COLUMBIA LIBRARIES OFFSITE HEALTH SCIENCES STANDARD HX00025585 >l<<4^;;;;;y; i'X///; .'.'/.'.' \::' •////A' :x/.vx-: . 5.^:<;!' the Sexual Organs, . . . . . . . . T-" CHAPTER XIX Dkvki.opmknt 766 The Changes in the Ovum, 766 The Fa^tal Membranes 779 The Devel()|)M)ent of the Organs, ......... 78K APPENDIX «2r, INDEX «3y FAHEENHEIT and CENTIGEADE SCALES. F. 500° 401 39a 383 374 356 347 338 329 320 311 302 284 275 266 348 239 230 212 2a3 194 176 167 140 122 113 105 101 100 C. 260° 205 200 195 190 180 175 170 365 160 155 150 140 135 130 120 115 110 100 95 90 80 75 60 50 45 40.54 40 37.8 98.5 95 86 77 68 50 41 32 23 14 + 5 - 4 -13 -22 -40 -76 36.9 35 30 25 20 10 5 0 - 5 - 10 - 15 -20 -25 -30 -40 -eo 1 cleg.F. = .W°C. 1.8 " - 1°C. 3.6 " = 2°0. 4.5 " ^ 2.5°C 5.4 " = 3°C. To convert de- grees F. into de- grees C, subtract 32, and multiply To convert de- grees C. into de- grees F., multiply by ?, and add 32°. MEASUREMENTS FKENCH INTO ENGLISH. LENGTH. 1 m^tre 1 10 dficimt'tres' I := 39.37 English 100 centimetres f inches. 1,000 millimetres J (or 1 jd. andSJ^in.) 1 d6clm6tre ) 10 centimetres V = 3.9.37 inches 100 millimetres ) (or nearly 4 inches.) 1 centimetre 10 millimetres 1 millimetre = .3937 or about (nearly i inch.) = nearly j'j inch. Or, One Metre — 39.37079 inches. (It is the ten-millionth part of a quarter of the meridian of the earth.) 1 Decimetre = 4 in. 1 Centimetre = 1*5 in. 1 Millimetre = V., in- Decametre = S2.80 feet. Hectometre - 109.36 yds. Kilometre = 0.62 miles. One incli = 2.539 Centimetres. One foot = 3.047 Decimetres. One yard = 0.91 of a Bietre. One mile = 1.60 Kilometre. The cubic centimetre (15.432 grains — 1 gramme) is a standard at 4° C, the grain at 16". 60 C. WEIGHT. (One gramme is the weight of a cubic centimetre of water at 4° C. at Paris). 1 gramme ] 10 decigrammes I = 15.432349 grs. 100 centigrammes f (or nearlj- 1514). 1,000 milligrammes J 1 decigramme ) 10 centigrammes - = rather more 100 milligrammes ) than IJ^ grain. 1 centigramme 10 decigrammes = i-ather more than j^j grain. 1 milligramme = rather more than ^ grain. Or 1 Decigramme = 2 dr. 34 gr. 1 Hectogrm. = 314 oz. (Avoir.) 1 Kilogrm. = 2 lb. 3 oz. 2 dr. (Avoir.) A grain equals about 1.16 gram., a Troy oz. about 31 gram., a lb. Avoirdupois about ^ Kilogrm. and 1 cwt. about 50 Kilogrms. CAPACITY. 1,000 cubic decimetres 1 — 1 cubic 1 .000.000 cubic centimetres C metre. ■ = 1 litre. 1 cubic decimetre or 1,000 cubic centimetres \ Or One Litre = 1 pt. 15 oz. 1 dr. 40. (For simplicity. Litre is used to signify 1 cubic decimetre, a little less than 1 English quart.) Decilitre (100 c.c.) = 314 oz. CVntilitre (10 c.c.) = n dr. Millilitre (1 c.c.) = 17 m. Decalitre = 2J ^al. Hectolitre = 22 gals. Kilolitre (cubic metre) — 3714 bushels. A cubic inch = 16..38 c.c. : a cubic foot = 28.315 cubic dec. and a gallon = 4.54 litres. CONVERSION SCALE. To convert (iRAMMES to OrxcES avoir- dupois, multiply by 20 and divide by 567. To convert Kilogrammes to Pof^'Ds, multiply by 1 ,000 and divide by 454. To convert Litres to Gallons, mul- tiplj' by 22 and divide by 100. To convert Litres to Pints, multiply by 88 and divide by 50. To convert BIillimetres to Ls'ches. multiply by 10 and divide by 254. To convert Metres to Yards, multi- ply by 70 aud divide by 64. SURFACE MEASURE. 1 square metre = about 1550 sq. inches. Or 10.000 sq. centimetres, or 10.75 sq. ft. 1 sq. inch = about G 4 sq. centimetres. 1 sq. foot 930 ENERGY MEASURE. 1 kilogrammetre=about7.24ft. pounds. 1 foot pound = " .1381 kg:m. 1 foot ton = '• 310 kgni. HEAT EQUIVALENT. 1 kiloealorie = 424 kilogrammetres. ENGLISH MEASURES Apothecaries Weight. Avoirdupois Weight 7000 grains = 1 lb. Or 437.5 grains = 1 oz. 16 drams = 1 oz. 16 oz. = 1 lb. 28 lbs. = 1 quarter. 4 quarters = 1 cwt. 20 cwt. = 1 ton. Measure of 1 deciiii<-'ti-e, or 10 centiiiiBtres, or 100 iiiilliuif!tres. — Cranium --7 Cervical Vertebr» — Clavicle. — Scapula. 12 Dorsal Vertebrae. Humerus. 5 Lumbar Vertebrae, nium. Ulna. Radius. Pelvis. — Bones of the Carpus. Bones of the Meta- carpus. Phalanges of Fingers, Femur. Patella^ .Tibia. .Fibula. _ Bones of the Tarsus. _ Bones of the Meta - Phalanges of Toes. THE SKELETON (after Houjkn;. SjinpTiysia Pubis. DIAGRAM OF THORACIC AND ABDOMINAL REGIONS. A. Aortic Valve. M. Mitral Valve. P. Pulmonary Valve. T. Tricuspid Valve. Handbook of Physiology. CHAPTER I. THE PHENOMENA OF LIFE. Human" Physiology is the science which treats of the various pro- cesses or changes which take place during life in the organs and tissues of the body of man. These processes, however, must not be considered as by any means peculiar to the human organism since, putting aside the properties which serve to distinguish man from other animals, as well as those which mark out one animal from another, the changes which go on in the tissues of man go on much in the same way in the tissues of all other animals as long as they live. Furthermore it is found that similar changes proceed in all living vegetable tissues; they indeed constitute what are called vital phenomena, ^iiiiX are those proper- ties which mark out living from non-living material. Tlie lowest types of life, whether animal or vegetable, are found to consist of minute masses of a jelly-like substance, which is now gener- ally known under the name of ])rotoj)hmn. Each such minute mass is called a cell, so that these minute elementary organisms are designated unicellular. Not only is it true that the lowest types of life are made up of protoplasm, but it has also been shown that the tissues of which the most complex organisms are composed consist of protoplasmic cells. Thus, for example, the human body can be shown by dissection to be constructed of various dissimilar parts, bones, muscles, brain, heart, lungs, intestines, etc., and these on more minute examination with the aid of the microscope, are found to be composed of different tissues, such as epithelial, connective, nervous, muscular, and the like. Each of these tissues is made up of cells or of their altered equivalents. Again, we are taught by Embryology, the science which treats of the growth and structure of organisms from their first coming into being, that the human body, made up of all these dissimilar structures, commenced its life as a minute cell or ovum (fig. 2) about ^l-^ih of an inch in diame- ter, consisting of a spherical mass of protoplasm in the midst of which was contained a smaller spherical body, the nuclcua or fjenninal vesicle. 1 2 HANDBOOK OF PHYSIOLOGY. The plienomeua of life then are exhibited in cells, whether existing alone or developed into the organs and tissues of animals and plants. It must be at once evident that a correct knowledge of the nature and activities of the cell forms the very foundation of physiology; cells being, in fact, physiological no less than morphological units. The prime importance of the cell as an element of structure was first established by the researches of the botanist Schleiden, and his conclu- sions, drawn from the study of vegetable histology, were at once ex- tended by Theodor Schwann to the animal kingdom. The earlier observers defined a cell Space cou- '^^ ^ morc or less spherical body limited by a tainiug ■ liquid. membrane, and containing a smaller body Protoplasm. .Nucleus. Cell-wall. Nucleus or germinal , vesicle. Nucleolus or germi- nal spot. Space left by retrac- tion of yelk. .Yelk or vitellus. ■Vitelline membrane. Fig. 1.— Vegetable cells. Fig. S. — Semidiagrammatic representation of a hiunan ovum, showing the parts of an animal cell. (Cadiat.) termed a nucleus, which in its turn incloses one or more still smaller bodies or nucleoli. Such a definition applied admirably to most vege- table cells, but the more extended investigation of animal tissues soon showed that in many cases no limiting membrane or cell-wall could be demonstrated. The presence or absence of a cell-wall, therefore, was now regarded as quite a secondary matter, while at the same time the cell-substance came gradually to be recognized as of primary importance. Many of the lower forms of animal life, e.g., the Ehizopoda, were found to con- sist almost entirely of matter very similar in appearance and chemical composition to the cell-substance of higher forms; and this from its chemical resemblance to flesh was termed Sarcode by Dujardin. When recognized in vegetable cells it was called Protoplasm by Mulder, while Eemak applied the same name to the substance of animal cells. As the presumed formative matter in animal tissues it was termed Blastema, and in the belief that, wherever found, it alone of all substances has to do with generation and nutrition, Beale has named it Germinal matter or Bioplasm. Of these terms the one most in vogue at the present day, as we have already said, is Protoplasm, and inasmuch as all life, both in the animal and vegetable kingdoms, is associated with protoplasm, we THE PHENOMENA OF LIFE. 3 are justified in describing it, with Huxley, :is the "physical basis of life," or simply " living nuitter/' A cell may now be defined as a nucleated mass of protoplasm, of microscopic size, varying in the human body from the red blood-cell which is about 3-13^10- of an incli in diameter to the ganglion cell, 3/,-,,- of an inch, which ^iossesses sufficient individuality to have a life-history of its own. Each cell originates from a pre-existing cell, grows, produces other cells, and dies, going through the same, though briefer, cycle as the whole organism. 8ome of the lower forms of life seem to consist of non- nucleated protoplasm, but the above definition holds good for all the higher plants and animals, though some few cells lose their nuclei in the course of develoj^mentj e.g., the red blood-cells of all mammals. Properties of Protoplasm. Protoplasm is a semi-fluid substance, which swells up but does not mix with water. It is transparent and generally colorless, with refrac- tive index higher than that of water but lower than that of oil. It is neutral or weakly alkaline in reaction, but may under special circum- stances be acid, as, for example, after activity. It undergoes stiffening or coagulation at a temperature of about 54.5° 0. (130° F.), and hence no organism can live when its own temperature is raised above that point; it is also coagulated and therefore killed by alcohol, by solutions of many of the metallic salts, by strong acids and alkalies, and by many other substances. Under the microscope it is seen almost universally to be granular, the granules consisting of different substances, either albuminous, or fatty, or glycogenous matters, or more rarely of inorganic salts. The granules are not equally distributed throughout the whole cell-mass, as they are some- times absent from the outer part or layer, and very numerous in the interior. The granules may exhibit an irregular shaking, dancing move- ment, which is not vital and is knowai as the Brownian movement. In addition to granules, protoplasm generally exhibits spaces or vacuoles, generally globular in shape, excepting during movement when they may be irregular, filled with a watery fluid. These vacuoles are more numer- ous and pronounced in vegetable than in animal cells. Gas bubbles also sometimes exist in cells. It is impossible to make any definite statement as to the exact chem- ical composition of living protoplasm, since the methods of chemical analysis necessarily imply the death of the cell; it is, however, stated tliat protoplasm contains 75 to 85 per cent of water, and of the 15 to 25 per cent of solids, the most important part belongs to the classes of sub- stances called jyro^t^/rfo or «/^?i6'. Proteids contain the chemical ele- ments carbon, hydrogen, nitrogen, oxygen, sulphur, and phosphorus, the 4: HANDBOOK OF PHYSIOLOGY. last two In small quantities o'n]y. A proteid-like substance, nuclein, found in the nuclei of cells, contains pliosphorns in greater abundance. In cell protoplasm ti compound of nuclein with proteid, called nucleo- proteid, forms the most abundant proteid substance. Other bodies are frequently found associated with the proteids, such as glycogen, fiiji(i/)I(is)n or reticu- lum forms a fine network, increases in relative amount as tlie cell grows older, and has an aftinity for staining reagents. The liyaloplaam is less refraetile, elastic, or extensile, and has no aftinity for stains; it pre- dominates in young cells, is thought to be tiuid, and tills the interspaces of the reticulum. The nodal points of the reticulum, witli the granules (niicrosomrsi) found in the protoplasm, cause the granular appearance. Butschli has recently asserted that protoplasm is an emulsion made up of numerous microscopic vacuoles whose walls are in close apposition and are seen under the microscope in optical section only, thus causing the reticular appearance. This idea is accepted by few. The arrangement of the reticulum varies considerably in different cells, and even in different parts of the same cell. Sometimes, for ex- ample (fig. 8), the meshwork has an elongated radial arrangement from 10 HAISTDBOOK OF PHYSIOLOGY. the nnciens; at others, the mesh work is more evenly disposed, as in fig. 9. At the jimctious of the fihrils there are usually slight enlarge- ments or nodes. In some cells, particularly in plants, but also in some animal cells, there is a tendency toward a formation of a firmer external envelope, Membrane of cell Reticulum of cell — . Membrane of nucleus. Achromatic substance of nucleus. Chromatic substance of nucleus. Fig. 8. — Cell with its reticulum disposed radially ; from the intestinal epithelium of a worm. (Carnoy.) constituting in vegetable cells a membrane distinct from the more central and more fluid part of the protoplasm. In such cases the reticu- lum at the periphery of the cell is made up of very fine meshes. The membrane when formed is usually pierced with pores by which fluid may pass in, or through which protrusion of the protoplasmic filaments form- ing the cell's connection with other cells surrounding it may take place. It is an exceedingly interesting qiiestion whether in cells the one Fig. 9. — (a.) The colorless blood-corpuscle showing the intra-cellular network, and two nuclei with intra-nuclear network, (b.) Colored blood-corpuscle of newt showing the intra-cellular net- work of fibrils. Also oval nucleus composed of liniiting membrane and fine intra-nuclear network of fibrils. X 800. (Klein and Noble Smith.) part of the protoplasm can exist without the other. Schafer summar- izes the matter thus: — "There are cells, and unicellular organisms both animal and vegetable, in which no reticular structure can be made out, and these may be formed of hyaloplasm alone. In that case, this must be looked upon as the essential part of protoplasm. So far as amoeboid phenomena are concerned it is certainly so; but whether the chemical THE PHENOMEJTA OP LIFE. 11 changes which occur in many cells are effected by this or by spongio- plasm is another matter." Another question about which there is some difference of opinion is, which part of the protoplasm is chiefly contractile. It is usually con- cluded that this property rests iu the mesh work, but there seems a considerable amount of evidence in favor of the view that part if not all of the contractility resides in the hyaloplasm; for example, in amoe- boid cells the pseudopodial protoplasm are certainly made of this and not of spongioplasm, and when the corpuscle is stimulated the hyalo- plasm flows back into the reticular network. If the view that the hyalo- plasm is chiefly contractile be a correct one, the special condition of an amoeboid cell must be considered to be condition of contraction, and the flowing out of the process to be relaxation. The Cell Nucleus. All cells at some period of their existence possess luiclei. As has been incidentally suggested the origin of a nucleus in a cell is the first trace of the differentiation of protoplasm. The existence of nuclei was first pointed out in the year 1833 by Robert Brown, who observed them in vegetable cells. They are either small transparent vesicular bodies containing one or more smaller particles (nucleoli), or they are semi-solid masses of proto23lasm always in the resting condition bounded by a well-defined envelope. In their relation to the life of the cell they are certainly hardly second in importance to the protoplasm itself, and thus Beale is fully justified in comprising both under the term "ger- minal matter." They control the nutrition of the cell, and probably- initiate the process of subdivision. If a cell be mechanically divided, that 2)ortion not containing the nucleus dies. Histologists have long recognised nuclei by two important char- acters : — (1.) Their power of resisting the action of various acids and alkalies, particularly acetic acid, by which their outline is more clearly dclined, and they are rendered more easily visible. This indicates some chemi- cal difference between the protoplasm of the cells and nuclei, as the former is destroyed by these reagents. (2.) Their quality of staining in solutions of carmine, hannatoxylin. etc. Nuclei are most commonly oval or round, and do not generally conform to the diverse shapes of the cells; they are altogether less vari- able elements than cells, even in regard to size, of which fact one may see a good example in the uniformity of the nuclei in cells so multiform as those of epithelium. But sometimes nuclei occupy almost the whole of the cell, as in the lymph corpuscles of lymphatic glands, and iu some small nerve cells, and may even jn-ojoot alxive the surface. 13 HANDBOOK OF PHTSIOLOGY. Their position in the cell is verj- variable. In many cells, especially where active growth is progressing, two or more nuclei are present. Structure of Nuclei. The nucleus when in a condition of rest is bounded by a distinct membrane, the nuclear membrane, possibly derived from the spongio- plasm of the cell, which encloses the nuclear contents or haryoplasm. The membrane consists of an inner, or chromatic, and an of outer, or Xode of meshwork - Node of meshwork — »-Nuclear membrane. Nucleolus. Nuclear matrix. Nuclear meshwork. Fig. 10. — The resting nucleus — diagrammatic. (Wakleyer. achromatic layer, so called from their reaction to stains. The karyo- plasm is made up of a reticular network, or chroinoplasni, whose in- ters2Daces are filled by the karyolymph, or midear matrix, a homogeneous substance which is rich iu proteids, has but slight affinity for stains, and is supposed to be fluid. The network is composed of linin, or achromatin, a transparent unstainable framework; and of cliromatin, which stains deeply, is sup- A B p.cj Fig. 11. — Diagram of nucleus showing the arrangement of chief chromatic filaments, a. Viewed from the side, the polar end being uppermost, p.c.f.. Primary chromatic filaments; n., nucleolus; n.o.m., node of meshwork. b. Viewed at the polar end. i.c.f., Looped chromatic filament; /./., ir- regular filament. (Rabl.> ported by the linin, and occurs sometimes in the form of granules, but usually as irregular anastomosing threads, both thicker primary fibres and thinner connecting branches. The threads often form thickened nodes, karyosemes or false nucleoli, at their points of intersection. It THE PHENOMENA OF LIFE. 13 is now quite geDerally believed that tlie chromatin occurs as short, rod- like and highly refractive masses, which are embedded in the linin in a regular series. The nucleoli, or plasmosomes, are spherical bodies of unknown func- tion. They stain deeply, and may either lie free in the nuclear matrix or be attached to the threads of the network. Attraction Sphere. In addition to the nucleus, a minute spherical body called the coitro- so/iie is believed to be constantly present in animal cells, though some- times too small to be demonstrated. The centrosome is smaller than the nucleus, close to which it lies, and exerts a peculiar attraction for Fig. 11a.— Leucocyte of Salamander Larva, sliowin^ attraction sphere. (After Klemming.) the protoplasmic tilanients and granules in its vicinity, so that it is sur- rounded by a zone of fine radiating fibrils, forming the atiraction sphere or archoplasm. Some authorities assert that the centrosome lies within the nucleus in the resting state, and only passes into the cell proper in the earlier stages of cell division. The attraction sphere is most dis- tinctly seen in cells about to divide. It plays an important role in nuclear division, but it is doubted if it gives the initial impulse to the process. Cell Division. The division of a cell is jireceded by division of its nucleus, which may be either direct or indirect. Direct or simple division, amitosis or akinesis (zivr^rvis-, movement), occurs witliout any change in the arrange- ment of the intranuclear network; it is probalily lin\ited to the anurbas. 14 HANDBOOK OF PHYSIOLOGY. A constriction develops at the centre of the nucleus, possibly preceded by division of the nucleoli, and gradually divides it into two equal daughter nuclei. A similar constriction of the protoplasm of the cell occurs between the daughter nuclei and divides it in two parts. FJg. 13. — Akinesis, amitosis, or direct cell division. A, Constriction of nucleus; B, division of nucleus and constriction of cell body; G, daughter nuclei still connected by a thread, division being delayed ; jD, division of cell body nearly complete. (After Arnold.) Indirect division, mitosis (//iro?, a thread), or karyokinesis (xapuv^, a kernel), is the almost universal method, and consists of a series of Fig. 12a.— Karyokinesis, mitosis, or indirect cell divisioq (diagrammatic). .4, Cell with rest- ing nucleus; B, wreath, daughter centrosomes and early stage of acliromatic spindle; C, chromo- somes; D, monaster stage, achromatic spindle in long axis of nucleus, cliromosomes dividing: E, chromosomes moving toward centrosomes ; F, diaster stage, cliromosomes at poles of nucleus, commencing constriction of cell body ; G, daughter nuclei beginning return to resting state; H, daughter nuclei .showing monaster and wreath; i, complete division of cell body into daugliter cells whose nuclei have returned to the resting state. (After Bohm and von Davidoff. ) changes in the arrangement of the intranuclear network, resulting in the exact division of the chromatic fibres into two parts, which form the THE PHENOMENA OF LIFE. 15 chromoplasm of the daughter nuclei. The changes follow a closely similar course in both plant and animal cells. The process has been divided by different authorities into a varying number of stages, with varying names, but for the sake of simplicity it seems best to accept the Achromatic spiral Fig 13.— Early stages of karyokinesis. a. The thicker primary fibres remain and the achro- matic spindle appears, b. The thick fibres split into two and the achromatic spindle becomes longi- tudinal. (Waldeyer.) authority of Yerworn and recognize two stages only — a progressive one in which the changes in the nucleus advance to a maximum, and a retro- gressive one in which the resulting nuclear halves revert to the resting state. Progressive stage. The resting nucleus becomes somewhat enlarged, and the centrosome (according to those who regard it as lying normally centred. (CyLctsier) ckrcintci.( '"I'l'Jj loruft' clear area of nucleus'" Fig. 1-1.— Monaster stage of karyokinesis. (Rabl. ) within the nucleus) migrates into the cell protoplasm. The centrosome then divides into two daughter centrosomes which lie near the nucleus but are separated by a considerable interval. Each is surrounded by the radiating fibrils of the attraction sphere, and some of these fibrils pass continuously from one centrosome to the other, forming the achromatic spindle. At the same time (prophases) the intranuclear network be- comes converted into a fine convoluted coil (spircm or skein) which may be either continuous or else broken up into several threads. Tlie thread 16 HANDBOOK OF PHYSIOLOGY. or threads then shorten and become thicker, while the convolutions, which haye become less numerous, arrange themselves in a series of con- necting loops, forming the ■wreath. The nuclear membrane and the nucleolus disappear, the latter passing at times into the cell protoplasm and disintegrating. The wreath then breaks up into V-shaped segments, iVt Fine uniting • ' • " filaments. Fig. 15.— Stages of karyokinesis. (Rabl.) A. Commencing separation of the spUt chromosomes. B. The separation fuither advanced. C. The separated chromosomes passing along the fibres of the achromatic spindle. the chrumosoines, of which each species of animal has a constant and characteristic number. This varies from two to thirty-six in the differ- ent animals, but is sixteen in man. The two centrosomes migrate to the poles of the nucleus, while the achromatic spindle which connects them occujDies the long axis of the Remains of spindle. Line of separation of the two cells. Antipole of daugh- ter nucleus. Lighter substance of the nucleus. Cell protoplasm. Hilus. Fig. 16.— Final stages of karyokinesis. In the lower figure the changes are still more advanced than in the upper. (Waldeyer.) nucleus. The chromosomes, becoming much shorter and thicker, gather around the spindle in its equatorial plane, with their angles directed toward the centre, forming the aster or monaster. The actual division of the nucleus is begun at this time (vietaphases) by the splitting of each chromosome longitudinally into halves which lie at first close together so that each seems doubled. Soon afterward the fibrils of tho achromatic spindle begin to contract, and thus separate the THE PHENOMEXA OF LIFE, l7 halves of the chromosomes iu such a way that one-half of each is turned toward one pole, and the other half toward the other. As this con- tinues, the two groups, which are equal in size, draw away from each other and from the equator, each group being formed of daughter chromosomes. Ivetrogressive stage {(inaphases and telo^jhases) . The two groups (daughter chromosomes) now gradually aj^proach their respective poles, or centrosomes, and the equator becomes free. On reaching the jiole, each group gathers iu a form which is similar iu arrangement to the monaster and is known as the diaster. During this time the cell bodv becomes slightly constricted by a circular groove at its equatorial j^lane. Soon afterward the fibrils of the achromatic spindle which connect the two groups begin to grow dim and finally disappear. The daughter chromosomes assume the form of threads twisted in a coil and develop each a nuclear membrane and a nucleolus, forming a daughter nucleus. The nuclei enlarge and the nuclear threads assume the appearance of the resting state of the nucleus. Meanwhile, the constriction about the body of the cell has become deeper and deeper until the protojilasm is divided into two equal parts, or daughter cells, each with its daughter nucleus, and the process of karyokinesis is completed. Differences between Animals and Plants. Having considered at some length the vital j^roperties of protoplasm, as shown in cells of vegetable as well as of animal organisms, we are now in a position to discuss the question of the differences betwee7i jjlanta and animals. It might at the outset of our inquiry have seemed an unnec- essary thing to recount the distinctions which exist between an animal and a vegetable as they are in many cases so obvious, but, however great the differences may be between the higher animals and plants, in the lowest of them the distinctions are much less plain. In the first j^lace, it is important to lay stress upon the differences between vegetable and animal cells, first as regards their structure and next as regards their functions. (].) It has been already mentioned that in animal cells an envelope or cell-wall is by no means always present. In adult vegetable cells, on the other hand, a well-defined cellulose wall is highly characteristic; this, it should be remembered, is non-nitrogenous, and thus differs chemically as well as structurally from the contained protoplasmic mass. ]\Ioreover, in vegetable cells (hg. 17, b), the jirotoplasmic contents of the cell fall into two subdivisions: (1) a continuous film which lines the interior of the cellulose Avail; aud (2) a reticulate mass contain- 18 HANDBOOK OF PHYSIOLOGY, ing the nucleus and occupying the cell cavity; its interstices are filled with fluid. In young vegetable cells such a distinction does not exist; a finely granular protojDlasm occupies the whole cell-cavity (fig. 17, A). Another striking difference is the frequent presence of a large quan- tity of intercellular substance in animal tissues, while in vegetables it is comparatively rare, the requisite consistency being given to their tissues by the tough cellulose walls, often thickened by deposits of lignin. As an example of the manner in which this end is attained in animal tissues, may be mentioned the deposition of lime salts in a matrix of intercellular substance which occurs in the formation of bone. (2.) As regards the respective functions of animal and vegetable cells, one of the- most important differences consists in the power which vege- table cells possess of being able to build up new complicated nitrogenous Fig. 17.— (a.) Young vegetable cells, showing cell-cavity entirely filled tvith granular protoplasm inclosing a large oval nucleus, with one or more nucleoli, (b.) Older cells from same plant, show- ing distinct cellulose-wall and vacuolation of protoplasm. and lion -nitrogenous bodies out of very simple chemical substances ob- tained from the air and from the soil. They obtain from the air, oxy- geUj carbonic anhydride, and water, as well as traces of ammonia gas; and from the soil they obtain water, ammonium salts, nitrates, sulphates^, and phosphates, and such bases as potassium, calcium, magnesium, so- dium, iron, and others. The majority of plants are able to work up these elementary compounds into other and more complicated bodies. This they are able to do in consequence of their containing a certain coloring matter called cMorojjliyll, the presence of which is the cause of the green hue of plants. In all plants which contain chlorophyll two processes are constantly going on when they are exposed to light : one, which is called true respiration and is a process common to animal and vegetable cells alike, consists in the taking of the oxygen from the at- mosphere and the giving out of carbon dioxide; the other, which is peculiar apparently to bodies containing chlorophyll, consists in the taking in of carbon dioxide and the giving out of oxygen. It seems that the chlorophyll is capable of decomposing the carbon dioxide gas and of fixing the carbon in the structures in the form of some new com- THE PHENOMENA OF LIFE, 111 pound, one of the most rapidly formed of which is starrJi. 'J'lie tiri^t step in the formation of starch is the union of carbon dioxide and water to form formic aldehyde, COaH-H20 = CH20 + 02, oxygen being evolved; then by polymerization the formation of sugar thus, G CIl20 = C6Hi206; and by dehydration, CbHi^Oo— H20 = C6Hio05, the production of starch. In this way is starch synthesized or built up. Vegetable protoplasm by the aid of its chlorophyll is able to build up a large number of bodies besides starch, the most interesting and important being proteid or albumin. It appears to be a fact that the power which bodies possess of being able to synthesize is to a large extent dependent upon the chlo- rophyll they contain. Thus the j)0wer is only present to any marked extent in the plants in which chlorophyll isfoiind and is absent in those which do not possess it; while on the other hand it is present in the extremely few aninuils which contain it and is absent except in certain rare instances as one of the properties of animal protoplasm. It must be recollected, however, that chlorophyll without the aid of the light of the sun can do nothing in the way of building up substances, and a plant containing chloroph3dl when placed in the dark, as long as it lives, and that is not as a rule long, acts as though it did not contain any of that substance. It is an interesting fact that certain of the bac- teria have the chlorophyll rejjlaced by a similar pigment which is able to decompose carbon dioxide gas. Animal cells, except in the very rare cases above alluded to, do not possess the power of building up from simple materials; their activity is chiefly exercised in the opposite direction, viz., they have brought to them as food the complicated compounds produced by the vegetable kingdom, and with them they are able to perform their functions, set- ting free energy in the direction of heat, motion, and electricity, and at the same time eliminating such bodies as carbon dioxide and water, and producing other bodies, many of which contain nitrogen, but which are derived from decomposition, and only in very rare cases from building up. It must be distinctly understood, however, that there are instances of animal cells performing synthetic functions and of combining two simpler compounds to produce one more complex, and it is quite possi- ble that many of the processes performed by the cells of certain organs are instances of synthesis, and not as they have been described of break- ing down; and the reverse is undoubtedly the case with vegetable cells, so that it is impossible to generalize to a greater extent than to say that the tendency of the activity of the vegetable cell is chiefly toward syn- thesis, and of the animal cell toward analysis. With reference to the substance chlorophyll it is necessary to say a few words. It has been noted that the synthetical operations of vege- 20 HANDBOOK OF PHYSIOLOGY. table cells are peculiarly associated with the possession of chlorophyll and that these operations are dependent npon the light of the sun. It has been further shown that a solution of chlorophyll has a definite absorption spectrum when examined with the spectroscope, and that it is particularly those parts of the solar spectrum corresponding to these absorption bands which are chiefly active in the decomposition of car- bonic anhydride, and that, moreover, the position of the maximum absorp- tion corresponds with the maximum of energy of light. In the synthet- ical processes of the plant then, by aid of its chlorophyll, the radiant energy of the sun's rays becomes stored up or rendered potential in the products formed. The potential energy is set free, or is again made kinetic, when these products simply by combustion produce heat, or when they are taken into the animal organism and used as food and to produce heat and motion. The influence of light is not an absolute essential to animal life; in- deed, it is said not to increase the metabolism of animal tissue to any extent, and the animal cell does not receive its energy directly from the sun's light, nor yet to any extent from the sun's heat, but from the products formed by vegetable metabolism supplied as food, either di- rectly, as in the case of herbivora, or indirectly in the case of carnivora.. The potential energy of these food stufEs is set free in the destructive metabolism of the animal cell already alluded to. But it must be always recollected that anabolism is not peculiar to vegetable, or katabolism to animal cells ; both processes go on in each, but the cliief function, as far as we know at present of the former, is to transform kinetic into po- tential energy, and of the latter to render potential energy kinetic, as in heat, motion, and electricity. With reference to the food of plants, it should not be forgotten that some of the lowest forms of vegetable life, e.g., the bacteria, will live only in a highly albuminous medium, and in fact seem to require for their growth elements of food stuffs which we shall see later on are es- seutial to animal life. In their metabolism, too, they very closely ap- proximate to animal cells, not only requiring an atmosphere of oxygen, but giving out carbonic anhydride freely, and secreting and excreting many very complicated nitrogenous bodies, as well as forming proteid, carbohydrates, and fat, requiring heat but not light for the due perform- ance of their functions. It must be added, however, that certain bac- teria grow only in the absence of oxygen. (3.) There is, commonly, a difference in general chemical composition between vegetables and animals, even in their lowest forms; for associated with the protoplasm of the former is a considerable amount of cellulose, a substance closely allied to starch and containing carbon, hydrogen, and oxygen only. The presence of cellulose in animals is much more rare THE PHENOMKNA OF JJFE. 21 than in vegetables, but there are many aninials in wliich traces of it may be discovered, and some, the Ascidians, in which it is found in consider- able quantity. The presence of starch in vegetable cells is very charac- teristic, though, as we have seen above, it is not distinctive, and a sub- stance, glycogen, similar in composition to starch, is very common in the organs and tissues of animals. (4.) Inherent power of movement is a quality which we so commonly consider an essential indication of animal nature, that it is difficult at first to conceive it existing in any other. The capability of simj^le mo- tion is now known, however, to exist in so many vegetable forms, that it can no longer be held as an essential distinction between them and animals, and ceases to bo a mark by which the one can be distinguished from the other. Thus the zoospores of many of the Cryptogamia ex- hibit ciliary or amoeboid movements of a like kind to those seen in amoebae; and even among the higher orders of plants, many, e.^., i)io?ifea Muscijnda (Venus's fly-trai)),and Mimosa sensitlva (Sensitive plant), ex- hibit such motion, either at regular times, or on the application of external irritation, as might lead one, were this fact taken by itself, to regard them as sentient beings. Inherent poAver of movement, then, although especially characteristic of animal nature, is, when taken by itself, no proof of it. CHAPTER II. THE FUNCTIONS OF ORGANIZED CELLS. As we proceed upward in the scale of life from unicellular orgauisms, we find that another phenomenon is exhibited in the life history of the higher forms, namely, that of Develojjment. An amoeba comes into be- ing derived from a previous amoeba; it manifests the properties a,nd performs the functions of its life which have been already enumerated; it grows, it reproduces itself, whereby several amoebae result in place of one, and it dies. It cannot be said to develop, however, unless the for- mation of a nucleus can be considered as an indication of such a process. In the higher organisms it is different; they, indeed, begin as a single cell, but this cell on division and subdivision does not form so many Fi^. 18.— Transverse section through embryo chick (26 hours), a, Epiblast; 6, mesoblast; c, hypoblast; d, central portion of mesoblast, which is here fused with epiblast; e, primitive groove; /, dorsal ridge. (Klein.) independent organisms, but produces the material from which, by devel- opment, the complete and perfect whole is to be derived. Thus, from the spherical ovum, or germ, which forms the starting-point of animal life and which consists of a protoplasmic cell with a nucleus and nucle- olus, in a comparatively short time, by the process of segmentation which has been already mentioned, a complete membrane of cells, polyhedral in shape from mutual pressure, called the Blastoderm, is formed, and this speedily divides into two and then into three layers, chiefly from the rapid proliferation of the cells of the first single layer. These layers are called the Epiblast, the Mesoblast, and the Hypoblast (fig. 18). It is found in the further development of the animal that from each of these layers is produced a very definite part of its completed body. For example, from the cells of the epiblast are derived, among other 22 THE FUNCTIONS OF ORGANIZED CELLS. 23 structures, the skin and the central nervous system; from themesoblast is derived the flesh or muscles of the body, and from the h3'iDoblast the epithelium of the alimentary canal and some of the chief glands, and so on. It is obvious that the tissues and organs so derived exhibit in a vary- ing degree the primary properties of protoplasm. The muscles, for example, derived from certain cells of the mesoblast are particularly con- tractile and respond to stimuli readily, while the cells of the liver, although possibly contractile to a certain extent, have to do chiefly with the processes of nutrition. Thus, in development, we see that as the cells of the embryo in- crease in number it speedily becomes necessary for the organism to depute to different groups of cells, or to their equivalents {i.e., to the tissues or organs to which they give rise), special functions, so that the various functions which the original cell may be supposed to discharge, and the various properties it may be supposed to possess, become divided up among various groups of resulting cells. The work of each grouj) is specialized. As a result of this division of labor, as it may be called, these functions and properties are, as might be expected, developed and made more perfect, while the tissues and organs arising from each group of cells are developed also, with a view to the more convenient and effective exercise of their functions and employment of their prop- erties. In studying the functions of the human body it is necessary flrst of all to know of what it is composed, of what tissues and organs it is made up; this can of course only be ascertained by the dissection of the dead body, and thus it comes that Anatomy {ijyariijyw, to cut up) the science which treats of the structure of organized bodies, is closely associated with physiology; so closely, indeed, that Histology {1e. (\ ColuTimar epithelium with striated border; g. goblec cell, with its mucus partly ^xrruded: I, lyinph-L-orpusoies between the epi- thelial cells; b. basement membrane; e, sections of blood capillaries; //i, section of plain muscle fibres; c. /, central lacteal. (Schafer.) the cells themselves being supposed in many cases after discharge to recover their original shape. Ciliated epithelium consists of cells which are generally cylindrical in form (figs. 29, 30), but may be spheroidal or even almost squamous. This form of epithelium lines — (a.) the mucous membrane of the respiratory tract beginning just beyond the nasal aperture and com- pletely covering the nasal passages, except the upper part to which the olfactory nerve is distributed, and also the sinuses and ducts in connec- tion with it and the lachrymal sac; the upper surface of the soft palate and the naso-pharynx, the Eustachian tube and tympanum, the larynx, except over the vocal cords, to the finest subdivisions of the bronchi. In part of this tract, however, the epithelium is in several layers, of which only the most superficial is ciliated, so that it should more accu- rately be termed transitional (p. 37) or stratified, (b.) Some portions of the generative apparatus in the male, viz., lining the " vasa efferentia" of the testicle, and their prolongations as far as the lower end of the THE STRUCTURE OF THE ELKMKXTARY" TISSUES. 35 epididymis; in the fem.ile (c.) commencing about the middle of the neck of the uterus, and extending tliroughout the uterus and Fallopian tubes to their fimbriated extremities, and even for a short distance on the peritoneal surface of the latter, (d.) The ventricles of the brain and the central canal of the spinal cord are clothed with ciliated epithe- lium in the child, but in the adult this epithelium is limited to the central canal of the cord. In the embryo the pharynx, oesophagus, and part of the stomach may also be lined with ciliated cells, (e.) The ex- cretory ducts of certain small glands in different localities, (f.) In certain animals, especially the lower vertebrates, ciliated cells line the beginning of the tubes of the kidneys. The Cilia are fine hair-like processes Avhich give the name to this variety of epithelium; they vary a good deal in size in different classes Fig. 20. Fig. 30. Fig. 29.— Spheroidal ciliated cells from the mouth of the frog. X 200 diameters. (Sharpey.) Fig. 30.— Ciliated epiclieliuiu from the liuniau trachea. «, Large, fully formed cell, (j, Shorter cell; c, developiu;^' cells wuh more than one nucleus. (Cadiat. ) of animals, being very much smaller in the higher than among the lower orders, in which they sometimes exceed in length the cell itself. The number of cilia on any one cell ranges from ten to thirty, and those attached to the same cell are often of different lengths, in the human trachea measuring jy-J-rj- to u-gVir of '^^i inch, but nearly ten times tlie length in the cells of the epididymis. The cilia themselves are fine rounded or fiattened processes, appar- ently homogeneous, pointed toward their free extremities. According to some observers these processes are connected through intervening knob-like junctions with longitudinal fibres which pass to the other end of the cell, but which arc not connected with the nucleus. When living ciliated epithelium, e.ij., from the gill of a mussel, or oyster, or from the mouth of the frog, or from a scraping from a polypus from the human nose, is examined under the microscope in a drop of 0.6 per cent solution of common salt {normal saline solution), the cilia are seen to be in constant rapid motion, each cilium being fixed at one end, and swinging or lashing to and fro. The general impression given 36 HANDBOOK OF PHYSIOLOGY. to the eye of tlie observer is very similar to tliat produced by waves in a field of corn, or swiftly rnuniug and rippling water, and the result of their movement is to produce a continuous current in a definite direc- tion, and this direction is invariably the same on the same surface^ being always, in the case of a cavity, toward its external orifice. Ciliary Motion. — Ciliary, which is closely allied to amoeboid and muscular motion, is alike independent of the will, of the direct influence of the nervous system, and of muscular contraction. It may contiune for several hours after death or removal from the body, pro- vided the portion of tissue under examination be kept moist. Its inde- pendence of the nervous system is shown also in its occurrence in the lowest invertebrate animals apparently unprovided with anything analogous to a nervous system, in its i^ersistence in animals killed by prussic acid, by narcotic or other poisons, and after the direct applica- tion of narcotics, such as morphia, opium, and belladonna, to the ciliary surface, or of electricity through it. The vapor of chloroform arrests the motion; but it is renewed on the discontinuance of the application. The movement ceases when the cilia are deprived of oxygen, although it may continue for a time in the absence of free oxygen, but is revived on the admission of this gas. Carbonic acid stops the movement. The contact of various substances, e.g., bile, strong acids, and alkalies, will stop the motion altogether; but this seems to depend chiefly on destruction of the delicate substance of which the cilia are composed. Temperatures above 45° C. and below 0° C. stop the movement, whereas moderate heat and dilute alkalies are favorable to the action and revive the movement after temporary cessation. The exact explanation of ciliary movement is not known; whatever may be the exact cause, however, at any rate the movement must depend upon some changes going on in the cell to which the cilia are attached, as when the latter are cut ofl from the cell the movement ceases, and when severed so that a j^ortion of the cilia are left attached to the cell, the attached and not the severed portions continue the movement. Some authorities consider it due to actual contraction of the cilia themselves; others assert that it is caused by movements in the cell protoplasm acting upon the rootlets of the cilia. Schiifer suggests a very plausible ex- jjlanation, viz., that a cilium is either a curved hollow extension of the cell, which is filled by hyalo2:)lasm and invested by a delicate membrane, or else a straight one whose investing membrane is thicker (or otherwise Jess extensible) along one side than along the other. In either case a rhythmic flowing of the hyaloplasm into and out of the cilium would cause its alternate flexion and extension. As a special subdivision of ciliary action may be mentioned the motion of spermatozoa, which may be regarded as cells with a single cilium. THE STRUCTURE OF THE ELEMENTARY TISSUES. 37 (b) Transitional Epithelium. This term has been applied to cells, which are neither arranged in a single layer, as is the case with simple epithelium, nor yet in many superimposed strata as in laminated; in other words, it is employed when epithelial cells are found in two, three, or four superimposed layers. The upper layer may be either single columnar, columnar ciliated, or squamous. AVhen the upper layer is columnar or ciliated the second layer co2isists of smaller cells fitted into the inequalities of the cells above them, as in the trachea (fig. 30). The epithelium which is met with lining the urinary bladder and ureters is, however, the transitional par excelle)ice. In this variety there 3 "'-<:^-''Q^4^y Fig. 31. Fig. 32. Fig. 31.— Epithelium of the bladder, a. One of the cells of the first row; h, a cell of the second row; c, cells in situ, of first, second, and deepest layers. ( Obersteiner. ) Fig. 3x!.— Transitional epithelial cells from the mucous membrane of the bladder of a rabbit. Highly magnified. «, Large flattened cell of superficial layer; a', similar cell in profile; 6, pear- shaped cell of second layer. cKleiu.) are two or three layers of cells, the upper being more or less flattened according to the full or collapsed condition of the organ, their under surface being marked with one or more depressions, into which the heads of the next layer of club-shaped cells fit. Between the lower and narrower parts of the second row of cells are fixed the irregular cells which constitute the third row, and in like manner sometimes a fourth row (fig. 31). It can be easily understood, therefore, that if a scrajiing of the mucous membrane of the bladder be teased, and examined under the microscope, cells of a great variety of forms may be made out (fig. 32). Each cell contains a large nucleus and the larger and superficial cells often possess two. (c) Stratified Epithelium. The term stratified epithelium is employed when the cells forming the epithelium are arranged in a considerable number of superimposed layers. The shape and size of the cells of the different layers, as well as the number of the layers, vary in different situations. Thus th^ 38 HANDBOOK OF PHYSIOLOGY. superficial cells are as a rule of the squamous, or scaly variet}', and the deepest of the columnar form. The cells of the intermediate layers are of different shapes, but those of the middle layers are more or less rounded. The superficial cells are broad and overlap by their edges (figs. 33 and 34). Their chemical com- Fig. 33.— Squamous epithelium scales from the inside of the mouth. X 260. (Henle.) position is different from that of the underlying cells, as they contain keratin, and are therefore horny in character. The nucleus is often not apparent. The really cellular nature of even the dry and shrivelled scales cast off from the surface of the epi- dermis can be proved by the application of caustic potash, which causes them rapidly to swell and assume their original form. The squamous cells exist in the greatest number of layers in the epi- dermis or superficial part of the skin; the most superficial of these are being continually removed by friction, and new cells from below supply the place of those cast off. The intermediate cells approach more to the flat variety the nearer ihey are to the surface, and to the columnar as they approach the lowest Fig. 34.— Vertical section of the stratified epithelium covering the front of the cornea. Highly magnified. (Schafer.) c, Lowermost columuar cells; js, polygonal cells above these; /?, flattened ceils near tue suj facj. The intercellular channels, bridged by minute cell processes, are well seen. layer. There may be considerable intercellular intervals; and in many of the deeper layers of epithelium in the mouth and skin, the outline of the cells is very irregular, in consequence of processes passing from cell to cell across these intervals. Such cells (fig. 35) are termed "ridge and furrow,^' " cogged " or " prickle " cells. These " prickles " are prolongations of the intracellular network which run across from cell to cell, thus joining them together, THE STRUCTURE OF THE ELEMENT A KY TISSUES. 39 the interstices being filled by the transpurent intercclluhir cement-sub- stance. When this increases in quantity in inflaniination the cells are pushed further apart, and the connecting fibrils or '" ])rickles" elongated and therefore more clearly visible. The columnar cells of the deepest layer are distinctly nucleated; they multiply rapidly by division; and as new cells are formed beneath, they press the older cells forAvard to be in turn pressed forward themselves toward the surface, gradually altering in shape and chemical composition until they are cast off from the surface. Stratified epithelium is found in the following situations: (1.) Form- ing the epidermis, covering the whole of the external surface of the body; (3.) Covering the mucous membrane of thenose, tongue, mouth, pharynx, and oesophagus; (3.) As the conjunctival epithelium, covering the cor- nea; (4.) Lining the vagina and the vaginal part of the cervix uteri. Fig. 35.— Jagged cells from the middle layers of pavement epithelium, from a vertical section of the gum of a new-born infant. (Klein.) Functions of Epithelium. — According to function, epithelial cells may be classified as: (1.) Protective, e.g., in the skin, mouth, blood- vessels, etc. (2.) Protective and moving — ciliated epithelium. (3.) Spxreting — glandular epithelium; or. Secreting formed elements — epi- thelium of testicle secreting spermatozoa. (4.) Protective and secreting, e.g., epithelium of intestine. (5) Sensorial, e.g., olfactory cells, rods and cones of retina, organ of Corti. Epithelium forms a continuous smooth investment over the whole body, being thickened into a hard, horny tissue at the points most ex- posed to pressure, and developing various appendages, such as hairs and nails, whose structure and functions will be considered in a future chapter. Epithelium lines also ihe sensorial surfaces of the eye, ear, nose, and mouth, and thus serves as the medium through which all impressions from the external world — touch, smell, taste, sight, hearing — reach the delicate nerve endings, whence they are conveyed to the brain. The ciliated epithelium which lines the air-passages serves not only as a protective investment, but also by the movements of its cilia pro- motes currents of the air in the bronchi and bronchia, and is enabled to liropcl fluids and minute particles of solid matter so as to aid th(/ir ex- 40 HANDBOOK OF PHYSIOLOGY. pulsion from the body. In the case of the Fallopian tube, chis agency assists the progress of the ovum toward the cavity of the uterus. Of the purposes served by cilia in the ventricles of the brain nothing is known. The epithelium of the various glands, and of the whole intestinal tract, has the power of secretion, i.e., of chemically transforming certain materials of the blood ; in the case of mucus and saliva this has been proved to involve the transformation of the epithelial cells themselves; the cell-substance of the epithelial cells of the intestine being discharged by the rupture of their envelopes, as mucus. Epithelium is likewise concerned in the processes of transudation, diffusion, and absorption. It is constantly being shed at the free surface and reproduced in the deeper layers. The various stages of its growth and development can be well seen in a section of any laminated epithelium such as the epidermis. II, The Connective Tissues. This group of tissues forms the Skeleton with its various connections — bones, cartilages, and ligaments— and also affords a supporting frame- work and investment to the various organs composed of nervous, mus- cular, and glandular tissue. Its chief function is the mechanical one of support, and for this purpose it is so intimately interwoven with nearly all the textures of the body that if all other tissues could be removed, and the connective tissues left, we should have a wonderfully exact model of almost every organ and tissue in the body, correct even to the small- est minutiae of structure. Structure of Connective Tissues in General. Connective tissue is made up of two chief elements, namely, cells and intercellular substance. (A.) Cells. — ^The cells are of two kinds: {a.) Fixed Cells. — ^Theae are of a flattened shape, with branched pro- cesses, which are often united together to form a network: they can be most readily observed in the cornea, in which they are arranged, layer above layer, parallel to the free surface. They lie in spaces in the inter- cellular or ground substance, which are of the same shape as the cells they contain, but rather larger, and which form by anastomosis a sj'stem of branching canals freely communicating (fig. 36). To this class of cells belong the flattened tendon corpuscles which are arranged in long lines or rows parallel to the fibres (fig. 42). These branched cells, in certain situations, contain a number of pig- ment granules, giving them a dark aj)pearance; they form one variety of pigment cell. Branched pigment cells of this kind are found in the outer layers of the choroid (fig. 37). In many of the lower animals. THE STRUCTURE OF THE ELEMENTARY TISSUES. 41 such as the frog, they are found widely distributed, not only in the skin, but also in internal parts, e.g., the mesentery and sheaths of blood- vessels. In the web of the frog's foot such cells may be seen with pig- ment granules evenly distributed throughout the body of the cell and its processes; but under the action of light, electricity, and other stim- uli, the pigment granules become massed in the body of the cell, leaving the processes quite hyaline; if the stimulus be removed, they will grad- ually be distributed again throughout the processes. Thus the skin in the frog is sometimes nniform.ly dusky, and sometimes quite light-colored, with isolated dark spots. In the choroid and retina the pigment cells absorb light. {b.) Amcehoid Cells, of an approximately spherical shape; they have a threat general resemblance to colorless blood-corpuscles, with which FiR. 3(1.— Horizontal preparation of the cornea of frog:, stained in gold chloride; showing the network of branched cornea corpuscles. The ground substance is completely colorless. X 400. (Klein.) some of them are probably identical. They consist of finely granular nucleated protoplasm, and have the property, not only of changing their form but also of moving about, hence they are termed migratory. They are readily distinguished from the branched connective-tissue corpuscles by their free condition, and the absence of processes. Some are much larger than others, and are found especially in the sublingual gland of the dog and guinea-pig, and in the mucous membrane of the intestine. A second variety of these cells called plasma cells are larger tban the amoeboid cells, apparently granular, less active in their movements. They are chiefly to be found in the inter-muscular septa, in the mucous and sub-mucous coats of the intestine, in lymphatic glands, and in the omen- tum. (B.) Intercellular Substance. — This may be fibrillar, as in the fibrous tissues, and in certain varieties of cartilage; or homogeneous, as in hyaline cartilage. 42 HANDBOOK OF PHYSIOLOGY, The fibres conijDosiiig the former are of two kinds — (a.) White fibrea (b.) Yellow elastic fibres. {a.) White Fibres. — These are arranged parallel to each other in wavy bundles of various sizes: sach bundles may either have a parallel ar- Fig. 37. Fig. 38. Fig. 39. Fig. 37.— Ramified pigment cells from the tissue of the choroid coat of the eye. X 350. a, Cell with pigment; 6, colorless fusiform cells. (Kolliker.) Fig. 38. — Flat, pigmented, branched connective-tissue cells from the sheath of a large blood- vessel of the frog's mesentery: the pigment is not distributed uniformly throughout the substance of the larger cell, consequently some parts of it look blacker than others (uncontracted state). In the two smaller cells most of the pigment is withdrawn into the cell-body, so that they appear smaller, blacker, and less branched. X 350. (Klein and Noble Smith.) Fig. 39. — Fibrous tissue of cornea, showing bundles of fibres with a few scattered fusiform cells (a) lying in the inter-fascicular spaces. X 400. (Klein and Noble Smith.) fine, varying from -^^^ rangement (fig. 39), or may produce quite a felted texture by their inter- lacement. The individual fibres composing these fasciculi are exceedingly to ^^W inch, i.e., -^-^ to -^^ mm.,* or 0.5 to 1//, homogeneous, unbranched, and of the same diameter throughout. They can readily be isolated by macerating a portion of white fibrous tissue {e.g., a small piece of tendon) for a short time in lime, or baryta-water, or in a solution of common salt, or of potassium permanganate: these reagents possess the power of dissolving the cementing inter- fibrillar substance and of thus separating the fibres from each other. By prolonged boil- ing the fibres yield gelatin. (b.) Yellow Elastic Fibres (fig. 40) are of all sizes, from excessively fine fibrils, -jj-^o inch, up to fibres of considerable thickness, ■^-^j^ inch (i.e., from about 1/j. to 6/^) : they are distinguished from white fibres by the following characters: (1.) Their great power of resistance even to the prolonged action of chemical reagents, e.g., caustic soda, acetic acid, etc. (2.) Their well-defined outlines. (3.) * 1 millimetre = 1 micron, which is represented by the Greek //. Fig. 40.— Elastic fibres from the ligamenta subflava. x 200 (Sharpey.) THE STRUCTURE OF THE ELEMENTARY TISSUES. 43 Their great tendency to branch and to form networks by anastomosis. (4.) Their twisted corkscrew-like appearance, and that their free ends usually curl up, (o.) Their yellowish tint and considerable elasticity. (6.) Their resistance to hfematoxylin and similar reagents, and their affinity for magenta and other aniline staining colors. These fibres yield on boiling not gelatin, but a gelatinous substance called elastin. The chief varieties of connective tissues may be thus classified : I. The Fibrous Connective Tissues. A. — Chief Forms. a. White fibrous. b. Elastic. c. Areolar. B. — Special Varieties. a. Gelatinous. b. Adenoid or Eetiform. c. Adipose. II. Cartilage. III. Bone a)id dentine. I. Fibrous Connective Tissues. A. — Chief Forms. — (rr.) White Fibrous Tissue. Distrihuti'ni. — It is found typically in tendon; also in ligaments, in the periosteum and perichondrium, the dura mater, the pericardium, the sclerotic coat of the eye, the fibrous sheath of the testicle; in the fasciae and aponeuroses of muscles, and in the sheaths of lymphatic glands. Structure. — To tlie naked eye tendons and many of the filirous membranes, when in a fresh state, present an appearance as of watered silk. This is due to the arrangement of the fibres in wavy parallel bun- dles. Under the microscope the tissue appears to consist of long, often parallel, bundles of fibres of different sizes. The fibres of the same bun- dle now and then intersect each other. The cells in tendons (fig. 42) are arranged in long chains in the ground substance separating the bun- dles of fibres, and are more or less regularly quadrilateral with large round nuclei containing nucleoli, which are generally placed so as to be contiguous in two cells. Each of these cells consist of a thick body, from which processes pass in various directions into, and partially fill up the spaces between, the bundles of fibres. The rows of cells are separated from one another by lines of cement substance. The cell spaces can be brought into view by silver nitrate. The cells are gener- 44 HA] Spherical or, from pressure, polyhedral cells with large central nucleus, surrounded by a finely reticulated bui)- 8ta™e staining uniformly with haematoxylin. b. Similar cells with spaces from which the fat hab been removed by oil of cloves, c. Similar cells showing how the nucleus with inclosing protoplasni Lsbeing^r^sed toward peripherj^. d. Nucleus of endotlieUum of investing capillaries. (McCarthy.) Drawn by^Tr^v^s^^^^^^ connective-tissue corpuscles, developing into fat-ceUs. (Klein.) THE STKUCTURE OF THE ELEMENTARY TISSUES. 51 h. That part of the fat which is situate beneath the skin must, by its want of conducting power, assist in preventing undue w-aste of the heat of the body by escape from the surface. c. As a packing material, fat serves very admirably to fill up spaces, to form a soft and yielding yet elastic material wherewith to wrap ten- der and delicate structures, or form a bed with like qualities on which such structures may lie, not endangered by pressure. As examples of situations in which fat serves such purposes may be mentioned the palms of the hands and soles of the feet and the orbits. d. In the long bones fatty tissue, in the form known as yellow mar- row, fills the medullary canal, and supports the small blood-vessels which are distributed from it to the inner part of the substance of the bone. Basemext Membranes. Basement membranes are a special structure upon which the epi- thelium of mucous membranes rests. They are of homogeneous appear- ance, and are developed from flattened connective-tissue corpuscles, joined at their edges, or from a concentrated cement substance. Some basement membranes possess elasticity, e.g., in the cornea. II. Cartilage. General Structure of Cartilage. — All kinds of cartilage are composed of cells imbedded in a substance called the matrix : the apparent differ- ences of structure met with in the various kinds of cartilage are more due to differences in the character of the matrix than of the cells. Among the latter, however, there is also considerable diversity of form and size. With the exception of the articular variety, cartilage is invested by a thin but tough firm fibrous membrane called the pericJiondrium. On the surface of the articular cartilage of the foetus, the perichondrium is represented by a film of epithelium; but this is gradually worn away up to the margin of the articular surfaces when by use the parts begin to suli'er friction. Nerves are probably not supplied to any variety of cartilage. Cartilage exists in three different forms in the human body, viz., 1, Hyaline cartilage, 2, Yellow elastic-cartilage, and 3, White fibro-cartilage. 1. Hyaline Cartilage. Distribution. — This variety of cartilage is met with largely in the human body — investing the articular ends of bones, and forming the costal cartilages, the nasal cartilages, and those of the larynx wath the exception of the epiglottis and cornicula laryugis, as well as those of the trachea and bronchi. Structure. — Like other cartilages it is composed of cells imbedded in 52 HANDBOOK OP PHYSIOLOGY. a matrix. The cells, which contain a nucleus with nucleoli, are irregular in shape, and generally grouped together in patches (fig. 53). The patches are of various shapes and sizes and placed at unequal distances apart. They generally appear flattened near the free surface of the mass of cartilage in which they are placed and more or less perpendicular to the surface in the more-deeply seated portions. The matrix of hyaline cartilage has a dimly granular appearance like that of ground glass, and in man and the higher animals has no appar- ent structure. In some cartilages of the frog, however, even when ex- amined in the fresh state, it is seen to be mapped out into polygonal blocks or cell-territories, each containing a cell in the centre, and repre- ^K, Fig. 53. Fig. 54. Fig. 53.— Hyaline articular cartilage (human). The cell bodies entirely All the spaces in the matrix. X 340 diams. (Schafer.) Fig. 54.— Fresh cartilage from the Triton. (A. Rollett.) senting what is generally called the capsule of the cartilage cells (fig. 54). Hyaline cartilage in man has really the same structure, which can be demonstrated by the use of certain reagents. If a piece of human hyaline cartilage be macerated for a long time in diluted acid or in hot water 35°-45° C. (95°-113° F.), the matrix, which previously appeared quite homogeneous, is found to be resolved into a number of concentric lamellae, like the coats of an onion, arranged round each cell or group of cells. It is thus shown to consist of nothing but a number of large systems of capsules which have become fused with one another. The cavities in the matrix in which the cells lie are connected to- gether by a series of branching canals, very much resembling those in the cornea: through these canals fluids may make their way into the depths of the tissue. In the hyaline cartilage of the ribs the cells are mostly larger than in the articular variety and there is a tendency to the development of fibres in the matrix (fig. 55). The costal cartilages also frequently be- THE STKUCTURE OF THE ELEMENTARY TISSUES. 53 come calcified in old age, as also do some of those of the larynx. Fat- globules may also be seen in many cartilages (fig. 55). In articular cartilage the cells are smaller and arranged vertically in narrow lines like strings of beads. In the foetus cartilage is the material of which the bones are first constructed; the '"model" of each bone being laid down, so to speak, in this substance. In such cases the cartilage is termed temporary. It closely resembles the ordinary hyaline kind; the cells, however, are not grouped together after the fashion just described, but are more uniformly distributed throughout the matrix. A variety of temporary hyaline cartilage which has scarcely any ma- Fig. 55, Fig. 56. Fig. 55.— Costal cartilage from an adult dog, showing the fat globules in the cartilage cells. (Cadiat.) Fig. 56.— Yellow elastic cartilage of the ear. Highly magnified. (Hertwig.) trix is found in the human subject and in the higher animals generally, in early fo3tal life, when it constitutes the chorda dorsalis. Nutrition. — Hyaline cartilage is reckoned among the so-called non- vascular structures, no blood-vessels being supplied directly to its own substance; it is nourished by those of the bone beneath. When hyaline cartilage is in thicker masses, as in the case of the cartilages of the ribs, a few blood-vessels traverse its substance. The distinction, however, between all so-called vascular and non-vascular parts is at the best a very artificial one. 2. Yellow Elastic Cartilage. Distribution. — In the external ear, in the epiglottis and cornicula laryngis, and in tlie Eustachian tube. Structure. — The cells in this variety of cartilage are rounded or oval, with well-marked nuclei and nucleoli (fig. 56). The matrix in which they are seated is composed almost entirely of fine elastic fibres, which 54 HANDBOOK OF PHYSIOLOGY. form an intricate interlacement about the cells, and in their general characters are allied to the yellow variety of fibrous tissue : a small and variable quantity of hyaline intercellular substance is also usually present. A variety of elastic cartilage, sometimes called cellular, is found to form the framework of the external ears of rats, mice, or other small mammals. It is composed, as its name implies, almost entirely of cells which are packed very closely with little or no matrix. When present the matrix consists of very fine fibres which twine about the cells in various directions and inclose them in a kind of network. Elastic car- tilage seldom or never ossifies. 3. White Fibro-Cartilage. Distribution. — White fibro-cartilage is found to occur: — 1. As inter-articular fibro-cartilage, e.g., the semilunar cartilages of the knee-joint. 2. As circumferential or marginal cartilage, as on the edges of the acetabulum and glenoid cavity. 3. As connecting cartilage, e.g., the inter-vertebral fibro-cartilages. 4. In the sheaths of tendons and some- ^-j; M'i'iWillii times in their substance. In the latter situ- ation the nodule of fibro-cartilage is called a sesamoid fibro-cartilage, of which a specimen Cells of cartilage. Very fibrous matrix. Fig. 57. ' U*Jliy||liili!aHll!'.ir:til!»l Fig. 58. Fig. .W.— White fibro-cartilage. (Cadiat.) Fig. 58. — White fibro-cartilage from an inter-vertebral ligament. (Klein and Noble Smith.) may be found in the tendon of the tibialis posticus in the sole of the foot, and usually in the neighboring tendon of the peroneus longus. Structure. — White fibro-cartilage (fig. 58), which is much more widely distributed throughout the body than the foregoing kind, is composed, like it, of cells and a matrix; the latter, however, being made up almost entirely of fibres closely resembling those of white fibrous tissue. In this kind of fibro-cartilage it is not unusual to find a great part of its mass composed almost exclusively of fibres, and deriving the name THE STRUCTURE OF THE ELEMENTARY TISSUES. 55 of cartilage only from the fact that in another portion, continuous with it, cartilage cells may be pretty freely distributed. By prolonged boiling, cartilage yields a substance called cliondriti — which gelatinizes on cooling. The cells of white fibro-cartilage are as a rule rounded or somewhat flattened but in some places are distinctly branched. Functions of Cartilage.— Cartilage not only represents in the fcetus the bones which are to be formed {temporary cartilage) but also offers a firm, yet more or less yielding, framework for certain parts in the developed body, possessing at the same time strength and elasticity. It maintains the shape of tubes as in the larynx and trachea. It affords attachment to muscles and ligaments; it binds bones together, yet allows a certain degree of movement, as between the vertebrae; it forms a firm framework and protection, yet without undue stiffness or weight, as in the pinna, larynx, and chest walls; it deepens joint cavities, as in the acetabulum, without unduly restricting the movements of the bones. Development of Cartilage. — Cartilage is developed out of an em- bryonal tissue, consisting of cells with a very small quantity of intercel- lular substance: the cells multiply by fission within the cell- capsules, while the capsule of the parent cell becomes gradually fused with the surrounding intercellular substance. A repetition of this process in the young cells causes a rapid growth of the cartilage by the multiplication of its cellular elements and corresponding increase in its matrix. Thus we see that the matrix of cartilage is chiefly derived from the cartilage cells. III. Bone. Chemical Composition. — Bone is composed of earthi/ and animal mat- ter in the proportion of about 67 per cent of the former to 33 per cent of the latter. The earthy matter is composed chiefly of calcium phos- phate, but besides there is a small quantity (about 11 of the 67 per cent) of calcium carbonate and calcium fluoride, and ma g nesi urn phosphate. The animal matter called collagen is resolved into gelatin by boiling. The earthy and animal constituents of bone are so intimately blended and incorporated the one with the other that it is only by cliemical action, as for instance by heat in one case and by the action of acids in another, that they can be separated. Their close union too is further sliown by the fact that when by acids the earthy matter is dissolved out, or on the other hand when the animal part is burnt out, the shape of the bone is alike preserved. The proportion between these two constituents of bone varies in different bones in the same individual and in the same bone at different ages. Structure. — To tlie naked eye there appear two kinds of structure 56 HANDBOOK OF PHYSIOLOGY. in different bones, and in different parts of the same bone, namely, the dense or comiMct, and the s-pongy or cancellous tissue. Thus, in making a longitudinal section of a long bone, as the humerus or femur, the articular extremities are found capped on their surface by a thin shell of compact bone, while their interior is made up of the spongy or cancellous tissue. The shaft, on the other hand, is formed almost entirely of a thick layer of the compact bone, and this surrounds a central canal, the medullary cavity — so called from its con- taining the medulla or marrow. In the flat bones, as the parietal bone or the scapula, one layer of the cancellous structure lies between two layers of the compact tissue, and in the short and irregular bones, as those of the caijms and tarsus, the cancellous tissue alone fills the interior, while a thin shell of comj^act bone forms the outside. Marrow. — There are two distinct varieties of marrow — the red and yelloiv. Fig. 59. — Cells of the red marrow of the guinea-pig, highly maguified. a, A large cell, the nu- cleus of which appears to be partly dividedinto three by constrictioiis; b. a cell, the nucleus of which shows an appearance of being constricted into a numVier of smaller nuclei; c, a so-called giant cell, or myeloplaxe, with many nuclei; d, a smaller myelo-plaxe, with three nuclei; e-i, proper cells of the marrow. (E. A. Schafer.) Red marrow is that variety which occupies the spaces in the cancel- lous tissue; it is highly vascular, and thus maintains the nutrition of the spongy bone, the interstices of which it fills. It contains a few fat-cells and a large number of marrow-cells, many of which are undis- tinguishable from lymphoid corpuscles, and has for a basis a small amount of fibrous tissue. Among the cells are some nucleated cells of very much the same tint as colored blood-corpuscles. There are also a few large cells with many nuclei, termed giant-cells or myelopla'xes, which are derived from over-growth of the ordinary marrow-cells (fig. 59). Yellow marrow fills the medullary cavity of long bones, and consists chiefly of fat-cells with numerous blood-vessels; many of its cells also are in every respect similar to lymphoid corpuscles. THE STRUCTURE OF THE ELEMENTARY TISSUES. 57 From these marrow-cells, especially those of the red marrow, are de- rived, as we shall presently show, large quantities of red blood-corpuscles. Periosteum and Nutrient Blood-vessels. — The surfaces of bones, except the part covered with articular cartilage, are clothed by a tough, fibrous membrane, the periosfeuin ; and it is from the blood-vessels which are distributed in this membrane, that the bones, especially their more compact tissue, are in great part suf)plied with nourishment, — minute branches from the periosteal vessels entering the little foramina on the surface of the bone, and finding their way to the Haversian canals to be immediately described. The long bones are supplied also Ijy a proper nutrient artery which, entering at some part of the shaft so Fig. 60.— Transverse section of compact bony tissue (of humerus^ Three of the Haversian canals are seen, with their concentric rings; also the lacuna?, with the canaliculi extending from them across the direction of the lamella. The Haversian apertures were filled with debris in grind- ing down the section, and therefore appear black in the figure, which represents the object. as viewed with transmitted light. The Haversian systems are so closely packed in this section, that scarcely any interstitial lamellee are visible. X 150. (Sharpey.) as to reach the medullary canal, breaks up into branches for the supply of the marrow, from which again small vessels are distributed to the interior of the bone. Other small blood-vessels pierce the articular extremities for the supply of the cancellous tissue. Microscopic Strucfnve of Bone. — TS"ot«'ithstanding the differences of arrangement Just mentioned, the structure of all bone is found under the microscope to be essentially the same. Examined with a rather high power its substance is found to contain a multitude of small irregular spaces, approximately fusiform in shape, called lacunoe, with very minute canals or canaliculi, as they are termed, leading from them, and anastomosing with similar little prolongations from other lacunar (fig. GO). In very thin layers of bone, no other canals than these may be visible; but on making a transverse section of 58 HANDBOOK OF PHYSIOLOGY. the compact tissue as of a long bone, e.g., the humerus or ulna, the arrangement shown in fig. 60 can be seen. The bone seems mapped out into small circular districts, at or about the centre of each of which is a hole, around which is an appearance as of concentric layers — the lacunm and canaliculi following the same con- centric plan of distribution around the small hole in the centre, with which indeed they communicate. On making a longitudinal section, the central holes are found to be simply the cut extremities of small canals which run lengthwise through the bone, anastomosing with each other by lateral branches (fig. 61), Fig. 61. — Longitudinal section from the human ulna, showing Haversian canal, lacunjB, and canaliculi. (RoUett.) and are called Haversian canals, after the name of the physician, Clopton Havers, who first accurately described them. The Haversian canals, the average diameter of which is -^^^^ of an inch, contain blood-vessels, and by means of them blood is conveyed to all, even the densest parts of the bone; the minute canaliculi and lacunae- absorbing nutrient matter from the Haversian blood-vessels and con- veying it still more intimately to the very substance of the bone which they traverse. The blood-vessels enter the Haversian canals both from without, by traversing the small holes which exist on the surface of all bones be- neath the periosteum, and from within by means of small channels which extend from the medullary cavity, or from the cancellous tissue. The arteries and veins usually occupy separate canals, and the veins,, which are the larger, often present, at irregular intervals, small pouch- like dilatations. THE STBUCTURE OF THE ELEMENTAKY TISSUES. 59 The lacunae are occupied by branched cells, which are called hone- cells, or hone-corpuscles (fig. 62), which very closely resemble the ordi- nary branched connective-tissue corpuscles; each of these little masses of protoplasm ministering to the nutrition of the bone immediately sur- rounding it, and one lacunar corpuscle communicating with another, and with its surrounding district, and with the blood-vessels of the Haversian canals, by means of the minute streams of fluent nutrient matter which occupy the canaliculi. It will be seen from the above description that bone is essentially connective-tissue impregnated with lime salts : it bears a very close re- semblance to what may be termed typical connective-tissue such as the substance of the cornea. The bone-corpuscles with their pro- Fig. 62. Fig. 63. Fig. 62.— Bone-corpuscles with their processes as seen in a thin section of human bone. (Rollett.) Fig. 63.— Thin layer peeled off from a softened bone. This figure, which is intended to represent the reticular structure of a lamella, gives a better idea of the object when held rather farther off than usual from the eye. X 400. (Sharpey.) cesses occupying the lacunaj and canaliculi correspond exactly to the cornea-corpuscles lying in branched spaces. Lamellae of Compact Bone. — In the shaft of a long bone three distinct sets of lamella? can be clearly recognized. (1.) General or fundamental lamellae ; which are most easily tracea- ble just beneath the periosteum, and around the medullary cavity, form- ing around the latter a series of concentric rings. Ac a little distance from the medullary and periosteal surfaces (in the deeper portions of the bone) they are more or less interrupted by (2.) Special or Haversian lamellae, which are concentrically arranged around the Haversian canals to the number of six to eighteen around each. (3.) Interstitial lamellae, which connect the system of Haversian lamellae, filling the spaces between them, and consequently attaining 60 HANDBOOK OF PHYSIOLOGY. their greatest development where the Haversian systems are few, and vice versa. The ultimate structure of the lamellae appears to be reticular. If a thin film be peeled ofE the surface of a bone, from which the earthy matter has been removed by acid, and examined with a high power of the mici'oscope, it will be found composed of a finely reticular struc- ture, formed apparently of very slender fibres decussating obliquely, but coalescing at the points of intersection, as if here the fibres were fused rather than woven together (fig. 63). In many places these reticular lamellge are perforated by tajDcring fibres called the Claviculi of Gagliardi, or the perforating fibres of Sharpey, resembling in character the ordinary white or rarely the elastic Fig. 64. — Lamellae torn off from a decalcified human parietal bone at some depth from the sur- face, o, a, Lamellae, showing reticular fibres; h, b, darker part, where several lamellae are super- posed; c, perforating fibres. Apertures through which perforating fibres had passed, are seen es- pecially in the lower part, a, a, of the figm'e. (Allen Thomson.) fibrous tissue, which bolt the neighboring lamellse together, and may be drawn out when the latter are torn asunder (fig. 64). These perforating fibres originate from ingrowing processes of the periosteum, and in the adult still retain their connection with it. Development of Bone. — From the point of view of their develop- ment, all bones may be subdivided into two classes. {a.) Those which are ossified directly or from the first in membrane or fibrous tissue, e.g., the bones forming the vault of the skull, parietal, frontal, and a certain portion of the occipital bones. (b.) Those whose form, previous to ossification, is laid down in 7ii/a- line cartilage, e.g., humerus, femur. The process of development, pure and simple, may be best studied in bones which are not preceded by cartilage, i.e., membrane- formed {e.g., THE STEUCTURE OF THE ELEMENTARY TISSUES. 61 parietal) ; and without a knowledge of this process (ossification in mem- brane), it is impossible to understand the much more complex series of changes through which such a structure as the cartilaginous femur of the foetus passes in its transformation into the bony femur of the adult (ossification in cartilage). Ossification in Membrane. — The membrane, afterward forming the periosteum, from which such a bone as the parietal is developed, consists of two layers — an extemdl fibrous, and an internal cellular or osteo-genetic. The external layer is made up of ordinary connective-tissue, being composed of layers of fibrous tissue with branched connective-tissue corpuscles here and there between the bundles of fibres. The internal layer consists of a network of fine fibrils with a large number of nucle- ated cells with a certain addition of albuminous ground or cement sub- stance between the fibrous bundles, some of which are oval, others drawn out into long branched processes: it is more richly supplied with capillaries than the outer layer. The relatively large number of its cellular elements, which vary in size and shape, together with the abundance of its blood-vessels, clearly mark it out as the portion of the periosteum which is immediately concerned in the formation of bone. In such a bone as the parietal, which is represented then when ossi- fication commences by the species of fibrous connective tissue with many cells above indicated, the deposition of bony matter, which is preceded by increased vascularity, takes place in radiating spiculfe, starting from a centre of ossification, and shooting out in all directions toward the periphery. These primary bony spiculae consist of the fibres of the tis- sue which are termed osteogenetic fibres, composed of a soft transparent substance called osteogen, in which calcareous granules are deposited. The fibres are said to exhibit in their precalcified state indications of a fibrillar structure, and are likened to bundles of white fibrous tissue, to which they are similar in chemical composition, but from which they differ in being stiffer and less wavy. The deposited granules after a time become so numerous as to fill up the substance of the fibres and bon^ opiculfe result. Calcareous granules are deposited also in the in- terfibrillar matrix. By the junction of the osteogenetic fibres and their resulting bony spiculse a meshwork of bone is formed. The osteo- genetic fibres, which become indistinct as calcification proceeds, are believed to persist in the lamellas of adult bone. The osteoblasts, being in part retained within the bone trabeculte thus produced, form bone corpuscles. On the bony trabecul® first formed, layers of osteoblastic cells from the osteo-genetic layer of the periosteum are developed side by side, lining the irregular spaces like an epithelium (fig. 65, b). Lime- salts are deposited in the circumferential part of each osteoblast, and thus a ring of osteoblasts gives rise to a ring of bone with the remaining. 62 HANDBOOK OF PHYSIOLOrxY. uncalcified portions of the osteoblasts imbedded in it as bone corpuscles, as in the first formation; then the central portion of the bony plate becomes harder and less cancellous. At the same time, the plate in- creases at the iDerijihery not only by the extension of the bony spiculae, but also by deposits taking place from the osteogenetic layer of the periosteum. The primitive spongy bone is formed, and its irregular branching spaces are occupied by processes from the osteogenetic layer of the peri- osteum consisting of numerous blood-vessels and osteoblasts. Portions of this primitive spongy bone are re-absorbed. The osteoblasts are arranged in concentric successive layers and give rise to concentric Haversian lamellae of bone, while the irregular space in the centre is reduced to a well-formed Haversian canal, containing the usual blood- vessels, the portions of the primitive spongy bone between the Haversian Fig. 65.— Osteoblasts from the parietal bone of a human embiyo, thirteen weeks old. a, Bony septa with the cells of the lacunae; b, layers of osteoblasts; c, the latter in transition to bone cor- puscles. Highly magnified. (Gegenbaur.) systems remaining as interstitial or ground-lamellge (p. 59). The bulk of the j)rimitive spongy bone is thus gradually converted into compact bony-tissue of Haversian systems. Those portions of the ingrowths from the deeper layer of the periosteum which are not converted into bone remain in the spaces of the cancellous tissue as the red marrow. Ossification in Cartilage. — Under this heading, taking the femur as a typical example, we may consider the process by which the solid cartilaginous rod which represents the bone in the foetus is converted into the hollow cylinder of compact bone with expanded ends formed of cancellous tissue of which the adult femur is made up. We must bear in mind the fact that this foital cartilaginous femur is many times smaller than the medullary cavity even of the shaft of the mature bone, and, therefore, that not a trace of the original cartilage can be present in the femur of the adult. Its j^urpose is indeed purely temporary; and, after its calcification, it is gradually and entirely absorbed as will be presently explained. THE STRUCTURE OF THE ELEMENTARY TISSUES. 63 The cartilaginous rod which forms the foetal femur is sheathed in a membrane termed the perichondrium, which, so far resembles the peri- osteum described above, as to consist of two layers, in the deeper one of which spheroidal cells jDredominate and blood-vessels abound, while the outer layer consists mainh' of fusiform cells which are in the mature tissue gradually transformed into fibres. Thus, the differences betweeu the foetal perichondrium and the periosteum of the adult are such as ^i':l-^i r-j-'-v., usually exist between the embry- f^" tI: =- ' " onic and mature forms of conuec- %^ ^ ^- ^ '; tive tissue. %\ .ts Between the hyaline cartilage ll'v -: '- _ of v.liich the foetal femur consists ^ §m and the bony tissue forming the i%| :. ' adult femur, there are tv:o cliief ac/fif:^ intermediate stages — viz. (1) of ■!=.<^ <=r =*-^