0 Columbia 53niDer^ftPcJk.j College of S^i)piitiani anb burgeons Hibrarp Digitized by the Internet Archive in 2010 with funding from Open Knowledge Commons (for the Medical Heritage Library project) http://www.archive.org/details/humananatomyincl19192pier Human Anatomy HUMAN ANATOMY INCLUDING STRUCTURE AND DEVELOPMENT AND PRACTICAL CONSIDERATIONS BY THOMAS DWIGHT, M.D., LI..D. J. PLAYFAIR McMURRICH, PH.D. PARKMAN PROFESSOR OK ANATOMY IN HAKVARI) PROf-liSSOR OK ANATOMY IN THK UNIVtRSITV OV INIVKRSITY MICHIGAN CARL A. HAMANN, M.D. GEORGE A. PIERSOL, M.D.. SC.D. PROKKSSOR OF ANATOMY IN WliSl liRN RliSERVK PROKliSSOR OK ANATOMY IN THK UNIVERSITY OF VNIVERSITY PENNSYLVANIA AND J. WILLIAM WHITE, M.D., PH.D., LL.D. JOHN RHEA BARTON PROFESSOR OK SURGERY IN THE UNIVERSITY OK FENNSYLVANIA WITH SEVENTEEN HUNDRED AND THIRTY-FOUR ILLUSTRATIONS, OF WHICH FIFTEEN HUNDRED AND TWENTY-TWO ARE ORIGINAL AND LARGELY FROM DISSECTIONS BY JOHN C. HEISLER, M.D. PROFESSOR OK ANATOMY IN THE MEDICO-CHIRURGICAL COLLEGE EDITED BY GEORGE A. PIERSOL VOL. II. SEVENTH EDITION PHILADELPHIA & LONDON J. B. LIPPINCOIT COMPANY Copyright, 1906, 1907, 1908, 191 1, 1913, 1916, 1918, 1919, by J. B. Lippincott Company. Entered at Stationers' Hall, London, England. AU Rights Utstrved. Gk vv^2. ELECTROTTPF.D AVD PRIKTED BT J. ». LIPPIXCOTT COMPAST, PHILADELPHIA, P. 8. A. CONTENTS. \OL. II. THE NERVOUS SVSTICM. PACK General Considerations 996 The Nervt)iis Tissues 997 The Nerve-Cells 997 The Nerve-Fibres looo Neuroglia 1003 The Nerve-Trunks 1006 The Ganglia 1007 Development of the Nervous Tissues . . . 1009 Nerve-Terminations 1014 Motor Endings 1014 Sensory Endings 1015 The Central Nervols Svstem. 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 1030 Microscopical Structure 1030 White Matter 1036 Fibre Tracts 1039 Blood-\'essels of Spinal Cord 1047 Development of Spinal Cord 1049 Practical Considerations : Spinal Cord. . 105 1 .Malformations 105 1 Injuries 1052 Localization of Lesions 1053 The Brain 1055 General Description 1056 General Development 1058 Derivatives from the Rhombencephalon 1063 The Medulla Oblongata 1063 Internal Structure 1068 The Pons \'arolii 1077 Internal Structure 1078 The Cerebellum 1082 Lobes and Fissures 1084 Architecture 1088 Internal Nuclei 1088 Cerebellar Cortex 1090 Cerebellar Peduncles 1093 The Fourth X'entricle 1096 Development of the Hind-Brain Derivatives iioo The Medulla 1 101 The Pons 1 103 The Cerebellum 1 103 The Mesencephalon 1105 The Corpora Ouadrigemina 1106 The Cerebral Peduncles 1107 The Sylvian Aqueduct 1108 Internal Structure of the Mid-Brain 11 12 The Tegmentum 1 1 r2 The Crusta 1115 The Median Fillet 11 15 The Posterior Longitudinal Fas- ciculus 1116 The Mesencephalon — Continued Development of Mid-Brain 1 1 17 The I-'ore-Brain 11 19 The Diencephalon 11 19 The Thalamus 11 19 .Structure 11 20 Connections 1121 The Epithalamus 11 23 The Trigonum Habenulje 1 123 The Pineal Body 1 1 24 The Posterior Commissure 11 25 The Metathalamus 1 1 26 The Hypothalamus 1 1 27 The Subthalamic Region 1 127 The Corpora Mammillaria IJ28 The Pituitary Body 1 129 The Third Ventricle 1 131 The Telencephalon 1 132 The Cerebral Hemispheres 1133 Cerebral Lobes and Interlobar Fissures 1 135 Lobes of the Hemispheres 1 139 Frontal Lobe 1 139 Parietal Lobe 1143 Occipital Lobe 1 145 Temporal Lobe 1 147 Insula 1 149 Limbic Lobe 1150 The Rhinencephalon 1151 The Olfacton,- Lobe 1151 Architecture of the Hemispheres 1155 The Corpus Callosum 1 155 The Fornix 1 158 The Septum Lucidum 1 159 The Lateral Ventricles 1160 Internal Nuclei of the Hemisphere 1169 The Caudate Nucleus 1169 The Lenticular Nucleus 1 169 The Ciaustrum 1172 The Amygdaloid Nucleus 1172 The Internal Capsule 1173 Structure of the Cerebral Cortex 11 75 The Nerve-Cells of Cortex 1176 The Nerve-Fibres of Cortex 11 79 Variations in Cerebral Cortex 1180 White Centre of the Hemisphere 1182 The Association Fibres 1182 The Commissural Fibres 1184 The Projection Fibres 1187 Development of the Derivatives of Fore- Brain 1 189 The Pallium 1189 The Sulci and Gyri 1190 Histogenesis of Cerebral Corte.x. .. . 1192 The Rhinencephalon 1193 The Corpus Striatum 1 193 The Diencephalon 1 193 The Cerebral Commissures 1194 Measurements of the Brain 1195 V VI COxXTENTS. The Membranes of the Brain The Dura Mater The Pia Mater The Arachnoid Tile Pacchionian Bodies The Blood-Vessels of the Brain Practical Considerations : The Brain and Its Membranes Con,2:enital Errors of Development. . The Menins^es Cerebral Hemorrhaj^e Cerebral Localization Cranio-Cerebral Topography The PEKirnHKAL Nervous Svstem The Cranial Nerves The Olfactory Nerve The Optic Nerve The Oculomotor Nerve The Trochlear Nerve The Trigeminal Nerve The Gasserian Ganglion The Ophtiialmic Nerve and Branches The Ciliary Ganglion The Maxillary Nerve and Branches The Spheno-Palatine Gang- lion The Mandibular Nerve and Branches The Otic Ganglion The Submaxillary Ganglion Practical Considerations : The Tri- geminal Nerve The Abducent Nerve The Facial Nerve Practical Considerations The Auditory Nerve The Glosso-Pharyngeal Ner\'e The \'agus or Pneumogastric Nerve Practical Considerations The Spinal Accessory Nerve Practical Considerations The Hypoglossal Nerve Practical Considerations The Spinal Nerves The Posterior Primary Divisions The Cervical Nerves The Thoracic Nerves The Lumbar Nerves The Coccygeal Nerve The Anterior Primary Divisions The Cervical Nerves The Cervical Plexus and Branches The Phrenic Nerve Practical Considerations The Brachial Plexus and Branches The P^xternal Anterior Thoracic Nerve .• The Musculo-Cutaneous Nerve The Median Nerve 197 19S 202 203 205 206 207 207 208 209 210 214 220 220 223 225 228 230 232 233 236 237 240 242 246 247 24S 249 250 254 256 260 265 272 274 275 275 277 27S 279 281 282 2S2 2S4 284 285 2S6 290 292 292 297 298 298 PAGE The Brachial Plexus and Branches — Cotiiinued Practical Considerations 1301 The internal Anterior Thoracic Nerve 1303 The Lesser Internal Cutaneous Nerve 1303 The Internal Cutaneous Nerve 1303 The Ulnar Nerve 1303 Practical Considerations 1306 The Subscai)ular Nerves 1306 The Circumflex Nerves 1307 Practical Considerations 1308 The Musculo-Spiral Nerve 1308 Practical Considerations 1374 The Thoracic Nerves 1314 Practical Considerations 1318 The Luml)ar Plexus and Branches. ...'.. 1319 The llio-Hypogastric Nerve 1320 The Uio-lnguinal Nerve 1321 The (ienito-Crural Nerve 1322 The External Cutaneous Nerve 1324 The Obturator Nerve 1324 The Accessory Obturator Nerve. .. . 1324 The Anterior Crural Nerve 1327 Practical Considerations: Lumbar Plexus 1330 The Sacral Plexus and Branches 1331 The Great Sciatic Nerve 1335 The External Popliteal Nerve 1336 The Anterior Tibial Nerve 1336 The Musculo-Cutaneous 1338 The Internal I'opliteal Nerve 1339 The Posterior Tibial Nerve 1342 The Pudendal Plexus and Branches 1345 The Small Sciatic Nerve 1348 The Pudic Nerve 1349 The Coccygeal Plexus 1352 Practical Considerations : Sacral Plexus 1352 The Sympathetic Nerves 1353 General Constitution and Arrange- ment 1355 The Gangliated Cord 1356 Kami Communicantes 1356 Cervico-Ceplialic Portion of Gangliated Cord "^ 1358 Tlie Superior Cervical Ganglion 1359 The INIiddle Cervical Ganglion 1362 The Interior Cervical Ganglion 1362 Thoracic Portion of Gangliated Cord. .. . 1364 The Splanchnic Nerves 1364 Lumbar Portion of Gangliated Cord. .. . 1366 Sacral Portion of Gangliated Cord 1367 The Plexuses of the Sympathetic Nerves 1367 The Cardiac Plexus 1367 The Solar Plexus 1368 Subsidiary Plexuses 1369 The Hy]KDgastric Plexus 1374 Subsidiary Plexuses. 1374 Practical Considerations : The Sympa- thetic Nerves 1375 Development of the Peripheral Nerves. . 137.5 THE ORGANS OF SENSE. The Skin. General Description 138 1 Structure 1 3S2 The Hairs 13S9 Structure 1391 The Nails " I394 The Cutaneous Glands 1397 The Sebaceous Glands 1397 The Sweat Glands 1398 Development of the Skin and its Append- ages 1400 CONTENTS. Vll The Nose. The Outer Kosc i.W4 Cartilages of tlie Nose 1404 Practical Considerations : The External Nose 1 4*^'7 The Nasal I-'oss^e i4"9 The X'estibule 14^9 The Seinuin 14 10 The Lateral Wall 141U The Nasal Mucous Membrane 1413 The (Olfactory Resiion 1413 The Respiratory Region 141 5 Jacobson's Or.uan 1417 Practical Consitlerations : The Nasal Cavities 1417 The Accessory Air-Spaces 1421 The Maxillary Sinus 1422 The hVontal Sinus 1423 The Ethmoidal Air-Ceils 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 Eve. The Orbit and its Fascia; 1436 Practical Considerations 143S The Eyelids and Conjunctiva 1441 Practical Considerations 1446 The Eyeball 1447 Practical Considerations 144S 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 — Coittittued The Retina Practical Considerations The Optic Nerve Practical Consitlerations The Crystalline Eens Practical Considerations The \'itreous Body Practical Considerations The SusjK-nsiJry Apparatus of the Lens. The A(|ue()us Humor and its Chamber. Practical Considerations The Lachrymal Ajjparatus The Lachrymal C.land The Lachrymal Passages Practical Considerations Development of the Eye The Ear. The External Ear The Auricle The External Auditory Canal Practical C(jnsiderations The Middle Ear. The Tympanic Cavity The Membrana Tympani The Auditory Ossicles The Mucous Membrane The Eustachian Tube The Mastoid Cells Pract. Consid.: The Middle Ear The Tymi^uiic Cavity The Tympanic Membrane The Eustachian Tube The Mastoid Process and Cells. .. . The Internal Ear The Osseous Labyrinth The X'estibule The Semicircular Canals The Cochlea The Membranous Labyrinth The Ctricle The Saccule , . The Semicircular Canals The Cochlear Duct The Nerves of the Cochlea Development of the Ear 1462 1468 1469 1470 1471 1473 1473 1474 [475 [476 1476 1477 1477 [478 1479 1480 •484 '4S4 t487 1490 1492 1492 1494 1496 [500 1 501 [504 '504 1504 [505 1507 1508 1510 t5ii 15" ;i2 1513 '514 [514 1515 '515 ■517 521 [523 THE GASTRO-PULMONARV SVSTEAL General Considerations 1527 Mucous Membranes 1528 Structure 1 52S 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 ALniEXTARV Canal. The Mouth 153S The Lips, Cheeks and Vestibule 153S The Teeth 1542 Description of Individual Forms. .. . 1543 Structure of the Teeth 154S The Enamel 1 54S The Dentine 1550 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 157.5 The Glands of the Tongue 1575 The .Muscles of the Tongue 1577 The Sublingual Space 15S1 The Salivary Glands 15.S2 The Parotid Gland 1582 The Submaxillar}- Gland 1583 The Sublingual Gland 1585 Structure of the Salivary Glands. . . . 1585 Development of the Oral Glands. . . . 1589 Vlll CONTENTS. f'AGE Practical Considerations : The Mouth . . . 15S9 Malformations : Hareli|) and Cleft Palate 1589 The Lips 1590 The Gums 1550 The Teeth 1591 The Roof of the Mouth 1592 The F"loor of the Mouth 1593 The Cheeks 1594 The Tongue .. 1 594 The Pharynx 1596 The Naso-Pharynx 159S The Oro-Pharynx 1598 Tlie Laryn.<;o-Pharynx 1598 The Lymphoid Structures 1599 The Faucial Tonsils 1600 The Pharyn<:^eal Tonsil 1601 Relations of the Pharynx 1601 Development and Growth of Pharynx 1603 Muscles of the Pharynx 1604 Practical Considerations: The Pharynx. . 1606 The disophagus 1609 General Description 1609 Course and Relations 1609 Structure 161 1 Practical Considerations : CEsophagus . . 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 Ulcers and Cancer 1631 Dilatation and Displacement 1631 Operations on the Stomach 1632 The Small Intestine 1633 General Description 1633 Structure 1 634 The Duodenum 1644 Duodeno-Jejunal Fosste 1647 Interior of the Duodenum 1648 The Jejuno-lleum 1649 The Mesentery and Topography .... 1650 Meckel's Diverticulum 1652 Practical Considerations: The Small In- testine 1652 The Peritoneal Coat 1652 The Muscular Coat 1653 The Mucous and ^ibmucous Coats 1653 Ulcers of the Duodenum 1653 Infection 1654 Typhoid Ulcers 1654 Contusion and Rupture 1654 Obstruction 1 655 Operations 1 656 The Large Intestine 1657 General Description 1657 Structure 1 657 The Caecum 1 660 The Vermiform Appendix 1664 Peritoneal Relations 1665 Pericsecal Fossa; 1666 Tlie Large Intestine — Cotitinued Retro-Colic P'ossce 1667 The Colon 1668 General Description 1668 Peritoneal Relations 1670 The Sigmoid l'"lexure 1671 Development and (irowth 1671 The Rectum 1672 The Anal Canal 1 673 The Anus 1673 Muscles and Fascia,' of Rectum and Anus 1675 The Ischio-Rectal Fossa 1678 Pract. Consid.: The Large Intestine .. . 1680 The CiLCum 1680 The X'ermiform Appendix 1681 Etiology of Api)endicitis 1681 Anatomical Points relating to the Symptoms and to the Treat- ment of Appendicitis 1683 Operations for Appendicitis .... 1685 The Colon and Sigmoid Flexure. .. . 1685 Distention and Rupture 1686 Displacements 1686 Obstruction and Stricture 1687 Wounds 16S8 Operations 1688 The Rectum and Anal Canal 1689 Development of the Alimentary Canal . . 1694 Formation of the Mouth 1694 Formation of the Anus 1695 Differentiation of the Body-Cavity . . 170a Development of the Peritoneum. .. . 1702 The Liver 1705 General Description 1705 Borders and Surfaces 1707 Blood-Vessels 1709 Structure 1712 The Hepatic Duct 1718 The Gall-Bladder 1719 The Common Bile-Duct 1720 Peritoneal Relations of the Liver . . . 1721 Position of the Liver 1722 Development and Growth 1723 Practical Considerations : The Biliary Apparatus 1726 Anomalies in Form and Position of the Liver 1726 Hepatoptosis and Hepatopexy 1726 Obstruction of Hepatic Circulation. . 1727 Wounds and Hepatic Ab.scess 1727 Malformations of Gall-Bladder 1729 Wounds and Rupture 1729 Distention and Cholecystitis 1729 The Cystic and Common Bile-Ducts. 1731 Operations on Gall-Bladder and Bili- ary Ducts 1732 The Pancreas 1732 General Description 1732 Structure i734 Pancreatic Ducts 1736 Development 1737 Practical Considerations : The Pancreas. 1738 Malformations 1 738 Injuries 1738 Pancreatitis 1 739 The Peritoneum 174° General Considerations 1740 The Anterior Parietal Peritoneum . . 1743 The Anterior Mesentery 1744 The Posterior Mesentery : Part I . . . 1746 The Posterior Mesentery : Part II . . 1751 The Posterior Mesentery : Fart III.. 1753 CONTENTS. Practical Considerations : The Perito- neuni AnatDinical Routes for Infections. . . IVritonitis anatomically considered. . Abiloniiiial Hernia (ieiK-ral Considerations .'.... I'redisposinj; anatomical conditions. Inguinal I icrnia Anatomy of Injjuinal Canal Anatomy of Indirect Inguinal Hernia X'arieties of Inguinal Hernia. .. . Anatomy of Direct Inguinal Her- nia Anatomical Considerations of Treatment Femoral I lernia Anatomy of Femoral Canal. .. . Anatomical Considerations of Treatment Umbilical Hernia \'entral I lernia Lumbar Hernia Obturator Hernia Sciatic 1 lerni:e Perineal I lernice Diaphragmatic Hernice Intraabdominal Herni:e Accessory Organs of Nutrition. The Spleen General Description Structure Peritoneal Relations Development and Growth Accessory Spleens Practical Considerations: The Spleen. . . . The Thyroid Body General Description Structure Development Accessory Thyroids Practical Considerations : The Thvroid Body '..... The Parathyroid Bodies General Description Structure The Thymus General Description Structure 754 7. SI 756 759 759 759 763 763 766 767 773 773 774 775 776 777 777 77« 778 77S 779 781 7S1 7S3 ■785 7S7 787 787 789 789 791 793 793 794 795 795 795 796 796 798 The Thymus — Con/iniird Development and Changes 1800 The Suprarenal Hoilies 1801 General I )escriplion 1801 Structure 1S02 Development and Growth 1804 Accessory .Sujjrarenals 1805 Practical Considerations: The Suprarenal Bodies 1806 The Anterior Lobe of the Pituitary Body, 1807 Development 1808 The Carotid Body 1809 The Coccygeal Body 181 1 The Aortic Bodies 1812 The Organs of Respiration. The Laryn.x 1813 Cartilages, Joints and Ligaments. .. . 1813 Form of Laryn.x and Mucous Mem- brane : 1818 Muscles of the Larynx 1825 Changes with Age and .Se.x 1828 Practical Considerations : The Larynx. . 1828 The Mediastinal Space 1832 Practical Considerations 1833 The Trachea 1834 General Description 1S35 Structure 1835 Relations 1836 Grf)wth and .Subsequent Changes 1837 Bifurcation of Trachea and Roots of Lungs 1837 The Bronchi 183;^ Practical Considerations : The Air-Pas- sages 1840 The Lungs 1S43 General Description 1843 Lobes and Fissures 1845 Physical Characteristics 1846 The Bronchial Tree 1847 The Lung Lobule 1S49 Structure ' 1851 Blood-Vessels 1853 Relations to Thoracic Walls. .. . 1855 The Pleurae 1858 General Description 1858 Relations to the Surface 1859 Structure i860 Development of the Respiratory Tract . . 1S61 Practical Considerations : The Lungs and Pleurae 1864 THE URO-GENITAL SYSTEM. The Urinary Organs. The Kidneys General Description Position and Fixation Relations Architecture Structure , Practical Considerations: The Kidneys . . Anomalies of Form, Size or Num- ber Anomalies of Position Renal Calculus Injuries and Tumors Operations The Renal Ducts Pelvis of the Kidney The Ureter Structure 1S69 1S69 1870 1S73 1875 1877 1S87 1 887 1887 1S90 1893 1893 1S94 1894 1S95 1896 Practical Considerations: The Ureters . . 1898 Congenital Anomalies 1898 Ureteral Calculus 1S99 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 191 1 Retention of Urine 1912 Rupture and Wounds 1913 Cystitis and Vesical Calculus 1914 The Male Perineum 1915 The Triangles 1916 CONTEXTS. PAGE Pract. Consid. : The Bladder — Continued The Perineal Interspaces 1916 Landmarks 191S Lateral Lithotomy 1919 Median Lithotomy 1921 Suprapubic Lithotomy 1921 The Female Bladder 1922 The Urethra 1922 The Prostatic Portion 1922 The Membranous Portion 1923 The Spon.s:>- Portion 1923 The Female Urethra 1924 Structure 1924 Practical Considerations: Male Urethra. . 1927 Congenital Abnormalities 1927 Clinical Division of Urethra 192S Rupture of Urethra 1930 Anatomical Consideration of Ure- thritis 1930 Stricture of Urethra 1931 Urethral Instrumentation 1933 Development of the Urinarv- Organs. .. . 1934 The Pronephros 1934 The Mesonephros (Wolffian Body) . . 1935 The Metanephros ( Kidney) 1937 The Bladder and Urethra 193S The Male Reprodictive Organs. The Testes 194^ General Description 1941 Architecture 1942 Structure 1942 Spermatogenesis 1944 The Spermatozoa 1946 The Epididymis 1947 General Description 1947 Structure 1 947 The Appendages of the Testicle 1949 The Appendi.x Testis 1949 The Appendi.K Epididymidis 1949 The Paradidymis 1950 The Vasa Aberranlia 1950 Practical Considerations: The Tes- ticles 1950 Congenital Anomalies 1950 Orchitis 1951 Epididymo-Orchitis 1952 Castration 1952 Hydrocele 1953 The Spermatic Ducts 1953 The \'as Deferens 1954 The Ejaculatory Duct 1955 Structure of Spermatic Duct 1956 The Seminal Vesicles 1956 General Description 1956 Structure 195S Practical Considerations: The Seminal Vesicles 1959 The Spermatic Cord i960 Practical Consideraticms: The Spermatic Cord 1961 The Scrotum 1961 General Description 1961 Coverings of the Testicle 1963 Practical Considerations: The Scrotum . 1964 The Penis 1965 General Description 1965 The Corpora Cavernosa 1966 The Corpus Spongiosum 1967 The Glans Penis 1968 Structure 1968 Practical Considerations: The Penis.... 1972 Congenital Abnormalities 1972 Pract. Consid.: 'I he Penis — Continued Circumcision Contusions and Wounds Amputation The Prostate Gland General Description Position and Relations Structure Development Pract. Consid.: The Prostate Gland . Relations to Generative System . . . Injuries Hypertrophy Operations The Glands of Cowper General Description Structure Development. .• The Female Reproductive Organs The Ovaries General Description Position and Fixation Structure Follicles and Ova The Human Ovum Corpus Luteum Development Variations Practical Considerations : The Ovaries. . The Fallopian Tubes General Description Course and Relations Structure Development and Changes Variations Practical Considerations : The Fallopian Tubes Rudimentary Organs The Epoophoron Gartner's Duct The Paroophoron \'e5icular Appendages The Uterus General Description Attachments and Peritoneal Rela- tions The Broad Ligament The Round Ligament Position and Relations Structure Development and Changes Menstruation and Pregnancy Practical Considerations : Uterus and Attachments Compartments of Pelvis Displacements of Uterus The Broad Ligament The Round Ligaments The \'agina General Description Relations Structure Development Variations Practical Considerations: The Vagina. Relations to Uterine Cer\ix Fistulse The Labia and the Vestibule The Labia .Majora The Mons Pubis The Labia Minora The Vestibule 973 974 975 975 975 976 977 979 979 979 979 9S0 982 9S4 9S4 9S4 9S4 9S5 9S5 986 987 988 990 990 993 995 995 996 996 997 997 999 999 1999 2000 2000 2001 2002 2002 2003 2003 2004 2004 2005 2007 2007 2010 2012 2012 2013 2014 2014 2015 2Ql6 2016 2016 2017 2019 20J9 2019 2019 2020 2021 2021 2021 2022 2022 CONTENTS. xi The Clitoris 2024 The iUilhiis V'estihiih 2025 The Cilaiuis of 15arlliolin 2026 Pract. Consid.: Tlie Mxtenial Genitals . . 2027 The Mammary Glaiuls 2027 ( "leiu-ral I )escrii)lioii 2027 Stnictiire 2029 Milk ami Coloslruin 2030 1 )cvelopment 2032 \'ariations 2033 Practical Considerations : The Maniniary Glands 2033 The NipiJle 2033 Paths of Infection 2034 Carcinoma 2035 I'ractical Considerations : The Mammary (jlaiuls — ConiiiiiuJ Kt-moval of the lireast 2036 Development of Reproductive Organs. . . 2037 ( iencral Consitkralions 2037 Tin- Indilfcrent Sta^e 2038 Dilferenli.ilion of the Male Type. .. . 2038 Descent of the Testis 2040 Differentiation of the I-emale Type. . 2042 Descent of the Uvary 2043 The IC.xternal Orj^ans 2043 In the 1-Y-male 2044 In the .Malt 2044 Summary of Development 2045 The I'emale Perineum 2046 VOLUME II. THE CENTRAL NERVOUS SYSTEM THE NERVOUS TISSUES THE SPIXAL CORD THE BRAIN THE PERIPHERAL NERVOUS SYSTEM THE CRAN'IAL, SPIN'AL AXD 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. 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 ner\'ous 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 ner\-ous 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 nei^ve-Jibres, 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 neu7-one, {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 cent7-ipelal or afferent fibre (a). The impressions thus carried are transferred to the second element, the motor 7ieurone (B), which in response sends out the impulse originating within the cell-body (nerve-cell) along the process known as the centri- ftigal 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 aflerent impressions are distributed to ner\e-cells situated not only in the immediate vicinity of the first neurone, but at diflerent and often distant levels. Although ver\' exceptionally the relation between the neurones may perhaps be that of actual continuitv 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, — conti_guit\- 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 ner\'Ous system is often divided, therefore, into a central Sind a pen'plieral portion. The former, also known as the cerebro-spinal axis, includes the brain and spinal cord and contains the chief axial collections of ner\'e-cells ; the peripheral portion, on the contrary, 996 Diagram showing fundamental units of ner\-ous system. A. sensory neurone, conducting afferent impulses by its pro- cess ia) from periphery ^S\\ B. motor neurone sending: efferent impulses by its process {e) to muscle. THE NERVOUS TISSUES. 997 contains the ncive-cclls of the sensory ganglia and is princijjally composed of the nerve-fibres that ])ass to antl 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 inner\ation 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 a])pear- ance, but are seen to be made up of a darker and a lighter substance. The fcjrmer, \\\G. grav 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 7vhite matter, is (\\\ki to its chief constituents, the medullated nerve-tibres, in conjunction with its relatively meagre blood supi^ly. Fig. 835. THE NERVOUS TISSUES. The Neurones. — The neurones, the essential morphological units of the nervous system, consist of the cell-body and the process^. The latter, as seen in the case of a typical motor neurone (Fig. 835 J, 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 {^b) 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 Dendrites Aiborization of axone Telodendriou 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). 998 HUMAN ANATOMY. Fig Semidiagrammatic representation of structure of neurone ; a, axone. On reaching- their destination the axones terminate in end-arborizations (telodendria) of various forms, in a manner similar to the dench'ites. According to the distriljution of their axones, tlie neurones are divided into two classes. In those of the first, known as ccl/s of type I, the axone is continued as a nerve-fibre and is, therefore, relatively long. Soon after leaving the cell-body such axones give off delicate lateral processes, the collaterals, which, after 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 class, cells of type If possess short axones that are not continued as nerve-fibres, l)ut almost immediately break up into complex end-arborizations or neio'opodia (Kolliker), limited to the gray matter. The processes of the se?isory neurones, as in the case of those constituting the spinal and other ganglia connected with afferent nerves, are so modified during development (Fig. 839) that later both dendrites and axones arise in common from the single 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 belonging 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 with a conspicuous Fig. 838. nucleolus. Their cytoplasin varies in appearance with the method of fixation and staining to such an extent that considerable uncertain- ty exists as to the relation of many described details to the actual structure of the cells. It may be accepted as established, however, that the cell-body of the neurone consists of 2. ground substa?ice, homogeneous or finely granular, in which delicate yf(^;77/r^ and masses of chromatophilic granules are embedded ; in addition, a variable amount of brown or blackish pig7nent is com- monly present in the vicin- ity of the nucleus. The presence of the fibrillae within the nerve-cell, long ago maintained by Max Schultze but later disre- garded, has been placed beyond question by the researches of Apathy, Bethe, Cajal and others. The signifi- cance and relations of the fibrillae to the nerve-cell, however, have given rise to warm Nerve-cells of human spinal cord stained to show Nissl bodies ; D, dendrites ; .,4, axones; C, implantation cone ; A^, nucleus ; ^1/, nucleolus. X 400. Til)'; Ni:R\()rs TissrEs. 999 discussion. The ()l)scrvati<)iis hasnl [.\\>ii\\ llic iiii|)r()\(.(l inctliods of silver-staining intioiliicctl by Cajal have coiUiibiili-il nuich towards the sohition of these questions, and, at |)resent, the most experienced histcjloj^isls inehne towards the view tiiat tlie tibrilhe demonstrable within the nerve-cell are limiltd to the body and processes of that particular neurone and do not unite with the hbrillic* of other neurones. When adequately dilTerentiated by successful staining, the fibrilla; 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 arborizatifjns (Retzius). After special staining with methylene blue, or other basic anilines, the chro matophilic granules ajjpiar dee])ly colored and arranged in groups or masses of vary- ing form and size. Such aggregations, known as A7ss/ bodies, after the German histologist whose elaborate studies and thet)ries concerning the structure (jf the nerve- cell have gi\'en prominence to these masses of " stainai^le substance," are usually most conspicuous in the vicinity of the nucleus. Collecti\ely, 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 a.xis-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, eithei at its base or at a point some distance from the cell -body. * Notwitlistandiiii^ the elaborate classification of nerve-cells and the theories based upon the Nissl bodies, their si.uniiicance is still debatable, althouj^h in the lijjht 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 i)itya 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. 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 multipolar. 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 cercbro-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. lOOO HUMAN ANATOMY. Fig. S40. connected with the acoustic nerve. An interesting modification of bipolar neurones is presented by the olfactory cells, whose dendrites are represented by the extremely short processes embedded within the nasal mucous membrane, whilst the axones are prolonged as the fibres of the olfactory nerves into the cranial cavity to end in tclodendria 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. 841). 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 j. Within the cerebellar cortex are likewise found examples of Bipolar neurones; a, from olfactory miicous membrane — dendrite is above; by from retina. {Modified from Cajal.) Fig. 841. Multipolarnerve-cells of various forms; A, from spinal cord ; B, from cerebral cortex; C, from cerebellar cortex (Purkinje cell) ; a, axone ; c, implantation cone. the multipolar neurones of Golgi's type II, whose axones 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 tlie nerve-fibres are not independent elements, but that all are the processes of neurones — either the axones of those that are prolonged into fibres (type I), or the dendrites of those situated within the spinal and other sensory peripheral ganglia. Although neurones exist which are not continued as nerve-fibres, the latter are always connected THK NKR\'(KS rissri:s. lOOI Axis-cylinders Axolciiiina Medullary sheath Node of Rainier Neurilemma Medullated nerve-fibres, as seen in longi- tudinal seclions of spinal nerve. >' 500. with neurones. Rccoj^iii/.in^, llicrcloic, thai the ncrvc-liljix-s art,- only processes of neurones, their separate description is justified only as a matter of convenience. The fundamental |)art of every nerve-fibre is the central cord, commcjiilv kncnvn as the axis-cylinder, which is composed of threads of ^reat delicacy, the axi's- Jibrillcc, proloni;\'d from the nerve-cell and embedded within a scmilluid interfibrillar substance, the neuroplasm , the entire cord per- haps beinu^ enclosed by a delicate structureless sheath, the axo/cniina. The existence of the a.volemma as a distinct sheath, lunvever, is (|ues- tionable, the a|)pearance of such investment not improbably beinji;^ due to a local condensation of the framework of the medullary coat immediately around the a.xis-cylinder. In the case of the typical fibres, such as form the chief constituents of the peripheral nerves distributed to varit)us parts of the bod)', the a.xis- cylinder is surrounded by a relati\'ely thick coat, known as the vicdullary sheath, outside of which lies a thin structureless enx'elope, the neurilemma or sheath of ScJnvann, 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 XitViCwXwx frame'cL'ork and a fatty substance, the myelin, 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 Evvald and Kiihne as possessing properties similar to the keratin of horny substances and, hence, was named by them neurokeratin. 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 indi\'idual 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 Hardesty ^ 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 thev thus divide into a series of intcryiodal 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 Axis-cylinder Neurilemma Medullary sheath Medullated ner\ e-fibres ni transverse section. X 550. 'Amer. Journal of Anatomy, vol. iv., 1905. I002 HUMAN 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 ajjpears broken by clear narnjw clefts that extend obliquely from the neuri- lemma to the axolemma and thus subdivide each internodal segment into a number of smaller tracts, known as the Schviidt- Lantcrviann segments (Fig. 844). The oblique clefts do not all extend in the same direction, even within 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 as well as of the adjoining seg- ments. The significance of this subdi\ision is un- certain ; many regarding the details as artefacts. According to Capparelli', however, the apparent clefts are in reality unstained membraneous septa that pass obliquely from the axolemma to the inner surface of the neurilemma and serve to hold the axis-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 neurilemma-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-cells) that during the growth of the nerve-fibre were active in providing its envelope (page loiij. Fig. S45. Node of Rainier MeduUated nerve-fibres after treatment with osmic acid; A, fibre showing relic u- lum within medullar>- coat ; B. one show ing same coat divided into segments. /. 500. MeduUated nerve-fibres becoming nonmeduUated on approaching their termination. X 235. Depending upon the presence or absence of the medullary sheath throughout the greater part of their course, nerve-fibres are distinguished as meduUated or non- ^ Archiv f. mikros. Anat. u. Entwick., Bd. 66, 1905. journal of Comparative Neurology, vol. xiii., 1903. THE NERVorS TISSUES. 1003 Nonmcdullatcd nerve- fibrcsin longitudinal si.'ction ol splenic nerve. / 310. nicduUilfil. The medullated fibres constitute tin- j^^rcat niajority of iIkjsl* makinj^ up tlie peripheral nerves and tlie tracts of the cerebro-sjjinal a.xis ; the component fibres of the latter, however, while medullated are without the neurilemma. The nonmedullated fibres, on the other hand, are chieHy prolongations (a.xones) from the i^ani^lioii cells of the sym|)athetic system, alth(ju^h 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 "Fio. .S46. absolute, since everv medullatcti ner\e-til)re becomes nonmed- ullated before ••'•achiiii; its terminatinn, cc-iUral or ])crij)li(ral. Medullated lurve-fibres vary j^reatly in tliickness, the smallest haviiijr a dinnu'ter of only .txji mm., whilst the larj^est may measure as much as .020 mm. According to tluir diameter, as determined by ILolliker, the medullated filjres may be j^rouped as line (.002-.004 mm.), medium (.U05-.009 nmi.), and coarse (.oio-.o2u mm.). In general, the thicker fibres are the lonj^^er and are the processes of large nerve- cells ; conversely, the finer have shorter courses and belong to small cells. Allhough subject to many e.xceptions, the motor fibres are usually the thicker and the sensory the smaller. Since there are many more nerve-iibres 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 apj)roaching their peripheral termination the branching becomes more frecjuent and the medullary sheath thinner until it ends, after which the a.xis-cylinder continues invested witii 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 telcKlendrion. The nonmedullated nerve-fibres proper, also X.&x\\\it(\ pale fibres or fibres ofi Retnak, 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 invagi- nated ectoblast lining the neural tube, certain elements, the spongioblasts, being devoted to the production of the neuroglia, while others, the neuroblasts, gi\-e 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 partlv retained as the ependymal cells, and ib^ those which have early migrated to more peripheral locations and given rise to stellate cells that are con\-erted into spider-like elements, the astrocytes. 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,^ the astro- cytes are transformations from larger branched gliogenetic cells, by the conversion of whose robust protoplasmic processes the delicate JibrillcB that later form the chief 'Archiv f. mikros. Anat. u. Entwick., Bd. 64, 1904. Fig Young neuroglia cells; astrocytes, from brain of child. X 300. I004 HUMAN ANATOMY. constituents of the neuroglia arise. So long as neuroglia is being produced, as in the ner\'ous a.\is of young animals, the large gliogcnetic cells are present and directly concerned in the production of additional til^rilke, their cytoplasm becoming pro- gressively less granular and reduced through the various transition [)hases until in the final condition, as the small glia cells, little more than the nucleus remains. During these changes very many tibrilUe lose their connection with the cells and, in conjunction with the glia threads still attached to the astrocytes, form an elaborate interlacement in which the neuroglia cells, now reduced and for the most part devoid of processes, 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. Everywhere the chief part of the sup- porting* tissue consists of the intricate felt-work of fibrilUe, glla-Jibrcs, 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. Fig In the immediate vicinity of the neurones the felt-work of the fibrilla." is unusually close, so that the- Cfll-bodies and the roots of the processes are surrounded by a protectins^ slieath, the glia-capsulc. Tliis diminishes along the dendrites, 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 nenrogliar 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 nerve 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 (Rubaschkin). Beneath the pia mater the neuroglia is especially dense and forms the external stibpial layer that every- where invests the nervous mass, following all the inequali- ties of its surface, bi this manner the pia mater is excluded and, except where its connective-tissue strands accompany the blood-vessels that enter the ner\ous 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, biternally 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 o! 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. J ji * vi' ^ Ependymal cells and adjacent neuro- glia surrouiuliiig central canal of spinal cord ot cat. X 75. (^Rubaschkin.) Where the neuroglia borders the neural tube (especially the central canal of the spinjil cord) it constitutes the ependymal layer, the peculiari- ties of which call for sj^iecial mention. The imme- diate lining of the tube consists of a single layer of pyramidal epithelial elements, the ependymal cells, whose free surfaces or bases look towards the lumen, and the apices towards the surrounding ner\'ous tissue. At least during the earlier years in man, and throughout life in many lower mammals, the free surface of each cell is beset with a number of hair-like processes that in their relations with the cytoplasm correspond to ordinary cilia. The pointed distal end of the ependvmal cell is prolonged into a conical process that is direcdy continued into usually a single neurogliar fibre which, after a course of uncertain length becomes Till-: NERVOUS TISSUES. 1005 lost in the siirroiindinuf com[)lex of u:lia-til>ics. In younj^^ tissue the apical processes often exhibit e\iclences of l)reakini^ up into a nunii>er of line fibrilhe. Where the processes enter robust tracts of neuroglia, as in tiie posterior longitutiinal sej)tuin of the spinal cord, they are of unusual lenjj^th. In addition to the radially directed fibres connected with the ependymal cells, the fibre-complex of the ependynial zone includes many fibrilUe that are circularly and longitudinally disposed. Scattered p^lia cells, some stellate but mostly small, are also present and represent the elements from which the neuroi;lia-til)rilke have been derived. Ill tlif prc-ci-iling accoiiiil of tlic ek-iiK-iits composiiit,^ tliL- nervous tissues the neurones have been resarclt-d as tlie nu)ri)lu)logical units, eacli retainin.L; its individual anatomical inde|)en- denie, altlioujjh fuiutionally closely related with otlar similar units. This cfjnception, com- monly rtk-rrLcl to as the Neurone Doctrine and strikiiiijly fornuilaled by W'akleyer in 1.S91, 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 establisiied kir;^ely tiirough the convincing embryolo.<;ical investit^ations of His and the renewed study of the nerve-cells as demonstrated by the improved apjilications of the Golj^i silver-impre.!:^nations, supi^lemented by the method of vital staininji by methylene blue introduced by Eluiicii. The Neurone Doctrine has gained wide acceptance and the support of the most distinguished anatomists, among those who have materially strengtiiened its position being Kolliker, Ramon y Cajal, Retzius, Lenhoss^k, Waldeyer, van Gehnchten, 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 ApAthy^ published his observations on the structure of the ganglia of certain invertebrates, as revealed by a new mercuric gold-ciiloride method, and thereby established the important fact that the cell-body and processes of the neurone are pervaded by fine neurofibrilke, thus confirming the fibrillar structure of the nerve-cell advanced by Max .Schultze more than a quarter of a century before. Following ApAthy, Bethe''' investigated the tissues of the higher animals and succeeded in dem- onstrating the existence of the neurofibrilla; 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 neurofibrillar, 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 neurofibrillse 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. Apdthy, 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 criti ism made by the newer school, that the supporters of the neurone theory relied upon methods which inadecjuately 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 ^ 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 Apdthy, Bethe and Nissl,* Retzius rests his case with little concern as to the verdict of those to whom facts and not speculation most appeal. ' Mitteilungen aus d. Zoolog. Station zu Neapel, Rd. xii., 1897. ' Allgerneine Anat. u. Physiol, des Nervensystems, 1903. ^ Biologische Untersuchungen, N. F., Bd. xii., 1905. ■* Die Neuronenlehre und ihre Anhanger. 1903. I006 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 (integument or mucous membranes), as, for example, the median or the third division of the trigeminal nerve, three sets of fibres are included : ( i ) the efferent axones of motor neurones whose cell-bodies are situated within the spinal cord or brain ; (2) the afferent dendrites of sensory neurones within the spinal and other sensorv ganglia ; and (3) the efferent axones of neurones within the sympathetic ganglia that accompany the spinal fibres to the periphery and ser\e for the innervation of the involuntary muscle of the blood-vessels and of the skin and the glands. The nerve-fibres, the various kinds usually more or less intermingled, are grouped into bundles, the funiculi, which differ in number and diameter according to the size of the entire trunk that they form. Each funiculus is surrounded by a definite sheath of dense connective tissue, the perineurium, which is directly con- tinuous with the delicate fibro-elastic tissue prolonged between the individual nerve- fibres as the endoneuriuni. When well represented, the sheath of the funiculus consists of concentric lamellae of fibrous tissue which enclose pei'ineurial lymph- spaces. Fig. S49. ) Spinal cord Spinal ganglion Fig. 853. Xerve-fibres, cut transversely N'erve-cell Anterior (motor) root Common trunk of spinal nerve Anterior division Section of spina! nerve, showing its roots, ganglion, common trunk and primary divisions. X 10. present. The chief ganglion-cells are from .060-. 080 mm. 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 capsule which is continuous with the sheath of the nerve-fibres. Most of the many other oval nuclei that are conspicuous in sections of the ganglia belong to the neurilemma of the ner-\e-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 sens(jry neurones, whose processes course as medullated fibres within the spinal nerves, many more are small cells, whose axones never acquire a medullary coat and, dividing into peripheral and central branches, run within the trunks and posterior roots of the nerves as nonmeduUated fibres. Based largely on the behavior of their axones, DogieU has described eleven types of cells within the ganglia. Ranson regards the "large" and "small" cells, whose axones become medullated and nonmeduUated fibres respectively, as an important grouping, 1 Der Bau der Spinalganglien, Jena, 190S. Dogiel describes eleven varieties of nerve-cells. Capsule Nerve-fibres Section of spinal ganglion, showing nerve-cells surrounded by nucleated capsules. X .'^oo. DEVFXOPMEXT (W THE NI'.RXOl S SYSTEM. 1009 •nnd li.'is tnircd the noniiu-diillatc 1 fibres aloiii^ llu- dorsal roots into tlv» s[>inal cord. Tli«' ])rcscnce of fibres i)r()bal)ly derived from syin[>athetic neurones has been demonstrated. The sympatlutic miuiilia are represented by those of the great ganji;liated cords, certain cranial ^anj^^lia (ciliary, spheno-palatine, otic, and subma.xillary;, the j^anjj^lia within the three prevertebral ])le.\uses, and the innumerable small and often micro- sco|)ic j^ani^lia associated with the muscular tissue of the dij^estivc, respiratory and uro-j^enital tracts, in the heart antl in the varif)us j^lands. In their i^eneral structure the sympathetic j^anj^lia are similar to those connected with the spinal ner\'es, ftjrmini^ definite masses enclosed by a fibrous capsule, from which connective-tissue processes pass into the interior of the j^suij^lion for the support and separation of the nervous elements. The individual j^^anLjli- Fig. S54. on-cells — unipolar, bipolar or multi- polar— are ensheathed by nucleated capsules continuous with the neuri- lemma of the ner\'e-fibres. The sympathetic <^ano;Iion-cells are \ari- ously related to the terminal ramifi- cations of {a) 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 i)lexus, 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 ple.xus (Huber'). The broader relations of the component nervous elements of the spinal ganglia are considered in connection with the Sympathetic System (page 1354). Diagram of coiisiilueiits 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 Kb) around sensory cells; (/, sympathetic neurone; AR, PR, anterior and posterior roots; AD, I'D. anterior and posterior primary divi- sions of spinal nerve; RC, ramus communicaiis. 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 J -=:ii--,— ;_^.. -:r--^x^"^^^^"^>I^^ to the labors of His, whose account, supplemented by the ir^^^^P-h^^''^^^^^ -^^iB-^^^''^ ' important contributions of Kfilliker, Cajal, Lenhoss^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 or.e 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 syncytiuvi fHardesty^). The large dividing elements within the latter, the s:ernii7ial cells of His, are conspicuous on account of their mitotic figfures 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 ' Journal of Morpholog>-, 1899. ='Amer. Journal of Anatomy, vol. iii., 1904. ex ihn Segment from lateral wall of neural tube of pig embryo of 5 mm. ; syncytium replacing distinctly outline'l cells, a, inner zone; g, ferminai cells; Urn, internal limiting mem- rane; m, peripheral zone ; r, radial strands of cytoplasm. X 690. (Hardesty.) lOIO HIM AN ANATOMY. been sustained (Kolliker, Scliaper and others) since the primary germinal cells probably only represent proliferating elements engaged in forming what for a time is an undifTerentiated tissue. The cells composing the neural wall are at first in close contact, their blended cytoplasm (syncytium) forming an almost unbroken sheet. Soon, however, this continuity is interrupted in consequence of the longitudinal expansion of the tissue and the appearance of spaces, and the cell-substance is resolved into a delicate reticulum, the myclospougium of His, which becomes condensed at the inner and outer margins of the wall of the neural tube into the internal and external limiting membrane. The meshes of the reticulum enlarge, the intervening nucleated tracts of cytoplasm elongate and the increasing nuclei become radially disposed. By reason of these changes the elements next the lumen of the tube assume a columnar form and radial arrangement and become the primary ependymal cells. The remaining elements, appropriately named the indifferent cells (Schaper), increase in number in conseciuence of the continued division of the germinal cells and gradually become collected as the nuclear layer at some distance beycjnd the ependymal zone. Meanwhile and very early, the peripheral portion of the supporting framework adjoining the outer border of the neural wall becomes denser and free from nuclei and is converted into the marginal zone { Randschleier of Fig. S56. . ^ K ilm elm His), that is continuous with the delicate reticulum pervading the other parts of the wall. The in- different cells later differentiate into (a) the spongioblasts from which the characteristic constitu- ents of the definite supporting tissue, the neuroglia, are derived, and [b) the neuroblasts that are directly converted into the neu- rones. Within the resulting cell- comple.x 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-cells and fibres) that the primary syncytial meshwork undergoes Segment of wall of neural tube of pig embryo of 10 mm. ; radial strands (r) of syncytium and differentiation of ependymal (ai, nuclear {b) and marginal {m) layers; j'/w. <"/»«, internal and external limiting membrane ; ^, dividing cell ;/, pia mater. X 690, (Hardesty.) Fig. 857. Transverse section of ventro-lateral segment of de\tlopinjj spinal cord from pig embryo of 30 mm., upper part of figfure from chrome-silver preparation, lower part from one stained with toluidin blue: c, central canal; <■;>, ependymal layer; n. nuclear layer; w, marginal layer; r, radial fibres; v. ventral iilate uniting halves of cord. X 240. (Hardesty.) (Hardesty), the gradual transformation of the spongioblasts and their descendants into fibrillae establishes a more definite framework that replaces the primary net-work f myelospongium), and eventually, in conjunction with the fibrillae derived from the processes of the ependymal cells, DEVELOl'MKNT OF I HI-: XKRVOL'S TISSUES. lOII gives rise to the lieliiiilc suppurtiiij; tissue, tiie iitiiri)^Iia. A< cording to I lardesty, the >iha-fibres arise within the syncytial tissue iiulependeiitly of the iKuroj;ha cells, a view in direct (opposition to the observations ot Kubaschkin, who attributes to the descendants of the spoiiKioblasls, the ^Jlia^;enelic cells, a i)ositive rCAc in the production of the fibres. Accepting; the conclusiijiis of the last-named investigator, the successive sta);es of the cells concerned in the production of the jjeneral neurojiliar tissue are representeil by the spi>nxnu/>/asfs, the ^(/iuj^uitelic cells, the aslroivlrs, and, (inally, the f^lia cells. The primary cpciulymal clctnctils are succeeded by the epithelium which lines the ventricles and the central canal of the spinal (ord. Their peri|>h- erally directed processes are in larj^e |)art transfcormed into >;lia-libres and thus, ahjuj^ with the |)rocesses of the spider cells, contribute to the formation of the neuro};liar felt-work. The accompanying illustration {V\<^. 857), taken from Hardesty's paper, affords an instructive com|)arison ot the appearance of the younj^ supportinj^ tissue after true slaininj^ with approved reagents ( lUnda | and after silver precipitation methods (Clolj^ij upon which so much reliance has been placed. The silver picture shows the classic loiijj neurogliar fibres extending the entire thickness, but fails to reveal the wealth of supp(jrting tissue and nuclei. To what extent the mesoblastic ingrowths that follow the penetrating young blood-vessels into the neural wall take part in the |)roduclion of the distinctive neurogliar framework is admittedly difficult to determine (Hardesty) ; that such tissue, however, contributes to the sui)port (jf the nervous elements is certain. Histogenesis of the Neurones. — The neurobla.sts are distinguishable with certainty from the spongioblasts as soon as they are provided with nerve-processes. The latter appear as out- growths from the pointed and perii^herally directed ends of the developing nerve-cells, invade the marginal zone, and later emerge from the wall of the immature cord as the ventral or anterior root-fibres of the spinal nerves (Fig. .S5.S). The deeper tint of their distal ends after staining, their tendency to collect in con- verging groups, and the uniform width of the outgrowing nerve- processes are distinctive charac- teristics of the neuroblasts ( His ' ). The first, and for a considerable lime the only processes with which the neurones are provided cor- respond to the axones that be- come the axis-cylinders of the efferent (motor) nerves. Subse- quently other processes, the den- drites, grow out in various direc- tions from the cell-bodies of the Fig. 8.sS. '«£■> ■Neuroblasts ^^ '-%t m ■■,.'Ah"* y C Eflerent axones Portion of spinal cord of human embryo, showing development of ventral root-axones as outgrowths from ventral neuroblasts. X 300. (After His.) young neurones. Development of the Peripheral Nerves. — According to the teaching of His, accepted by most anatomists, the a.xis-cylinder of the entire future nerve-fibre is formed by the peripheral growth of the original nerve-process of the neuroblast. The assumed development of the ner\e- fibre by the union of a number of segments ( Balfour, Dohrn, and others, and, more recently, Bethe and O. Schultze) is not in accord with renewed investigations, and the findings upon which the composite theory of the fibre is based are open to different interpretation (Kolliker, Retzius). According to Bardeen,^ the development of the peripheral spinal nerves is briefly as follows: The motor neuroblasts and the sensory spinal ganglion-cells send out processes of considerable thickness, all of which soon begin to give rise at their extremities to groups of Jibrillcr, which increase in thickness and length and, in turn, at their extremities give rise to new groups of fibrils. At first these proceed as naked bundles, but soon become surrounded with nucleated fusiform sheath-cells which thus enclose the early embryonic nerve, and may contain hundreds of fibrilhe. After a nerve has become distended by ingrowth of new fibrils from behind, the proliferating sheath cells begin to wander from the periphery in among the fibrilla.' and give rise by anastomosis of their processes to a net-work that divides the original fasciculus into a number of ."jpcondary bundles. The intrafascicular cells increase rapidly, the process of subdivision ' Die Entwickelung des menschlichen Gehirns, 1904. 'Amer. Journal of Anatomy, vol. ii., 1903. IOI2 HUMAN ANATOMY, Developing intercostal ner\'e of pie embryo of lo mm. ; tip of nerve is composed of fibrils surroutuled by sheath-cells. X 360. (Bardeen.) Fig. 860. L^ continues and the bundles of fibrillae become progressively smaller and more compact until, surrounded by membranous septa, they correspond to the a.iis-iy/indcrs of the individual nerve- fibres, enclosed by the iieuri/enmta and its cells. The endoncnnuui appears comparatively late and, like the neurilemma, is a product of the mesoblast. Later, condensations of the mesoblast around the definite bundles of nerve-fibres and about the entire nerve-trunk provide \\\^ perincuriuui and the cpincuriiini respectively. During its course to the periphery the young nerve gives rise to numerous branches, the points of outgrowth being indicated by a preparatory increase of the peri])heral cells which often form a tubular projection into which the nerve- fibrillffi grow. The proximal plexuses (such as the brachial or lumbar) are formed during the outgrowth of the nerves from the region of the central nervous system ; the coarser distal plexuses arise during the e.xtension of the branches to the various parts for which they are destined ; whilst the finer lerininal plexuses are established during the development of functional unity between the nerve-fibres and the structures to which they are distributed. The medullary sheath is a comparatively late acquisition, since it does not appear until about the fourth month of fcetal life. Within the central nervous system the tracts of nerve- fibres obtain their medullar)- coat at different times (some not until after birth), a variation that is of much service in enabling the anatomist to trace the course of the indi\idual 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 meduUated fibres within the cerebro-spinal axis are devoid of this sheath, and, further, that the myelin sometimes appears before the neurilemma ( Roister, Bardeen). While it is doubtful whether the myeliii is directly formed from the outer part of the axis-cylinder, as suggested by K511iker, it is probable that this structure exerts some influence resulting in the deposit of the myelin- droplets either from the blood (Wlassak), or from the apparently fluid substance that after a time surrounds the axis-cylinder (Bardeen). Regarding the formation of \\\Q framezvork supporting the droplets of myelin, Hardesty' inclines to the view that certain sheath cells, which appear during medullation, 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 origin of the afferent (sensory) neurones, whose cell-bodies are situated within the spinal and other ganglia, is entirely different from that of the efferent (motor) ones above described, bi the case of the spinal nerves, the development of the ganglia pro- ceeds from a group of ectoblastic cells that form a ridge, the ganglion-crest, on the margin of either lip of the still open neural tube (Fig. 860), just where the general ectoblast passes into that lining the groove. On approximation of the lips of the latter, the cells of the ganglion-crests fuse into a wedge-shaped mass that completes the closure of the neural tube and constitutes a centre of proliferation from which the cells migrate outward over the dorso-lateral wall of the tube. The proliferation is not uniform but most marked at points that correspond to the mesoblastic somites, in consequence of which a series of segmentally arranged cell-aggregations appears on each side of the neural tube. These collections are the anlages of the spinal ganglia. Within them certain cells soon become fusiform and, assuming the r61e ol •neuroblasts, send out a process from either end. One process— the axone— grows centrally, while the other— the dendrite— extends peripherally and becomes the chief part of a sensory nerve-fibre. The subsequent growth of the neurone is not symmetrical, but to one side, and so h Transverse sections of dorsal region of human embryos, showing early differ- entiation of spinal ganglion \ A,B, neural tube still open ; t", D. tube closed ; a, ganglion-ridges ; *, fused ridges ; c, out- growth to form ganglion ; d, ectoblast. X 230. (Lenhossik.) 'Amer. Journal of Anatomy, vol. iv., 1905. DEVELOPMENT ()1- Till': NERVOUS TISSUES. 1013 Fk;. 861. ''^^^> dr^ y.- "'&m Cross-section of part of dorsal region of human embryo, showing developing spinal ^ianglion; dz, vz, niz, 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. orclcrci.1 that tlir two processes are approximated and tiiially joined t(^ tlie cell-b(jdy by a common stalk ( l-i^- ^39), the neurone beinj; thus converted into an unipolar j^anj^lion-cell. The centrally tlirecteil |)rocesses, the later posterior root-libres ot a spinal nerve, jjrow into the develop- ing cord and enter the peripheral /one (later the white matter) to end, when their develojjment is completed, at various levels in relation with neu- rones formed within the neural axis. The [)eri- pherally directed ])rocesses of the spinal sensory neurones, on tlu- other hand, mingle with the axones from the motor neurones to form the mixetl nerves distributed to the various parts of the body. The essential parts of the sensory neurones, the cell-body ami 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 — tlK)se of tl.e 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, nor by the 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 fcetal month as aggregations of cells at the distal ends of the visceral rami of the developing spinal nerves. From these cells are derived the definite sympa- thetic neurones of the gangliated cord, as well as those which follovv' 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 by 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 a.xones centrally and give rise to the efferent splanchnic nerves, whilst the axones of still others pass to the growing spinal nerxes. Fig. 862. -r I *v> I V "^^ .«?;>, ■\ Spinal ganglia \-j^ Sagittal section of rabbit embryo showing several developing spinal ganglia and nerve-trunks ; ^, aorta; 5, intersegmental artery. X 52. IOI4 HUMAN ANATOMY. NERVE-TERMINATIONS. The terminations of the fibres composing the peripheral ner\-es — the axones of certain motor neurones situated within the cerebro-spinal axis and the sympathetic system and the dendrites of the neurones of the sensory ganglia — supply the means by which the various structures of the body are brought into intimate relation with the ner\'Ous system. Some of these terminations transfer impulses resulting in muscular contractions ; others convey impressions that produce various sensations fpain, pressure, muscle-sense, Fig. 863. temperature;. The nerve- Xerve A terminations, therefore, may be grouped according to func- ^Ov *■> 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 ; (3j terminations of sympathetic neurones that end in involuntar\- muscle and (c) in cardiac muscle. Endings in Voluntary Muscle. — On approaching their peripheral destination the medullated nerve-fibres branch repeatedly, each fibre in this manner coming into relationwith a number of mus- cle-fibres. When the med- ullated ner\e-fibre reaches the muscle-fibre which it supplies, its medullary sheath abruptly ends and the neurilemma becomes inseparably fused with the sarcolemma, whilst the axis-cylinder passes beneath this sheath to terminate in an end-plale. The latter appears as an oval area, from .040-. 060 mm. in its greatest diameter, which is applied to the muscle-substance ; in profile it shows a slight F'g. S64. projection beyond the contour of the muscle-fibre, although this is often wanting. Embedded within a general nucleated sheet of granular protoplasm, the sole-plate, lie the brush-like terminal arborizations of the axis-cylinder formed of irregular varicosites and club- shaped ends. From the details of the development of the motor end plates, as described by Bardeen, it is probable that the granular sole-plate and its nuclei are differentiated from the sarcoplasm and the nuclei of the muscle-fibre respectively. The much discussed relation of the end-plate to the sarcolemma — whether outside or beneath — seems to be decided in favor of a subsarco- lemmal position, since the muscle-sheath appears sub- sequently to the formation of the motor-ending, a fact that explains the apparent piercing of the sarcolemma by the axis-cylinder. Usually each muscle-fibre is pro- vided with a single motor end-plate, which may lie at an equal or unequal distance from the ends of the fibre. Exceptionally two end-plates m.ay be found on one muscle-fibre, in which case the endings lie near each other. Eiid- plate Motor nerve-endings in voluntarv- muscle : bundle of ner\e-fibres is seen separating to supply the individual muscle-fibres. X i6o. muscle : plate ; ■' 400 Motor iier\e-ending in voluntary axone terminatmg in enc neurilemma ; s, sole-plate. NERVK-TERM I NATIONS. 1015 Endings in Involuntary Muscle. — The tcniiiiuitions (;f the axoncs (jf the syiTipathclic neurones supplyini^' the nonstriated muscle are comparatively simple. The neurones contribulini; the immediate fibres of distributicjn usually occupy the notlal j)oints of j)le.\uses from which bundles Fig. 865. of nonmcdullatetl nerve-lilires extend to and enclose tlie nuiscle fasciculi. Enterinm the latter the nerve-tibres divide intU? Corpuscle of Meissner lying within papilla of corium of skin from finger; only deeper layers of overlying epidermis are shown; n, entering nerve-fibre. X 270. ioi8 HUMAN ANATOMY. Cylindrical end-bulb from con- nective tissue layer of skin. X iSo. {Szymonowicz .) a number of bold curves and twistin^s. After the disappearance of their sheaths, the naked axis-cyhnders undergf) repeated (hvisions, tin- resulting- fibrilhe becoming varicose and intertwined 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, a second group is distinguished by the possession of a thick laminated capsule tliat encloses a cylindrical core or inner bulb containing the slightly branched axis-cylinder. These endings, of which the Pacinian corpuscle is repre- sentative, are relatively large and ellipsoidal. A transitional form, connecting them with the spherical end-bulbs, is j)resented 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 medullary 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. S74. the deeper layers of the connective tissue layer of the skin, especially on the palmar and plantar aspects of the lingers and toes, in the connective tissue in the vicinity of the joints, in tendons, in the sheath of muscles, in the periosteum and in the tunica propria of the serous membranes, the peritoneum, pleura and pericardium. They are particularly large in the mesentery of the cat, where they may be readily de- tected with the unaided eye as oval pearly bodies some- times two millimeters or more in length. The most conspicuous part of the Pacinian body is the robust capsule that constitutes almost the en- tire bulk of the corpuscle and consists of from one to three dozen thin con- centric lamellee of fibrous tissue. The surfaces of the . iamelte are covered with endothelial plates whose nuclei appear as fusiform thicken- ings, along the concentric striae of the corpuscle. The axis of the Pacinian body Vater-Pacinian corpuscles from skin of child's finger; A, longitudinal; B, transverse section ; n, nerve entering capsule to reach inner bulb. X 105- NERVK-Ti:RML\ATI()NS. 1019 Corpuscles of Herbst from bill of duck ; a, longitudinal, b, transverse section ; w, nerve traversing lamellae of capsule ; axis-cylinder within core is surrounded by cells. X 360. is occuj)iecl by a core or inner bulb of seniitluid sul)st:ince in which the naked axis-cylintlcr is emhi-'tUU-d. On joining the proximal pole of tlic corpuscle, the fibrous ( Henle's) sheath of the nerve-fibre bleiuls with tlie outer hinielhe of the capsule, while the medullary coat is retained durintj the somewhat tortuous path of the fibre thrc^uj^h the cajjsule as far as the ct)re. Here the remaining envelope of the nerve-fibre disapjjears, the terminal part of its course, through the core, being as Fig. S75. the naked a.\is-cylinder. At a 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 expandetl 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 .1-3 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 I020 HUMAN ANATOMY. Nerve-fibre- »:• ; ^.'l Sheath- I**'! Ivmph-space. Each spindle receives usually several medullated ner\e-fibres, which, after incorporation of their sheaths of Henle with the capsule, pierce the latter at various points and proceed to the individual muscle-hbres. The terminal relations of the nenes to the intrafusal fibres have been studied by means of the newer methods especially by Rufifini, ^ Fig. 876. Huber and DeWitt and Dogiel. I ., . After repeated division during their course through the cap- sule and periaxial space, the nerve-libres pierce the a.xial 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 DeW'itt, in their general architecture resemble closely the senson,- endings in muscle. They lie embedded within the interfascicular con- nective tissue and are usually found in the vicinit)' of the junction of muscle and tendon. Like the neuromuscular end- ings, the teyidon-spijidles 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 fasciadi, which are smaller and apparently less mature than those of the sur- rounding tendon-tissue. The intrafusal fasciculi are invested bv a fibrous axial sheath be- tween which and the capsule lies a periaxial lymph-space. On reaching the spindle, after repeated branching, the medullated ner\e-fibres pene- trate the capsule, with which their fibrous fHenle's; sheaths blend, and undergo further division. The medullar^' coat is lost after thev pierce the axial sheath, the naked axis- cylinders breaking up into smaller fibrils that extend along the intrafusal fasciculi. The terminal ramifications, applied to the surface of the fasciculi, vary in details (Huber). Some arise as short lateral branches that partly encircle the fasciculi and end in irregular plate-like expansions, while others terminate between the smaller fasciculi. .\xial_ sheath 'A- Xerve-fibre :^\\ A n^vromuscular ending; .ff, neurotendinous ending in longitudi- nal section, meihylene-blue staining. X 260. (Drawn from preparation made by Professor Huber.) THE CENTRAL NERVOUS SYSTEM. TiiK central nervous system includes the s])inal cord and the brain. In princiiile tlusc i)arts are to be rej^ardet! as the walls of the primary neural lube, modified by unequal Ljrowth and expansion, which even after acquirinj": their defmite relations enclose tlie remains t)f the canal, as re[)resented by the system of ventricular spaces. In contrast to the spinal sej,Mnent of the neural tube, which always remains a rela- tively simple cylinder, the spinal cord, the cephalic segment early differentiates into three pn'man' cerebral vesicles, the anterior and posterior of which subdivide, so that five secondary brain-vesicles are present. Coincidendy marked flexure of the cephalic segment occurs at certain ])(>ints and in consequence this part of the n(-ural 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 Rexed 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 i36o), 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 reprcsentati\'c of the oijce 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 \ertebral 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'/i 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 i-2uoo 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. 102 1 I022 HUMAN ANATOMY. Skull Metiulla Laniiiuc, cut Transverse i» ^— ' processes Dnral sheath Posterior divisions of sacral nerves Sheath of filiim End of dural sheath . Cn P'lid of filum Coccyx Spinal cord enclosed in unopened dural sheath lying within vertebral canal ; neural arches completely removed on right side, partially on left, to expose dorsal aspect of dura: first and last nerves of cervical, thoracic, lumbar atitl sacral groups are indicated by Italic figures; corresponding vertebrae by Roman numerals. The Membranes of the Cord. — riu; .sj)inal cord, loL^clhcr with tlie rc)t)ts of the thirty-one j)airs of sj)inal nerves, lies within the vertebral canal enclosed by three protecting membranes, or meninges, which, from without inward, are ( i ) the dura viatcr, (2) the arachnoidca, and (3) the />/« mater, 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 fibro-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 plexiform veins and loose fatty con- nective tissues (Pig. •'^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, xhit Ji/ion tei'- viinale, 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 of an outer fibrous and an inner vascular layer, the connective tissue of the latter ac- companying the blood-vessels into the s'ubstance of the cord. The arachnoid, a delicate veil- like structure made up of interlacing bundles of fibro-elastic tissue, lies between the other two membranes and invests loosely the inner surface of the dura nnd closely the outer surface of the pia. It effectually subdivides the considerable space between the external and internal sheaths into two compartments, the one beneath the dura, the subdural space, being little more than a capil- lary cleft filled with modified lymi>h, and the other, the subarachnoid space, between the arachnoid and TlIK CKNTRAL NMRXOIS SWSTKM. 1023 The spinal cord, therefore, hany^s iMO. S7S. / Pons Arailinoid . Vertebral artery Mcecula' and thus imper- fectly sulxli\itled into secondary coni- partnu'iits, all of which arc linc-d wilh endotlu'liuni. The spinal cord is fixed within the loose dural sheath not only l)y the rocjt- fibres of the spinal nerves that j)ass between the cord and the outer cnveloije, but also by two lateral fibrous bands, the liiyatnc?ita detiticulata, that arc continu- ous with the pia aloni;- 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 tij)s t)f 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 nerv^es, 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 F'iG. 879. Anterior roots of spinal nerves % Dura, reflected Spinal cord, covered wilh arachnoid and pia Upper part of spinal cord within dural sheath, which has been opened and turned aside; lijjamenta denticulata and nerve-roots are shown as they pass outward to dura. Dural sheath Periosteum Spinal cord Posterior root Ligamentum denticulatum \nterior root Spinal ganglion Spinal nerve Extradural areolar tissue Vertebral artery Body of fourth cervicil vertebra Transverse section of vertebral ai K \el of fourth cervical vertebra, spinal cord in position. with the dura and partially subdi\ides the subarachnoid space. Lower, this partition, I024 HUMAN ANATOMY. Fig. 8So. Skull Vertebral artery / en Spinal accessory iier\'e Pedicles, cut Medulla / en Spinal accessory ,, , , ner\ e hdge of cut dural sheath I T- / tn- -Spinal cord - ^' en -I T -/ In Pedicles : lidge of cut dural sheath Spinal cord End of con us niedullaris Filum terminale Descending ner\es End of dural sheath Filum externum Cn End of filum Coccvx Posterior wall of vertebral canal has been removed and dural sheath opened to expose spinal cord and dorsal roots of attached nerves ; / c«, I C, fir?t cervical ner\-e and vertebra respectively; Cn, coccygeal nerves. which may transmit blood-vessels, is imperfect or altogether absent.' As they cross the subarachnoid space the bundles of root-fibres of the spinal nerves are enclosed by prolongations of the pia and arachnoid. These sheaths are retained by the nerves for only a short distance after the latter receive an additional investment from the dura as they leave the vertebral canal. The dural sheath becomes continuous with the epineurium of the spinal nerves. The Cord - Segments. — Although no suggestion of such sub- division is to be seen as constrictions on its surface, in principle the spinal cord consists of a series of segments, each of which gives origin to the anterior f motor) and recei\'es i\\e pos- terior (sensory) root-Jibres of one pair of spinal nerves. These nen-es, usually thirty-one pairs in number, are classified as eight cei'vical, tweh'e thoracic, fi\e lumbar, fi\e sacral, and one coccygeal. Corresponding to the attachment of the nerves the cord is conventionally divided into cervical, tho7acic, lumbar, and sacral regions. Of the entire length of a cord measur- ing 43 cm., approximately lo cm., or about 23.5 per cent., belonged to the cer\'ical region; 24 cm., or 55.5 per cent., to the thoracic; 6 cm., or 14 per cent., to the lumbar; and 3 cm., or 7 per cent., to the sacral region. The spinal ner\es are attached to the lateral surfaces of the cord by fan-shaped groups of anterior and pos- terior root-fibres that are gathered into compact strands as they converge to form a common trunk (Fig. 884). The portion of the spinal cord with which the root-fibres of a spinal nerve are connected constitutes its cord- segment, the limits of which lie in the inter\al separating the extreme fibres of the nerve and those of the adjacent nerves. In the thoracic cord these intervals are very evident, since the segments are relatively long ; in the cer\ical and lumbar regions, on the contrary, the groups of root-fibres are so crowded that they form almost unbroken rows. The length of the individual cord- segments varies ; thus, according to the measurements of Liideritz, those of the cervical region, are from 11-13.5 mm. ; rill'. CENTRAL NRR\'()US SNSTEM. 1025 those of the thoracic re- Fig. 881. gion from 12-26 mm., the longest iK-loiiniiij; to the V - \M I tl)t>racic lu-rvcs ; those of the himl)ar region rapiilly decrease from 15.5 -5.5 mm., followed by a more grailual dimiiuition to less than 4 mm. in the sacral region. In conseiiuence of the disproportion between the lenglii of llu- sjiinal conl and that of the vertebral canal, tiie discrepancy be- tween the level at wiiich the nerves are attached to the cord and that of the intervertebral foramina through which lliey leave the canal becomes more marked towards the lower end of the series. The growth of the cord, how- ever, is not uniform since, as shown by Pfitzner, during tlie later years of childhood elongation Huiullcs ncrvc-fiD Uiiral stieatli Extradural areolar tissue Veins 'r...:>i^ Transverse section ot veile.-ial can vertebra; spinal cord (conus niedullaris) witliin dural sheath. 1, al level of middle of first lumbar surrounded by nerve-bundles, is seen Fig. 882. Conus meduUaris Descending spinal nerves Front wall of dural sheath Filum internum Coccygeal Coccvx End of spinal cord with roots of lower nerves descend- ing in Cauda equina to gain their respective foramina ; T-s ht, 7-5 sn, en, lumbar, sacral and coccygeal neri-es. of the thoracic region occurs to such an extent that this part of the cord once more equals, if indeed not e.xceeds, the corresponding portion of the spine. While the cervical cord keeps fairly abreast the cervical portion of the vertebral column, the lumbar and sacral segments are left far behind. The results of these changes are seen in the course of the root-fibres, which in the neck, below the third nerve, run somewhat downward to their points of emergence, and in the thoracic region pass more horizontally, while those of the lumbar and sacral nerves descend almost vertically for a considerable distance — in the case of the last sacral nerve 28 cm. (Testut)— before reaching their appropriate levels. The large and conspicuous leash of descending root-fibres, seen upon open- ing the dural sheath, constitutes the Cauda equina, in the midst of which the glistening silvery filum terminale is distinguishable. It is evident, there- fore, that in most cases the level of the cord-segment and that of the vertebra bearing the same designation do not correspond. Likewise, it must be re- membered that, although in general the spinal nerves are named in accordance with the vertebrae immediately below which they escape, in the neck there are eight cervical spinal nerves and onlv seven vertebrae, the first or sub- occipital nerve emerging between the atlas and the skull, and the eighth between the last cer\'ical and first thoracic vertebra : hence, except the last one, they correspond with the \ertebra below. 6s I026 HUMAN ANATOMY. Form of the Fig. S83. Medulla V* Cenicai Thoracic Sacral Coccygeal Filuni V Cord. — After remo\al of its membranes and the root-fibres, the spinal cord is seen to differ from a simple cylinder in the following respects. It is somewhat flattened in the antero-posterior direction, so that the sagittal diameter is always less than the trans\erse diameter, and its outline in cross-sections, therefore, is not circular but more or less 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 cenicalis) begins just below the ujjper end of the cord and ends opposite the second thoracic \ertebra, ha\ing its greatest expansion at the level of the fifth and sixth cer\ical vertebrae, where the sagittal diameter is about 9 mm. and the transverse from 13-14 mm. The lower or lumbar enlargement (intumescentia lumbalis j begins opposite the tenth thoracic vertebra, slightly above the origin of the first lumbar nerve, and fades away in the conus 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 ner\-e-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, are 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 dural sac, which it pierces at the level of the second sacral vertebra and, ensheathed by a prolongation of dura {vagina terminalis), as the filum terminale extermon, proceeds downward through the lower end of the sacral canal for a distance of about 8 cm. (2,}i in.), finally to be attached to the periosteum covering the posterior surface of the coccyx. The part within the dural sac, the filu/n terminale intermmi, is about 16 cm. (6^ in.) in length and surrounded by the nerve-bundles of the Cauda equina (Fig. 882), from which it is readily dis- tinguished by its glistening silvery appearance. Spinal cord denuded of mem- branes and nerves, showing pro- portions of its length contributed by different regions and position and relative size of enlargements, as viewed from before; semidiagram- niatic, based on measurements ; one-third actual size. coccygeal nerves, homologous The upper half or less of the internal filum contains the terminal part of the central canal of the spinal cord wailed by a thin and variable layer of nervous substance in which small nerve-cells are usually present. The minute bundles of nerve- fibres often found adhering to the filum. which sometimes may be followed to and even through the dural sheath, are regarded by Rauber as representing one or two additional ( second and third) with the caudal nerves of the lower animals. THI-: CliNTRAL NKKXOLS SNSTKM. 1027 The Columns of the Cord. Inspection of the surface and particularly of cross-sections ()f the spinal cord ( V'l^. .SH5 ) shows the latter to he partially divided into a symmetrical rij^hl and left half l)y a median (left in front anil a partition in the mill-line behind. The cleft, tin- anterior median fissure ( lissiira mcdiaiia anterior; extends the entire lenq^th of the cord, and is continued on the uppei j>art of the filiim terminale. It is narrow, from 2-3.5 '"'"• >'i tl<-"pth, penetrating.^ for less than one-third of the ventro-dorsal diameter of the cord, and occupied hy a process of pia mater. AIoiiilJ its floor, which lies immediately in front of the white commissure, it is fre(iuently deflected to one side of the mid-line and presents a slight exjjansion. The separation into halves is completed by the posterior median septum (septum mcdlaiiuin postcriiis), the "^o-coWcd poslirior nia^iuti Jissurr. With the ( x- ce[)ti(in of a shallow j^roove in the upper cervical cord, the lumbar cnlarj.^emenl and the conus medullaris, no fissure exists, but in its |)lace a dense partition extends from the posterior surface to the mitldle of the interior cjf the cord, endinj^ inclose relation to the gray commissure. The character of the sejitmn is a subject of dispute, accordinj^ to some anatomists con- sisting exchisively of condensed neurojjlia, wliile others regard it as compo.sed of pial tissue blended with tlie neuroglia and, therefore, of belli niesoblastic 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 straiy^ht and narrow, and marked by a slight furrow, the postero- lateral sulcus (sulcus lateralis posterior) that lies from 2.5-3.5 rn'ii- 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 ■^ — Medulla distinct furrow nor con- Fig. 884. tinuous. 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 at)out 6 mm. in width in the thoracic cordand 8mm. and 7 mm. in the cervi- cal and lumbar enlarge- sheath ; cord-segments are indicated by groups of converging tiundles of posterior . ^ root-fibres; spinal ganglia are seen lying within the intervertebral foramina; mentS respectl\'ely. spinal accessory nerve is seen ascending on each side. The tracts included 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 Ganglion on 4 nerve Dorsal roots of 5 cerv. nerv-e tapper end of spinal cord, viewed from behind after partial removal of dural "jundles of posterior ertebral foramina ; I028 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 autero-lateral colum?i, that on each side embraces something more than two-thirds of the sem {circumference of the cord. In the lower cervical and upper thoracic cord, each posterior column is subdivided by a shallow furrow that lies from 1.5-2 mm. lateral to the posterior medium septum. This, the paramedian sulcus (sulcus intcrmedius posteriory, 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 1, 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 ^' Posterior median septum Poslerior column 'Posterior root-furrow \X' Substantia gelatinosa .i Rolandi ^ Caput con. ', Cervix con :^ .Posterior rool-fibres Lateral cor- Basis cornu -r^ i Lateral column Central canal in gray commissure Caput cornu Anterior median fissure .Anterior column -Anterior white commissure Transverse section of thoracic cord, showing disposition of RTay and white matter and division of latter into anterior, lateral 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 grisea) 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, Xhe posterior ^nd the anterior gray eom7)iissure, which lie behind and in front of the tube respectively. 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- missura anterior 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 grisea centralis). THE CENTRAL NF.RVOrS SN'STEM. 1029 I c- rr^ Each crescent of j^ray matter is (livisil)le into three parts — tlie ventral and the dorsal extremity, that project l)eyond the transverse ^niy commissure and constitute the antcrioy and posterior horns ox cornua of the ^ray matter (columnac yriseae), and the intermediate portion (pars intermedia) that connects the cornua and receives the commissure. The two horns dilTer markedly from each other and, althouj^h varying in details in different levels, retain their distinctive features throui^hout the C(;rd. The anterior cornu (cdlinniia urisea anteiior) is short, thick and rounded, and separated by a consitlerable layer of white matter from the surface of the cord, throuj^h which the ventral root-fibres proceed to their points of emerj^^ence in the root-areas. The blunt tip of the anterior horn is known as the eaput eornu, and the dorsal por- tion by which it joins the commissure and the pars intermedia as the basis coryiu. The posterior cornu (cohnniia jjrisea i)osteri()r) presents a marked contrast in bein^ usuallv relatively lonj^, narrow and pointed, and in e.xtendini; peripherally almost to the postero-lateral Fig. 886. sulcus. The tip or apex of the dorsal horn is formed £gs?gg2§g8 of a A-shaped stratum of peculiar character, the sub- stantia gelatinosa Rolandi, that appears lig^hter in tint ( I' ii;'. J~'S5) and st)nK-\\ hat less o])aque 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 cornn (cervix columnac posteriorls) 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. Tliis 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- inatio 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 J and i s- known as the lateral cornu (columna lateralis). 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 cohimn. 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 ner\'e 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 nerv'es, 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-segments; 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 ana 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.) I030 HUMAN ANATOMY. of tlie last sacral nerve, it reaches three-fourths. The absolute amount of j^ray matter is greatest within the cervical and lumbar enlargements of the cord, where it is directly related to the large nerves supplying the limbs. On comparing the tracts of white matter and the gray column it follows tliat while in the lower third ot the lumbar cord these are of approximately equal area, below this level the gray matter e.xceeds the white. In the remaining regions, on the other hand, the white matter predominates, in the greater part of the thoracic cord exceeding the gray from four to five fold and in the cervical cord being from two to three times greater. The Central Canal. — Where well represented, the central canal (canalis cen- tralis), the remains of the once conspicuous neural tube, appears as a minute opening in the gray 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 of the filum 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 being partially or completely obliterated for longer or shorter stretches, the lumen last disappearing in the lower part of the cord. Within the conus meduUaris, the central canal 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 obliteration of the central canal, complete in about 50 per cent, of subjects beyond middle life (Schulz), 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 appears 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 be 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 Dendy and to Nicholls,' the fibre is concerned in automatically regulating flexion of the body, by transmitting to the brain stimuli due to changes in tension. MICROSCOPICAL STRUCTURE OF THE SPINAL CORD. The three chief components of the spinal cord — the nerve-cells, the ner\'e-fibres and the neuroglia — vary in proportion and disposition in the white and gray matter. It is, therefore, desirable to consider the general structure of the cord before describ- ing its detailed characteristics at different levels. The Gray Matter. — The most distinctive elements of the gray matter are the midtipolarnerve-cclls which lie embedded within a complex sponge-like matrix formed by the various processes — dendrites, axones and collaterals — from other neurones, the supporting neuroglia and the blood-vessels. In two localities — immediately around the central canal and capping the dorsal cornu — the gray matter varies in its appearance and constitution and exhibits the modifications peculiar to the central and Rolandic substantia gelatinosa, the details of which call for later description (page 1034). The nerve-cells of the anterior horn are multipolar, in cross-sections the cell-bodies appearing irregularly polygonal and in longitudinal sections fusiform in out- 1 Anatomischer Anzeiger, Bfl. xl., 1912. MlCKoSaM'ICAL S TRIC riRi: Ol- Sl'lNAL CORlX 1031 line. They may vary from .065-. 135 in cliamctcr, uiilcss unusually small, when they measure from .o3f)-.().So mm. ( K(')lliker). In a ty|)i(al example, as represented by one of the \entral radicular cells ^ivin^ orijLjin to anterior root-libres, from three to ten dendritic processes rachale in various planes, divide dichotomously with decreasing width and linaliy end in terminal arhorizaticjns. In contrast to the robust dendrites beset with spines, the a.\one is smooth, slender and directly continuous with the a.xis-cylinder of a root-fibre of a spinal nerve and unbranched, with the exceptions of delicate lat(;ral processes that are ^iven ofT almcwt at ri^ht angles. These processes, the collaterals, arise at a variable distance from the cell-lxxiy, but usually close to the latter and always before leaving the .tjray matter. They rej)eatedly divide and follow a recurrent course within the anterior horn. After a[)propriate stainins^ tlie cytoplasm of the nerve-cells exhibits cons])icuous accumulations of the deeply staining; tiyroiil su!)stance that lie within the meshes of the reticulum formed by delicate neurotibrilhe. Fig. S87. ■-^ n- -y which not only occupy the cell-body but also extend into the \arious processes. The fibrilke, however, do not pass beyond the limits of the neurone to which they belonu:(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 usuallv 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. In addition to the con- 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 diflfer 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 grray matter, showing multipolar nerve-cells. X 120. I032 HL'MAX ANATOMY the axone divides into two, rarely 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 not 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 not 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 ynesial group, containing many commissural cells, and a lateral group composed chiefly of ventral radicular cells. These collections, however, varv in extent and definition in different parts of the cord and, where well marked, are often Fig. 888. '^^ 4 \ Cells of substantia gelatinosa Rolandi Posterior horn cells -Accessoo' dorso- lateral groups _ Dorso-lateral group Ventro-lateral group Mesial group Transverse section of lower cer\ical cord, showing grouping of nerve-cells ; Nissl staining. X 20. 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 recognized as the veyitro-lateral and the dorso-lateral group 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 lejitro-mesiaJ and a dor so-mesial group, of which the latter is variable and at many levels wanting. In a general 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 conspicuouslv exemplified in the cervical and lumbo-sacral enlargements, in which the presence of large lateral cell-groups is directly associated with a marked increase in the transverse diameter of the anterior horn. Con\'ersely, when these cell-columns become smaller or disappear, the corresponding elevations on the surface of the anterior horn diminish or are absent. Owing to such variations the contours of the gray core are subject to constant and sometimes abrupt change. MICROSCOPICAL STRrCTURI-: OI" SPINAL CORD. 1033 The ventro-medi.in cell-column is tlie most constant, since, as enipliasized by the pains- taking stiuliis of itnice,' it is interrupted only between tiie levels of the fifth lumbar and first sacral nerve in its otherwise unbroken course throuf^h the lenj^th of the cord, as far as the level of the fifth sacral nerve. An augmentation ui this tract in the fourth and fifth cervical segments is probably associated with the s|)inal origin of the phrenic nerve (Mruce). The dorso-mesial cell-column is much less constant, bein^ represented only in the thoracic region, in a few cervical se}.;ments and at the level of the first lumbar nerve. In aj;reement with van (iehuchten and others, Hruce rejjards the continuity of the mesial j;roup as presumj)- tive e\ idence of its close relation to the dorsal extensor muscles of the trunk. The ventro-lateral cell-column ajipears first at the level of the fourth cervical nerve, increases rapidly in the succeed injj: se}j:ments and fades away at the lower part of the eij^hth cervical segment. It reappears in the lumbar enlarj^ement, reaching its ma.ximum 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 tiie 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 a/ci'ssorv dorso- lateral or post-f>ostcro- lateral frroup. HeUjw the level of the second th(jracic ner\'e the dorso-lateral cell-column is unrepre.sented 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 cer\ical and lumbo-sacral enlargements — it is evident that they are 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 witli 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 ner\'e, the cells of the anterior horn become so diminished in number, 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 siibstayitia gelatmosa Rolajidi, and (3) the inyier 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 cf 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 cervical tmclei of 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. I034 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 .006-.020 mm., althous^h exceptionally of larger size. Their numerous short dendrites are irrejjularly 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 a.\es lying parallel or the apices directed tf)wards the Rolandic substance respectively, and constitute a one-celled layer enclosing the substantia gelatinosa, into whicli many of their tangentially coursing dendrites penetrate. Their axones pass through 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 ptjsterior 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 Fig. 8S9. White matter of posterior column ■J, , $1 Cells of Clarke's column Substantia gelatinosa centralis Central canal 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 or in curves, mostly into the lateral column of the same side ; some, howe\er, have been followed into the posterior or anterior columns of the same side (Kolliker), and, rarely, mto the opposite anterior column (Cajal). Exceptionally type II cells— those in which the axone is not prolonged as the axis-cylinder of a nerve-fibre, but soon breaks up into an elaborate end- arborization confined to the gray matter— are found within the gray matter of the posterior horn. Their number is, however, much less than often assumed (Ziehen). The nervous character of most of the cells seen within the substantia gelatinosa Rolandi has been established only since the introduction of the Golgi methods of silver-impregnation. Previously, these elements were regarded as glia cells, an exceptionally large amount of neuroglia 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 neurogliar elements and numerically rich in nerve-cells. The nerve-cells of the pars intermedia of the oray matter, which connects the dorsal and ventral horns and lies opposite the gray 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 sTRrcrrRi-: oi- sri.\.\i, cord. i'J35 Those t)f the first class, or intcrmcdio-latcral cells, are assoc iated with the formatio reticu- laris and its ci)iKlensation, tlie lateral iiorii, and lu-nce are often spoken of as the j^roup or column of the Uilcnil horn. These cells form a slender tract of small closely packed elements that is representetl throiij;'! alnnjst the entire lenj^th of the cord, althouj;h best marketl in the upjK-r third of the thoracic region and |)artia!ly inlerriipted in the cervical and lumbo-sacral seRments. Where the ft)rmati() reticularis is condensed with a tlistinct lateral horn, as in the thoracic region, the cells occupy the projection, hut elsewhere lie within the base of the jjray net- work. As a continuous cell-column the tract extends from the lower |)art ui the eij^hth cervical segment to the ujiper part of tiie third luml)ar,l)eing most c, 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 tho.se 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 .020-.040 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 fibrillce 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 I03S Hl'MAX A.\ATUM\'. true of tlie bundles of root-fibres of the spinal ner\-es. 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 aflords slight indication of a subdivision of the columns of white matter into areas corresponding with definite fibre-tracts, yet the combined evidence of anatomical, pathological, embr\-ological and experimental investigation establishes the existence of a number of such paths of conduction. With few exceptions, they are, however, without sharp boundaries and illv 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 necessan,- 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 endogeywus tracts. It is evident that some of these fibres constitute pathways for the transmission of impulses from lower to higher levels and hence form ascendijig tracts, while others, which conduct impulses in the opposite direction, form descending tracts. Since it is impossible to distinguish between these fibres by mere insp>ection 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 continuit\- of a ner\-e-fibre is destroyed, either by a pathological lesion or by the experimenter's knife, the portion of the ner\-e-fibre (the axone of a neurone) beyond the break, and therefore isolated from the presiding ner\e-cell. undergoes secondar>- 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 ner\'e-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 i^by the methods of Marschi or of Weigert), certain groups of ner\e- fibres will present degenerative changes. It will be seen, however, that the degenerated tracts in sections 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 ner\e-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 has 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 ner\ous pathways, is based on the fact, first demonstrated by Meckel, that the ner\e-fibres of the central ner\ous system do not all acquire their medullar}- sheath at the same time. Taking advantage of such variation, as suggested by Meynert and later extensively carried out by Flechsig and others, upon staining sections of embryonal tissue with reagents that color especially the medullary- substance, it is possible to differentiate and follow certain fibre-tracts in the fcetal 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 medullar^- coat accords with the sequence in which ner\ous function is assumed by the fretus and child. Thus, the paths required for spinal reflexes (the posterior and anterior root-fibres) are first to become medullated (fourth and fifth fcetal months); those bringing into association the different segments of the cord next (from the fifth to the seventh month 1 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. wiiiTi: matti:r ov thk si'INAL cord. 1039 Based on llie collective evidence contributed \)y these methods — anatomical, physiolotj^ical, and developmental — it is possible to locate and trace with fair accuracy a iiuinbir of fibre-tracts in the cerebro-spinal axis. Since they are underj^oinj^ continual au_i,'^nicntalion or decrease, their actual area and position are subject to variation, so that the detailed relations in one re,i,non of the cord difler from those at other lc\i'ls. The accoin|)anyini^ schematic fij^urc, 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 fil^re-tracts. It must also be appreciated that the definite limits of these tracts in such diagrammatic Fig. 89;,. .Association tracts Cerebro-spinal trad Spino-thalamic tract Spino-olivary tract (Helweg) \'estibulo spinal tract Sulco-marginal fibres Diagram of spinal cord, showing position of chief tracts and relations of their component fibres to ner\-e-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 ; c, 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 Gowers' tract ; 9, 10, fibres from lateral and direct pyramidal tracts; 11, 11, anterior root-fibres ; V.F., ventral field; O.F., oval field; C.B., comma bundle. representations seldom e.xist 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 mav be shared bv 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 GoU (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 sensor)'- 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. S94. 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 ape.x of the posterior horn. Soon after gaining the posterior column, with few exceptions, each dorsal root-fibre undergoes a >- or ^— like divi- sion 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 which 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 oft 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 the III or even higher order, in the sequence of the chain required to complete the path for the conduction and distribution of the impulse. The 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 to the cells of the column of Clarke. 3. The collaterals passing to the anterior horn cells. 4. The fibres entering the posterior horn from the tract of Burdach and of Lissauer to end about the neurones of the II order situated within the gray matter of the posterior horn and the intermediate gray matter. The direct ascending posterior tract includes the dorsal root-fibres that pass uninterruptedlv 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 graduallv displaced medianly 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 collaterals to anterior root cells; other fibres end at different levels around cells in gray matter of posterior horn ; S. G., spinal ganglion. wmri-: maitior oi- tiiic spinal cord. 1041 sections of the cord in the ciT\ical ii-^ion the lim^ ril)r(-s entering; hy the lower nerve- roots oecnpy tile iiiiur part of (ioll's cohiniii. In the liinil)ar cord, thev are exchuied from the median septum hy a narrow hcinielli])tical area, which with that of the opposite side forms the oval field of I'lechsij^. The fibres entering by the lower thoracic nerves lie more laterally, while those eiiterinj^ by the upper thoracic and cervical nerves appropriate the adjoining part of Hurdach's tract, the lateral area of which, next the postc-rioi' horn, is occupied chielly hv the posterior root-l'ibres. It must l)e mulerstoiHl liial wiiik- in a };e'ierai way tlie fibres of the lunjj asceiidinj; tracts have the disposition just intlicaled, they are ho intertwined and miii|;led with the strands passing to and from the }jray matter that the defmite outlines of their conventional area, as represented in diagrams, are wanting. Collectively the fibres coniposing this tract are of medium or small size, but acciuire their medullary coat very early, myelination beginning about the fourth fdtal montli, althougii not completed until the ninth (Bl-cIiUtl-w). Fk; A \. Section of spinal cord at level of second cervical seg- ment; forniatio reticularis fills bay between posterior and anterior cornua ; substantia gelatinosa caps apex of pos- terior cortiu. Drawn from Weigert-Pal preparation made by Professor Spiller. ,-; 6. The termination of tlie long ascend- ing; hl)res is chiefly in relation with the neurones within the lower part of the medulla — the fibres of Goll's tract end- inof about the cells of the nucleus s^racilis and those of Burdach's tract about the cells of the nucleus cuneatus. From these stations paths of the II order convev the im])ulses 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, Scolder 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 1042 HUMAN ANATOMY. Fig. S96. 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 hitrher levels by the ascendino^ stem-fihres. Althnu<;h 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 the same or opposite sides ( Ziehen). 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 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 thev soon terminate in end-arborizations around the neurones of the 1 1 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 chiefiy by the cells of the substantia gelatinosa Rolandi, including the marginal cells, and the inner cells of the caput cornu. The secondan,- 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. In 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 cord the descending limbs of the long posterior fibres are collected into the comma bundle of Schultze, 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 wiiiri': .M.\rii:i< oi' iiii': simnai. coki). 1043 Section of spinal cord at level of seventh cervical segment ; anterior cornua are less robust ; root-zone is seen just behind Lissauer's tract. X 6. Preparation by Professor Spiller. likely that these areas re|>reseiit llu- i)rin( ii)al aj^i^reKalioiis of llie dounuard coursiiiK limbs uf the axoiies, derivecl from the posterior horii-cells of tlie same and opjiosile sides. In the cervical rejijioii the desceiul- iiiK Iiml)S of tile posterior ^ l<^- ^97- root lil>riS appear as the comma trad; in tiie lower thoraiic cord these are re- placed by, witlioiit beiiiK iH- rectly contimions with, those formiii}^: the oval field, and tiu'se in turn by the a.xones of tlie triangular iinmile. No one ol these fields is exclusively devoted to the desceiuliii}? limbs of endoj;enous fibres, since in all the presence of e.\»\rol)al)ly about the end of the sec(jnd week of fcetal life, the neural tube i)resents three regions : — the relatively thick lateral ica/ls and the thin ven- tral and dorsal iiUervening bridges, iW- Jloor- and rooj-plales, that in front and behind C(jmplele the boundaries of the canal in the mid-line. By the hftli week the lateral walls exhibit a distinct dilTerentiatiou into three /ones — the inner cpcnilyinal layer, the middle nuclear layer and the outer inanritial laver, surrounded by the external li)iiili)ii^ inonliranc. bi contrast to the other two, the marginal zone is almost devoid of nuclei and, beyond affording supjiort and perhaps assisting in providing a medullary coat, plays a passive role in the production of the nervous elemeiUs. ]5y this time the former general oval contour of the developing cord, as seen in cross-sec- tions, has become modified by the c(jnspicuous tliickening 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 Ventral root-fibres Neuroblasts - Floor-plate Develojsing spinal cord of about four weeks. X 100. {His.) Fig. 905. Roof-plate Dorsal zone Dorsal root-fibres 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 diis thickened ventro-lateral part of the nuclear lajer, 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, bi this manner a longitudinal furrow appears by which the side walls of the tube are differentiated into two tracts, the dorsal ^ndi. the ventral zones (the alar and basal laminte 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- I050 HUMAN ANATOMY. ing of the dorsal part of the canal in consecjuence 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 tiie partition is formed by the union of the inner (ependymal) layers, it is probable that the septum is to be regarded as essentially 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 unclosecl 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 accjuires 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 markedlv increases, especially behind where the posterior horn appears as a projection beneatli tlie broadening mass of the ingrowing dorsal root-fibres. As the posterior horn becomes better defined, the root-bundle becomes meso-laterally displaced, lying behind the horn, and then constitutes the tract of Biirdach. GoU's tract is formed somewhat later and at about the 'bird month appears as a narrow v\'edge-shaped area that is introduced between the mid-line and Burdach's tract. Towards the end of the second month, the anterior white coinmissiire is Indicated by the oblique transver.se 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. Posterior columr Root-fibres GoU's tract Burdach's tract rosterior median septum Root-fibres ,^ Dorsal zone Developing spinal cord of about seven and one-half weeks. X 44- {His.) Anterior median fissure Anterior Root-fibres with pial process column Developing spinal cord of about three months. X 30. {//is.) become deeper and narrower in consequence of the increased bulk of medio-ventral parts of the cord. As the fissure 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. UiUil the third month the gray matter, derived from the nuclear layer, is much more voluminous than the surrounding marginal layer, which, so far as the contribution of nervous elements is concerned, is passive, since its conversion into the white matter depends upon the ingrowth of axones from the neurones situated either within or outside the cord. The development of the individual fibre-tracts includes two stages, between the comple- tion of which a considerable, and sometimes a long, period interxenes. The first marks the invasion of the supporting tissue of the marginal zone by the ingrowing axones as naked axis- cylinders ; the second witnesses the clothing of these fibres with myelin. The period between the appearance of the tract and the development of the medullary coat is variable. In some cases, as in the great cerebro-spinal motor paths, although the fibres grow into the cord during the fifth month of fcetal life, myelination does not begin until shortly before birth and is not completed until after the second year. 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 fibies and in the direction of conduction. I'RACnCAL CONSIDKRATIONS : Sl'lNAL C(JRD. 1051 Based on tlie i)l)Sfrvalions of I'lcchsiji, His, IJcLlUcrew, and ollicrs, the* time ot the appearance and of tlie development of tlie medullary coat of some of the fibres within the spinal cord may !)<.• Ki^'*-""- Fibres of Appear Myelinate Anterior root about 4th week durin>( 5th month Burdach's tract during 4th week end • Fortunately, it is this variety which offers the ber.t prognosis, since the cord ends usually just below the lower border of the first lumbar vertebra, and the cauda equina beini,^ more movable and toU|L,dier than the cord itself, it can better evade the encroachment on the canal, althouv^di in spite f)f tliL'se facts, it is not infre(iuently injured in such lesions. The bodies of the lumbar vertebne are the lari^est 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 jiortion of the spine. In caries of the spine (Pott's disease) the lesion is situated in the bodies of the vertebr.e, 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 resultint,^ will a.ijain affect the motor jjortion of the cord, first producini^ a paralysis of motion in the i)arts below, varyin_i^ in dej^ree 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 diiiticult 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 vertebne, and then gradually decreases to the first thoracic, where it merges into the thoracic portion of the cord. Only in the thoracic re J-ion 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 s>-mptoms 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 nen-es arise from the cord between the lower margin of the foramen magnum and the sixth cervical spine, the first six thoracic I054 HUMAN ANATOMY. Fig. 908. First cer\-ical. vertebra -Skull First thoracic _ vertebra ' • First thoracic spine ^ u^ nerves between the latter spine and the fourth thoracic, the lower six thoracic nerves between the fourth and ninth dorsal spines, 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 ner\e-roots, applicable to the prelumbar nerves, is gi\en 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 ( l-\') thoracic nerves subtract one ; for the lower (\'I-XII) thoracic nerves subtract two. All the cer- vical ner\es pass out through the intervertebral foramina above the vertebrse after which they are named, except the eighth cervical, which emerges between the se\'enth cer- \ical and the first dorsal vertebrae. All the other spinal ner\'es 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 le\el, as from a fracture-dislocation of the spine, may be associated with a paralysis of the ner\-e-roots passing out at or btlow 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- vertebral 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, pass 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 to their destination. These ganglion cells constitute, at least functionally, the trophic centres for the muscles. Lesions of the anterior horns, therefore, besides causing First lumbar vertebra -First lumbar spine III First sacral veriebr Diagram, based on frozen section, showing relations of tx)dies and spines of vertebrae to levels at which spii.al nerves escape Irom vertebral canal. '11 11. i;rai.\. 1055 paralysis {polio-mytlitis), will k-aosterior rocjts, and some of them soon cross to the op[)osite side of the cord, others ascending in the posterior column. The lemniscus is probably the chief sensory tract in the medulla oblonjrata, pons, and cerebral peduncles. Kvery segment of the spinal cord contains centres for certain groujjs of muscles, and for refle.x movements associated with them. A refle.v begins in the stimulation of a sensory nerve. The impulse thus created jtasses to a centre in the cord and thence is transmitted to a motor nerve, thus producinjj^ a contraction of the muscle suj>plied bv that ner\e. The complete jiath of this imj^ulse is called a rrjiex arc. The sensor\- impulse maybe transmitted to different segments fjf the cord and thence out through the corresponding motor roots. Tims a complicated refle.x arc is produced, h is to be assumed, however, that the impulse will take the shortest route, so lliat simple reflexes will have their reHex arc chieHy in those segments of the cord in which the posterior root enters. Each segment of the cord is connected with libres from tlic brain to which must be ascribetl the hmction of reflex inhibition. If the inhibitory libres 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). Hinuato-rhachis, or hemorrhage into the membranes of the cord fextramedullary hemorrhage), may result from an injurvto the spinal column, as a fracture or a severe sprain. The bleeding may be from the ple.xus of veins between the dura and fjony 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 hyperccsthesia. 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 nenous axis and that the de^^ree 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 involvmg association is conspicuous, the antero-superior parts of the brain, the pallium, as the regions particularly concerned are called, are so enormously develoi)ed that the human brain is thereby di^stinguished 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 clegree by the thickening, unequal growth and expansion of its walls. Even when most'complex. as in man, \he 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 01faclor>- tract Stalk of pituitarj- 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. Preparatory to entering upon a description of the fully formed brain, it is desirable to consider briefly the broad plan according to which the organ is laid down and the general lines klong which its evolution proceeds. Before doing so, however, it will be necessary to take a general survey of the relations of the several divisions composing the brain. . , Denuded of its investing membranes and the attached cranial nerves, and viewed from below (Fig. 909"). the encephalon is seen to consist of a median brain-stem, that inferiorly is directlv continuous with the spinal cord through the foramen magnum and above divides into two diverging arms that disappear within the large overhang- ing mass of the cerebrum. The brain-stem includes three divisions, the interior o which, the medulla oblongata, is the uninterrupted upward prolongation of the spinal cord and above is limited by the projecting lower border ot the quadrilateral mass Till-: liRAIN. 1057 of the next division, the pons Varolii. Hey, cerebral peduncle; cf, cervical flexure ; bf, cephalic flexure. Drawn from His model. it may be pointed out here, in a general way, that the pallium gi\x*s 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 are thus gradually covered over. The striate area thickens into the corpus striatum, 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 surrounding 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 OE Till-. P.RAIN. 1061 bined ci)ntril)iiti(>ii of tin- tclfiiccphalon and (lic-iKc])hai()M. During the fifth week the dicnccphaloii expands into a relatively laij^e ine^nilar space (I'i^. 913;. vvhose roof and floor are thin and whose latc-ral walls are thickened by the masses of the de\'elopin' Anterior part of third ventricle Diencephalon Optic thalanii Optic nerves and tracts Subthalamic tegmenta Interpeduncular structures Pineal and pituitary bodies Posterior part of third ventricle Middle vesicle Mesencephalon or Mid-brain Meseiicei)halon Cerebral peduncles Aqueduct of Svlvius Corpora quadrigeniiiKi Posterior vesicle Rhombencephalon or Hind-brain Isthmus Superior cerebellar peduncles Superior medullary velum Metencephalon Pons Cerebellum Fourth ventricle Myelencephalon Medulla Inferior 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 UE THE BRALN. 1063 Kpithalamus Thalamus / Mctathalamus I'ars mammillaris hypothalami leseticephalon I'edunculi cerebri Isthmus erebcUum Pons Medulla corpus c;ill(jsuiii, ihc fuiulaiiKiUal rchititnislups indicated by cnihryolrjo^y arc of such value that, even in the ilcscription of the aduU orj^^an, j^roupinj^ of tlie various jxirts of the brain upon a develop- mental basis is found acKan- F'tJ- 9'4- tai^eous. Althoutjh strict aclherence to such a plan would be at times inconven- ient, and, therefore, will not be followeil, constant refer- ence to |)rimarv relations is imperative. It will be con- venient, therefore, at this place, to call attention to the accompanying outline diaijrams which illustrate the principles established by His in his epoch-makinjr studies of the human brain. In adtlition to showing the five cerebral vesicles, Fig. 913 indicates the relati\e 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 lamin^E, 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 deri\-ations of the fore-brain, particularly of those which are differentiated from the diencephalon and later are found connected with the third \entricle. In order to set forth the developmental relations of the fore-brain, the following table from His, slightly modified, will be of service : (Pallium CHemisphserium ' Corpus striatum (■Telencephalons ( Rhinencephalon Fore-Brain I (^ Pars optica hypothalami Rhinencephalon Pars optica hypothalami ^ Oorsal zone Ventral zone Diagram showing chief derivatives from cerebral visicles ; based on brain of embryo of third month. (His.) Prosencephalon [die.n CEPHALON Pars mammillaris hypothalami f Thalamus Epithalamus I Hahenula Thalamencephaloii •! Corpus pineale Commissura post. .Mctathalamus Corpora geniculata Parts of the Brain derived from the Rhombenxephalox. THE MEDULLA OBLONGATA. The medulla oblongata, sometimes called the bidb 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 \allecula 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 bv the prolongation of the anterior and posterior median fissures ; each half is subdi\ided bv a ventro-lateral and a dorso-lateral line of ner\e-roots into tracts that seeminglv are continuations of 1064 HUMAN 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 cr^rd. 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 Tdecussatio 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 Optic tract Mammillary body Pons (basilar groove) Middle cerebellar peduncl Anterior median fis> Cerebellum Root-bundles of ninth and tenth nerves Inhindibulum 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 triangxilar depression, \\\& foramen ccecum. The posterior median fissure, the direct continuation of the corresponding groove 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 di\ergence of the dorsal tracts of the bulb, which below enclose the fissure, to form the Ir^wer lateral boundaries of the lozenge-shaped fourth ventricle ffossa rhomboidalis), and {b) 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. Each 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. One of these, the antero-lateral furrcw, marks the line of emergence of the root-fibres of the hypoglossal nerve, which, being entirely THE MLDILI.A ()liL()N(;ATA. 1065 motor, correspond to the ventral roots of the spinal nerves with which they are in series. Tiie other j^roove, the postero-lateral furrow, continues upward in a general way the line of the dorsal spinal root-til>res and marks the attachment of the fibres of the ninth, tenth and bulbar part of the elevinth cranial nerves. Unlike the posterior root-til)res of the cord, which are exclusively sensory, those attached along this groove of the metiulla 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 pneuniogastric include both and, therefore, are mixed. The Anterior Area. — This subdivision of the medulla, also known as the/jva- 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. I'Lach Vw,. 916. pyramid constitutes a robust strand, which cerebral cortex belowbeginsat 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-s])inal 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. Th2 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). decussation Lateral ramidal tract 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. io66 HUMAN ANATOMY. The Lateral Area. — This region is (Jofined on the surface by the antero-lateral and postero-laleral furrows in front and behind respecti\ely, and includes a narrow strip on the hiteral 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 ), i)roduced 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 uj>per 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, \.\\e 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 aiiterior supej-ficial arcuate fibres, which cover for a variable distance the inferior part of the olive in their course backward to gain Thalamus Median geniculate body Inferior brachium Superior coUiculus 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 Roland i 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 de, showing lateral aspect of medulla, pons, and midbrain. 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 Goll and of Burdach, which within the medulla are known as the funic- ulus gracilis and funiculus cuneatus respectively, and are separated from each other by the paramedian sulcus. Beginning with a width of about 2 mm. , the gra- cile funiculus 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 gray matter. Then, diverging from its fellow of the opposite side to bound the ventricle, after a short course it loses its identity as a distinct strand and becomes continuous with the THr: MEDULLA OBLONGATA. 1067 inferior cerebellar peduncle or restiform body. The expansion within the U|)|)ii part i)t thf tiMiiciilMs urac iHs, thi- ela\a, contains tlic nucleus gracilis ( nucleus funiculi iLjrucilis), the reception station in uhiih the lon^ sens(jry fibres of Gull's tract are interrupted. The trianj.;nlar interval included Ix-twc-en the ^racile funiculi, where these henin to diverge, corresponds to the level at which the central canal of the cord ends bv openinjj^ out into the fourth ventricle. A thin lamina, thi- obex, closes this interval and is continuous with the ventricular roof. Alonji^ the outer side of the j^racik- fasciculus and separated from it by the jjara- median furrow, extends a second longitudinal tract, the funiculus cuneatus, which at the lower end of the medulla recei\es the column of linrdach. .Slij^htly above the Icjwer level of the clava, the cuneate strand alstj exhibits an e.xj)ansi matter Isolated anterior cornu Pvramidai decussation Transverse section of medulla at level B, Fig. 919; pyramidal decus- sation well established; posterior cornua are displaced laterally by posterior columns. X 5H. Preparation by Professor Spiller. I070 HUMAN ANATOMY. gracilis and, even after the disapiK-arance of the latter, continues as a striking collec- tion of gray matter beneath the dorsal surface of the medulla, from which it is sei)arated by the posterior suj^erticial 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 7incleus cuneahis cxternus, and the deeper and more broken one, the micleiis ciineahis internus. Owing to the increased bulk of the fasciculi (jf 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 le\ el with the central canal. Meanwhile the i)OSterior cornua themselves, especially the cajjping substantia gclatinosa, materially gain in bulk and ncnv ajjpear as two club-shaped masses of gray matter that cause the dorso-lateral projections of the Rolandic tubercles seen on the Fig. 922. Funiculus gracilis Funiculus cuneatus _ / Spinal root of V nerve -~.,^ Substantia gelatinosa-j >- Nucleus gracilis Nucleus cuneatus #' nucleus Pyramidal tract " Root-fibres of hypoglossal Anterior superficial arcuate fibres' Arcuate nucleus Transverse section of medulla at level D, Fig. giq. showing posterior nuclei, inferior olivarv 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 grav matter — the inferior oli\ary nucleus and the two accessory olivarv nuclei. Beneath the prominent olivary eminence lies a corrugated sack-like lamina of gray 1072 HUMAN ANATOMY. Ventral Dorso-lateral aspect of inferior olivary nucleus as reconstructed by Dr. Florence R. Sabin. X 5- matter, the inferior olivary nucleus ( nucleus olivaiis inferior), which in favorable transverse sections appears as a conspicuous sinuous C-hkc tigure. The nucleus resembles a greatly crumpled bag, of which the closed end lies beneath the corresponding superficial protuberance and the mouth, or hiliim, 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 rnr^i- > its transverse diameter is about 6 mm., and its vertical one about one millimeter lesL. 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 a.xone, 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 oli\ary 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 uncertain ; according to Kolliker it is prob- ^^' able that from some of 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 niesial accessory olivary nu- cleus (nucleus olivaris accessorius mesialis) is a sagittally placed the chain of conduction are Cerebello-olivary strands Gray matter '^^-^•'V? .*; Core of white matter M •i^ ■ #. lamina, from I o-i i mm. in length, which lies between the tract of the fillet and the root-fibres of the hypoglossal ner\'e. It extends be- low the inferior olive and, therefore, is encountered in transverse sections at a lower le\el— 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 cerebello-olivary fibres. X loo- Till': MKIHJI.LA (JHLoNciATA. lo: which the lower and mitldlc arc coiithiuous aiul the upper is unconnected, and four small isolated masses of ,uray matter alont; the dorsal border of the nucleus. The inferior or si)inal L-m\ of the nucleus is thickened and bent outward, so that its plane is oblique anil parallel with the ventral surface of the chief olive. Hi^dier, when the latter is well established, the mesial accessory nucleus is represented by a narrow broken tract, that corresponds more closely with the saj,dttal plane. In this situa- tion the nucleus lies between the fillet and the iimer end oi the chief olive and across Dorsal nucleus of vakils Fig. 927. Vc-iitricular roof Funiculus cuiu-a- tus, overlaid by- rest 1 lor 111 body Fasciculus^/;// solitarius"^"^' ^ N X- Substantia gela- tinosa overlaid by root of \' Nucleus ambiguus Nucleus lateralis Nucleus cuneatus .««■>'■ ■ -I fife; . '''i'^^Tf^^ ^ - ' ■■-■ .^sf^S^t^^ :.-■ im-mv ■Li I lypoglossai ncleus Post, longitudinal fasciculus Root-fibres of XII Inferior olivar>- nucleus Tract of mesial fillet Pyramidal tract Anterior superficial arcuate fibres Transverse section of medulla at level E, Fig. giq ; central canal has opened into fourth ventricle ; restiform body appearing. X 5- Preparation by Professor Spiller. its hilum. The dorsad 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-glosso- pharyyigeal 7iucleus, partly sensory and pardy 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). \'iewed 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 68 I074 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 vestibidar tierve 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 fascicnlus solitarins, or Fig. 928. Ventricular roof Fasciculus solitarius Dorsal nucleus of X Nucleus of XII I Post. long. Restiform Root-fibres v of X nerve Nucleus ambiguus Root-fibres of XII -^*^ 'Restiform body Descending ■ vestibular root Gray column of vestibular root Form.retic.grisea Form, retic. alba Interolivary stratum (median fillet) Inferior olivary nucleus O- Nerve-cell Pyramidal tiacts 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 by 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 descendbig root of the trigeminal 7ierve 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 accessojy olivary nncleus, by the feltwork of fibres producing the reticular formation. In con- trast to that within the hiG. 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) \.\\efor7natio reticularis grisea. Accessions to the irregularly distrib- uted nerve-cells occur as two moredefinitecol- lections ; one of these, (4) the nucleus am- bigtais. consists of an inconspicuous group of large 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 ner\'e. The other (5), the nucleiis lateralis, includes an uncertain aggregation of medium sized cells, situated near the periphery and ventral -*^f^- - f J. Portion of formatio reticularis grisea, showing nerve-cells and transverse and longitudinal fibres. X 130- interlacing Till': Ml-.Dl l.l.A olU.ONCiATA. 1075 fiDin the trij^cminal rcjot. A scijaratc- ^\ini\> of somcwli.it larger cells, nearer the ventral bonier of the trifacial root, has been desij^nated the nucleus lateralis dot sails, and 1)V K()lliker regarded as belon^in.n to the oritL^in of the spinal accessory nerve. Cothlrar fibre* croulog mlifuriii body Descenilini; root at vestibular nerve \ -\- V\c.. 930. Strlx BcuMicae Mc.ll.in Nucleus vestibular of IX nucleus Delten' nucleus Krstiforin hotly >sl. lont;. fasciculus Koot of IX nerve Substantia cdatinosa Honnatio reticularis all >.i Tract of mesial fillet — : Inferior olivary botly V \%. \ Fibres of IX nerve ^ Spinal root of V neive Substantia t^cUtinosa rormatio reticularis grisca V \. \- \'raniidal tracts Transverse section of medulla at level G, Fig. 919; ventral part is narrower, whilst dorsal part is expanded owing to increased size of restifoim Dodies. 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 trad, 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 Portion of transverse section of medulla, showing median raphe and adjacent formatio reticularis alba. < 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 7mcleus. The latter Hes at first chiefly on the ventral and, hij^her, 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 t?-ad of the median fillet, composed of longitudinal fibres that are the upward continu- ation of the deep arcuate hbrcs, which, from the sensory decussation to the upper limit of the cuneate nucleus, bend sharply brainward after crossing the mid-line. The hlk't-tracts are also known as the interolivary 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) 'Y\\q. posterior long itudi^ial fasciculus appears in cross-section as a compact oval or laterally flattened stranci, 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 11 16). 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-oli\'ary 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 Gowers' 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 ner\'e-cells that are found within the anterior area. The details of a transverse section passin,^ 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 Ijy only a few irregular plications. The pyramids, likewise, are narrower, and separated by 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 mesial vestibular 7!ucleus. External to the latter the lateral or Deiters" tmclcus 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 vestibular part oi the auditory nerve pass backward to gain the vestib- ular nuclei. The outer surface of the restiform body is closely related to a considerable THK TONS VAROLII. 1077 tract of K^ay niattt-r that collectively (oiislilutes the receptioii-iuicleiis of the cochlear division of the aiulitorv nerve. This j^aiiKlion is siiixlivided into a superior and an inferior portion, these beinK the iioisa/ lochhar nmUus and the ventral loihliar uiuhus res|K,'ctively. They both receive the libres of the cochlear or lateral division of the auditory nerve. The ventral cochlear nucleus is the starting pt)int of a tract of transverse fibres, that pass horiz(jnlally inward, many traversing the fillet and crossinj,' the raphe, and inlerniinKle with those from the opposite side. They thus form a broad strand, the corpus tnipfzuidis, that within the pons occupies the lower limit of the tegmental region, which it separates from the ventral. In Fig. 932 Tu;. 932. ntscinil Sul'st.inli.i liii; root Kclutinosa ofVMI (ir.-iy sutislaiicc uf lloor of ventricle Mesial vcslll.iiUr nuclcui Vestibular nerve Detters' nucleus kcstif.rin l«ly Cochlear nerve Dorso-lateral ~ cochlear nucleus _ Ventral cochlear nucleus Wyr., "' Cochlear nerve and ventral cochlear nucleus Spinal root of V Trapezoidal fibres' Inferior olivary nucleus Median fillet Pjraniidal 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 restiforni 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 restiforni body and proceed beneath the ventricular floor to the mid-groove ; these mark the course of the strice acusticce seen crossing the ventricle. \'entro-mesiaI to the spinal root of the trigeminus and the associated Rolandic substance the nucleus 0/ 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 dow^nward 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 tran.sversely 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 frf)m the cerebral peduncles to the medulla. The transverse striation indicates the general ctjiirse 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 f fasciculus obiiquus pontisj frfjm 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 midrlle jjc-duncle, the sides of the pons are blended with the overlying superior cerebellar peduncles, which, in conjunction with the intervening superior viedidlary velum, complete dorsally the ring of tissue sur- rounding the narrowed superior end of the fourth ventricle. INTERNAL STRUCTLR?: OF THE PONS VAROLII. Viewed in transverse sections the pons is seen to include two clearly defined areas, the ventral and the dorsal fFig. 933;. The ventral part Tpars basilaris; presents a characteristic picture in which the large pyramidal tracts are covered in Abducent fibres Inferior cerebellar }]eduncle Facial fibres Substantia gelatinosa S[/inal root of V Facial nucleus Trapezoidal fibres Superior olive .■.\: Fif- 933- erior ce Post. \> iascicu Interior cerebellar peduncle Nucleus Post. long N-rl-r*! of VI fasciculus -'"■ V ^^;-%-,' - ■ ^f^'frnerjring fecial fibres • '.>5tibular fibres --^s pinal root of V •Oliiary peduncle -Superior olive Formatio reticularis ^ cftegmestum K'. Pyrami'l : I ransverse fibres Transverse sectioB.jof x>ons at level I, Fig. 910 ^«ho)»ing general subdivision into ventral and dorsal 'tegmental) areas and nuclei dt sixth and seventh nerves. X 3. and excluded from the surface by a conspicuous layer of superficial transverse fibres Cstratum superficiale pontis;, that laterally sweep backward into the cerebellar peduncle and are traversed by the root-fibres of the seventh and eighth nerves. The pyra- mids no longer appear as compact fields, but are broken up into smaller bundles by the transverse strands of ponto-cerebellar fibres. This subdivision becomes more marked at higher levels of the pons ("Fig. 936), in which the interweaving of the longitudinal and transverse bundles 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 of the cerebral peduncle. The dorsal limit of the ventral field is occupied by a well marked deeper layer of transverse fibres Tstratum profundum pontis). A considerable amount of gray matter, collectively known as the pontile nucleus nil'. J'ONS VAROLII. 1079 ^■>^iy' m Portion of cross-section of pons, showing cells of pontine nucleus. X 300. (nucleus poiilis) i.s clislrilnUucl vvithiii the iiitcrsliccs Ijclwccn the bundles of ncrve- fihics. The cells of this nucleus, small in size and stellate in form, arc closely relateil to the ponto-cerehellar fibres (jf the same and of the oppcjsite side, many constitutiniL; stations of intc-rruption in the cortico-cerebellar paths. riu' dorsal or tegmental part of the pons (pars dorsaiis pontisj resembles to a considiTabli- exli-iU in its miu lal structure thi; formatio reticularis ^risea 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 jT^ray matter and of nerve- ^'"'' '''^^• fibres, together with chantjes ._ ^__.,.«^ - in the position of the fillet, _-l__'^ " ' '~--- -"'~' produce di-lails that vary Z.. with the level of the section. ;; ■; When this passes above the lower margin of the pons (Fik^- 933). t\\" diverging and oblicjuely cut strands of fibres, coursing from the ventricular tioor towards the ventral aspect, mark the root- fibres of the si-xth 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 i^Dunded prominence of the eminentia teres, which it helps to j)roduce, 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 si.xth 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, parti}' lodged within an indentation on the dorsal surface of the conspicuous tract of transx'erse 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. 933J 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 oli\'e a small o\'al bundle of longitudinal fibres, the central tegmental fascicuhis, 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 lioor of the fourth ventricle. Here they converge into io8o HUMAN ANATOMY. a compact strand that, as the ascending portion of tlie 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 a.xis directed dorso-ventrally, but approximately horizontal. The tract now appears as a modified oval, somewhat compressed from before back- ward, the thicker inner end of which reaches the rajjhe while the tapering outer end lies near the superior oli\e. 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. o^^. ____ — Superior cerebellar peduncle *r'if _- Inferior cerebellar peduncle Mesencephalic root of V- Posterior longitudinal fasciculus Sensory trigeminal nucleus ^'■ddle cerebellar peduncle ?^» Motor trigeminal %' nucleus lotor fibres of V V ' 4 '''^Trigeminal nerve Superior olive Median fillet i »/&t^ O tp transverse pontine fibres — ^ Psramidal tracts Middle transverse pontine fibres Transverse section of pons at ievel J, Fig. 919, showing root of trigeminal nerve with its nuclei. X 3- Preparation by Professor Spiller. the lateral pontine area, appear the substantia gclatijiosa and the associated spinal root of the trigeminal nerve. Just behind the latter the descending vestibular root lies close to the inner side of the restiform body. The collection of nerve-cells marking Deiters' nucleiis 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 w ith the nuclei and roots of the trigeminal verve. At this level the nuclei and roots of the sixth and seventh ner\-es are no longer seen. The median fillet appears on each side as a compressed oval, the long axis of which is hori- zontal 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 trapezoid body— pass. The lateral boundary of the ventral part of the pons is defined by a hugh tract of obliquely cut 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 getatinosa, which, it will be remembered, is to be seen in all the preceding lower levels intimately related TIIK l'()NS VARCJLII. 1081 to tlif dt-sttndini^^ or spina/ root of tlitr fiftli iit-rve. A slcoikI and more compact fjanKlion, the motor nm/fiis of llu- lriK<--mimis, lies to tlie inner side and sli^litly farther batk. It contains large multipolar cells, extends to a somewhat hi^^her level than the sensory nucleus, and is se|)arated from the latter by a strand