferior vena cava aud the hepatic plexus. The suprarenal plexus (plexus suprarenalis) arises from the lateral aspect of the semilunar ganglion and is joined by filaments from the diaphragmatic and renal FIG. 1138. inferior surface Gastro-epiploica dextrawith plexus Pyloric artery with plexus Castro-duodenal artery with plexus Hepatic artery with plexus Inf. pancreatico- duodenal artery Sup. pancreatico- duodenal artery Pancreas, Sup. mesenteri' artery with plexu Stomach, turned up 4Gastro-epiploica sinistra with plexus Right vagus nerve Gastric artery 'with plexus .Splenic artery ith plexus Spleen Dissection showing gastric, hepatic and splenic plexuses; stomach has been turned up and part of pancreas removed. plexuses. It consists mainly of medullated fibres and, while very short, is made up of a number of filaments and is of considerable size. Numerous tiny ganglia are scattered throughout the meshes of this plexus. The renal plexus (plexus renalis) is derived mainly from the aortico-renal ganglion, additional fibres being contributed by the smallest splanchnic nerve, some- times by the small splanchnic, and by the aortic and suprarenal plexuses ; there is occasionally present a twig from the first lumbar ganglion. Entering the hilum of the kidney with the renal artery, the plexus splits up and ramifies in the renal sub- stance. In its course along the artery a number of ganglia of varying size, called the renal ganglia, are found. In addition to supplying the kidney, filaments are furnished to the spermatic plexus and to the ureter, and on the right side to the inferior vena cava. 1372 HUMAN ANATOMY. The spermatic plexus (plexus spermaticus) follows the course of the spermatic artery through the abdomen, inguinal canal and scrotum, inosculating with filaments which arise in the pelvis and accompany the vas deferens and its artery to the scrotum. It is derived from the renal and aortic plexuses, a small spermatic gang- lion being situated at the point of origin of the fibres contributed by the aortic plexus. The ovarian plexus (plexus ovaricus), arising similarly to the spermatic, accompanies the ovarian artery and is distributed to the ovary, the oviduct, the broad ligament and the uterus. In the broad ligament it inosculates with those pelvic fibres which constitute the uterine plexus. FIG. 1139. Hepatic artery and plexus Pancreas Duodenum Middle colic artery Transverse colon enic artery perior mesen- eric artery and plexus Termination of ileum Caecum Dissection showing hepatic and superior mesenteric plexuses ; transverse colon has been turned up. The superior mesenteric plexus (plexus mesentericus superior) (Fig. 1139), firm in texture and containing a large admixture of medullated fibres, is continuous with the cceliac plexus above and with the aortic below. Its fibres are derived from the semilunar ganglia, the cceliac plexus and the right vagus. Situated in the root of the plexus and lying below and to the right of the origin of the superior mesen- teric artery is the superior mesenteric ganglion (g. mesentericum superius), from which a number of the fibres of the plexus arise. Accompanying the superior mesenteric artery, the plexus gives off subdivisions which correspond to and follow the course of the branches of that artery, supplying filaments to the small intestine, the ccecum, the vermiform appendix and the ascending and transverse colons. As THE SYMPATHETIC SYSTEM OF NERVES. 1373 the fibres approach the distal edge of the mesentery some of them leave the vessels and form minute independent plexuses from which filaments pass to the gut. The aortic plexus (plexus aorticus abdominalis) (Fig. 1136) is the direct downward extension of the solar. Embracing the aorta, it extends from the origin of the superior mesenteric artery above to that of the inferior mesenteric below, and is connected with the semilunar ganglia and with the renal and superior mesenteric plexuses superiorly and with the hypogastric inferiorly. It consists of a pair of Nerve from aortic plexus Ovarian vein Ovarian artery Ureter - Branches from renat plexus Vena cava inferior FIG. 1140. Aorta Renal ganglion ^Ovarian artery Ovarian vein Ureter Part of inferior mesenteric plexus Inferior mesenteric " artery Hypogastricf — plexusj - External, iliac artery Internal iliac_ artery V. lumbar nerve_ (partly hidden) I. sacral nerve — II. sacral_. nerve Pelvic plexus- Ill. sacral__ nerve" Branches ot ri^ht pelvic plexus to- re ctum Right Ovary- Fallopian tube- Ligament of_ ovary Peritoneum covering bladder Dissection showing hypogastric and pelvic plexuses. symmetrically placed nerve trunks situated at the sides of the aorta and connected with each other by several branches which lie anterior to that vessel ; filaments from the lumbar ganglia join the main cords of the plexus. It gives off the inferior mes- enteric plexus, sends contributions to the suprarenal, renal and spermatic or ovarian, supplies filaments to the aorta and inferior vena cava and terminates in the hypo- gastric plexus. The inferior mesenteric plexus (plexus mesentericus inferior) is derived from the left portion of the aortic plexus and follows the course and distribution of the artery for which it is named. Situated a short distance beyond its origin is the small inferior mesenteric ganglion. From this plexus branches are i374 HUMAN ANATOMY. distributed to the descending and sigmoid colons and to the upper portion of the rectum. The hypogastric plexus (plexus hypogastricus) (Fig. 1140), the continuation of the aortic, lies on the posterior wall of the pelvis in the angle between the common iliac arteries, and enclosed in a firm investment of fibrous tissue. In addition to the fibres derived from the aortic plexus, others are contributed by the lumbar ganglia, and the resulting intricate interlacement, in which there are no ganglia, constitutes the hypogastric plexus. It supplies the pelvic contents and at its lower end divides into the two pelvic plexuses. The pelvic plexuses (plexus hypogastrici inferiores), (Fig. 1140) the terminal divisions of the hypogastric, are situated lateral to the rectum and to the vagina in the female. They comprise fibres derived from the hypogastric plexus and from the upper part of the sacral portion of the gangliated cord, aided by the visceral branches of the pudendal plexus ', all of these forming an elaborate net-work, in which are dotted numerous small ganglia. The completed structure follows the course of the internal iliac artery, around whose branches it sends derivatives for the supply of the pelvic contents. The hemorrhoidal plexus (plexus hemorrhoidalis medius) arises from the upper portion of the pelvic plexus and after inosculating with the superior hemorrhoidal branches (nn. hemorrhoidales superiores) of the inferior mesenteric plexus, are distributed to the rectum. The vesical plexus (plexus vesicalis) consists of branches of the pelvic which accompany the vesical arteries to the lateral and inferior portions of the bladder, after reaching which they leave the vessels and split into small twigs for the supply of the bladder, some filaments going to the ureter, the vas deferens and the seminal vesicle. The prostatic plexus (plexus prostaticus) comprises a number of nerves of con- siderable size and is situated between the lateral aspect of the prostate gland and the mesial surface of the levator ani muscle. After furnishing twigs to the prostatic urethra, the neck of the bladder and the seminal vesicle, it continues forward as the cavernous plexus. The cavernous plexus (plexus cavernosus penis) extends forward through the triangular ligament and the compressor urethrse muscle to the dorsum of the base of the penis, where it receives some communicating filaments from the pudic nerve. After supplying branches to the apex of the prostate gland and the membranous urethra, the plexus terminates by breaking up into (i) the small and (2) large cavernous nerves of the penis. 1. The small cavernous nerves (nn. cavernosi penis minores) pierce the fibrous envelope of the crus penis and end in filaments which supply the erectile tissue of the corpus cavernosum. 2. The large cavernous nerve (n. cavernosus penis major), consisting mainly of medullated fibres, passes directly along the dorsum of the penis, giving off fila- ments which enter the substance of the corpus cavernosum. At about the middle oi the body of the penis it inosculates with the dorsal nerve of the penis, both of these nerves sending twigs to the corpus spongiosum. The utero-vaginal plexus (plexus uterovaginalis) corresponds to the prostatic plexus of the male and consists of two portions : (i) the uterine plexus and (2) the vaginal plexus. 1. The uterine plexus (plexus uterinus) is derived from the pelvic plexus and is supplemented in its distribution by the visceral branches from the pudendal plexus. These fibres accompany the uterine vessels along the side of the uterus, most of them entering the cervix and the lower portion of the body of the uterus. They inoscu- late with fibres from the ovarian plexus and in their meshes are found many small i^an^lia. a collection of which is located near the cervix uteri and is called the gang- lion cervicale. 2. The vaginal plexus (plexus vajjinalis) arises from the lower part of the pelvic and comprises mainly fibres derived from the visceral branches of the puden- dal plexus. It supplies the vagina and the urethra and continues forward as the (.i\< rnous plexus of the clitoris (plexus cavernosus clitoridis). DEVELOPMENT OF PERIPHERAL NERVES. 1375 Practical Considerations. — The cervical sympathetic may be injured by deep wounds of the neck, or may be compressed by tumors, abscesses or aneurisms. It supplies motor fibres to the involuntary muscles of the orbit and eyelids, vasomotor fibres to the face, neck and head, dilator fibres to the pupil, accelerator fibres to the heart and secretory fibres to the salivary glands. If it is irritated, some or all of the following symptoms will be present : the palpebral fissure will open wider, the eyes will be protruded, the skin of the face and neck will be pale and cold, the pupils dilated, and the sweat, nasal secretion and saliva diminished. Section or destruction of the cervical sympathetic will give the opposite symptoms. The cervical sympathetic has been removed for epilepsy, glaucoma and exoph- thalmic goitre. The greatest success has been obtained in the last condition, espe- cially by Jonnesco, who advises this procedure in hysteria, chorea, and tumors of the brain, as well as in the above-mentioned conditions. It may be excised through an incision anterior to the sterno-mastoid, as it lies posterior to the carotid sheath on the prevertebral fascia. The superior cervical ganglion is the largest and lies opposite the transverse processes of the second and third vertebrae. Branches of it go upward along the external and internal carotid arteries, the ascending branch passing along the internal carotid artery through its bony canal in the base of the skull to form the carotid and cavernous plexuses, both of which are really parts of one plexus arranged around this artery. Other branches communicate with the cranial nerves, the pharyngeal nerves and the superficial cervical cardiac nerve. The middle cervical ganglion is the smallest, lies on the inferior thyroid artery oppo- site the sixth cervical vertebra and is in danger in the ligation of that artery. The inferior ganglion, intermediate in size between the other two, lies in a depression between the neck of the first rib and the transverse process of the seventh cervical vertebra. The branches of the upper four or five thoracic ganglia of the sympathetic enter into the supply of the thoracic viscera, but the branches of the lower seven or eight form the splanchnic nerves and go to the supply of the abdominal viscera through the solar plexus and its extensions into other sympathetic plexuses of the abdomen. It is of interest and importance to observe that those intercostal nerves corresponding in their origin from the spinal cord with the ganglia giving off the splanchnics, together with the first two lumbar nerves, the ilio-hypogastric and ilio-inguinal, supply the abdominal wall with motor and sensory branches. In this way the same segments of the spinal cord supply the abdominal viscera as well as the skin and muscles over them. A similar arrangement of the nerves is seen in the joints, where the same nerves supply the skin covering the joint, the muscles which move it, and the joint structures. As a result of this, when necessary, all parts of the joint act in sympa- thy. In an inflammation of the joint the skin becomes sensitive, tending to ward off interference, and the muscles become rigid, preventing motion and favoring rest. In a similar manner the abdominal muscles become rigid to protect inflamed viscera underneath, the muscles of one side only if the inflammation is localized to one side, but the muscjes of both sides if a general peritonitis is present. DEVELOPMENT OF THE PERIPHERAL NERVES. The manner in which the nerve-fibres composing the peripheral nervous system develop from the primary cells, the neuroblasts, has been indicated in the previous sketch of their histogencsis given on page ion. It remains, therefore, to describe briefly at this place the more important features of their morphogenesis. The fundamental fact has been repeatedly empha- sized, that efferent or motor fibres are outgrowths from neurones situated within the cerebro- spinal axis, whilst all afferent or sensory fibres arise from cells placed outside this axis and within the ganglia located along the course of the nerves. It is evident, furthermore, that the efferent constituents of the peripheral nerves have their nuclei of origin within the spinal cord or brain and grow outward, as axones, to their destinations. The afferent fibres, on the other hand, proceed in both directions, the axones early growing centrally to join the nervous axis, hence, having usually a short course, being represented by the entering sensory roots. The dendrites grow in the opposite direction and contribute the sensory fibres that extend often to remote parts of the body. Whilst in the lowest vertebrates, the amphioxus and the cyclos- tomes, the ventral and dorsal roots of the spinal nerves remain distinct, in the higher types they join to form the mixed nerve, which typically divides into the anterior, posterior and 1376 HUMAN ANATOMY. visceral divisions. Such typical division, however, is displayed only by those spinal nerves dis- tributed to that part of the trunk in which the primary segmentation is retained, namely, the thoracic region, where the skeletal muscular, and vascular segments, as well as the nerves, retain their identity. In the other parts of the spinal series, the cervical and the lumbo-sacral, where provision is made for the supply of the highly differentiated musculature of the ex- tremities from a number of cord-segments, the nerves early unite to form plexuses from which the limb- trunks grow out, an arrangement well adapted for the distribution of fibres from different sources without undue multiplication of nervous paths. Concerning the factors which guide the young nerve to its destination with such remarkable constancy, nothing is known, but it may be assumed that these are probably influences of a physical character, the developing nerve taking the path offering least resistance. The visceral division of the spinal nerve, to which reference has been made, corresponds to the white ramus communicans given off by certain of the thoracic and lumbo-sacral nerves. These splanchnic fibres differ from the somatic efferent ones in taking their origin from cells which occupy a more lateral position within the gray matter of the spinal cord than do the root-cells giving rise to the motor fibres destined for the skeletal muscles. Whilst the great majority of the splanchnic fibres reach the ramus of communication by way of the anterior root, some few perhaps traverse the posterior or sensory root and its ganglion before continuing their course to the sympathetic. The sensory fibres described within the anterior roots of the spinal nerves are not actual constituents of these roots, which are exclusively motor, but recurrent meningeal twigs destined for the membranes of the cord. The Cranial Nerves. — From the preceding account of these nerves, it is evident that the optic nerve differs morphologically widely from an ordinary' nerve, since it may be regarded as a modified outlying portion of the brain. Its development may be omitted, therefore, from this series and appropriately considered in connection with the development of the eye (page 1482). There is sufficient reason, as will appear later, for regarding the hypoglossal nerve as a cranially displaced member of the spinal series. Of the remaining nerves, only the olfactory and auditory are purely sensory ; the third, fourth, sixth and eleventh are exclusively motor ; and the fifth, seventh, ninth and tenth are mixed, the motor strands taking origin from the neu- rones within the brain-stem, while the sensory ones are derivations from the neurones lying within the ganglia connected with the afferent fibres. Although at first sight the trigeminus closely corresponds to a spinal nerve in the possession of a gangliated sensory and a motor root, critical examination of the origin of its motor fibres discloses an important differ- ence, namely that they arise from the lateral nuclei and not from the mesial, which correspond to collections of ventral root-cells. A similar difference also appears between the efferent trigeminal fibres and those of the eye-muscle nerves, the latter arising from groups of root-cells occupying a position close to the mid-line. In order to appreciate the significance of this differ- ence, reference must be made to the primary division of the musculature of the head already referred to in connection with the grouping of the muscles (page 472 ). It was there pointed out that it may be assumed that the segmented condition of the trunk musculature, as expressed by the metameres, is continued into the cephalic region but with subsequent suppression of the middle members of the possible nine or ten segments which constituted the original quota of head-metameres. Of those persisting two groups are recognized— one including the first three metameres, giving rise to the ocular muscles and being supplied by the third, fourth and sixth nerves ; the other including the last three or four, producing the tongue-muscles, and being sup- plied by the twelfth nerve. To these groups of cephalic meiameres is added a third, the branchiomeres, which are regarded as representing a supplementary series connected with the branchial arches and not present in the trunk. The branchiomeres receive the'mixed cranial nerves, whose motor filaments supply muscular masses surrounding the visceral tubes (digestive and respiratory), and arise from the lateral motor nuclei. It follows that none of the cranial nerves contain fibres from all these sources, in the case of the fifth, seventh, ninth and tenth, the fibres being derived from the lateral motor and the sensory nuclei, and in the case of the third, fourth and sixth, from the mesial (ventral) nuclei alone. From the primary conditions, as revealed by studies on the lower vertebrates, it is probable that the dorsal fibres also are by no means of similar morphological value, since some represent a somatic sensory system, as those distributed to the integument, and others belong to a visceral sensory one, as those distributed to the walls of the mouth, pharynx and larynx. Following the principle already emphasized, the motor fibres of tin- cranial nerves grow from the brain outward, while tin- sensory ones extend centrally from the ganglia of the nerves associated with the brain. The cranial and spinal nerves appear on the surface of the neural tube at a very early period, their presence being conspicuous by the end of the fourth week (Fig. 901). The olfactory nerve is developed in connection with the epithelial lining of the primary olfactory pit (page 1429). As early as the end of the first fu-tal month, in the human embryo, cells corresponding to netiroblasts appear in the anlage of the olfactory organ. From these elements processes soon j;n>\v brainward, nucleated tracts indicating the formation of the later olfactory fibres. The cell-bodies of the youn;; neurone migrate so that for a time their position DEVELOPMENT OF PERIPHERAL NERVES. 1377 is no longer within the primary epithelium, but deeper and within a cell aggregation known as the olfactory ganglion. The neurones, however, retain connection with the olfactory epithe- lium by means of their peripherally directed processes, which correspond to dendrites, and with the brain by means of their axones. With the thickening of the olfactory epithelium which sub- sequently occurs, the peripheral fibres and their nuclei comes to lie entirely within the epithelial stratum and persist as the olfactory cells, whose centrally directed processes form the olfactory filaments that end as arborizations within the characteristic olfactory glomeruli. The first cranial nerve is peculiar in the superficial position of its cell-bodies and in the extreme shortness of its dendrites, which are represented by the rod-like fibres of microscopic length extending from the cell-bodies toward the free surface of the olfactory mucous membrane. This superficial position of the olfactory neurones is regarded as an unusual persistence of the primary condition of all sensory elements and as evidence of the archaic nature of the olfactory nerves. FIG. 1141. Superior colliculus Mid-brain ineal body iencephalon Median geniculate body Pallium inferior colliculus Oculomotor nerve Trochlear nerve Glosso-pharyngeal Vagus nerve Spinal accessory nerve Cerebellum Trigeminal nerve Auditory nerve iryngeal nerve -JL- Rhinencephalon Optic stalk nferior part of III. ventricle Facial nerve Hypoglossal nerve Reconstruction of brain of human embryo of four and one half weeks (10.2 mm.); outer surface, showing developing nerves. X 12. Drawn from His model. The optic nerve is so inherently a derivative of the cerebral and optic vesicle, that its develop- ment is appropriately considered with that of the eye (page 1482) ; moreover, its morphological significance being so at variance with that of the other nerves, it may be omitted from further discussion in the series now being described. The oculomotor nerve being strictly a motor nerve has much in common in its mode of formation with the ventral root of a spinal nerve, with which it is homologous. The nerve originates as an outgrowth from a group of neuroblasts, which occupies the ventral zone about the middle of the mesencephalon. From these neurones, visible in the fourth week in the human embryo, the axones proceed as a converging group of fibres which, piercing the wall of the brain-tube close to the mid-line, appear on the ventral surface of the brain-stem as the fibres of the third nerve. Although by some regarded as possessing a transient rudimentary dorsal root that early entirely disappears, thus bringing the nerve of a cranial myomere into close correspondence with those of the spinal series, it is doubtful whether such structure is usually present, the suppression of the dorsal portion of the nerve being complete. Soon after its for- mation, the main trunk undergoes division into a smaller upper and a larger posterior limb, which foreshadow the superior and inferior divisions of the mature nerve. The trochlear nerve, although springing from a central group of neuroblasts in close proximity with those giving rise to the third, is peculiar in the course of its axones. Instead of maintaining a ventral course, these proceed dorsally and become superficial on the upper (dorsal) aspect of the hind-brain, piercing the plate which later becomes the superior medul- 8? 1378 HUMAN ANATOMY. lary velum. As in the case of the third, so for the trochlear an abortive transient dorsal ganglion and root have been described (Martin). If present these must be regarded as ex- ceptional and not constant features. The trigeminal nerve is a mixed nerve and therefore takes its origin differently for its two roots. The motor one is developed from a series of neuroblasts, which lie at some distance from the mid-line within the wall of the neural tube, at a position corresponding to the junction of the dorsal and ventral zones of the mid-brain and metencephalon. The axones of these neuroblasts grow forward and converge to the surface of the later pons at a position close to where the ingrowing sensory fibres join the neural tube. The sensory fibres are the axones of neurones located within the Gasserian ganglion. The latter is derived as a ventrally directed outgrowth from the ectoblast of the roof of the hind-brain, with which it remains attached for a short time, but later becomes entirely separated. The neuroblasts acquire a bipolar form, one set of processes, the axones, growing centrally to establish secondary connections with the hind-brain as the large sensory root, while the others, the dendrites, extend peripherally into the substance of the fronto-nasal and maxillary processes to form the ophthalmic and maxillary nerves and into the mandibular process to form, in conjunction with the smaller motor root, FIG. 1142. Reconstruction of brain and cranial nerves of pig embryo ; cranial nerves indicated by figures ; ci-cs, cervical spinal nerves ; in connection with seventh nerve., / s.fl, large superficial petrosal ; ch.ty., chorda tympani : fa., facial ; j., «., vagus ganglia of root and trunk ; com., commissural extension of ganglion of root ; f, Froriep's hypoglossal ganglion. (/•". T. Lewis.) the mandibular division of the trigeminus from the ganglion ridge. Provision for the ciliary ganglion is made early by the migration of cells from the major ganglion along the de- veloping ophthalmic division. Similar migrations along the other divisions give rise to the spheno-palatine, the otic and the submaxillary ganglia. The later histological characteristics of these cells, as well as their mode of origin, warrant the view that the ciliary ganglion, as well as the others connected with the trigeminus, belong to the sympathetic system. On entering the wall of the brain-tube, the bulk of the sensory trigeminal fibres assume a longitudinal course and early establish the tract of the spinal cord. The abducent nerve developes, in a manner identical with the third and fourth, from a median group of cells occupying the ventral zone of the upper part of the hind-brain. In the human embryo of about four and a half weeks (Fig. 1141), the nerve appears at its super- ficial origin mesial to the Gasserian ganglion. The root-fibres early consolidate into a compact strand. The facial nerve being a mixed one also arises from a double source, its motor fibres taking origin from efferent m-uroblasts situated in the ventro-lateral wall of the metencephalon. In contrast to the direct ventral course of the axones of the mesial motor nerves, those of the facial pursue a path to the surface of the brain-stem even more indirect than that taken by the lateral motor fibres of the other mixed nerves. Proceeding as the axones of neuroblasts lying within the lateral part of the ventral /one of the wall of the hind-brain, they are directed dor- sally, then grow forward, turn outward and, finally, ventrally to gain emergence from the brain. The sensory portion of the facial is topographically closely connected during its development with the auditory, the nuclei of the two nerves often being designated the facial-acoustic com- plex. The three components of this aggregation — the genicnlate, the cochlear and the vestibu- inglia— are primarily derived from an ectoblastic cell-mass in the vicinity of the otic vesicle. DEVELOPiMENT OF PERIPHERAL NERVES. 1379 Vagus root gang. Accessory root gang. Froriep The neuroblasts of the facial constituent, the geniculate ganglion, send their centrally directed processes to the brain-stem as the pars intermedia, whilst their peripherally growing dendrites contribute the sensory fibres, passing by way of the chorda tympani and the greater and lesser superficial petrosal nerves. The geniculate ganglion and the pars intermedia correspond, therefore, to a dorsal root. The auditory nerve, although for a time closely related in position (Fig. 1103) with the facial (geniculate) ganglion, developes entirely independently and at no time has more than an incidental relation. The primary auditory nucleus is defined in human embryos by the begin- ning of the fourth week as an elongated ellipsoidal mass in contact with the anterior wall of the otic vesicle. According to Streeter l, the nucleus very shortly exhibits a differentiation into a superior and an inferior part, from the latter of which soon appears a third portion. This third portion, the later ganglion spirale, early manifests a tendency to coil in consequence of its close relations with the duct us cochlearis. The pIG major part of the primary acoustic complex, including the superior and most of the inferior part, becomes the vestibular ganglion, from the neuroblasts of which centrally directed a x o n e s pass to the young brain- stem as the vestibular nerve, while the dendrites become connected at certain places with the semicircular canals, the utricle and the saccule. The grouping of the vestibular rami seen in the adult is early foreshadowed in the develop- ing nerve, since from the upper part of the vestibular ganglion grows out the su- perior division of the vestib- ular nerve which, supplies the utricle and the ampullae of the superior and external semicircular canals (Fig. 1070) . The lower part of the ganglion, in addition to fur- nishing the anlage for the cochlear nerve, gives off the inferior division of the vestib- ular nerve, by which the saccule and the posterior canal are supplied. During the subsequent growth of the structures, the neurones of the spiral ganglion send ax- ones towards the brain which become the cochlear nerve, whilst their dendrites-grow peripherally into the ductus cochlearis and are represented by the minute filaments extending from the cells of the spiral ganglion to the auditory cells of Corti's organ. The glosso-pharyngeal nerve is a mixed nerve and has, therefore, a double origin. Its motor fibres arise from neuroblasts situated in the dorsal part of the ventral zone of the wall of the hind-brain just posterior to the otic vesicle. The sensory part of the nerve, along with that of the vagus, offers greater complexity, since it is developed, as shown by Streeter 2, from two sources. The ganglion of the root (g. superius or jugular ganglion) arises very early as a small mass of cells derived from the ganglion-crest of the hind-brain. It varies in size and soon ceases to grow, which behavior, in connection with the preponderating ingrowth of the motor fibres, accounts for the well-known inconstancy of the structure. The ganglion of the trunk (g. petrosum) arises, according to Streeter, not from the neural crest, but in relation with the ectoblast of the second visceral furrow. At first ununited with the smaller ganglion superius, the ganglion of the root subsequently becomes joined to it, the two nodes 1 Amer. Jour, of Anatomy, vol. vi., 1907. 2Amer. Jour, of Anatomy, vol. iv., 1904. IX. root gang. N. tymp. Gang, petros. IX. Gang, nodos. N. laryg. sup. XII. with r. descend. Sympathetic Vagus Reconstruction of peripheral nerves of human embryo of five weeks (14 mm.) X 13. (Streeter.) 1380 HUMAN ANATOMY. being later closely related, both as to position and fibres. An outgrowth of distally directed fibres establishes the main trunk of the nerve, while a forwardly growing strand represents the later tympanic branch. The vagus and spinal accessory nerves are so inseparably related in their development that their origin must be regarded as proceeding from a common vagus complex. The latter comprises three elements: (a) a series of motor roots, which arise from the ventral zone of the hind-brain and extend from near the glosso-pharyngeal anlage in front as far as the third or fourth spinal segment below ; (b) a partially subdivided, but at first continuous, ganglionic mass, which arises from the ganglion-crest of the hind-brain and represents the root-ganglia ; (c) a secondary' ventral cell-mass, the primitive ganglion of the trunk, which, as in the case of the glosso-pharyngeal nerve, is developed in close relation with the ectoblast of the posterior branchial furrows. Whilst the motor rootlets persist and become the efferent root-fibres of the later vagus and accessory nerves, the dorsal or crest-ganglia soon exhibit differences in their growth, the one situated farthest forward outstripping the others and becoming the vagal gang- lion of the root, and the remaining ones becoming the accessory root -ganglia. These latter constitute a chain which below meets with the spinal dorsal ganglia. Primarily, therefore, the entire length of the vagus complex is occupied by a series of mixed nerve strands possessing both motor and sensory elements. The head-end of the series later becomes predominatingly sensory, while in the tail-end of the same the motor character prevails. The ventral vagus nucleus is attached secondarily to the dorsal nucleus by centrally growing fibres, while from its distal end extend the dendritic processes which constitute the trunk of the vagus and its branches. In consequence of the intergrowth of these afferent and efferent fibres, the definite tenth nerve in the usual sense, with its two ganglia, becomes established. Although for a short period the accessory part of the complex is provided with both motor and sensory parts, the latter are subsequently overpowered by the efferent fibres, so that the presence of the rudimen- tary ganglionic elements within the accessorius can be demonstrated only by microscopic exam- ination (Streeter) . From the preceding facts it is evident that the estimate of the eleventh nerve as an integral part of the vagus is well founded. The hypoglossal nerve appears in the human embryo, towards the close of the third week, as several strands which grow from the ventral zone of the wall of the hind-brain and are in series with the ventral root-fibres of the upper cervical spinal nerves. Soon the separate root- lets converge and consolidate into a common trunk, from which, by the end of the fifth week, the chief branches of distribution arise. The production of the wide-meshed net-work which distinguishes the communications between the upper cervical and hypoglossal nerves results from the separation of fibres which are at first closely adjacent, the subsequent migration of the growing tongue-muscles drawing the hypoglossal fibres away from the spinal nerves, except at such points where they have become enclosed in a common sheath. There is good reason for regarding the hypoglossal nerve as representing the ventral roots of trunk-nerves, which have been cephalicly displaced and drawn within the cranium. Moreover, the observations of Froriep and others upon adult mammals and of His upon the human embryo have shown the presence of a rudimentary dorsal ganglion and abortive dorsal root-fibres. The occasional presence of a rudimentary ganglionic mass, known as Froriep's ganglion, attached to the fibres of the adult hypoglossal nerve in man is to be interpreted as the persistent dorsal element which ordinarily disappears. From the preceding sketch it is evident that in no instance, as observed in the usual adult condition in man, is there complete correspondence between the members of the cephalic series and those of the trunk. The group of purely sensory nerves — the olfactory, optic and auditory — includes one, the optic, which is so exceptional in its fundamental relations as to lie without the pale of peripheral nerves in their strict sense. The remaining two sensory nerves are held to be primarily the equivalents of constituents of a peculiar system of sensory organs, best developed in fishes, known as the organs of the lateral line. The third, fourth, sixth and twelfth, the ventral motor nerves, are undoubtedly associated with head-somites, although the exact number and nerve relations of such mesoblastic segments are uncertain ; in fundamental significance, therefore, these nerves agree with those of the trunk-series, although modified by the suppression of their dorsal or sensory constituents. The mixed nerves— the fifth, seventh, ninth and tenth (the eleventh being reckoned as part of the vagus) — are unrepresented in the spinal series and belong to the brunchiomeres represented by the visceral arches. Of these nerves, the trigeminus most nearly accords in constitution with a typical spinal n<-rve, since, with the exception of ventral motor constituents which are wanting, it pos- as does the typical spinal nerve, both somatic (general cutaneous) sensory and visceral sensory fibres. A further resemblance is found in the character of the gray matter constituting the reception-nucleus for the sensory fibres of the trigeminus, since this column is composed of sulistautia iM-latinosa continuous with the Rolandic substance capping the posterior cornu of the cord. A similar, alth< >ugh less intimate, arrangement is seen in the column of gray matter accom- panying the descending root (funiculus solitarius)of the facial, glosso-pharyngeal and vagus nerves. THE ORGANS OF SENSE. THE cells directly receiving the stimuli producing the sensory impressions of touch, smell, taste, sight and hearing are all derivations of the ectoblast — the great primary sensory layer from which the essential parts of the organs of special sense are differentiations. The olfactory cells — nervous elements that correspond to ganglion cells — retain their primary relation, since they remain embedded within the invaginated peripheral epithelium lining the nasal fossae, sending their dendrites towards the free surface and their axones into the brain. Usually, however, the nerve cells connected with the special sense organs abandon their superficial position and lie at some distance from the periphery, receiving the stimuli not directly, but from the epithelial receptors by way of their dendrites. In the case of the most highly specialized sense organs, the eye and the ear, the percipient cells lie enclosed within capsules of mesoblastic origin, the stimuli reaching them by way of an elaborate path of conduction. THE SKIN. Since the extensive integumentary sheet that clothes the exterior of the entire body not only serves as a protective investment, an efficient regulator of body temperature and an important excretory structure, but also contains the special end- organs and the peripheral terminations of the sensory nerves that receive and convey the stimuli producing tactile impressions, the skin may be appropriately considered along with the other sense-organs of which it may be regarded as the primary and least specialized. On the other hand, the correspondence of its structure with that of the mucous membranes, with which it is directly continuous at the orifices on the exterior of the body, emphasizes the close relation of the skin to the alimentary and other mucous tracts. This general investment, the tegmentum commune, includes the skin proper, with the specialized tactile corpuscles, and its appendages — the hairs, the nails and the cutaneous glands. Its average superficial area is approximately one and a half square meters. The skin (cutis), using the term in a more restricted sense as applied to the covering proper without its appendages, everywhere consists of two distinct portions — a superficial epithelial and a deeper connective tissue stratum. The former, the epi- dermis, is devoid of blood-vessels, the capillary loops of which never reach farther than the subjacent corium, as the outermost layer of the connective tissue stratum is called. The thickness of the skin, from .5-4 mm., varies greatly in different parts of the body, being least on the eyelids, penis and nymphae, and greatest on the palms of the hands and soles of the feet and on the shoulders and back of the neck. In general, with the exception of the hands and feet, the skin is thicker on the extensor and dorsal surfaces than on the opposite aspects of the body. Of the entire thick- ness, the proportion contributed by the epidermis is variable, but in most localities it is about . i mm. Where exposed to unusual pressure, as on the palms of laborers or on habitually unshod soles, the epidermis may attain a thickness of 4 mm. As seen during life, the color of the skin results from the blending of the in- herent tint of the tissues with that of the blood within the superficial vessels. When the latter are empty, as after death, the skin assumes the characteristic pallor and ashen hue. Where the capillaries are numerous and the overlying strata thin, the skin exhibits the pronounced rosy color of the lips, cheeks, ears and hands. Where, on the contrary, the contents of fewer vessels shimmer through the epidermis, the paler tint of the limbs and trunk is produced. In certain localities — especially over the mammary areolae after pregnancy, the axillae, the external genital organs and around the anus — the skin presents a more or less pronounced brownish color owing to the unusual quantity of pigment within the 1381 I382 HUMAN ANATOMY. Imprint of dorsal surface of left hand near ulnar border; radiating lines are produced by creases connecting points at which hairs emerge. FIG. 1145. epidermis. The amount of skin-pigment not only differs permanently among races (white, yellow and black) and indi- FIG. 1144. viduals (blond and brunette), but also varies in the same person with age and exposure, as contrasted by the rosy tint of the infant and the bronzed tan of the weather beaten mariner. Unless bound down to the underlying tissues, as it is over the scalp, external ear, palms and soles, the skin is freely movable. Its physical properties include con- siderable extensibility and marked elasticity. By virtue of the latter the temporary displacement and stretch- ing produced by movements of the joints and muscles is overcome and the smoothness of the skin, so con- spicuous in early life, is maintained. With advancing age the elasticity becomes impaired and folds are no longer effaced, resulting in the perma- nent wrinkles seen in the skin of old people. Certain folds and furrows, however, are not only permanent and ineffaceable, appearing in the foetus, but are fairly constant in position and form. One group, produced by flexion of the joints, includes the conspicu- ous creases on the flexor surface of the wrist, palm and fingers, and the similar markings on the soles of the feet. The other group, more extensive but less striking, includes the fine grooves that connect the points of emergence of the hairs and cover the trunk and extensor surface of the limbs with a delicate tracery (Fig. 1144). The surface modelling of the skin covering the palms, soles and flexor aspects of the digits is due to the disposition of numerous minute riclges (cristae cutis) and furrows (sulci cutis). The cutaneous ridges, about . 2 mm. in width, correspond to double rows of papillae which they cover, the sweat glands opening along the summit of the crests. The patterns formed by the cutaneous ridges (Fig. 1145) remain throughout life unchanged and are so distinctive for each individual that they afford a reliable and practical means of identi- fication. In addition to the various longitudinal, trans- verse and oblique ranges of ridges that cover the greater part of the hand, groups of concentrically arranged ridges occupy the volar surface over the distal phalanges, the pads between the metacarpo-phalangeal joints and the middle of the hypothenar eminence. These highly characteristic areas, the so-called tactile pads < tonili tnctiles) are most strikingly developed over the bulbs of the lingers, where the ridges are often disposed in whorls rather than in regular ovals. The markings of corresponding areas of the two hands are symmetrical and sometimes identical. Structure.— The two parts of which the skin is everywhere composed — the epidermis and the connec- tive tissue stratum — are derivatives of the ectoblast and of the mesoblast respectively. The connective tissue portion includes two layers, Imprint of palmar surface of left middle finger, showing arrangement of cutaneous ridges; transveise m terruptions are produced by flexion creases over joints. THE SKIN. the corium and the tela subcutanea, which, however, are so blended with each other as to be without sharp demarcation. The corium or derma, the more superficial and compact of the connective tissue strata, lies immediately beneath the epidermis from which it is always well defined. With the exception of within a few localities, as over the forehead, external ear and perineal raphe, the outer surface of the corium is not even but beset with elevations, ridges, or papillae, which produce corresponding modelling of the opposed under surface of the overlying epidermis. The pattern resulting from these eleva- tions varies in different regions, being a net-work with elongated meshes over the back and front of the trunk, with more regularly polygonal fields over the extremi- FIG. 1147. Portion of corium from palmar surface of hand after removal of epi- dermis ; each range includes a double row of papillae, which underlie the superficial cutaneous ridges and en- close openings of sweat glands ; latter appear as dark points along ranges of papillae. X 5. Sweat glands Small portion of preceding specimen, showing papillae under higher magnifica- tion ; orifices of torn sweat glands are seen between papillae. X 24. ties and with small irregular meshes on the face (Blaschko). The best developed papillae are on the flexor surfaces of the hands and feet, where they attain a height of .2 mm. or more and are disposed in the closely set double rows that underlie the cutaneous ridges on the palms and soles above noted. The papillae afford favorable positions for the lodgement of the terminal capillary loops and the special organs of touch and are accordingly grouped as vascular and tactile. In recognition of the elevations, which in vertical sections of the skin appear as isolated projections, the corium is subdivided into an outer papillary stratum (corpus papillare), containing the papillae, and a deeper reticular stratum (tunica propria), composed of the closely interlacing bundles of fibrous and elastic tissue that are continued into the more robust and loosely arranged trabeculae of the tela subcutanea. These two strata of the corium, however, are so blended that they pass insensibly and without definite boundary into each other. Although composed of the same histological factors— bundles of fibrous tissue, elastic fibres and con- nective tissue cells — the disposition of these constituents is much more compact in the dense reticular stratum than in the papillary layer, in which the connective tissue bundles are less closely interwoven. While the general course of the fibrous bundles within the corium is parallel or oblique to the surface, some strands, continued upward from the underlying subcutaneous sheet, are vertical and traverse the stratum reticulare either to bend over and join the horizontal bundles or to break up and disappear within the papillary stratum. The elastic tissue, 1384 HUMAN ANATOMY. which constitutes a considerable part of the corium, occurs as fibres and net-works, which within the reticular stratum form robust tracts corresponding in their disposition with the general arrangement of the fibrous bundles. Towards the surface of the corium, the elastic fibres become finer and more branched and beneath the epidermis anastomose to form the delicate but close subepithelial elastic net-work that is present over the entire surface of the body with the exception, possibly, of the eyelids (Behrens). The tela subcutanea, the deeper layer of the connective tissue portion of the skin, varies in its thickness, and in the density and arrangement of its component bundles of fibro-elastic tissue, with the amount of fat and the number of hair-follicles and glands lodged within its meshes. The latter are irregularly round and enclosed by tracts of fibrous tissue, some of which, known as the retinacula cutis, are prolonged from the corium to the deepest parts of the subcutaneous stratum. Here they often blend into a thin but definite sheet, the fascia subcutanea, which forms the innermost boundary of the skin and is FIG. 1148. Epidermis Papillary stratum Reticular stratum Hair follicle Retinaculum Fat Section of skin, showing its chief layers — epidermis, corium and tela subcutanea. X 17. connected with the subjacent structures by strands of areolar tissue. Where such loose connection is wanting, as on the scalp, face, abdomen (linea alba), palms and soles, the skin is intimately bound to the underlying muscles or fasciae and lacks the independent mobility that it elsewhere enjoys. The integument covering the eye- lids and penis is peculiar in retaining to a conspicuous degree its mobility although devoid of fat. Where the latter is present in large quantity, the term panninUus ad i f>osus is often applied to the tela subcutanea. In places in which the skin glides over unyielding structures, the interfascicular lymph-spaces of the tela subcutanea may undergo enlargement and fusion, resulting in the production of the subcutaneous mucous bursae. These are found in many localities, among the most constant bursae being those over the olecranon. the patella and the metatarso-phalangeal joints of the little and the great toe. The bursa in the latter situation, when abnormally enlarged, are familiar as bunions. In addition to the strands of involnutary muscle associated with the hairs as the arrectores pilorum, unstrined muscular tissue is incorporated with the skin in the mammary areolre and over the scrotum and penis (tunica dartos"). The facial muscles having largely cutaneous insertions, the skin covering the fare is invaded by tracts of striated muscular tissue that penetrate as far as the corium. THE SKIN. 1385 The epidermis or cuticle, the outer portion of the skin, consists entirely of epithelium and, being partly horny, affords protection to the underlying corium with its vessels and nerves. The thickness of this layer varies in different parts of the body. Usually from .08-. 10 mm., it is greatest on the flexor surfaces of the hands and feet, where it reaches from .5-. 9 mm. and from 1.1-1.3 mm- respectively (Drosdoff). The cuticle consists of two chief layers, the deeper stratum germinativum, con- taining the more active elements, and the stratum corneum, the cells of which undergo cornification. Between these layers lies a third, the stratum intermedium, that is FIG. 1149. =r Stratum corneum Spiral duct of *^~ sweat gland •/; : ..--/-; ,': •: ';'-; ;.r';-v';-.:- . • iff ' • •. -• ' • •- */';••>>,; • • "".-' .-<'•' -: ,/ /•,' ', ',• »-:'^ ".'£'.' -.-: :V j £'J Stratum lucidum germinativum Portion of section of skin from sole of foot, showing layers of epidermis. X 70. ordinarily represented by only a single row of cells to which the name, stratum, granulosum, is usually applied. This layer marks the level at which the conversion of the epithelial elements into horny plates begins and also that at which the separation effected by blistering usually occurs. On the palms and soles, where the epidermis attains not only great thickness but also higher differentiation, four distinct layers may be recognized in vertical sec- tions of the cuticle. From the corium outward, these are: (i) the stratum germina- tivum, (2) the stratum granulosum, (3) the stratum lucidum and (4) the stratum corneum. The first two represent the portion of the epidermis endowed with the greatest vitality and powers of repair and the last two the horny and harder part. The stratum germinativum, or stratrtm Malpighi, rests upon the outer sur- face of the corium, by the papillae of which it is impressed and, hence, when viewed from beneath after being separated, commonly presents a more or less evident net-work 'of ridges and enclosed pits, the elevations corresponding to the 1 386 HUMAN ANATOMY. •: -r. Stratum corneum Stratum lucidum Stratum granulosum Stratum germinativum Deepest cells of epidermis Corium Portion of preceding preparation, showing in more detail layers of epidermis only deeper part of stratum corneum is represented. X zbo. interpapillary furrows and the depressions to the papillae. In recognition of this reticulation the name, rete Malpighi, is sometimes applied to the deepest layer of the epidermis. As in other epithelia of the stratified squamous type, the deepest cells are columnar and lie with their long axes perpen- dicular to the supporting connective tissue. The basal ends of the colum- nar cells are often slight- ly serrated and fit into corresponding indenta- tions on the corium. Their outer ends are rounded and received between the super- imposed cells. Succeed- ing the single row of columnar elements, the cells of the stratum germinativum assume a pronounced polygonal form, but become some- what flatter as they approach the stratum granulosum. The num- ber of layers included in the germinal stratum is not only uncertain, but varies with the rela- tion to the papillae, being greater between than over these projections. The finely granular cytoplasm of the cells of the stratum germinativum contains delicate but distinct fibriUce, which, longitudinally disposed in the deep columnar cells, in the polygonal elements (Fig. 1151), radiate from the nucleus towards the periphery (Kromayer). The fibrillas are not confined to the cells, but extend beyond and pass across the intercellular lymph -clefts as delicate protoplasmic bridges that connect the units of the various layers of the stratum and confer upon them the character- istics of the so-called ' ' prickle cells. ' ' The stratum granulosum is exceptionally well marked on the palms and soles and in these localities includes from two to four rows of polygonal cells, somewhat horizontally compressed, that stand out conspicuously in stained sections by reason of the intensely colored particleswithin theircytoplasm. The nature of the peculiar substance, deposited within the body of the cells as particles of irregular form and size, is still uncertain. To it Ranvier gave the name of eleidin and Waldeyer that of koatohyalin. Since the nuclei of the cells in which the deposits occur always exhibit evidences of degenera- tion, it is probable that keratohyalin is in some way derived from disintegra- tion of the nucleus (Mertsching) and represents a transition stage in the process ending in cornification of the succeeding layers of the cuticle (Brunn). The stratum lucidum, usually wanting in other localities, in the palm and sole appears as a thin, almost homogeneous layer, separating the corneous from the FIG. 1151. Fihrillre Intercellular, cleft Portion of horizontal section of skin, showing intracolliilar fibrillre within cells of stratum germinativum. X 800. THE SKIN. 1387 Pigmental epidermis Duct of sweat gland granular layer. With the latter it constitutes the stratum intermedium. As indicated by its name, the stratum lucidum appears clear and without distinct cell boundaries, although suggestions of these, as well as of the nuclei of the component elements, are usually distinguishable. The cells of the stratum lucidum are but little cornified and differ, therefore, from those of the overlying layers ; moreover, the eleidin within the cells of the stratum lucidum probably is in a fluid condition. The stratum corneum includes the remainder of the epidermis and consists of many layers of horny epithelial cells that form the exterior of the skin. Where no stratum lucidum exists, as is usually the case, the corneous layer rests upon the stratum granulosum, from which its horny elements are being continually recruited. During their migration towards the free surface, the cells lose their vitality and become more flattened until the most superficial ones are converted into the dead horny scales that are being constantly displaced by abrasion. The pigmentation of the skin, which even in white races is conspicuous in certain regions (page 1381), depends upon the presence of colored particles chiefly within the epidermis, although, when the dark hue is pronounced, a few small branched pigmental connective tissue cells may appear within FIG. 1152. the subjacent corium. The dis- tribution of the pigment particles varies with the intensity of color, in skins of lighter tints being principally, and sometimes en- tirely, limited to the columnar cells next the corium. With increasing color the pigment particles invade the neighboring layers of epithelium until, in the dark skin of the negro, they are found within the cells of the stratum corneum but always in diminishing numbers towards the free surface. Even when the cells are dark and densely packed, the colored particles never encroach upon the nuclei, which therefore annear as con Section of skin' surrounding anus, showing pigmentation of deeper uui u Ie> appeal layer of epidermis. X 50. spicuous pigment free areas. The source of the pigment within the epidermis is uncertain, by some being found in an assumed transference of the colored particles from the corium, by means of wandering cells or of the processes of pigmented connective tissue cells that penetrate the cuticle, and by others ascribed to an independent origin in situ within the epithelial elements. While it may be accepted as established that at times the connective tissue cells are capable of modifying pigmentation (Karg), it is equally certain that the earliest, and probably also later, intracellular pigmenta- tion of the epidermis appears without the assistance of the connective tissue or migratory cells. The blood-vessels of the skin are confined to the connective tissue portion and never enter the cuticle. The arteries are derived either from the trunks of the subjacent layer as special cutaneous branches destined for the integument, or indi- rectly from muscular vessels. When the blood supply is generous, as in the palms and soles and other regions subjected to unusual pressure or exposure, the arteries ascend through the subdermal layer to the deeper surface of the corium where, having subdivided, they anastomose to form the siibcutaneous plexus (rete arteriosura cutaneum). From the latter some twigs sink into the subdermal layer and contribute the capillary net-works that supply the adipose tissue and the sebaceous glands. Other twigs, more or less nunierous, pass outward through the deeper part of the corium and within the more superficial stratum unite into a second, subpapillary plexus (rete arteriosum subpapillare), that extends parallel to the free surface and I388 HUMAN ANATOMY. Papillary loops beneath the bases of the papillae. The latter are supplied by the terminal twigs which ascend vertically from the subpapillary net-work and break up into capillary loops that occupy the papillae and lie close beneath the epidermis (Fig. 1153). With the exception of the loops entering the hair-papillae, the capillaries enclosing the hair- follicles arise from the subpapillary plexus. The arrangement of the cutaneous veins, more complex than that of the arteries, includes four plexuses (retia venosum) lying at different levels within the corium and extending parallel to the FIG. 1153. surfaces. The first and most superficial one is formed by the union of the radicles returning the blood from the papillae. The component veins lie below and parallel to the rows of papillae and im- mediately beneath the bases of the latter. At a slightly lower level, in the deeper part of the stratum papillare, the ve- nous channels proceeding from the subpapillary net- work join to form a second plexus with polygonal meshes. A third occurs about the middle of the corium, while the fourth shares the position of the subcutaneous arterial plexus at the junction of the corium and subdermal strata. The deepest plexus receives many of the radicles returning the blood from the fat and the sweat glands, the re- mainder being tributary to the veins accompany- ing the larger arteries as they traverse the tela subcutanea. The lymphatics of the skin are well repre- sented by a close super- ficial pic. \ us within the papillary stratum of the corium into which the terminal lymph-radicles of the papilla- empty. The Section of injected skin, showing general arrangement of blood-vessels. X 4°- elation of these channels to the interfascicular connective tissue spares is one only of indirect communication, since the lymphatics are provided with fairly complete endothelial walls. It is probable that the lymph-paths within the papilla- are closely related to the intercellular clefts of the epidermis, according to Unna,^ indeed, direct communications existing. Migratory leucocytes often find their way into the cuticle where thev then appear as the irrc-giilarly stellate cells of Langerhans seen between the epithelial elements. A u ide-meshed \Ar/> plc.vns of lymphatics is formed within the subdermal laver, from which the larger lymph-trunks pass along with the subcutaneous Mood vessels. THE HAIRS. 1389 The numerous nerves within the highly sensitive integument are chiefly the peripheral processess of sensory neurones which terminate in free arborizations between the ephithelial elements of the cuticle, or in relation with special endings located, for the most part, within the corium or subdermal connective tissue. Some sympathetic fibres, however, are present to supply the tracts of involuntary muscle that occur within the walls of the blood-vessels or in association with the hairs and the sweat glands. On entering the skin the medullated nerves traverse the subdermal layer, to which they give off twigs in their ascent, and, passing into the corium, within the papillary stratum divide into a number of branches. Those destined for the epidermis beneath the latter break up into many fibres which, losing their medullary substance, enter the cuticle and end in arborizations that ramify between the epithelial cells as far as the outer limits of the stratum germinativum. The ultimate endings of the fibrillae, whether tapering or slightly knobbed, always occupy the intercellular channels and are never directly connected with the substance of the epithelial elements. According to Merkel, special tactile cells, (Fig. 867) occur in the human epidermis, particularly over the abdomen and the thighs. These cells, spherical or pyriform in shape and composed of clear cytoplasm, occupy the deeper layers of the cuticle and, on the side directed towards the corium, are in contact with the end-plate or meniscus of the nerve. The nerve-fibres particularly concerned with the sense of touch terminate within the connective tissue portion of the skin, either within the corium in special end-organs — the tactile bodies of Meissner, the end-bulbs of Krause, the genital corpuscles and the end-organs of Ruffini, or within the subdermal layer in the Vater-Pacinian cor- puscles, or their modifications, the Golgi-Mazzoni corpuscles. The structure of these special end-organs is elsewhere described (pages 1018, 1019), their chief locations being here noted. Meissner's corpuscles (Fig. 872) are especially numerous in the tactile cushions on the flexor surface of the hands and feet. While much more plentiful in all the tactile pads than in the intervening areas, the touch corpuscles are most abundant in those on the volar surface of the distal phalanges, where they approxi- mate twenty to the square millimeter (Meissner). Their favorite situation is the apex of the papillae, where they appear as elongated elliptical bodies, sometimes in pairs, whose outer pole lies immediately below the epidermis. These corpuscles are additionally, although sparingly, distributed on the dorsum of the hand, the flexor surface of the forearm, the lips, the eyelids, the nipple and the external genital organs. The Vater-Pacinian corpuscles (Fig. 874) are well represented in the hands and feet and usually occupy the subdermal tissue, although sometimes found within the corium. Their distribution corresponds closely to that of Meissner's corpuscles, they being most numerous beneath the tactile cushions in the order above described. The Golgi-Mazzoni corpuscles are modifications of the Pacinian bodies and, like the latter, are found within the subdermal tissue. The end-bulbs of Krause (Fig. 869) occur within the corium, either slightly below or within the papillae, on the lips and external genital organs, as well as probably in other regions. The genital corpuscles (Fig. 870) lie within the corium of the modified skin covering the glans penis and the prepuce and the clitoris and surrounding parts of the nymphae. The end-organs of Ruffini resemble the sensory terminations in tendons (page 1017) and lie within the deeper parts of the corium, often associated with the Pacinian bodies. The mode of ending of the nerves supplying the hairs and sweat glands will be described in connection with those structures (pages 1394, 1400). THE HAIRS. The appendages of the skin — the hairs, nails and cutaneous glands — are all specializations of the epidermis and are. therefore, exclusively of ectoblastic origin. The hairs (pili) are present over almost the entire body, the few localities in which they are absent being the flexor surface of the hands and feet, the extensor aspect of the terminal segment of the fingers and toes, the inner surface of the 1390 HUMAN ANATOMY. prepuce and of the nymphae and the glans penis and clitoridis. With the exception of those regions in which the growth is sufficiently long to constitute a complete cover- ing— the scalp, bearded parts of the face in the male, axillae and mons pubis — the hairs are for the most part short and scattered, although subject to great individual variation and sometimes to remarkable redundance. The hairs in various locations are known by special names ; those of the scalp being capilli ; of the eyebrows, supercilia ; of the eyelashes, cilia; of the nostrils, vibrissce ; of the external ear, tragi ; of the beard, barba ; of the axillae, hird ; of the pubes, pubes ; while the fine downy hairs that cover other parts of the body are designated lanugo. The closest set hairs are on the scalp, where, according to Brunn, on the vertex they number from 300-320, and in the occipital and frontal regions from 200-240 per square centimeter. On the chin 44 were counted, on the mons pubis 30-35, Epidermfs Sebaceous gland Erector muscle — - Sweat gland Root Hair-papilla Inner root-sheath Outer root-sheath Bulb Papilla Paniculus adiposus Section of scalp, showing longitudinally cut hair-follicles. X 14. on the extensor surface of the forearm 24 and on the back of the hand 18 for like areas. Even where their distribution is seemingly uniform, close inspection shows the hairs to be arranged in groups of from two to five. The length of the hairs includes the extremes presented by the lanugo, only a few millimeters long, on the one hand, and by the scalp-growth, sometimes meas- uring 150 cm. (59 in.) or more, on the other. Their thickness, likewise, shows much variation, not only in different races, individuals and regions, but also in the same person and part of the body, as on the scalp where fine and coarse hairs may lie side by side. The thickest scalp-hairs have a diameter of .162 mm. and the finest one of .011 mm., with all intermediate sizes. The hairs of the beard vary from .101-. 203 mm. and those on the pubes from .054-. 135111111. (Falck). In a general way hairs of light color are finer than dark ones, the respective diameters of blond, brown and blaek hairs being .047, .054 and .067 mm. . (Wilson). On attaining their full growth without mutilation, hairs do not possess a uniform thick- ness throughout their length, since they diminish not only towards the tip, where the shatt ends in a point, but also towards the root. This feature is most evident in short hairs, as in those of the eyebrows. The color of the hair, which varies from the lightest straw to raven black, i closely associated with racial and individual characteristics, being usually, but by no THE HAIRS. 1391 means always, in harmony with the degree of general pigmentation. The latter is commonly uniform throughout the length of the hair, but in rare cases it may be so variable that the shaft presents a succession of alternating light and dark zones (Brunn). The straight and curly varieties of hair depend chiefly upon differences in the curvature of the follicle l and the form of the hair. In the case of straight hairs the follicle is unbent and the shaft is cylindrical, and therefore circular in cross- section ; hairs that are wavy or curly spring from follicles more or less bent and are flattened or grooved, with corresponding oval, reniform, irregularly triangular or indented outlines when transversely cut. Arrangement of the Hairs. — Since the buried part of the hair, the root, is never vertical but always oblique to the surface of the skin, it follows that the free part, the shaft, is also inclined. The direction in which the hairs point, however, is by no means the same all over the body, but varies in different regions although constant for any given area. This disposition depends upon the peculiar placing of the hair-roots which in certain localities incline towards one another along definite lines, an arrangement that results in setting the shafts in opposite directions. As these root-lines are not straight but spiral, on emerging from the skin the hairs diverge in whorls (vortices pilorum), the position and number of which are fairly definite. Such centres include: (i) the conspicuous vertex whorl on the head, usually single but sometimes double; (2) the facial whorls surrounding the openings of the eyelids; (3) the auricular whorls at the external auditory meatus ; (4) the axillary whorls in the armpits ; and (5) the inguinal whorls, just below the groin ; additional (6) but less constant lateral whorls may be located, one on each side, about midway between the axilla and the iliac crest and somewhat beyond the outer border of the rectus muscle. These whorls, all paired except the first, apportion the entire surface of the body into certain districts, each covered by the hairs proceeding from the corresponding vortex. The whorl-districts, moreover, are irregularly subdivided into secondary areas by lines, the hair- ranges (flumina pilorum), along which the hairs diverge in opposite directions. Additional lines, the converging hair-ranges, mark the meeting of tracts pointing in different directions and in places also assume a spiral course. In consequence of these peculiarities the body is covered with an elaborate and intricate hair-pattern, that is most evident on the foetus towards the close of gestation ; later in life the details of the pattern are uncertain owing to its partial effacement by the constant rubbing of clothing. Structure. — Each hair consists of two parts, the shaft, which projects beyond the surface, and the root, which lies embedded obliquely within the skin, the deepest part of the root expanding into a club-shaped thickening known as the bulb. The root is covered with a double investment of epithelial cells, the inner and outer root- sheaths, which, in turn, are surrounded by a connective tissue envelope, the theca. The entire sac-like structure, consisting of the hair-root and its coverings, constitutes the hair-follicle (folliculus pili). At the bottom of the latter, immediately beneath the bulb, the wall of the follicle is pushed upward to give place to a projection of connective tissue, the hair-papilla, which carries the capillary loops into close relation with the cells most active in the production of the hair. 'Save in the case of the finest hairs (lanugo), which are limited to the corium, the hair-follicles traverse the latter and end at varying levels within the fat-laden subdermal layer (panniculus adiposus). In a general way the follicle may be regarded as a narrow tubular invagi- nation of the epidermis, at the bottom of which the hair is implanted and from the entrance of which the shaft projects. The most contracted part of the follicle, the neck, lies at the deeper end of the relatively wide funnel-shaped entrance to the sac. Closely associated with the hair-follicle, which they often surround, are the sebaceous glands that pour their oily secretion at the upper third of the follicle into the space between the shaft and the wall of the sac. The Hair-Shaft. — In many thick hairs, but by no means in all, three parts can be distinguished — the cuticle, the cortex and the medulla. The latter, however, is usually wanting in hairs of ordinary diameter, being often also absent in those of large size. 1 Frederic : Zeitschr. f. Morph. u. Anthropo!., Bd. ix., 1906. 1392 1 1 I'M AN ANATOMY. The cuticle of the hair appears as a transparent outermost layer marked by a net-work of fine sinuous lines, the irregular meshes of which have their longest diameter placed obliquely transverse. These lines correspond to the free borders of extremely thin glassy cuticle-plates that overlie the hair as tiles on a roof, the imbrication involv- ing from four to six layers. Seen in profile (Fig. 1155), the contour of the hair-shaft, therefore, is not smooth but serrated, the minute teeth formed by the free margins of the scales being directed towards the tip of the hair. After isolation by suitable reagents, the cuticular elements appear as transparent structureless cells, quadrilateral in outline and curved to con- form to the hair-shaft which they cover. The cortical substance, often indeed constituting practi- cally the entire shaft, consists of elongated fusiform cells so compactly arranged that the individual elements are only dis- tinguishable after the action of disassociating reagents. In addition to the remains of the shrunken nuclei the hair- spindles, as these modified epithelial cells are called, possess fibrillse that pass between adjacent cells similar to the inter- cellular bridges in the epidermis. A variable amount of pigment, present either as a diffuse tint of the spindles, or as granules within or between the same, is a constant constituent of the cortical substance. In blond hair the color is chiefly- diffuse, the pigment granules being often entirely wanting ; in hair of darker shades, the granules predominate and increase in intensity of color as well as in quantity. As the hair grows outward from the bulb, it loses much of its moisture, and in consequence later contains minute air-vesicles that replace the fluid previously occupying the clefts between the hair-spindles. Even when conspicuous, the medulla does not extend the entire length of the hair, often being interrupted and always disappearing before reaching the tip. The medulla, when well represented, is seen as an axial stripe, somewhat uneven in outline, that varies with illumination, with transmitted light appearing as a dark band and with reflected light as a light one. This peculiarity depends upon the presence of air imprisoned between the shrunken and irregular medullary cells — dried and cornified epithelial elements which are con- nected by branching processes into a net-work incompletely filling the medulla. The air within the shaft is a factor modifying the color of the hair, since the resulting reflex tends to lessen the intensity of the tint directly referable to the pigment ; FIG. 1156. this diminution affects par- ticulary the lighter shades, as in dark hairs the large amount of pigment masks the reflex. Portion of shaft of hair; k, shaft covered with cuticle ; s, cuticle re- moved to expose cortical substance; tn, medulla X 125. a, 6, isolated cells of cuticle and of cortical substance respectively. X 240. Outer root-sheath Hair surrounded by inner root-sheath KvMox •Adipose tissue The Hair-Folli- cle.— This structure consists essentially of ( i ) a connective tissue sheath, the theca, con- tributed by the corium ; (2) an epithelial lining, the outer root-sheath, continued from the deepest layer of the epidermis; and (3) the inner root- sheath, an epithelial investment probably differentiated within the follicle, and not a direct prolonga- tion from the cuticle. The theca folliculi includes three strata : an outer, composed of loosely dis- posed longitudinal bundles of fibrous tissue with few cells and elastic fibres ; a middle one, made up of closely placed circular bundles ; and a very thin, homogeneous inner coat, the _;'/y two or more, which approaches the follicle immediately below the level of the mouth of the sebaceous glands. After penetrating the fibrous sheath as far as tin- glassy membrane, the nerve-fibre separates into two divisions that encircle more or less completely the follicle and on the opposite side break up into numerous fibrillae constituting a terminal arborization. The nerve-endings usually lie on the outer surface of the glassy membrane within the middle third of the follicle and only exceptionally are found within the outer root-sheath or the hair-papilla. THE NAILS. The nails ( unties), the horny plates overlying the ends of the dorsal surfaces of the linger-; and toes, correspond to the claws and hoofs of other animals and, like them, are composed exclusively of epithelial tissue. They are specializations of the Papillary ' twig Portion of section of injected scalp, s net-works surrounding hair-follicles and papillae. X 20. owing capillary twigs entering THE NAILS. 1395 epidermis and, therefore, may be removed without mutilation when the cuticle is taken off after maceration. The entire nail-plate is divided into the body (corpus unguis), which includes the exposed portion, and the root (radix unguis), which is embedded beneath the. skin in a pocket-like recess, the nail-groove (sulcus unguis). The modified skin supporting the nail-plate, both the body and the root, constitutes the nail-bed (solum unguis), the cutaneous fold overlying the root being the nail-wall (vallum unguis). The sides of the quadrilateral nail-plate are straight and parallel and at their distal ends connected by the convex free margin (margo liber) that projects for a variable distance beyond the skin. The proximal buried border (margo occultus) is straight or slightly concave, more rarely somewhat convex, and often beset with minute serrations (Brunn). Both surfaces of the transversely arched nail are smooth and even, with the exception of the longitudinal parallel ridges that often mark the upper aspect. Inspection of the latter during life shows color-zones, the translu- cent whitish crescent formed by the projecting portion of the nail being immediately followed by a very narrow yellow band that corresponds to the line along which the stratum corneum of the underlying skin meets the under surface of the plate. The FIG. 1159. B c Distal portions of fingers, showing relations of nail ; A was drawn from living subject ; B and C are lateral and under views respectively of inner surface of cuticle with nail ; nothing but the epidermal structures are present, the cuticle and nail having been removed together, a, 6, distal and proximal borders of nail ; c, under surface of nail ; rf, nail in section ; e, line of deflection of cuticle to under surface of nail ; f, lunula ; g, nail-wall ; h, cuticle in section. succeeding and larger part of the nail is occupied by the broad pink zone which owes its rosy tint to the blending of the color of the blood in the underlying capillaries with that of the horny substance. On the thumb constantly, but on the fingers often only after retraction of the cuticle, is seen a transversely oval white area, the so-called lunula, which marks the position of the underlying matrix. Additional white spots, irregular in position, form and size, are sometimes seen as temporary markings. The thickness of the nail-plate — greatest on the thumb and large toe and least on the last digits — diminishes towards the sides, but in the longitudinal direction, between the lunula and the free margin of the nail, is fairly uniform ; beneath the white area, however, the under surface of the nail shelves off towards the buried border, where it ends in a sharp edge. Structure. — The substance of the nail-plate (stratum corneum unguis) consists entirely of flattened horny epithelial cells, very firmly united and containing the remains of their shrunken nuclei. These cornified scales are disposed in lamellae, which, in transverse section, pursue a course in general parallel with the dorsal sur- face. In nails which possess the longitudinal ridges, however, the latter coincide with an upward arching of the lamellae dependent upon the conformation of the nail matrix (Brunn). In longitudinal section the lamellation is oblique, extending 1396 HUMAN ANATOMY. from above downward and forward, parallel to the shelving under surface beneath the white area that rests upon the matrix. Minute air-vesicles, imprisoned between the horny scales, are constant constituents of the nail-substance. When these occur in unusual quantities, they give rise to the white spots in the nail above mentioned. Corresponding respectively to the colored zones — the white, rosy and yellow- seen on the dorsal surface of the nail, the nail-bed is divided into a proximal, FIG. i i 60. Subcutaneous tissue Stratum germinativum Stratum corneum v-x Eponychium and Corium of nail-bed Nail-plate Epidermis- .Transformation zone ^Matrix Longitudinal section of proximal part of nail lying within the nail groove. X 3°- a middle and a distal region, each of which exhibits structural differences. The most important of these regions is the proximal, known as the matrix, which lies beneath the white area and alone is concerned in the production of the nail. The corium of the nail-bed varies in the different regions in the arrangement and si/e of its elevations. Within the proximal third of the matrix, these elevations occur in the form of low papillae, which decrease in height and number until they disappear, a smooth field occupying the middle of the matrix. This even field is succeeded by one possessing closely set, low, narrow longitudinal ridges, that at the distal margin of the lunula suddenly give place to more pronounced, but less numerous broader, linear elevations. These continue as far as the distal end of the nail-bed and are then replaced by papilla?. Owing to the strong fibrous bands and the absence of the usual layer of fatty subdermal tissue, the corium of the nail-bed is closely attached to the bone. The fibrous reticulum formed by the interlacing of the longitudinal with the vertical bundles contains few elastic fibres, since these are entirely wanting beneath the of the nail and only present in meagre numbers within the matrix. In view of its genetic activity, the relations of the epidermis underlying the nail are of especial interest. While the stratum germinativum of the skin covering the finger tip passes directly and insensibly onto the nail-bed, the entire extent of which it invests (stratum liviim unguis), the stratum corneum ends on reaching the under surface of the nail-plate, the Hi of apposition corresponding to the narrow yellow zone which defines the distal boundary ot rosy area. Heiieath tin- latter, therefore, the epidermis of the nail-bed consists of the strati germinatn urn alone, which, without cornification of any of its cells, rests against the under snr- • f the nail, llenrath the white /one, that is, within the matrix, the epidermis includes a halt do/en or more layers of the usual elements of the stratum germinativum, surmounted by a like number of strata of cells distinguished by a peculiar brownish color. On reaching the nail these modified epithelial elements, which appear white by reflected light, are not circumscribed, but pass over into the siibst;uice of the nail, into the constituent cells of which they are directly con- verted. Their cytoplasm presents a marked fibrillation to which, according to Hrunn, the light appearance of the cells is referable as an interference phenomenon and not as a true pigmenta- tion. This peculiarity of the cells, coupled with the relatively small size of subjacent capillii THE CUTANEOUS GLANDS. 1397 probably accounts for the tint distinguishing the white area. Since the transformation of the cells of the stratum germinativum into those of the nail-plate is confined to the matrix, it is evi- dent that the continuous growth 'of the nail takes FIG. 1161. place along the floor and Nail-bed Nail-plate bottom of the nail-groove, the last formed increment of nail-substance pushing forward the previously dif- ferentiated material and thus forcing the nail to- wards the end of the digit. The relation of the epi- dermis of the nail-wall to the substance of the plate is one of apposition only, production of the nail oc- curring in no part of the fold. Over the greater extent of the latter all the typical constituents of the cuticle are represented, but within the most proximal portion the stratum germi- nativum alone is present, the stratum corneum fad- ing away. Where the horny layer exists, it rests directly upon the nail, but is differentiated from the latter by being less dense Transverse section of nail-wall and adjacent part of nail-plate and nail-bed. X'go. and by its response to stains. As the nail leaves the groove, a part of the stratum germinativum of the nail-wall is prolonged distally for a variable distance over the dorsal surface of the nail-plate as a delicate membranous sheet, the eponychium, which usually ends in a ragged abraded border. Stratum corneum and Stratum germinativum of nail-wall Eponychium — — Margin of nail — Corium — THE CUTANEOUS GLANDS. These structures include two chief varieties, the sebaceous and the sweat glands^ together with certain modifications, as the ceruminous glands within the external auditory canal, the circumanal glands, the tarsal and ciliary glands within the eyelid and the mammary glands. In all the epithelial tissues — the secreting elements and the lining of the ducts — are derivatives of the ectoblast and, therefore, genetically related to the epidermis. THE SEBACEOUS GLANDS. Although these structures (glandulae sebacae) are chiefly associated with the hair-follicles, in which relation they have been considered (page 1394), sebaceous glands also occur, if less frequently, independently and in those parts of the skin in which the hairs are wanting, as on the lips, angles of the mouth, prepuce and labia minora. The size of these glands bears no relation to that of the hairs, since among the smallest (.2-. 4 mm.) are those on the scalp. The largest, from .5-2.0 mm., are found on the mons pubis, scrotum, external ear and nose. Conspicuous aggre- gations, modified in form, occur in the eyelid as the Meibomian glands. Depending upon the size of the glands their form varies. The smallest ones are each little more than a tubular diverticulum, dilated at its closed end. In those of larger size the relatively short duct subdivides into several expanded compartments, which, in the largest glands, may be replaced by groups of irregular alveoli, with uncertain ducts that converge into a short but wide common excretory passage. Structure. — The structural components of these glands include a fibrous envelope, a membrana propria and the epithelium, the first two being continuous with the corresponding coverings of the hair-follicle. The epithelium continued 1398 HUMAN ANATOMY. Duct Alveoli Sebaceous glands from skin covering nose. X 60. into the ducts and alveoli of the sebaceous glands is directly prolonged from the outer root-sheath of the epidermis, where associated with the hair-follicles, or from the epidermis where the hairs FIG. 1162. are wanting. The periphery of the alveolus is occupied by a single, or incompletely double, layer of flattened and imper- fectly denned basal cells, that rest immediately upon the mem- brana propria and are distin- guished by theirdark cytoplasm and outwardly displaced oval nuclei. Passing towards the centre of the alveolus, the next cells contain a number of small oil drops which, with each suc- cessive row of cells, become larger and appropriate more and more space at the expense of the protoplasmic reticulum in which they are lodged. In consequence, the cells occupy- ing the axis of the alveoli, which are completely filled and with- out a lumen, contain little more than fat. As the cells are escaping from the glands they lose their nuclei and individual outlines and, finally, are merged as debris into the secretion, or sebum, with which the hairs and skin are anointed. The necessity for new cells, created by the continual destruction of the glandular elements that attends the activity of the sebaceous glands, is met by the elements recruited from the FIG- 1163. proliferating basal cells, which in turn pass towards the centre of the alveolus and so displace the accumulating secretion. THE SWEAT GLANDS. These structures (glandulae sudoriferae), also called the sudoriparous glands, are the most important representatives of the coiled glands (glandulae gloini- fornies) often regarded as constituting one of the two groups (the sebaceous glands being the other) into which the cutaneous glands are divided. They occur within the integument of all parts of the body, with the exception of that covering the red margins of the lips, the inner surface of the prepuce and the glans penis. They are especially numerous in the palms and soles, in the former locality numbering more than uoo to the square centimetre (Horschelmann), and fewest on the back and buttocks, where their number is reduced to about 60 to the square centimetre ; their usual quota for the same area is between two and three hundred. Modified simple tubular in type, each gland consists of two chief divisions, the ' (corpus) or gland-coil, the tortuously wound tube in which secretion takes place, and the c.\-i-n-tnry duct (dtictus sudorifenis ) which opens on the surface of the skin, exceptionally into a hair-follicle, by a minute orifice, the sweat poit i poms siulnrifiTiis . often distinguishable with the unaided eye. The body of the gland, irregularly spherical or tlattened in form and yellowish red in color, consists of the windings of a single, or rarely branched, tube and com- monly occupies the deeper part of the curium, hut sometimes, as in the palm and Cells from alveoli of sebaceous eland, slum-in.^ ictiru'au-il protoplasm due to invsrm-e of oil droplets. X 700. THE CUTANEOUS GLANDS. 1399 Spiral part of d scrotum, lies within the subdermal connective tissue. The coiled portion of the gland is not entirely formed by the secretory segment, since, as shown by the recon- structions of Huber, about one fourth is contributed by the convolutions of the first part of the duct. On leaving the gland-coil, in close proximity to the blind end of the gland, the duct ascends through the corium with a fairly straight or slightly wavy course as far as the epidermis. On entering the latter its further path is marked by conspicu- ous cork-screw-like windings, which, where the cuticle is thick as on the palm, are close and number a dozen or more and terminate on the surface by a trumpet-shaped orifice, the sweat-pore. In its course through FlG- Il64- the corium the duct never traverses a papilla or ridge, but always enters the cuti- cle between these ele- vations. On the palms and soles, where the pores occupy the sum- mit of the cutaneous ridges, the ducts enter the cuticle between the double rows of papillae. Structure. — The secreting portion of the gland-coil, called \heampulla on account of its greater diameter, possesses a wall of remarkable structure. The thin external sheath, composed of a layer of dense fibrous tissue andelastic fibres, supports a well defined mcmbrana propria. Immediately within the latter lies a thin but compact layer of invol- untary muscle whose longitudinally disposed spindle - shaped ele- : duct Stratum "corneum S. lucidum S. granulosum S.germinativutK Duct of sweat-gland Corium -. Fat-cells Coiled part of sweat-gland Section of skin from palm, showing different parts of sweat-glands extending from surface into tela subcutanea. X 65. ments in cross-section appear as a zone of irregularly nucleated cells that encircle the secreting epithelium and displace it from its customary position against the basement membrane. This muscular tissue enjoys the distinction, sharing it with the muscle of the iris, of being developed from the ectoblast. The secreting cells constitute a single row of low columnar epithelial elements, that lie internal to the muscle and surround the relatively large lumen. Their finely granular cytoplasm contains a spherical nucleus, situated near the base of the cell, and in certain of the larger glands, as the axillary, includes fat droplets and pigment granules. These are liberated with the secretion of the gland and when present in unusual quantity account for the discoloration produced by the perspiration of certain individuals. In the case of the ceruminous glands, the amount of oil and pigment is constantly great and confers the distinguishing characteristics on the ear-wax. The sudden and conspicuous reduction in the size of the tube which marks the termination of the secreting segment and the beginning of the duct, is accompanied by changes in the structure of its wall. In addition to a reduction of its diameter to 1400 HUMAN ANATOMY. FIG. 1165. Muscle-cell Secreting-cells Parts of duct one-half or less of that of the ampulla, the duct loses the layer of muscle and becomes flattened, with corresponding changes in the form of its lumen. The single row of secreting elements is replaced by an irregular double or triple layer of cuboidal cells, which exhibit an homogeneous zone, sometimes described as a cuticle, next the lumen. On entering the epidermis, the duct not only loses its fibrous sheath and membrana propria, but the epithelial constituents of its wall are soon lost among the cells of the stratum germinativum, so that its lumen is continued to the surface as a spiral cleft bounded only by the cornified cells of the cuticle. Apart from mere variations in size, certain glands — the circumanal, the cilia rv and the ceruminous — depart sufficiently from the typical form of the coiled glands to entitle them to brief notice. The circumanal glands, lodged chiefly within a zone from 12-15 mm- wide and about the same distance from the anus, are not all the same, but include, according to Huber, four varieties. In addition to ( i ) the usual sweat glands and (2) some (Gay's) of e.\ tour years and the eyelashes for only a few months ( Pincusi. During the years of greatest vitality not only are the discarded hairs replaced by new ones, but the actual number of hairs may increase in consequence of the development of additional follicles from the epidermis after the manner of the primary formation. When from age or other cause the hair-follicles loose their productive activity and, therefore, are no longer capable of replacing the atrophic hairs, more or less conspicuous loss of hair results, whether only tem- porarv or permanent evidently depending upon the recuperative powers of the follicles. The change of hair that is continually and insensibly occurring in man, in contrast to the conspicuous periodic shedding of the coat seen in other animals, includes the atrophy of the old hair on the one hand, and the development f i • , TM- , • . , . , /greatest breadth X ioo\ mankind. 1 his relation, the cephalometric nasal index \ \ greatest length / varies with different races, according to Topinard the index of the white races being below 70 {leptorhines\ that of the yellow and red races between 70 and 85 (mesorhines} , and that of the black races above 85 (fi/atyrhincs} . THE CARTILAGES OF THE NOSE. The cordiform nasal opening ( apcrtura pvriformis ) of the facial skeleton, bounded by the free margins of the nasal and superior maxillary hours, is enclosed and continued to the anterior nares by the nasal cartilages and contiguous fibrous tissue. These cartilages are usually considered as including five chief plates, the unpaired and the paired upper and hncer lateral, and a variable number of smaller THE CARTILAGES OF THE NOSE. 1405 supplemental pieces (cartilagines minores). The conventional division of the first three, however, is unwarranted, since embryologically and morphologically they constitute one piece (cartilage niediana nasi), which even in the adult is represented by the connected septal and upper lateral plates. The cartilage of the septum (cartilago septi nasi) (Fig. 1171) completes the median partition that divides the right and left nasal fossae from each other and represents the anterior extremity of the primordial cartilaginous cranium. It is irregularly rhomboidal in form and so placed that its superior angle lies above, received between the nasal bones and the median plate of the ethmoid, and its inferior angle below, resting upon the incisor crest of the maxillae. The anterior angle is directed forward and the posterior, much the more pointed, is prolonged as the sphcnoidal process (processus sphenoidalis septi cartilaginei) for a variable distance between the mesethmoid and the vomer towards the body of the sphenoid, which exceptionally it may reach. The antero-superior margin of the septal carti- lage, thickest above, is attached to the under surface of the internasal suture for a FIG. 1171. ^Perpendicular plate of ethmoid Frontal sinus - Septal cartilage ) ^~ — - Sphenoidal sinus Mesial crus of left lower lateral cartilage Vomerine cartilage" / f HR^IB \ A^' Posterior naris ; \ Sphenoidal process Vomer Nasal septum viewed from left side ; mucous membrane has been partially removed. distance of from 12-15 mm- Below the nasal bones, the margin of the septal cartilage is continuous with the upper lateral cartilages which form ring-like expan- sions (alae) of the median plate. Still lower, the free-margin of the latter extends between the lower lateral cartilages to within about a half inch from the tip of the nose which, however, it does not reach, the medial crura of the lower lateral plates intervening. The postero-superior margin, the thickest part of the cartilage, is attached to the free margin of the perpendicular plate of the ethmoid bone. The postero-inferior margin rests upon the anterior part of the upper margin of the vomer and the incisive crest as far as the anterior nasal spine, where the border passes into the rounded antero-inferior margin that joins the nasal spine with the anterior angle. This border is always convex and does not reach the lowest part of the partition between the nostrils, which being devoid of septal cartilage, is freely movable and constitutes the septum mobile. The upper lateral cartilages (cartilagines nasi laterales) (Fig. 1172) are two triangular plates, one on either side, that by their median and longest border are attached to the septal cartilage, with which in their upper part they are directly continuous. The upper margin of each is joined to the free border of the nasal bone, which it slightly underlies, and, exceptionally, the adjacent edge of the maxilla. The lower margin is embedded in fibrous tissue which connects it with the adjoining plates. The median parts of the cartilages are markedly convex and separated by a slight groove that is, for the most part, obliterated by fibrous tissue. 1406 HUMAN ANATOMY. N Upper lateral cartilage Small alar cartilage Lower lateral . cartilage Nasal . aperture Cartilage at tip The lower lateral cartilages (cartilagines alares majores) (Fig. 1172) area pair of thin curved plates that encircle the apertures of the nostrils anteriorly and constitute the framework of the tip of the nose. Each cartilage consists of an inner plate (crus mediale), from 6—7 mm. FIG. 1172. broad, which, with its fellow of the opposite side, embraces the lower and anterior part of the septal cartilage and aids in com- pleting the partition separating the nares. In front it narrows, bends sharply outward, and passes seotai more or less abruptly into a broader outer plate (crus laterale), which is of very uncertain form and size, although of a general elongated oval shape and some 12 mm. broad. The triangular space between the varyingly prolonged posterior end of the lateral plate, the maxilla and the upper lateral cartilage is filled out by fibrous tissue in which Nasal bone Septal "cartilage Lower lateral "cartilage Mesial crusof -lower lateral cartilage Bony and cartilaginous framework of nose, front aspect. are embedded two, three or more small cartilaginous pieces (cartilagines alares minores). These vary greatly in size and form, but in a general way tend to complete the ring of cartilage surrounding the lateral wall of the nares. They do not, however, reach the lower border of the nasal ring, which, as well as the remaining part of the lower boundary of the aperture of the nostril, is devoid of cartilage and composed of integument and fatty connective tissue. The rounded anterior angles of the lower lateral cartilages occupy the tip of the nose, close together when this is pointed, but separated by a space that shows externally as a more or less evident groove when the tip of the nose is blunt and broad. The median plates approach the septal cartilage closer in front than behind, where they curve outward to end in a rounded and upward curving hook. The fibrous tissue uniting the median borders of the lower lateral plates with the anterior edge of the septal cartilage usually contains two small sesamoid cartilages (cartilagines sesamoideae nasi) that partly fill the triangular intervals on either side of the median line. The vomerine cartilages (cartilagines vomeronasales) are two narrow strips, from 1-2 mm. wide and from 10-15 mm- l°ng> tnat ne> one on either side, along tlu lower border of the septal cartilage in the vicinity of the nasal crest. They are attached to the carti- lage and bone by fibrous tissue and situated beneath the mucous membrane lining the nasal fossae. Their chief interest is their rela- tion to the rudimentary or^an of Jacobson (page 1417) below which they lie. In animals in which the organs are well devel- oped these cartilages fc >rm protect- ing and supporting scrolls ; in man, however, both organ and cartilaiM- are so feebly developed that they loose their close relation. The internment covering the outer nose is in general thin and closely bound down to the underlying fibrous tissue, being particularly unyielding over the tip and alae. With the exception of within the al;e and lateral borders of the nostrils, the Lower lateral cartilage Upper lateral cartilage Small alar cartilage Cartilage of tip Lateral crus Mesial crus cartilage. Nasal aperture Si-ptal cartilage Cartilages of nose, viewed from below. PRACTICAL CONSIDERATIONS: THE EXTERNAL NOSE. 1407 fatty tissue is very meagre. The sebaceous glands, on the other hand, are well developed and open in many instances in conjunction with the follicles of the delicate hairs that cover all parts of the surface. On the alae the closely placed glands are of exceptional size and open by ducts readily seen as minute depressions. Vessels. — In order to compensate for the exposed position, the external nose is generously supplied with arteries^ derived chiefly from the facial and ophthalmic, which are united by numerous anastomoses with each other as well as with branches from the infraorbital. The veins are all tributary to the angular vein, which begins at the inner canthus and descends along the side of the nose to the facial trunk, receiving in its course the dorsal, lateral, and alar branches. The angular vein communicates with the ophthalmic and the veins of the nasal fossa. The lymphatics are arranged in three sets (Kuttner). The first, beginning at the root of the nose, passes above the upper eye-lid and along the supraorbital ridge to the parotid nodes. The second group, formed by the superficial and deep lym- phatics at the nasal root, skirts the lower margin of the orbit and ends in the lower parotid nodes. The third and most important set includes from 6 to 10 trunks that follow the blood-vessels and end in the submaxillary nodes. The nerves supplying the outer nose include the motor branches of the facial to the muscles and the sensory twigs from the trifacial to the skin, distributed by the infratrochlear and nasal branches of the ophthalmic and by the infraorbital of the superior maxillary. PRACTICAL CONSIDERATIONS : THE EXTERNAL NOSE. The Nose may be congenitally absent, or bifid, or imperfect, as from absence of the septum or of one nostril, or — very rarely — of both nostrils. As to its external aspect it may be of various types, e.g. : Grecian, when the dorsum is on a practi- cally continuous straight line with the forehead, with no marked naso-frontal groove ; aquiline, with the dorsum slightly arched ; rounded, with the arch much more pronounced; foetal — "pug" — with the bridge depressed and the nostrils directed somewhat forward. The foetal type is simulated in the new born by the subjects of inherited syphilis in whom the bridge of the nose is often much depressed as a result either of (a) imperfect development following the severe specific coryza that affects the nasal mucosa and, through the close apposition of the latter to the periosteum of the fragile nasal bones, interferes with their nutrition ; or (£) by actual caries or necrosis of those bones or of the septum favored by the same conditions. In acquired syphilis the similar nasal deformity is practically always the result of the destruction of the septum, or, less frequently, of the nasal bones, by late (tertiary) lesions. As a consequence of faulty development in the anterior mid-portion of the frontal bone the membranes of the brain may protrude, forming a meningocele, which is more common at the naso-frontal junction than elsewhere. Occasionally the defect permitting the protrusion exists in the cribriform plate of the ethmoid, and the meningocele occupies the nasal fossa, having under these circumstances been mistaken for a nasal polyp and removed, death resulting from subsequent septic meningitis. The cosmetic importance of the nose is so great, the diseases producing deformity so frequent, and the susceptibility of the organ to injury so marked, that much ingenuity has been expended upon devices to restore it when lost, or to improve its appearance. In the Tagliacotian operation a cutaneous flap is taken from the arm which is held close to the nose by a complicated dressing until the flap is firmly united in its new position, when its pedicle is detached from the arm. The Indian method is more particularly anatomical, since the flap taken from the fore- head is so fashioned that it receives intact the blood from the frontal branch of the ophthalmic artery from the internal carotid, the ophthalmic receiving at the origin of the frontal an important anastomosis from the angular branch of the facial artery, which is given off from the external carotid artery. For partial deformities flaps may be taken from the sides according to the size and situation of the deficiency. I4o8 HUMAN ANATOMY. As upon other parts of the face, plastic operations are very successful owing to the free blood supply. Acne rosacc-a is common on account of the ready response in vascularity of the nose to external irritating influences, and to internal disturbances of the circulation, as from heart and lung disease, chronic gastritis, and alcoholism. Furuncles and superficial infections are frequent because of the number of sebaceous and sweat glands present. Lupus and — in the alar sulcus — rodent ulcers are com- mon because of the constant exposure of the nose to external irritation and to lowering of temperature, depressing its vital resistance. Frost-bite of the nose is also common, especially about the tip, because of its exposed position and the lack of protection to the delicate vessels from overlying tissues. The nerve supply to the nose is likewise very free, as is shown in a practical manner by the pain which accompanies inflammatory conditions, especially those involving the lower cartilaginous portion where the skin and subcutaneous tissues are very adherent. The resulting exudate is therefore much confined, pressing upon the nerves ; this accounts also for the frequency with which gangrene occurs under these circumstances. Watering of the eyes from irritation of the skin or mucous membrane of the nose is due to the free nerve supply, and to the fact that the same nerve, the tri- geminal, supplies the nose and the lachrymal apparatus ; as a portion of the nasal chamber is supplied by a branch of the ophthalmic nerve, raising the eyes to the sun will often give the added irritation necessary to precipitate a sneeze when the nasal stimulus suggests one, but is not quite strong enough unaided. Cough and bronchial asthma have resulted from nasal affections due to the indirect relations between the fifth cranial nerve and the pneumogastric. As the olfactory portion of the nasal fossa is in the upper portion of the cavity, an earnest effort to recognize an odor or to enjoy one to the utmost, is accompanied by a deep inspiration through the nose with dilatation of the nostril. In paralysis of the facial nerve, the involvement of the dilatores naris has been thought to explain the lessening of the olfactory sense sometimes seen in this condition. Paralysis of the levatores alae nasi muscles has permitted the nostrils to close during inspiration, causing stridor and mouth-breathing. The loss of the sense of smell is a not uncommon result of severe blows, especially on the forehead, and may be due to (#) concussion of the olfactory bulbs ; (£) fracture of the cribriform plate of the ethmoid ; (c~) injury to the olfactory roots where they cross the lesser wing of the sphenoid ; or (d ) lesion of the olfactory nerves where they traverse the cribriform foramina. Sneezing from irritation of the nose is probably due to the indirect relationship between the fifth pair and the vagus and may be so violent that serious injury may result, as in cases in which a subcoracoid luxation of the shoulder, a fracture of the ninth rib, and the rupture of all the coverings of a large femoral hernia were produced by this act (Treves). The abundant sweat and sebaceous glands in the skin of the nose account for the frequency with which acne vulgaris attacks it. The alae, the only movable por- tions, take part in the movements of expression, as in contempt and scorn. Fractures of the nose are common because of its exposed position, and of the frequency of blows and other forms of violence applied to the face. Their chief importance depends upon the prominence of the nose as a feature of the face, any change in its shape attracting general attention. The fracture occurs most com- monly in the lower part, because of the greater weakness of the bones and their greater prominence at that level. In its upper part, the relative depression of the dorsuir, the greater thickness of the bones, and their more firm support, make fracture less common. On the other hand, the higher fractures are more dangerous because of their possible relation with the cribriform plate and sinuses of the ethmoid bone, tin- frontal sinuses and the nasal duct. Involvement of the cribriform plate is in effect a compound fracture of the base of the skull, exposing the meninges to the danger of infection. Fractures of the nose are almost always compound, because of the intimate adhesion of the mucous membrane to the bone, with little intervening tissue, so that when the bone breaks the overlying adherent tissue is torn through. This accounts for the practically uniform occurrence of cpistaxis, on account of which it is often difficult to detect the presence of escaping cerebro-spinal fluid when the THE NASAL FOSS^. 1409 cribriform plate is also fractured. On the other hand, the rich glandular supply of the mucous membrane, which makes the usual mucous secretion exceptionally free, may, in a post-traumatic coryza, result in a watery discharge of such quantity as to suggest the escape of the cerebro-spinal fluid. Emphysema within the orbit and under the skin may result from the communication of the nose with the ethmoidal or frontal sinuses. In the effort to keep the nose clear of blood by blowing, the air is forced into the subcutaneous tissues. In fractures at the lower part, the deformity is frequently lateral, because of the greater exposure to side blows, and the tendency of the cartilaginous alae and septum to avoid crushing. In the upper part depression is more likely, because of the tendency to escape any but forces from in front, the greater force necessary to produce the fracture, and the presence of a bony septum underneath, which crushes rather than bends. When the deformity has been replaced there are no strong muscles to repro- duce it, so that little or no effort is necessary to maintain the fragments in position. The deformity must be reduced early and the reduction maintained, because owing to the free blood supply, union is usually rapid, sometimes occurring in a week. One must bear in mind in reducing the deformity that the roof of each nasal fossa is not more than 2-3 mm. wide, and that, therefore, a narrow rigid instrument is necessary to press the fragments upward into their normal positions. THE NASAL FOSS^. The cavity of the nose is divided by the median septum into two nasal fossae which extend from the anterior to the posterior nares, or choana;, through which they open into the naso-pharynx. They communicate more or less freely with the accessory air-spaces within the frontal, ethmoid, sphenoid and maxillary bones, into which, as a lining, the mucous membrane of the nasal fossae is directly continued. Seen in frontal section (Fig. 1 176), each fossa is triangular in its general outline, the apex being above at the narrow roof and the base below on the floor. The smooth median wall is approximately vertical and meets the floor at almost a right angle, while the sloping lateral wall is modelled by the projecting scrolls of the three turbinates, which overhang the corresponding meatuses. In sagittal sections (Fig. 1 174) the contour of the fossa resembles an irregular parallelogram from which the upper front corner has been cut off, so that in front the upper border slopes downward to correspond with the profile of the outer nose. The greatest length of the fossa, measured along the floor, is from 7-7.5 cm. (2^-3 in.) and its greatest height from 4—4.5 cm. The width is least at the roof, where it is less than 3 mm., and greatest in the inferior meatus a short distance above the floor, where it expands to from 15—18 mm. The Vestibule. — The anterior part of the fossa, immediately above the open- ing of the nostril and embraced by the outer and inner plates of the lower lateral cartilage and adjoining portion of the septum, is somewhat expanded and constitutes the vestibule (vestibulum nasi), a pocket-like recess prolonged towards the tip being the ventricle (recessus apicis). These spaces are lined by delicate skin, directly con- tinuous with the external integument and tightly adherent to the underlying cartilage, and, in the lower half of the vestibule, containing numerous sebaceous glands and hairs. In the vicinity of the nostril the hairs, known as vibrissa, are coarse and long and curved downward to afford protection to the nasal entrance. Over the upper part of the vestibule, the skin is smooth and closely attached to the lower lateral cartilage, the upper margin of the outer plate projecting as a slightly arching ridge, the limen vestibuli, which forms the superior and lateral boundary of the vesti- bule and marks the line of transition of the skin into the mucous membrane that lines the remaining parts of the nasal fossa. Above and beyond the vestibule, the nasal fossa rapidly expands into a triangular space, the atrium nasi, that lies in advance of the entrance into the middle nasal meatus. Above and in front the atrium is bounded by a low and variable ridge, the agger nasi, that represents a rudimentary naso-turbinate, which in many mammals attains a large size. The space lying in front of the agger, extending 1410 HUMAN ANATOMY. from the limen to the cribriform plate of the ethmoid and roofed in by the forepart of the arched upper boundary of the fossa, is long and narrow in consequence of the approximation of the median and lateral walls. It leads from the nasal aperture to the summit of the nasal fossa and to it Merkel applied the name carina nasi. The Nasal Septum. — The median wall consists of the partition formed chiefly by the perpendicular plate of the ethmoid, the vcmer and the septal cartilage, cov- ered on both sides by mucous membrane. The extreme lower and anterior part of the septum, consisting of the alar cartilage and the integument, is flexible, and there- fore called the membranous portion, or septum mobile ; the terms bony and cartilagi- nous portions are applied to the remaining parts of the septum supported by bone and cartilage respectively. While during early childhood its position is median, in the great majority of adults the septum presents more or less asymmetry and lateral deflection, most often FIG. 1174. Frontal sinus Superior turbinate Spheno-ethmoidal recess Opening of sphenoidal sinus Superior meatus Opening of Eustachian tube Vestibule Limen vestibuli \ Posterior limit of nasal fossa Middle meatus Middle turbinate Inferior meatus Right nasal fossa, lateral wall ; and naso-pharynx. to the right. This deviation may affect the septal cartilage alone, may be limited to the bones (in 53 per cent, according to Zuckerkandl), or may be snared by both. The most common seat of the deflection is the junction of the ethmoid and vomer, in the vicinity of the spheno-ethmoidal process, or along the union of the vomer and the septal cartilage. The asymmetry may involve the entire septum, which then is oblique ; or it may take the form of a simple bulging towards one side, a double or sigmoid projection ; or be an angular deflection resembling a fold, crest or spur that projects into one, sometimes both, of the fossae (Heymann). Although the mucous membrane covering the nasal septum is generally smooth and of fairly constant thickness, its surface is marked by inequalities caused chiefly by variations in the amount and development of the glandular and vascular tissue. One such accumulation, the tube re i< /inn s<-/>//\ is relatively constant and on the septum about opposite the anterior end of the middle turbinate. During early life a series of from four to six or more oblique ridges, plica septi, often model the lower and posterior part of the septum, extending from below upward and forward. Slightly above the anterior nasal spine, the septal mucosa presents the minute openings lead- ing into the rudimentary organ of Jacobson. Behind, the margin of the bony septum is covered by mucous membrane of unusual thickness which, therefore, forms the immediate free edge of the partition separating the posterior nares. The Lateral Wall. — The lateral wall of the nasal fossae is characteristically modelled by the projecting scrolls (conchac nasi) of the three turbinates. The latter partly subdivide each fossa into three lateral recesses, the superior, middle, and THE NASAL 1411 inferior meatuses. These are overhung by the corresponding bony concha, the superior meatus being roofed in by the upper turbinate and the inferior lying between the lower turbinate and the floor of the fossa. That part of the nasal fossa between the conchae and the septum, into which the recesses open medially, is sometimes called the meatus nasi communis. The details of the nasal fossa as seen within the macerated skull have been described in connection with the skeleton (page 223). In the recent condition, when the soft parts are in place, while their general contour is preserved, the compartments of the fossae are materially reduced in size by the thickness of the mucous membrane and the erectile tissue that cover the bony framework. The Superior Meatus. — Corresponding to the small size of the upper turbinate, the superior meatus (meatus nasi superior), or ethmoidal fissure, is narrow and groove-like and little more than half the length of the middle one. It is directed downward and backward and is floored by the convex upper surface of the middle concha. When the upper turbinate is replaced by two scrolls (conchae superior et suprema) — a condition that Zuckerkandl regards as very frequent, if indeed, not the more usual — the meatus is accordingly doubled. Into the upper and front part of the superior meatus the posterior ethmoidal air-cells open by one or more orifices FIG. 1175. Frontal sinus Probe in infundibulum Middle turbinate, partly, removed Hiatus semilunari Ethmoidal bulla Openings Agger nasi of maxillary sinus into infundibulum Ventric Limen nasi Vestibul Probe in naso-lachrymal duct Opening of middle ethmoidal cells Superior turbinate, partly removed ning into spheno-ethmoidal recess Oper Sphenoidal sinus Opening of posterior ethmoid cells into superior meatus Xaso-pharynx Opening of Eustach- ian tube Inferior meatus Inferior turbinate, partly removed Middle meatus Lateral wall of nasal fossa ; portions of turbinate bones have been removed to expose openings into air spaces. of variable size. Above and behind the upper turbinate and in front of the body of the sphenoid bone lies a diverticulum, the spheno-ethmoidal recess, into the posterior part of which opens the sphenoidal sinus. The Middle Meatus. — The recess beneath the middle turbinate (meatus nasi medius) is spacious and arched to conform with the contour of the middle and inferior conchae which constitute its roof and floor respectively. On elevating, or still better removing close to its attachment, the middle turbinate bone, a deep crescentic groove, the infundibulum, is seen on the outer wall of the fossa overhung by the anterior half of the concha. The crescentic cleft leading from the middle meatus into the infundibulum is the hiatus semilunaris? which extends from above downward and backward, with its convexity directed forward. Its anterior boundary is a sharp crescentic ridge due to the uncinate process of the ethmoid covered with thin mucous membrane, while behind it is limited by a conspicuous elevation produced by the corresponding underlying bony projection of the ethmoidal bulla. 1 Some confusion exists in the use of this term, since it is often applied to the entire groove and not merely to the cleft which leads from the meatus into the groove. The name is here employed as indicating the lunate cleft and not the groove (which is the infundibiilum), as originally used by Zuckerkandl, who introduced it. See Antomie der Nasenhohle, Wien, 1882, page 39. 1412 HUMAN ANATOMY. When the infundibulum does not end blindly above, which it often does (page 194), its upper extremity, usually somewhat expanded, receives the opening of the frontal sinus, ostium frontale. The sinus is, however, not dependent upon the infundibulum for its communication with the middle meatus, since, as pointed out by Zuckerkandl, between the front of the attachment of the middle turbinate bone and the uncinate process of the ethmoid there exists a passage which leads to the ostium frontale. Into the upper part of the infundibulum usually open some of the anterior ethmoidal air-cells ; lower in the groove lies the oval or slit-like ostium maxillare, the chief communication of the antrum of Highmore. When the latter is provided with an additional orifice, as it is in 10 per cent. (Kallius), the smaller accessory communication opens into the infundibulum a few millimeters behind the principal aperture. Above the hiatus semilunaris, either on or above the bulla, is usually seen the slit-like opening through which the middle ethmoidal cells communicate with the meatus. The Inferior Meatus. — This passage (meatus nasi inferior), the largest of the three, measures from 4. 5-5.5 cm. in length, its anterior end lying from 2.5-3.5 cm- behind the tip of the nose. At first relatively contracted, it abruptly expands, not FIG. 1176. Scalp Cerebral hemisphe Superior longitudinal sinus Bone Falx cerebri Ethmoidal cells Lower end of probe lying in hiatus semilunaris Middle turbinate— Probe passing from antrum into.- infundibulum Inferior turbinate Nasal septum Right eyeball Hiatus semilunaris Middle meatus -Maxillary sinus —Inferior meatus Floor of nasal fossa Oral mucous membrane Tongue Frontal section of head, viewed from behind, showing nasal fossae and communication) with frontal and maxillary sinuses. only in height, in correspondence with the arched attached border of the loud turbinate, but also in width. Farther backward, it gradually diminishes and is again reduced at its choanal end. On tin- lateral wall of the inferior nu-atus, usually from 3-3.5 cm. behind the posterior margin of the nostril, after removal of the lower turbinate, may be seen the opening of the naso-lachrymal duct. The position and form of the orifice are subject to much variation. When close to the arching attached border of the concha, the aperture is usually oval or even round ; when its position is lower, it is narrow and slit-like, obliquely vertical, and often guarded by a fold of mucous membrane, the so-called ra/ri- of llasucr. The arched roof of the nasal fossa is divisible into a naso-frontal, an ethmoidal and a sphenoidal part in accordance with the bones over which the THE NASAL MUCOUS MEMBRANE. 1413 mucous membrane stretches. The lower part of the naso-frontal division, below the nasal bone, is cutaneous and cartilaginous. Anteriorly the roof is reduced to little more than a groove on account of the approximation of the lateral and median walls, but posteriorly broadens towards the choana. The median part of the roof, formed by the cribriform plate of the ethmoid, is very thin and makes a sharp angle with the steeply descending sphenoidal division. Between the latter and the superior turbinate bone lies the spheno-ethmoidal recess. The floor of the nasal fossa, much broader than the roof and supported by the palatal process of the maxilla and the horizontal plate of the palate bone, from before backward is approximately horizontal, but from side to side is distinctly con- cave. Anteriorly this wall is robust, but rapidly diminishes in thickness as it passes backward. About 2 cm. behind the posterior margin of the nostril and close to the septum, the floor of each nasal fossa presents a slight depression, sometimes narrow and funnel-shaped, that leads into a small canal lined with a prolongation of mucous membrane. This canal converges towards the septum with its fellow of the opposite fossa, descends almost vertically, and passes through the incisive foramen in the hard palate to end on the roof of the mouth as a minute slit at the side of the incisive pad or papilla palatina. Although the two tubes of mucous membrane may join to form a single incisive canal, they usually retain their independence (Leboucq, Merkel). They are often closed and impervious ; sometimes, however, even in the adult communication is retained between the nasal and oral cavities. The posterior nares or choanae, the apertures through which the nasal fossae communicate with the naso-pharynx, one on either side of the septum, resemble in form somewhat a Gothic arch (Fig. 1354). They are relatively much lower in the new- born child than in the adult, in which they measure about 3 cm. in height and 1.5 cm. in breadth (Zuckerkandl), although individual variation is considerable. Each opening is bounded below by the horizontal plate of the palate bone ; laterally by the inner surface of the internal pterygoid plate of the sphenoid ; above by the vaginal process of the sphenoid and the ala of the vomer ; and mesially by the vertical posterior borders of the vomer. Over this bony arch the nasal mucous membrane is continuous with that lining the pharynx. Laterally the posterior limit of the nasal fossa in the recent condition is indicated by a furrow (sulcus nasalis posterior) that extends from the under surface of the sphenoid downward to about the junction of the hard and soft palates. Behind this furrow, about on a level with the lower border of the inferior turbinate, lies the opening of the Eustachian tube (Fig. 1174). Since the turbinates end approximately 12 mm. in advance of the choanae, the outlines of these openings are unbroken by the scrolls that model the lateral wall of the nasal fossae, all three conchae, however, being visible through the posterior nares. THE NASAL MUCOUS MEMBRANE. Beyond the limen that marks the limit of the integument clothing the vestibule (page 1409), the nasal fossa is lined by mucous membrane continuous with that of the naso-pharynx through the choanae. Since in addition to lining the tract over which the respired air passes -the nasal mucous membrane contains the cells receiving the impressions giving rise to the sense of smell, it is appropriately divided into a respir- ator \> and an olfactory part. The Olfactory Region. — The highly specialized regio olfactoria is quite limited in extent and embraces an area situated over the middle of the upper tur- binate and the corresponding part of the septum. According to Brunn, J whose conclusions are here presented, the olfactory area of each fossa includes only about 250 sq. mm. , the septum contributing something more than one-half of the entire surface. Accordingly the specialized field is by no means coextensive with the upper turbinate bone, as it reaches neither its lower nor posterior border (Fig. 1177). The anterior margin of the area, which lies about i cm. behind the front wall of the nasal fossa, is irregular in outline owing to the invasion of the specialized region by the adjacent 1 Archiv f. mikros. Anat., Bd. 39, 1892. 1414 II I'M AN ANATOMY. FIG. 1177. respiratory mucous membrane, tongues or even islands of the latter projecting into or being surrounded by the former. Upon the evidence derived from careful dissection of the olfactory mucous membrane, however, it is difficult to avoid the conclusion that Brunn's areas are too limited, as nerve-fila- ments clearly attached to the olfactory bulb are usually traceable onto the upper part of the middle turbinate bone. In fresh preparations the olfactory area usually, but not always, can be approximately mapped out by the yellowish hue, lighter or darker, that distinguishes it from the respiratory region in which the mucous membrane exhibits a rosy tint. The epithelium contains two chief con- stituents— the supporting and the olfactory cells. The supporting cells are tall cylindrical elements, about .06 mm. in height, that extend the entire thickness of the epithelium. Their outer and broader ends are of uniform width and contain the oval nuclei which, lying approximately at the same line and staining readily, form a deeply colored and conspicuous nuclear stra- tum at some distance beneath the free margin. Between the latter and the row of nuclei, the epithelium presents a clear zone devoid of nuclei. The inner part of the supporting cells is thinner and irregular in contour and often terminates by splitting into two or more basal processes that rest upon the tunica propria. Between these ends lie smaller pyramidal elements, the basal cells, that FIG. 1178. Right nasal fossa, septum (s) has been partially separated and turned upward ; dark field shows olfactory area on lateral and mesial walls of fossa, as mapped out by Brunn.. oT,:: ce,ls Bundle of olfactory nerves Section of olfactory mucous membrane; epithelium displays outer nuclei-free and nuclear layers formed by supporting; cells and broad stratum containing nuclei of olfactory cells. X 300. probably represent younger and supplementary forms of the sustentacular cells. Tin- granular protoplasm of the basal processes often contains pigment particles. The olfactory cells, the perceptive elements receiving the smell-stimuli, con- .si>t of a fusiform body, lodging a spherical nucleus enclosed by a thin cn\-elope of cytoplasm, and two attenuated processes, a peripheral and a central. The olfactory cells are in fact sensory neurones that have retained their primitive position within the surface epithelium, as in many invertebrates, instead of receding, as is usual in THE NASAL MUCOUS MEMBRANE. the higher animals, to situations more remote from the exterior. The slender peripheral process of the olfactory cell, which corresponds to the dendrite of the neurone, is of uniform thickness and ends at the surface in a small hemispherical knob that projects slightly beyond the general level of the epithelium and bears from 6—8 minute stiff cilia, the olfactory hairs. The length of the peripheral processes, being dependent upon the position of the nuclei, varies, since the latter occupy different levels within the epithelium in order to accommodate their greater number — about 60 per cent, in excess of those of the supporting cells (Brunn). The central FIG. 1 1 80. FIG. 1179. Olfactory cell 4— Supporting cell Section of human olfactory mucous membrane, silver preparation ; two olfactory cells are seen, one of which sends nerve-fibre towards brain. X 335- (Brunn.) Isolated elements of epithelium of olfactory mucous membrane ; a, olfactory cells ; b, sup- porting cells. X 1000. (Brunn.) processes of the olfactory cells, much more delicate than the peripheral, are directly continued, as the axis-cylinders, into the subjacent nonmedullated nerve-fibres within the tunica propria, from which they pass through the cribriform plate to enter the brain and end in the arborizations within the olfactory glomeruli of the bulbus olfactorius (page 1152). The tunica propria is differentiated into a superficial and a deep layer by the adenoid character of the stratum immediately beneath the epithelium. The superficial layer, from .015 -.020 mm. thick, consists of closely packed irregularly round cells, resembling lymphocytes, and meagre bundles of delicate connective tissue. The deep layer, on the other hand, contains robust bundles of fibro-elastic tissue and relatively few cells. A distinct membrana propria is wanting within the olfactory region. The glands of Bowman (glandulae olfactoriae) are characteristic of the olfactory region and probably elaborate a specific secretion (Brunn). They open onto the free surface by very narrow ducts that lead into saccular fusiform dilatations, into which the tubular alveoli open. The ducts possess an independent lining of flattened cells that extend as far as the surface and lie between the surrounding epithelial ele- ments. The dilatations are clothed with flattened or low cuboidal cells, which are replaced by those of irregular columnar or pyramidal form within the tubular alveolar. From the character of their secretion the glands of Bowman are probably to be reckoned as serous and not mucous (Brunn, Dogiel). The Respiratory Region. — The mucous membrane lining of the respiratory region differs greatly in thickness in various parts of the nasal fossa. In situations where the contained cavernous tissue is well represented, as over the inferior turbinate, it may reach a thickness of several millimeters, while when such tissue is wanting, as on the lateral wall, it is reduced to less than a millimeter. 1416 HUMAN ANATOMY. The epithelium is stratified ciliated columnar in type, from .050 -.070 mm. thick, and includes the tall surface cells, bearing the cilia, between the inner ends of which lie the irregularly columnar basal cells. Numerous elements exhibit various stages of conversion into mucous-containing goblet cells. The current produced by the cilia is toward the posterior nares. Beneath the epithelium stretches the membrana propria or basement membrane, that varies greatly in thickness ; although in certain localities feebly developed, it is usually well marked and measures from .010-. 020 mm. in thickness (Brunn). FIG. 1181. Epithelium — Blood-vessel Glands Section of respiratory mucous membrane covering nasal septum. X 75- Under pathological conditions its thickness may increase fourfold or more. In many places the membrana propria is pierced by minute vertical channels, the basal canals, in which connective-tissue cells and leucocyctes are found, but never blood-capillaries (Schiefferdecker). The tunica propria consists of interlacing bundles of fibro-elastic tissue which are most compactly disposed towards the subjacent periosteum. The looser super- ficial stratum is rich in cells and here and there contains aggregations of lymphocytes that may be regarded as masses of adenoid tissue (Zuckerkandl). In certain parts of the nasal fossa the stroma of the mucous membrane contains vascular areas com- posed of numerous intercommunicating blood-spaces that confer the character of a true cavernous tissue. These specialized areas, the corpora cavernosa, as they are called, are especially well developed over the inferior and the lower margin and posterior extremity of the middle conchse, and less so over the posterior end of the upper turbinate and the tuberculum septi. When typical, they occupy practically the entire thickness of the mucous membrane from periosteum to epithelium, the interlacunar trabeculae containing the glands and blood-vessels destined for the sub- epithelial stroma. The blood-sinuses, the general disposition of which is vertical to the bone (Zuckerkandl), include a superficial reticular zone of smaller spares and a deeper one of larger lacunae. The engorgement and emptying of the cavernous tissue is controlled by nervous reflexes and probably has warming of the inspired air as its chief purpose (Kallius). The glands of the respiratory region are very numerous, although varying in size, tubo-alve01ar in form and, for the most part, mixed mucous in type. The chief ducts open on the free surface by minute orifices barely distinguishable with the unaided eye. Their deeper ends branch irregularly into tubes that bear the ovoid terminal alveoli. The latter are lined with mucous-secreting cells, between which lie PRACTICAL CONSIDERATIONS: THE NASAL CAVITIES. 1417 FIG. 1182. ICOUS mbrane Vomerine cartilage the crescentic groups of serous cells that stamp the glands as mixed (Stohr). Exceptionally exclusively serous glands are also encountered (Kallius). Jacobson's Organ. — Mention has been made of the rudimentary structure (organon vomeronasale) found in man, almost constantly in the new-born child and frequently in the adult, as a representative of the organ of Jacobson that is present, in varying degrees of perfection, in all amniotic vertebrates (Peter). In many animals possessing in high degree the sense of smell (macros- matic), the ojgan is well developed and functions, serving possibly as an accessory and outlying surface by which the first olfactory impressions are received (Seydel). In man the organ is represented by a laterally compressed tubular diverticulum, from 1.5—6 mm. in length, that passes backward and slightly upward to end blindly be- neath the mucous membrane on each side of the septum. The entrance to the tube is a minute aperture situated near the lower border of the septum, above the anterior nasal spine and the rudimentary vomerine cartilage. The median wall of the diverticulum is clothed with epithe- lium composed of tall columnar cells resembling those of the olfactory region, but the characteristic olfac- torv Cells are wanting" The eoithe- Portion of frontal section through nasal fossae of kitten, showing . . -1 . <• 1 1 organ of Jacobson. X 20. hum covering of the lateral wall corresponds to that of the respiratory region. In macrosmatic animals branches of the olfactory nerve are traceable to Jacobson's organ in which are found olfactory cells. PRACTICAL CONSIDERATIONS : THE NASAL CAVITIES. The nasal cavities have certain important clinical relationships which may be classified as (i) physiological — (a) respiratory, phonatory and olfactory ; (^) sexual ; (2) topographical — (a) the nasal chamber and the vestibule ; (3) the premaxillary, maxillary, and palatal portions ; (c) the septum, and the turbinate bones. i. (a) The air passing out from the pharynx, being confined to the plane of the posterior nares, is not carried up to the olfactory region, so that the odors on the expired breath are not appreciated. When the communication between the respira- tory and olfactory portions is cut off, as by swelling of the mucous membrane at the region of union of these portions, loss of smell supervenes. Discharge which may accumulate about the middle turbinate bone or in the upper portion of the vestibule cannot be removed by the act of blowing the nose, for the reason above assigned that the air of expiration cannot pass within the olfactory portion. The act of blowing the nose, or the process of washing out the nose by a current thrown in from the naso-pharynx, will wash out the inferior meatus with ease, provided the discharge is not inspissated, and the parts of the floor of the nose are normal (Allen). An abnormal width or patency of the respiratory portion of the fossa — especially of the inferior meatus — due to imperfect development of the inferior turbinates, has been thought (Lack), by diminishing the vis a tergo in blowing the nose and thus favoring the retention and decomposition of the nasal mucus, to contribute to the occurrence of atrophic rhinitis (ozaena). The value of the nose as an accessory organ of phonation consists in its action as a resonating cavity which adds quality, color and individuality to the voice. This function of the nose becomes strikingly 1418 HUMAN ANATOMY. apparent when, as during an acute coryza, the fossae are more or less completely obstructed and the voice becomes flat and entirely without resonance. (£) The relations between the nasal chambers and the sexual apparatus are of practical importance and have as an anatomical basis the analogy between the mucosa covering much of the turbinates and part of the septum, and the erectile tissue of the penis, and the sympathy between the erectile portions of the generative tract and erectile structures — e. g. , the nipple — in other parts of the body. 2. (a) The distinction between the nasal chamber and the vestibule is, in the main, based upon the difference in their lining membrane, that of the vestibule being simply a continuation inward of the external integument to the line {limen nasi~) at which the nasal fossa proper begins. The vestibular cavity is provided with rigid hairs (to aid in arresting foreign particles carried in with the air current), and sebaceous glands, and is especially susceptible to eczematous or furuncular affections. Diseases of the vestibule may, therefore, be dealt with as though they were affections of the skin ; while diseases of the mucosa of the nasal chambers are to be treated on the same principles as those of the mucous membranes generally, with special refer- ence to its erectile character and to its close relation to the underlying periosteum and bone. (£) The sutural lines of the premaxilla, of the maxilla, and of the palatal bones aid in determining the boundaries of the subdivisions of the nasal chamber, which are indicated to some degree by the production of the planes of the sutures of the roof of the mouth, vertically upward through the nasal chambers. (c) The morphological significance of the septum, placed as it is in the median line of the face of the embryo, with the turbinate bones lodged to its right and left sides, remains the same in the skull of the adult, notwithstanding the fact that, with cultivated races at least, the septum is usually deflected through the greater part of its course from the median line. This deflection has been said to be due to the persistent growth of the septal bones in a vertical plane after their edges have united — the apex of the deflection being often found at the junction of the ethmoid and vomer ; any preponderance in strength of one of these bones will cause bending of the weaker — usually the perpendicular plate of the ethmoid. The usual direction of the deflection is to the left, and this has been thought to be due to the habit of using the right hand in blowing the nose. Asymmetry of the nasal chambers is a result of the deflection. One of these chambers, commonly the left, is much smaller than its fellow of the opposite side, and may be occluded, when the right chamber will be larger than normal and possess both osseous and erectile structures which have undergone physiological hypertrophy. Care should be taken to distinguish between such hypertrophy and the effects of diseased action (Allen). The anterior nares are directed downward and are on a lower plane than the floor of the nose. To examine the interior of the nose the movable nostril must therefore be elevated and the head thrown backward. The speculum shaped for the purpose should not be passed beyond the dilatable cartilaginous portion. With good light one may see the anterior part of the middle turbinate bone, a larger portion of the inferior turbinate, the beginning of the middle meatus, and get a freer view of the inferior meatus, the septum and the floor of the nose. The lower orifice of the nasal duct cannot be seen, although it is only about an inch from the orifice of the nostril, and three-fourths of an inch above the floor of the nose. This is due to the fact that it is concealed behind the attached and depressed anterior end of the inferior turbinate. To expose better the structures in the external wall of the narrow and rigid nasal fossa, various procedures have been adopted. Rouge made an opening into the anterior nares from the mouth, by incising in the angle between the upper lip and the gum. By separating the alar cartilages from the hours and dividing the cartilag- inous septum the movable anterior portion of the nose can he turned upward, giving a full exposure of the nasal fossrv, without leaving an unsightly scar. To permit a freer exploration with tin- finger, Koeher divided the septum as far back as possible with scissors. He also divided the roof of the nose near the septum, turning the divided parts aside. An osteoplastic flap may be made by extending this incision upward, dividing the bone in this line and making a second incision around PRACTICAL CONSIDERATIONS: THE NASAL CAVITIES. 1419 the alae and along the side of the nose, again dividing the bone. The flap thus formed can be turned upward, after breaking the bridge of bone between the upper ends of the two incisions, exposing the nasal fossa. The ringer can be passed backward through the nostril far enough to meet the finger of the other hand passed to the posterior nares through the mouth. The posterior nares can be examined by the rhinoscopic mirror or by the finger introduced through the mouth. Posterior rhinoscopy, like laryngoscopy, is carried out with difficulty, because the region of the naso-pharynx is sensitive and is intol- erant of intrusion. In the act of swallowing, the epiglottis protects the larynx by closing the laryngeal opening, and the soft palate rises against the posterior wall of the pharynx, preventing regurgitation into the nose. When the rhinoscopic mirror is used the same thing occurs, so that the view of the larynx and naso-pharynx is shut off. Considerable difficulty is sometimes experienced in training the patient to overcome this tendency. The employment of the nasal douche is based upon the same mechanism. When the stream of fluid passed through one nostril reaches the posterior part of the nose, its progress toward the mouth is obstructed by the elevated soft palate, and it therefore passes around the posterior edge of the septum and back through the opposite nasal fossa. With the rhinoscopic mirror in good position, and the soft palate quiet, one may see the posterior nares divided by the septum, the turbinated bones, and the meati (especially the middle turbinate and the middle meatus), the roof of the naso- pharynx and the orifices of the Eustachian tubes. The finger introduced through the mouth can feel the same structures, and can recognize naso-pharyngeal adenoids, tumors, or abscesses. The mucous membrane over the turbinates, owing to the presence of a rich venous plexus, is one of the most vascular in the body, and resembles erectile tissue (page 1968). This and the general vascularity of the nose partly explain the great frequency of epistaxis. The excessive supply of blood to the mucosa may be (a) for the purpose of enabling it to raise the temperature and add to the moisture of the inspired air ; (£) to favor the activity of the numerous mucous glands, the free secre- tion of which together with the action of the cilia of the epithelial cells is required to remove the dust and the micro-organisms that are filtered from the air during inspi- ration by the vibrissae.and the cilia themselves ; (<:) to endow it with sufficient vitality to resist the pathogenic action of such micro-organisms. In spite of this defensive quality, the constant exposure to atmospheric irritants often leads to congestions and coryzas, which if long continued and frequently repeated result in hypertrophy of the mucous membrane. This may require removal by cauterization or excision to relieve the consequent obstruction. The mucous membrane is somewhat less closely attached to the septum than to the neighboring parts, and hence haematomata of the septal submucosa are not infrequent after an injury to the nose. Such haematomata are almost invariably infected and proceed to suppuration forming septal abscesses, the constitutional symptoms (toxaemia) of which may give rise to anxiety if their local cause is overlooked. Epistaxis is common not only because of (a) this vascularity of the mucosa, but also by reason of (£) the frequency of trauma to the nose ; the relation of its veins (r) to the general venous current so that they may be congested in cardiac or in pul- monary disease, or in straining, or in paroxysms of coughing, as in whooping cough ; and (d) to the intracranial sinuses, so that nose-bleed may be a symptom of cerebral congestion or tumor ; (e) the bleeding may be vicarious, as in cases of suppressed menstruation (an illustration of the sexual relations of the nasal apparatus); (/) it not uncommonly follows ulceration — simple, tuberculous or syphilitic — and in obstinate cases such ulcers should always be sought for. The source of hemorrhage from the nose is most frequently in the anterior part, particularly on the septum, and is then ordinarily controlled with ease. Usually the patient should be kept upright, with the head back, (not in the usual position lean- ing over a basin, increasing the tension of the vessels of the neck and head) and should be made to take deep breaths with the arms raised, thus fully expanding the thorax and depleting the cervical veins and, indirectly, the facial and ophthalmic into which the veins of the nose empty. If ordinary means fail, and this is more likely i42o HUMAN ANATOMY. if the bleeding point is posterior, the posterior nares may be plugged. For this purpose a long silk ligature is passed through the nose to the pharynx and out through the mouth, by means of a Bellocq's cannula or a soft catheter. To the middle of the ligature is attached a plug of gauze slightly larger than the posterior nares, which is then drawn by the anterior end of the ligature into the nasal fossa, which it should tightly fill. Postnasal adenoids originate in the normally excessive lymphoid tissue — pharyn- geal tonsil — of the postnasal space, of which tissue they are a simple hypertrophy. The growth forms a mass in the vault of the naso-pharynx and often extends down- ward and forward, filling up Rosenmiiller's fossae and involving the orifices of the Eustachian tubes. The tonsils are commonly also enlarged. The symptoms produced are : (a) obstructed nasal respiration, more marked during sleep, when the mouth is closed by the approximation of the tongue to the palate ; (£) as a result of this, broken rest and " night terrors" ; and (V) as a further consequence (and also from deficient oxygenation), deterioration of the general health, delayed or arrested growth, and anaemia ; (d) intermittent partial deafness and recurrent attacks of catarrhal or suppurative otitis media ; (/) pigeon-breast from inequality of intra- and extra-thoracic atmospheric pressure. The early removal of adenoids that produce any or all of these symptoms is usually indicated, and is facilitated by their friability and by the toughness and den- sity of the submucosa on which they lie, circumstances which permit of their usually easy enucleation either with the fingers or with the adenoid forceps and curette. Naso-pharyngeal growths may be either simple fibromata or fibro-sarcomata. They are usually dense, and contain large venous channels, which have no definite sheath and thus do not retract when severed. Incision into them may therefore be followed by severe hemorrhage with no tendency to spontaneous arrest. Ulceration or abrasion of the surface of these growths is not infrequent, and is also attended by repeated and often dangerous loss of blood. The nasal fossae, already very narrow, are frequently further obstructed by path- ological conditions, such as deviations of the septum, hypertrophy of the mucous membrane covering the turbinates, spurs on the septum, polypi and tumors. The septum is rarely straight after the seventh year, in about seventy-five per cent, of cases being turned to one or the other side, most frequently the left (vide supra). Both the bony and cartilaginous portions, more especially the anterior cartilaginous, are involved. The deflection is sometimes due to a fracture from blows or falls. The whole nose usually deviates more or less to one side. Spurs on the septum com- monly occur at the junction of the bony and cartilaginous portions. A deviation of the septum does not necessarily mean that the narrowed nasal fossa is seriously obstructed. It frequently, however, comes in contact with the surface of the turbin- ates, and may result in an adhesion or synechia from the irritative inflammation which is set up. Operations are often necessary to correct the difficulties arising from deviation of the septum. The concavity on the opposite side will differentiate it from a tumor. Hypertrophy of the ethmoidal labyrinth, or bulla ethmoidalis, is sometimes so far advanced as to obstruct the nasal fossa on that side. The middle turbinate over- lies and yields before this expanded cell, and may even press against the septum to such an extent as to make it bend and obstruct the opposite nasal fossa to a greater or lesser degree. The removal of the middle turbinate is sometimes practiced in these cases (Taylor), or the bulla itself may be obliterated by means of the cutting forceps or curette. Over-development of the bulla ethmoidalis may at times be so great as to occasion obstruction of the upper portion of the corresponding nasal fossa. The floor of the nose is the widest part, and slopes gradually backward and downward in the upright position, so that collecting mucus tends to run backward and drop into the throat. Rhinolitlis, which are incrustations usually about a foreign body, are most frequently found in the inferior meatus, which is the largest. The posterior nares are below the level of the respiratory portion, so that any discharge above the middle tnrbinate cannot be blown from the nose. The anterior portion of the inferior turbinate slopes downward and forward, and its anterior end is attached THE ACCESSORY AIR-SPACES. 1421 so near the floor of the nose that the roomiest portion of the inferior meatus is posterior. Therefore, the entrance of air into the lower part of the nasal fossa is obstructed, and is favored toward the upper — "respiratory" — portion, especially through the wide anterior opening of the middle meatus, which reaches as high as the tendo-oculi. This anatomical arrangement is the explanation of the fact already mentioned, that odors on expired air are not recognized. The relations of the nasal chambers explain why a coryza may cause (a) lach- rymation, by affecting the tear duct, lachrymal sac, and conjunctiva ; (6) dysphagia, by extending to the pharynx by way of the posterior nares ; (e) hoarseness or cough, by further extension to the respiratory tract ; (d) frontal headache, by involving the frontal sinuses ; (r>p«: The • Pigmented layer 2. Layer of rods and cones 3. Layer of bodies of visual cells or outer nuclear layer 4. Outer plexiform layer 5. Layer of bipolar cells, or inner nuclear layer 6. Inner plexiform layer 7. Layer of ganglion cells 8. Layer of nerve-fibres Neuro- • epithelial layer Cerebral layer FIG. 1219. To these nervous layers must be added two delicate membranes, ( i ) the membrana limitans interna, which bounds the inner surface of the retina, and (2) the membrana limitans externa, which lies between the outer nuclear layer and the layer of rods and cones. These membranes represent the terminal portions of the supporting neu- rogliar fibres, or fibres of Miiller. The pigmented layer, formed of deeply pigmented cells, constitutes the most external layer of the retina and represents the outer wall of the foetal optic vesicle. It is composed of hexagonal cells, from .012— .018 mm. in diameter, the protoplasm of which is loaded with fine, needle-shaped crystals of pigment {fuschi). The outer portion of the cells is almost free from pigment and con- tains the nucleus. From the inner border fine proto- plasmic processes extend inward between the rods and cones of the neuroepithelial layer. Under the influence of light, the pigment particles wander into these processes and, under such conditions, the pigmented cells may remain attached to the retina when the latter is separated from the choroid. Ordinarily, the pigmented layer ad- heres to the choroid and, hence, was formerly considered to be a part of that membrane. The pigmented cells are separated by a distinct intercellular cement substance and in some of the lower animals contain colored oil droplets and particles of a highly refracting myelin-like substance (inyeloid granules of Kiihne). The layer of rods and cones, although usually described as a distinct stratum, is only the highly specialized outer zone of the layer of visual cells and, therefore, constitutes the outer portion of the neuroepithelial division of the retina. It is com- posed, as its name indicates, of two elements, the rods and the cones, which are the outer ends of the rod and cone visual cells. They are closely set, with their long axes perpendicular to the surface of the retina. The rods far outnumber the cones, except in the fovea centralis, in which location cones alone are found. In the macula each cone is surrounded by a layer of rods ; elsewhere the cones are separated by intervals occupied by three or four rods. The rods of the human retina (Fig. 1221) have an elongated, cylindrical form, and measure approximately .060 mm. in length and .002 mm. in diameter. Each rod Pigmented cells from outer layer of retina ; surface view. X 250. 1464 HUMAN ANATOMY. is composed of an outer and an inner segment, of about equal length. The outer segment possesses a uniform diameter, is doubly refracting, and readily breaks up into minute disks. It is invested with a delicate covering of neurokeratin, contains myeloid (Ktihne) and is the situation of the visual purple or rhodopsin. The inner rod segment is somewhat thicker and has an ellipsoidal form. It is singly refracting, homogeneous in structure (rapidly becoming granular after death) and from its innei extremity sends the delicate rod-fibre through the external limiting membrane into the outer nuclear layer where the nucleus of the rod visual cell is found. The cone visual cell is composed of the same general divisions as the rod-cell, including the specialized outer part, the cone, and the body within the external nu- clear layer. The cones are shorter than the rods, and, except in the fovea, have a length of .035 mm. Each one (Fig. 1221) is composed of an outer narrow cone- shaped segment, and an inner broader segment, which is distinctly ellipsoidal in form, with a diameter of .007 mm. The inner segment is double the length of the outer, and is continued inward as FIG. 1220. the cone-jibre with its nucleus in the outer nuclear layer. In the fovea, where the cones alone are found, they are of approximately the same length as the rods, and possess about one half the usual diameter. The outer nuclear layer, the inner portion of the neuroepi- thelial layer, is composed of the bodies of the rod and cone visual cells, which show chiefly as the nuclei, the so-called rod- and cone- granules. The rod-granules oc- cupy an elliptical enlargement of the attenuated rod-fibres. They exhibit a transverse striation and are placed at varying levels within the layer. The rod-fibres are con- tinued as a thin protopla'smic pro- cess into the outer reticular layer, where they form small end-knobs which are associated with the outer terminals of the small nerve-cells, the rod-bipolars. The cone-gran- ules are less numerous than those of the rods, display no transverse markings, and are found only in the outer portion of the nuclear layer, near the external limiting mem- brane. The cone-fibres, the attenuated bodies of the cone visual cells, are broader than the corresponding parts of the rods and are continued through the outer nuclear layer as far as the outer portion of the external plexiform layer, where they end with a broad base, from which delicate processes extend inward to interlace with the terminal arborizations of the cone-bipolars. The outer nuclear layer is about .05 mm. in thickness. The outer plexiform layer is a narrow granular looking stratum, between the outer and the inner nuclear layer, and constitutes the first of the cerebral layers of the retina. It is composed of the dendritic arborixations of the bipolar nerve-cells of the succeeding layer, which lie in close relation with the centrally directed proces- ses from the foot-plates of the cone-cells and with the end-knobs of the rod-fibres. In addition to these constituents of the plexiform layer, numerous fibres arising from the protoplasmic processes of the horizontal cells of the inner nuclear layer also take part in its formation. Internal limiting membrane Ganglion cell Fibres of Miiller Bipolar nerve- cells Blood-vessel Layer of visual cells Nucleus of cone- cell Pigment layer Section of human retina from near posterior pole. X 230. THE NERVOUS TUNIC. 1465 FIG. 1221. b ~ The inner nuclear layer, the most complicated of the retinal strata, measures .035 mm. in thickness near the optic disc. It contains nervous elements of three main types — the horizontal cells, the bipolar cells, and the amacrine cells — and, associated with these, the nuclei of the sustentacular cells. The horizontal cells form the external layer, and were formerly included in the outer plexiform layer. They have flattened cell-bodies and send out from five to seven dendrites, which divide into innumerable branches and, passing into the outer plexiform layer, terminate in close association with the bases of the rod and cone visual cells. Each horizontal cell possesses also an axone, which is directed outward through the outer plexiform layer, and ends in a richly branched arborization about the visual cells. A second type of large horizontal cells is also described, some of which send axis-cylinder pro- cesses through the inner nuclear layer to form terminal arborizations in the inner plexiform layer. The function of the horizontal cells is not well understood, but they prob- ably serve as association fibres between the visual cells. The bipolar cells, the ganglion cells of this layer, are of two chief varieties, the rod-bipolars and the cone- bipolars. They are oval cells, each sending an axone inward toward the inner plexiform layer, which ends in communication with the large nerve-cells of the ganglion cell layer, and a dendrite outward which is associated with the end terminals of the visual cells and with the arboriza- tions of the horizontal cells. The dendrites of the rod- bipolars form an arborescence of vertical fibrils, which enclose from three to twenty end knobs of the rod-fibres, whilst their axis-cylinders pass entirely through the inner plexiform layer and usually embrace the cell-body of one of the large ganglion cells. The dendrites of the cone- bipolars, on the other hand, bear horizontal arborizations which interlace with the fibrils from the foot-plates of the cone-cells. Their axones penetrate less deeply into the inner plexiform layer than do those of the rod-bipolars, coming in contact at various levels with the peripherally directed dendrites of the ganglion cells. The amacrine cells are placed in the inner portion of the nuclear layer. Formerly considered as sustentacu- lar elements, they are now recognized as nerve-cells, although, as their name indicates, no distinct axone can be demonstrated. They possess, however, richly branched dendritic processes, which ramify in the inner plexiform layer and end either as the brush-like arborizations of the diffuse amacrines, or as the horizontally branching arborizations of the stratiform amacrines. A third type, known as association amacrines, is also described. They connect widely separated amacrine cells of the same layer (Cajal). The nuclei of the sustentacular cells, the fibres of Miiller, will be described later (page 1466). The inner plexiform layer, .04 mm. in thickness, appears granular, similar to the corresponding outer zone, and is composed of the interlacing axones of the bipolar, amacrine and horizontal cells from the inner nuclear layer and the dendrites of the large ganglion cells in the subjacent retinal layer. Intermingled with them are also the fibres of Miiller, which form conspicuous vertical striae, with lateral offshoots within the stratum. The layer of ganglion cells, consists, throughout the greater part of the retina, of a single row of large multipolar neurones, each with a cell-body containing a vesicular nucleus and nucleolus and showing, like many other ganglion cells of the central nervous system, typical Nissl bodies and a fibrillar structure. Near the macular region, the ganglion cells are smaller but more numerous and arranged as several superimposed layers; toward the ora serrata, on the contrary, the individual Visual cells from human ret- ina, A, cone-cell; £, rod-cell; «, b, outer and inner segments ; c, attenuated bodies (fibres), with nucleus (rf) and central ends (e); em, position of external limiting membrane. X 750. (Greeff.) 1466 HUMAN ANATOMY. FIG. 1222. cells are separated by considerable intervals. Their axones, or axis-cylinder pro- cesses, pass inward and become the nerve-fibres of the fibre layer. Converging toward the optic entrance, they become consolidated into the optic nerve and pass to the brain. The dendrites of the ganglion cells, one to three in number, run outward into the inner plexiform layer and end as richly branched arborizations. These, like those of the amacrine cells, terminate either diffusely, or in horizontal ramifica- tions limited to definite strata, in connection with the centrally directed processes from the bipolar cells. The nerve-fibre layer is composed almost entirely, but not exclusively, of the axones of the ganglion cells of the preceding layer. The individual fibres, from .005-. 05 mm. in diameter, are collected into bundles of varying size, which take a horizontal course and converge toward the optic disc. They are normally devoid of medullary sheaths, but acquire them after passing through the lamina cribrosa of the sclera. A few of the fibres are centrifugal, arising from ganglion cells within the brain, and terminate apparently in connection with the association amacrines of the inner nuclear layer. In the macular region, the nerve-fibres are prac- tically absent, those from the retinal area lying directly to the temporal side of the macula arching above and below the yellow spot. From the macula itself, a special strand, known as the maculo -papillary bundle and composed of about twenty-five fasciculi, passes directly to the nerve-disc. The sustentacular tissue, the neuroglia of the retina, exists in two forms — as \hefibres of M'uller and as the spider cells. The fibres of Miiller are modified neuroglia fibres which pass vertically from the inner surface of the retina through the succeeding layers as far as the bases of the rods and cones (Fig. 1222). The inner extremities of the fibres possess conical expansions, which are in apposition and form an incomplete sheet, known as the membrana limitans internet . As the fibres traverse the retinal layers, they give off delicate lateral offshoots, which break up into a fine supporting reticu- lum. Within the inner nuclear layer each fibre presents a broad expansion, in which is situated the oval nucleus of the sustentacular cell, the fibre of Miiller. After traversing the outer nuclear layer their broadened peripheral ends come into contact and form a continuous sheet, the membrana limitans externa. From the latter deli- cate offshoots continue outward and embrace the bases of the individual rods and cones. In addition to the robust fibres of Miiller, neuroglia cells, in the form of spider cells, are found in the nerve-fibre and ganglion cell layers. These cells send out long delicate processes which extend between the processes and cells and thus help to support them. The Macula Lutea. — The structure of the retina undergoes important modifi- cations in two areas, at the macula lutea and at the ora serrata. In the former the n-an^-lion cells increase rapidly in number as the macula is reached, so that instead of forming a single layer they are distributed in from eight to ten strata. The inner nuclear layer is also increased in thickness. Within the fovea centralis, however. in order to reduce to a minimum the layers traversed by the light-rays, the cerebral layers are almost entirely displaced, only the absolutely essential retinal strata— the pigment cells and the visual cells with their necessary connections — being retained within the area of sharpest vision (Fig. 1223). On approaching the fovea, the Uansjion cells rapidly decrease in number, until at the centre of the depression, they Bi« entirely absent and the nerve-fibre layer, therefore, disappears. The bipolar Supporting fibres of Miiller from retina of ox; Golgi preparation. (Cajal.) THE NERVOUS TUNIC. cells are present as an irregular layer within the fused remains of the two plexiform layers. The most conspicuous elements are the visual cells, which in this position are represented solely by the cones, which have about twice their usual length and thickness, the increase in length being contributed by the outer segments. The cone-cell nuclei become removed from the external limiting membrane ; the cone- fibres are therefore lengthened, pursue a radial direction, and constitute the so-called Internal limiting membrane Inner plexiform layer Ganglion cells FIG. 1223. Fovea centralis Bipolar cells Outer Pigmented Cone Cones plexiform layer layer visual cells Section of human retina through fovea centralis. X 80. fibre-layer of Henle. Opposite the centre of the fovea, the choroid is thickened by an increase in the choriocapillaris. The yellow color of the macula is due to a diffuse coloration of the inner retinal layers. The Ora Serrata. — The visual part of the retina ends anteriorly in an irregu- lar line, the ora serrata. Within a zone of about i mm. in width, the retina dimin- ishes in thickness from .50 to .15 mm., in consequence of the abrupt disappearance of its nervous elements. The rods disappear first ; then the cones become rudimen- tary, and finally cease ; the ganglion cells, nerve-fibre layer and inner plexiform layer fuse, and the two nuclear layers unite and lose their characteristics, most of the nuclei present being those of the supporting fibres of Miiller, which are here highly developed. These elements continue beyond the ora I224- serrata (Fig. 1224) as the transparent cylindrical cells composing the inner layer of the pars ciliaris retincz, the densely pigmented cells of the outer layer being a direct continuation of the retinal pigmented cells. These two strata of cells are prolonged over the ciliary body and the Pigmented cells ells Bipol Section of human retina through ora serrata, showing transition of pars optica into pars ciliaris. X 165. iris as far as the pupillary margin, over the iris constituting the pars iridica retinfs. As the columnar cells pass forward, they gradually decrease in height, and at the junction of the ciliary body and the iris the cells of both layers become deeply pig- mented, with consequent masking of the boundaries of the individual elements. The cells of the anterior layer are of additional interest as giving rise to the dilatator muscle of the iris. The aggregation incident to the convergence of the nerve-fibres from all parts of the retina produces a marked thickening of the fibre-layer around the optic disc, and as the fibres turn outward to form the optic nerve the other layers of the retina, together with those of the choroid, suddenly cease. On the temporal side a narrow meshwork of intermediate tissiie separates the nerve-fibres from the other retinal strata, but at the nasal side this tissue is absent. The ganglion cells dis- appear first, whilst the pigmented cells, with the lamina vitrea of the choroid, extend furthest inward. . The blood-vessels of the retina are derived from a single artery, the arteria centralis retina;, which enters the optic nerve at a point from 15-20 mm. behind the eyeball, and, with its accompanying vein, runs in the axis of the nerve and 1468 HUMAN ANATOMY. emerges slightly to the nasal side of the centre of the optic disc. Here the artery divides into two main stems (Fig. 1 225), the superior and inferior papillary branches, each of which subdivides at or near the disc-margin into superior and inferior nasal and temporal branches which run respectively mesially and laterally, dividing dichotomously as end arteries, no anastomosis existing. The macular region is supplied by special macular branches, the center of the fovea, however, being free from blood-vessels. The larger branches from the central artery course within the nerve-fibre layer, and send fine twigs peripherally inward to form an inner and an outer plexus, the former on the outer surface of the inner plexiform layer, and the latter within the inner nuclear layer. Beyond the outer plexiform layer the vessels do not penetrate, the visual cells being dependent for their nourishment upon the choriocapillaris of the choroid. At the nerve entrance an indirect communication exists between the arteria centralis and the posterior ciliary arteries, through the medium of the small branches which constitute the circulus arteriosus Zinni. FIG. 1225. Temporal Nasal Normal fundus of right eye as seen with ophthalmoscope ; central retinal vessels seen emerging from optic nerve; arteries are lighter, veins darker vessels; fovea centralis shows as light point in macular region, which lies in temporal field and is devoid of large vessels. The lymphatics of the retina are represented chiefly by the perivascular lym- phatic spaces which surround all the veins and capillary blood-vessels. These spaces may be injected from the subpial lymph-space of the optic nerve, and by the same method communications may be demonstrated between (i) this space and the interstices between the nerve bundles which converge toward the optic papilla, (2) a space between the membrana limitans interna and the hyaloid membrane of the vitreous, and (3) a narrow cleft between the pigmented cells and the layer of rods and cones. Practical Considerations. — All pathological conditions of the retina ap- pear as opacities, and thus interfere with sight. The medullary sheaths of the optic nerve-fibres end at the lamina cribrosa. Rarely the sheaths around these may extend some distance into the retina, showing as a white striated margin around the optic disc and continuous with it. Sometimes the blood-vessels of the retina may enter at the margins of the optic disc, instead of at its centre, as usual, which is then free of vessels and very pale. At the entrance of the optic nerve, the transparency of the retina is lessened by the thickening of its fibre-layer PRACTICAL CONSIDERATIONS: THE RETINA. 1469 The integrity of the central artery of the retina is necessary to the preservation of sight. The branches of this vessel are distributed to the retina only, and have no communication with those of the other coats, nor do they anastomose with one another. If the main artery or one of its branches is plugged with an embolus, the area supplied by the blocked vessel is then deprived of sight. The retina may undergo inflammatory change in nephritis, syphilis, diabetes, and other constitutional diseases. Of all these inflammations of the retina, that due to kidney disease (albuminuric retinitis) is the most characteristic. Besides the signs of general inflammation, as haziness of the retina, choked disc, distended retinal arteries, or hemorrhages into the retina, pure white or even silvery patches often occur ; they are due to fatty degeneration. Retinitis without these charac- teristic changes may occur from albuminuria, so that the urine should be examined in all cases of retinitis. The retina between the optic nerve and the ora serrata is held in apposition to the choroid only by the support afforded by the vitreous body. It may be readily detached from the choroid by such causes as injury, extravasation of blood or serum between the two layers, or by tumors of the choroid. In contusions of the eye the retina is sometimes torn alone, although this is rare. The retina does not tear as easily as the choroid, as is shown by the fact that in ruptures of the choroid the retina is generally not lacerated. Glioma is the only tumor found in the retina, and occurs exclusively in children, usually under three years of age. A rare tumor arising from the pars ciliaris retinae has been described, to which the name terato-neuroma has been applied by Verhoeff. The Optic Nerve. — The extraocular portion of the optic nerve has been de- scribed elsewhere (page 1223). Likewise, the three sheaths — the dural, the arachnoid FIG. 1226. Physiological excavation Lamina cribrosa Fibre-layer — Visual cells — \ Choroid — : Sclera — Dural sheath - I'_L_-V — Subarachnoid space Arachnoidal £ . f j 4— Subdural spa. sheath y Pial sheath i Y- '&'' r^^—^..-. , Hl, JH Central retinal vessels within optic nerve Section of eyeball through entrance of optic nerve. X 20. and the pial — which, with the subdural and the subarachnoid lymph-spaces, are con- tinued over the nerve as prolongations of the corresponding brain-membranes (page 949). On reaching the eyeball, the dural sheath bends directly outward, its fibres commingling with those of the outer third of the sclera (Fig. 1226) ; the arachnoid ends abruptly on the inner wall of the intervaginal space ; whilst the pia arches outward to form part of the inner third of the sclera, but sends longitudinal fibres as far as the choroid. As the nerve-fibres enter the eyeball, for convenience assuming that they are passing from the brain toward the retina, they traverse a fenestrated i4/o HUMAN ANATOMY. FIG. 1227. Blood-vessel Interfascicular connective tissue membrane, the lamina cribrosa, which is formed by interlacing bundles from the inner third of the sclera and from the pial sheath. As they penetrate the lamina cribrosa they lose their medullary sheaths ; in consequence the optic nerve is reduced one third in diameter. The intervaginal lymph-space ends abruptly, being separated from the choroid by the fibres of the pia which arch outward to join the sclera. The nerve projects slightly into the eyeball on account of the thickness of ., . the layer of arching nerve- fibres and forms, therefore, a circular elevation, known as the optic papilla or optic disc, about 1.5 mm. in diameter, the center of which is occupied by a fun- nel-shaped depression, the so-called physiological exca- vation. The axis of the nerve 4. / • is occupied by the central 'M^,-'>^.-. — •._, artery of the retina, which v gives off minute branches for Transverse section of part of optic nerve, showing several fasciculi of the nutrition of the nerve nerve-fibres. X 125. . , , ' that anastomose with the pial vessels, and, through the circulus arteriosus Zinni, with branches of the posterior ciliary arteries. When seen in transverse sections (Fig. 1227), the optic nerve appears as a mosaic of irregular polygonal areas composed of bundles of medullated nerve-fibres surrounded by connective tissue envelopes. Although provided with medullary sheaths, the optic fibres are devoid of a neurilemma, in this respect agreeing with the nerve-fibres composing the central nervous system. The entire nerve corresponds to a huge funiculus, the perineurium being represented by the pial sheath, and the endoneurium by the interfascicular septa of connective tissue prolonged from the pia between the bundles of fibres. Numerous connective tissue cells occur along the strands of fibrous tissue. Practical Considerations. Any disturbance of the optic nerve-fibres passing from the retina to the cortex of the brain (page 1225) will cause disturbance of vision, and within certain limits the lesion may be localized by the character of the symptoms produced. The most characteristic symptom from a lesion on one side behind the chiasm is a homonymous lateral hcmianopsia, — that is, the right or the left half of each eye will be blind. This is explained by the fact that the optic tracts are made up of fibres coming from the corresponding lateral halves of both retinae, — /'.y the lesions, no other symptoms will be present, but the hemianopsia will be complete and homonymous — that is, the corresponding halves of the two eyes will be blind. THE CRYSTALLINE LENS. 1471 FIG. 1228. If the lesion affect the chiasm, as from tumors of the pituitary body, periostitis of the body of the sphenoid bone, tuberculous or syphilitic exudate, causing pressure on the mesial portion of the chiasm involving the decussating fibres, the nasal half of each eye supplied by these fibres will be blind (heteronymous hemianopsia). Since the nasal half of each eye perceives the temporal half of the visual field, this variety of half-blindness is called bitemporal hemianopsia. If the optic fibres of one side in front of the chiasm are involved, the disturbance of vision will affect one eye only, so that the occurrence of absolute blindness of one eye, without other known cause, with good sight in the other, would suggest a lesion in front of the chiasm. Inflammation of the intraocular end of the optic nerve — that is, of the optic disc, or papilla — gives rise to the condition to which the name optic neuritis, or papillitis, is applied, which is then recognizable with the ophthalmoscope. If in addition to or independently of the signs of inflammation there are marked engorgement, oedema, and the evidence of mechanical compression, so that the swollen nerve-head protrudes into the vitreous beyond y2 to ^ mm., the phenomena of "choked disc" are pre- sent. This variety of papillitis, as well as more moderate grades of optic neuritis, constitutes one of the important symptoms of brain tumor, occurring in fully 80 per cent, of the cases. The development of the papillitis does not necessarily depend upon the size of the growth, nor upon its situation, except that tumors of the medulla are less apt to originate optic neuritis than those in other parts of the brain. Usually a bilateral condition, it is sometimes unilateral, and under such circum- stances it suggests that the cerebrum is the seat of the growth, and is, on the whole, in favor of the tumor being on the same side as the neuritis. With this exception, however, optic neuritis, although an important symptom of brain tumor, has no localizing significance. Other intracranial causes of optic neuritis are the various types of meningitis (when the ophthalmoscopic picture often appears in the form of the so-called "descending neuritis"), abscess and soft- ening of the brain, cerebritis, hydrocephalus and aneu- rism. In addition to the intracranial causes of papillitis, this phenomenon may arise from a general infection — for example, influenza, syphilis, rheumatism, small-pox, etc. — and is then known as infectious optic neuritis. It is also caused by various toxic agents, by anaemia, by menstrual disturbances, nephritis, and other constitu- tional disorders (de Schweinitz). Injuries of the optic nerve are most frequently the result of fractures of the base of the skull at the optic foramen, the nerve being injured by the fragments. It may be wounded by foreign bodies entering the orbit, with or without injury of the eyeball. THE CRYSTALLINE LENS. The lens, the most important part of the refractive apparatus of the eye, is a biconvex body situated on a level with the anterior plane of the ciliary body, from which it is suspended by the suspensory ligament, or zonule of Zinn. Its anterior surface supports the pu- pillary margin of the iris, and its posterior surface rests in a depression, the patellar fossa, on the anterior sur- face of the vitreous body. It is completely transparent and enclosed in a transparent elastic membrane, the lens capsule. Together with the capsule, the lens measures from 9-10 mm. in its transverse diameter, and about 4 mm. in thickness from pole to pole. The convexity of its two surfaces is not the same, that of the posterior being greater than that of the anterior. Neither are these convexities constant, since they are continually changing with the variations in lens-power incident to viewing distant or near objects. The radius of curvature of the anterior surface is approxi- mately 9 mm. and that of the posterior surface 6 mm. when the eye is accommodated Meridional section of human lens and its capsule ; anterior epithelium and transitional zone are seen. X 7- (Babuchin.) H72 HUMAN ANATOMY. Fragments of isolated lens-fibres; A, from superficial layers ; B, from deeper layers ; C, young fibres with nuclei. X 275. for distant objects ; these radii are reduced to about 6 and 5 mm. respectively in accommodation for near objects. The anterior surface is therefore more affected in the act of accommodation, the lens becomes more convex and its antero-posterior diameter increases from 4 to 4.4 mm. The superficial portion of the lens beneath the capsule is composed of soft compressible material, the substantia corticalis ; the consistency gradually increases toward the centre, especially in later life, so that the central portion, the nucleus lentis, is much firmer and dryer. The structure of the lens includes the capsule and its epithelium and the lens substance. The capsule, which entirely surrounds the lens, is a transparent, struc- tureless, highly elastic membrane, which, while resistant to chemical reagents, cuts easily and then rolls outward. It is thickest on the anterior surface, where it measures from .010— .015 mm., and thinnest at the posterior pole (.005-. 007 mm. ). In the adult the lens is devoid of blood- vessels, but during a part of foetal life it is surrounded by a vascular net-work, the tunica vasculosa lentis, which is supplied chiefly by the hyaloid artery. This temporary vessel is the terminal branch of the central artery of the retina and passes from the optic disc forward through the hyaloid canal or canal of Cloquet in the vit- reous to the posterior surface of the lens. The vascular lens tunic and the hyaloid artery are temporary structures and usually disappear be- fore birth. Exceptionally they may persist, the tunic being represented by the pupillary membrane and the artery by a fibrous strand within the vitreous, stretching from the optic disc towards the lens. The capsule probably represents an exudation product of the cuticular elements from which the lens- substance is developed. The anterior portion of the capsule is lined by a sin- gle layer of flat polygonal cells, the epithelium of the lens capsule, which represents morphologically the anterior wall of the original lens- vesicle (page 1481). On ap- proaching the equator of the lens, these cells become elongated, and gradually converted into the young lens- fibres, the nuclei of which form a curved line, with its convexity forward, in the superficial part of the lens. The lens-substance is composed of long flattened fibres, the cross-sections of which have a compressed hexagonal outline, from .oo5-.on mm. broad and from .002— .004 mm. thick, held FIG. 1231. together by an interfibrillar cement substance. These fibres are modified epithelial elements, which develop by the elongation of the original ectoblastic cells of the poste- rior layer of the lens-vesicle. The subsequent growth of the lens depends upon a similar modification of the anterior capsule-cells, the re- gion where this transforma- tion occurs being known as the transitional zone. The individual lens-fibres vary greatly in length, those form- ing the outer layers being longer and thicker than those which constitute the nucleus of the lens. The edges of the fibres are finely serrated, and, as the points of the serrations of adjacent fibres are in contact, fine intercellular channels are left for the FIG. 1230. Lens-fibres seen in transverse section. X 280. Adult crystalline lens, showing lens-stars; A, anterior; fi, posterior surface ; radiating lines of juncture meet at central area. X 4. (At until. ) THE VITREOUS BODY. 1473 passage of nutritive fluid. The fibres are so arranged that their ends terminate along definite radiating striae, or lens-stars, which in the young lens are three in number on each surface. In the adult lens additional rays increase the number to from six to nine, the striae being less distinct but distinguishable with the ophthalmoscope. The lens-fibres which come from the pole of one surface of the lens terminate at the end of one of the radial striae in the other, and conversely ; the intervening fibres take up intermediate positions. In adult life the lens-fibres become more condensed, the lens loses its clear appearance, and assumes a yellowish tint. This change affects the nucleus first and the periphery later, coincidently the lens becoming less elastic as the result of its loss of water. Practical Considerations. — The lens may be congenitally absent (aphakia), or it may be abnormal in size, shape, position, or transparency. Its anterior or posterior surface may be abnormally convex (lenticonus). Congenital anomalies of position (ectopia lentis) occur rarely. The lens may remain in its fcetal position in the vitreous chamber, or it may be displaced in an equatorial direction from faulty development and weakness of some part of the suspensory ligament. This weakness usually occurs below so that the lens moves upward. The ligament may be absent in its whole circumference, when the lens may be protruded into the anterior chamber. Coloboma or partial deficiency of the lens is very rare. It is with comparative frequency associated with a similar defect in the iris, ciliary body and choroid, and, like it, is usually in the lower portion. A defect of the corresponding part of the suspensory ligament is occasionally present. Traumatic luxation of the lens may take place into the vitreous or aqueous chamber. It may occur laterally through the coats of the eyeball into the capsule of Tenon or under the conjunctiva. That into the vitreous is most frequent. The capsule of the lens is strong and elastic. It is at the same time brittle, breaking like thin glass when torn as by a sharp instrument. For this reason it is sometimes called the vitreous membrane. The anterior layer of the capsule is con- siderably thicker than the posterior, and is more liable to pathological changes, pro- ducing opacities. Wounds of the capsule permit the aqueous fluid to reach the lens fibres, which then become swollen, opaque, and finally disappear from the dissolving action of the aqueous. Advantage of this is taken in the needling operation (dis- cission) for the removal of a cataract. In children the lens substance is of nearly equal consistency throughout, but as age advances the central portion becomes gradually more condensed, and is called the nucleus. A well-marked nucleus, however, does not exist until adult life. In old age the lens loses its elasticity so that the changes necessary for accommodation are interfered with, and sight is disturbed. The hardened nucleus permits a greater reflection of light than the outer portion, so that the lens is more readily seen in older people, and the pupil loses more or less its blackness. A cataract is an opacity of the lens, or its capsule, but that of the lens is so much more common than that of the capsule, that by the word cataract the lenticular is usually meant, unless the word is otherwise qualified. All cataracts are at sometime partial, and they are called according to their location, anterior polar or capsular, posterior polar or capsular, central or nuclear, lamellar, perinuclear and cortical. Cataract occurs sometimes in the young, and is then soft ; that is, the lens has no nucleus. THE VITREOUS BODY. The vitreous body (corpus vitreum) fills the space between the lens and the retina, being in close contact with the retina and acting as a support to it as far forward as the ora serrata. Here it becomes separated from the retina and passes to the posterior surface of the lens, presenting a shallow depression, the fossa hya- loidea or patellar fossa, on its anterior surface for the reception of the lens. The fresh vitreous is a semifluid, perfectly transparent mass which consists of about 98. 5 per cent, of water. The structure of the vitreous has been a subject of protracted dispute, but recent investigations have established beyond question that it possesses a framework, 93 1474 HUMAN ANATOMY. composed of delicate, apparently unbranched fibrils, which pass in all directions through the vitreous space and form the meshes in which the fluid constituents of the mass are held. The surface of the vitreous is enclosed by a delicate boundary layer, called the hyaloid membrane, formed by condensations of the fibrils, which are here arranged parallel to the surface, and closely felted. It is, however, not a true membrane, but only a con- densation of the vitreous fibres. The vitreous is attached firmly to the retina at the nerve entrance and at the ora serrata, between these points the hya- loid being indistinct. As the vitreous leaves the retina, the boundary layer becomes thicker, in some cases to be- come thin again or absent in the region of the patellar fossa. The central part of the vitreous is occupied by a channel, the hyaloid canal, also known as the canal of Slil- ling or the canal of Cloqiict, which is about one millimeter wride and extends from the optic entrance toward the pos- terior pole of the lens. During foetal life this canal lodges the arteria hya- Portion of adult vitreous body showing felt-work of fibres hidea, the continuation of the central and atrophic traces of cells. X 450. (Retzius.) . . . • • « artery of the retina, which passes to the lens and assists in forming the embryonal vascular envelope surrounding the lens. Usually the embryonal connective tissue, together with the blood-vessel, disappears ; occasionally, however, delicate remnants of this tissue can be detected. The normal adult vitreous ordinarily contains no cells, but some are occasionally seen near the surface, beneath or on the hyaloid membrane. They are amoeboid, often contain vacuoles and are to be considered as modified leucocytes. In addition a few branched connective-tissue cells may be present. Practical Considerations. — Congential abnormalities of the vitreous arc due either to a persistence of some part of its foetal vascular apparatus or to an atypical development of the tissue from which it is formed. The remains of these structures may occasionally be seen as a filamentous band, free at one end, which floats in the vitreous, the other end being attached to the optic disc behind, or the posterior sur- face of the lens in front. The strand may be attached at both ends, with or without a patent artery. Small rounded gray bodies, apparently cystic and attached to the disc, are occasionally seen. They are in some way the remains of the fcetal vascular apparatus. The congenital opacities sometimes seen at the posterior pole of the lens are probably derived from the posterior fibro-vascular sheath of the lens. Materials from the blood are readily absorbed by the vitreous, as the bile in jaundice. Muscte volitantes are the flocculi, seen by the patient as black spots fore the eyes, and are sometimes made up of inflammatory exudate from inflam- mation of the internal or middle coat of the eye. They may be due to blood from traumatic or spontaneous hemorrhage into the vitreous. Muscat volitantes are often seen independently of any vitreous disease and are due to the shadows thrown upon the retina by naturally formed elements in the vitreous body, perhaps the remains of embryonic tissue. Some of the vitreous may be lost and rapidly replaced with- out seriously disturbing sight. In the removal of cataract, the suspensory ligament may be divided and an embarrassing loss of vitreous may result. A foreign body in the vitreous chamber generally gives rise to a serious inflam- mation, which may destroy the eye. If loose, it tends by gravity to settle in tin- lower portion, and usually rests on the posterior part of the ciliary body (T. Collins). Rarely, in the absence of infection, it has remained for years without setting up inflammation. The rule is, however, to remove them, when recent, as early as possible, as inflammation may set in at any time. In most cases the foreign body lals be- im. SUSPENSORY APPARATUS OF THE LENS. 1475 Cornea Canal of Schlemm Sclera can be exactly localized by the X-ray, and if of iron or steel, may often be removed by a magnet. The accident is always serious and may be followed by a virulent inflammation, demanding an excision of the globe to prevent a sympathetic involve- ment of the other eye. Because of the risk of infection aatd loss of fluid, operative interference in the vitreous chamber is usually to be avoided; Sympathetic ophthalmitis, or more accurately, infective irido-cyclitis, or uveitis, is an inflammation of one eye, usually called the "sympathizer^ owing to injury or disease of the fellow eye, usually called the "exciter." Traumatisms of the ciliary region (danger zone) which have set up an irido-cyclitis or uveitis are responsible for fully 80 per cent, of the cases of so-called sympathetic inflammation. This disease was formerly supposed to be due to reflex action through the ciliary nerves, and this theory in a modified form is still maintained by a few clinicians. The " mi- gration theory ' ' propounded by Leber and Deutschmann that the inflammation is a progressive process in the continuity of the tissue of one eye to the other by way of the optic nerve apparatus and is of bacterial origin, has not been proved. It is believed by some investigators that the bacteria which enter the primarily affected eye produce a toxin which causes the disease, and by others that it represents an endogenous infection produced by invisible bacteria, that is, that it is a metastasis (de Schweinitz). THE SUSPENSORY APPARATUS OF THE LENS. The lens is held in position by a series of delicate bands, which pass from the vicinity of the ora serrata over the ciliary processes to be attached to the periphery of the lens. These fibres collectively con- FIG. 1233. stitute the suspen- sory ligament, or zonula of Zinn, a structure of impor- tance not only for the support of the lens but also in assisting the ciliary muscle in effecting the changes in the curvature of the lens incident to accommodation. The zonula is not, as for- merly believed, a con- tinuous membrane, but is composed of a complicated system of fibres. The latter, varying in thickness from .005-. 022 mm., arise chiefly from the cuticular membrane covering the pars ciliaris retinae in the vicinity of the ora serrata. The investigations of Retzius, Salzmann and others indicate that some fibres arise also from the mem- brana limitans interna of the pars optica retiny th<- proliferation of its cells, becomes converted into a solid structure and constitutes the crvNt.illine lens. At first the lens-vesicle fills the cavity of the optic cup completely, but with tlie deepening of the latter, a spare appears between its anterior wall and the lens-vesicle, which gradually widens and forms the vitreous cavity. The space between the lens-vesicle and the ectoMast is imaded l>v a process from the surrounding mesohlast. which pushes in from the side. From this in-n.uth is developed the cornea, with the exception of the surface epithelium, and the stroina of the iris. Almost from the tirst appearance of the inva.uination of the primary optic vesicle, the inva-inated portion of the \-.iH exhibits a maiked tendency to proliferation of its cells. The Part of frontal section of head of early rabbit embryo, showing optic vesicles e vagina ted from brain-vesicle. X 30. DEVELOPMENT OF THE EYE. 1481 Brain-vesicle Uninvaginated wall Invaginated wall Optic stalk Optic vesicle Lens-pit Lens-pit shows as depressed area of thickened ectoblast ; anterior wall of optic vesicle beginning to be invaginated ; optic stalk narrowing. X 30. uninvaginated portion of the wall, on the contrary, gradually becomes thinner, until it is repre- sented by a single layer of cubical cells. These soon assume a dark color in consequence of the appearance within their protoplasm of fine pigment particles. From this wall, there- fore, the layer of pigmented cells composing the outermost stratum of the retina is developed, whilst from the rapidly augmenting layers of the inner wall, the essential nervous elements of the retina, together with the supporting neurogliar tissue, are formed. The invagination of the optic vesicle is not confined to its outer wall, but also affects its lower wall, in consequence of which a groove, the fietal ocular cleft, appears in this position (Fig. 1240). This is continued backward to and along the under surface of the optic stalk, in the form of a furrow. By means of this slit a com- munication is established between the cavity of the FIG. 1238. secondary optic vesicle and the centre of the optic stalk, and through it blood-vessels from the sur- rounding mesoblast gain entrance to the interior of the nerve and the eyeball. The walls of this fcetal cleft gradually approximate and become fused. The imprisoned vessel, the hyaloid artery, later gives rise to the arteria centralis retinas. The vitreous body has been usually considered as a derivative exclusively of mesoblastic tissue which entered the eye in company with the blood-vessels. According to the investigations of Schon, Kolliker, Wolfrum and others, however, this view is inadequate, since at least the anterior or ciliary portion of the vitreous is a product of the cells of the inner wall of the secondary optic vesicle. The choroid and the sclera are differentiated from the mesoblast, which surrounds the eyeball. Development of the Lens. — Soon after the iso- lation of the primitive lens- vesicle from the surface ectoblast, the cells in the posterior wall begin to proliferate actively, while those on the anterior wall are reduced to a single layer. The latter persists as the lining epithelium of the adult lens-capsule. By the growth of the cells of the posterior wall and their elongation into lens-fibres, the hollow vesicle is gradually converted into a solid mass of lens-substance, the fibres extending forward until they come in contact with the anterior wall. Subsequently the growth of the lens proceeds by the application of additional layers of fibres to the surface of the primary nucleus, the new fibres developing from the cells lining the anterior capsule. Their con- FIG. 1239. version takes place at the equator of the lens, where the nuclei of the elongating lens-fibre are arranged in a convex line known as the nuclear zone (Fig. 1228). The capsule of the lens appears very early, even before the closure of the lens-vesicle, and long before the appearance of blood-vessels around the lens. It forms a sharp boundary line, at first along the posterior border, which gradually thickens and finally surrounds the entire lens. The capsule is to be considered as a secretion product of the lens-cells. The rapid early growth of the young lens requires an adequate blood supply. This is insured by the development of a vascular net-work, the tunica vasculosa lentis, which completely surrounds the lens from the second month until the close of fcetal life, when this temporary membrane is ab- sorbed. The chief supply of this vascular net-work is derived from the vessels of the vitreous, which, as already noted, enter the eye through the cleft in the optic nerve. Passing forward through the canal of Cloquet in the centre of the vitreous cavity, the chief vessel, the hyaloid artery, reaches the posterior pole of the lens, when it divides into numerous branches. These branches pass around the equator of the lens onto the anterior surface, where, joined by vessels from the mesoblastic tissue which is to constitute the future iris and ciliary body, they proceed to the centre of the pupil and break up into their terminal loops. The portion of the net-work covering the pupillary area is called the membrana pupil- laris, whilst the remainder is known as the membrana capsularis. This vascular sheet is usually entirely absorbed before birth, but occasionally portions of it may be seen persisting in the form of fine threads in the pupillary space, or on the posterior pole of the lens. The retention of such strands is sometimes associated with the persistence of portions of the hyaloid artery. ,r— Ectoblast Outer layer Lip of optic cup Inner layer Anterior wall Optic stalk Posterior wall of lens-sac Lens-sac closed ; outer and inner layers of sec- ondary optic vesicle now almost in contact. X 30. HUMAN ANATOMY. FIG. 1240. — Outer layer r— Inner layer — Posterior wall y of lens-sac - MrsobtaSt •— Lip of optic cup Foetal cleft Sagittal section of developing eye at same stage as preceding specimen, showing invagi- nation of optic vesicle along fcetal cleft. X 30. Development of the Retina.— As already pointed out, the retina develops from the walls of the optic vesicle, the pigmented layer being derived from the nninvaginated outer wall, the pig- ment appearing early and first near the anterior margin of the optic cup; the remainder of the retina comes from the rapidly growing cells of the inner wall. The first cells to be differentiated in the nervous portion of the retina are the spoitgioblasis which develop into the supporting neurogliar fibres, the fibres of Mi'illo: These are strengthened by the addition of mesoblastic elements, which enter the inner layers along with the blood- vessels. The neuroblasts develop from cells which correspond in position to those of the external nu- clear layer. As they divide, the cells are displaced inward, so that the ganglion-cells represent the oldest descendents. When seven or eight layers have been differentiated, the ganglion-cells send out axones, which form the fibre-layer and converge toward the optic nerve. The visual cells are the last to appear, the layer of rods and cones developing as cuticular outgrowths from the cells of the external nuclear layer. Anteriorly the walls of the secondary optic vesicle are reduced to a double layer of cells. For a certain distance, corresponding to the position of the future ciliary body (pars ciliaris retinae), the outer cells are pigmented, whilst the inner ones are trans- parent. Still farther forward, the rudimentary portion of the nervous tunic is continued over the posterior surface of the iris (pars iridica retinae) as a double layer of deeply pigmented cells which extends as far as the pupillary margin which thus corresponds to the anterior lip of the secondary optic vesicle. The optic nerve is developed secondarily and in close association with the early optic stalk, which is at first hollow, and later becomes grooved along its inferior surface. The walls of this foetal cleft become approximated and, after the entrance of the blood-vessels, the lips of the cleft fuse, the vessels being thus enclosed. Since the fibres of the optic nerve are for the most part axones of the ganglion-cells of the retina, it is evident that they are not developed within the nerve, but invade the latter as outgrowths of fibres from the retina, pushing along the optic nerve and tract to reach their cerebral connections. In addition to these centripetal fibres, a certain number of centrifugal ones appear later as outgrowths from cells within the brain. The supporting tissue is developed by proliferation of the cells of the optic stalks FIG. 1241. and their differentiation into neurogliar ele- ments, assisted by the mesoblastic elements from the surrounding pia and the portion which enters the cleft with the blood-vessels. The nerve-fibres are at first naked axis-cylin- ders, which later acquire medullary sheaths. Development of the Fibrous and Vascular Tunics. — With the separation of the lens- vesicle from the overlying ectoblast, the meso- blast insinuates itself between these structures, / in addition to surrounding the entire ecto- blastic optic vesicle. Tin- portion surrounding tin- optic vesicle posteriorly thickens rapidly and becomes differentiated into the vascular tunic, or choroid, whilst the outer layer be- comes the fibrous tunic, orsclera. The choroid appears first, the pigmentation of its cells being Upper eyelid Outer pigmented retinal layer Inner retinal layer __ Mesoblast _ Lens, now solid Optic nerve _\'asi-ular vitreous tissue — Ectoblast ^s Lip of retinal coat _ Mesoblast — Lower eyelid Much later stage, showing lens now solid ; lavers of optic vesicle converted into retinal coat ; vascular vitreous tissue; condensation and invasion of mesoblast. evident by the seventh month. The meso- blastic proc.-ss I iet \\eeti tlie lens and the ecto- Mast is verv thin at first, but subsequently splits into two layers. Tin- anterior of these becom.-s the sui.staiitia ptopria ,,f the cornea and its lining endothelium. The latter produces the membrane of DefCemet The posterior mesoblastic layer carries blood-vessels which help to form tin- capillarv net-work surrounding the lens. The space between the two mesoblastic lavers represents the future anterior chamber of the eye. About the fourth fcetal month the an- terior lip of the optic vesicle pushes forward in advance of the lens and carries with it additional iiiesoblastic tissue |- p,m this the iris is developed, the stroma being formed by the mesoblast, uhilst the posterior pi-inepted portion represents the anterior part of the op'tic vesicle, from which the dilatator muscle ' and. according to some authorities, also the sphincter pupillz) is derived. "I he ciliary processes are produced by the rapid lateral expansion of the walls of the THE EAR. 1483 optic vesicle, about the fourth or fifth month, in consequence of which folds in the membrane arise, into which blood-vessels and other mesodermic elements extend. The corneal stroma becomes blended with the sclera, thenceforth the two forming a continuous tunic. Development of the Vitreous Body. — As already stated, the vitreous body is at present re- garded as developing, at least in part, from the cells of the inner wall of the optic vesicle, especially from its anterior or ciliary portion. The suspensory ligament of the lens is derived from the same source. The cells develop into the fibres which form the fine net-work of the vitreous body; at the periphery these become condensed and form the boundary layer or hyaloid membrane. The vitreous is supplied with blood by branches of the hyaloid artery, which springs from the head of the optic nerve. An especially complete net-work is found at the periphery of the vitreous and these vessels pass forward to the equator of the lens and assist in forming the tunica vasculosa lentis. The retinal vessels are formed later as branches of the central artery, the vitreous vessels usually undergoing complete absorption before birth. The development of the eyelids begins with the production of folds of integument, which appear above and below the cornea during the second fcetal month. The folds approach each other and the epidermal cells fuse about the third month, the eyelids remaining united until shortly before birth. The Meibomian and other glands of the lids are produced by ingrowths of the surface ectoblast. The lachrymal gland arises during the third month as a solid ingrowth from the conjunctival epithelium close to the upper lid. The lachrymal canal begins as a solid process of epithelial cells from the lid, which dips inward along the lachrymal furrow, between the superior maxillary and nasal processes. This cord of cells becomes isolated from the sur- face, and later acquires a lumen, connecting by means of the canaliculi with the conjunctival sac above. The duct establishes communication with the nasal fossa just before birth. THE EAR. The ear (organon auditus) may be conveniently studied under its three natural subdivisions, which are conventionally described as the external, middle and the internal ear — structures lodged entirely or in part within the temporal bone. The FIG. 1242. Bone \ Malleus Incus Stapes Inner ear Semicircular canal Internal auditory canal Auditory nerve Endolymphatic sac Cartilage Diagram showing relations of three subdivisions of ear. {Modified from Schwalbe.) external ear includes the auricle and the external auditory canal ; the middle ear the tympanum, the Eustachian tube and the mastoid cells ; and the internal ear the labyrinth, with the peripheral ramifications of the auditory nerve. Such division, moreover, is justified by the developmental history of the organ, since the internal ear is developed essentially from the highly differentiated otic vesicle which gives rise to the complicated membranous labyrinth ; the middle ear largely from the first pharyngeal pouch ; whilst the external ear represents the deepened and modified boundaries of the first external visceral furrow. 1484 HUMAN ANATOMY. THE EXTERNAL EAR. The external ear, the outermost subdivision of the auditory organ, includes ( i ) the auricle, the funnel-shaped appendage attached to the side of the head for the collection of the sound-waves, and (2) the external auditory canal, which conveys these stimuli to the tympanic membrane, the flexible partition closing the canal and separating it from the middle portion of the ear. THE AURICLE. The auricle (auricula), also called ihe pinna, is attached to the side of the head around the opening of the external auditory canal, midway between the forehead and the occiput. It presents two surfaces, an external and an internal. The angle which its internal surface forms with the head, the cephalo-auricular angle, is usually about 30°, but varies from 20-45°. The circumference of the auricle is somewhat pyriform in outline, with the broadest part of the figure above. The external surface of the auricle is irregularly concave and presents for examination several well-marked depressions and elevations, which depend, for the most part, upon the corresponding modelling of the underlying cartilage. The concha, the largest and deepest of the concavities, surrounds the entrance or meatus to the external auditory canal. This funnel-like fossa is subdivided by an obliquely transverse ridge, the crus helicis, continuous with the helix, into the upper and smaller part, the cymba con- chae, and a lower and larger part, the concha proper or cavum conchae. The tragus is an irregular FIG. 1243. Fossa helicis Cavum conch Antitra Fossa triangularis Crura antihelicis —Cymba conchae -Crus helicis — Incisura anterior — Tragus ^Incisura intertragica Lobulus is an eminence in front of, and slightly overlapping, the meatus. At the upper extremity of the tragus, just below a notch, the incisura anterior, that separates the tragus from the upper part of the auricle, is sometimes seen a small elevation, the tuberculum supratrag- icum. The antitragus is an eminence behind the tragus and separated from it by a deep notch, the incisura intertragica. The lobule contributes the rounded lower ex- tremity of the auricle. In contrast to other parts of the pinna, it possesses no framework of cartilage and, hence, is soft and inelastic. The helix forms the scroll-like margin of the ear, sweeping from the upper part of the tragus in front to the lobule behind. It is more or less rolled upon itseli : so that its margin looks forward. On the anterior edge of the helix, near the junction of its upper and middle thirds, is sometimes found a small triangular ele- vation, the fur-point or titkt'itlf <>/' A^TCV'//, which is of interest as representing, ac- cording to the last-named authority, the erect pointed extremity in the expanded ears of certain quadrupeds. It is said to l»e constant in the foetus of about the sixth month and to U- more common in the male than in the female. In front of and parallel to the heli\, is a curved ridge, the antihelix which Ix-gins at the antitragus below, forms the concave posterior boundary of the concha, and divides above it into a superior ami an inferior cms between which lies the fossa of the antihelix or the fossa triangularis. A narrow groove between the helix and the antihelix marks the fossa of the helix or the scaphoid fossa. Tin- elevations on the external surface of the auricle are represented by depressions on the cranial surface, and conversely tin- depressions on the external Right auricle, outer aspect. THE EXTERNAL EAR. 1485 surface are represented by eminences. Thus, the concavity of the concha is represented on the cranial surface by the eminentia conchae; the antihelix by the fossa antihelicis ; the fossa triangularis by the eminentia fossae triangu- laris ; the scaphoid fossa, by the eminentia scaphae. The other elevations and depressions corresponding to those of the outer surface are not seen on the cranial surface, except in the dissected cartilage denuded of the integument. Structure of the Auricle. — The auricle consists of integument and an enclosed plate of yellow elastic cartilage continuous with that of the meatus. It is also provided with several unimportant ligaments and muscles. The lobule, however, contains no cartilage, but only fibrous tissue and fat enclosed within the integumentary fold. The skin of the auricle is thin and closely adherent to the cartilage, especially on the outer surface. In certain parts it contains fine hairs and sebaceous and sweat glands. The hair follicles are especially abundant over the tragus, antitragus and the notch lying between them, the hairs guarding the entrance into the external auditory canal, known as tragi, being exceptionally long. The sebaceous glands are especially well developed in the cavity of the concha. Cartilage of the Auricle. — The cartilage of the auricle may be divided into two parts : (a} the scroll-like plate forming the tragus and external auditory canal, and (£) the large irregular plate forming the main cartilage. These two divisions FIG. 1244. B Insertion of auricularis superior Obliquus Helicis major Spina helicis Tragicus Plate of tragus and external auditory canal Cauda helicis Cartilaginous framework of right auricle, with intrinsic auricular muscles; A, outer, B, inner surface. are connected by a cartilaginous isthmus lying between the incisura intertragica on its outer side and the deep fissure, (incisura terminalis auris), which in the isolated cartilage is seen between the posterior wall of the outer meatus and the anterior border of the lower part of the concha, on its inner side. Two smaller clefts, the fissures of Santorini, are found between the three plates which form the carti- laginous scroll supporting the tragus and outer end of the external auditory canal. The cartilage of the tragus is an irregular plate and subject to considerable variation. The depressions and elevations of the cartilage proper correspond in general to the surface modelling of the auricle, but are sharply marked, especially on the cranial aspect. A deep notch, the fissura antitragohelicina, separates the lower part of the helix from the antitragus, thus defining the caudal process (cauda helicis), as the lower extremity of the cartilage forming the helix is called. The spina helicis is a small conical projection, directed forward and down- ward, opposite the first bend of the helix. This serves for the attachment of the anterior ligament. The upper end of the tragus-plate fits into an angle formed by the junction of the beginning of the helix and the upper end of the anterior border of the concha. In addition to the elevations and depressions already referred to, on the mesial surface is found a ridge, the ponticulus, which extends downward and forward over the eminence of the concha and serves for the attachment of the posterior auricular muscle (Fig. 1244, B}. 1486 HUMAN ANATOMY. Ligaments of the Auricle.— The extrinsic ligaments of the auricle, those which attach the auricle t<» the temporal bone, form a more or less continuous mass ,,i fibres, These are separated somewhat arbitrarily and described as the anterior and posterior liganu-ms. The anterior ligament extends from the helix and the tra-us to the root of the zygoma. The posterior ligament extends from the emi- nence of the concha and ponticulus to the anterior part of the mastoid process. A number of bands of fibrous tissue, the instrinsic ligaments, bind the parts of the cartilage together. The Muscles of the Auricle.— These include the extrinsic and the intrinsic muscles. The extrinsic muscles of the auricle, those which extend from the head to the auricle and move it as a whole, have been described under the muscular system (page 483). They are the anterior, superior and posterior auricular muscles. The intrinsic muscles, six in number, consist of small strands of muscle-fibres attached to the skin, which extend from one part of the auricle to another and are confined to the auricle itself. Of these, FlG four are on the external surface of the auricle and two on the cranial. 1. The smaller muscle of the helix (m. helicis minor} lies upon the cms helicis and the beginning of the helix, its fibres running obliquely upward and forward. 2. The greater muscle of the helix (in. he/ids major] arises from the spine of the helix and extends upward along the anterior border of the helix and is inserted into the eminence of the triangular fossa. 3. The muscle of the tragus (in. tragi- cus) is a flat muscle on the outer surface of the tragus ; usually only its vertical fibres are distinguishable. Occasionally a separate bundle of muscular fibres (m. pyramidalis} extends from the tragus to the spine of the helix. Likewise another band, the in. in- cisurce Santorini, sometimes called the dilatator concha, bridges the greater incisura Santorini. Both of these, however, belong to the system of the tragus muscle. 4. The muscle of the antitragus (in. antitragicus) is attached to the outer surface of the antitragus. Its fibres run obliquely from the antitragus upward and backward and are inserted into the caudate process of the helix. On the cranial surface of the auricle are the transverse and the oblique muscles. 5. The transverse muscle (in. fransversus) bridges over the fossa antihelicis and extends from the eminence of tin- scaphoid fossa to the eminence of the concha. 6. The oblique muscle (in. ob/iquns}, considered by Gegenbauer as a part of the trans- verse muscle, extends from tin- bark of the concha to the eminence of the triangular fossa. Actions.— I )urhciine and /.iemssm found that by stimulating the muscles of the tragus and antitragus the external auditory canal was narrowed. Duchenne further demonstrated that the . i and lesM-r nmseles c it tin- helix wen- antagonistic to those of the tragus. The transverse intisel.- ,md the ol>li.|iie muscle by their contraction are said to cause a slight flattening of the auricle. Vessels of the Auricle. — Arteries. — The auricle receives its blood supply from branches of tin- Mipertinal temporal artery and the posterior auricular artery, and thus indirectly from the external carotid. The superficial temporal sends three branches to tin- outer surface' of the- auricle: (a) the artery of t lie helix to the i.lin^ part of the helix, fossa triangularis and the superior cms of the anti- helix: < /' .' th«- ," A ;T <>/'///<• ,-rns hdnis to the region of the crus helicis; (c) the ,ntii\' <>/ Hie li-ii^u* to the region of tin- tragus and lobule, the lobule receiving Plate of tragus Cartilaginous i-an.il Bony canal Dissection showing l>ony and cartilaginous portions of right external auditory canal ; seen from in front. THE EXTERNAL EAR. 1487 a branch, the anterior artery of the lobule, from the artery of the tragus. The pos- terior auricular artery supplies a variable number of branches to the auricle. Usually two of these are given off below and one above the posterior auricular muscle. These branches are larger and longer than those from the superficial temporal. After rami- fying over the cranial surface of the auricle, they reach its outer surface by piercing the auricle or by passing over its free margin. They supply the posterior part of the outer surface and anastomose with the branches of the superficial temporal. The veins of the auricle accompany the arteries and include : (a) the anterior auricular, which empties into the superficial temporal ; (b) the posterior auricular, three or four in number, which join a plexus behind the ear which empties principally into the external jugular vein, but also unites with the posterior facial vein. Com- munications with the mastoid emissary vein of the lateral sinus also frequently exist. The Lymphatics. — The lymphatics of the auricle form a close net- work within the deeper layers of the integument, from which lymphatic stems pass in three general groups. Those from the outer surface are afferents chiefly of the anterior auricular nodes, which are placed immediately in front of the tragus and beneath the parotid fascia ; a few, however, bend backward over the helix to end in the posterior auricu- FIG. 1246. (A. perforans fossae triangularis A. helicis A. temporal is A. perforans cymbse A. caudae helici A, tragi A. auricularis post. sup. Posterior auricular muscle A. auricularis post. inf. A. auricularis posterior Parotid branch Arteries of right auricle, A, lateral surface; B, postero-mesial surface. (Schwalbe.) lar nodes that overlie the insertion of the sterno-mastoid muscle. Those from the upper part of the cranial surface pass mainly to the posterior auricular nodes, some being tributary to the external jugular nodes. A number of stems from the lower part of the auricle and from the lobule terminate in the parotid nodes. Nerves of the Auricle. — The motor nerves supplying the intrinsic muscles of the auricle are from the temporal and posterior auricular branches of the facial nerve, the former being distributed to the muscles of the helix, tragus and antitragus, whilst the posterior auricular supplies the tranverse and oblique muscles. The sen- sory nerves include branches from : (a) the great auricular nerve, which supplies the integument of the lower three-quarters of the inner surface of the auricle, with the exception of a small portion near the meatus, and sends filaments to the outer surface of the lobule and adjacent area ; (3) the small occipital nerve, which supplies the upper one-quarter of the inner surface ; (sis between the vessels of the anterior and posterior surfaces THE EXTERNAL EAR. 1489 Sebaceous gland Cartilage of the ear.* At its inner end the cartilagino-membranous meatus is attached to the inferior and lateral edges of the osseous meatus, the fibrous part being continuous superiorly and posteriorly with the periostea! lining of the osseous canal. The osseous portion of the tube, about 14 mm. in length, is longer and narrower than the cartilagino-membranous part. At its inner end it presents a narrow groove, the sulcus tympanicus, for the insertion of the tympanic membrane. The sulcus extends around the sides and floor of the canal, but is deficient above. The skin lining the external auditory canal is closely attached to the underlying cartilaginous portion of the tube. The skin measures about 1.5 mm. in thick- ness, but is much thinner within the bony canal, except along the roof, where it remains relatively thick. Over the outer surface of the tympanic membrane the skin is reduced to a very delicate and FlG- I249- smooth investment, covered by a corre- spondingly attenu- ated epidermis, and a suggestion of sub- cutaneous tissue. Numerous fine hairs and large sebace- ous glands occur in the cartilaginous portion, but dimin- ish in size and fre- quency towards the bony canal, in which they are entirely wanting. Within the cartilaginous meatus and along the roof of the bony tube, the skin is closely be- set with the large coiled ceruminous glands, which re- semble in structure modified sweat glands. Like the latter, the cerumin- ous glands consist of a deeper and wider coiled portion, the secretory segment, and a long narrow excretory duct, which ends in most cases inde- pendently on the free surface of the skin, but sometimes, particularly in the very young child, it opens into the duct of a sebaceous gland. The cuboidal secreting cells contain yellowish brown pigment particles and granules resembling fat. The ear-wax or cerumen is, as usually found, the more or less dried mixture of the secretions derived from both varieties of glands, together with discarded squamous epidermal cells. Vessels. — The arteries distributed to the external auditory canal are from three sources: (a) anterior branches of the superficial temporal supply the external por- tion of the meatus ; (6) the deep auricular artery, a branch of the internal maxillary, passes to the deeper portions ; whilst {c} the posterior auricular provides branches for the posterior and superior surfaces. The arteries destined for the interior of the canal pierce the membranous roof of the cartilaginous meatus, the fissures of Santo- rini and the fibrous tissue connecting the cartilaginous with the bony portion of the tube. They form capillary net-works within the perichondrium and periosteum and, 04 Ceruminous gland Cartilage Hair-follicle Corium Transverse section of skin lining cartilaginous portion of external auditory canal. X 30. 1490 HUMAN ANATOMY. FIG. 1250- Concha External auditory canal Membrana flaccida Depression for malleus Umbo Tympanic membrane within the skin, around the glands and the hair follicles, some extending on to the upper part of the membrana tympani. The deeper veins of the meatus, which drain the bony and a small part of the cartilaginous meatus, empty into the venous plexus behind the articulation of the lower jaw, those from the upper wall of the meatus extending upward to join the venous plexus which spreads out over the skull. The lymphatics of the external auditory canal arise from a cutaneous net-work from which trunks pass in three general groups, as do those of the auricle, (i) The trunks of the posterior group arise in the posterior wall of the external meatus and empty, for the most part, into the posterior auricular (mastoid) nodes. Some, however, avoid this first station and join the efferent vessels of the upper nodes of the superior deep cervical chain. (2) The inferior group includes a vari- able number of trunks coming from the lower wall of the external audi- tory meatus, some of which pass to the nodes placed along the course of the external jugular vein at its exit from the parotid, whilst others end in the mastoid nodes. (3) The anterior group is from the concha and the anterior wall of the meatus. These vessels are tribu- tary to the parotid nodes, more particularly to the anterior auricular nodes situated immediately in front of the tragus. Nerves. — The sensory nerves supplied to the external auditory canal are derived from the auriculo-temporal branch of the trigeminus and from the auricular branch of the pneumogastric. The latter, also known as Arnold' s nerve, perforates the wall of the meatus and supplies its lining membrane. Practical Considerations : The Auricle. — The auditory mechanism may be said to consist of two portions — that which conducts the sound and that which receives it. The former is represented by the external and the middle ear ; the latter, by the internal ear. The function of the auricle is to collect and intensify the sound-waves and to direct them into the external auditory canal. That it does not play a very important part in hearing is shown by the fact that its removal has been followed by comparatively little loss in the acuteness of hearing (Treves). Complete absence of the auricle is exceedingly rare ; partial defect i miiiolia ) is more frequent ; while congenital fistulae are comparatively common. These fistula are considered to be due to a defective closure of the first branchial deft. According to His, however, they are due to deficient union of the crus helicis and tin- cms supratragirus. If a fistula closes at its orifices, a retention cyst, sometimes di-rmoid, may result. The ear may be abnormally large (macrotia), or, M a result <>f defective union of the rudimentary tubercles from which the auricle is developed, auricular appendages < polyotia'} may be met with. A supernumerary auricle m. iv \< iv rarely be found on the side of the neck at the orifice of one of the lo\\cr branchial clefts. Ouing to the rich blood -supply of the auricle, wounds heal rapidly. When, houever, they occur near the external auditory meatus and are large, cicatricial closure of the canal must be guarded against. Frost-bite is frequent because of the exposure to cold and the lack of protec- tion to the Mood vessels from overlying tissues, since little more than skin covers them. An intense reactive congestion follows, and frequently leads' to gangrene. Cast of right external auditory canal, seen from be- hind ; natural size. Drawn from cast made by Professor Randall. PRACTICAL CONSIDERATIONS : THE EXTERNAL EAR. 1491 The skin is closely adherent to the underlying tissues, especially on the anterior surface, so that the exudate is under much tension, interfering with the blood- supply. The nerves are also compressed, accounting for the great pain. H&matomata of the auricle are due to effusions of blood between the cartilage and its perichondrium. They occur usually on the concavity of the auricle from a blow, as in boxers, or foot-ball players. They may occur rarely, without traumatism, as in the insane, although some believe that injury is the exciting cause in these cases ; or even, in very exceptional instances, may appear without precedent trauma or mental disease. In those cases in which there is great tension, it may be neces- sary to incise and drain to prevent necrosis. Of the tumors, keloid, following punctures for ear-rings, is common in the negro ; capillary nsevi are frequent, whilst cirsoid aneurism may occur. Cysts in connection with the first branchial cleft have already been mentioned. The External Auditory Canal. — Congenital atresia is rare and is often associated with malformations of the auricle, the middle and the internal ear, so that correction of the external condition will usually fail to restore the hearing. The length of the external meatus is about \]^ inches, about ^ inch of which is bony and about % inch cartilaginous. In the new-born it consists of skin and cartilage only, and its lumen is very small. Owing to the obliquity of the tympanic membrane, that structure, in the new-born, is in close contact with the floor of the canal, so that the latter must be drawn away from the membrane to expose it. For this purpose the auricle should be drawn downward and backward. The skin of the cartilaginous portion is supplied with hair, sebaceous and ceruminous glands. Furuncles are frequent, the infection passing along the hair-follicles to the asso- ciated sebaceous glands. In some persons, one boil follows another from successive glandular infection. The skin of the bony portion is thinner than that of the car- tilaginous, except in the posterior part of the roof, where a thicker wedge-shaped piece containing glands extends as far as the drum-head. Ceruminous masses often collect, and frequently contain pathogenic bacteria. They may press upon the tympanic membrane, and through intralabyrinthine pres- sure may produce vertigo, or may lead to vomiting or convulsions. Interference by the mass with air conduction may result in loss of hearing. A diffuse infection of the meatus may be primary, but it is more apt to be a secondary result of otitis media. In severe cases the pus may extend to the bone separating the periosteum. It may then pass to the parotid region through the anterior bony wall, but it is more likely to do so through the fissures in the cartilag- inous portion. Abscesses in the parotid region more frequently extend by the same route in the reverse direction. The general direction of the canal is from without inward, downward, and slightly forward. The auricle and cartilaginous meatus are suspended from the margin of the bony portion so that an angle is formed opening downward. For a short distance from the external orifice the meatus inclines forward. In the remain- ing cartilaginous portion it turns backward, while in the bony portion it is again deflected forward. Therefore, to examine the tympanic membrane the cartilaginous meatus must be drawn upward to correct the vertical curve, and backward to straighten the antero-posterior curve. The diameter of the canal is greater at the two extremities than in the centre. The smallest diameter in the bony portion is at the inner third, where foreign bodies most frequently lodge, which have been known to remain in the canal for years without much discomfiture, or even, in some cases, without their presence being known. Care is necessary in their removal lest the tympanic membrane be injured. The anterior wall of the meatus is in relation with the temporo-maxillary articu- lation, and its bony portion has been fractured from blows upon the lower jaw. The parotid gland is in relation with this wall as well as with the floor, so that tumors of the gland may narrow or occlude the canal by pressure. Parotid abscesses opening into the canal are likely to pass through the deficiencies in the cartilage (fissures of Santorini). Since the lower jaw is in relation with the cartilaginous as well as with the bony portion of the meatus, the former is drawn forward when the mouth is opened. Hence the mouth is usually opened when one listens intently. 1492 HUMAN ANATOMY. The posterior wall is separated from the mastoid process by the tympano-mas- toid fissure. The auricular branch of the pneumogastric (Arnold's nerve) passes through this fissure to the posterior wall of the canal. The coughing, sneezing, or vomiting that sometimes follows irritation of the canal, as from cleaning the ear, or ex- amining it with instruments, is said to be due to a reflex effect upon the pneumogastric through this branch. The auriculo-temporal branch of the trigeminal nerve enters into its supply, and may explain the earache in cancer of the tongue or disease of the lower teeth. Between the posterior wall of the meatus and the mastoid cells is a thin plate of bone one or two millimeters in thickness. The sigmoid portion of the lateral sinus is usually about 12 mm. back of this wall, and the mastoid antrum about 5 mm, posterior to its deeper portion. The superior wall, which is from 4-5 mm. in thickness, often contains air- cells between two plates of compact bone. Pus may burrow through it from the canal to the interior of the cranium. At the posterior superior angle of the canal are a number of small openings for blood-vessels and some connective tissue fibres, through or along which pus may find its way from the mastoid antrum to the under surface of the periosteum in the meatus. THE MIDDLE EAR. The middle ear includes three subdivisions : the tympanic cavity, the Eustachian tube, and the mastoid cells. It is an irregular air-chamber, beginning on the lateral wall of the naso-pharynx with the Eustachian tube, which leads upward, backward and outward, for about one inch and a half into the temporal bone. Opposite the external auditory canal, it widens into the tympanic cavity and continues backward into the mastoid cells. THE TYMPANIC CAVITY. The tympanic cavity (cavum tympani), also called the tympanum, is an irregu- lar space within the temporal bone, lying between the internal ear and the external FIG. 1251. Superior ligament Head of malleus Tendon of tensor tympani Posterior semicircular canal' Facial nerve Vest i bull- ( Internal auditor}' canal Cochlea Probe in Eustachian tul>c Articular surface for incus Epitympanic space Lateral ligament Handle of malleus External auditory canal Tympanic membrane, cut Promontory Tympanic cavity Frontal section through right ear, viewed from behind. X 2^. audit.. i-y canal. It is lined with mucous membrane and contains, in addition to the air uhich enter> l»y way ot the Kust.irhiun tube, the chain of ear ossicles. Its short- cut diameter, that between the middle of the tympanic membrane and the wall of the labyrinth, is about 2 mm. Tin- antcro-|),.steri<.r diameter is about 12 mm., whilst the distance fmm the roof (tegnien tympani) to the floor, the supero-inferior diam- eter, is about 15 mm. THE MIDDLE EAR. H93 The cavity of the tympanum is subdivided into three parts : (i) the atrium or tympanic cavity proper; (2) the cavum epitympanicum, the upper part of the space which overlies the atrium ; and (3) the antrum, which leads into the mastoid cells. The atrium (Fig. 1251) resembles in shape a short cylinder with concave ends, the outer end being formed by the tympanic membrane and its bony margin, whilst the inner end is formed by the outer wall of the labyrinth. The cavum epitympanicum or attic occupies the space between the atrium and the roof and constitutes approximately one-third (about 5 mm. ) of the supero- inferior diameter. It contains the head of the malleus and the body of the incus (Fig. 1252). It extends considerably over the external auditory canal and is bounded laterally by a wedge-shaped portion of the temporal bone, called the scutum. The antrum tympanicum is an irregularly pyramidal space communicating with the upper back part of the tympanum by a triangular orifice. Its dimensions vary, but its average length is about 12 mm., its height 8.5 mm., and its width 6.7 mm. It is larger in the infant than in the adult, and its lumen is frequently lessened by bands of mucous membrane which stretch across it and thus encroach upon the space. Its roof is formed by the tegmen tympani, sometimes called the tegmen antri in this location. Its external wall is formed by the squamous portion of the FIG. 1252. Superior ligament of incus Superior ligament of malleus Head of malleus, Chorda tympani nerve Tensor tympani Processus cochleariformis Eustachian tube •Epitympanic space Incus -Orbicular process, for stapes Handle of malleus Tympanic membrane Inner aspect of outer wall of right tympanic cavity, sYiowing incus and malleus and tympanic membrane in position. X ?1A. temporal bone, and on its internal one is seen the outer wall of the horizontal semicir- cular canal. The thin mucous membrane of the antrum is closely united with the periosteum and possesses a layer of low nonciliated squamous epithelium. The walls of the tympanic cavity present many irregularities and depres- sions and the boundaries are not sharply defined. As the direction of the supero- inferior axis of the cavity is not perpendicular but oblique, it follows that the outer wall, composed of the tympanic membrane and its bony margin, is, accurately speaking, the infero-lateral wall, whilst that formed by the labyrinth is the dorso- mesial wall. For convenience of description, however, there may be recognized with advantage an external and an internal, a superior and an inferior, and an anterior and a posterior wall. The outer wall (paries membranacea) of the tympanic cavity proper (the atrium) is formed by the drum-head and the margin of bone into which it is inserted, whilst the outer wall of the epitympanic space is formed by the scutum. In the infant the bony external auditory canal consists of a ring of bone, the annulus tympani- cus. This ring, incomplete at its upper anterior part at the notch of Rivinus, possesses a well-marked groove, the sulcus tympanicus, for the reception of the tympanic membrane. At the notch of Rivinus, the tympanic membrane is attached to the bony margo tympanicus and the external lateral ligament of the malleus, and is continuous with the skin lining the bony auditory canal. 1494 HUMAN ANATOMY. The Membrana Tympani. — The tympanic membrane or drum-head is a delicate transparent disc, irregularly oval or ellipsoidal in outline and concave on its outer surface. It is placed obliquely with the horizontal plane, forming an angle of about 55°, opening outward. As the middle portion of the membrane is drawn in- ward, the inclination of its several parts differs. The obliquity of the membrane is about the same in the infant as in the adult. With the upper back wall of the external auditory canal the drum-head forms a very obtuse angle, whilst with the antero-inferior wall it encloses an angle of about 27°. The longest diameter of the membrane is directed from above and behind, forward and downward, and measures from 9.5-10 mm. ; the shortest is from 8.5-9 mm- The membrane is about . 10 mm. thick, except at the periphery, where it is thickened. Like the rest of the tympa- num and the labyrinth, it is practically as large in the infant as in the adult. The handle of the malleus is embedded in the tympanic membrane (Fig. 1252), and extends from a point near the middle, upward and forward toward the periphery, to end at the short process. At its lower end, the handle of the malleus is flattened laterally and broadened at the umbo, which corresponds to the deepest part of the concavity of the membrane. The short process of the malleus forms a conspicuous rounded projection at the antero-superior part of the drum- head. Extending from the short process of the malleus to the anterior and posterior ends of the tympanic ring are two straight striae. The part of the drum-head included between these striae and the Rivinian notch is known as the membrana flaccida (pars tlaccida) or Shrapnell's membrane. It is thinner and less tense than the remaining larger part of the drum-head which is called the membrana tensa ( pars tensa). The inner aspect of the drum-head presents two folds of mucous membrane which stretch horizontally backward and forward to the annulus and form an anterior and a posterior in verted pocket. The anterior pocket contains in its wall, in addition to the mucous membrane, the long process of the malleus, the chorda tympani nerve and the inferior tympanic artery, the nerve continuing along the lower border of the posterior fold. \ The structure of the tympanic membrane includes three main layers : ( i ) the middle fibrous stratum, or membrana propria ; (2) the external cutaneous layer, the prolongation of the skin lining the external auditory canal ; and (3) the internal mucous membrane, a continua- tion of the mucous membrane clotKing other parts of the tympanic cavity. The fibrous layer or membrana propria represents the mesoblastic portion of the drum-head and consists of an outer stratum of radially disposed fibres which diverge from the malleus towards the periphery of the membrane, and an inner stratum of circular fibres, concentrically arranged and test developed near the periphery of the membrane but absent at the umbo. The radiating fibres, on the contrary, become more dense at the umbo, partly through accumulation and partly thnmgh splitting ( . lierladi). Between the fibres of the two layers are seen connect- ive tissue corpuscles which are spindle-shaped in longitudinal and stellate in cross-section. The membrana pn.pria is absent within the pars flaccida or Shrapnell's membrane. At the periphery of the membrana propria, the fibres, especially those of the radial stratum, are con- nected with those of a ring of thick connective tissue, the annulus fibrosus which occupies the sulcus tympanicus. The fibres of the annulus fibrosus run in various directions, but for the most part converge toward tin- tympanic membrane proper (Fig. 1253). Round cells are found between these fibres. Tin- cutaneous layer consists of a thin epidermal stratum, composed of two or three rows of cells and a delicate sheet of connective tissue, but neither a definite corium nor papilla: are ;it. A thickened kind of subcpithelial connective tissue extends across Shrapnell's mem- brane and along the handle of the malleus and contains the large vessels and nerves which pass from the me.itus to the membrana tunpani. The mucous membrane covering the inner surface of the drum-head consists of a scanty of connective tissue, invested with a sheet of large low nonciliated epithelial cells. The vessels of the t\ mpanic membrane include aitn -/V.v which are arranged as an outer and inner set, separated b\ tin- membrana propria. The former set is derived from the deep auricu- lar branch of the internal maxillary artery ; the latter from the tympanic branch of the internal maxillary and from the stylo-mastoid branch of the posterior auricular. Each of these sets forms a plexus ot vessels with a large branch extending downward along the malleus handle, and another around the pet iphei v of the membrane, these two branches being connected by numerous radiating twigs. IVitoraiing vessels connect the two sets of arteries, especially along THE MIDDLE EAR. H95 the malleus handle and at the periphery of the membrane. The veins are most numerous at the handle of the malleus and periphery of the membrane and communicate with those of the exter- nal meatus and tympanic cavity. The lymphatics are arranged similarly to the blood-vessels in two sets, one under the skin and the other under the mucous membrane. They communicate freely with each other and probably empty partly into the lymph-nodes situated over the mastoid process and in the region of the tragus, and partly into the lymph-tracts of the Eustachian tube and thence event- ually into the retropharyngeal and deep cervical nodes. FIG. 1253. Epithelium of tympanic surface Circular fibres Radial fibres- Mucous membrane Blood-vessels Epidermis of drum-head Subepidermal layer External auditory canal Epidermis of canal Corium of skin lining canal . ' Epidermis passing onto drum-head O*' Bone Radial fibres of annulus fibrosus Section through attached margin of tympanic membrane, showing continuation of skin and mucous membrane over its outer and inner surfaces respectively. X 75- Drawn from preparation made by Dr. Ralph Butler. The nerves supplying the tympanic membrane are derived chiefly from the auriculo-tem- poral branch of the trigemlnus, supplemented by twigs from the tympanic plexus and by the auricular branch of the vagus. They accompany, for the most part, the blood-vessels and, in addition to supplying the latter, form both a subcutaneous and a submucous plexus. The inner wall (paries labyrinthica) of the tympanic cavity separates it from the internal ear. It presents for examination a number of conspicuous features. The promontory appears as a well-marked bulging of the inner wall near its middle (Fig. 1254) and corresponds to the first turn of the cochlea. The branches of the tympanic plexus are found in the mucous membrane covering it. At the bottom of a niche, whose anterior border is formed by the lower posterior margin of the promontory, lies the round window (fenestra cochlea). It is closed by the secondary tympanic membrane (membrana tympani secundaria), which separates the tympanic cavity from the scala tympani of the cochlea (Fig. 1259). The membrane is attached in an obliquely placed groove, is slightly concave toward the tympanum, and measures from 1.5-3 mm- m diameter. The oval window (fenestra vestibuli) lies at the bottom of a depression, the fossula vestibuli, in the upper back part of the inner wall, above the round window, and leads into the vestibule. It is somewhat kidney-shaped, its upper border being concave, its lower slightly convex. In the recent state the oval window is closed by the foot-plate of the stapes and the ligament which connects the ossicle with the sides of the window (Fig. 1260). The longest diameter of the latter is about 3 mm. and its shortest 1.5 mm. Abov° the oval window 'a well-marked 1496 HUMAN ANATOMY. ridge indicates the position of the facial canal or aquedudus Fallopn. This ridge is bordered posteriorly and superiorly by the elevation which corresponds to the wall of the horizontal semicircular canal (prominentia canalis semicircularis later- alis,. The sinus tympani, a well-marked depression, is behind the promontory, between the niche of the round window and the pyramid, below and behind the oval window. It is separated from the fossulse of the two windows by bony ridges. It varies in depth from 2-5 mm , with a vertical diameter of from 2-6 mm. The superior wall (paries tegmentalis) is formed by a plate of bone, the teg- men tympani, which forms part of the upper and anterior surface of the petrous portion of the temporal bone. Posteriorly it forms the roof of the antrum tympam- cum, and anteriorly contributes the roof of the canal for the tensor tympani muscle and of the adjoining part of the Eustachian tube. It varies in thickness and may be defective to a large extent from atrophy or arrested development. The inferior wall (paries juRularis), narrower than the superior, separates the typanum from the jugular fossa. Its bony plate may be incomplete and may lie considerably below the level of the membrana tympani. The anterior wall (paries carotica) separates the tympanum from the carotid artery and at times presents a fissure. At its upper part is the irregular trian- gular opening of the Eustachian tube and above this opening lies the small canal for the FIG. 1254. Outer end of horizontal part of facial cana Stapes lying in oval window Stapedius muscle Round window Facial canal Promontory Tensor tympani Eustachian tube Outer aspect of inner wall of right tympanic cavity; stapes lies within oval window. X 2^. tensor tympani muscle. The canaliculus caroticus tympanicus perforates the anterior wall just below the mouth of the Eustachian tube, and transmits the tympanic branch of the internal carotid artery and carotico-tympanic nerves. The posterior wall (paries raastoidea) of the tympanum at its upper part is occupied by the antrum tympanicum, which leads into numerous irregular cavities, the mastoid cells. At the lower border of the antrum is a saddle-shaped notch, the fossa incudis, which lodges the short process of the incus. Extending forward from the posterior wall, on a level with the lower border of the oval window, projects the small bony elevation, the pyramid (eminentia pyramidalis), which encloses the Mapcdius muscle (Fig. 1254). Its apex is pierced by a small round opening for the exit of the stapedius tendon. The canal within this eminence communicates posteriorly with the facial canal. On a level with the eminentia pyramidalis, close to the posterior margin of the drum-membrane, lies the apertura tympanica canaliculi chonlac t\mp;mi, the opening through which the chorda tympani nerve enters the middle ear, THE CONTENTS OF THE TYMPANUM. The Auditory Ossicles. — Three small bones (ossicula auditus) form a chain extending ICTOM the upper part of the tympanum from the tympanic membrane to the labyrinth. The outermost of these, the malleus (hammer), is attached to the tympanic membrane ; the innermost, the stapes (stirrup), is fixed in the oval window, ami between these t\\o hones and connected with both of them, lies the third link in tlie chain, the iii, a^ i anvil ). THE MIDDLE EAR. 1497 The malleus (hammer) is about 8 mm. long and consists of a head, a neck and three processes. The head is the upper club-shaped portion, lying in the epitympanic space ; the constricted portion just below the head is the neck, and below this is a prominence to which the three processes are attached. The posterior surface of the head bears, for the articulation with the incus, an oblong depressed surface with prominent margins extending in a spiral manner downward and inward to the neck. This articular surface consists of two principal facets separated by an oblique ridge, the upper facet looking backward, the lower, inward. The axis of the head forms with that of the handle an angle of about 140°, opening upward and inward. FIG. 1255. Head Head Point of insertion of lateral ligament Manubrium Processus gracilis Manubrium Right malleus;/}, seen from behind; /?, seen from in front. X The manubrium (handle), a tapering process extending downward, backward and inward, is embedded in the substance of the tympanic membrane (Fig. 1253). Near the upper part of the inner anterior surface of the handle is sometimes found a slight projection for the insertion of the tensor tympani muscle. The lower end of the manubrium is spatula shaped, flattened transversely. The long process is directed toward the Glaserian fissure, whilst the short process looks toward the external meatus. The processus breins (short process) is a small conical elevation situated at the upper end of the handle, below the neck of the malleus. Like the handle it is attached to the tympanic membrane and covered by a layer of cartilage, notably on its external surface. The processus gracilis (long process) arises from the anterior angle of the internal surface of the neck, close to the base of the short process, and extends downward and forward to the Glaserian fissure. It is well developed in the foetus and in young children, but is often rudi- mentary in the adult. FIG. 1256. Body Upper facet Lower facet Upper facet Lower facet Processus longiis Right incus; A, lateral; fi, anterior aspect. "X 41A- The incus (anvil) resembles a molar tooth with two widely separated fangs, rather than an anvil. It consists of a body, a long process and a short process. The body of the incus has two main facets on its anterior and antero-external surfaces, which correspond to those on the head of the malleus and articulate with them. The processus brevis (short process) is conical in shape, flattened laterally and projects nearly horizontally backward to a depression in the posterior wall of the tympanum at the entrance of the antrum, where its apex is attached. The processus longus (long process) runs downward and backward, behind and nearly parallel with the handle of the malleus, and forms nearly a right angle with the short process. At its lower end it is bent sharply inward and constricted into a neck, which terminates in a rounded tubercle, the processus orbicularis, that articulates with the head of the stapes. In the foetus this process is separated from the rest of the long process. HUMAN ANATOMY. The stapes (stirrup), as its name implies, is stirrup-shaped and consists of a head, neck, two crura and a base or foot-plate. The external surface of the small rounded head is hollowed out for articulation with the orbicular process of the incus. Just below this is the constricted neck, from which the two crura diverge to become attached to the foot-plate near its lower FIG. 1257. Vpper edge Posterior Lower edge Foot-plate Right stapes, A, seen from above; B, mesial surface of foot-plate. X *,%. margin. The anterior crus is shorter and straighter than the posterior, both being slightly curved. The foot-plate consists of a lamina of bone and corresponds to the bean-shape of the oval window, into which it nearly fits. The upper edge of the foot-plate is convex ; its lower edge is almost straight, being slightly hollowed out near its middle. Articulations of the Ossicles.— In the malleo-incudal joint, both articular surfaces are covered with a thin layer of hyaline cartilage. The fairly well-developed capsular ligament, reinforced mesially, is fastened to the depressed margins of the articular surfaces. A wedge- shaped meniscus of fibre-cartilage projects from the upper wall of the capsule between the sur- n faces of hyaline cartilage. "When the Sup. ligament Head of malleus External ligament Mem. flaccida or Shrapnell's membrane Prussak's space Neck Short process Chorda tympani Ten i Ion of tensor lympaiii Handle artilage Epidermis Membrana propria ' 1'°0g manubrium handle moves inward, its lower cog catches the corresponding cog of the incus and the long process of the latter must follow. If the handle moves outward, the lower cog moves away from the incus and the latter moves but little" (Politzer). The articulation of the incus and stapes is a very delicate but true joint. Both the slightly convex surface of the orbicular process of the incus and the slightly concave surface of the head of the stapes are covered with hyaline car- tilage and united by a capsular ligament made up largely of elastic fibres and thickened on the posterior surface. Sometimes a meniscus of fibro-cartilage separates the two articular surfaces. The articulation of the stapes and oval window is effected by the margins of the window and the foot-plate of the stapes. These surfaces, as well as the vestibular aspect of the stapes, are cov- ered with a layer of hyaline cartilage. The cartilage of the foot-plate and that of the window are connected by a liga- ment of elastic fibres, forming a syn- chondrosis. In addition to the ligaments con- cerned in the foregoing articulations, four bands attach tin- ossicles to the tym- panic walls and prevent their excessive through tnmlletMand tympanic moinbraiu'. movement; of these, three connect the Dmwfl tn» ,.„,-,, .,ii,,,, ,,,.,,1, »,>• i»,. Kai,,i, Butler. maiieus a,lcl one the incus- 1. The superior ligament of the malleus extends from the tegmen tympani to the head of the lll.lllells. ( l-'igs. I2.S'J and 12.SS. ) 2. The anterior ligament of the malleus is a strong, broad, fibrous band arising from the anteiior part oi the he, id ,md neck of the malleus. Some of its fibres are attached to the ante- in! oi the amiulus- tympani.-us t spina tympanica major \ and other fibres pass through the ( .laserian fissure t<> l>. < < uiie attarhed t,> the spine of the sphenoid. These fibres correspond to the remains . >f the embryonic proceH ot Meckel of the malleus and envelop the processus gracilis. membrane Mcmlirana ti-nsa Aiimilus tenclinoMis THE MIDDLE EAR. 1499 3. The lateral ligament of the malleus is somewhat fan-shaped and extends between the roughened neck of the malleus and the external wall of the tympanum above the Rivinian notch. The posterior fibres of this ligament are called the posterior ligament of the malleus (Helmholtz) , and, together with the fibres of the anterior ligament lying in the same plane, form the " axis- ligament of the malleus," since the axis on which the malleus turns passes through the attach- ment of these two fibrous structures. 4. The posterior ligament of the incus extends from the apex of the short process of the incus to the tympanic wall at the lower part of the mouth of the antrum. It is fan-shaped, the incudal attachment being less extensive than that of the tympanic. The superior ligament of the incus is variable and consists chiefly of a fold of mucous membrane. The Intratympanic Muscles. — The muscles within the tympanum connected with the" ossicles (musculi ossiculorum auditus) are : (i) the tensor tympani and (2) the stapedius. The tensor tympani is a diminutive spindle-shaped muscle, about 1.25 cm. long, lying in the bony canal directly above the osseous part of the Eustachian tube, from which it is partly FIG. 1259. Facial nerve Ramus utriculus i ampulla ris Utricle Foot of stapes Cisterna peri- '„; lymphatica Lowest part of _^ spiral lamina Beginning of posterior — ampulla Secondary tym- panic membrane Stapedius muscle External auditory canal [_ Handle of | malleus — Promontory fll HiKr Drum-head or tympanic membrane Tympanic cavity Vertical section through human middle and internal ear. X 5/4. Drawn from preparation made by Dr. Ralph Butler. separated by the bony scroll, the processus cochleariformis. The posterior fibres arise from the top of the cartilage of the Eustachian tube and the adjoining part of the great wing of the sphenoid. Some of the fibres are connected with the tensor palati muscle and others arise from the wall of the canal which the muscle occupies. The fibres converge in a feather-like manner to the tendon, which begins within the muscle about the middle of the canal, and, pass- ing through the tympanic opening of the canal, turns at nearly a right angle over the end or rostrum of the processus cochleariformis to be inserted into the anterior part of the inner margin of the malleus-handle just below the short process. The tendon is almost per- pendicular to the plane of the tympanic membrane, is oblique to the long axis of the manu- brium and is enveloped, along with the muscle-belly, in a fibrous sheath. The tensor tympani and tensor palati muscles receive their nerve supply from the same source, namely, the trigem- inus, through the otic ganglion. The stapedius muscle lies within the triangular canal of the eminentia pyramidalis, arising from its floor and sides. Its fibres converge to the tendon, which, passing through the opening at the apex of the canal, extends forward, slightly upward, and outward, to be inserted into the lower posterior part of the head of the stapes. Some of the fibres of the tendon also pass to the 1500 IITMAN ANATOMY. orbicular process and the capsular ligament. The tendon is frequently enveloped in a fold of mucous membrane. A branch of the facial nerve passes through a small orifice between the Fallopian canal and the canal for the stapedius to supply this muscle. Movements of the Ossicles. — When the tympanic membrane and malleus-handle are moved inward, the long process of the incus is also moved inward and pushes the head of the stapes inward, and slightly upward. This causes pressure upon the liquid within the labyrinth, and, since the bony walls of the labyrinth are inelastic, the membrane of the round window is bulged outward. As the tympanic membrane regains its normal position, these movements are re- versed. When on the other hand the tympanic membrane is moved outward, the movement of the long process of the incus is very slight because of the unlocking of the malleo-incudal articu- lation. Contraction of the tensor tympani muscle draws the centre of the tympanic membrane inward and in this way increases the tension of the membrane and of the posterior part of the axial ligament of the malleus, especially of its external portion. Contraction of the stapedius muscle pulls the head of the stapes backward, thus tilting the anterior end of the foot-plata out- ward, the posterior end acting as a fulcrum. The Mucous Membrane of the Tympanum. — The tympanic cavity is lined by a thin transparent mucous membrane, closely adherent to the periosteum and continuous with that of the Eustachian tube and naso-pharynx anteriorly, and FIG. 1260. Stapedius muscle Posterior crus of stapes Vestibule Footplate of stapes Internal audito- ry canal Vestihular ganglion Cochlear nerve Basal turn of cochlea Lower end of incus Malleus handle Tympanic cavitj ^^ X5K. Drawn from with that of the mastoid cells posteriorly. It covers the ossicles and their ligaments, the im.srlrs. the tendons and the chorda tympani nerve, and forms a number of folds extewting ,,,,_ lh, ,lvitv, Tll(.S(. follls vary in iocatiori) direction and number and form pouches within the tympanum. The attic is divided into M ,-xtcrnal and an internal compartment by the incus the yadof the malleus, the superior ligament of the malleus and the superior malleo- ttCUdal fold of mucoufl membrane. The extrnul compartment is bounded on the outer side by the external tympanic wall, and is itself subdivided into a superior and an m cn,,r space !>v the external ligament of the malleus. The inferior division is ra 1,, 1 Prussak s space am ,s bounded externally by Shrapnel!', membrane, inter- nally bythe m-ck ot tin- ma 1,-ns, mfer.orly by the short process of the hammer and superiorly by tin- .-xtt-rnal tiganeBt of the malleus (Fig. 1258). A number of THE MIDDLE EAR. 1501 inconstant folds of mucous membrane extend from the wall of the tympanum to the malleus and the incus. The most constant of these is the outer malleo-incudal plica, which stretches backward to the posterior ligament of the incus. Additional folds frequently extend between the crura of the stapes and from them to the wall of the tympanum. The epithelium of the tympanic mucosa varies in different parts of the cavity. Over the promontory, the ossicles and the tympanic membrane, it consists of a single layer of low cuboidal nonciliated cells, whilst over the other parts the cells are ciliated columnar in type. Small tubular glands occur within the lining of the anterior part of the cavity. The subepithelial connective tissue, which supports the vessels and nerves, comprises two layers, the outer forming the periosteum of the bony wall. The secondary tympanic membrane closing the fenestra cochleae, bulges somewhat toward the cochlea and is attached to the bony crest or ridge of the win- dow by its widened rim. It consists of three layers, of which the middle one is a distinct fibrous lamina propria, which is covered on the tympanic surface by mucous membrane, and on the other side by an extension of the lining of the perilymphatic space. The lamina propria is composed of radially disposed bundles of fibrous tissue. The outer mucous stratum is formed of a thin fibrous tunica propria, invested by a single layer of flattened nonciliated epithelial cells, similar to those covering the neighboring promontory. The innermost stratum of the membrane includes a thin layer of subendothelial fibrous tissue, over which stretches a layer of endothelial plates. Vessels and Nerves of the Tympanum. — The arteries supplying the tympanic cavity are from five sources. 1. The stylo-mastoid branch of the posterior auricular artery passes through the stylo-mastoid foramen and the Fallopian aqueduct, and sends a branch to the sta- pedius muscle and three branches to the posterior part of the tympanic cavity. One of these passes to the floor, one through the canal for the chorda tympani nerve, and one to the posterior part of the oval window. 2. The tympanic branch of the internal maxillary artery enters the tympanic cavity through the Glaserian fissure and supplies the anterior part of the cavity, including the anterior ligament of the malleus, the processus gracilis and the tympanic membrane. 3. The middle meningeal branch of the internal maxillary artery sends a branch through the hiatus Fallopii to anastomose with the stylo-mastoid artery, a branch through the canaliculus tympanicus to the promontory, and a branch to the tensor tympani muscle. 4. The ascending pharyngeal sends branches to the floor and the promontory, one of them accompanying Jacobson's nerve. 5. The internal carotid artery in its passage through the carotid canal gives off branches to the anterior wall of the tympanic cavity. The veins follow, in a general way, the course of the arteries. They are tribu- tary to the middle meningeal, the pharyngeal plexus and the jugulars. The lymphatics arise from a net- work within the mucous membrane and end chiefly in the retropharyngeal and the parotid nodes. The nerves supplying the mucous membrane of the tympanum are branches from the tympanic plexus formed by the tympanic branch of the glosso-pharyngeal nerve, in conjunction with sympathetic filaments from the net-work accompanying the carotid artery. The tensor tympani muscle receives its supply from the trigeminus; the stapedius muscle from the facial. Although the chorda tympani nerve has an intimate topographical relation to the space, which it traverses close to the outer wall, it gives no filaments to the structures within the tympanum. THE EUSTACHIAN TUBE. The Eustachian tube (tuba auditiva^ is a canal, partly bony and partly cartilagi- nous, extending from the lateral wall of the naso-pharynx backward, upward and out- ward to the anterior part of the tympanum. In the adult it measures about 37 mm. (iYz in.) in length, of which approximately the upper third (tympanic portion^) 1502 HUMAN ANATOMY. belongs to the bony division, whilst the remainder is contributed by the cartilaginous division of the tube. With the sagittal plane it forms an angle of 45 > and with the horizontal plane one of about 33°- With the long axis of the external auditory canal it forms an angle of from I35°-I45°, opening outward. The cartilaginous and bony divisions of the tube do not lie exactly in the same plane, but join at a very obtuse angle opening outward. The tube has somewhat the shape of an hour glass, being wider at the ends and narrowed at the junction of the cartilaginous and bony portions into the isthmus, where its height is about 3 mm. and its breadth about halt as much. . , , , The osseous or tympanic portion (pars ossea) about 12 mm. long, is bounc above by the tegmen tympani and the canal for the tensor tympam muscle, from which it is incompletely separated by the processus cochleanformis. Below and internal to it lies the canal for the carotid artery. Its lumen is irregularly triangular in cross-section. FIG. 1261. Tympanic membrane Tympanic cavity Antrum Condyle of jaw Basilar process External audi- tory canal Parotid eland Fo««a of Rosen- muller Cartilage of Eustachlan tube Eustachlan tube Levator palati Tensor palati Palatal raph Palatal ni Internal auditory canal Right internal carotid artery •Tympanic membrane External auditory meatus Parotid gland External pterygoid muscle Ramus of jaw Internal pterygoid muscle Soft palate Masseter muscle .Vestibule Buccinator muscle Anterior part of section through head at plane shown in small outline figure, viewed from below; left Eustachian tube exposed throughout its length. Drawn from preparation made by Professor Dwight. The cartilaginous or pharyngeal portion (pars cartilaginea) is about 25 mm. ( i in. ) in length and attached to the rough oblique margin of the anterior end of the osseou> portion of the tube. Its posterior wall is formed by a plate of cartilage (cartilage tubae auditivae), the upper margin of which is curled outward upon itself to form a gutter, which appears on transverse section as a hook, whose inner and outer plates are known as the mesial and lateral lamina respectively. The interval between the margins of this cartilaginous groove presents outward and forward and is rilled up with a strong fibrous membrane, thus completing the canal. Therefore part of the anterior wall and the posterior superior wall of the tube are formed by this cartilage and the rest of the anterior \\all and all of the inferior by fibrous tissue. The cartilage is attached to the base of the skull and frequently is deficient in places, sometimes being divided into several pieces. At birth the cartilage is entirely of the hyaline variety, but later this is more or less extensively replaced, particulary in the pharyngeal division, by librocartilagc, except in the upper part where the hyaline cartilage THE MIDDLE EAR. 1503 persists. It is this cartilage, covered by the cushion of mucous membrane, that confers the characteristic Gothic arch contour to the lower opening, the osteum pharyngeum, of the tube. The Mucous Membrane of the Eustachian Tube. — The Eustachian tube is lined throughout its length with mucous membrane, which differs some- what in the cartilaginous and osseous portions. That in the former resembles the mucous membrane of the naso-pharynx, with which it is directly continuous, whilst that of the osseous division resembles, to some extent, the mucous membrane of the tympanic cavity. The epithelium of both divisions consists of the ciliated stratified columnar type, with some goblet cells, but the cells in the pharyngeal division, especially in the lower part, are taller than those of the tympanic portion, which are low cuboidal. In the tympanic portion the mucous membrane is closely united with the perios- teum and contains very few mucous glands and little or no adenoid tissue. In the cartilaginous division, on the contrary, the epithelium overlies a layer of adenoid FIG. 1262. Lateral lamina — •".•'•' i . \- _Mesial lamina of cartilage .'••.'.' ' 'of tube Oblique muscle-fibres — ^BHHBrr -•'} • ' Lumen of tube -Glands '.• Tensor palati (dilator tubae)- Levator palati-- Transverse section of cartilaginous Eustachian tube. X 7- tissue, often called the tubal tonsil. This tissue is especially abundant in children, and beneath it are found numerous mucous glands which open on the free surface of the tube. These glands extend nearly to the perichondrium and sometimes can be traced even through the fissures in the cartilage into the surrounding connective tissue. A considerable amount of adipose tissue often occupies the submucosa of the lower and lateral walls. The submucous layer is well developed in the cartilaginous division of the tube, particularly in the outer membranous wall. It consists of loosely arranged fibro-elastic tissue, which supports the mucous glands and the larger vessels and nerves. The muscles of the Eustachian tube are the levator and the tensor palati, the contractions of which not only affect the palate, but also produce changes in the position of the floor and in the lumen of the tube. These muscles are described in connection with the palate ('page 1570), suffice it here to note their close relations to the Eustachian tube, beneath and to the inner side of which the levator lies, and to the outer side of which the tensor extends. By reason of the intimate attachment which both muscles have to the cartilage of the tube, since both take partial origin from this structure, contraction of their fibres tend to draw apart the walls of the canal and they thus serve as dilators. Such action is particularly true of the tensor palati, many of 1 504 HUMAN ANATOMY. whose fibres are inserted into fibrous tissue completing the lateral wall of the tube (Fig. 1262), this part of the muscle being designated the dilator tub a. In addition to opening the tube, the levator palati elevates its floor. The blood-vessels of the Eustachian tube include the arteries, which arise from the ascending pharyngeal and from the middle meningeal and the Vidian branches of the internal maxillary; and the veins, which communicate with those of the tym- panum and of the pharynx and also form a plexus connecting with the cavernous sinus. The nerves are supplied from the tympanic plexus and from the pharyngeal branches from the spheno-palatine ganglion. THE MASTOID CELLS. The antrum tympanicum communicates posteriorly with a variable number of irregular pneumatic cavities, the mastoid cells (cellulae mastoideae), so called because the majority of these spaces occupy the mastoid process. Unlike the antrum, these cells are not developed at birth. As the mastoid process develops, the original diploetic structure is usually more or less replaced by larger cavities forming the pneumatic type. In a study of one thousand bones, Randall found that scarcely two per cent, of mastoids could be classed as diploetic, and only some ten per cent, as combining a notable amount of diploe with pneumatic spaces ; further, that no mastoid is absolutely pneumatic, although some senile bones show a single thin-walled cell occupying the greater part of the process. The pneumatic cells of this region may extend to the sigmoid portion of the lateral sinus ; into the occipital bone ; into the squamous portion of the temporal bone and above the external auditory canal ; into the root of the zygomatic process ; into the floor of the Eustachian tube close to the carotid canal, and occasionally as far as the apex of the petrous portion of temporal bone. These spaces are lined by a very thin mucous membrane, which is continu- ous with that of the antrum and of the tympanic cavity. It is closely united with the periosteum and possesses a layer of low nonciliated squamous epithelium. The blood-vessels supplying the mastoid cells are the arteries derived from the stylo-mastoid and the middle meningeal, and the veins, which communicate with those of the tympanum and the external wall of the mastoid process. Some of the veins are tributary to the mastoid emissary and the lateral sinus, whilst others pass beneath the superior simicircular canal through the cranial wall to join the dural veins. The nerves are the mastoid ramifications of the tympanic plexus. Practical Considerations : The Tympanum. — This cavity is continuous anteriorly with the nasopharynx by way of the Eustachian tube, and posteriorly with the mastoid antrum and air cells by way of the attic, so that infection, which is very common in the pharynx, may extend throughout this whole tract. The tympanic cavity extends above the limits of the membrane about 5-6 mm. as the attic, and about 2-3 mm. below as the "cellar" or hypotympanic recess. Secre- tions on the floor, therefore, may not be seen through the membrane. The defective drainage which results from the lower level of the floor of the tympanum, as com- pared with that of the external meatus, is one of the causes of the frequency of chronic otitis media with purulent discharge, even after the early evacuation of the products of inflammation in the acute stage. On tin- internal wall tin f,ni,i/ nerve passes in a curve over the vestibule in the anide between the roof and inner wall of the tympanum, then downward in the slightly projecting Fallopian canal with a concave turn above and behind the oval window, continuing its course downward at the junction of the posterior and inner wall to emerge Mow from the skuH at the stylo-mastoid foramen. This canal otttn ' "iiMderal.le re>i-taiice to caries in its immediate neighborhood, although the di^easr not infrequently communicates itself to the nerve. Such involvement of the nerve is often tin- prodromal symptom of a fatal cerebral affection (Politzer). At birth this portion <>f the Fallopian canal is very thin and translucent, and is deficient as it an h<> over the oval window, so that involvement of the nerve is much more common in children than in adults. PRACTICAL CONSIDERATIONS : THE MIDDLE EAR. 1505 Roofing in the antrum and the passage leading into it from the attic is a thin layer of bone (tegmen antri), which is particularly thin over the antrum and separates these spaces from the middle fossa of the skull. Not infrequently there are membranous defects in the tegmen, upon which the dura rests (Macewen). Pus frequently passes through this bony plate, or its deficiencies, to the temporo- sphenoidal region of the brain, which is the most frequent seat of brain abscess. Fractures of the base of the skull in the middle fossa may pass through the tegmen, rupturing the adherent dura, and permitting cerebro-spinal fluid to pass into the tympanum. If there is coincident rupture of the tympanic membrane, the fluid will likely appear at the external auditory meatus, or if the membrane remains intact, the fluid may pass to the pharynx through the Eustachian tube. Often the hearing in chronic plastic otitis media is better during a great noise than when the surroundings are more quiet, because the stiffened ossicles transmit additional ordinary sounds more readily after they have been loosened by the more violent vibrations; or it may be because the auditory nerve, owing to the greater irritation, becomes more sensitive (Urbantschitsch). The various relationships of the tympanum as involved in infectious disease should be understood from the standpoint of etiology and from that of sequelae or complications. Infection may reach the tympanum from (a) the naso-pharynx through the Eustachian tube (scarlatina, diphtheria, pharyngitis, tonsillitis, rhinitis); or (3) the mastoid antrum and cells posteriorly. It may extend from the tympanum («) upward, by perforation of the tegmen, often deficient at places, leading to external pachymeningitis, or to subdural abscess ; the dura, arachnoid, and pia mater at this level are fused, so that when the dura is ulcerated through, a diffuse meningeal infection does not ensue, but the process tends rather to spread into the brain along the perivascular lymphatic sheaths of the pial vessels, resulting in an abscess in the temporal lobe (Taylor); (^) to the internal jugular vein through venules that penetrate the fundus tympani to empty into the jugular bulb, and thence to the lateral sinus ; (rana ids •nterior us pensory ment irt process of malleus Anterior fold lower portion, the umbo being slightly below the middle of the membrane. By the two lines the membrane is divided into unequal quadrants. This arrangement into quadrants is a very important one since the pathological appearances occurring in each differ greatly. The antero-suflerior quadrant corresponds to the tympanic opening of the tube, the canal for the tensor tympani muscle, and the anterior pouch of the drum-head. The antero-inferior quadrant corresponds to the carotid canal. The postcro-superior quad- rant contains the long process of the incus, the stapes, and the articulations of these bones, the oval window, the pyramid, and stapedius muscle, the posterior pouch of the drum-head, the chorda tympani, and the posterior fold (pathologic). 1\\zpostcro- inferior quadrant contains the round window, the tympanic cells in the floor of the tympanic cavity and the bulb of the jugular vein. The flaccid portion or Shrapnell's membrane corresponds to the neck of the malleus and Prussak' s space (Briihl-Politzer). The bulb of the jugular vein may be larger than usual in which case it may encroach upon the posterior half of the membrane. Moreover, it may have an imperfect bony covering when it will be in danger during paracentesis tympani, the place of election of which is in this portion of the membrane. For the same reason, pus in the middle ear may more readily encroach upon the vein. The posterior inferior quadrant is selected for openings to evac- uate effusions in the tympanum, because it is less sensitive and vascular than the rest of the membrane and corresponds to less important structures. The opening also gives better drain- age than through any other portion. It should be borne in mind that the floor of the tympanum is 2-3 mm. below the inferior margin of the drum head, so that in the upright position perfect drainage can- not be obtained. The tym- panic membrane has an internal mucous lining, an external cutaneous and an intervening fibrous layer. It, therefore, has little elasticity, so that, while small openings often heal rap- idly, large openings close slowly, or not at all. A permanent opening, however, does not of necessity produce deafness. With an aural speculum and good light, one may locate the various structures as follows : Above and in front is seen the short process of the malleus as an appar- ently prominent point. From this point two streaks pass to the periphery, showing the division between the tense portion of the membrane and its flaccid portion (Shrapnell's membrane), seen only in a roomy meatus. Extending backward and downward from this point is seen a whitish streak ending at the umbo. This is the long process or handle of the malleus. Directed downward and forward from the iimbo is an area of liidit with its apex at the umbo and its base near the periphery of the membrane. It is triangular in shape and is due to the funnel shape of the membrane and the resulting light-reflex. Above and in front of the short process of the malleus is the membrane of Shrapnell. Through the grayish translucent tym- panic membrane the contents of the tympanum may sometimes be seen, changing apparently the color of the membrane. Its conical shape has been proven by trial and mathematically to be the most favorable for the reception of sound waves. ' The vibrations are transmitted through the ossicles to the labyrinth by way of the oval uindow. Tin- malleus rests in the membrane, the stapes occupies the oval window and the incus lies between and articulates with the two. Light reflex Normal drum-head of right side as seen with mirror. X 6. PRACTICAL CONSIDERATIONS : THE MIDDLE EAR. 1507 The Eustachian Tube. — The superior orifice of the Eustachian tube is in the upper part of the anterior wall of the tympanum, and is therefore, not well adapted for drainage of that cavity. The tube is directed downward, forward, and inward to the side of the naso-pharynx, where it is on a level with the posterior end of the inferior turbinate bone. In children it is wider, shorter, and more horizontal, so that in infection of the middle ear drainage in them is better, but, for the same anatomical reasons, otitis media is more likely to follow pharyngeal and tonsillar infections. The pharyngeal orifice is bounded above and at the inner side by the prominent cartilaginous arch which encloses a funnel-shaped opening. The mucous membrane over this projection is thickened by a cushion of adenoid tissue, hyper- trophy of which is frequently associated with pharyngeal adenoids and enlarged tonsils, and may occlude the tube, ultimately causing deafness. The upper border of the pharyngeal opening of the tube is a half inch above the soft palate, and the same distance below the basilar process, below the hinder end of the inferior turbi- nate bone and in front of the posterior pharyngeal wall (Tillaux). Immediately behind this orifice is the well-marked depression called Rosenmuller's fossa, the depth of which is increased in cases of enlargement of the pharyngeal tonsil and which may then lead to difficulty in the passage of a catheter into the Eustachian tube. It may also', when recognized, serve as a useful guide to the orifice of the tube. Injury to the orifice of the tube during operations in the naso-pharynx, or at the posterior ends of the turbinates, may lead to cicatricial contraction and occlusion, thus causing defective hearing. Ulcerations in the naso-pharynx may produce a like effect. The length of the tube is about 37 mm. (ij^ in.) and its pharyngeal opening is about 25 mm. (i in.) lower than the tympanic. Its upper third (12 mm.) is bony, and its lower two-thirds (25 mm.) cartilaginous. The narrowest part, the isthmus, is at the junction of these two portions. The lumen of the cartilaginous portion forms a somewhat S-shaped slit, the walls being in actual contact, except during the act of swallowing, when the slit opens so that air may reach the tympanum and equalize the atmospheric pressure on the two sides of the tympanic membrane. In the bony portion, though the lumen is smaller, it is open. In cases of obstruction of the tube at its pharyngeal end — as by pressure from a growth, or from a thickened mucosa — the outside pres- sure predominates, the tympanic membrane is pushed inward, and buzzing or ' ' singing in the ears ' ' results. Whenever the palate is raised or deglutition takes place, the tensor palati and levator palati contract, and in so doing open the Eustachian tube by traction on the fibrous tissue which unites the outer borders of the fibre-cartilaginous scroll of which the tube is composed. Concussion of the tympanic membrane from loud reports, as from the firing of great guns, is minimized by breathing with the mouth open, thus elevating the soft palate, opening the Eustachian tube, and equalizing the pressure on the two sides of the membrane. Inflation of the tympanum is accomplished through the Eustachian tube, and is employed for diagnostic, prognostic, and therapeutic purposes. Several methods are in use. Valsalva's consists of a vigorous expiratory effort while the nose and mouth are kept closed. Politzer inflates the tympanum through one nostril' by a vigorous compression of a rubber air-bag, while the patient is in the act of swallow- ing. The opposite nostril and mouth are closed. The most satisfactory method in difficult cases is by means of the Eustachian catheter. The instrument is passed tip downward along the floor of the nose until it drops into the post-nasal space and the posterior wall of the pharynx is reached. The tip is then turned gently outward and withdrawn about i cm. when the slight resistance of the cartilaginous rim is felt. After gliding forward over this prominence, it will engage in the orifice of the tube. The ring at the proximal end of the catheter — which is in the plane of the the curve of the beak and thus shows the position of the latter — is then directed toward the external meatus of the same side (Bonnafont). The catheter may be withdrawn, and the tip at the same time be turned to the opposite side from the one to be catheterized, so that the beak of the instrument catches on the edge of the vomer. It is then turned upward through 180°, and thus enters the tubal opening (Frank, Lowenberg). I5o8 HUMAN ANATOMY. Foreign bodies may lodge in the tube during vomiting, or a broken piece of the bougie may be left in. They will usually escape during vomiting or hawking, or they may be removed by an instrument if visible. If the tube is normal, a bougie i % mm. in diameter will easily pass the isthmus, the narrowest part. Strictures may be dilated or applications made by bougies. Narrowing of the lumen may occur near the isthmus from chronic inflammation or, at the pharyngeal orifice, from the pressure of pharyngeal adenoids, tumors, or polypi. Mastoid Process and Cells. — The mastoid process which is formed by the posterior extremity of the petrous bone, is relatively small at birth and contains no air cells except the antrum. The antrum is almost constant, although its size varies. In the infant it will hold a small pea, while in the adult its average length is from 12- 15 mm. (one-half inch or slightly more), its height 8-10 mm., and its width about 7 mm. (Briihl). It is the means of communication between the tympanum and the mastoid cells, so that infection finds an easy passage from the former to the latter. Its distance from the external surface of the mastoid process will depend upon the size of its cavity. This is usually from 12-14 mm- Anteriorly the antrum opens into the attic portion of the tympanum, and is in almost a direct line through that cavity with the Eustachian tube. A probe passed up the tube from the pharynx would pass through the attic into the antrum and would strike th*e joint between the incus and the stapes. The axis of the external canal would strike the line at an angle of about thirty degrees. The floor of the antrum is below the level of the entrance into the attic, so that pus in the antrum tends rather to enter the mastoid cells. Sometimes nearly all the mastoid cells are pneumatic ; more frequently they are diploetic at the tip of the mastoid process, and pneumatic above (page 184). Pus in the air spaces may reach the diploetic region by breaking down the thin intervening septa. Those cases in which there are no mastoid spaces are probably sclerotic from pathological causes. Thus a chronic inflammation of the mastoid may give rise to new bone formation, filling the diploe and causing eburnation. This would tend to prevent the outward progress of pus and would favor its extension toward the interior of the cranium. The suprameatal spine is about 10-12 mm. above the floor of the antrum, which corresponds to a point about half way up the posterior wall of the bony meatus, and lies about 5 mm. posterior to the inner end. Thus bulging of the posterior wall of the meatus may result from disease in the antrum. The squamo-mastoid suture is frequently seen on the surface of the mastoid process in children, and may give pas- sage to pus from the antrum to the surface. Through deficiencies in the mastoid process near its tip pus may find its way into the sheath of the sterno-cleido-mastoid muscle, or along the large blood-vessels into the neck. The bony wall between the antrum and posterior fossa of the skull is thin and cancellous, and may show deficiencies through which pus may reach the posterior fossa. In the fossa on the posterior surface of the mastoid process is the groove for the sigmoid sinus, which is frequently infected from disease of the antrum. Such infection may extend from the antrum to the posterior or cerebellar fossa of the skull, causing nu-ningitis, septic thrombus of the lateral sinus, or a subdural or cerebellar abscess. The possible lines of extension of mastoid inflammation may be summarized as follows ( alter Taylor) : (i) Upward, from absorption of the thin tegmen antri, or through tin- veins pa^in^ up through foramina in the tegmen (causing external pa. h\ -meningitis in the floor of the middle cranial fossa), or through the remains of the petro-s<|uam<>u^ suture ( causing thrombosis of the superior petrosal sinus). (2) •>,/. !>v emissary veins, or through a sinus at the lower part of the mastoid in the digastric t rising cellulitis beneath the sterno-mastoid, or travelling along the ^tvlo.^losMis, styl.. pharvn-eus and stylo-hyoid to the retro-pharyngeal region). .in/, through tin- thin l>ony layer separating the external auditory meatus fn.in the antrum and the mastoid cells (causing discharge from the meatus if the perforation is complete, or if it remains subperiosteal, directing the pus outward to a point jus\ back of the pinna). (4) Outward— especially in children— through the thin poM auditory process of the squamous bone, or through the open masto- PRACTICAL CONSIDERATIONS: THE MIDDLE EAR. 1509 squamous suture (causing a fluctuating adenomatous postauricular swelling, pushing the pinna forward and making it unduly prominent). (5) Inward, either through venules passing to the sigmoid sinus, or through caries of the wall of the sigmoid groove (causing external pachy meningitis, or subdural abscess, or suppurative basal meningitis, or cerebellar abscess — by way of the cerebellar veins emptying into the lateral sinus — or, most frequently, sigmoid sinus thrombosis). The sigmoid sinus is usually about i cm. behind the suprameatal spine, but is occasionally so far forward as to lie just beneath the external surface of the mastoid process, and immediately behind the bony wall of the meatus. Owing to its close relation to the mastoid antrum and cells, no other cranial sinus is so frequently the seat of infective inflammation. In infants, however, it is seldom seen, owing to the following facts : First, the mastoid cells are not developed in them, though the antrum exists ; secondly, the squamous covering of the antrum is not yet soldered to the mastoid, and therefore, purulent matter finds a ready exit, not being enclosed in a complete bony casing ; thirdly, more numerous exits for the venous blood exist in infants than in adults ; and fourthly, the sigmoid sinus rests on a flatter osseous surface than in adults, the bony gutter which imbeds the adult sinus being not yet fully formed. In infants the internal ear is more exposed than in adults to pathological encroachments from the middle ear, hence in them leptomeningitis is apt to ensue, which frequently ends fatally, and that so rapidly as to prevent the formation of sigmoid sinus thrombosis (Macewen). When the sigmoid sinus is infected, extension may occur to the venous channels associated with it, especially to the internal jugular, anterior condylar, and deep veins of the neck into which the anterior condylar empty themselves. Evidence of involve- ment of these may be found in two areas, — along the internal jugular, and in the upper third of the posterior cervical triangle. Pain on pressure over the inflamed veins may be elicited even when the patient is deeply somnolent or semi-conscious. Thrombosis of the internal jugular when marked, is very easy of detection, as it lies so super- ficially. The finger perceives a cord-like formation to the inner side of the sterno- mastoid on the outer side of the artery, though the latter is sometimes overlapped by it. This may extend the whole length of the internal jugular, but it is frequently confined to the upper third. The entire thrombus may be disintegrated and its par- ticles carried by the current to the lung, where they may set up infective infarction. They may be carried to the lungs by the veins passing into the posterior cervical triangle which flow through the vertebral and other channels to the subclavian (Macewen). The complication most to be feared in middle ear disease is the spread of the infection to the interior of the cranium. This may occur by direct extension of the carious process through the bone ; more rarely through the labyrinth and internal auditory canal or the aqueducts ; or, still more rarely along the small blood-vessels or connective tissue fibres which pass through the bone between the middle ear and the dura. Very exceptionally the pus may find its way through the thin anterior wall into the carotid canal and along this to the cranial cavity. Although otitis media appears to occur on both sides with equal frequency, the right side of the head has been said to be more frequently affected by intracranial sequelae. If so, this is probably due to the greater size of the lateral sinus and the sigmoid sinus on the right side. Consequently the right sigmoid sinus encroaches more upon the petrous and the mastoid portions of the temporal bone, especially at the sigmoid knee, and the distance between the lower border of the tympanum and the antrum on the one hand and the sigmoid sinus on the other, is less than between the corresponding points on the left side (Macewen). Involvement of the internal ear from otitis media is comparatively rare. This portion of the ear is developed independently of the rest, and, after necrosis, may be extruded in sequestra, in which may be recognized the structure of the labyrinth. If the pus associated fails to escape externally, there is danger of its passing through the internal auditory meatus and aquaeductus vestibuli to the brain. Affections of the semi-circular canals produce disturbances of equilibrium. The sinus is in danger in operations on the antrum, the external opening for which should be immediately behind the meatus, and the centre of the opening 2-3 I5IO HUMAN ANATOMY. mm. below the level of its upper wall. If the sinus is in an abnormally anterior posi- tion, the posterior wall of the meatus must be removed to gain more room The facial nerve is also in threat danger in these operations, and has frequently been injured. It lies in the inner wall of the mouth of the antrum, and is therefore, in front of it. The antrum is approximately about 12 mm. (one-half inch) in a direc- tion very slightly inward, forward, and upward from a point on the external surface, 5 mm. posterior to tin- suprameatal spine. The anterior edge of the opening made to reach the antrum should be at this point, and its upper edge 3 mm below t spine. It should never be carried deeper than i# cm. (ffc in.) from the anterior edge of the external opening, for fear of injuring the facial nerve or external semi- circular canal. As the situation of the mastoid antrum is the key to the position in all operations upon either the antrum itself or the mastoid cells, Macewen has noted three points in the anatomy of the mastoid that may govern the surgeon in reaching the antrum without (a) opening the sigmoid groove and injuring its enclosed sinus ; (£) encroaching upon the Fallopian canal and destroying the facial nerve ; (c~) invading the middle cerebral fossa ; (d) injuring the semicircular canals. 1. The suprameatal triangle — the lower border of which corresponds with the level of the roof of the antrum, and is, therefore, a few lines below the level of the base of the temporo-sphenoidal lobe — is bounded above by the posterior root of the zygoma, below by the postero -superior segment of the bony external meatus, and behind by a line uniting these two and drawn vertically from the posterior border of the meatus to the zygomatic root. The opening is made within this triangle and close to the last line — the base of the triangle. 2. The excavation of the bone is carried inward and a little forward, in the direc- tion of the posterior wall of the bony meatus, as shown by a probe passed into it from behind between the skin and the osseous wall. The more oblique the direction of this wall from behind forward, the more anterior the situation of the antrum. 3. The depth of the inner wall of the tympanic cavity from the level of the skull at the bony external meatus should be determined by introducing a probe through the external ear (and through the tympanic membrane previously per- forated by pathological processes) until it touches the inner wall of the tympanum. If this cavity lies deeply, the more superficial mastoid antrum will be relatively deep also. Of forty brain abscesses, the bone was diseased directly to the dura in thirty-seven (92 per cent.), the bone was diseased, but not the dura, in one (2.5 per cent.), and the bone was healthy in two (5 per cent.) (Korner). It follows from this list of cases, that after a thorough exposure of the antrum and the ear cavities, the carious process should be followed inward to the dura or brain. In case an abscess in the temporo-sphenoidal lobe cannot be reached in this way the skull may be opened by a trephine, or by an osteo-plastic resection immediately above the ear. A cerebellar abscess might be reached by an opening one and one-half inches behind the centre of the bony meatus and one inch below Reid's base line. THE INTERNAL EAR. The internal ear consists essentially of a highly complex membranous sac, con- nected with tin- peripheral ramifications of the auditory nerve, and a bony capsule, which encloses all parts of the membranous structure and is embedded within the substance of the petrous portion of the temporal bone. These two parts, known tivelv as the membranous and the bony labyrinth, are not everywhere in close apposition, but in most places are separated by an intervening space filled with a fluid, the yVr//W/>//, the inner sac lying within tin; osseous capsule like a shrunken within a mould. The membranous labyrinth is hollow and everywhere filled with a fluid, called the endofympkt which nowhere gains access to the cavity occupied by the pcrilymph. The internal ear is closely related, on the one side, with the bottom of tin- internal auditory canal, which its inner wall contributes, and with the inner wall of the tympanic cavity on the other. Its entire length is about 20 mm., and its long axis corresponds closely with that of the pyramidal or petrous THE INTERNAL EAR. portion of the temporal bone. The position of approximately its posterior third is indicated by the transverse ridge that crosses the upper surface of the temporal bone a short distance behind the internal auditory meatus. The irregular cavity of the bony labyrinth, hollowed out in the temporal bone, comprises three subdivis- Tympanic cavity- Facial canal Cochlea Semicircular canals Vestibule Internal auditory canal Right temporal bone, upper part of petrous portion has been removed to show bony labyrinth lying in position. ions : — a middle one, the vestibule, an anterior one, the cochlea, and a posterior one, the semicircular canals. Both the front and hind divisions communicate freely with the vestibule, but neither communicates with the membranous labyrinth nor, in the recent condition, with the tympanic cavity. Although corresponding in its general form with the bony compartments of the cochlea and semicircular canals, the membranous labyrinth less accurately agrees in its contour with the bony vestibule, since, instead of presenting a single cavity, it is subdivided into two unequal compartments, known as the saccule and the utricle, which are lodged within the bony vestibule. The divisions of the membranous labyrinth are, therefore, four, which from before backward are : the membranous cochlea, the saccule, the utricle and the membranous semicircular canals. THE OSSEOUS LABYRINTH. The Vestibule. — The vestibule (vestibulum), the middle division of the bony labyrinth, lies between the cochlea in front and the semicircular canals behind and communicates freely with both. It is an irregularly elliptical cavity, measuring about 5 mm. from before back- ward, the same from above FlG- I2°5- downward, and from 3-4 mm. from without inward. The lateral (outer) wall separates it from the tympanic cavity, and contains the oval window with the foot-plate of the stapes. The medial (inner) wall, directed toward the bottom of the internal audi- tory canal, presents two depressions separated by a ridge, the crista vestibuli, the upper pointed end of which forms the pyramidalis vestibuli. The anterior and smaller of these depressions is the spherical recess (recessus sphaericus) and lodges the saccule. In the lower part of this fossa, about a dozen minute peiforations mark the position of the macula cribrosa media for the passage of branches of the vestibu- lar nerve from the bottom of the internal auditory canal to the saccule. The posterior and larger depression is the elliptical recess (recessus ellipticus). Behind the lower Superior canal Horizontal canal Superior ampulla Common crus Lodges utricle Lodges saccule' Cochlea Posterior canal Posterior ampulla Cast of right bony labyrinth, mesial aspect. X 2. 1512 HUMAN ANATOMY. part of the spherical recess, the crista vestibuli divides into two limbs between which is the recessus cochlearis, which lodges the beginning of the ductus cochlearis and is pierced by a number of small openings for the passage of nerve filaments to this duct. The numerous minute holes piercing the crista (pyramid) and the elliptical collectively form FIG 1266. Cms commune Aqujeductus. vestibuli Recessus ellipticus' Macula inferior Crista vestibuli Recessus sphseri- cns with macula media Ampulla recess the macula cribrosa superior (Fig. 1266) and transmit branches of the vestibular nerve to the utricle and to the ampullae of the superior and hori- zontal semicircular canals. Below and behind the re- cessus ellipticus lies a groove, the fossula sul- ciformis, which deepens posteriorly into a very small canal, the aqueduct of the vestibule (aquae- ductus vestibuli) which runs in a slightly curved course to the posterior surface of the petrous portion of the temporal bone, where it ends in a slit-like opening, the apertura externa aquaeductus vestibuli, situated between the internal opening of the internal auditory canal and the groove for the lateral sinus. The canal transmits the ductus endolymphaticus and a small vein. The anterior wall of the vestibule is pierced by the large opening leading into the scala vestibuli of the cochlea. Near this aperture is seen the beginning of the lamina spiralis ossea which lies on the floor of the vestibule below the oval window. Posteriorly the vestibule directly communicates with the semicircular canals by five round openings. The Semicircular Canals. — The three bony semicircular canals — the superior, the posterior and the horizontal — lie behind the vestibule and are perpendicular to one another (Fig. 1 265). Their disposition is such that the planes of the three canals Macula crib- rosa superior Facial canal window Lamina spiralis Section of right bony labyrinth passing through plane of superior semi- circular canal ; anterior wall of vestibule is seen from behind. X 4- FIG. 1267. Ampulla of superior canal Small end of posterior canal Cms commune correspond with the sides of the corner of a cube, suggestively re- calling the relations of the three cardinal planes of the body— the sagittal, frontal and transverse. Each canal possesses at one end a dil&tation, called the osseus ampulla. The superior canal (ca- nalis superior i lies farth- est front and in a nearly vertical plain- at ri^ht angles to the lon^ axis of the primus portion of the temporal bone, vvhiUt tli«' nl-uif of the longest canal, llu- pOS- terior < canal is posterior) is approximately parallel to it. The external portion of tin- hori/oiital semicircular canal lorms a prominence on the inner wall of the middle ear above the facial canal, while the upper part of the superior semicircular canal produces the •onspictious elevation, the eminentia arcuata, seen on the superior Ampulla of horizontal l Facial canal i )v:il Round (cochlear) window Section of right bony labyrinth passing through plane of superior semicircular canal; posterior wall of vest.huU- Is wen from before. X 4- Small end of horizontal canal Ampulla of posterior canal THE INTERNAL EAR. 1513 surface of the petrous bone. The semicircular canals open into the posterior part of the vestibule by five apertures (Fig. 1267), the undilated ends of the superior and posterior canals joining to form a common limb (crus commune). The horizontal canal (canalis lateralis) alone communicates with the vestibule by two distinct open- ings. Its ampulla is at its outer end and lies at the upper part of the vestibule above the oval window, from which it is separated by a groove corresponding to the facial canal. Lying above and close to this opening is placed the ampullary end of the superior canal. The ampullary end of the posterior canal lies on the floor of the vestibule, near the opening of the non-dilated end of the horizontal canal and of the canalis communis. In the wall of the ampulla of the posterior canal, a number of small openings (macula cribrosa inferior) provide for the entrance of the special branch of the vestibular nerve destined for this tube. The Cochlea. — The bony cochlea constitutes the anterior part of the labyrinth and appears as a short blunt cone, about 5 mm. in height, whose base forms the an- terior wall of the outer end of the internal auditory meatus. Its apex is directed hori- FIG. 1268. Scala vestihuli Scala tympai Modiolus Area cochlearis Area vestibularis inferior. Internal auditory can Foramen Einrjulare Haimtlus, overlying helicotrema Lamina spiralis ossea Canalis spiralis modioli Facial canal Crista falciformis Area vestibularis superior Cochlea and bottom of internal auditory canal exposed by vertical section passing parallel with zygoma; prepara- tion has been turned so that cochlea rests with its base downward and apex pointing upward. X 5. zontally outward, somewhat forward and downward, and reaches almost to the Eusta- chian tube. Its large lower turn bulges into the tympanic cavity and produces the conspicuous elevation of the promontory seen on the inner wall of the middle ear (Fig. 1269). The bony cochlea consists essentially of a tapering central column, the modiolus, around which the bony canal, about 30 mm. long, makes something more than two and a half spiral turns, the basal, middle and apical. The conical modiolus has a broad concave base which forms part of the base of the cochlea (basis cochlea), and a small apex which extends nearly to the apex of the cochlea, or cupola (cupula). It is much thicker within the lowest turn of the canal than above, and is pierced by many small canals for the nerves and vessels to the spiral lamina (Fig. 1268). The axis of the modiolus, from base to apex, is traversed by the central canal, whilst a more peripherally situated channel, the canalis spiralis, encircles the modiolus and contains the spiral ganglion and a spiral vein. Project- ing at a right angle from the modiolus into the canal of the bony cochlea is a thin shelf of bone, the lamina spiralis ossea, which is made up of two delicate bony plates between which are fine canals containing the branches of the cochlear nerve. The spiral lamina begins between the round window and the lower wall of the 1 5 14 HUMAN ANATOMY. vestibule (Fig. 1269), and after winding spirally around the modiolus to the apex of the cochlea, ends in a hook-like process, the hamulus, which forms part of the the boundary of the helicotrema (Fig. 1269). The partial division of the canal of the bony cochlea effected by the osseous spiral lamina is completed by the membranous spiral lamina, which stretches from the free edge of the osseous lamina, to which it is attached, to the outer wall of the canal (Fig. 1271). The upper division of the canal is called the scala vestibuli and communicates with the vestibule, whilst the lower division, the scala tympani, would open into the tympanic cavity, were it not separated from that space by the secondary tympanic membrane. These scalae communicate with each other through an opening, the helicotrema, at the apex of the cochlea. Close to the beginning of the scala tym- pani at the round window is the inner orifice of the aquaeductus cochleae (ductus pcrilvinptiaticus), its outer opening being in a depression on the lower surface of the pyramid near its posterior edge. It transmits a small vein and establishes a communication between the subarachnoid space and the scala tympani. The internal auditory canal communicates with the cranial cavity by an oval opening on the posterior surface of the pyramidal portion of the temporal bone, from which it extends outward to the internal ear. Its outer or lateral end, the fundus, is divided into a smaller superior and a larger inferior fossa by a transverse ridge, the crista falciformis. In the anterior part of the superior fossa (area facialis) is the opening of the facial canal (aijiiaeductus Fallopii) for the transmission of the facial nerve. In its posterior part are the openings (area vestibularis superior) for the branches of the vestibular nerves which supply the utricle and the ampullae of the superior and horizontal semicircular canals. These openings appear in the macula cribrosa superior on the inner surface of the bony labyrinth (page 1512). The ante- ri< >r part of the inferior fossa is called the area cochlearis and is perforated about its middle by the opening of the central canal of the modiolus. Surrounding this are the numerous small apertures of the tractus spiralis foraminosus for the trans- mission of branches of the cochlear nerve to the two lower turns of the cochlea. Behind the area cochlearis and separated from it by a ridge, lies the inferior area of the vestibule (area vestibularis inferior) with its small openings for the passage of nerves to the saccule. The macula cribrosa media, described above, is formed by these openings. Behind the inferior fossa is a large opening, the foramen singu- lare, which leads into a canal at the other end of which are the small openings of the macula cribrosa inferior. It transmits the branch of the vestibular nerve des- tined for the ampulla of the posterior semicircular canal. THE MEMBRANOUS LABYRINTH. The membranous labyrinth (labyrinthus membranaceus) lies within the bony labyrinth, which it resembles in general form. This agreement is least marked \v it'h in the vestibule, since here the single division of the bony capsule is occupied by two compartments of the membranous sac, the utricle and the saccule. The membranous labyrinth comprises: (i) the utricle and the saccule, which, with the din-lux cndolyntfihaticHs, lie within the vestibule; (2) the three membranous semi- circular canals lodged within the bony semicircular canals; and (3) the mem- branous cochlea enclosed within the bony cochlea. The membranous labyrinth is attached, especially in certain places, by connective tissue to the inner wall of the bony capsule. The inter\al l»et \\eeii the membranous and bony labyrinths, largest in the BCftke tympani and vestilmli of the cochlea and in the vestibule, constitutes the perilymplutie ipOM < sp.-itiuiii pci il\ mphaticiiin ) and contains a modified lymphatic tluid, the perilymph. The fluid within the membranous labyrinth, appropriately called the endolymph, can pass from one part of the labyrinth to another, although the saccule and utrirule are only indirectly connected through the ductus endo- lymphaticus and a narrow channel, the canalis utriculo-saccularis. The Utricle.— The utricle uitriculus) occupies the recessus ellipticus in the upper lurk part of th. • \ i-Mibulr. It is larger than the saccule and communicates with tlie three membranous semicircular canals. Attached to the upper and inner walls of the vestibule by connective tis.Mie, it extends from the roof of the vestibule THE INTERNAL EAR. 1515 backward and downward to the opening of the posterior ampulla, a distance of from 5. 5-6 mm. The utricle is made up of three subdivisions, the uppermost of which is respresented by a blind sac, from 3-3.5 mm. in length and breadth, called the recessus utriculi, whilst the two lower divisions together form the utriculus pro- prius, which measures 3 mm. by from 1.5-2 mm. The lower part of the utricle proper is prolonged into the tube-shaped sinus posterior, which connects the am- pulla of the posterior semicircular canal with the utricle. The openings of the semicircular canals into the utricle are disposed as follows : into the recessus utriculi open ( i ) the ampulla of the superior semicircular canal and (2) that of the horizontal canal. Into the ritriculus proprius open (3) the sinus superior, which lies within the crus commune and receives in turn the nonampullated ends of the superior and posterior semicircular canals; (4) the non- ampullated end of the horizontal semicircular canal ; and (5) the ampulla of the posterior semicircular canal through the sinus posterior. On the antero-lateral wall of the recessus utriculi is placed the macula acustica of the utricle, whilst from its FIG. 1269. Hamulus Helicotrema Facial canal Vestibular( oval) window Pyramid Tympanic cavity/' / - / Promontory Probe passes through cochlear (round) window Lamina spiralis secutidaria Right bony cochlea partially exposed by section passing through outer wall of apex and of first turn. antero-mesial wall springs the canalis utriculo-saccularis, the small canal from the utricle that joins even a smaller passage from the saccule to form the ductus endolymphaticus. The Saccule. — The saccule (sacculus) is an irregularly oval compartment, about 3 by 2 mm. in size, which occupies the recessus sphaericus in the lower and anterior part of the vestibule, to which it is attached by connective tissue. It is somewhat flattened laterally and at its lower end gradually narrows into a passage, the canalis reuniens, which connects the saccule with the ductus cochlearis. Its upper end bulges backward forming the sinus utricularis, whose wall comes in contact with that of the utricle. The small canal, already mentioned as helping to form the ductus endolymphaticus, arises from the posterior wall of the saccule. The ductus endolymphaticus passes through the aquaeductus vestibuli to end in a blind dilated extremity, the saccus endolymphaticus, lying between the layers of the dura mater below the opening of the aqueduct. Through the openings in the recessus sphaericus branches of the vestibular nerve enter and pass to the macula acustica sacculi on the anterior wall of the saccule. The canalis reuniens is the very small tube passing from the lower part of the saccule into the upper wall of the cochlear duct near the caecum vestibulare, as its blind vestibular end is called. The Membranous Semicircular Canals. — These tubes (ductus semicircu- lares) occupy about one third of the diameter of the osseous canals and correspond 1516 HUMAN ANATOMY. Trabeculse Perilymphat space Membranous canal Trabeculoe Bony wall Transverse section of superior semicircular canal, showing relations of membranous to bony tube. X 35. to them in number, name and form. They are closely united along their convex margins with the bony tube (Fig. 1270), whilst their opposite wall lies free in the penlymphatic space, FIG. 1270. being attached only by irregular vascular con- nective tissue bundles, ligamenta labyrin- th! canaliculorum, which stretch across this space. Like the bony canals, each of the membranous tubes possesses an ampulla, which in the latter is relatively much larger than in the former, being about three times the size of the rest of the tube. The part of the ampulla corre- sponding to the con- vexity of the semicir- cular canal is grooved on the outer surface at the entrance of the ampullary nerves. On the corresponding in- ternal surface is a pro- jection, the septum transversum, which partially divides this space into two parts and is surmounted by the crista acustica, which contains the endings of the vestibular nerves. The crescent-shaped thickening beyond each end of the crista is called the planum semilunatum. Structure of the Utricle, Saccule and Semicircular Canals. — The vestibule and the bony semicircular canals are lined by a very thin periosteum composed of a felt-work of resistant fibrous tissue, containing pigmented connective tissue cells. Endothelium everywhere lines the perilymphatic space between the membranous and osseous canals, covering the free inner sur- face of the periosteum, the fibrous trabeculae, and the outer or perilymphatic surface of this part of the membranous labyrinth. The walls of the utricle, saccule and membranous semicircular canals are made up of (a) an outer fibrous connective tissue lamella and (b) an inner epithelial lining, the latter consisting throughout the greater part of its extent of a single layer of thin flattened polyhedral cells. Be- neath the epithelium, especially in the region of the maculae, is (c) a thin, almost homogeneous /n;i/int- »tt-»if>ra>i<\ \vith few cells. This middle layer presents in places on its inner surface sin. ill papillary elevations covered by epithelium. On the concave side of each of the semicircular canals is a strip, the raphe, of thickened epithelium in which the cells become low cylindric.il in type-. In the plana semilunata they are cylindrical in type. Over the regions receiving the m i \> tilm >, tin.- ma< uke acnsticce and the cristie acusticae, the epithelium Undergoes a marked alteration, changing from the indifferent covering cells into the highly specialized neoroephhelium. The maculae acusticae are about 3 mm. long by 2 mm. broad, the macula of the saccule being .1 little narrou. r i 1.5-1.6 mm. ) than that of tin- utricle (2 mm.). At the margin of these areas the cells an- at liist cuboid. il. next low columnar, and then abruptly increase in length, until they measure from .o3o-.o}.s mm., in contrast with their usual height of from .oo3-.oo4 mm. The a. oustic an-a in- cells are lon.n. rather narrow, irregularly cylindrical elements and extend the entire thickness of the epithelial la\er. resting upon a well-developed basement-membrane by their expanded or di\ ide.l basal process«-s At a variable distance from the base, they present a swelling CIK -losing an oval nucleus and terminate at the surface in a cuticular zone. The cylin- drical hair-i'i-ll\ nv broader but shorter than tin- snstentacular cells, and reach from the free sin-fa, e only as i.u as the middle of the epithelial layer, where each cell terminates usually in a THE INTERNAL EAR. 1517 rounded or somewhat swollen end containing a spherical nucleus. The central end, next to the free surface, exhibits a differentiation into a cuticular zone, similar to that covering the inner ends of the sustentacular elements. From the free border of each hair-cell, a stiff robust hair (.020-.025 mm. long) projects into the endolymph. This conical process, however, is resolv- able into a number of agglutinated finer hairs or rods. The free surface of the neuroepithelium within the saccule and the utricle is covered by a remarkable structure, the so-called otolith membrane. This consists of a gelatinous membrane in which are embedded numberless small crystalline bodies, the otoliths or ear-stones. Between it and the cuticular zone is a space, about .020 mm. in width and filled with endolymph, through which the hairs project to the otolith membrane. The otoliths (otoconia) are minute crystals, usually hexagonal in form, with slightly rounded angles, and from .oog-.on mm. in length. They are composed of calcium carbonate with an organic basis. On reaching the macula the nerve-fibres form a subepithelial plexus, from which fine bundles of fibres pass toward the free surface. The fibres usually lose their medullary substance in passing through the basement membrane and enter the epithelium as naked axis-cylinders. Passing between the sustentacular cells to about the middle of the epithelium, they break up into fine fibrillae, which embrace the deeper ends of the hair-cells and give off fine threads that pass as free axis-cylinders between the cells to higher levels. The crista acustica and the planum semilunatum are covered with neuroepithelium similar to that of the maculae. The hairs of the hair-cells, however, are longer and converge to and are embedded within a peculiar dome-like structure, known as the cupola, which probably does not exist during life, but is an artefact formed by coagulation of the fluid in which the ends of the hairs are bathed. Otoliths probably do not exist in the cristae acusticas. The Cochlear Duct. — The membranous cochlea (ductus cochlearis) lies within the bony cochlea, and like it includes from two and one-half to two and three- quarter turns, named respectively the basal, middle and apical, the latter being FIG. 1271. Organ of Corti Ligamen turn spiral Basilar membrane Corti's membrane s/Ganglion spirale v. Scala vestibuli Ductus ' cochlearis ill Scala ZJ"tympani Ganglion spirale Modiolus Cochlear nerve in interal auditory canal Section of human cochlea passing through axis of modiolus. X 12. three-fourths of a turn at the apex of the cochlea. The tapering tube of the bony cochlea, winding spirally around the modiolus, is subdivided into three compart- ments by the osseous spiral lamina and two membranes, namely, the membranous spiral lamina and Reissner's membrane. The membranous spiral lamina (lamina basilaris) or basilar membrane extends from the free border of the lamina spiralis ossea to the outer wall of the cochlea, where it is connected to an inward bulging of the periosteum and subperiosteal tissue, called the spiral ligament. The lower of the two tubes thus formed is the scala tympani and communicates, in the macerated skull, with the tympanum through the round window. The upper tube is subdivided into two compartments by an exceedingly delicate partition, known as Reissner's membrane (membrana vestibularis) which extends from the upper surface of the osseous lamina near its outer end, obliquely upward and outward, to the external wall of the cochlea. • The compartment above this membrane is the I5I8 HUMAN ANATOMY. scala vestibuli and communicates with the perilymphatic space of the vestibule. The scala- tympani and vestibuli communicate only at the apex of the cochlea through tin- helicotrema. They contain perilymph and are brought into relation with the stibarachnoid space through the aqu;eductus cochleae. They are lined by a delicate fibrous periosteum, usually covered on the surface which is in contact with the enclosed perilymph, by a single layer of endothelial plates. In some localities, however, as on the tympanic surface of the basilar membrane, the lining cells retain their primitive mesoblastic character and never become fully differentiated into endothelium. The third compartment, the ductus cochlearis, is triangular on cross-section (Fig. 1271), except at its ends, and bounded by Reissner's membrane above, by the basilar membrane and a part of the osseous spiral lamina below, and by the outer wall of the bony cochlea externally. Save for the narrow channel, the canalis reuniens, by which it communicates with the saccule, the cochlear duct is a closed tube and contains endolymph. It begins below as a blind extremity, the caecum vestibulare, lodged within the recessus cochlearis of the vestibule and, after making two and three-quarter turns through the cochlea, ends above at the cupola of the cochlea in a second blind extremity, the caecum cupulare, or lagena, which is attached to the cupola and forms a part of the boundary of the helicotrema. Architecture and Structure of the Cochlear Duct.— Reissner's membrane (membrana yestib- ularis), the delicate partition separating the cochlear duct from the scala vestibuli, begins on the upper surface of the lamina spiralis, about .2 mm. medial to the free edge of the bony shelf, and extends at an angle of from 40-45° with the lamina spiralis ossea to the outer wall of the cochlea, where it is attached to the periosteum. Notwithstanding its excessive thinness (.003 mm.), it consists of three layers : (a) a very delicate middle stratum of connective tissue, (d) the endothelium covering the vestibular side, and (c] the epithelium derived from the coch- lear duct, and contains sparingly distributed capillary blood-vessels. The outer wall of the cochlear duct (Fig. 1272) is bounded by a part of a thickened cres- centic cushion of connective tissue, whose convex surface is closely united with the bony wall and whose generally concave surface looks toward the cochlear duct. This structure, the liga- mentum spirale, extends slightly above the attachment of Reissner's membrane and to a greater distance below the attachment of the basilar membrane, thus forming part of the outer walls of the scalae vestibuli and tympani. At its junction with the basilar membrane it presents a marked projection, the crista basilaris, whilst a very slight elevation marks the point of attach- ment of the membrane of Reissner. The part of this ligament lying between these projections corresponds to the outer wall of the cochlear duct. Its concave free inner surface is broken by a third elevation, the prominentia spiralis, or accessory spiral ligament, distinguished usually by the presence of one large (vas prominens) or several small blood-vessels. The lower and smaller of these two divisions of the outer wall is called the sulcus spiralis externus and is lined by cuboidal epithelium, whilst the larger upper division is occupied by a peculiar vascular structure, the stria vascularis, which contains capillary blood-vessels within an epithelial struc- ture. Its surface is covered with pigmented irregular polygonal epithelial cells, and its deeper strata consist of cells which, especially in the superficial layers, resemble the surface epithelium, but in the deeper layers assume more and more the character of connective tissue. Over the prominentia spiralis the cells become flat and polyhedral. The ligamentum spirale is composed of a peculiar connective tissue, rich in cells and blood- vessels. Its thin outer layer forms the periosteum and is denser than the adjacent loose con- nective tissue. The latter is broadest opposite the scala tympani, where its fibres converge towards the crista basilaris. Opposite the outer wall of the cochlear duct it again becomes more compact and is rich in cells and blood-\ vssels. An internal layer extending from near the prominentia spiralis to tin- basilar membrane, consists of a hyaline, noncellular tissue. Some authors claim to have found smooth muscle-fibres in the ligamentum spirale. The tympanic wall or floor of the cochlear duct (Fig. 1272) comprises the basilar uicm- tirtini-, extending from the basilar crest to the outer end of the bony spiral lamina, and the limbus lamiiur .\pim/is, which includes this wall from the attachment of Reissner's membrane to the end of tin- bony lamina. The limbus uiisi.i sph.ilis) is a thick mass of connective tissue upon the upper surface of the outer end of the osseous lamina spiralis. Its outer extremity is deeply • . d to form a -utter, the sulcus spiralis internus, the projections of the limbus above and below the sulcus formin;.; respectively its superior (vestibular) and inferior (tympanic) labia. The upper surface of the limbus is marked by clefts and furrows which are most conspicuous Hearth- -outer margin of the upper lip I lal.imn \i-stiimlare), where the irregular projections between THE INTERNAL EAR. 1519 the furrows form the so-called auditory teeth, because of their fancied resemblance to incisor teeth. The lower lip (labiuin tympanicum) is continuous externally with the basilar membrane and is perforated near its outer end by some 4000 apertures (foramina nervosa) transmitting minute branches of the cochlear nerve. The epithelium covering the elevated portions of the limbus, including the auditory teeth, is of the flat polyhedral variety, the intervening furrows and clefts being lined by columnar cells. The epithelium of the sulcus spiralis consists of a single layer of low cuboidal or flattened cells, continuous with the epithelium of the auditory teeth above and with the highly specialized elements of Corti's organ below. The basilar membrane consists of a median (inner) and a lateral (outer) part. The former, known as the zona arcuata, is thin and supports the modified neuroepithelium constituting the organ of Corti; the outer part, named the zona pectinata, is the thicker division and lies external to the foot-plates of the outer rods of Corti. The basilar membrane is made up of three distinct layers, the epithelium, the substantia propria and the tympanic lamella. The substantia propria is formed of an almost homogeneous connective tissue with a few nuclei and fine fibres, which radiate toward the outer edge of the spiral lamina. The fibres of the zona arcuata are very fine and interwoven, appearing to be an extension of those of the lower lip of the limbus, whilst straight and more distinct fibres stretch from the outer rods of Corti to the spiral ligament and constitute the so-called auditory strings. According to the estimate of Retzius, there are 24,000 FIG. 1272. Bony capsule of cochlea Stria vascularis Reissner's membrane • Prominentia spiralis Membrana tectoria Spiral ligament Crista basilaris Nerve- fibres •< v- -i- Vas spiralis Bone membrane Cross-section of ductus cochleaiis from human cochlea. X 90. Drawn from preparation made by Dr. Ralph Butler. of these special fibres. Their length increases from the base toward the apex of the cochlea, in agreement with the corresponding increase in breadth of the basilar membrane. The tympanic lamella contains numbers of fusiform cells of immature character interspersed with fibres. In this location the differentiation of the mesoblastic cells lining the tympanic canal has never advanced to the production of typical endothelial plates, the free surface of the lamella being invested by the short fusiform cells alone. The inner zone of this layer contains capillaries which empty into one, or sometimes two, veins, frequently seen under the tunnel of Corti and known as the vas spirale. The epithelium covering the inner zone of the basilar membrane forms the organ of Corti, the highest example of specialization of neuro-epithelium. The Organ of Corti. — The organ of Corti (organon spirale) consists in a general way of a series of epithelial arches formed by the interlocking of the upper ends of converging and greatly modified epithelial cells, the pillars or rods of Corti, upon the inner and outer sides of which rest groups of neuroepithelial elements — the auditory and the sustentacular cells. The triangular space included between the converging pillars of Corti above and the basilar membrane below constitutes the tunnel of Corti, which is, therefore, only an intercellular space of unusual size. It contains probably a soft semifluid intercellular substance serving to support the nerve-fibrils traversing the space (Fig. 1273). The pillars or rods of Corti, examined in detail, prove to be composed of two parts, the denser substance of the pillar proper, and a thin, imperfect proto- plasmic envelope, which presents a triangular thickening at the base directed toward the cavity of the tunnel. Each pillar possesses a slender slightly sigmoid, longitudinally striated body, whose 1 520 HUMAN ANATOMY. upper end terminates in a triangular head, and whose lower extremity expands into the foot resting upon the basilar membrane. The inner pillar is shorter, more nearly vertical and less curved than the outer ; its head exhibits a single or double concave articular facet for the recep- tion of the corresponding convex surface of the head of the outer rod. The cuticular substance of both pillars adjoining the articular surfaces is distinguished by a circumscribed, seemingly homogeneous oval area of different nature. The upper straight border of the head of each pil- lar is prolonged outwardly into a thin process or head-plate, that of the inner lying uppermost and covering over the head and inner part of the plate of the outer pillar. The head-plate of the latter is longer and projects beyond the termination of the plate of the inner rod as the phalan- geal process, which unites with the adjacent phalanges of the cells of Deiters to form the mem- brana reticularis. The inner pillars of Corti are more numerous, but narrower than the outer elements, from which arrangement it follows that the broader outer rods articulate with two and sometimes three of the inner pillars, the number of the latter in man being estimated by Retzius at 5600, as against 3850 of the outer rods. Immediately medial to the arch of Corti, resting upon the inner rods, a single row of spe- cialized epithelial elements extends as the inner auditory or hair-cells. These elements, little more than half the thickness of the epithelial layer in length, possess a columnar body contain- ing an oval nucleus. The outer somewhat constricted end of each hair-cell is limited by a FIG. 1273. Membrana tectoria Nuel's space Inner hair-cells Outer hair-. cells Hensen's cells Oll-of Dritrr- Section showing details of Corti's organ from human cochlea ; owing to slight obliquity of section, width is some- what exaggerated. X 375- Drawn from preparation made by Dr. Ralph Butler. sharply defined cuticular zone, from the free surface of which project, in man, some twenty-five rods or hairs. The inner hair-cells are less numerous (according to Retzius about 3500), as well as shorter and broader, than the corresponding outer elements. Their relation to the inner rods of Corti is such, that to every three rods two hair-cells are applied. The inner sustentacular cells extend throughout the thickness of the epithelial layer and exhibit a slightly imbricated arrangement as they pass over the sides of Corti's organ to become continuous with the lower cells of the sulcus spiralis. The cells covering tin- Kisilar membrane from the outer pillar to the basilar crest comprise three groups: (a) those composing thr outer part of Corti's organ, including the outer hair- «•//*• and cells of Deiters ; (/>) the outer supporting cells, or cells of Hensen ; (c) and the low cuboidal elements, the cells of Claudius, investing the outermost part of the basilar membrane. The outer auditory or hair-cells are about five times more numerous (approximately 18,000 according to Waldeyer) than tin- corresponding inner elements, and in man and apes are dis- posed in thre, -or t'.uir rows. They alternate with tin- peculiar end-plates or "phalanges" of 1 tellers' cells, which separate the ends of the hair-cells and join to form a cuticular mesh-work, the nifintimmi »•//-,ed and indented octagonal areas of the end-plates of Deiters1 cells THE INTERNAL EAR. 1521 FIG. 1274. (Fig. 1274). The outer hair-cells are cylindrical in their general form, terminating about the mid- dle of the epithelial layer in slightly expanded rounded ends, near which the spherical nuclei are situated. The outer sharply defined ends of the cells are distinguished by a cuticular border sup- porting about twenty-five rigid auditory rods or hairs which project beyond the level of the mem- brana reticularis. The deeper end of each outer hair-cell contains a dense yellowish enclosure, known as the body oj 'Retzizis, which is triangular when seen in profile. The bodies are absent in the inner hair-cells. The cells of Deiters have much in common with the rods of Corti, like these being special- ized sustentacular epithelial cells which extend the entire thickness of the epithelial stratum to terminate in the peculiar end-plates or phalanges. It follows, that whilst the free surface of Corti's organ is composed of both auditory and sustentacular cells, the elements resting upon the basi- lar membrane are of one kind alone — the cells of Deiters. The bodies of the latter consist of two parts, the elongated cylindri- cal chief portion of the cell, con- taining the spherical nucleus and resting upon the basilar mem- brane, and the greatly attenuated pyramidal phalangeal process. A system of communicating in- tercellular clefts, the spaces of Nuel, lie between the auditory and supporting cells ; like the tunnel of Corti, these spaces are occupied by a semifluid intercel- lular substance. The cells of Deiters are arranged, as a rule, in three rows, although in places within the upper turns four or even five alternating rows are sometimes found. Each, cell contains a fine filament, ihejibre ofRetzius, which begins near the Cells of Hensen '— Deiters' cells Outer hair-cells Plate-like processes of inner pillar-cells Outer pillar cells Inner hair-cells Corti's organ viewed from above, showing mosaic formed by pillars and Deiters' cells ; outer ends of auditory cells occupy meshes of cuticular net-work. (Retzius). middle of the base with a conical expansion, and extends through the cell-body to the apex of the phalangeal process, where, according to Spec, it splits into seven or more fine end-fibrils, that extend into the cuticular superficial layer under and about the phalanges. The membrana tectoria or Corti's membrane stretches laterally from the upper lip of the limbus, above the sulcus spiralis and Corti's organ, as far as the last row of outer hair-cells. The membrane is a cuticular production, formed originally by the cells covering the region of the auditory teeth and the spiral sulcus. Medially it rests upon the epithelial cells, but farther outward it becomes separated from the free edge of the auditory teeth and assumes its conspic- uous position over the organ of Corti. The membrane seems to be composed of fine resistant fibres, held together by an interfibrillar substance. During life the membrane is probably soft and gelatinous, and much less rigid than its appearance indicates after the effect of reagents. The lower surface of the free portion of the membrane, opposite the inner hair-cells, is mod- elled by a shallow furrow, which indicates the position of a spirally arranged band known as the stripe of Hensen. Like the basilar membrane, the membrana tectoria increases in width from the base towards the apex of the cochlea. The outer sustentacular cells or cells of Hensen form an outer zone immediately external to the last Deiters' cells. These elements resemble the inner sustentacular cells, but differ somewhat in form and arrangement. In consequence of their oblique position, the bodies are not only greatly elongated, but also imbricated. They do not contain the fibres of Retzius. The cells of Claudius are the direct continuations of Hensen's cells, and laterally pass uninterruptedly into the low columnar elements covering the remaining part of the basilar membrane. They consist of a simple row of cuboidal cells possessing clear, faintly granular protoplasm and spherical nuclei. The Nerves of the Cochlea. — The branches of the cochlear division of the auditory nerve enter the base of the cochlea through the tractus spiralis foraminosus (page 1514), those destined for the apical turn traversing the central canal of the modiolus. From the modiolus a series of stout lateral branches diverge at quite regular intervals through canals which communicate with the peripheral spiral canal within the base of the bony spiral lamina. Within the peripheral canal the nerve- fibres join numerous aggregations of bipolar nerve-cells, which continue along the 96 1522 HUMAN ANATOMY. spiral canal and collectively constitute the ganglion spirale. From these cells numerous dendrites are given off, which pass along the canals within the spiral lamina towards its margin, the twigs meanwhile subdividing to form an extensive plexus contained within corresponding channels in the bone. At the edge of the spiral lamina bundles of fine fibres are given off, which escape at the foramina nervosa of the labium tympanicum and enter the epithelial layer close to the inner rod of Corti. During or before their passage through the foramina, the nerve-fibres lose their med- ullary substance and proceed to their destination as fine naked axis-cylinders. The radiating bundles pass within the epithelium to the mesial side of the base of the inner pillar ; here they divide into two sets of fibrillae, one, the mesial spiral fasciculus, going to the inner hair-cells and the other, the lateral spiral fasciculus, passing between the inner pillars to reach the tunnel of Corti. Within this space fibrillae are given off which, after crossing the tunnel, escape between the outer rods into the epithelium lying on the lateral side of the arch. The further course of the fibrillae seems to be such that some extend between the outer pillar of Corti and the first rows of hair-cells, whilst succeeding groups of fibrillae course between the rows of Deiters' FIG. 1275. £ .-••= \& 1- /^\ =i h 5JS If 5| • uperior canal Ductus endolymphaticus Ligamentum spirale ""* Basilar^ membrane Branches of cochlear nerve to \ Posterior canal Corti's organ Membranous cochlea | Blind sac of ductus cochlearis Canalis reuniens opening into cochlear duct Branch of vestibular nerve to posterior canal Membranous labyrinth of five months foetus, postero-mesial aspect ; u, utricle ; ss, sf>, superior and posterior utric- ular sinus; j, saccule; us, utriculo-saccular canal ; cr, canalis reuniens; pa, posterior ampulla. X 6. (Ketzius). cells to reach the remaining hair-cells. The relation between the nerve-fibrils and the auditory cells is in all cases probably close contact and not actual junction with the percipient elements. The paths by which the impulses collected from the audi- tory cells are conveyed to the cochlear nucleus, and thence to the higher centres, are described in connection with the Auditory Nerve (page 1258). Blood- Vessels of the Membranous Labyrinth. — The arteries supplying the internal ear arise from the internal auditory artery, supplemented to a limited extent by branches from the stylo-mastoid. The auditory artery, a branch of the basilar, after entering the internal auditory meatus divides, according to Siebenmann, into three branches : — (i) the anterior vestibular, (2) the cochlear proper, and (3) the vestibuh-cochlmr inacul.i. th. s.iccule and the cristae of the upper and outer ampullae of the corre- sponding semicircular canals. Hie cochlear artery pursues a spiral course. It gives off three branches, two ,,f which are distributed t.) the lower turn of the cochlea, whilst the third sup- plie^ the middle and apical turns. The vestibulo-cochlear artery arises either from the cochlear artery or independently and divides, within the spiral lamina, into a cochlear and a vestibular DEVELOPMENT OF THE EAR. 1523 branch. The cochlear branch is distributed to the lower turn of the cochlea and anastomoses with the cochlear artery proper. The vestibular branch is distributed to the lower part of the vestibule, including the lower part of the saccule and utricle, to the crus commune and part of the semicircular canals, and to the lower end of the cochlea. According to Siebenmann, the macula of the saccule receives its arterial supply from a blood-vessel which usually arises from the common stem of the vestib- ulo-cochlear artery, or, more rarely, runs independently through the whole internal meatus. A similar origin applies to the artery supplying the nerve of the posterior ampulla. In the base of the spiral lamina the arteries are connected by capillary loops especially in the lower turn of the cochlea. As mentioned above, one or more spiral vessels are often seen under the tunnel of Corti within the tympanic covering of the basilar membrane. The region of the stria vascularis and prominentia spiralis are especially well supplied with blood-vessels. Those seen in the scala tympani are principally veins, while a larger number of arteries are found in the scala vestibuli. The blood-supply of the lower turn of the cochlea is much more generous than that of the others. The veins by which the blood escapes from the cochlea include : ( i ) the vein of the vestibular aqueduct, which empties into the superior petrosal sinus ; (2) the vein of the cochlear aqueduct, which empties into the internal jugular and (3) the venous plexus of the inner auditory canal, which empties either into the transverse or inferior petrosal sinus. The first of these channels collects the blood from the semi- circular canals; the second from the whole cochlear canal through the anterior, pos- terior and middle spiral veins and from most of the vestibule through the anterior and posterior vestibular veins. The veins of the internal auditory canal form collat- erals to the other veins of the labyrinth and receive the large central cochlear vein (Siebenmann), which leaves the cochlea near the border of the central foramen of the modiolus, as well as tributaries corresponding to the branches of the acoustic nerve. FIG. 1276. Hind-brain THE DEVELOPMENT OF THE EAR. The development of the ear includes the formation of two morphologically distinct divis- ions, the membranous labyrinth, the essential auditory structure, and the accessory parts, com- prising the middle ear, with its ossicles and associated cavities, and the external auditory canal and the auricle. The developmental history of the organ of hearing proper in its early stages is largely an account of the growth and differentiation of the ectoblastic otic vesicle, since from this is produced the important membranous tube, the enveloping fibrous and osseous structures being comparatively late contributions from the mesoblast. Development of the Labyrinth. — The internal ear appears as a thickening and soon after depression of the ectoblast within a small area on either side of the cephalic end of the neural tube, at a level correspond- ing to about the middle of the hind-brain (Fig. 1276). This depression, the auditory pit, is widely open for a considerable time and distinguished by the greater thickness of its depressed wall, which contrasts strongly with the adjacent ectoblast. After a time the lips of the pit approximate until, by their final union, the cup-like depression is converted into a closed sac, the otic vesicle. This sac, after severing all connection with the ectoblast, gradually recedes from the sur- face in consequence of the growth of the intervening mesoblastic layer ; it next loses its sphe- roidal form and becomes somewhat pear-shaped, with the smaller end directed dorsally. The smaller end rapidly elongates into a club-shaped diverticulum, the recessus endolymphaticus, which later becomes the ductus and the saccus endolymphaticus. The remainder of the otic sac soon exhibits a subdivision into a larger dilatation, the vestibular pouch, and a smaller ventral one, the cochlear pouch (Fig. 1279). Auditory pit Dorsal aorta Oropharynx I visceral furrow I visceral arch Frontal section of early rabbit embryo, showing otic pits. X 40. 1524 HUMAN ANATOMY. FIG. 1277. Hind-brain Otic sac The semicircular canals differentiate from three folds which grow from the vestibular pouch opposite the attachment of the ductus endolymphaticus. The central parts of the two walls of each fold unite and undergo absorption, while the peripheral part of each fold remains open, thus forming a semicircular tube, one end of which becomes enlarged to form the ampulla. The superior vertical canal appears first, and the horizontal or external last. The growth of the epithelial diverticula is later accompanied by a condensation of the surrounding mesoblast, which differentiates into an external layer, the future cartilaginous and later bony capsule ; a layer internal to this becomes the perichondrium and later periosteum. A second mesoblastic layer is formed from the cells immediately surrounding the otic vesicle, whilst the space between these fibrous layers is filled by a semi-gelatinous substance which later gives place to the perilymph occupying the perilymphatic space. Within the ampullae, which early develop, the epithelial lining undergoes specialization, accompanied by thickening of the meso- blastic wall within circumscribed areas, to form the cristae acusticae. Coincidently with the development of the semicircular canals, a diverticulum, the cochlear canal, appears at the lower anterior end of the membranous sac. This tube, oval in section, grows forward, downward, and inward, and represents the future cochlear duct. After attaining considerable length, further elongation is accompanied by coiling and the assumption of the permanent disposition of the tube. The epithelium of the cochlear tube early exhibits a distinction, the cells of the upper surface of the somewhat flattened canal becoming attenuated, whilst those on the lower wall undergo thickening and further differentiation. The flattened cells form the epithelial covering of Reissner's membrane and of the outer wall, and the taller elements are converted into the complicated structures of the tympanic wall of the ductus cochlearis, including the crista, the sulcus, and the organ of Corti. The development of these structures includes the differentiation of two epithelial ridges ; from the inner and larger of these is derived the lining of the sulcus spiralis and the overhanging membrana tectoria. The outer ridge is made up of six rows of cells, the inner row becoming the inner hair-cells, the outer three rows becoming the outer hair-cells, whilst the two rows between these two groups form the rods of Corti. The crista appears between the sulcal cells and the cochlear axis as a thickening of the spiral lamina. The cochlear outgrowth of the primary otic vesicle forms the membranous cochlea, or scala media, alone, the walls of the adjacent divisions, the scala vestibuli and scala tympani, resulting from the changes within the surrounding mesoblast. The latter differentiates into two zones, an outer, which Incomes the cartilaginous, and finally osseous, capsule, and an inner, lying immediately around the membranous canal, which for a time constitutes a stratum of deli- cate connective tissue between the denser capsule and the ectoblastic canal. Within this layer clefts appear, which gradually extend until two large spaces bound the membranous cochlea above and below. Tlu-se spaces, the scala vestibuli and the scala tympani, are separated fora time from the scala media by a robust septum consisting of a mesoblastic layer of considerable thickness and the u.ill of tlie ectoblastic tube. With the further increase in the dimensions of the lymph- •paces, the partitions separating them from the cochlear duct are correspondingly reduced, until, finally, the oner broad layers are represented by frail and attenuated structures, the membrane of Reissner and the basilar membrane, which consequently include an ectoblastic stratum, the epithelial layer, strengthened by a mesoblastic lamina, represented by the sub- stantia propria and its endothelioid covering. The main s.ic of the ,,tic vesicle from which the foregoing diverticula arise constitutes the primitive membranous vestibule, and later subdivides into the saccnle and utricle. This separa- tion begins as an annular constriction of the primitive vestibule, incompletely dividing the vesicle into tuo compartments. The still relatively large ductus endolymphaticus, the direct successor "' '''• ' •iid.'lvmphaticus, unites \\ith the narrow canal connecting these vesicles in such a 111. inner that each space receives one of a pair of converging limbs, an arrangement foreshad- owing the permanent relations of the parts. K\en before the subdivision of the primitive vestibule is established, the vestibular end of the cochlear canal beeomes eotistricted, so that communication between this tube and the future saccule is maintained bv only a narrow passage, later the canalis reunions. The devel- opment of the macula; acusticae of the saccule and utricle depends upon the specialization of Pan of frontal section of head of rabbit embryo ; otic sac is separated from ectoblast and beginning to elongate. X 40. DEVELOPMENT OF THE EAR. 1525 Endolymphatic recess Otic vesicle shows differentiation into three subdivisions, endo- lymphatic, vestibular and cochlear. X 40. the epithelium within certain areas associated with the distribution of the auditory nerves. The nerve-fibres form their ultimate relations with the sensory areas by secondary growth into the epithelial structures. Development of the Auditory Nerves. — The vestibular and cochlear nerves, according to Streeter1, develop from a ganglion-mass first seen at the anterior edge of the otic vesicle. This consists of an upper and lower part from the dorsal and ventral portion pIG I2y8. of which peripheral nerve branches are developed, whilst a single stem connects it with the brain. The nerves destined for the utricle and the superior and external ampullae develop from the upper part of the ganglionic mass, while the nerves which supply the saccule Wal1 °f ,; j , r brain-vesicle and posterior ampulla develop from the lower part of this mass. The stem extending centrally from the ganglion toward the brain becomes the vestibular nerve. The spiral ganglion begins its development at the ventral border of the lower part of this mass, the cochlear nerve growing toward the brain while the peripheral division containing the ganglion extends into the membranous cochlea. From the foregoing sketch, it is evident that the membranous labyrinth is genetically the oldest part of the internal ear, and that it is, in fact, only the greatly modified and specialized closed otic vesicle surrounded by secondary mesoblastic tissues and spaces. Development of the FIG. 1279. Middle Ear.— The tympanic cavity and the Eustachian tube are formed essentially by the backward prolonga- tion and secondary expansion of the inner entoblastic por- tion of the first branchial fur- row, the pharyngeal pouch. The dorsal part of the latter, in conjunction with the adja- cent part of the primitive pharynx, gives rise to the sec- ondary tubo-tympanic space (Fuchs); the posterior end of this becomes dilated to form the tympanic cavity, while the segment interven- ing between the tympanic diverticulum and the pharynx is converted into the Eusta- chian tube. The first and second branchial arches con- tribute the roof of the tym- panic cavity. The ear ossicles are de- veloped in connection with the primitive skeleton of the visceral arches. The malleus Wall of brain- vesicle c duct alicular recess ar pouch face Cochlear duct Further differentiation of otic vesicle into endolymphatic duct, utriculo- saccular pouch and cochlear duct. and incus represent specialized parts of the cartilaginous rod of the first arch, the tensor tym- pani being developed from the muscular tissue of the same arch. The stapes is developed from the second arch. The mesoblast which surrounds the structures of the tympanic cavity during their development becomes spongy and finally degenerates toward the end . 1 Amer. Jour, of Anatomy, Vol. VI., 1907. 1526 HUMAN ANATOMY. The air-cells of the temporal bone, including those of the mastoid process, are formed later by a process of absorption. The tympanic membrane results principally from changes which take place in the first branchial arch ; it is originally thick and consists of a mesoblastic middle stratum, covered on its outer surface by the ectoblast and on its inner surface by the entoblast Development of the External Ear. — The median portion of the ectoblastic groove of the first branchial furrow becomes deepened to form the outer part of the external auditory canal, FIG. 1280. •- •?/- -cochlea 10 vilEEKS +. Diagram illustrating development of human membranous cochlea; primary otic vesicle subdivides into vestibular and cochlear pouches and endolymphatic appendage ; cochlear pouch becomes ductus cochlearis ; from vestibular pouch are derived utricle, saccule and semicircular canals ; whilst endolymphatic appendage gives rise to endo- lymphatic sac and duct. (Slreelfr.) while the surrounding parts of the first and second arches develop into the auricle. About the fourth week of fcetal life, the thickened posterior margin of the first arch is broken up into three tubercles by two transverse furrows. Similarly on the adjoining margin of the second arch, a second vertical row of three tubercles is formed and, in addition, behind these a longitudinal groove appears marking off a posterior ridge. From these six tubercles and the ridge are differ- entiated the various parts of the auricle, the lowest nodule of the first arch becoming the tragus, t!u remaining ones with the ridge giving rise to the helix, whilst from the three tubercles of the second arch are developed, from above downward, the antihelix, the antitragus and the lobule. THE GASTRO-PULMONARY SYSTEM. GENERAL CONSIDERATIONS. THE food-stuffs required to compensate the continual loss occasioned by the tissue-changes within the body are temporarily stored within the digestive tube. During this sojourn the food is subjected to the digestive processes whereby the sub- stances suitable for the nutritive needs of the animal are separated by absorption from the superfluous materials which, sooner or later, are cast out as excreta. Closely associated with digestion, and in a sense complementary to it, is the respiratory func- tion by which the supply of oxygen is assured. In the lowest vertebrates these two life-needs, food and oxygen, are obtained from the water in which the animal lives, this medium containing both nutritive materials and the air required for the perform- ance of the respiratory interchange of gases (oxygen and carbon dioxide). FIG. 1281. Wolffian body i j Spleen / Notochord Neural canal / / / Oral cavity Pharyngeal / *W , \ \ \ pouches Heart Luilg|-f \ ,Llve^ V \ Stomach Pancreas Cloacal orifice Mid-SUt Hind-gut Sagittal section of schematic vertebrate (Modified from Fleischmann.) Since, therefore, in these animals both food and oxygen are secured from the same source, the water, the digestive and respiratory organs form parts of a single gastro-pulmonary apparatus. This close relation is seen in the lower vertebrates (fishes), in which the anterior segment of the digestive tube is connected on either side with a series of pouches and apertures, the branchial clefts, bordered by the vascular gill-fringes by means of which the blood-stream is brought into intimate relation with the air-containing water. When the latter element is forsaken as a permanent habitat and the animal becomes terrestrial, a more highly specialized apparatus, suited for aerial respiration, becomes necessary. This need results in the development of the lungs. The latter, however, retain the intimate primary relation to the digestive tract, and are formed as direct ventral outgrowths from the gut-tube. The vertebrate digestive tract early becomes differentiated into three divisions : fore-gut, mid-gut, and hind-gut. The first includes the mouth, pharynx, oesopha- gus, and stomach, and serves for the mechanical and chemical preparation of the food materials. The second comprises the longer or shorter, more or less convoluted small intestine, and forms the segment in which absorption of the nutritive materials chiefly takes place. The third embraces the large intestine, and contains the super- fluous remains of the ingested materials which are discarded from the body at the 1527 1528 HTM AN ANATOMY. anal opening. Associated with the mid-gut are two important glands, the ^liver and the pancreas. Greater complexity in the character of the food and in the manner of securing it necessitates increased Specialization in the first segment of the festive tube ; hence the addition of accessory organs, as the lips, oral glands, tongue, and teeth the latter often serving as prehensile as well as masticatory organs. Reference to the early relations of the embryo to the vitellme sac (page 32) recalls the important fact that the greater part of the gut-tract is formed by the con- striction and separation of a portion of the yolk-sac by the approximation and closure of two ventral folds, the splanchnopleura. Since the latter consists of two layers, the entoblast and the visceral lamina of the mesoblast, the tube resulting from the union of the splanchnopleuric folds possesses a lining directly derived from the inner germ- layer, supplemented externally by mesoblast. The latter contributes the connective tissue, muscular and vascular constituents of the digestive tube, while the epithelium and the associated glandular elements are the products of the entoblast. MUCOUS MEMBRANES. The apertures of the digestive, respiratory, and genito-urinary tracts mark loca- tions at which the integument becomes continuous with the walls of cavities and passages communicating with the exterior. The linings of such spaces constitute FIG. 1282. fe_Epithelium Papilla of tunica propria occu- pied by blood- vessels Connective- tissue stroma Section of rane. X 35°- mucous membranes. The latter, however, not only form the free surface of the chief tracts, but also that of the ducts and tubes continued into the glands which are developed as outgrowths from the mucous membranes. Temporarily in the higher types and permanently in such of the lower animals as possess a common cloacal space, all the mucous membranes of the body are con- tinuous. After acquiring the definitive arrangement whereby the uro-genital tract Becomes separated ti..iu tin- di-i -sti\ <- tul>«-. tln-M- mrinliruncs in man and mammals (except monotremata) form two great tracts, the gastro-pulmonary and the genito- urinary. The free surfaces of the mucous membranes are kept continually moist by a viscid, somewhat tenacious secretion, the -mucus, derived from the glands ; they are thus protected from the drying :md irritating influences of the air, foreign substances, and s< •( it-ted or excreted matters with which they are brought into contact. Structure. KM TV mucous membrane comprises two distinct parts: the epi- thelium, which forms the immediate free surface and furnishes protection for the more delicate tis>iies beneath ; and the tunica propria, a connective-tissue layer which constitutes the slroma and gives place and support to the terminal branches of the MUCOUS MEMBRANES. 1529 FIG. 1283. Epithelium nerves and the blood-vessels and the beginnings of the lymph-radicles. Thus it will be seen that the general structure of a mucous membrane corresponds closely with that of the integument, the protecting epidermis of the latter being represented by the epithelium of the former, while both the corium and the tunica propria include the connective-tissue basis over which the epithelial layer stretches. A stratum of submucous tissue, corresponding with the subcutaneous layer in the skin, connects the mucous membrane with the surrounding structures. The epithelium may be squamous or columnar, simple or stratified. Its char- acter is usually determined by the conditions to which it is subjected ; thus, where covering surfaces exposed to mechanical influences of foreign bodies, it is commonly stratified squamous, as in the upper part of the digestive tract. Where, on the other hand, the mucous membrane is concerned in facilitating absorption, as in the intestinal tube, the epithelium is simple columnar in type. In localities in which the existence of a current favors the function of an organ, either as a means of freeing the surface from secretion or particles of foreign matter, as in the respiratory tract, or of propul- sion through a tube, as in the epididymis or the oviduct, the epithelium is of the ciliated columnar variety. Modifications of the epithelial cells, due to the presence of pigment or of secretion, distinguish certain mucous membranes, as those clothing the olfactory region and the large intestine respectively. The tunica propria or stroma consists of interlacing bundles of fibro-elastic tissue which support spindle or stellate connective-tissue cells. The latter usually lie within the uncertain clefts between the stroma bundles, which may be re- garded as lymph-spaces. In many localities the surface of the tunica propria is beset with numerous ele- vations or papilla, over which the epithelium extends. Such irregu- larities, when slight, may not modify the free surface of the mucous mem- brane, since the epithelial layer com- pletely fills the depressions between the elevations ; when more pro- nounced, the papillae or folds of the connective tissue produce the con- spicuous modelling of the surface seen in the papillae of the tongue or the rugae of the vagina. The papillae contain the terminal loops of the blood-vessels and the nerves supplying the mucous membrane. Where especially concerned in ab- sorption, the mucous membranes often gain increase of surface by cylindrical eleva- tions, or villi, as conspicuously seen in the small intestine. These projections, consisting of the stroma covered by epithelium, contain the absorbent vessels, or lacteals, in addition to the blood-capillaries. A more or less well-defined line separates the epithelium from the subjacent tunica propria. This demarcation is the basement membrane, or membrana propria, a detail which has been variously interpreted. Usually the basement membrane appears as a mere line beneath the epithelium, and is then, probably, formed by the apposition of the basal processes of the epithelial cells. When surrounding glandular tissue it is better developed, presenting a distinct and much more robust structure. In these positions the basement membrane is probably a product of the tunica propria and occurs in two types, sometimes being homogeneous, at other times reticular (Flint1). In many localities the deepest part of the mucous membrane, next the submu- cous tissue, is occupied by a narrow layer of involuntary muscle, the muscidaris mucosee. While not everywhere present, it is especially well developed in the intes- tinal tract from the gullet to the anus, and in places consists of two distinct layers, 1 American Journal of Anatomy, vol. ii., No. i, 1902. Section of mucous membrane of oesophagus. X 55. 1530 HUMAN ANATOMY. a circular and a longitudinal. The inner surface of the stratum is often broken by processes of muscular tissue which penetrate the tunica propria well towards the epithelium. The muscularis mucosae belongs to the mucous membrane, and there- fore must be distinguished from the muscular coat proper, which is frequently a conspicuous additional layer in the digestive tract. Mucous membranes are attached to the surrounding structures by a submucous layer of areolar tissue. The latter varies in thickness and density, consequently the firmness of the union between the mucous and submucous strata differs greatly in various localities. Usually the attachment is loose, and readily permits changes in position and tension of the mucosa, which, in the relaxed condition, is often thrown into temporary folds or rug&, as in the oesophagus and stomach. In other places the folds are permanent and not effaced by distention of the organ ; a conspicuous example of such arrangement is seen in the valvulae conniventes of the small intestine, in which the submucous tissue forms the basis of the elevation. The blood-vessels supplying mucous membranes reach the latter by way of the submucous tissue, in which the larger branches divide into the twigs which pass into FIG. 1284. Non-vascular epithelium Terminal capillary loops Tunica propria Larger branches within suomucosa Section of injected oral mucous membrane. X 60. the mucosa. Within the deeper parts of the tunica propria the smaller arterial ranches break up into the capillaries forming the subepithelial and papillary net- works, the vascular loops being limited to the connective tissue stroma and never entering the epithelium. The venous stems usually follow the arteries in their gen- When glands are present, the capillaries surround the tubules or alveoli :s in close relation to the basement membrane The /ynifihatics within .nu.-ous im-ml.runes are seldom present as definite chan- smce Acy begin as the uncertain interfascicular clefts between the bundles of s the deeper parts of the mucosa the lymph-paths become •, ittd exfe as ,1, u-ately walled varicose passages which converge towards Within the latter the lymph-vessels form net-wSrks richly provided with valv.-s an, coiled; c-d, increasingly complex compound tubular; e, tubo-aiveolar ; f, simple ; g-k-i, progressively complex compound alveolar. or liver, since these organs supply special secretions for particular ends. Aggrega- tions of the secreting elements vary greatly in size, form, and arrangement, as well as in the character of their products. The simplest type is the unicellular gland found in the lower forms; in principle this is represented in man and the higher animals by the goblet-cells seen in pro- fusion in mucous membranes covered with columnar epithelium. The secretion poured out by these goblet-cells serves to protect and lubricate the surface of the mucous membranes in which they occur. The term ' ' gland, ' ' however, usually implies a more highly developed organ composed of a collection of secreting epithe- lial elements. Glands are classified according to their form into two chief groups, the tubular and the alveolar, each of which occurs as simple or compound. It should be empha- sized that in many instances no sharp distinction between these conventional groups 1532 HUMAN ANATOMY. FIG. 1286. Opening on mucous membrane Excretory duct exists, some important glands, as the salivary, being in fact a blending of the two types; such glands are, therefore, appropriately termed tubo-alveolar. ..... In the least complex type, the simple tubular, the gland consists of a cylindrical depression lined by epithelium directly continuous with that covering the adjacent sur- face of the mucous membrane, as an outgrowth of which the gland originally devel oped. In such simple gland the two fundamental parts, \tefundus and the duct , are seen in their primary type. The fundus includes the deeper portion of the gland i which the epithelium' has assumed the secretory function, the cells becoming larger and more spherical in form, while in structure the distinction between the spongioplasm and hyaloplasm is usually marked in consequence of the particles of secretion stored up within the meshes of the spongioplastic net-work, which is often sharply displayed. The duct connects the fundus with the free surface and carries off the products elabc rated within the gland. It is lined with cells which take no part in secretion and hence retain for some distance the character of the adjacent surface epitheln Dilatation of the fundus of the primitive type produces the simple alveolar or saccular gland ; division of the fundus and part or the duct gives rise to the compound tubu- lar variety ; repeated cleavage and subdivi- sion of the duct, with moderate expansion of the associated terminal tracts, lead to the production of the tubo-alveolar type. Simple tubular glands may be minute cylindrical depressions of practi- cally uniform diameter, as the crypts of Lieberkiihn in the intestine, or they may be somewhat wavy and slightly expanded at the fundus, as often seen in the gastric glands towards the cardiac end of the stomach. When the torsion becomes very pronounced, as in the sweat-glands, the coiled variety results. Compound tubular glands pre- sent all degrees of complexity, from a simple bifurcation of the fundus and ad- jacent part of the duct, as in the pyloric or uterine glands, to the elaborate duct- system ending in terminal divisions either of a tubular form, as in the kidney and tes- ticle, or of a modified, somewhat dilated, alveolar form, the tubo-alveolar type, as in the salivary glands. Tubo-alveolar glands, modified compound tubular, constitute a very im- portant group, since they embrace many of the chief secretory organs of the body. They are made up by repetition of similar structural units, differences in the size of the organ depending upon the number of those associated to compose the gland. These unit-. < -m respond to the groups of terminal compartments, or alveoli, con- nected with a single ultimate division of the duct-system. The alveoli or acini contain the secreting cells, and arc limited externally by a basement membrane, often well developed, which supports the glandular epithelium and separates the latter from the blood- and lymph-vessels that surround the acinus. The alveoli belonging to the same intermediate duct, held together by delicate connei me tissue, constitute a pyramidal mass of glandular tissue, \hz. primary lobules. The latter are assembled into larger groups, or secondary lobules, which in turn are united by interlobular connective tissue into the lobes composing the entire gland. The lol.es .ue held together more or less tirmly by the interlobar areolar tissue continuous with the general fibrous envelope, which forms a capsule for the entire organ and separates it from the surrounding structures. Beginning of duct in alveoli Terminal alveolus Diagram showing relations of various portions of duct- system in glands of tubo-alveolar type. GLANDS. 1533 The interlobar tissue and its interlobular continuations contain the blood-vessels, lymphatics, and nerves supplying the gland and, in addition, the major portion of the excretory ducts. In the larger glands the latter form an elaborate system of pas- sages arranged after the general plan shown in the accompanying diagram (Fig. 1285). Traced from the terminal compartments, or alveoli, of the gland, the duct- system begins as a narrow canal, the intermediate duct, lined by low cuboidal or flat- tened cells directly continuous with the glandular epithelium of the alveoli. After a short course the tube increases in diameter and becomes the intralobular duct, which is often conspicuous on account of its tall and sometimes striated or rod-epithelium. The further path of the excretory tubules lies within the connective tissue separating the divisions of the glandular substance, and embraces the interlobular and the inter- lobar ducts, the latter joining to form a single main excretory duct which opens upon the free surface of the mucous membrane. The last-named passage is lined for some distance by cells resembling those covering the adjacent mucous membrane ; where these are stratified squamous in type, this character is maintained for only a limited FIG. 1287. Mucous alveoli . I '( , ,«B**a, '>«.••' ' /f%m&7/f-- '±'±1 ' - .'•' /-.'•'/ x; *?G3t^sf TilS''1*?.'- -.r- Serous alveoli Section of posterior part of tongue, showing alveoli of serous and mucous types of glands. X 60. extent, before the interlobar ducts are reached gradually giving place to a simple, sometimes at first double, layer of columnar epithelium which extends as far as the intralobular tubules. The walls of the larger ducts consist of a fibro-elastic coat, lined by epithelium, and sometimes, in the case of the large glands, as the parotid, liver, pancreas, or testicle, are strengthened externally by a layer of involuntary muscle. In the case of the large ducts the latter is usually disposed as a transverse and longi- tudinal layer, to which, as in the hepatic duct (Hendrickson), a third oblique one may be added. Differential stains show the presence of a large amount of elastica. The glandular epithelium lining the alveoli rests upon the limiting basement membrane as a single layer of irregularly spherical or polygonal secreting cells ; these do not completely fill the alveolus, but leave an intercellular cleft into which the product of the cells is poured and in which the system of excretory ducts begins. Depending upon the peculiarities of the cells and the character of their secretion, glands are divided into serous and mucous. 153* HUMAN ANATOMY. The serous glands are distinguished by cells which are distinctly granular, generally pyramidal in form, with nuclei situated in the vicinity of the centre. The secretion elaborated by such glands is thin and watery. The general appearance of the cells depends upon the number and size of the granules stored within their cyto- plasm and changes markedly with the variations of functional activity of the gland. When a serous gland is in a condition of rest, the cells are loaded with secretion, and appear, therefore, larger and coarsely granular. After active secretion, on the contrary, the cells are exhausted and smaller and contain little of their product, often exhibiting differentiation into a clear outer zone, free from granules, and a darker inner zone, next the lumen, in which the granules still remain. The mucous glands elaborate a clear, viscid, homogeneous secretion, which, when present in considerable quantity, as during rest, distends the cells, crowding the nuclei to the periphery against the basement membrane, and gives to the glandu- lar epithelium a clear and FIG. 1288. transparent appearance in marked contrast to the granular character of the elements of a serous gland. During rest, when loaded and distended with mucoid secretion, the transparent cells possess well-defined outlines, and present a nar- row peripheral zone con- taining the displaced nuclei and granular protoplasm. After prolonged activity the exhausted cells contain rela- tively little mucoid secre- tion, and hence the threads of spongioplasm are no longer widely separated, but lie closely ; in consequence of these changes the cells lose their former transpar- ency and resemble the elements of serous glands, becoming smaller, darker, and more granular than the cells of the quiescent mucous gland. The alveoli of mucous glands often contain small crescentic groups of small granular cells lying between the usual larger clear ele- ments and the basement membrane ; these are the Demilune of serous cells Duct Mucous cells Demilune Section of human sublingual gland, showing serous cells arranged as demi- lunes. X 300. crescents of G ianu ."/, or detniluncs of Heidcnhain, the interpretation of which has caused nun -h disi -u^ion. Tin older view regarded the crescents as groups of cells ditierin^ fnnn the surrounding ones only in their stage of activity and not in their essential characters, all the cells within the alveolus being of the same nature. The opposite view, advanced by Ebner over a quarter of a century ago, has received sup- pent iroin more recent critical studies by Kuchenmeister, Solger, Oppel, R. Krause, and others, who have shown that the cells composing the crescents differ from the mucus-containing elements, elaborate a special secretion, and are similar to, if not identical with, those filling the alveoli of serous Clauds. According to these observers, the crescent^ an- Croups of serous cells compressed and displaced by the predomi- nating mucous elements, but not excluded from the lumen of the alveolus, as was GLANDS. 1535 formerly thought to be the case, since extensions of the lumen pass between the mucous cells to reach the demilunes. In addition to the main alveolar lumina, always narrow in serous and wider in mucous acini, the existence of intercellular passages, or secretion-capillaries, has been established for many glands, especially by the employment of the Golgi and other special methods. These clefts penetrate laterally be- tween the glandular epithelium from the axial lumen • I289- towards the basement membrane, partially enclosing the secreting cells with a branching system of minute canals. Alveoli containing exclusively mucous cells do not possess these intercellular canaliculi, the axial lumen alone being present. In acini of the serous type the accessory channels are represented by minute branching passages which penetrate between the cells, but seldom reach the basement membrane. The most conspicuous of the secretion-capillaries occur in alve- oli containing the demilunes, the product of the serous cells escaping into the main lumen by means of the lateral intercellular canals which pass between the mucous elements to reach the peripheral group of Section of several alveoli of submax- serous cells composing the crescent. The view that SJg ftli8g£8&?2gS the secretion-capillaries normally extend into the cyto- ceiis^x"^ (s$?!?i*™/)roilps °* serous plasm of the glandular epithelium, and are, therefore, also intracellular, must be regarded as doubtful and still undecided, although sup- ported by many able histologists. Depending upon the distribution of the two varieties of alveoli, the tubo- alveolar glands may be divided into four groups (Ebner): 1 . Pure serous glands, in which only serous alveoli occur, as the parotid. 2. Mixed serous glands, in which a few mucous alveoli are intermingled with the serous, as the submaxillary. 3. Mixed mucous glands, in which the serous cells occur as crescentic groups or demilunes, as the sublingual and buccal. 4. Pure mucous glands, without serous alveoli or demilunes, as the palatal. Simple alveolar or saccular glands in their typical flask-like form, as seen in the skin of amphibians, are not found in man. The dilated spherical fundus is lined with clear and distended secreting cells, in which the nuclei are displaced towards the periphery by the mucus elaborated within the epithelial elements. In the higher animals this type of gland is represented, somewhat modified, by the simple sebaceous follicles. Compound alveolar or saccular glands constitute a group much less exten- sive than formerly supposed, since careful study of the form and arrangement of many organs, as the salivary glands, pancreas, etc. , has shown that these are more appro- priately regarded as tubo-alveolar than as branched saccular glands. The latter, however, still have representatives in the larger sebaceous and Meibomian glands. The most conspicuous example of the compound saccular or racemose type is the lung, which in its development and the arrangement of the air-tubes and the sac-like terminal compartments corresponds to this variety. The blood-vessels distributed to glands are always numerous, since secretory activity implies a generous blood-supply. In the case of the smaller and simpler glands, the capillaries within the mucosa form a mesh-work outside the basement membrane enclosing the glandular epithelium. The large compound glands are pro- vided with a vascular system which usually corresponds in its general arrangement to that of the excretory ducts, following the tracts of the interlobar and interlobular areolar tissue and its extensions between the groups of the alveoli. On reaching the individual acini, the capillaries form net-works which surround the basement mem- brane enclosing the alveoli, thus bringing the blood-current into close, but not direct, relation with the secreting cells, an arrangement favoring the selection by the proto- plasm of the particular substances required for the function of the gland. When the relation between the glandular epithelium and the capillaries is unusually intimate, 1536 HUMAN ANATOMY. as in the case of the liver, a distinct basement membrane is sometimes wanting a delicate supporting reticulum alone intervening between the blood-stream and the protoplasm of the cells. Although subject to local deviations, conspicuously excep- tional in the liver, the veins follow in general the course of the arterial branches the larger blood-vessels, together with the main excretory ducts, the lymphatics and the nerves occupying the principal extension of the connective tissue into the glandular The lymphatics are represented by the larger trunks which follow the excretory ducts and freely anastomose within the interlobular areolar tissue. After the mtra- lobular portion of the vessel is reached, its definite character is gradually lost until the lymphatic channels are to be recognized only as the clefts between the bundles of connective tissue separating the alveoli. • FIG. 1290. FIG. 1291. ni-t-\\iiik surrounding tubular glands. X 55- Section of submaxillary gland of rabbit ; upper half of figure shows distribution of nerve-fibres to alveoli ; lower half shows terminal ducts and secretion-capillaries. X 290. (Retzius.) The nerves supplying the larger glands include fibres from two sources, the cranial or spinal nerves and the sympathetic. They follow the interlobular excretory ducts, around which plexuses are formed, ganglion-cells being frequent at the points of junction. The stronger twigs contain a preponderating proportion of thick medullated libre-,, which become progressively less in size and number in their course towards the alveoli. Upon reaching the latter the nerves consist almost entirely of nonmedullatcd fibres, and in the end-plexuses around the alveoli such fibres alone are present. Tin- terminal distribution, as demonstrated by the Golgi and methylene- bluc methods, includes t-f)i/fnii>nir and hypoloumar fibrilla:, the former lying upon and the latter beneath the basement membrane. The hypolemmar fibrillae pass into the acini from the extra alveolar plexus formed by the filaments surrounding the base- ment membrane. The ultimate relation between the terminal fibrillae and the glandu- l.ir epithelium is -.till uncertain, but it may be regarded as established that the nerves extend between and around the cells ; an intracellular termination, on the contrary, is doubtful. Ret/ins, Kbm-r, and others agree in picturing the delicate perialveolar as consisting of tortuous and convoluted filaments which end in occasional GLANDS. 1537 FIG. 1292. delicate varicosities. Arnstein l has described a special minute plate-like end-organ as a widely occurring mode of nerve-ending in glands. W. Krause 2 has noted in certain glands a form of end-capsule resembling a simplified Pacinian corpuscle. The sympathetic fibres are distributed especially to the involuntary muscle of the blood- vessels and the ducts, the peristaltic wave within the muscular coat of the latter facili- tating emptying of the secretion. Development. — Since glands are only extensions of the mucous membrane or integument upon which they open, their development begins as an outgrowth or budding from the epithelium covering such surfaces. In the simple tubular glands the minute cylinders are closely placed and composed of densely packed cells. In the case of the larger compound glands, as the salivary or pancreas, the first anlage consists of a solid cylindrical plug which, penetrating into the mesoblast, soon begins to branch. The ends of the terminal divisions enlarge and eventually become the alveoli. Meanwhile the sur- rounding mesoblast undergoes condensation and forms the interlobular and other septa, as well as the general envelope, or capsule, thereby giving definite form to the general glandular aggregation. The vascular and other structures usually found within the interparenchymatous tissue are secondary and later formations. The develop- ment of the gland involves a double process of active growth, — not only the extension of the epithelial pro- cesses, but also a coincident invasion and subdivision of the latter by the mesoblast to form the constituent units of the organ. The lumen of the gland appears first in the main excretory duct, from which it extends into the secondary tubes and, finally, into the alveoli. Growth, separation, and more regular arrangement of the cells composing the epithelial cylinders are the chief factors in producing the lumen. In the early condition of the glands, before the assumption of functional activity, the cells later constituting alveoli of the serous or mucous type are similar and without histological distinction. Upon the establishment of their different roles, however, the characteristics distinguishing the varieties of glands appear, the differences de- pending upon physiological rather than upon inherent anatomical variation. 1 Anatom. Anzeiger, Bd. x., 1895. 2 Zeitschrift f. rational. Med., Bd. xxiii., 1865. Duct, with lumen Alveoli,' still solid Section of foetal oral mucous membrane, showing developing tubo-alveolar gland. X 50. THE ALIMENTARY CANAL THIS is a long and complicated tube extending from the mouth to the anus. Excepting the two ends, each of which is at first a pouch from the ectoblast, it is developed from the entoblast with a mesoblastic envelope. It consists of the mouth, pharynx, and cesophagus 'above the diaphragm, and of the stomach and small and large intestines below it. There are many accessory organs connected with it whose primary function is to assist in the process of nutrition. The chief ones above the diaphragm are the teeth, the tongue, and the salivary glands ; those below it are glands of various kinds, mostly so small as to be contained in the mucous membrane. But two distinct organs, the liver and the pancreas, belong to this class, both being originally outgrowths from the gut. The trachea and lungs have a similar origin, but their physiological function is so different that they are treated of under a separate heading. The general structural plan of the digestive tube, presenting in places great mod- ifications, is : (i) a lining of mucous membrane ; (2) a submucous layer of areolar tissue, into which glands may penetrate from the former ; (3) a double layer of non- striped muscular fibres, of which, as a rule, the inner is circular and the outer longi- tudinal ; (4) below the diaphragm, a serous covering from the peritoneum, which, although originally complete, is in the adult wanting in certain parts. The length of the alimentary canal is, on the average, not far from 9 m. (ap- proximately 30 ft.), of which not more than 45 cm. (about 18 in.) is above the diaphragm. A preliminary sketch of the divisions above the diaphragm may be con- venient. The vestibule of the mouth is the space between the lips and cheeks exter- nally and the jaws and teeth internally. The (potential) cavity of the mouth is within the arches of the gums and teeth. It is bounded above by the hard palate and its backward continuation the soft palate. The greater part of the floor is occupied by the tongue. There is a free horseshoe-shaped space beneath the tongue within the lower jaw, called the alveolar-lingual groove or, better, the sublingual space. The pharynx joins the mouth at the anterior pillar of the fauces, a fold passing outward and downward from the soft palate to the tongue. The pharynx extends from the base of the skull to the lower border of the larynx. The upper part, the naso- pharynx, is behind the nasal chambers which open into it, the oro-pharynx is behind the mouth, and the laryngo- pharynx behind the larynx. At the lower border of the larynx it is followed by the oesophagus, a long tube which, piercing the diaphragm, opens into the stomach. THE MOUTH. The framework of the mouth is made by the hard palate and the alveolar processes of the upper jaw, by the greater part of the body (including the alveolar processes) of the lower jaw and part of the ramus, and by the hyoid bone, to which may be added the mylo-hyoid muscle forming the floor. When the lips are opened and the lower jaw dropped, the mouth is a true cavity extending to the pharynx ; when these parts are closed, the tongue fills practically the whole space. It is convenient, however, to speak of the cavity of the mouth. This space is subdivided into the vestibule or preoral cavity and that of the oral cavity or mouth proper. The former is the region between the closed lips and cheeks in front and the closed jaws and teeth behind. When the lips are closed, it communicates with the mouth proper only by a small passage behind the wisdom-teeth, in front of the ramus of the jaw. THE LIPS, CHEEKS, AND VESTIBULE. The orifice of the mouth (rima oris) is a transverse slit of variable length, hounded by pr..j«-c -ting folds,— the lips. These, like the cheeks, with which they are continuous, are composed of complicated layers of muscle, covered externally by and internally by mucous membrane. 153* THE LIPS, CHEEKS, AND VESTIBULE. 1539 Fat is found irregularly disposed among the muscles of the cheeks in varying quantity, but in the depression in front of the masseter and superficial to the buccinator there is a distinct ball of fat enclosed by a capsule, which is the remnant of the so- FIG. 1293. Frontal sinus Sphenoidal sinus phenoid Orifice of Eustachian tube Orbicularis oris {Esophagus Trachea Sagittal section of head ol young adult, three-fourths natural size. called "button" of infancy, — a collection which gives resistance to the cheek and pre- vents it from being flattened by atmospheric pressure during nursing. The mucous membrane is reflected from the cheeks onto the jaws, where it covers the gums. This line of reflection at the middle of the lower jaw is 7 or 8 mm. from the alveolar 154° HUMAN ANATOMY. border and about twice as far from it in the upper. In both jaws, but especially m the lower, the line approaches the teeth as it passes backward. There is a distinct fold or fnniou of mucous membrane passing from the anterior nasal spine to the middle Of the upper lip. The free edge is often irregular, and may have a nodular enlargement. A much smaller fold is often found on each side in the region of the bicuspids. A median fold to the lower lip is small and inconstant Externally the lips present a red region of modified mucous membrane, intermediate between the skin of the face and the mucous membrane of the mouth. A sagittal section through either lip shows these three parts. In the new-born the intermediate part is subdivided into two, of which the inner— rather the broader— more closely resem- bles true mucous membrane than the latter. After death in the young child it assumes a brownish color, which has been mistaken for the effect of acid, adult these two subdivisions lose their distinctness. The lower lip is the larger and FIG. 1294. Mucous mem- brane cover- ing palate Tongue amaxillary duel Lingual nerve Sublingual gland tibmental artery Mylo-hyoid \ » Platysma Anterior belly of digastric Genio-hyoid' Genio-glossus Frontal section, showing oral cavity and lower part of nasal fossae ; plane of section passes through anterior end of zygoma. Three-fourths natural size. fuller, showing more red except towards the angles of the mouth, where it disap- pears. Its lower border is slightly indented in the middle. The upper lip shows a more marked indentation below a little gutter, \.\\e philtrum, running down from the nasal septum. A slight median prominence of the lower edge of the upper lip is the tn/tfrcle, which interrupts the straightnessaf the cleft when the lips are closed, making the line resemble a Cupid's how. The muscles of the lips are a complicated interlacement from many sources. The orbicnltiris <»/.<;, formerly supposed to form a sphincter, has no separate exist- ence. The general plan is as follows. The upper fibres of the buccinator enter the lower lip and pa-s out at the opposite angle to ascend into the upper part of the other Inn -i iuator. Those of the lower part traverse the upper lip in a similar manner. The layer funned by the buccinator lies under the mucous membrane near the border of the lips, and bends forward so that its edge is nearest the skin at about its junction 1 ( >tto Neustatter : Ueber den Lippensaum, etc., Inaug. Dissert., Munich, 1894. THE LIPS, CHEEKS, AND VESTIBULE. FIG. 1295. with the free red surface. In the lower lip the quadratus (depressor labii inferioris) runs upward under the skin to break up into fibres ending in the lips. The tri- angularis (depressor anguli oris) passes at the angle of the mouth into the upper lip and ends as a series of Separate fibres inserted into the mucous membrane, many of them crossing the middle line. This muscle, before it breaks up, is in the same plane as the buccinator, but farther from the edge of the lips. Some German au- thors, by grouping together the various muscles of the upper lip, have made a superior quadratus and triangularis which are disposed in a similar manner to the lower ones. Besides these there are two muscles, the zygomaticus, de- scending, and the risorius, ascending, which meet at the oral angles and end there in the skin or mucous membrane, or in both. There are also numerous fibres, seen only with the microscope in sagittal sections, passing from the skin to the mucous membrane ; these consti- tute the rectus.1 Philtrum Tubercle Labial region, from life, reduced one-fifth. FlG. 1296. -. , Fibres of "rHmlarie f : jh j :'i; Integument Sebaceous gland Transition into true — i mucous membrane Modified mucous membrane Transition into modified skin Sagittal section of lip of young child. X 20. The mucous membrane, which is smooth, is so closely attached to the muscles that it follows the movements of the latter. Mucous glands are lodged in its 1 Aeby : Archiv f. mikro. Anat, Bd. xvi., 1879. I542 HUMAN ANATOMY. deeper parts and in the scanty submucous tissue. They are named labial, buccal, ^\ molar, according to their situation. The labial glands are gathered into a series of groups near the inner border of the lips, the buccal glands are smaller and scattered and the molar glands are well-defined groups opposite the molar teeth. The duct of the parotid gland (q.v.) opens into the vestibule, the space between the lips and cheeks externally, and the teeth and alveolar processes internally Separating the vestibular space from that of the mouth proper behind the alveolar processes is a prominent fold of mucous membrane over the pterygo-maxillary ligament. appears at the inner side of the last upper molar and runs downward and outward to that of the lower. The space behind the teeth when the mouth is closed is small, but a tube some 5 mm. in diameter can be passed through it. Vessels.— The arteries supplying the lips, which are very vascular, are chiefly the coronary branches of the facial arteries, each of which forms an arch meeting its fellow in each lip The vessel lies between the muscles and the glands of the mucous membrane, nearly opposite the line of junction of the latter and the intermediate por- tion The pulsation is easily felt through the mucous membrane. The veins less regular, lie on the outer side of the muscles. The lymphatics empty into the glands at the angle of the jaw, excepting those near the median line of the lower lip, which run into the suprahyoid glands. Nerves.— The mucous membrane of the cheek is supplied by the buccal branc of the inferior maxillary division of the fifth cranial nerve, the lips by the terminal branches of its second and third divisions. THE TEETH. In form the teeth present three parts, — the body or crown, coated with enamel ; a somewhat constricted part, the neck, covered by the gums ; and the root or fang, which, covered by the cementum, is fixed in the socket. The greater part of the tooth is composed of the dentine and surrounds the pulp-cavity, to which minute openings in the root or roots transmit vessels and nerves. The shape of the crowns is the basis of classification. Thus, in the front teeth the crown is flattened so as to have a chisel-like shape, adapted to cutting, hence these are termed incisors ; the canine teeth have the crown forming a single point or cusp ; the bicuspids have two, and the multicuspids ; or molars, several cusps. The crowns of all the teeth may be considered as modifications of a simple cone, or as combinations of several cones. * In man the teeth come in two sets, the temporary or milk and the permanent teeth ; the total number of the former is twenty, that of the latter thirty-two. The number and arrangement of the teeth of any animal is expressed in its dental formula ; this for man, for the left half of the mouth, may be written as follows : Temporary Teeth : i* c1 nt-(=^.X2 = 2o\ . 21 z\ 5 / Permanent Teeth : i2 cl bi2 m 3 (= - X 2 = 32^. 2 i 2 3 \ 8 / It will thus be seen that in the milk-teeth there are no bicuspids and one molar less. Since the typical mammalian dental formula is t^. c- bi^ m~, it may be assumed 3143 that in man three pairs have been suppressed. These suppressed teeth are occasion- ally represented l>y supernumerary ones ; from the position of the latter it is probable that the missing teeth are the second incisors and the first and fourth bicuspids. To avoid confusion in the nomenclature of the teeth from the curve of the jaws, it is customary to speak of the labial and lingual surfaces of the incisors and canines, and of the facial, or bncrul, and lingual surfaces of the bicuspids and molars. The sides against the other teeth are often (-ailed the median and distal, supposing the teeth to be implanted in a straight transverse line. This is not satisfactory in all 'See Homologies, page 1566. THE TEETH. 1543 cases. We shall speak instead of the inner and outer sides of the incisors and canines and of the anterior and posterior sides of the bicuspids and molars. If the position of the tooth in the jaw be remembered, no confusion is possible. The Incisors. — The crowns are characterized by slightly convex quadrilateral labial surfaces, rather broader than the lingual ones, and ending in straight cutting edges, slightly concave lingual surfaces slanting forward and bevelled at the edge, triangular lateral surfaces, and single roots. The labial and lingual surfaces of the crowns are bounded at the root by curved lines, the convexity being towards the gums. At the sides these borders are continued as straight lines towards the free FIG. 1297. Partly developed fangs of last molar, Crown of last molar. Permanent teeth, showing their forms and relations; outer surface of jaws partly removed. Last molars are only partially formed. edge, and meet at an acute angle. The enamel is continued farther on the lingual surface, especially in the lateral incisors of both jaws. The cutting edge shows three small scallops on its first appearance, but they speedily wear away (Fig. 1298). The superior median incisors are much the largest. The labial surface of the crown is nearly square. The inner half of this surface is more strongly convex than the lateral. Traces of three swellings are often found on the labial side of the lower half of the crown extending to the three primitive scallops on the edge. The free edge meets the internal border at nearly a right angle, but the outer angle is rounded. The lingual surface, narrower than the labial, is a little concave. Some- times the edges are raised so as to leave a distinct V-shaped depression, in the middle of which runs a vertical ridge, the cingulum, which ends below in a tiiberde. 1544 HUMAN ANATOMY. Often the cingulum of the incisors is represented merely, by the tubercle. There are all kinds of intermediate stages between this and a nearly plane surface. Sometimes the tubercle is triple. The fang is nearly conical, and usually has an outward slant. The superior lateral incisors are more cusp-shaped, the angles, especially the outer, tending to be rounded. The lingual surface is less plane than in the median incisors and the cingulum larger. Sometimes it is almost a distinct cusp. The fang is also conical, with an outward inclination. The inferior incisors are smaller than the superior, and the median ones the smallest of all. The crowns broaden from the neck to the edge. This feature is more marked in the lower races, and still more in apes. The labial surface is more nearly plane than in the upper ones; the lingual surface is more even cingulum is small, often not very evident. The angles of the free edge are sharper than those of the upper jaw, excepting the outer one of the lateral tooth, which is generally rounded. The fangs are compressed 'from side to side and their tips turn a little away from the median line. This is particularly true of the lateral one, but FIG. 1298. Unworn surfaces of upper and lower permanent incisor teeth, lingual aspect. X 2. Median incisor teeth of left side, labial (A) and lateral ( /•' i aspects. (Lfidy.) Temporary incisor teeth of left side. A, median ; B, lateral in- cisors. (Leidy.) The is a constant feature of neither. The sides of the fangs are often grooved. external groove is the deeper, and when only one is present it is on that side. The pulp-cavity is relatively large in the superior median incisors, in which it presents three expansions towards the free edge. It is smaller in the others, and has usually but two distinct diverticula. The canal of the lower teeth, especially when the roots are deeply grooved, often divides below the pulp-cavity into an anterior and a posterior branch, which usually reunite before reaching the tip of the fang.1 The upper incisors occupy in all more space than the lower, which is due chiefly to the great size of the upper median ones. In the lower jaw the median incisors are the smaller, but there is no great difference between them and the laterals. The superior Literals an- but slightly larger than those below them. The temporary incisors differ only slightly, save in size, from the permanent ones. The cd^cs, however, arc originally straight, except those of the inferior median ones, which show the irregularities. The Canines. — These, called by the Germans the "corner teeth" as marking tl- • point where the alveolar arch changes direction most suddenly, are characterized by a crown with a single cusp, a long conical root somewhat compressed laterally and marked l>v a groove on each side. The crown, convex on the labial side, expands An.itmnir Mensi hlichen Gebisses, Leipzig, 1891. 1 /urkcrk. null : Anatomic drr MmulliohK-, mil hesoiulere I'.eriicksichtigung der Ziihne. Wien, THE TEETH. 1545 Canine teeth of left side, labial (A) and lateral (B) as- pects. C, temporary canines. (Leidy.) from the root and suggests that of an incisor with the angles taken off. The lingual side of the crown of the upper tooth tends to be convex, often having a ridge running down to the small tubercle at the base. In the lower tooth this side is plane or con- cave, with a distinct tubercle, which exceptionally is enlarged so as to hint at a secondary cusp. The sides of the crown are triangular. The borders of the enamel are convex to the gum on the labial side, less so on the lingual, and slightly concave laterally. The_/awg- of the upper tooth is the longer and the less compressed ; it very rarely ends in a bifurcation, but this is less uncommon in the lower. The direction of the end of the fang is uncertain. The whole tooth is broader on the labial than on the lingual side. The pulp-cavity is most marked in antero-posterior sections, which show an en- largement of its continuation at the beginning of the root, just beyond the neck. The milk canines are much like the second ones, only smaller. The labial surface of the upper tends to divide into an outer and an inner facet. The root is approximately triangular on section, with rounded edges. The Bicuspids or Premolars. — These teeth, of which the second is the larger in both jaws, are characterized by crowns with two cusps, one on the buccal and one on the lingual side. The upper ones, being very much the more typical, will be used for the general description. Both the labial and the lingual aspects of the crowns are convex ; they expand laterally from the neck, and each ends in a pointed cusp of which the anterior border is the shorter. This is used in determining the side, but we agree with Testut that the guide is often useless. The buccal cusp is the larger. The cusps are separated by a furrow from which small ramifications often run onto the buccal one. The lin- gual cusp has an unbroken surface. The buccal cusp of the first bicuspid is more prominent than the lingual, but in the second they reach the same plane. The bor- der of the enamel is convex towards the root on both the buccal and lingual aspects, the ends of these curves meeting on the other sides. The fang is compressed with a groove on the sides next its neighbors. That of the second is often bifid just at the tip, but that of the first is very often, per- haps usually, divided into two throughout, having a buccal and a lingual root. Sometimes the former is subdivided, so that it has three like a molar. The root has in general a backward slant. The lower bicuspids have smaller grinding surfaces on the crowns than the upper, but the roots are longer, and the crowns, seen from the side, are at least as large. The first has a well-developed buccal cusp, curving in from the buccal surface, and a very small lingual one connected to the former by a ridge interrupting the fissure between them, which gives the tooth something of the effect of a small canine. The second, like that of the upper jaw, has the two cusps in one plane ; the lingual one is sometimes double, and the plane is often obscure. The flattened fang is but faintly grooved, if at all, and is rarely bifid. The pulp-cavity of the bicuspids ends in an expansion below each cusp, that under the buccal being the larger.- In the upper teeth the cavity is much compressed from side to side in the root. In the first upper bicuspid there are usually two pro- longations to the point of the fang, even when the root is not split. In the second the cavity generally agrees with the conformation of the root. In the lower teeth the cavity is less compressed and is tolerably roomy as it enters the root. It is usually single, but may split. First premolar teeth of left side, labial (A) and lateral (B) aspects. (Leidy.) 1546 HUMAN ANATOMY. The Molars.— These teeth— three on each side— are distinguished by the large crown, into which the neck expands, the number of cusps on the surface and the ereater subdivision of the root. Those of the lower jaw are the larger ; and in both jaws the first is the largest and the last (called from its late appearance the wisdom- tooth} the smallest. The crowns are convex on both the buccal and lingual sides, but nearly plane on the others. The enamel ends in a nearly straight line all the way round The grinding surfaces are four-sided ; those of the upper are somewhat dia- mond-shaped, the buccal anterior angle being rather in front ; those of the lower are nearly parajlelograms, the long diameter being antero-postenor. Typical upper molars have four cusps at the angles ; typical lower ones have an additional cusp at the posterior border ; but in the upper jaw the first is the only one that can be calle In the upper molars the largest cusp is the anterior lingual, which is connected by a ridge (the cingulum) to the posterior buccal. The posterior lingual cusp is the smallest. A minute rudimentary cusp is found on the lingual surface of the anterior lingual cusp, usually too small to reach the grinding surface, and often hard to recog- nize. Not counting this, the first upper molar has four cusps in more than 90 per cent. Owing to the cingulum, the grooves on the grinding surface are best described as two oblique ones, the first from the anterior border to the middle of the FIG. 1304. Upper molars First Another first Second Lower molars First Another first Second Second molar teeth of left side, labial (A) and lateral (//(aspects. (-Leidy.) Triturating surfaces of molar teeth of right side. The upper margin of the figures corresponds to the labial surface. (Ltidy.) buccal, the second from the lingual border to the middle of the buccal. They are deepest at the middle. They appear on the buccal and lingual sides, deeper on the former, but rarely reach the gum. They may end in a pit, a favorite seat of caries (Tomes). The crown of the second upper molar presents three chief forms (Miihl- reiter). It may have four cusps and differ but slightly from the first molar. The lingual surface is relatively narrower and the posterior lingual cusp smaller. In the second form the last-mentioned cusp is wanting. The cingulum persists and the grinding sin-face is approximately triangular. The third form is compressed from side to side into a very narrow diamond, with the anterior buccal cusp in front and tin- |>.>st«-ri«>r lingual behind. Three and four cusps are about equally common in this tooth in Caucasians, but tin- lower races have more often four. The crown of the upper wisdom tooth presents many remarkable variations. The posterior lingual cusp is wanting in about t\\o thirds of the cases. The crown may be strongly com- -• •7i'ns of the lower molars are divided by a crucial fissure, the main line running anteio posteriorly. The hind part of this splits so as to enclose the fifth cusp, which is near or artu.illv at the buccal side. The effect of this is to form a cavity at the crossing of the lines in the middle of the crown. The lines on the sides THE TEETH. 1547 of the crowns are less deep than in the upper jaw. Sometimes the fifth cusp is wanting, in which case the posterior part of the furrow does not divide and the arrangement is remarkably symmetrical. Very rarely the first molar has a sixth cusp on the lingual side. The first molar has five cusps in more than 90 per cent. ; the second four only in more than 80 per cent. ; the third four rather more often than five. The buccal cusps of the lower molars are worn down earlier than the lingual ones. The following tables from the independent researches of Rose l and of Zuckerkandl show the percentage of frequency of different groupings of cusps. Although there is some discrepancy in the percentages, both agree as to the most and least common arrangement in both jaws. These statistics, like those of the separate teeth, apply to Europeans. (It is to be remembered that a certain percentage of teeth cannot be included.) Cusps . Cusps . Cusps . UPPER JAW. Molars. •444 •443 •433 Per Cent. Rose. Zuck. LOWER JAW. Molars. 123 Per Cent. Rose. Zuck. 19.9 28.9 37-9 9.6 28.7 60. i Cusps Cusps Cusps 555 19-8 ii. 5 545 30-4 30-5 544 40-4 50-0 The fangs of the first and second upper molars are two buccal and one lingual, which latter is much the largest. It is often, especially in the first molar, grooved on the lingual side. It is conical and strongly divergent. It often shows a tendency to subdivision, which may actually occur, although rarely. The two buccal ones are compressed antero-posteriorly and nearly vertical. The front one is the broader, and is grooved before and behind. This is often the case with the other. The roots of the upper wisdom-tooth are smaller ; the lingual is less divergent, and may be connected by a plate with one of the buccal ones. All may be fused more or less completely into one. The roots of the inferior molars are two : an anterior and a posterior, of which the former is rather the larger, both compressed from before backward and, especially the first, deeply grooved, suggesting the fusion of two. Sometimes, again especially in the first, each root is bifid. Those of the wisdom-tooth are usually nearer together, and are frequently fused into a common coni- cal root. Apart from their position in the jaws, the roots of the molars, excepting the upper wisdom-tooth, have a back- ward slant of varying degree. Their twists and curves are remarkably uncertain. Sometimes they converge and some- times diverge unduly, hooking in either case under bone, so as to make extraction difficult or impossible. The pulp- cavity of the molars is large, especially at the level of the neck. In the upper teeth it is distinctly wider transversely than from before backward. It has as many prolongations towards the surface as there are cusps. There is a canal in each root of the upper teeth. Those in the buccal fangs are compressed, that in the lingual cylindrical. The anterior fang of the lower molars has two canals which develop from a single one. The posterior fang has but one. The milk molars are two in number. Like the perma- nent ones, the lower are the larger ; but, unlike them, the second tooth is larger than the first in both jaws. The crown of both first molars presents a prominence on the buccal sur- face near the root. The crown of the first upper molar is rather suggestive of a bicuspid, although there are two buccal cusps and one lingual. The first inferior molar is relatively narrow and long from before backward. The length of the buccal side is greater than that of the second permanent one. The second molars resemble very closly the first permanent ones. The upper has four cusps and a cingulum, the lower five cusps. The hollow in the crown of the tem- porary molars is relatively deeper than that of the permanent ones, but smaller and more divergent. They straddle the crowns of the developing bicuspids. 1 Anatom. Anzeiger, Bd. vii., 1892. Temporary molar teeth (A, first; B, second) of left side. Triturating surfaces of crowns also shown. (Leidy.) HUMAN ANATOMY. TOOTH-STRUCTURE. In principle, and among the lower vertebrates in fact as well, teeth may be regarded* hardened papilla of the oral mucous membrane ; they consist, therefore, of two chief parts,— the connective-tissue core and the epithelial capping. < three constituents present in typical mammalian teeth, the enamel is the derivative of the ectoblastic epithelium, the dentine, with the pulp, and the cementum being con- tributions of the embryonal connective tissue. . The Enamel.— This, the hardest tissue of the body, covers the crown, being thickest on the cutting edge or grinding surface of the tooth. It gradually thins away FIG. 1306. Stripes of Retzius (longitudinal) Contour lin Schreger's li Neck Prism-stripes of Schreger (light and dark) Gum Pulp-tissue Dentine Cementum Alveolar periosteum Z Osseous tissue of jaw Root-canal Sagittal section of canine tooth in situ. Semi-diagrammatic. the neck, around which its terminal border appears as a more or less distinct ami nl'trii serrated eil-e. The external surface of the enamel, especially in young teeth, nfti-n <-\hil>its a line ^triation composed of horizontally disposed lines. Under a hand-ijas^ tin >< lines are seen to be minute elevations, the enamel-ridges, which encircle the (i-own. The remarkable hardness of this tissue is due to the large amount (97 per cent.) of earthy material ami the small proportion of organic matter, which latter in adult enanx 1 averages only about 3 per cent.; in infantile enamel the amount of animal material i- from h\e to six times greater ( Hoppe-Seyler). STRUCTURE OF THE TEETH. 1549 The enamel — the product of epithelial cells, the ameloblasts— consists of an aggre- gation of five- or six-sided columnar elements, the enamel-prisms, which measure from .0035-0045 mm. in diameter and from 3-5 mm. in length. Their general disposition is at right angles to the surface of the dentine upon which they rest, on the one hand, and to the exterior of the crown on the other. They usually extend the entire thickness of the FIG. 1307. enamel, and are of slightly larger diameter at the surface of the tooth than next the dentine, in this manner com- pensating for the increase in the external circumference of the crown. The assumption that additional prisms are intercalated at the periphery is not supported by the manner of the production of the enamel-columns. The latter run for a short distance almost at right angles to the surface of the dentine, then bend laterally for a considerable part of their course, but assume a vertical disposition on approaching the external surface. In addition to these general curves, the ranges of enamel- columns possess a spiral arrangement, in consequence of which the parallelism of the prisms, as seen in ground- sections, is disturbed and their bundles are apparently interwoven. In thin accurately transverse sections enamel pre- raiiges^orenamei-prfsms1.' Sx°5oong sents a mosaic in which the hexagonal areas represent the ends of the individual prisms. Critically examined, the areas consist of a darker central portion surrounded by a narrow lighter peripheral zone. The interpreta- tion of the latter has been various, many observers regarding such lines as cement- substance holding together the prisms. According to Walkhoff,1 however, what is usually regarded as cement-substance is a cortical, apparently homogeneous layer of less thoroughly calcified material which encloses the denser central portion of the prism and acts as a cushion, thereby reducing the effect of pressure. After the decalcifying action of acids, the prisms may be outlined by stains which color the very meagre amount of true cement-substance which exists between the enamel- columns and appears as delicate lines defining the prisms. Under favorable conditions, especially, but not only, after the action of acids, the enamel-prisms exhibit alternate light and dark transverse markings. The true rela- tions of these bands are to be appreciated only by accurate focusing in thin sections passing exactly parallel to the axes of the prisms ; otherwise the obliquity of section produces the optical distortions often represented in the assumed wavy contour of the enamel-rods. The varicose appearances commonly seen depend upon the beaded form and consequently scalloped border of the denser central portion of the prisms, which give a corresponding arrangement to the lighter cortical substance which fills the minute inequalities of that portion ; the true outline of the enamel-prism, how- ever, is smooth and straight, and not varicose, as the optical impressions lead one to believe and as usually pictured. According to Williams, the apparent varicosities depend upon the spherical form of the enamel-globules of which the prisms are built up. When an axial longitudinal section of a tooth is examined by reflected light, the enamel displays a series of alternate dark and light bands, — the prism-stripes of Schreger. These markings extend generally vertical to the surface of the enamel, and depend upon the relation of the ranges of the enamel-prisms to the axes of the light-rays. Rotation of the illuminating pencil through 180° effects the change of the dark stripes to light ones and vice versa. Each stripe includes from ten to twenty enamel-prisms, and is invisible by transmitted light. In addition to the foregoing markings, the enamel often presents, in radial longi- tudinal sections, brownish parallel lines, the stripes of Retzius, which run in the general direction of the contour of the tooth, but at an angle of from 15° to 30° with the free surface. Seen in sections cut at right angles to the tooth-axis, these stripes appear as a series of concentric lines encircling the crown parallel to and near the surface ; in the middle and deeper parts of the enamel they are less evident or entirely 1 Normale Histologie mensch. Zahne, 1901. 1550 HUMAN ANATOMY. The FIG. 1308. Longitudinal ground-section of enamel, treated with acid, showing disposition of ranges of enamel-prisms (p,p') in stripes of Schreger. Left third of figure shows alternate light (s) and dark (s') bands as seen by re- flected light. X 200. (Ebner.) absent The interpretation of the stripes of Retzius is still a subject of dispute. brTwn appearance of the stripes by transmitted light only, by reflected light appear- ing bluish white, disproves the assumption that they depend upon the presence of pilment within the enamel. The widely accepted view of Lbner that the stripes ^are due to air contained in the interfascicular clefts, has been modified by Walkhoff, wh< regards the markings as due to local diminution in the calcification of the enamel-prisms during certain periods in the growth of the tissue when the central as well as the cortical substance of a great number of columns fails to take up sufficient lime salts. The enamel-cuticle, or membrane of Nasmyth, forms a continuous investment of the crown of the newly erupted tooth. In the course of time it dis- appears from the areas exposed to wear, but over the protected surfaces it may persist during life. The membrane (.oo9-.oi8 mm. in thickness) is transparent and remarkably resistant to the action of acids, less so to alkalies, affording admirable protection to the underlying enamel. After separation from the latter by acids it appears structureless, or at most granular. The inner surface of the membrane presents markings and slight irregularities which correspond to the free ends of the subjacent enamel-prisms. The origin of the enamel-cuticle has been much discussed, and even now is not without some uncer- tainty. It may be regarded as established that it rep- resents the remains of part of the tissue once concerned in the production of the enamel. The latter is formed, as more fully described on page 1561, through the agency of the epithelial cells constituting the inner layer of the enamel-organ. With the completion of their task as enamel builders, these cells produce a continuous cuticular envelope which persists as Nasmyth' s membrane, the epithelial elements of the enamel-organ, so far as they are concerned in forming enamel, subsequently degenerating. The enamel-cuticle is continuous with the cortical substance of the prisms, with which it agrees in optical and chemical properties, — a relation which confirms the identity of origin of Nasmyth' s membrane and the enamel-columns. The Dentine. — The dentine or ivory resembles bone both in its genesis and chemical composition, being a connective tissue modified by the impregnation of lime salts. Dentine exceeds bone in hardness, containing a larger proportion (72 per cent.) of earthy matter and a smaller amount (28 per cent.) of organic substance. When decalcified by acids, the remaining animal material retains the previous form of the dentine and yields gelatin on prolonged boiling. Dentine, like bone, is formed through the agency of specialized connective-tissue cells, the odontoblasts , but differs from osseous tissue in the small number of these cells which become imprisoned in the intercellular matrix. When this occurs, as it exceptionally does in normal human dentine ami more frequently in pathological conditions or in the lower animals, the dfntine-ctlls correspond to the bone-corpuscles, both being connective-tissue elements lying within lymph-spaces in the calcified intercellular substance. Kxamined in dried sections under low magnification, the dentine presents a radial striation composed of fine dark lines which extend from the pulp-cavity internally to the enamel or the cementum externally. These dark lines are the dentinal tubules, filled with air, which arc homologous with the lacunae and canaliculi of bone, and contain the processes of the odontoblasts. In the crown, as seen in longitudinal sections, the course of the dentinal tubules is radial to the pulp-cavity ; in the root their disposition is hori/ontal and almost parallel. The canals, however, are not straight. l>ut si^moid, the first convexity being directed towards the root, the second towards the crown. In addition to these primary curves, which are especially marked in the crown, the dentinal tubules present numerous shorter secondary curves which STRUCTURE OF THE TEETH. impart to the individual canaliculi a spiral course. The cause of the latter Kollmann refers to the more rapid growth of the dentinal fibres than of the slowly forming dentinal matrix. In consequence of the correspondence of the curvature of the den- tinal tubules, the tooth-ivory exhibits a series of linear markings, Schreger' s lines, which run parallel to the inner surface of the dentine. These markings must not be confounded with the contour lines of Owen (page 1552), also within the dentine, or with Schreger' s prism-stripes within the enamel (Fig. 1306). The dentinal tubules are minute canals, from .0013-. 002 mm. in diameter, which begin at the pulp-cavity with the largest lumen and extend to the outer surface of the dentine, to end beneath the enamel or cementum. Each spirally coursing canal undergoes branching of two kinds, a dichotomous division at an acute angle in the vicinity of the pulp-cavity, resulting in two canaliculi of equal diameter, and a lateral branching during the outer third of their course whereby numerous twigs are given off with a corresponding dimi- nution in the size of the cana- FIG. 1309. liculi ; the terminal tubes, often reduced in diameter to mere lines, frequently anas- tomose with one another or form loops. The dentinal tubules are occupied by the delicate dentinal fibres, the processes of the odonto- blasts, which in the young tooth constitute a net-work of protoplastic threads through- out the dentine of importance for the nutrition of the tis- sue. The relation of the den- tinal tubules on the external surface of the dentine varies on the crown and root. In the former situation the free surface of the dentine pre- sents crescentic depressions, filled by the enamel, in which the tubules appear as ab- ruptly terminating or cut off ; on the root, on the contrary, where the dentinal surface is smooth, the tubules stop in curved ends or loops beneath the cementum, only in very exceptional cases communi- cating with the canaliculi of the latter. The immediate wall of the dentinal tubules is formed by a delicate membrane, the sheath of Neumann, which in appropriate transverse sections appears as a con- centric ring. On softening the decalcified dentine by acids or alkalies, the sheaths may be isolated, since they resist the action of the reagents which attack the sur- rounding intertubular substance. The sheaths of Neumann are formed through the agency and at the expense of the dentinal fibres, the latter being smaller in old than in young dentine. The sheaths, therefore, may be regarded as specialized parts of the intertubular matrix, distinguished by less complete calcification and greater density. The intertubular ground-substance of dentine resembles that of bone in being composed of bundles of extremely delicate fibrillae of fibrous connective tissue. The latter, best seen in decalcified tissue, swell on treatment with water containing acids or alkalies, and yield gelatin after prolonged boiling. The disposition of the bundles Enamel _ Crack, in enamel Junction of enamel and dentine Dentinal tubules Ground-section of dried tooth including adjacent enamel and dentine. X 3°°- 1552 HUMAN ANATOMY. of fibrillce — more regular in dentine than in bone — is longitudinal and parallel to the primary surfaces of the dentine. In addition to the fibres which extend lengthwise, others run obliquely crosswise in the layers of dentine. The bundles of fibrillae measure from .002-. 003 mm. in diameter, and appear in transverse sections as small punctated fields. The fibrillae are knit together by the calcified organic matrix, in which the lime salts are deposited in the form of spherules, the interstices between which are later filled and calcification thus completed. When, as often happens, the latter process is imperfect, irregular clefts, the interglobular spaces, remain, the con- tours of which are formed by the spheres or dentinal globules of calcareous material. The interglobular spaces are of irregular form and uncertain extent, being usually largest in the crown. At the border between the dentine and the ce'mentum there exists normally a distinct zone, the granular layer of Tomes (Fig. 1311). composed of FIG. 1310. Pulp-tissue Granular layer of dentine Cementum Alveolar periosteum Transverse section of root of lower canine tooth. X 30. closely placed interglobular spaces of small size ; under low magnification in ground- sections the spaces appear as dark granules, hence the designation of the zone. Since the existence of these spaces depends upon imperfect calcification of the intertubular ground-substance, the dentinal tubules are unaffected and pass through the spaces on their course to tin- surface of the dentine, several of the canals traversing the larger spa. The contour lines of Owen, or the incremental lines of Sailer, appear as linear markings, which usually run obliquely to the surface of the dentine (Fig. 1306). They probably depend upon variations in calcification incident to the growth of the dentine, and resemble the interglobular spaces in their origin. The contour lines •re bi-st marked in the crown and are only exceptionally seen in the fang. As pointed out by WalkhotT. tin- lines i if Owen and those of Retzius in the enamel are usuallv present at the same time, since both are expressions of imperfect calcification. The Cementum. — The cement, or crusta petrosa of the older writers, forms an investment of slightly modified osseous tissue from the neck of the tooth to its STRUCTURE OF THE TEETH. 1553 Dentine apex. Beginning where the enamel ceases, or overlapping the latter to a small extent, as a layer only .02-. 03 mm. thick, the cement gradually increases in thick- ness until over the root, especially between the fangs of the molars, its depth reaches several millimetres. When well developed the cement usually presents two layers, — an inner, almost homogeneous stratum next the dentine, in which the cement-cells are absent, and an outer supplemental layer which exhibits the appearance of true bone- tissue. The ground-substance of cemen- tum differs from that of ordinary bone FIG. 1311. in containing, according to Bibra, slightly less organic matter and a great number of fibre-bundles that extend vertically to the lamellae, corresponding to Sharpey's fibres. The lacunae are larger than those of bone and vary greatly in their number and form ; their processes, the canaliculi, are unusually long and elaborate. As in bone, so these lymph-spaces contain con- nective-tissue cells, the cement-corpuscles. The lamellae are so disposed that the lacunae lie generally parallel with the long axis of the tooth, their processes extend- ing vertically to the free surface. While connecting with one another by means of the canaliculi, the lacunae very rarely communicate with the dentinal tubules, the latter terminating in blind endings. The union between the outer surface of the cement and the pericementum is in- timate, since the latter is in fact the alve- olar periosteum from which the cement was derived ; this close relation is indi- cated by the roughness which the outer surface of the cement presents when macerated. Although at times feebly developed under normal conditions, typical Haversian canals are found only in con- ditions of hypertrophy. The Alveolar Periosteum. — The periosteum investing the jaws likewise lines the sockets receiving the roots of the teeth, which are by this means securely held in place. The name pericementum is often applied to this special part of the peri- osteum, which clothes the alveoli on the one hand and covers the cement on the other, thereby fulfilling the double role of periosteum and root-membrane. The latter consists of tough bundles of fibrous tissue, elastic tissue being almost want- ing, which are prolonged into the penetrating fibres characterizing the cementum on one side and into the fibres of Sharpey of the alveolar wall on the other. The fibrous bundles run almost horizontally in the upper part of the root, but become more oblique towards the apex of the fang. In the latter situation the pericemen- tum loses its dense character and becomes a loose connective tissue through which the blood-vessels and nerves pass to reach the tooth. The less dense portions of the root-membrane between the penetrating bundles of fibrous tissue contain, in addition to the vessels and nerves, irregular groups of epithelial cells which appear as cords or net-works within the connective-tissue stroma. These groups are the remains of the epithelial sheath which surrounded the young tooth during its early development. They have sometimes been described as glands, lymphatics, and other structures, their true nature being unrecognized. At the alveolar mar- gin the pericementum is directly continuous with the tissue composing the gum, the fibrous bundles being so disposed immediately beneath the enamel-border that they form an encircling band of dense fibrous tissue, the ligamentum circulare dentis of Kolliker, which aids in maintaining firmer union between the tooth and the alveolar wall. Granular layer of Tomes Cementum Lacuna Ground-section of root of dried tooth including adjacent dentine and cementum. X 300. 1554 HUMAN ANATOMY. The Pulp. — The contents of the pulp-cavity is the modified tissue of the mesoblastic dental papilla remaining after the completed formation of the dentine. The major part of the adult pulp consists of a soft, very vascular connective tissue containing few or no elastic elements, but numerous irregularly distributed cells of uncertain form. The general type of the tissue resembles the embryonal, both in the character of the fibrous tissue and of the cells, which are round, oval, or stellate with long processes. The fibrous bundles and the more elongated cells are most regu- larly disposed around the blood-vessels and nerves, which they invest in delicate fibrous sheaths. The peripheral zone of the pulp, next the dentine, presents the greatest special- ization, since in this situation lie the direct descendants of the dentine-producing cells, the odontoblasts. FIG. 1312. Dentine In this locality the pulp, especially in older teeth, presents three layers. The outer (.04-. 08 mm. thick) consists of several rows of large cylindrical elements, of which the most superficial are arranged vertically to the free surface of the pulp, after the manner of an epithelium. These are the odontoblasts, now no longer active, about .025 mm. in length and .005 mm. broad, which send out long, delicate processes (the dentinal fibres) into the den- tal tubules externally, and shorter ones towards the pulp-tissue. When very young they probably possess also lateral processes. The deeper cells of the odontoblastic layer are less regularly disposed and less cylindri- cal in form. The second, or Weil' s layer, best seen in older teeth, is characterized by absence of cells, the fibrous tissue and the cell-processes forming a clear, cell-free zone which separates the striking layer of odontoblasts from the subjacent third or in- termediate layer. The latter consists of nu- merous small round or spindle-cells, closely placed, but irregularly disposed, which grad- ually blend with the ordinary pulp-tissue. The blood-vessels supplying the pulp are from three to ten small arteries which soon after entering the pulp-cavity break up into very numerous branches from which a rich capillary net-work is derived. In human teeth the capillaries usually do not invade the layer of odontoblasts, although at times the vascular loops may extend between these cells. The venous radicles form larger veins which follow the course of the arteries. Lymphatics have been demonstrated as networks within the pulp. The nerves are numerous, each fang receiving a main stem and several additional smaller twigs, which in a general way accompany the blood-vessels in their coarser distribution. On reaching the crown-pulp the larger twigs are replaced by Finer brandies, which divide into innumerable interwoven fibres. The latter, on reaching th<- margin of the pulp, form a peripheral plexus beneath the layer of odontoblasts, fn.ni which terminal iion-mednllated tibrilhe are given off. Some of these end beneath the odontoblasts in minute knot-like swellings ; others penetrate the odonto- blastic layer to terminate in pointed free endings. There is no trustworthy evidence supporting the view that the nerves directly communicate with the odontoblasts, but they have been traced into the dentinal tubules. IMPLANTATION AND RELATIONS OF THE TEETH. The Permanent Teeth. — Each fang is implanted in a socket corresponding to it in shape, so that the pressure is transmitted from the surface of the conical fang throughout, except at the very tip. which has a hole for the vessels and nerves. A corresponding hole in the socket communicates with the dental canals. The human Section of periphery of pulp-tissue of young tooth. X 175- IMPLANTATION AND RELATIONS OF THE TEETH. 1555 teeth are all in contact with their neighbors, there being no break or diastema in the upper jaw between the incisors and canines for the points of the canines of the lower jaw. The canines project very little beyond the line of the free edges. The crowns increase in size from the incisors to the first molars and then decrease. The ver- tical distance from the gum to the free edge regularly diminishes from the median incisors backward, with the exception of the canines. The lines of the teeth above and below are practically of the same length. When the mouth is closed the superior canines lie to the outer side of the FIG. 1313. inferior ones, opposite the ends of m b c the lips ; thus the median upper incisors impinge on both the lower ones of the same side, and the upper lateral incisors strike both the lower lateral and the canine. In the same way the point of the cusp of the upper first bicuspid rests between the points of both the inferior ones, and that of the second on both the second lower and the first molar. The first upper molar has, perhaps, a quar- ter of its grinding surface on that of the inferior second molar, but a smaller part of the second upper molar rests on the lower wisdom- tooth. The smaller size of the upper wisdom-tooth brings its posterior border into line with that of the lower. This arrangement causes the opposed crowns to interlock to a certain extent, but not so closely that grinding movements cannot occur between them. The advantage of each tooth coming in contact with two is evident after the loss of a tooth, as the one cor- responding to it is not rendered useless. In the upper jaw the incisors have a marked FIG. 1314. Dental arches seen from before. Letters in this and subsequent cuts indicate the groups of teeth : i, incisors ; c, canines ; b, bicus- pids ; m, molars. Dental arches seen from behind. forward inclination, and overlap the lower, concealing nearly a third of their crowns, the mouth being closed. The crowns of the upper bicuspids look pretty nearly downward and those of the molars slant outward. This is very marked in the wisdom-tooth and may be very slight in the first molars. The lower incisors have the front surfaces nearly vertical ; the molars have an inward slant, so as to bring their axes into the same line 1556 HUMAN ANATOMY. FIG. 1315. Dental arches seen from the side, showing relations of upper and lower teeth. as those of the upper ones ; hence it follows that the alveolar arches of the upper and lower teeth are in different curves, the latter having a great transverse distance between the necks of the wisdom-teeth. The right half of the jaw is usually the stronger and the teeth form a smaller curve It has been pointed out in the section on the motions of the lower jaw that the line between the molars, and probably the bicuspids, is a part of the circumference of a circle the centre of which is near the top of the lachrymal bone ; it may now b added that the line of the cutting edges of the lower incisors is a part of a transverse curve with the convexity upward. There is no corresponding concavity in the line of the edges of the upper incisors, for the lower do not naturally meet them ; but the convexity plays along the lingual surfaces of the upper ones. The position and shape of the superior incisors make their inner surface a part of a vault. A transverse section of this is necessarily a curve with an upward convexity. The wearing ot the outer corners of the lateral incisors is evidence of this 'action. The fact that there is no purely lateral motion, but an oblique one, modifies, without invalidating, this con- ception. The relations of the roots of the su- perior teeth to the antrum are very impor- tant. The incisors have no relation with it whatever. The long fang of the canine is opposite the wall between the antrum and nose, and separated by diploe from the former. The first bicuspid is usually sepa- rated in the same manner. The second is very close to its front wall and may indent the floor. The first and second molars always do this. The wisdom-tooth also in- dents it at the junction of the floor with the posterior wall. Its relation, owing in part to its varying development, is less certain. Exceptionally the first bicuspid and even the canine may be in contact with the antrum. Thus caries of the roots of any of the molars, but especially of the first and second, sometimes of the second bicuspid and exceptionally of the first, or even of the canine, may lead to inflamma- tion of the antrum. In certain cases pus may pass directly into it from the root. The Temporary Teeth. — In the first dentition the dental arches differ from the permanent ones in showing a broader curve, more nearly approaching half a circle, symmetrical on both sides, in having the upper incisors less slanting, and the molars of each row more nearly vertical. This implies less difference in curve between the jaws. The line of meeting of the teeth is more horizontal. The crowns increase in size from the incisors backward. In the young child the antrum is but a small pouch, and the roots of the first teeth and the sacs of the second lie in diploetic tissue. The first permanent molar, as its fangs grow, is nearest the antrum, having extended above it by the end of the second year. In its early stages the first bicuspid is too far forward to have any relation to the antrum, and the second reaches only its extreme anterior border. The second permanent molar is at first behind rather than below it, and the third is still higher. As these descend they swing around the antrum. Thus the roots of only the first permanent molar are in approximately the same relation to the antrum throughout DEVELOPMENT OF THE TEETH. About the beginning of the seventh week of foetal life the ectoblastic epithelium presents a thickening along the margins of the oral cavity. The ridge-like epithelial proliferation, or lahio-dt-ntal strand, so formed grows into the surrounding mesoblast ami >'• X^ >s.V > ..>.•;•• D i!vlM_ ^ : ' ' , , ;,;. ^ — — 1^ Outer layer ' ' ' ' ' • ' of c-name! ; '.. '.;,, "ixan Inner layer of enamel- organ Dental . papilla ES(LQjf^SK^ I'j—Mi.lille laver IVntal papilla i Kpillu-lial .'.' sheath Frontal sections, showing four early stages of tooth-development. A, B, X 100; C, D, X 60. 1.1.i->t Ix-ncath ilx- epithelial iiionuvtli. The papilla consists for a time of a close -ati.ui ..(' small, round, proliferating r«-lls ; with the diftVivntiation of the layers of the enaiiK-1 oi-Maii, the elements occupying the periphery of the dental papilla I.e. ..me elon-atet/,>n/,i/n',^2;.;~1 Enamel ; Epithelial sheath— % Position of mesoblastic dental papilla Reconstruction of developing lower incisor tooth from embryo of 30 cm. length, about twenty-four weeks. (Drawn from Rose's model.) enamel-organ. The investment thus formed constitutes the epithelial sheath ( Fig. 1320), a structure of importance in determining the form of the tooth, since it serves as a mould in which the young dentine is subsequently deposited ; there is, however, insufficient evidence to regard the epithelial sheath as an active or necessary factor in the production of the dentine. The formation of the enamel, in contrast to that of the dentine, results from the activity of ectoblastic epithelium, and may be regarded as a cuticular development carried on by the ameloblasts. The earliest stage in the production of enamel is the appearance of a delicate cuticular zone at the inner end of the ameloblast ; this fuses with similar structures tipping the adjoining cells to form a continuous homo- geneous mass. The latter soon exhibits differentiation into rod-like segments, the enamel-processes, or processes of Tomes, which are extensions from the ameloblasts and are the anlages of the enamel-prisms, and the interprismatic substance. The latter becomes greatly reduced in amount as the development of the enamel-columns progresses ; the major part, becoming incorporated with the processes of Tomes, forms the cortical portion of the enamel-prisms, while the remainder persists as the cement-substance which exists in meagre quantity between the mature prisms. The enamel-processes are for a time uncalcified, but with the more advanced formation of the enamel-prisms the calcareous material, which is deposited as granules and spherules, appears first in the axis of the prism, later invading the periphery (Ebner). The 1562 HUMAN ANATOMY. enamel increases in thickness by the addition oi the last-formed increments at the inner ends of the umeloblasts, the same cells sufficing for the deposit of the entire Owin<' to the expansion of the external surface of the crown, the diameter ol mass. FIG. 1321, Intermediate layer of enamel-organ meloblasts .Young enamel with Tomes's processes Dentine Last-formed dentine Odontoblasts Embryonal pulp-tissue FIG. 1322. Section of developing tooth through junction of enamel and dentine. X 4°°. the enamel-prisms augments towards their outer ends to compensate for the increased area which they must fill, since no additional prisms are formed. The complex curvature of the enamel-prisms and the oppositely directed ranges of the latter, producing the appearance of Schreger's stripes, result from changes in the position of the enamel-cells incident to the growth of the crown, since the axes of the newly formed prisms correspond with those of the ameloblasts, variations in the direction of which affect the disposition of the enamel-columns. The earliest formed enamel lies in close apposition with the oldest dentine con- stituting the membrana praeformativa ; the last devel- oped immediately beneath the ameloblasts. The enamel, therefore, is deposited from within outward, or in the reversed direction followed by the growth of the dentine. The oldest strata of both substances lie in contact ; the youngest on the extreme outer and inner surfaces of the tooth. After the requisite amount of enamel has been pro- duced, differentiation into prisms ceases, in consequence of which the last-formed enamel remains as a continu- ous homogeneous layer investing the free surface of the crown, known as the membrane of Nasmvth. The Tooth-Sac. — Coincidently with the develop- ment of the enamel organ and the growth of the dental papilla, tin- surrounding mesoblast undergoes differen- tiation into a connective-tissue envelope known as the dental or to, till sac. Tin- Litter not only closely invests the enamel-organ, but is intimatelv related to the base of the dental papilla, with which it is continuous. In •or.ii. i, i to die epithelial enamel-organ, which is entirely without blood-vessels, the Isolated anirlc>H:i-.ts frmu in- ..I MI w limn . hild. ,/, li;i--:il plate; /'. iiitiiul.it litmli-r; i, pro- ,/. li..ini>Ki-iu-i.iis ! ill i "\ i-lllii; plm-rss. • .(i«i. DEVELOPMENT OF THE TEETH. 1563 inner part of the tooth-sac is richly provided with capillaries, and therefore is an important source of nutrition to the developing dental germ. The part of the sac opposite the root of the young tooth is at first prevented from coming into direct contact with the dentine by the double layer interposed by the epithelial sheath. This relation is maintained until the development of the cement begins, when the vascular tissue of the dental sac breaks through the epithelial sheath to reach the surface of the dentine, upon which the cementum is deposited by the mesoblast. In consequence of this invasion, the epithelial sheath is disrupted into small groups or nests of cells which persist for a long time as epithelial islands within the fibrous tissue of the alveolar periosteum into which the dental sac is later converted. The formation of the cementum takes place through the agency of the mesoblastic tissue in a manner almost identical with the development of subperiosteal FIG. 1323. Jaws of child of six years, showing all temporary teeth in place with permanent teeth in various stages of development. bone, the active cement-producing cells, or cementoblasts, corresponding to the osteo- blasts which deposit the osseous matrix upon the osteogenetic fibres of the periosteum. A conspicuous feature of cementum is the unusual number of transversely disposed bundles of fibrillae, or Sharpey's fibres, among which many are imperfectly calcified. The cementum appears first in the vicinity of the neck of the toqth, and progresses towards the apex of the root as the dentine of the fang is deposited. After the tooth is fully formed, the layer of cement continues to grow until thickest at the apex, which it completely invests, with the exception of the canal leading to the entrance of the pulp-cavity. The cement being deposited directly upon the homogeneous layer con- stituting the external surface of the dentine, the firm connection between the two portions of the teeth is one of adhesion rather than of union. Later secondary changes may exceptionally bring the canaliculi of the cement into communication with the terminations of the dentinal tubules. During the changes incident to the 1564 HUMAN ANATOMY. completed tooth-development the tissue of the dental sac becomes denser, the part opposite the root persisting as the pericementum which intimately connects the cementum with the alveolar wall, while the more superficial part blends with the tissue forming the gum. The development of the permanent teeth is early provided for by the dif- ferentiation of the anlages of the secondary dental germs during the growth of the first. This provision includes the thickening and outgrowth of the dental bar to form the enamel-organ of second dentition, and later the appearance of a new dental pa- pilla beneath the epithelial cap. The enamel-organ for the first permanent molar appears about the seventeenth week of foetal life, followed soon by the corresponding dental papilla. The germs of the permanent incisors and canines, including the papillae, are formed about the twenty-fourth week ; those for the first bicuspids are seen at about the twenty-ninth week, and those for the second bicuspids about one month later. The interval between the formation of the enamel-organ and the asso- ciated dental papilla increases in the case of the last two permanent molars. While the enamel-germ of the second molar appears about four months after birth and the corresponding papilla two months later, the enamel-organ for the third molar, or wisdom-tooth, which is visible about the third year, precedes its papilla by almost two years. The First and Second Dentition and Subsequent Changes. — At birth the jaws contain the twenty crowns of the milk-teeth, the still separate cusps of the first permanent molars, one of which has begun to calcify, and the uncalcified rudi- ments of the permanent incisors and canines behind and above the corresponding milk-teeth of the upper jaw, behind and below those of the lower. At birth the bony plate above the alveoli of the upper jaw is separated by a little diploe from the floor of the orbit. The milk-teeth come through the gum in five groups at what are called dental periods, separated by intervals of rest. The grouping is more regular than the time of eruption. The teeth of the lower jaw have a tendency to precede their fellows of the upper. TABLE OF ERUPTION OF MILK-TEETH.1 Dental Periods. Groups of Teeth.. I. Six to eight months. Two middle lower incisors. II. .Eight to ten months. Four upper incisors. III. Twelve to fourteen months. Two lateral lower incisors and four first molars. IV. Eighteen to twenty months. Four canines. V. Twenty-eight to thirty-two months. Four second molars. The interval between the first and second periods is practically nothing. It is very common to have the first two groups appear together. After this every interval is longer than the preceding one. In the matter of time no part of development is more irregular than that of the teeth. The first incisors occasionally appear early in the fifth month and sometimes not till the tenth, or even later. The first dentition is sometimes complete at or shortly after the close of the second year. The roots are m»t fully formed when the crowns pierce the gums. The first set of teeth is in its most perfect condition between four and six years. ( 'alcitication of the second set begins in the first molar before birth, in the incisors and canines at about six months, the bicuspids and the second upper molar in the third year, the second lower molar at about six, and the wisdom-tooth at about twelve. The first ]>< rmanent molars come into line with the milk-teeth, piercing the gums b.-i,,iv any < •!" tin- Litter are lost. Before eruption the upper first molars lie nearer the median line and farther forward than the lower. The roots of the incisors are absorbed and the crowns fall out to make way for their successors. The molars do the same f..r the bicuspids which grow between their roots. The permanent superior canines are developed al.ove the interval between the lateral permanent incisors and the first Dl< USpid, which are almost in contact. An expansion of the jaw is necessary for them to come into place. The inferior ones have more room. Both are somewhat external to their predecessors. The second upper molar comes down from above and behind, 1 1 i.-iii Kou-h's Pediatrics. DEVELOPMENT OF THE TEETH. 1565 and so does the wisdom-tooth much later. The inferior second molar is formed almost in the angle between the body and ramus. The inferior wisdom-tooth, before it cuts the gum, faces forward, inward, and slightly upward. To the table from FIG. 1324. Permanent molars Permanent molar Permanent canine Bicuspids Permanent incisors Temporary canine Temporary molars Permanent incisors Temporary canine Permanent canine Bicuspids Jaws of child of ten years, showing partially erupted permanent teeth with temporary canines and molars still in place. Rotch we add one from Livy,1 who made observations on several thousand children of English and Irish operatives. TABLE OF ERUPTION OF PERMANENT TEETH.2 Years. Groups. 6 Four first molars. 7 Four middle incisors. 8 Four lateral incisors. 9 Four first bicuspids. Years. Groups. 10 Four second bicuspids. 11 Four canines. 12 Four second molars. 17 to 25 Four wisdom-teeth. TABLES SHOWING TIME OF ERUPTION OF PERMANENT TEETH.3 BOYS. Ages. 9 10 ii Lateral incisors .... 2 42 9 First bicuspids i 76 12 Second bicuspids .... . . 59 36 Canines . . . Second molars 1 British Medical Journal, 1885. i8 28 25 5 42 67 275 184 78 12 663 16 Total. 59 90 101 79 2 From Rotch. 3 From Livy. I566 HUMAN ANATOMY. GIRLS. AKes. 9 jo ii 12 13 14 '5 l6 Total. Lateral incisors 24 8 4 First bicuspids 56 13 2 I I • • • • 73 Second bicuspids 51 16 2 2 Canines 3" 34 12 5 • • \ Second molars 5 44 80 288 249 66 14 746 ( It seems possible from the method employed that, especially in the case of the second molars, the tables may err on the side of overstating the age. ) Livy's researches show that in the first dentition the first molars, incisors, and canines come through first in the lower jaw. In m, n theory, according to which the many cusps of the molars have arisen as outgrowths from a primitive cone. This is based on comparative anatomy and paleontology. According to this, there was first the cone, in the upper jaw called the protocone and in the lower \\\e prolo- conid. Two secondary cusps next appeared respectively before and behind it : the paracone and metacone of the upper teeth and the paraconid and metaconid of the lower. The next change is for these to move to the labial side in the upper jaw and to the lingual in the lower. Thus the primitive cone and these two secondary ones form the points of a triangle with the base outward in the upper jaw and inward in the lower. A prolongation, the talon or heel, is next developed on the posterior end of the tooth, and rises into a single cusp, the hypocone in the upper jaw and the hypoconid in the lower. The last, however, has two secondary cusps spring from it, the cntoconid and the hypoconid. According to this theory, the paraconid of the lower teeth has disappeared in the human molars owing to want of room consequent on the develop- ment of the talon of the upper teeth. The following table shows the homologies of the cusps of the human molars according to Osborn. UPPER MOLARS. Anterior lingual Protocone. \ Anterior buccal Paracone. J- Forming the triangle. Posterior buccal Metacone. j Posterior lingual Hypocone. The talon. LOWER MOLARS. of triangle. Posterior buccal Hypoconid. ~\ Posterior lingual Entoconid. > The talon. ior Hypoconulid. J lias advanced, in support of his theory of concrescence, that calcification begins sepa- rately for each cusp. < (shorn points out that Rose has shown that they ossify very nearly in tin- order of their alleged evolution. Schwalbe8 professes himself unable to decide on the relative merits of the t\\o theories. Variations. Variations of the cusps and of the fangs ha\e been described with the teeth. Those of number affect chielly the incisors and molars. An additional incisor may occur on one or both sides iu either dentition, not verv rarely in the upper jaw, but extremely SO in the louet, tlie condition in the latter being more stable. K\tra upper incisors are often more or less displaced to the rear and implanted obliquely. They are particularly common in cases of cleft palate; not impossibly the presence of additional teeth predisposes to the non-union of the 1 Aiiatoin. An/, i-er. I'.d. \ ii., r8o9. "• [.-naische /eitschrift, I'.d. \\viii., 1893. ;1 loiirnal of Morphology, isss. iS.s,,. 4 \mei-ic.ni Naturalist, isss, and International Dental Journal, 1895. 5 Anatom. An/ek'.er, |id. jx., 1894. THE GUMS. 1567 premaxillary and the maxillary bones, or to the non-union of two parts of the former, supposing that two such parts really exist. The extra incisor may apparently appear on the median side of the first, between the first and second, or between the latter and the canine. To account for this Rosenberg1 asserts that the typical number is five, as in the opossum, of which the second and fourth are the two persistent ones, and that either the first, third, or fifth may occasionally present itself. Th. Kolliker 2 records a case of right cleft palate in which, besides the four regular incisors, three were found between the cleft and the right canine. As cases of excess of incisors are much more common than of deficiency, the disappearance of the upper lateral one does not seem imminent ; still, there are signs of degeneration. The crown is less square than that of the central, it is occasionally pointed, often unusually small, sometimes not reaching the line of the other crowns. It may be absent, and then a series of cases can be made ranging from those in which the remaining incisor is separated both from its fellow of the other side and from the canine beside it by large gaps to those in which the teeth are regular and continuous. Very rarely one of the lower incisors is wanting, and, according to Rosenberg, either may fail. A fourth molar is very uncommon ; but not at all rarely the wisdom-tooth is late in coming through the gum, and occasionally it never does. It seems sometimes to be wanting and often is rudimentary. It has been seen represented by three detached cusps, an apparent confirmation of Rose's views of the homology of the teeth. The entire dental series may be unusually large or small. In the former case the face is prognathous, probably as a result of the increase of space required for the teeth. The upper central incisors are occasionally very large without increase in size of the other teeth. The same is true of the molars ; in which case the number of cusps is generally greater, but the converse does not occur when the molars are unusually small.3 The points of the canines may project beyond the line of the other teeth and the molars may increase in size from the first to the third. Teeth are sometimes remarkably displaced. The superior canines, owing to their high origin in the second dentition, are particularly subject to it. They may appear on the front of the jaw, in the antrum, the nose, or the back of the mouth. The molars, and especially the wisdom-teeth, are also erratic. THE GUMS. This term is used rather vaguely to indicate the mucous membrane and sub- mucous tissue covering the alveolar processes and closely attached to the necks of the teeth. Whether the neck is entirely surrounded by it varies in different indi- viduals as the teeth are not in all equally close ; as a rule, owing to the ordinary expansion of the crown from the neck, at least a little of the gum is found between the teeth. It is some 3 mm. thick, dense, firmly fastened to the bone, and is neither very vascular nor very sensitive. In structure the gums resemble other parts of the oral mucous membrane, con- sisting of the epithelium and the connective-tissue layer. The latter, directly con- tinuous with the periosteum of the alveolar border and the pericementum, is composed of closely fitted bundles of fibrous tissue and beset with numerous papillae. On young teeth the epithelium is prolonged for from .5-1 mm. over the enamel and often for a short additional distance over the cement, ending in an abrupt margin. In the immediate vicinity of the tooth the papillae sometimes exhibit infiltrations of lym- phoid cells. The gums are without glands. The structures sometimes described as such, as the "glands of Serres," consist of nests of epithelial cells derived from the remains of the atrophic embryonal epithelial sheath (page 1563). THE PALATE. The Hard Palate. — The shape and proportions of the hard palate have been discussed with the bones (page 228), so we have here to do only with its mucous covering. This is very firmly fastened to the rough surface of the bones by dense connective tissue which is particularly thick at the sides, doing much to fill up the angle between the roof and the alveolar process. On either side near the front, extending onto the inner surface of the alveolar processes, is a series of raised ridges (Fig. 1325), in the main transverse, although slightly convex anteriorly, the analogues of the palatal ruga of most mammals. They never extend behind the first molar tooth, are numerous and prominent in childhood, but much reduced in middle age, and occasionally wholly lost. 1 Morphol. Jahrbuch, Bd. xxii., 1895. 2 Nova Acte des Leopold. Carol. Akad. der Naturforscher, Bd. xliii., 1882. 3 Magitot : Traite" des Anomalies du Systeme Dentaire, 1887. 1568 HUMAN ANATOMY. Just behind the incisors, at or before the incisor canal, there is a small raised pad or fold of mucous membrane, on either side of which the orifice of the incisor canal \s often found. When pervious, it is very minute, admitting merely a bristle. Behind this the palate presents a median raphe of paler color than the rest, which may FIG. 1325. Orifices of palatine glands Incisor pad with orifice of incisor canal Raphe Mucous membrane removed to show layer of glands Soft palate Uvula Superior dental arch and palate ; palatal rugae occupy anterior part. Soft palate partially cut away. run to the root of the uvula or may stop short of it, being often deflected to the left. A little behind the pad this line may be interrupted by a pale oval elevation or more often a depression. The membrane of the roof of the mouth is nowhere bright red ; that of the hard palate, however, is paler than the rest. There are no glands in the oval white space, but there is a continuous layer on either side of it. The orifices of the glands are easily seen with a lens, sometimes with the naked eye. A little in FIG. 1326. oft palate Supratonsillar fossa Uvula Posterior pillar of fauces Epiglottis Muscular fibres of tongue Dorsal surface of Anterior pillar of fauces Tonsil Sagittal section through p.ilat.-, uvvila. and tongue, showing right lateral wall of fauces; tongue has been pulled downward by honk fr.int «>f the origin of the s, ,ft palate the mucous membrane becomes deeper colored. Thc-sc differences in color an- more striking in children. The Soft Palate.— This structure consists of a fold of mucous membrane, con- tinuous with the hard palate, enveloping several layers of interlacing muscular fibres, at lent i cm. in thickness at its origin. Its lower border is the edge of the fold. THE PALATE. 1569 This is concave on each side, and presents a median elongation, the uvula, which varies from a short prominence to a cord 2 cm. in length. Thus the palate has a lower surface looking downward and forward and an upper one looking upward and backward. When the mouth is closed the palate and uvula rest against the tongue ; when open they hang free, but the muscles inside can modify their shape and position. Median sections show the tip of the uvula often reaching within half FIG. 1327. Pharyngeal mucous membrane Azyg°s uvulae Tendon of tensor palati \ Artery / Levator palati Palato-pharyngeus Masses of glands Oral mucous membrane Transverse section of soft palate near its anterior attachment. X 4. an inch of the tip of the epiglottis. Possibly muscular relaxation allows it to descend somewhat farther than in life, but it is certain that no very great elongation is neces- sary for it to touch that organ and give rise to great discomfort. The soft palate can be raised so as to touch the back of the pharynx and close all communication between the nose and the mouth. Two folds, the pillars of the fauces, each the reflection of the mucous membrane over a muscular bundle, start from the palate on either side. The anterior pillar, enclosing the palato-glossus muscle, arises from the front of the palate near the uvula, some distance anterior to the edge, and, curving downward, runs to the tongue at the junction of the middle and posterior thirds, separating the mouth from the pharynx and forming the posterior border of the sublingual space. The posterior pillar starts from the lower border of the palate on either side of the uvula, covering \\\zpalato-pharyngeus, and runs down the throat to the superior cornu of the thyroid cartilage, the lower part being indistinct. Some of the muscular fibres within it go to the upper border of the thyroid cartilage in front of the horn, but the fold is not often found so low, except in frozen sections, in which it appears at the sides of the back of the pharynx. A deep triangular recess on either side, between the anterior and posterior pillars, contains the tonsil. This region is often vaguely described as the isthmus of the fauces, one being left in doubt whether it belongs to the pharynx or to the mouth. In the preceding pages the pharynx is described as beginning at the anterior pillar. The reasons for this divi- FIG. 1328. sion are developmental, Fibres of azygos uvulae Pharyngeal mucous membrane morphological, and phys- iological. The part of the tongue anterior to this fold is of mandibular (buccal) origin, while the part behind it comes from the pharynx. The sur- Masses of glands Oral mucous membrane face of the former is SUp- Transverse section of soft palate near base of uvula. X 4. plied by the mandibular nerve, the third division of the fifth, and the latter by the glosso-pharyngeal. The mucous membrane of the posterior third does not bear papillae (except the circumvallate papillae near the junc- tion of the two regions), but is rich in adenoid tissue and glands, differing in both respects from the part in front of it. The arrangement of the transverse fibres of the glosso-palati muscles in the substance of the tongue suggests a sphincter at the entrance of the pharynx. Finally, in deglutition it is in passing this line that the bolus ceases to be under the control of the will. 99 1570 HUMAN ANATOMY. The following layers compose the soft palate from above downward : ( i ) The pharyngeal mucous membrane. (2) A nbro-muscular layer. The fibrous portion is the expansion of the tendons of the tensor palati muscles. It is strong and tense near the hard palate, gradually dwindles lower down, and joins the pharyngeal aponeurosis at the sides. Below this is the complex of the muscles. (3) A glan- dular layer opening into the mouth. This is some 5 mm. thick at its origin and practically continuous throughout most of the palate. It is interrupted at the median line near the hard palate by a septum of muscular and fibrous tissue, is wanting near the free edge of the palate a little on either side of the root of the uvula, and is con- tinued down the uvula as a cylindrical string of glands nearly to the tip, through and about which run the fibres of the azygos uvula? muscle. Irregular glandular collections are found near the latter, especially at the base of the uvula. (4) A lower layer of mucous membrane. The mucous membrane of the soft palate is red on the pharyngeal and pale on the buccal surface ; on both sides it presents papillae, those on the upper surface FIG. 1329. «£v iiik/;^Giands .7; Jt-a* • \*r**S*'~ Glands Aponeurotic tissu Oral mucous membrane Obliquely cut muscles Sagitto-lateral section of soft palate. X 15. especially being near the base. The most common form, slender and elongated, is scattered over the entire buccal surface and the front of the uvula (Riidinger). Thicker short papilhe are also found near the beginning of the pharyngeal surface. Small adenoid collections occur on the upper surface, as well as small glands situated in the depth of the mucous membrane. The orifices of the chief glandular layer pierce the inferior palatal surface. The Muscles of the Soft Palate. — Some of the muscles arise in the soft palate : others run into it. Isolation of the individual sets of fibres is not always il>le. The tensor palati ( dilatator tubes} (Fig. 1330) arises from the scaphoid fossa at tin- root of the internal ptery^oid plate, from the spine of the sphenoid, and from the outer ineinlnanous part of the Knstarhian tube. It descends vertically along the internal pterv^oid plate as a round, red, and distinct muscle, which becomes tendinous a^ it turn> imvard under the hanmlar process at right angles to its previous course, alter which it broadens into the fibrous expansion in the soft palate already described, above the other muscles. A bursa lies between the tendon and the hamular process. THE PALATE. The levator palati (Fig. 1330) arises from the base of the skull at the apex of the petrous portion of the temporal bone and from the cartilaginous part of the Eustachian tube beside it. At first thick, it passes downward, forward, and inward with the tube, and, leaving it, expands into a layer which spreads out through the soft palate. Some of the anterior fibres from the tube go to the back of the hard palate, constituting the salpingo-palatinus, while others descend in the lateral wall of the pharynx, covered by mucous membrane, beneath the salpingo-pharyngeal fold. The great body of the fibres crosses the middle line in the front part of the soft palate. Most of them descend in the opposite side. Some seem to form loops with an upward concavity with fibres from the fellow-muscle. Near the hard palate this decussation completely divides the glandular layer (Fig. 1327). The azygos uvulae (Fig. 1331), although probably a double muscle originally, soon (even at birth) becomes practically a single one. Arising from the tendinous fibres of the tensor palati just behind the posterior nasal spine, it soon becomes mus- cular and increases in size. Its course is downward into the uvula, but on reaching the base it is already broken up into separate bundles which pass about and through FIG. 1330. Hard palate Hamular process Tensor palati Levator palati Soft palate (cut) External pterygoid plate Posterior nares Opening of Eustachian tube Cut edge of pharynx Mass of adenoid tissue Eustachian tube Fossa of Rosenmiiller (opened) Styloid process Occipital condyle Inferior surface of skull with upper part of opened pharynx and palatal muscles attached ; viewed from behind. the glandular core of the uvula. The belly of the muscle lies near the dorsal surface, between the fibrous expansion of the tensor palati and the levator palati, which decus- sates on its oral surface. The palato-pharyngeus (Fig. 1331) has a complicated origin in more than one layer from the border of the hard palate, from the lower surface of the apo- neurosis, and perhaps from fibres of the levator palati. Certain fibres, either arising in the middle line or coming from the other side, pass downward and outward over the azygos uvulae ; others lie beneath the glandular layer. Some of the fibres seem to continue the course of the salpingo-pharyngeus of the opposite side, with- out being directly continuous. The muscle passes down near the edge of the soft palate and then in the posterior pillar into the side of the pharynx, where it min- gles with the stylo-pharyngeus. A part is inserted into the upper border of the thyroid cartilage, and sometimes into the superior horn. It also expands, together with the stylo-pharyngeus, into a thin layer just beneath the mucous membrane of the back of the pharynx, which meets its fellow in the median line where it is inserted into the pharyngeal aponeurosis. Its lower limit is a curved line with the concavity looking upward and outward, behind the larynx (Fig. 1361). (This part 1572 HUMAN ANATOMY. of the muscle must be dissected from behind, after removing the constrictors of the pharynx.) ... The palato-glossus (Fig. 1339) is a small bundle arising from near the midd line of the oral side of the lower part of the soft palate, forming by its projection the anterior pillar of the fauces, in which it runs to the tongue, where it joins the trans- verse fibres. The pair of muscles act as a sphincter tending to close the passage from the mouth to the pharynx. A thin expansion from this muscle passes over the tonsil. Vessels. — The arteries of the palate (both hard and soft) come chiefly from the descending palatine, which, emerging from the posterior palatine canal, runs for- ward along the inner side of the base of the alveolar process. It sends a few branches FIG. 1331. Nasal septum Eustachian tube Salpingo- pharyngeus Levator palat Palato-pharyn External pterygoid Levator palati ___Tensor palati Internal pterygoid •f— Hamular process Tensor palati zygos uvulae 'osterior surface of tongue Palato-pharyngeus Stylo-pharyngeus Superior orifice of larynx Posterior crico-arytenoid OZsophagus Muscles of palate and pharyn rom behind ; pharynx laid open. inward and backward to the front of the soft palate, which is supplied on the side by a branch either from the facial or from the ascending pharyngeal. It is to be noted that no vessel is likely to interfere with the division of the tensor palati at the inner side <>f the hamular process. The -'fins of tin- hard palate follow in the main the arteries. Those of the upper side of the soft palate join the plexus of the zygomatic fossa. The larger ones of the under ^ide connect with the veins of the tonsil and the root of the tongue. The lyntphatii-* of the hard palate and of the under side of the soft palate form a rich plexn-. Those on the upper side of the latter are small. The chief current is to the deep glands of the neck. THE TONGUE. 1573 Nerves. — The tensor palati is supplied by the mandibular division of the fifth pair, the other muscles by the pharyngeal plexus. The mucous membrane of the hard palate is supplied by the anterior palatine nerve and terminal branches of the naso-palatine. That of the soft palate is supplied by the other palatine nerves and by branches from the glosso-pharyngeal. THE TONGUE. The tongue is a median muscular organ attached to the floor of the mouth, the symphysis of the jaw, and the body and both horns of the hyoid, covered with mucous membrane, which when the mouth is closed it practically fills (Fig. 1339). The root is the attached portion, extending from the hyoid to the symphysis, com- posed of the genio-glossi and the hyo-glossi muscles. The tip is the free anterior end, flat both above and below when extended, and surrounded by mucous mem- brane. Behind this the tongue is a solid mass. The dorsum in its anterior two- thirds is convex from side to side, and rests against the hard and soft palates ; the posterior third, nearly vertical, looks backward, forming the front wall of the pharynx when the mouth is closed. There is a median groove in the upper part of this pos- terior third, continued for a little distance onto the top, in which the uvula rests. This hind portion is so broad that the edges of the tongue reach quite to the sides FIG. 1332 FIG. 1333. Floor of the mouth and pharynx of an embryo of 7.5 mm. (From a reconstruction.) cop., copula; P., furcula; /., anlage of the body of the tongue; Ti., tuberculum impar; l-III, branchial arches. Plica mediana Plica sublingualis t Plica-l fimbriata i Fibres of- genio-glossus Under surface of tongue of new-born child. (Gegtnbawr.) of the pharynx. In the anterior two-thirds the edges of the tongue are prominent, overhanging the sides. Development shows that the tongue has a double origin. The body arises from a paired anlage near the midline in the anterior part of the mouth, while the root develops from a median elevation, the copula, and the adjoining portions of the second visceral arches. The tuberculum impar of His plays probably only a sub- ordinate role. The thyro-glossal duct comes to the surface at the junction of these two parts, which, in the infant, are still separated by the sulcus terminalis. The mucous membrane of the lateral and inferior surface is thin and smooth with small papillae at the tip. In the middle it forms a fold, the frenum, running from near the tip to the floor of the mouth. In infancy this is occasionally so short as to restrain the tip of the tongue from the motions necessary for nursing. Often it is hardly visible. The plica fimbriata and the plica sublingualis are two folds on either side of the front part of the under surface, of which the former with ragged edges is the outer, the longer, and the larger. Both are distinct in the infant and (especially the latter) lost or poorly marked later. The plicae fimbriatae bound a triangular space which Gegenbaur considers a rudiment of the under-tongue of some mammals. The mucous membrane of the dorsum is divisible into two wholly differ- ent regions : the one comprising the anterior two-thirds, the other the posterior ver- tical third. The line of separation, or sulcus terminalis, is, however, not transverse, but, starting at the side from the anterior pillar of the fauces, runs backward and inward to meet its fellow. This is not usually visible in the adult ; but its place is 1574 HUMAN ANATOMY. asilv recognized, as just before it is a V-shaped arrangement of circumvallate papillae the medbn apex being at or near a small depression, the foramen '.ccecum which mark™ Une termination of the foetal duct through the tongue from the thyroid. In the adult this may be a short tunnel or a depression, into which the ducts of several glands open. According to Munch,1 it is always behind the hindmost circumvallate FIG. 1334- Cut zygoma Pterygoid plates Eustachian tube Cut and reflected __ SOft nnl:lte__— IK- Half of uvula Anterior pillar of fauces Posterior pillar of fauces Spine of phenoid Zygoma Fossa of Rosenmuller ^^_ Posterior wall of pharynx >ft palate cut and turned aside Lymph-nodules constituting lingual tonsil Anterior portion of head has been removed by frontal section passing through plane of posterior nares ; the soft palate i-ut in mill inn- .in.i (uiii.-.i .IM.IL-, exposing posterior wall of pharynx ; ton.uiR- drawn forward and downward. papilla. Tin- mucous membrane covering the dorsum of the tongue is closely beset with elevations, or />///,/-, of which there are three varieties, the filiform, fungiform, an-1 cin-uiuvallate. In general they consist of a core of connective-tissue stroma cov- en-d with stratified squamous epithelium ; the projection formed by the connective • ue bean minute secondary papilla-, which, however, do not model the free sur- ;.; Arbeiten, I'-d. vi., THE TONGUE. J575 face of the mucous membrane. The anterior two-thirds of this surface are rough with fungiform and filiform papilla ; the former, less numerous, appear as red points chiefly near the edges, while the filiform are everywhere, but arranged in par- allel rows continuing forward the lines of the circumvallate papillae. At the edges of the tongue, just in front of the end of the anterior pillar of the fauces, close inspec- tion, especially with a lens, will generally show a small series of minute transverse parallel ridges, corresponding to fat papilla foliatce of rodents in a rudimentary con- dition. The papilla circumvallate are fungoid papillae surrounded by a depression bounded externally by a low annular wall. The usual number of these papillae is from nine to ten, ranging from six to sixteen (Munch). The sides of the V in which they are disposed are not very symmetrical. Usually there is at least one median papilla behind the apex, and very rarely one or two before it. The circumvallate papillae are of especial interest as being the most important seat of the gustatory end- 1335 Filiform papilla tissue of Section of lingual mucous membrane, showing filiform and fungiform papillae. X 75- organs, or taste-buds, which lie embedded within the epithelium lining the groove encircling the central elevation. A detailed description of the taste-buds is given with the organs of special sense (page 1433). The surface of the vertical posterior third of the tongue is smooth, in the sense that there are no papillae nor roughnesses, but it is studded with masses of lymphoid tissue, sometimes called the lingual tonsil (¥'\g. 1334), which make numerous eleva- tions on its surface. The mucous membrane of the back of the tongue is continued in a thinner layer onto the front of the epiglottis. It presents the median glosso- epiglottic fold, containing fibre-elastic tissue and muscular fibres of the genio-glossi, which separate two little depressions, the glosso-epiglottic fossa. These may be with- out any definite lateral boundary, or may be embraced by the small lateral glosso- epiglottic folds, the internal borders of which are concave. The mucous membrane is firmly attached to the subjacent muscles in the anterior two-thirds of the tongue, but less firmly behind. Glands of the Tongue. — The lingual glands include both serous and mucous varieties, which are distributed as three groups : (i) serous glands, (2) posterior mucous glands and (3) anterior mucous glands. HUMAN ANATOMY. The tubo-alveolar glands surrounding the circumvallate and the foliate papillae are the only ones of a purely serous type ; their thin, watery secretion is no doubt an important medium in conveying sapid substances to the taste-buds situated in this FIG. 1336. Epithelium covering filiform papillae Capillary loops within connective-tissue basis of papillae Mucous membrane Muscular tissue Injected mucous membrane and subjacent areolar and muscular tissue from upper surface of tongue. X 60. FIG. 1337. Epjtl hum "••,»:>;;' o. •;•;•''• Taste-bud Annular wall Duct of gland Central/ -^ ,,«. portion r. i pa- • > - • •:•••-' ''''". MO live tissue S VtflKjr~Ii ''':,- erous gland • <*••»* * Section across circumvallate papilla from child's tongue, showing central portion and encircling fold. X 75- vicinity. The glands mrirrliruj the circumvallate papillae constitute an annular group some 4 nun. wide and alxmt tuicc as deep. Those about the papillae foliata form an elongated group, about 3.5 mm. in width, which extends from 8-15 mm. in front of THE TONGUE. 1577 the base of the palato-glossal fold. Anteriorly towards the dorsum the serous glands remain isolated ; posteriorly they come into contact with the mucous glands, so that alveoli of both varieties may be included within a single microscopical field ( Fig. 1287 ). The posterior third of the dorsum, from the circumvallate papillae backward, possesses a rich, almost continuous layer of mucous glands, 5 mm. or more in thick- ness, which lie beneath the mucous membrane and mingle with the lymphoid tissue. Since the alveoli lie among the muscles at some depth, the excretory ducts often attain a length of from 10-15 mm., and open on the free surface in close association with the lymph-follicles. The anterior mucous glands (Fig. 1352) are disposed principally as two elon- gated groups, glandules linguales anteriores, or glands of Nuhn, or of Blandin (from 15-20 mm. in length, 7-9 mm. in width, and somewhat less in thickness), which lie on either side of the mid-line, near the tip of the tongue, among the mus- cular bundles. They meet in front, but diverge behind, where they may be con- FIG. 1338. Lymph-node Glands Interlacing fibrous and muscular bundles Glands. Section from posterior third of child's tongue, showing lymph-nodes constituting a part of lingual tonsil. X 30. tinued backward by additional collections of mucous glands along the edges of the tongue. The ducts — five or six in number — open on the folds occupying the under surface of the tongue near the frenulum. Muscles of the Tongue. — These include two groups, the extrinsic and the intrinsic muscles. The former pass from the skull or hyoid bone to the tongue ; the latter comprise the particular muscles both arising and ending within the organ. Their general arrangement is as follows. Under the mucous membrane is a dense sheath of longitudinal fibres, surrounding the others completely near the apex, and farther back wanting at the middle of the under surface where the fibres of the genio-glossi and hyo-glossi enter the organ. This outer layer is the cortex. The inner part is divided into two by a vertical median septum of areolar tissue, which is quite dense in its upper part. It is sickle-shaped, with the point in front and not reaching the apex. The inner portion, or medulla, is composed of transverse muscle-fibres inter- posed between layers of those called vertical, which in fact present many degrees of obliquity. The extrinsic muscles are fatgenio-glossus, the kyo-glossus, the stylo-glossus, and the palalo-glossus, to which may be added, from its position, the genio-hyoid. All of these are in pairs and symmetrical. 1578 HUMAN ANATOMY. The genio-hyoid (Fig. 1339) is a collection of fleshy fibres extending close to the median line, from the inferior genial tubercle to the anterior surface of the body of the hyoid bone. It is a thick band, four-sided on transverse section, with rounded angles, and expands laterally on approaching its insertion. A layer of areolar tissue separates it from its fellow. Nerve. — The nerve-supply is from the hypoglossal, but probably consists of fibres derived from the cervical nerves. Action. — To draw the hyoid forward and upward ; or, when fixed below, to depress the mandible. The genio-glossus (Fig. 1339) arises just above the preceding by short ten- dinous fibres from the superior genial tubercle. Its inferior fibres run horizontally backward to the base of the tongue, passing over the hyoid bone to the base of the epiglottis ; the fibres above these, inserted successively into the mucous membrane of FIG. 1339. Stump of masseter Tensor palati _ Levator palati Hainuhir process Styjoid process Superior constrictor Pterygo-mandibular ligament Stylo-glossus Stylo-pharyngeus Stylo-hyoid (cut) Hyoid bone Thyro-hyoid Inferior constrictor Genio-hyoid Hyo-glossus Pharyngeal and extrinsic lingual muscles the dorsum of the tongue near the middle line, are at first oblique, then vertical, and finally concave anteriorly as they approach the apex, so that the muscle is fan-shaped when seen from the side. Each muscle is separated from its fellow by the median septum. Nerve. — The hypoglossal. Action. — The complex action of this muscle includes retraction of the tongue by the anterior fibres, drawing forward and protrusion by the posterior fibres, and depres- sion, with increa.sed concavity, of the dorsum by its middle part. The hyo-glossus (Fig. 1339), external to the preceding, from which it is sepa- rated by aicolar tissue, arisefl from the side of the body of the hyoid, the whole of the greater horn, and the lesser horn. The last portion, rather distinct from the rest, is de-M ritx-d sometimes separately as the chondro-glossus. The whole muscle, applied to the side of the tongue, forms a layer of fibres directed upward and for- THE TONGUE. 1579 ward ; towards the front its fibres are almost longitudinal. The fibres from the lesser horn run on the dorsum beneath the mucous membrane, forming a part of the super- ficial longitudinal system. Nerve. — The hypoglossal. Action. — To depress the sides of the tongue, thereby increasing the transverse convexity of the dorsum ; the muscle also retracts the protruded tongue. The stylo-glossus (Fig. 1339) arises from the tip of the styloid process and from the beginning of the styio-maxillary ligament. It is a small ribbon-like muscle with an anterior and a posterior surface, but as it descends it twists so as to lie along the outer side of the tongue, which it reaches in the region of the circumvallate papillae. On joining the tongue the fibres divide into an upper and a lower bundle, both of which are chiefly longitudinal, although some fibres blend with the transverse series. It is soon lost in the sheath of longitudinal fibres. Nerve. — The hypoglossal. Action. — To retract the tongue and to elevate the sides, thus aiding in pro- ducing transverse concavity of the dorsum. The palato-glossus (Fig. 1339) arises from the anterior or buccal aspect of the palate, and descends within the fold forming the anterior pillar of the fauces to the tongue, where it joins the transverse fibres, passing between the two parts of the stylo-glossus. Nerve. — From the pharyngeal plexus, the motor fibres coming probably from the spinal accessory nerve. Action. — To elevate the tongue, to depress the soft palate, and, with its fellow by approximating the anterior pillars, to close the fauces. FIG. 1340. Longitudinal fibres Longitudinal fibres /;- ^Transverse fibres Plica fimbriata Glands Vertical fibres Transverse section of tongue of child, near tip. X 3. The intrinsic muscles are the lingualis, the transversus, and the perpendicu- laris (Fig. 1340). The lingualis, sometimes divided into a superior and an inferior, comprises the greater number of the longitudinal fibres, — all, in fact, that do not come from the extrinsic muscles. The thickness of this layer is some 5 mm. The transversus furnishes nearly all the transverse fibres, the most important extrinsic contribution being from the palato-glossus. It arises from the septum and runs outward to the mucous membrane ; as it approaches the cortex the fibres break up into bundles, among which pass groups of the fibres of the lingualis. The trans- versus is arranged in a series of horizontal layers, between which pass layers of the vertical set. Thus a horizontal section has the effect of a series of transverse fibres like the bars of a gridiron with the cut ends of the vertical fibres between them and the longitudinal fibres of the lingualis at either side. Near the apex fibres of this system run directly from the mucous membrane of one side to that of the other. The perpendicularis is the name given to the few vertical fibres that do not come from the extrinsic muscles. They occur chiefly at the tip and sides, passing from the lower to the upper mucous membrane. Nerve. — All the intrinsic muscles are supplied by the hypoglossal. Action. — The tongue is protruded chiefly by the action of the posterior fibres of the genio-glossus, drawing the posterior part of the tongue forward, assisted, perhaps, by the contraction of the transversus. It is withdrawn by its own weight. The I58o Hl'MAN ANATOMY. longitudinal system, the various parts of which can act separately, turns the tip m any direction. The stylo-glossus and palato-glossus raise the posterior portion, particularly at the edges, but the latter probably acts more on the palate than on the tongue. , £ ., Vessels.— The principal arteries supplying the tongue are branches of the lingual, elsewhere described (page 735)- Although there may be a trifling anasto- mosis at the tip between the vessels of the opposite sides, there is no communication sufficient to re-establish the circulation at once, so that ligation of either artery will render that half of the tongue bloodless for an operation. The veins consist of four sets on each side, communicating freely with one another. They are (i) the dorsal veins forming a submucous plexus on the back of the tongue above the larynx and joining those of the tonsil and pharynx, (2) two veins accompanying the artery and sometimes forming a plexus about it, (3) two with the lingual nerve (4) two with the hypoglossal nerve. Of these latter, the one below the nerve is the larger and is the ranine vein, running on the under surface of the tongue on either side of the frenum. The lymphatics present a rich net-work on the anterior two-thirds of the dorsum. The multitude of spaces throughout the organ communicate with lym- FIG. 1341. Longitudinal fibres Glands Portion of sublingual gland .Vertical fibres Transverse fibres Septum Genio-glossus Hyo-glossus Transverse section of tongue of child, through middle third. X 3. phatics. Some from the median part empty into the suprahyoid glands, but most go to the submaxillary and to the deep cervical glands. Nerves. — The motor fibres are supplied by the hypoglossal, aided probably by the facial through the chorda tympani. Those of common sensation are from the lingual branch of the fifth for the anterior two-thirds and from the glosso-pharyngeal for the remainder, excepting the region just in front of the epiglottis, which is supplied by the superior laryngeal from the vagus. The glosso-pharyngeal area somewhat overlaps the posterior third, as it supplies the circumvallate and foliate papillae. The chief fibres of special sense are derived from the glosso-pharyngeal, their principal distribution being to the taste-buds on the circumvallate papillae. Re- 14. inlin^ the source «>f tin- taste-fibres to the anterior parts of the tongue opinions still differ. According to many anatomists, these fibres reach their destination through tin- flu ml. i tympuni, since the latter nerve is supposed to receive taste- fibres from the ninth by way of tin- pars intermedia of Wrisberg, which accompanies the facial. According to Zander,1 Dixon," Spiller,8 and others, however, the view attributing tihn ^ of special sense- for the anterior part of the tongue partly to the tit'th nerve- i> correct Growth and Changes. — At birth the tongue is remarkable chiefly for its want of depth, as shown in a median seetion, which depends on the undeveloped condition of the jaws. This is gradually corrected coincidently with the growth of the face. 1 Anatotnischer Aii/eiv;er, Hd. xiv., 1897. 2 Kdinlmn_pli Medical Journal, 1897. •University i>l Pennsylvania Medic.il P.ulletin, March, 1903. THE SUBLINGUAL SPACE. 1581 The circumvallate papillae l are imperfectly developed for some time after birth, so much so that it is not easy to recognize them. The foliate papillae are also relatively undeveloped. On the other hand, the fungiform papillae are proportionately both larger and more numerous than in the adult. The development of the adenoid tissue at the back of the tongue occurs during the last two months of foetal life. In places the connective tissue surrounding the ducts of the mucous glands becomes infiltrated with leucocytes and is transformed into lymphoid tissue (Stohr). THE SUBLINGUAL SPACE. This space is between the lower jaw and the tongue, above the mylo-hyoid, and bounded behind by the fold of the anterior pillar of the fauces passing to the tongue. It is lined with thin, smooth mucous membrane reflected from the mandible to the tongue and attached lightly to the parts beneath. With the mouth closed, this space is filled by the tongue. It is best examined in the living subject when the tip of the tongue is against the upper incisors. A fold of mucous membrane, the frenum, FIG. 1342. Plica fimbriata Frenum ublingual ridge .Orifices of submaxillary and sublingual ducts Sublingual space, tongue pulled up. if well developed, passes in the middle line from the tongue to end over the floor of the mouth. Close to its termination on either side is a smooth elevation caused by the sublingual gland, which in the present position is drawn upward under the tongue. A varying number of gland-ducts perforate the mucous membrane with orifices hardly visible to the naked eye. Internal to these swellings at the lower end of the frenum is a small enlargement on each side of the median line, so closely blended, however, as to seem but one ; these elevations, the carimcula: salivares, mark the point at which the duct of the submaxillary gland opens on each side. This duct runs along the floor of the sublingual space between the mylo-hyoid muscle and the mucous membrane, a small part of the gland usually accompanying the duct a short distance over the muscle, forming a prominence, the sublingual ridge (plica sublingualis). A constant group of glands is found in the mucous membrane below the incisors.2 1 Stahr : Zeitschrift fiir Morph. und Anthrop., Bd. iv., Heft 2, 1902. 2 The sublingtial bursa alleged to exist on either side of the frenum has not been described, since it is at most extremely uncommon. 1582 Iir.MAN ANATOMY. THE SALIVARY GLANDS. These besides the mucous follicles of the mouth, are \he parotid , the submax- illary and the sublingual glands of the two sides. They are all reddish gray in color and of about the'same firmness, excepting the parotid which is denser. The Parotid Gland.— The parotid is the largest of the salivary glands, weigh- ing from 20-30 gm., with a considerable range beyond these limits It is situate behind the upper part of the ramus of the lower jaw, which it overlaps both within and without. Its limits in both directions are very variable. 1 he prolongation for- ward over the masseter muscle may become nearly distinct from the rest of the gland, FIG. 1343. parotid Inframandibular. hranoh of facial nerve External jugular vein ial artery Mylo-hyoid Digastric, anterior belly Submaxillary gland Su|>ertH i;il ilissiTtioti, showing parotid and suhmaxillary glands undisturbed. and is thru known as the soi'itt />/ii/i.\\ The s/ic-nt/t of tin- parotid is a strong fibrous envelope continuous with the cervical fascia in front of the sterno-mastoid, closely applied to tin- glandular substance and continuous with the partitions that pass through the or^an, so that it can t>e dissected off from the gland only with difficulty. Tin' parotid is divided into nianv small compartments or lobules by these resisting septa of fibrous tissue, the ejii.mtity of which gives it toughness. The shape of the parotid, as well as it-, si/e, is variable, since it 14 rows where it can among more or less resisting structures. Its shape and relations, therefore, may be considered together. Relations. The parotid occupies a cavity hounded in front by the ramus of the jaw, covered by the masseter and internal ptcrygoid muscles ; behind by the THE SALIVARY GLANDS. 1583 external auditory meatus, the tympanic plate, the base of the styloid process, and the front of the atlas. These two walls meet above at the Glaserian fissure. The pos- terior wall is prolonged laterally by the posterior belly of the digastric, the stylo- hyoid, and more externally by the sterno-mastoid muscles. The styloid process as it descends becomes internal, and the stylo-glossus and stylo-pharyngeus, together with the fascia known as the stylo-maxillary ligament, bound the posterior part of the gland internally. In front of the styloid process there is no wall to the space occupied by the parotid, the gland resting against the areolar tissue mixed with fat that lies on the outer wall of the pharynx. The widest part of this cavity is at the surface, where the fascia is connected with the capsule of the gland. The largest expanse of the parotid is, therefore, external. It overlaps the jaw and may reach down to the angle and be separated merely by fibrous tissue from the submaxillary gland. A constant, but very variable, prolongation on the face below the zygoma accompanies the duct. The parotid gland reaches upward between the joint of the jaw and the external auditory meatus and tympanic plate. Internally it lies against the structures above described, always resting on the inner side of the internal pterygoid muscle and extending to the great vessels and nerves which separate it from the side of the pharynx. There may or may not be a higher prolongation inward through the space in front of the styloid process. The internal carotid artery, inter- nal jugular vein, and pneumogastric nerve are close against the lower part of the inner surface of the gland. The external carotid artery enters the gland from the inner side and divides into its temporal and internal maxillary branches, besides giving off the posterior auricular, and sometimes the occipital arteries, within its substance. The external jugular vein is formed within the gland and emerges from its lower side. Near the skull the great vessels and nerves are separated from the gland by the styloid process. The facial nerve enters the gland on its posterior side and passes through it obliquely so as to become more superficial as it travels forward, lying external to the external carotid artery and jugular vein. Before emerging from the gland the facial nerve breaks up into its two great divisions, the branches of which begin to subdivide within the glandular mass. The auriculo-temporal nerve also passes through the upper part of the gland, emerging on its outer aspect. A varying number of lym- phatic glands lie in the substance of the parotid, mostly in the more superficial part. They are small and not easy to find. A larger one, said by Sappey to be constant, is in the gland just in front of the ear. The parotid or Stenson's duct is formed by two chief tributaries, and emerges from the front of the gland, above its middle, running forward and a little down- ward across the masseter muscle to turn in sharply at its anterior border. It then crosses a collection of fat and runs obliquely through the buccinator muscle and the oral mucous membrane to empty into the vestibule of the mouth opposite the second, often the first, superior molar tooth. The length is some 40 mm. and the diameter 3 mm. The termination is a mere slit. Its walls are firm and resistant. The general direction of the duct is that of a line from the lower side of the concha of the ear to midway between the border of the nostril and the red edge of the lip. The transverse facial artery lies above it, on leaving the gland, and a plexus of veins surrounds it. Vessels. — The arteries of the parotid gland are derived from several sources ; although numerous, none of them is large. Besides several small branches from the external carotid itself while in the gland-substance, there are twigs from the temporal, especially from its transverse facial branch, from the posterior auricular, the internal maxillary, and probably from an occasional branch that may pass through the gland. The veins form quite a plexus through the gland and open into the sys- tem of the temporo-maxillary and of the external jugular. Of the lymphatics much remains to be learned, but they probably empty into both the deep and the super- ficial cervical nodes. Nerves are from the facial, auriculo-temporal, and great auricular, besides sym- pathetic fibres from the carotid plexus. The Submaxillary Gland. — This gland, weighing from 7-10 gm. , lies largely under cover of the lower jaw, just before the angle, in a fossa on the inner side of the bone. As, however, the skin is carried inward under the jaw at this HUMAN ANATOMY. point the gland appears on the surface. It projects but little, if at all, on the outer side of the jaw, but curls around the posterior border of the mylo-hyoid muscle and extends for some distance in the floor of the mouth, under the mucous mem- brane in the angle between the mylo-hyoid and the hyo-glossus, sometimes reach- ing the sublingual gland (Fig. 1344)- It: lies in a capsule derived from the cervical fascia, which is so loosely attached that the gland can easily be isolated. I he anterior end of the posterior belly of the digastric and of the stylo-hyoid pass behind and beneath it. The hypoglossal nerve and the lingual vein lie beneath it, as does the first part of the lingual artery, until the latter passes under the hyo-glossus. Its sublingual branch runs along the inner side of the prolongation of the gland, FIG. 1344. Accessory parotid gland Parotid duct Masseter Buccinator Parotid glan Internal pterygoid- (cut) Superior constrictor- Digastric- Stylo-hyoid- Stylo-glossus- Stylo-pharyngeus- Occipital artery- Internal carotid. Middle constrictor- Facial artery. External carotid- Lingual artery- Superior thyroid_ artery Inferior constrictor- Lingual nerve Facial artery Oral mucous membrane Deeper portion of sub- maxillary gland Cut mandible Submaxillary duct .Sublingual gland ^ ^ Genio-glossus \Mylp-hyoid (cut) Genio-hyoid Stump of digastric, anterior belly 'Submental artery Submaxillary gland, superficial part \ Great cornu of hyoid bone Hyo-glossus Thyro-hyoid Deeper dissection, showing relations of salivary glands. tn which it sends vessels. The facial artery lies beneath the gland before reaching the bonier of the jaw. The facial vein is superficial to it. The lingual nerve lies above the prolongation. 1 In Submaxillary or Wharton's duct runs from the front of the main body of tin- ^1. ind alon^ the tloor of the mouth under the mucous membrane, often accom- panied externally by the prolongation of the gland. It is from 4-5 cm. long, with a diameter of ;, nun. Its walls an- decidedly thinner than those of the parotid duct. Anteriorly it rises to open into the month l>y a little papilla on the side of the frenum lintMi.i-, the last feu millimetres running in a fold of mucous membrane. The lingual nerve pas-M-s under the duct from without inward soon after it leaves the gland. The sublingual artery is beside it and a plexus of veins around it. STRUCTURE OF THE SALIVARY GLANDS. 1585 Vessels. — The arteries of the submaxillary gland are derived from the facial and the sublingual branch of the lingual. The veins are from the corresponding ones. The lymphatics go to the submaxillary glands. Nerves. — The gland receives filaments from the sympathetic plexus accompa- nying the facial artery, from the lingual nerve, and from the submaxillary ganglion. The Sublingual Gland. — This differs from the two preceding glands in having no capsule. It lies in loose areolar tissues on the mylo-hyoid muscle, at the front part of the sublingual space. Its weight is 3 or 4 gm. Each gland rests internally against the genio-glossus, and anteriorly they touch one another. They are more readily separated into lobes than the others. Testut regards them as aggregations of separate glands. The sublingual glands are covered by the mucous membrane of the floor of the mouth, which they press upward into rounded swellings on either side FIG. 1345. Opening of anterior, lingual glands Frenum Tongue, pulled upward Caruncle and opening of submaxillary duct Genio-glossus Sublingual gland Genio-hyoid Mylo-hyoid Cut fibres of digastric Section across anterior part of floor of mouth, showing relations of sublingual glands to mucous membrane and muscles. of the beginning of the frenum. The lingual nerve and the submaxillary duct are on the inner side. The sublingual or Rivinus' ducts vary in number from four to twenty or more. They open for the most part in the floor of the mouth, but some may join Wharton's duct. Bartholin' s duct is an inconstant one, larger than the others, that usually opens close to the outer side of Wharton's duct, which it follows. Vessels. — The arteries are from the sublingual branch of the lingual and the submental branch of the facial, which latter sends minute twigs through the mylo-hyoid muscle. The blood escapes into the ranine vein. The lymphatics run to the sub- maxillary nodes. Nerves are from the sympathetic, the lingual, the submaxillary ganglion, and, according to many, from the chorda tympani. STRUCTURE OF THE SALIVARY GLANDS. The three chief salivary glands possess in common the tubo-alveolar type of structure; depending upon the character of their secreting cells and products, the func- tionating organs represent both the serous and mucous varieties. The parotid is a pure serous gland ; the submaxillary is a mixed one, the alveoli containing serous cells predominating ; the sublingual, also a mixed gland, consists chiefly of mucous alveoli, the serous cells being limited to the marginal groups constituting the demilunes of Heidenhain, I586 HI 'MAN ANATOMY. The parotid gland consists entirely of serous alveoli, although mucus-pro- ducing acini may occur in the accessory lobules situated along the duct of Stenson. The primary lobules are made up of alveoli, from .015 to .020 mm. in diameter, lined with epithelial cells, which are somewhat pyramidal in form, since they are broader next the basement membrane and narrower towards the cleft-like lumen. 1 he rest- ing cells, fresh and examined without the addition of reagents, appear filled with numerous minute, glistening granules which lie embedded within a less strongly refracting substance. The granules, however, are readily affected by reagents, often undergoing partial or complete solution; hence the reticulated appearance of the pro- toplasm frequently observed in glandular epithelium after fixation. The nuclei of the serous cells are usually of spherical form and contain distinct nucleoli and delicate Interlobular duct Artery Small Intralobular duct Interlobular septum _ Duct Section of small lobule of parotid gland. X So. chromatin net-works. The system of excretory canals begins at the alveoli as the intermediate tubules, which in the parotid are relatively long, about .010 mm. in •li.unctcr, and lined with low, flattened cells, directly continuous with the taller alveolar epithelium, on the om- hand, and with that of the intralolmlar ducts on the other. The latter, oi Sii/trun- tubules of Pfliiger, of larger diameter (about .035 mm.) than that of the immediately preceding or succeeding segments of the canal, are clothed with a single laver of columnar cells, some .014 mm. in height, which present a jie, uli.ir differentiation into an inner and an outer zone. The former, next the lumen of the tube and containing the nucleus, appears finely granular or almost homogeneous, while the outer or basal /one exhibits a longitudinal striation composed of rows of minute granules. Alter treatment with certain reagents, the striated zone STRUCTURE OF THE SALIVARY GLANDS. 1587 breaks up into delicate rod-like processes, in recognition of which the cells lining the intralobular tubules are often designated rod-epithelium. An active secretory role has been ascribed to these cells, R. Krause ' having succeeded in demonstrating an excretory function by means of sodium sulphindigotate. The interlobular and inter- lobar ducts gradually increase in size and possess a lining of columnar cells which are usually arranged as a single layer. In the larger canals, however, the epithelium consists of two imperfect rows, since smaller cells lie next the basement membrane, wedged in between the larger typical elements. The columnar cells continue until near the termination of the main excretory duct, where they give place to the stratified squamous epithelium prolonged from the oral mucous membrane. FIG. 1347. Intermediate duct \ a V- • _.._ 1 ubular alveolus Alveolar lumen ; VI. ;-• ' Connective tissue Section of parotid gland, showing serous alveoli. X 270. The submaxillary gland differs in structure from the parotid in possessing both serous and mucous alveoli, the latter forming approximately one-fifth of the entire organ. The alveoli containing serous cells correspond closely with those of the parotid, being from .020 to .030 mm. in diameter and filled with elements loaded with minute granules. Not infrequently the cells exhibit differentiation into an inner granular and an outer almost granule-free zone. The mucous alveoli are often some- what larger than the serous, reaching a diameter of .040 mm. or more. The mucus- producing cells present the usual appearance and share the acinus with typical demi- lunes consisting of cells identical with those lining the serous alveoli. The mucous acini are directly connected with those of the serous type. Intermediate tubules connect alveoli of both kinds with the intralobular canals; those beginning in mucous acini are shorter (.035-. 060 mm. ) and less richly branched than the tubules originating in serous alveoli. The latter measure from .060-. 140 mm. in length, and repeatedly divide ; they are lined with low cubical cells which are gradually transformed from the alveolar epithelium in contrast to the abrupt transition seen in the tubules connected with mucous acini. The cells lining the intralobular tubules of the submaxillary gland exhibit the characteristic rod-like striation seen in the parotid, the rod-epithelium sometimes containing yellowish pigment granules. The interlobular and interlobar ducts resemble those of the parotid gland. The chief excretory duct possesses, in addition to a subepithelial elastic layer, a weakly developed stratum of longitudinally disposed involuntary muscle. Goblet-cells appear between the columnar elements lining the duct. The sublingual gland, being of the mixed mucous type, resembles in structure the labial and buccal glands, and consists of a series of individual lobules, opening by half a dozen or more separate ducts, rather than a compact single organ. In com- 1 Archiv f. mikro. Anat., Bd. xlix., 1897. I588 II r: MAN ANATOMY. mon with other mucous glands, the sublingual lobules do not possess intralobular tubules lined with the characteristic rod-epithelium. The interlobular ducts subdi- vide into smaller canals which extend within the primary lobules and give off wider passages lined with cubical epithelium. Towards the end of these terminal canals Duct Mucous alveoli Serous alveoli Section of submaxillary gland, showing serous and mucous alveoli. X 270. the mucous cells appear, at first isolated or in groups, increasing in numbers until they form the entire lining of the passage and become the secreting elements occupy- ing the tubular alveoli of the gland. The latter vary from .030— .060 mm. in diam- eter, and are clothed with cells averaging .015 mm. high. The condition of the FIG. 1349. Duct Crescents of serous cells -:< B <>f sutilingtial gland, showing st-nuis cells grouped as crescents. X 270. alveoli as n-anls tin- mucus bearing cells varies greatly even in the- same lobule At tiiu.-, ,,u entire primary lobule is ,-, >mp..srd of acini tilled with mucous cells ; at others emptv ami iM.rgrd alveoli alternate, or the depleted acini may predominate. Uncer- tainty a> to the presence of the demilunes also r\i>ts, since these may be absent in PRACTICAL CONSIDERATIONS: THE MOUTH. 1589 certain well-developed alveoli filled with large mucous cells, or they may be present in considerable numbers. Mucous cells are much less numerous in the sublingual glands of young infants than in the adult organ. The relatively wide lumen of the alveoli and the more reticulated appearance of their epithelium serve to distinguish the exhausted sublingual gland from the parotid of similar condition. The normal secretions of the oral glands, mucous as well as serous, contain no formed elements ; occasionally accidental granules or cell remains are present. The characteristic spherical so-called salivary corpuscles which occur in varying numbers in the mixed oral secretion have no relation to the salivary glands, since they are only modified leucocytes escaped from the lymphoid tissue of the faucial and lingual tonsils. On gaining the oral cavity, these cells are affected by the saliva and become greatly swollen, the granular remains of their cytoplasm exhibiting molecular motion in a marked degree. Development of the Oral Glands. — The earliest traces of the salivary glands are seen during the second fcetal month. The anlage for the submaxillary gland first appears about the sixth week ; next that for the parotid about the eighth week ; a little later that for the sublingual. The parotid anlage develops from the oral ectoblast along the lateral groove separating the upper and lower jaws. The submaxillary and sublingual glands arise from a ridge-like anlage of the buccal epithelium occupying the furrow marking the angle between the tongue and the floor of the mouth, the anlage for the sublingual lying nearer the tip of the tongue. At first the parotid and submaxillary lie about equally removed from the oral opening, but later migration occurs, the former passing backward and the latter forward. The development of the gland in each case begins as a solid cylindrical out- growth from the deeper layer of the oral epithelium, which presents a local thicken- ing. The cylinder rapidly lengthens and branches, so that by the eighth or tenth week the submaxillary and parotid glands respectively consist of a main stalk and terminal buds. The anlage of the sublingual gland gives off epithelial buds on acquiring a length of about i mm. The primary sprouts of the anlage subdivide and eventually become the smaller ducts and the glandular tissue. Meanwhile the imme- diately surrounding mesoblast undergoes condensation, and contributes the connective- tissue envelope with its prolongations between the lobules and acini supporting the blood-vessels and nerves. Towards the close of the third month, while the gland- tubules are still solid, the lumen of the future main excretory duct appears in the epithelial cylinder, extending from the free surface towards the alveoli. The latter acquire their lumen during the fifth month. The smaller oral glands, including those of the lips, cheeks, tongue, and palate, develop much later than the larger salivary, since their anlages appear during the fourth month. The details of their development correspond in general with those attending the formation of the larger oral glands. PRACTICAL CONSIDERATIONS : THE MOUTH. The chief congenital deformities of the mouth are harelip and cleft palate. Harelip results from a failure of the developmental procedures concerned in forming and differentiating the nasal and buccal cavities. These processes have already been described in connection with the formation of the face (page 59). Upon the down- growth of the fronto-nasal process depends the formation of the vomer, the perpen- dicular plate of the ethmoid and the external nose, and of the intermaxillary bone and that portion of the upper lip corresponding to the four incisors. The partition separating the nasal from the oral cavity, later the hard and soft palates, is formed by the union of the horizontal palatal plates from the buccal aspect of the two maxillary processes (Fig. 76). When the frontal and maxillary processes fail to unite on one side, single harelip results, the cleft in one side of the lip lying opposite the space between the upper canine and lateral incisor, or between the latter and the central incisor. When union between the maxillary and the frontal processes fails on both sides, double harelip follows, the lateral incisors often being absent and the inter- maxillary bone with the central incisors and the median portion of the lip occupying a position beneath the nasal septum. i590 HUMAN ANATOMY. Cleft palate is caused by faulty union between the palatal processes of the maxillary arches. The cleft is always in the middle line, and may involve only the uvula and soft palate, may extend to the posterior margin of the intermaxillary bone, or may diverge from that point on one or both sides and run forward through the alveolus, being then associated with single or double hare- FIG. 1350. iip( the cleft or clefts in the alveolus corresponding in position to the deficiencies in the lip (page 63). The Lips. — The mucous membrane of the lips and the adjacent skin are often affected by herpes labialis, which may be associated with gastro-intestinal disturbance, or may be purely ~~T neurotic in its origin, following mental depression or anxiety. It is found in the distribution of the second and third divisions of the fifth pair which supply sensation to the upper and lower lips re- spectively. The vascularity of the lips, while it leads to excessive exudate and large swelling after contused or lacerated wounds, favors rapid heal- ing and the avoidance of infection after surgical wounds. In few places equally exposed to con- New-born child with double harelip. tact with infectious organisms was healing by ' ' first intention" so common before the introduction of antisepsis. The coronary arteries run between the mucous membrane and the orbicu- laris oris. They are therefore more often severed by wounds extending from within outward — usually made by the teeth — than by those beginning externally. The coro- naries anastomose very freely. In arresting hemorrhage from them by direct ligature both ends should be tied. If a wound of the lips is united by pins and figure-of- eight sutures, the pins should be passed close to the inner edges of the wound so that the coronaries may be compressed between the pins and the sutures. The vascu- larity of the lips renders chancres of that region, like those of the face, exceptionally large both in depth and in superficial area. It also adds greatly to the extent of funmcular or carbuncular infection in this region, the occurrence of which is favored by the large number of hair and sebaceous follicles present. The danger of infective sinus thrombosis ( intracranial) as a result of such infection here or elswhere on the face is much increased by the free anastomosis between the valveless facial vein and its tributaries and the ophthalmic vein, which is also without valves. As might be expected, nzevi are frequent in the lips. In the male the lower lip is the favorite scat of epithelioma. Either infection or diminished tissue resistance from minor trauma- tisms, or from tobacco-irritation in smokers, is supposed to explain this clinical fact. The muroiis glands of the lip are not rarely the seat of retention-cysts from obstruc- tion of their ducts. The Gums.— The mucous membrane of the lips is continuous with that cover- ing the fibnms tissue of the gums, but the latter is slightly less vascular and much less sensitive. The gums are sometimes congenially hypertrophied ; the condition is usually associated with defective or aberrant developmental processes often affecting the mentality. They are also often found hypertrophied in edentulous old persons or in persons with badly titling artitieial dentures. They are the frequent seat of inflam- mation from various causes, the most common of which are the decomposition of f 1 .uul the deposition of calcium salts — tartar — about the necks of the teeth. Infec- tion frequently follows tin- hvper.emia produced by these forms of irritation. \Yheii it is contined to the space between the mucous membrane and the fibrous tissue, it Cause* a limited superticial abscess, — "gum-boil;" if it guins access to the sub- peiiosteal space, it mav cause a form of ak eolar abscess, tin- usual variety of which is, however, due to inf.-i -lion secondary to dental caries, and is situated about the root of a tooth ' rit/f in! • Tartar is toimd m..-,i abundantly near the openings of the submaxillary and sub- lingual ducts,— i.,-., near the inner sm faces of tin- lower incisor teeth. Mercury and lead cause gingivitis prot.al.lv by the actual presence of their salts in quantity Suffi- cient t.. a. i as irritants, their deposition from terminal capillaries being favored "by the PRACTICAL CONSIDERATIONS : THE MOUTH. 1591 frequent hyperaemia due to the vascularity and the warmth and moisture of the region, together with slight but repeated trauma during mastication. The gingivitis of scurvy or of purpura is merely a local evidence of a constitutional condition, and is hemorrhagic rather than inflammatory. During dentition the resistance of the gums may cause backward pressure upon the nervous and vascular supply of the pulp of the tooth, giving rise to some pain and sometimes to grave reflex disturbances, especially in infants. The insensitive gum then becomes exceedingly tender and is swollen and cedematous. The wide-spread relations of the fifth nerve render long-continued irritation of its dental branches dan- gerous. "Lancing" the gums is the obvious remedy. It is especially apt to be needed over the molars and cuspids, and the lines of incision should be planned so as to release fully the presenting surfaces of those teeth. The Teeth. — Alveolar Abscess. — The line of penetration in dental caries is often in the direction of the pulp, through which infection extends to the "apical space' ' between the root of the tooth and its socket, containing the vessels and nerves and some loose connective tissue. This space soon becomes filled with pus, the cavity enlarges, and reaches the compact bone on the surface of the alveolus (the density of which impedes the process somewhat) ; but finally the bone is perforated, usually through the thinner external or buccal wall of the alveolus. The periosteum usually yields opposite the gum immediately over the apex of the tooth, where it is reinforced by mucous membrane only. If the root of the tooth is a long one or the abscess has gone deeply into the bone, the pus may reach the periosteum at a point where it is supported by the muscular and fibrous tissues of the cheek. The pus may then strip the periosteum from the bone so as to cause extensive necrosis. This is less likely to occur in the alveolus of the upper jaw or in the hard palate, on account of their free blood-supply derived from several sources. In cases of this type in either jaw, a sinus followed by a depressed, adherent, and disfiguring cicatrix is liable to result (Rough ton). Alveolar abscess is also influenced in its course by the situation of the particular tooth involved. In the maxilla, abscesses connected with the canines or incisors may point into the nasal cavity or on the under surface of the hard palate. The pus FlG- I3SI- is more likely, however, to descend by gravity A alongside of the root to the edge of the gum, or to follow the canal of the root into the pulp-cavity. Abscesses connected with the upper molars, es- pecially the first, or, more rarely, those in relation to the cuspids, may point in the antrum. They occasionally open on the face in front of the an- terior border of the masseter. The relation of the apex of the root to the mucous membrane of the gum often determines the point of opening. *, If the apex in the case of the lower teeth is above, or in that of the upper teeth is below the line of reflection of the mucous membrane from the cheek to the gum, the abscess tends to point in the mouth. If the contrary is the case, pointing on the face or neck may result. Tn wkli-tlis trip firc;t tpptri pvriihit malfnrma Characteristic teeth of inherited syphilis. in Sypmm me ma- A upper permanent central incisors deeply tions characteristic of perversions of nutrition Or notched; lateral incisors show no defect; right r • n . • r . i rr • .1 canine has deep notch ; exposed dentine has 01 inflammation OI the gums Sufficiently Severe tO become discolored. B, upper incisors onlv re- affprt trip h1nr>H-<;iinn1v to trip tontri-«;ar^ Tfip cently erupted ; central notch marked out but supply I not yet cleared OHt by breaking away of unpro- enamel may be deficient, Opaque Or chalky, the tected dentine; four lower incisors present peg- j rf r • i, .i .«* • i • • like excrescences due to loss of enamel and dentine soft or friable, the teeth irregular in size exposure of dentine. (Hutchinson.) and uneven in position. The permanent teeth may show the same general aberrations as to growth and nutrition that are produced by stomatitis from digestive derangements or from local irritation. After mercurial stomatitis, for example, the teeth are irregularly outlined, horizontally seamed, scraggy, malformed, deficient in enamel, separated too widely, and dirty yellow in color. 1592 HUMAN ANATOMY. The typical (and pathognomonic) syphilitic teeth — " Hutchinson's teeth" — are the upper permanent central incisors. The type is observed in its perfection soon after the extrusion of these teeth. The essential characteristic is a crescentic notch (Fig. 1351 ,< / ) in the free edge of the tooth, the anterior border of the notch being bevelled from above downward and from before backward, — i.e., at the expense of the anterior >nrfaci- and border of the tooth. Typical Hutchinson's teeth are, fur- thermore, reduced in length and narrowed, — "stunted ;" their angles are rounded off, the lateral and inferior borders merging in a curved line ; they deviate from nor- mality in direction, their axes being obliquely convergent, or more rarely divergent, instead of parallel. The other surgical relations of the teeth and of the dental tissues which are of chief importance are concerned with the new growths originating in dental elements. The odontomata are divided by Sutton as follows, and the classification should be remembered in studying the anatomical development of the teeth : (i) Persistent portions of the epithelial sheath (page 1561), taking on over- growth, may give rise to an epithelial odontomc (multilocular cystic tumor). (2) Expansion of the tooth-follicle with retention of the crown or root of an imperfectly developed tooth results in a follicular odoniome (dentigerous cyst). (3) Hyper- trophy of the fibrous tooth-sac causes a fibrous odontome, especially frequent in rickets, which usually affects the osteogenetic fibrous membranes. (4) If the fore- going hypertrophy occurs and the thickened capsule ossifies, a ccmcntome results. (5) If this takes place irregularly, small malformed teeth — "denticles" — may form in large numbers and occupy the centre of the tumor (compound follicular odontome}. (6) Tumors of the root, after the full formation of the crown, are of necessity com- posed of dentine and cementum only, enamel not entering into them (radicit/ar odontomata). (7) Tumors composed of irregular conglomerations of enamel, den- tine, and cementum, and often made up of two or more tooth-germs fused together, constitute composite odontomata. All these growths can be understood only by careful study of the normal development of the teeth. They are rarely diagnosed before operation, which is therefore in some cases needlessly severe. Sutton says very truly, " In the case of a tumor of the jaw the nature of which is doubtful, par- ticularly in a young adult, it is incumbent on the surgeon to satisfy himself, before proceeding to excise a portion of the mandible or maxilla, that the tumor is not an odontome, for this kind of tumor only requires enucleation. In the case of a follicular odontome it is usually sufficient to excise a portion of its wall, scrape out the cavity, remove the tooth if one be present, stuff the sac, and allow it to close by the process of granulation. ' ' The Roof of the Mouth and the Palate. — The mucous membrane cov- ering the hard palate is so fused with the periosteum as practically to be inseparable from it. It is dense, resistant, and comparatively insensitive. A vertical trans- verse section of the roof of the mouth (Fig. 1294) shows the mucous membrane to be thickest laterally and thinner in the median line. Cleft palutc (page 1590) results from imperfect fusion between the horizontal palatal plates of the maxillary processes of the first visceral arch. It is always in the middle line. It mav involve the soft palate and uvula. If it extends forward as far as the alveolus, it follows the line between the maxilla and the premaxillary bone, uMi.illy terminating in a harelip (page 1589) opposite the interval between the lateral im-isor and (-.mine teeth. If it separates the maxillae on both sides from the pre- m.ixill.irv hone, it is almost always associated with double harelip. The toughness of the muco-periosteum of the hard palate facilitates the forma- tion of Maps in operation-, for the closure of such a cleft. In dissecting up the flaps it is well to keep dose t<> the bone and to avoid the descending or posterior pala- tine branches of the internal maxillary artery. These vessels, on which the nutri- tion of the Maps as \\ell .,s of the bone depends, emerge from the posterior palatine canal at a point on the liiu- of junction of the hard and soft palates 8 mm. (l/$ in.) anterior to tin- hamular pioeos and a little to the inner side of the last molar tooth. They run forward in a shallow groove just internal to the outer border of the hard palate. Thcv .ire nearer to the Lone than to the mucous surface, but their pulsa- tions can ott;-n be felt by the finger. For these reasons incisions in uranoplastv PRACTICAL CONSIDERATIONS: THE MOUTH. 1593 FIG. 1352. should be made close to the alveolus and the bone should be hugged as the flaps are raised. In troublesome bleeding from these arteries the posterior palatine canal may be plugged by a sharpened stick, which should previously be sterilized. When the clelt involves only the soft palate, staphylorrhaphy is required. The muscles that tend to pull the edges apart are the tensor palati and levator palati. The former turns around the hamular process and passes almost horizon- tally towards the median line, the latter lies close to the posterior surface of the soft palate and runs obliquely from above downward and inward. These muscles may be divided by various incisions, the simplest being a section of the velum near its lateral border and parallel with the cleft. The hamular process may be felt behind and a little internal to the last molar tooth. The pterygo-mandibular ligament may be felt passing from the hamular process to the posterior end of the mylo-hyoid ridge of the lower jaw just behind the last molar tooth. The fold of mucous membrane covering it may be seen when the jaws are separated widely. The lingual branch of the fifth nerve may be felt between the mucous membrane and the bone anterior to the base of the pterygo-mandibular ligament and below the last molar. With a finger passed behind the last molar, the swell of the alveolar ridge can be recognized as it nar- rows to pass into the ram us. The nerve is below and parallel with that ridge. It is sometimes divided for the relief of the unbearable pain of carcinoma of the tongue. This may be done by entering the point of a curved bistoury a little less than three-quarters of an inch be- hind and below the last molar and cutting on the bone towards the tooth. The Floor of the Mouth. —The mylo-hyoid muscle, extend- ing from the symphysis to the last molar tooth, separates the buccal cavity from the neck. Infections or neoplasms beginning above this muscle are first recognized through the mouth ; those below it in the neck. The sublingual gland, for example, lies altogether above it and directly beneath the mucous membrane of the floor of the mouth ; the duct of the submaxillary gland occupies a similar position. Affections of these structures, therefore, manifest themselves in the mouth. The submaxillary gland, however, lies partly beneath the poste- rior border of the mylo-hyoid. Accordingly, disease of this gland is apt to show most markedly beneath the jaw (Fig. 267, page 247). " Ludwig's angina" (page 553) may spread to the loose connective tissue between the mylo-hyoid muscle and the mucous membrane of the floor of the mouth. That membrane is reflected from the under surface of the tongue to the alveoli and is divided anteriorly by the frenum linguae. On either side of this may be seen the ridges indicating the situation of the sublingual glands, and close to the frenum at the inner end of the ridge the papillae at the opening of Wharton's ducts, into which a fine probe may be passed (Fig. 1352). The inelastic character of the walls of the latter should be remembered as explaining in part the intense pain caused by an impacted submax- illary calculus. This is also in part due to the close relation of the duct to the Anterior lingua! gland — Cut surface of mucous membrane — Lingual vein — Lingual artery Submaxillary duct — Sublingual gland Dissection of under sunace of tongue and sublingual space; mucous membrane removed and tongue drawn upward and for- ward from mouth. i594 HUMAN ANATOMY. lingual nerve. The relation of that nerve to the floor of the mouth posteriorly has already been described ( page 1249). The fold of mucous membrane constituting the frenum may be abnormally short and prevent the free movements of the tongue, interfering with sucking during infancy and with articulation later. When its division is necessary, it should be cut through close to the jaw, and with blunt-pointed scissors directed away from the tongue so as to avoid the ranine veins which may be seen close to it on the under surlace of the tongue. The ranine arteries lie farther out and are more deeply situated, being placed beneath two converging raised fringed lines of mucous membrane, the plica flmbriatte. A sublingual bursa is described by Tillaux as a triangular space situated between the genio-hyo-glossus and the mucous membrane, its tip being at the frenum, its base at the sublingual gland. Its existence, by no means constant, is said by Tillaux to explain the occurrence of the acute cystic tumor (grenouillette} , " acute ranula,'7 which is occasionally met with in this region. Ranulae — ordinary retention cysts — are common in the floor of the mouth, and branchiogenic cysts, due to the incomplete closure of the first branchial cleft, are sometimes found there. The Cheeks. — The buccal limits of the cheeks are accurately indicated by the reflections of mucous membrane lining them. By making outward traction on the angle of the mouth that membrane can be seen and palpated, and ulceration, as from a jagged tooth or beginning epithelioma, or mucous patches, or abscess, or new growths, can easily be detected. The papilla indicating the opening of the parotid duct may be seen or felt opposite the upper second molar tooth. A fine probe may be made to enter the duct for a short distance, the normal curves then interfering with its passage (Fig. 1343). Lipoma originating in the "boule de Bichat" (page 493) can be recognized. As the jaws are separated and closed the anterior border of the masseter may be seen and felt. The important structures of the cheek — the facial vein and artery and the parotid duct — are all anterior to this line (Fig. 691). The Tongue. — Congenital deformity of the tongue is rare. Forked tongue — normal in some birds and reptiles and in seals — is rare ; it is usually in asso- ciation with other developmental defects, as cleft palate. Congenital absence has been noted (de Jussieu). Macroglossia {lymphangioma cavernosum, Virchow) is a congenital affection in which the lymph-channels and lymph-spaces are dilated and the lymphoid tissue throughout the tongue, but especially at the base, greatly increased. The tongue may attain an enorntous size, and has even, by pressure, caused deformities of the teeth and alveolar arches and luxation of the mandible. The foramen caecum, indi- cating the junction of tin- pharyngeal and buccal parts of the tongue, is the superior termination of the fcetal thyro-glossal duct. " Ducts lined with epithelium have been found leading from the foramen caecum to accessory glands about the hyoid bone. It is probably from these glandular and epithelial collections about the hyoid bone that crrtain deep-seated forms of cancer of the neck are developed. Some of these take th«- form of malignant < ysts" ( Treves). The upper surface of the tongue lias for centuries been the object of especial observation in disease. The practical value of these observations is not univer- sal! v conceded, and too much weight has been placed upon them ; but there can be no doubt that some help in prognosis and even in diagnosis in digestive de- rangements, in fevers, and in various toxaemias may be obtained by inspection of tile tiillglle. The " fur," so carefully studied, consists of a mixture of d-esquamated epithelial cells, food particles, and micro < .rganisms of various kinds overlying living epithelium which mav be abnormally proliferating. The surface between the circumvallate papilla- is apt to be the most heavily coated, either in health or disease, because it is the least mobile part of the tongue and is not kept .lean by f.i.-tion, as are the sides and tip. The appearance of PRACTICAL CONSIDERATIONS: THE MOUTH. 1595 the coating and of the tongue itself varies greatly, but it may be said that dry- ness not due to mouth-breathing, but from deficient secretion, as in fevers ; dark- ness, from decomposition and desiccation of the coating, or from imperfect oxy- genation of the blood ; roughness, from papillary overgrowth with marked epithelial proliferation and desquamation ; redness, from epithelial denudation ; and stiff- ness, slowness, or tremulousness in protrusion, from either thick, inflexible coating, muscular weakness, or mental hebetude, are uniformly regarded as unfavorable conditions. Unilateral furring of the tongue has been observed in cases of dental caries, of fractured skull, and of intracranial disease, in all three instances the furring being on the side on which there was irritation of the branches of the fifth pair of nerves. In some of them it was confined to the anterior two-thirds of the upper surface, — i.e. , to the distribution of the lingual branch of the fifth (Hilton). In tonsillitis the tongue will often be furred over its posterior part only — i.e. , the portion which, like the tonsil, receives its nerve-supply from the glosso- pharyngeal (Jacobson). Unilateral furring in the presence of toothache may be due partly to the instinctive immobilizing of that side of the tongue nearest the painful tooth (Hutchinson). In chronic superficial glossitis the epithelium thickens at places into rounded, whitish patches, which are difficult to heal on account of the constant exposure to warmth, moisture, infection, and minor traumatisms, and the impossibility of securing rest. This condition (leukoplakia) may precede the development of epithelioma. In rare cases the epidermis covering the filiform papillae undergoes hypertrophy, producing the so-called " hairy tongue." The lymphoid tissue behind the circumvallate papillae, from overgrowth, forms an irregular rounded mass just beneath the mucous membrane, — the lingual tonsil, — which from its proximity to and interference with the epiglottis may require removal. The connective tissue of the tongue is scanty, but is abundant enough to permit of great swelling in cases of acute glossitis, and this is favored by the vascularity of the organ. The cause is always infection through a surface solution of continuity either traumatic or during some disease attended by drying and fissuring of the tongue. On account of the vascularity, naevoid growths are frequent. Carcinoma of the tongue is exceedingly common, and Treves calls attention to the fact that it usually affects the anterior two-thirds or that portion which is derived from the mandibular arch, as is the lower lip, which is also one of the commonest sites of epithelioma. Cancer of the fore part of the tongue may follow the lym- phatics of that region into the submaxillary glands, or pass by the main lymphatic channels into the deep cervical glands. Those first demonstrably enlarged, what- ever the site of the cancer, are apt to be in the group beneath and behind the angle of the jaw. The pain in cancer of the tongue is almost always associated with what are described as "earache," "toothache," " faceache, " and sometimes with spasm of the muscles of mastication. These symptoms are due to the connection of the lingual branch of the fifth pair with other branches of the third division of the fifth, especially the auriculo-temporal and inferior dental, with the tympanic branch of the glosso-pharyngeal, and with the chorda tympani from the facial. Pressure upon, or disease of, the hypoglossal nerve may cause unilateral atrophy of the tongue. The various paralyses should be studied in connection with the nervous supply of the tongue. As the tongue depends upon muscular and not ligamentous attachments for the preservation of its position in the mouth, its tendency to drop backward by gravity during complete anaesthesia or some other forms of profound unconsciousness in which muscular relaxation or paralysis occurs should not be forgotten. If it is allowed to fall back, the pressure on the epiglottis may close the opening into the larynx. During anesthetization it is well to press the lower jaw well forward, carry- ing the tongue with it through the attachments of the genio-glossi, and to elevate the chin, which still farther advances the tongue and removes it from close proximity to 1596 HUMAN ANATOMY. the epiglottis. Often this does not suffice, and direct traction on the tongue itself is required. /: \ cision of the entire tongue necessitates division of the muscles of the tongue, its connections by mucous membrane with the soft palate, the alveoli, and the epiglottis, the lingual arteries and veins, and the glosso-pharyngeal, lingual, and hypoglossal nerves. In opening abscesses of the tongue the position of the lingual arteries — much nearer the lower than the upper surface— should be remembered. Hemorrhage from wounds or during operation may temporarily be controlled by pressure from behind forward on the base of the tongue by two fingers thrust well below and behind it in the pharynx. By this procedure, or by forcing up the soft tissues between the inferior maxilla and the hyoid bone with the finger or thumb, the cut surface during partial excision may be brought well into view and the hemorrhage controlled while the vessels are sought and secured. THE PHARYNX. The pharynx is a bag, open in front, with musculo-membranous walls, lined with mucous membrane, extending from the base of the skull to the lower border of the larynx, near the level of the top of the seventh cervical vertebra. Thus it is bounded behind by the spine, covered by the prevertebral muscles and fascia, and by the basilar process of the occipital bone, which, especially in the median line, is separated by much areolar tissue, as well as by muscles from the posterior wall. The steep rise of the basilar process, together with the downward growth of the face, forms the deep recess known as the naso-pharynx. The roof is formed by a little of the front of the basilar process and by the back part of the basi-sphenoid. The anterior wall is formed by the back of the framework of the face, the soft palate, the back of the tongue, the hyoid bone, and the larynx. The pharynx communicates in front with the nasal chambers and the mouth ; the Eustachian tubes open into it on either side near the top ; and below it contains the opening of the larynx, behind which it passes into the oesophagus. The framework consists of the pharyngeal aponeurosis, a dis- tinct fibrous membrane above, placed between the mucous membrane and the mus- cular layer, which grows weaker below and is continued into the gullet. This is attached above to the pharyngeal tubercle and to the occipital bone on either side of it, to the cartilage between the petrous portion of the temporal and the basilar process, to the Eustachian tube which passes over it, and to the base of the internal pterygoid plate. This fascia is wanting in front. The parts forming most of the anterior wall — the soft palate and the back of the tongue — are capable of changing their relations. The pharynx is enclosed by a layer of fascia, the bucco-pharyngeal (not to be confounded with the pharyngeal aponeurosis), the front part of which is connected with the pterygo-mandibular ligament and covers the buccinator muscle. This fascia lies beneath the parotid gland and mingles with the cobweb-like tissue of the carotid sheath to make a large amount of rather dense areolar tissue on either side. At the bark it is very lax, allowing the pharynx to move on the smooth prevertebral fascia. The condition then- approaches that of a serous bursa. The pharynx is divided into the naso-, ore-, and laryngo-phan-nx by folds on the anterior and lateral walls. The uninterrupted posterior wall is covered with smooth mucous membrane, which, behind the larynx, tends to be puckered into longitudinal folds. The ntiso-pharyn\ is that part above the free edge of the soft Tin- .»,• p/tnn-nv communicates at the anterior pillar of tin- fauces with the mouth. The isl/imns, a niche between the faucial pillars containing the tonsils, is its anterior part. It is separated from the larvngo pharynx by fat pharyngO-epiglottic fold, which extends fn.ni the epiglottis to the side of the pharynx, as more particu- larly described later. The length of the male pharynx is about 13 cm. (about 5 in.), which is i.uelv much exceeded The greatest breadth (4-5 cni. ) is near the top of the l.irvn-o pharynx, rather In-low the greater horns of the hyoid bone. The greatest breadth in the naso-pharvn\, between tin- deepest points of the fossa of Rosenmiiller, is 3.5 <•'»•. '"' perhaps .1 little more. P.ehind the upper margin of the cricoid cartilage the lire.ulth is not over ;, cm., In-low which it abruptly diminishes. The antero- THE PHARYNX. 1.597 posterior diameter in the median line is greatest in the naso-pharynx, — about 2 cm. The back of the lower part of the soft palate is less than half that distance from the FIG. 1353. Frontal sinus Ha turcica Naso-pharyngeai fold >ssa of RosenmiiUer Eustachian tube 'haryngeal tonsil .-•5-. Salpingo-palatine fold ' pharyngeal fold Pterygo-mandibular fold Anterior pillar of fauces sSS* ••' i ^Of-'~~i Posterior ; ij|^~ pharynx / aJ "*n .-pharynx when the soft palate is not raised so as to cut off communication. Anteriorly are the nasal openings, described with the nose. The separation of the two regions on the lateral wall is determined by the naso- p/itin'nifi-al fold which runs from the base of the skull to the beginning of the soft palate. This fold is very irregular in course and development. It occasionally is grooved so as to present a furrow. Sometimes the furrow takes the place of the fold and at other times the fold joins that in front of the opening of the Eustachian lube Tliis orifice is on a level with the end of the inferior turbinate bone and less than i cm. behind it. It is usually a triangular opening without a distinct border below, although it may be oval or even round. The longest diameter is about i cm. The end of the cartilage of the tube curves over the top of the opening from the front and descends along its posterior border, producing a strong fold of the mucous mem- brane, the salpingo-pharyngeal, which descends to be lost in the lateral wall of the oro-pharynx, or even sooner. The salpingo-palathie fold in front of the opening of the Eustachian tube is, as a rule, less prominent and very variable. It is formed above by the bent end of the cartilage, and below by a small band of fibrous tissue, the salpingo-palatine ligament, running from the cartilage into the soft palate. The fossa of RosenmiilltT is a deep pocket at the angle of the pharynx between the posterior wall and the back of the projection of the cartilage of the tube. Its anterior and posterior walls are almost in contact and are often connected by accidental adhesions. This is the broadest part of the naso-pharynx. Adenoid collections-- the tubal tonsils — are found in varying degree about the orifice of the tube, especially over the fold behind it. The belly of the levator palati muscle makes a prominence in the lateral wall below the tubal orifice. The oro-pharynx opens into the mouth at the anterior pillar of the fauces. The posterior pillar, covering the palato-pharyngeus muscle, runs down the side of the pharynx as the palato-pharyngeal fold. It may be traced to the base of the superior horn of the thyroid cartilage, or, as is most common, it is lost on the lateral wall a little higher. The pharyngo-epiglottic fold above mentioned arises from the front of the epiglottis near the lateral edge and runs upward and backward across the pharynx. It may end soon, or it may reach the palato-pharyngeal fold, or, crossing this, may extend even as far as the salpingo-pharyngeal one. It contains muscular or tendinous fibres from the .stylo-pharyngeus. If well marked, it may bound below the niche containing the tonsil. The anterior wall of the oro-pharynx is formed, the mouth being closed, by the posterior vertical part of the tongue. The respiratory tract, passing through the nose, and the digestive, passing through the mouth, cross each other in the oro-pharynx, so that the former is the anterior below this point. The laryngo-pharynx, the lowest part of the pharynx, is, roughly speaking, the part below the level of the hyoid bone. It is separated from the oro-pharynx by the pharyniM) epi^lottic fold. In the middle of it is the opening of the larynx behind the epiglottis and enclosed by the aryteno-epiglottic and interarytenoid folds. The sinus pyrifortnis is a depression on either side of the entrance of the larynx between the arvteno epiglottk fold and the arytenoid cartilage internally and a part of th<- -n .it wing of the thyroid cartilage and the thyro-hyoid membrane externally. It is open behind. The thin mucous membrane lining the sinus has a transverse fold, formed 1>\ the superior laryngcal nerve, in front between the hyoid bone and the thyroid cartilage. The lower part of the palato-pharyngeal fold is seen in fro/en tions near the Superior horn of the thyroid cartilage at the lateral aspect of the deft, which is all that appears of the pharynx. The anterior wall behind the aryte- noid cartilages and the structures between them slants backward as it descends. I'.ehind the eric,, id cartilage it is vertical. Here the pharynx narrows to join the oesophagus, The mucous membrane of the pharynx is smooth, except for the elevations flections of lymphoid follicles. It is more loosely attached and more THE PHARYNX. 1599 disposed to be thrown into folds in the lower part. Mucous glands, on the other hand, are numerous in the upper part, scarce below ; they lie partly within the mucosa and partly in the submucous tissue and between the muscular bundles. The character of the pharyngeal epithelium varies in different localities. In the nasal pharynx the stratified ciliated columnar cells of the nasal fossa are continued as the covering of the pharyngeal mucous membrane, while the oro-pharynx is clothed with stratified squamous epithelium continued from the mouth. The last-named type of epithelium likewise covers the greater part of the laryngeal portion. The exact distribution of the two varieties of cells is subject to considerable individual variation. The ciliated columnar type extends laterally to include the openings of the Eustachian tubes, but lower clown gives place to the squamous. By no FIG. 1354. Base of skull Nasal septum „ • Naso-pharyngeal fold, 'i?. Lymphoid tissue Posterior pillar of fauces^ j, Faucial tonsil Pharyngo-epiglottic fold Cut edge of pharynx. L Uvula — Dorsuni of tongue losso-epiglottic fossa i — Median glosso-epiglottic fold Epiglottis, turned back Sinus pyriformis 'osterior surface of larynx Pharynx opened from behind ; epiglottis turned back. means the entire posterior surface of the soft palate is clothed with ciliated colum- nar cells, since the entire uvula and the edges of the palato-pharyngeal folds are invested with stratified squamous epithelium. The latter also covers the posterior wall of the pharynx and extends above as far as the vault. When covered with ciliated epithelium, the mucous membrane is redder, thicker, and contains more glands, but fewer papillae, than in those parts in which the squamous cells prevail. While containing much lymphoid tissue, fat is limited to a few deeply seated lobules of adipose tissue. Lymphoid Structures. — The upper part of the pharynx contains many lymphoid collections which make the surface uneven. They are much less frequent below. The larger and more constant masses are called ' ' tonsils. ' ' These include the faucial tonsils in the oro-pharynx, between the pillars of the fauces, the pharyn- i6oo HTM AN ANATOMY. FIG. 1355. Muscular fibres geal tonsil in the upper part of the pharynx, the tubal tonsils at the openings of the Eustachian tubes, especially on the posterior fold, and the lingual tonsil, con- sisting of the scattered adenoid collections over the posterior third of the tongue. Many additional lymph-nodules are scattered over the sides and roof, so connected as to form a lymphoid ring at the upper part of the pharynx. The faucial tonsils (Figs. 1326, 1353) are theoretically two almond-shaped masses of adenoid tissue, placed one on each side of the oro-pharynx, between the pillars of the fauces. The long diameter is vertical, and they have an outer and an inner surface and an anterior and a posterior border. The length is conventionally put at from 20—25 mm., the breadth at 15 mm., and the thick- ness at 10 mm. Practically, however, there is no definite shape nor size. In childhood the tonsil generally projects as a globular mass. If it extends more than slightly be- yond the level of the faucial pillars, it is said to be enlarged. After middle life it rises usu- ally but little from the floor of the niche. The shape of the free surface gives no clue to the size of the deep surface. In structure the tonsil is a mass of adenoid tissue en- closed in a fibrous capsule which is crossed on both the deep and free surfaces by a thin layer of muscular fibres. The superficial layer belongs to the palato-glossus ; the deep or external layer arises from the superior con- strictor and passes to the tongue. Beyond this externally are fat and areolar tissue. The closely adherent mucous membrane covers the free surface, which is full of pits from i or 2 mm. to i cm. in depth. The larger ones often expand be- FIG. 1356. jow the orifice, so that they may collect and retain secretions. A small free space, the snpratonsillctr fossa, lies above the tonsil at the apex of the niche containing it ; at the front of this there is very often a series of crypts with detached adenoid tissue about them, bur- rowing under the anterior pillar from behind and making a pouch beneath a fold, the plica trian- gular is. The adenoid tissue is continuous below with that of the tongue. The mucous membrane of the oro-pharynx shows many si altered lymphoid follicles in its walls, especially on the sides at and above the level of the tonsils. Vessels. — The arteries sup- plying the faucial tonsil are de- rived from several source*, and the arrangement of the vessels is extremely irregular ; the branch from the ascending pharyngral and that from the facial artery— one or both enter its base, while twigs from the lingual and descending palatine arteries, Section through faucial tonsil, showing general dis- position of lymphoid tissue. X 20. . •«' * J' ••&&&** Lymphocytes, invading .Epithelium .Blood-vessel . Lymphoid tissue v .• '- Portion of faucial tonsil, showitu; t-pitli.-lial lining of crypt invaded by esr;iping Kmplioi vies. X 325. THE PHARYNX. 1601 and perhaps others, reach it beneath the mucous membrane. Under ordinary cir- cumstances the tonsil is not very vascular, but receives a large quantity of blood when inflamed. There is a venous plexus communicating with the veins of the pharynx. The lymphatics probably communicate both with those of the dorsum of the tongue and with the glands near the angle of the jaw. Nerves. — The nervous supply is from the fifth and the glosso-pharyngeal. (The relations of the tonsils are given with those of the pharynx, page 1602.) The pharyngeal tonsil (Fig. 1353), sometimes called the third tonsil, is a median mass of adenoid tissue in the postero-superior wall of the pharynx, which reaches its greatest development in early childhood, generally dwindling after the twelfth year. When well developed, it lies below the occipital and the basi-sphenoid, nearly filling the space from the nasal septum to the back of the pharynx and almost touching on either side the folds made by the tubal cartilages. Its thickness in the median line is nearly i cm. Thus without being hypertrophied it nearly fills the naso- pharynx. The pharyngeal tonsil is a lobulated organ, the swellings being often regu- Foramen caecum FIG. 1357. Crista gall Cartilage of septum Vomer Permanent incisor Tongue Frenum of tongue Genio-hyoid Mylo-hyoid Hyoid bone Thyroid cartilage! Pituitary body Cranio-pharyngeal canal Pharyngeal tonsil Occipital bone Pharyngeal tonsil Anterior arch of atlas .Odontoid process .Uvula Epiglottis Third cervical vertebra Ventricle of larynx &\ -Cricoid cartilage Anterior portion of mesial sagittal section of child's head, probably of about three years. Reduced one-fourth. larly arranged around a central depression ; consequently it presents many pockets. The central one, which varies widely, is often improperly called the bursa pharyngea. It has absolutely nothing to do with the canal from the mouth to the sella turcica, through which a process of the oral tissue passes in early foetal life to the pituitary body (Fig. 1357), being decidedly behind that passage. Neither is it the true bursa pharyngea, since this term is more properly applied to a structure of uncommon occurrence, — namely, a still more posterior pocket in the mucous membrane leading from the roof of the pharynx, just behind its tonsil, into a small recess not over 1.5 cm. in length, on the under side of the basilar process. Relations of the Pharynx. — The structures behind the posterior wall have been mentioned (page 1596). The tip of the normal uvula hangs on a level near the lower part of the axis or the top of the third cervical vertebra. The tip of the epi- glottis is usually opposite the lower part of the third. The second and third cervical vertebrae are those behind that part of the pharynx seen through the open mouth. The pharynx ends at about the top of the seventh cervical vertebra. The lateral wall of the pharynx is very narrow, except in the region of the tonsils, where it reaches for- ward to the anterior pillar of the fauces. From the top of the thyroid downward it i6oa HTM AN ANATOMY. FIG. 1358. Pharyngcal tonsil of child one year old. (Schwabach.) Lymph-nodule is nothing more than the fold around the end of a transverse linear cleft. The whole lateral aspect is covered by a thick layer of areolar tissue, continuous with that of the carotid sheath. It is most convenient to give the relations of the lateral wall from below upward, excepting the nerves. The upper part of the lobes of the thyroid gland comes very close to the lower part of the pharynx, and may even touch it without undue enlargement. They separate the common carotid from the pharynx. A little higher this vessel is on the outer side of the great wing of the thyroid cartilage, but if the head be turned to one side the vessel of the other side will rest on the pharynx. The common carotid artery is very close to the pharynx just before its division. The inter- nal carotid lies against it until it reaches the skull. The beginning of the external carotid with its lingual and facial branches is also against it. The ascending pharyngeal artery runs along it, the middle meningeal lying against its upper part. The internal jugular vein is, probably, nowhere in direct contact with the pharynx unless just below the skull. The submaxillary gland touches it at the angle of the jaw. The sympathetic nerve comes in contact with the back or side of the pharynx. The vagus lies against the pharynx behind the internal carotid ; on reaching the common carotid, however, FIG- »359- it is in less direct contact. Its superior laryngeal branch crosses the pharynx to reach the thyro-hyoid membrane. The spinal accessory and the glosso-pharyngeal nerves lie against the upper part of the pharynx. The faucial tonsil lies about 2. 5 cm. above the angle and opposite a vertical line di- viding the ramus of the jaw into a front and a back half. It lies between the pillars of the fauces, and is separated from the mucous membrane by a thin layer of muscular fibres. The lower end reaches the tongue, the adenoid tissue being at times continuous between them. The tonsil is covered by the superior con- strictor. External to this is a yielding mass of areolar tis- sue, continuous with that of the carotid sheath, into which the tonsil may force its way if enlarged. This areolar tis- sue is bounded in front by tin- internal pterygoid muscle, and is pierced by the stylo- •; s,-, •,„,„,,, P..S,,.,,,,, u.,11.,1 ,,h.,M, 1X ,,t .i.n.i .showing part of Rl°ssus and the stylo-pha- ptenratw! tOMiL ryngrus, which subdivide it, leaving a small part of it be- tween them and the tonsil. At this level both carotids are at a considerable dis- tance from the tonsil. The internal is posterior and external, about 2 cm. distant. According to Zuckcrkandl, a transverse line through the posterior pillar will pass Bundles of muscular tis- sue of constric- tors -I'h.uviiKeal aponeurosts MO* — iu in THE PHARYNX. 1603 2 cm. in front of the vessel. The external carotid is placed more directly outward and is rather the nearer of the two. The parotid gland, according to Tillaux, sends a process in front of the styloid process, which reaches the lateral wall. This extension, however, does not seem to be by any means constant. Development and Growth of the Pharynx. — An account of the formation of the primitive pharynx is included in the Development of the Alimentary Tract (page 1694), the later changes being here noted. In the section on the bones it was shown that the chief peculiarities of the infant skeleton in this region are due to the small size of the face and the more horizontal base of the skull. The naso-pharynx has very little height, while, owing to the peculiar disposition of the parts, it has nearly the same antero-posterior diameter as in the adult. It is relatively broad and long, but very shallow. The tongue, in proportion, is much less thick at the base than later. The larynx is small, and, moreover, is placed higher in relation to the vertebral column, so that the termination of the pharynx is also higher. The position of the larynx at different ages is considered with that organ (page 1828). The soft palate is in the main horizontal at birth and about on a level with the top of the atlas. The uvula is rudimentary. In a child of probably not over three years we have found the tip of the uvula rather below the middle of the body of the axis. In Symington's section of a girl of thirteen it is pretty nearly in the adult position. In infancy the soft palate probably closes the passage into the naso-pharynx from below less perfectly than later. The opening of the Eustachian tube, although necessarily in the naso-pharynx, is in the foetus below the level of the hard palate. At birth it is at about that level, but rather below than above it. According to Disse, there is but little change for nine months, after which the opening is on the level of the inferior meatus. Proba- bly the adult position is generally reached after puberty. The opening is small in the infant and young child, and, owing to want of development of the cartilage, there is but a slight elevation about it and consequently but a small fossa of Rosen- miiller. The entire adenoid system of this region l has made but little progress before birth. At birth the pharyngeal tonsil is a very small collection of adenoid tissue at the back of the roof, covered by more or less converging folds of the mucous membrane. It is not necessarily present. During the first year it grows rapidly, and particularly forward, so that by the end of that time it extends to the back of the upper margin of the choanae. Under normal conditions the pharyngeal tonsil retains its relative size to the cavity of the pharynx up to twelve years ; but during this time the total amount of adenoid tissue has decidedly increased, owing to the development of the tubal tonsils. The faucial tonsils are developed in a recess of the primitive pharynx between the second and third visceral arches. By the fourth foetal month the tonsillar anlage presents a number of slit-like depressions, lined with entoblastic epithelium, from which secondary epithelial sprouts invade the neighboring mesoblast. This process continues after birth during the first year. The young connective tissue surrounding the epithelial sprouts — the latter being at first solid, but later possessing a lumen — becomes infiltrated by accumulating leucocytes and gradually assumes the character of adenoid tissue, the differentiation into distinct lymph-nodes, however, being delayed until after birth. The source of the lymphoid cells is a matter of dispute. Accord- ing to some, these elements are leucocytes from the circulation qaught within the young connective tissue ; others maintain that they are derived from the transforma- tion of the epithelium, the lymphoid tissue resulting from the mutual invasion and in- tergrowth between the en to- and mesoblastic elements. According to Hammar,2 who has carefully studied the development of the tonsils, the lymphoid cells are derived chiefly from the fixed connective-tissue elements. At birth the tonsils are insignifi- cant, but grow rapidly during the first year. At from the twelfth year to puberty the entire adenoid system of the pharynx enters upon a stage of retrogression. In the adult the pharyngeal and tubal tonsils are much smaller ; after middle age they undergo atrophy. 1 Escat : Evolution de la Cavit£ Naso-Pharyngienne, 1894. 2 Archiv f. mikro. Anat., Bd. xli., 1902. 1604 HUMAN ANATOMY. THE MUSCLES OF THE PHARYNX. The arrangement of the muscular tissue differs from the ordinary one of the digestive tract, inasmuch as the outer layer is approximately circular and the longi- tudinal fibres are largely internal. The chief elements are the three constrictors, which overlap one another from below upward, the stylo-pharyngeus, the palato- pharyngeus, and certain accessory and rather irregular bundles of muscular fibres. FIG. 1360. Condyles Internal carotid artery Internal jugular vein Central attachment of pharynx Internal pterygoid Styloid process posterior bully Stylo- pharyngeus Stylo-gloss us Stylo-hyoid — | Stylo-hyoid ligament Pharyn- geal aponeu- rosis Tip of great cornu of hyoid hum- Thyro-hyoid ligament Superior cornu of thyroid cartilage Middle constrictor Inferior constrictor uritiulinal muscle of orsophajrtis MinrI«-« of pharynx from In-hind ; jx.ition ,.t minim i-unstiictur has been removed. The superior constrictor < Fi^s. i ->V). 1360) arises from tlu- lower part of the intern. d ptcrv-oi.l pl.itr. hom tin- hamular procc-ss, tin- iHi-rygo-mandiluilar liiianu-nt which is stretched jron it to tlu- linmila of the lower jaw, from the neighboring- end of the invlo -hvoi.l rid^.-, and from the side of the tongue. From this origin the fibres pass backward to meet their fellows in a median raphe, which extends almost the THE PHARYNX. 1605 entire length of the posterior wall of the pharynx, being attached above to the pharyngeal tubercle on the under side of the basilar process. The upper edge of the muscle is concave on either side, not reaching the base of the skull and passing under the Eustachian tube, the vacant space being filled by the pharyngeal aponeu- rosis. The lower fibres pass somewhat downward as well as backward. The pterygo- mandibular ligament separates the superior constrictor from the buccinator, with which it would otherwise be continuous, forming a circle around the alimentary canal. FIG. 1361. Anterior margin of foramen magnum Styloid process Pharyngeal aponeurosls urn- — Stylo-hyoid ligament ''< Ml/ttliK 1 Stylo-glossus Stylo-hyoid Deep fibres of superior constrictor Palato-pharyngeus Great cornu of hyoid bone L— Stylo-pharyngeus _J_Thyroid cartilage Pharyngeal aponeurosis — CEsophagus Pharyngeal aponeurosis and longitudinal musculature, seen from behind. The middle constrictor (Figs. 1339, 1360) arises from the lower end of the stylo-hyoid ligament, from the lesser horn of the hyoid bone, and from the upper border of the greater horn. The fibres diverge from this narrow origin, the upper reaching the pharyngeal tubercle, the lower going to nearly the lower end of the pharynx, and all meeting their fellows in the median raphe. It conceals a consider- able part of the preceding muscle. 1606 HUMAN ANATOMY. The inferior constrictor (Figs. 1339, 1360), the thickest of the three, arises from the posterior part of the outer aspect of the cricoid cartilage, from the oblique line and the triangular surface below and behind it on the thyroid cartilage, including the inferior horn. It overlaps the preceding muscle, its upper fibres reaching to some 3 cm. below the base of the skull and the lower ones being nearly horizontal. The median raphe, which receives almost all the fibres, is wanting below. The lowest fibres are circular and continuous with the circular fibres of the gullet. The stylo-pharyngeus (Fig. 1361) arises from the inner side of the styloid process near its root and descends to the interval between the superior and middle constrictors near the hyoid bone, where it passes under the latter and ends by expand- ing in the side of the pharynx, some of its fibres going to the posterior border of the thyroid cartilage and others joining the expansion of the palato-pharyngeus. A bundle from the thyroid division passes to the side of the epiglottis, forming on the wall of the pharynx the fold known as the plica pharyngo-epiglottica. The fibres of the superior constrictor may be inseparable from the upper part of this layer. The salpingo-pharyngeus has been described in connection with the levator palati (page 1571). Variations. — Additional muscles are very common, being chiefly longitudinal bundles due to splitting of one of the normal muscles, especially the stylo-pharyngeus, or to new bundles of fibres arising from the base of the skull in the vicinity of the upper insertion of the pharyngeal fascia. There may be a pair of occipito-pharyngeal muscles, arising from the occipital bone on either side of the median line and descending to be lost in the posterior pharyngeal wall ; or there may be an asygos muscle instead. Bands may arise at the side from the petrous portion of the temporal bone or the spine of the sphenoid. Actions. — The general action of the pharyngeal muscles is sufficiently evident ; the constrictors decrease the size of the pharynx, probably drawing the larynx upward and backward at the same time. The longitudinal muscles raise the larynx and pharynx, acting chiefly on the latter. Vessels. — The arteries of the pharynx are from many sources and are irregu- lar. The chief is the ascending pharyngeal, which runs up near the posterior lateral angle. Occasionally, when enlarged, it is seen pulsating on the posterior wall. Branches from the facial play an uncertain part. The veins form the pharyngeal plexus situated outside of the constrictors and communicating in all directions. The chief outlets are by a pair of veins on each side, one going up to the internal jugular near the base of the skull and the other down to the external jugular or some of its tributaries (Luschka). A submucous plexus is particularly developed in the lower posterior wall, which opens into the pharyngeal plexus by several branches piercing the inferior constrictor. The following are nearly constant : a superior and posterior one near the middle line, one running outward on each side near the back of the thyroid cartilage, forming a part of the origin of the pharyngeal vein, and one passing forward to the superior thyroid vein.1 The lymphatics, which are numerous, run in the upper part to the prevertebral nodes and to the deep cervical system, as do the lower ones at another level. The presence of lymphatic nodes behind the naso-pharynx is of practical importance, as they are sometimes inflamed and may suppurate. They lie near the fossa- of Rosenmiiller. Nerves. — The constrictors are supplied by the pharyngeal plexus, the lower receiving fibres also from the recurrent laryngeal. The stylo-pharyngeus is supplied by the glosso-phai yngeal. The nerves of the mucous membrane are from the glosso- pharyngeal, the pneumogastric, and the sympathetic, to a great extent in a plexiform arrangement. PRACTICAL CONSIDERATIONS : THE PHARYNX. Tin- pharvnx in.iv !>.- said to present only three sides for consideration, but its eontinuitv al.ove with tin- naies, anteriorly with the mouth, and below \\ith the ori- ..f the l.uvnx and u-suphagiis associates it intimately with the diseases of those reoi,,ns. Tli-- naao pharvnx and tin- laryngeal relations \\ ill he considered with the Resirator I ^e 182. Respiratory I p^e 1829). 1 I'.iiuar ft I.ap.-yn- : Cnmpte- rendus de 1'Acad.