Chapter V: Vertebrata (1)

The most complicated condition in which matter exists is where, under the influence of life, it forms bodies with a structure of tubes and cavities in which fluids are incessantly in motion, and producing continuous changes. These have been rightly designated “organised bodies,” because of the various organs they contain. The two principal classes into which organised bodies have been divided are recognised as vegetable and animal. It was Bichat who taught that our animal life is double, while our organic life is single. In organic life, to stop is to die; and the life we have in common with vegetables never sleeps, and if the circulation of the fluids within the animal body ceases for a few seconds, it ceases for ever. In the vertebrate body, however, the combination of organs attains to the highest development, in striking contrast with that of the class we have previously considered, the Invertebrata, the animal kingdom being divided into Vertebrates and Invertebrates.

The Vertebrata are distinguished from all other animals by the circumstance that a transverse and a vertical section of the body exhibits two cavities completely separated from one another by a partition. A still more characteristic feature separates the one from the other; it is the specialisation of the chief nervous centres, and their peculiar relation to the other systems of the body.

The dorsal cavity of the body contains the cerebro-spinal nervous system, the ventral, the alimentary canal, the heart, and usually a double chain of ganglia; these pass under the name of the sympathetic system. It is very probable that this sympathetic nervous system represents, wholly or partially, the principal nervous system of the Annulosa and Mollusca. In any case, the central parts of the cerebro-spinal nervous system--_i.e._, the brain and the spinal cord--would appear to be unrepresented among invertebrate animals. Likewise, in the partition between the cerebro-spinal and visceral tubes, certain structures which are not represented in Invertebrates are contained. During the embryonic condition of all Vertebrates, the centre of the partition is occupied by an elongated cellular cylindrical mass, the notochord, or chorda dorsalis. This structure persists throughout the life in some Vertebrata, but in most it is more or less completely replaced by a jointed, partly fibrous, cartilaginous, and bony vertical column. All vertebrate animals have a complete vascular system. In the thorax and abdomen, in place of a single perivisceral cavity, in communication with the vascular system, and serving as a blood-sinus, there are one or more serous sacs. These invest the principal viscera, and may or may not communicate with the exterior, recalling in the latter case the atrial cavities of the Mollusca. In all Vertebrata, except Amphioxus, there is a single valvular heart, and all possess a hepatic portal system, the blood of the alimentary canal never being wholly returned directly to the heart by the ordinary veins, but being more or less completely collected into a trunk (the portal vein), which ramifies through and supplies the liver.

With reference to one other point of importance, the development of the ova of Vertebrates, these have the same primary composition as those of other animals, consisting of a germinal vesicle containing one or more germinal nuclei, and included within a vitellus. But as this forms a part of general anatomy, and as my object is simply the investigation of the fundamental and microscopical structure of animal organisms, I shall not further pursue the morphological part of the subject, especially as so many excellent text-books are within reach of the student who desires to fully acquaint himself with precise information.

Notwithstanding, then, the apparent diversity in the structure of the vertebrate and the invertebrate and the various tissues of which animals and vegetables are constituted, microscopical research has satisfactorily demonstrated that all textures have their origin in cells; in fact, when the formative process is complete, the animal cell is seen to consist of the same parts and almost the same chemical constituents as the typical cell of the plant--namely, a definite cell-wall enclosing cell contents, of which the nature may be diverse, but the cell nucleus is precisely the same and is the actual seat and origin of all formative activity. The cell and nucleus grow by assimilation or intersusception, that is, by inflowing of nutrition among all parts, the new replacing the old, yet maintaining its original structure and composition. That which was once thought special to animals is now found to be common to both plants and animals: they are found to be alike fundamentally in internal structure, and in the discharge of the mysterious processes of reproduction and of nutrition, although the latter forms a convenient line of separation. Life in plants goes on indefinitely; cuttings may be taken without injury to their vigour and duration of life. The same may be said of some of the lower forms of invertebrate life; for example, the hydra, the anemone, and some other well-known animals, may be cut up, divided into several parts, each one of which will form a new animal, provided a nucleus be included in the section. Nevertheless, the organisation of the amœba and the hydra is as complete for its purpose as that of man for his, and the evidence of continuity forbids the drawing of hard and fast lines, as was formerly done between the two kingdoms, the animal and vegetable. The amount of similarity or agreement in the organisation of animals is various. Animals indeed differ from each other in slight points only, for the discovery of which the microscope must be brought into requisition. Living matter in its earliest stage and simplest form appears to the naked eye as a homogeneous structure, but when placed under the highest powers of the microscope, it is seen not to be so.

But perhaps the most marked feature of the age has been the increasing attention given to the study of the lower forms of life, using their simpler structures and more diffuse phenomena to elucidate the more general properties of living matter. To understand life we must understand protoplasm. Of this there can be no doubt, as we have seen in a previous chapter that a whole family, the Monera, consists of this simple living, microscopic, jelly-like substance, which has not even begun to be differentiated, as in the amœba, which has as yet no special organs, and every speck becomes a mouth or a stomach, and which can be turned inside out and shoot out tongues of jelly to move and feel with. “Reproduction is the faculty most characteristic of life, and sharply distinguishes the organic from the inorganic.” It is, then, the corpuscles of protoplasm, called cells (cellulæ), which have so much interest for the physiologist, and these, like the cytods, may form independent organisms, which are then termed unicellular. Again, cells form other cells, and a multicellular organism results, and goes on increasing in geometrical progression. In the Vertebrata the cell retains its characteristic spheroidal shape, as seen in Fig. 423, and undergoes division by virtue of its living protoplasmic mass.

1. Newly formed cell structure; 2. Division of the nucleus; 3. It changes its situation in the cell; 4. Subdivides and breaks up; 5. Cell-walls increase in thickness; 6. Branch out into stellate cells; 7. Two cells coalesce; 8 and 9. Become multicellular.]

_Epithelial Cells._--All free surfaces of the human body, both internal and external, are to a very considerable extent covered by epithelium cells. These cells are everywhere the same, but with modifications in shape and arrangement. Epithelial cells are nucleated and always joined by their surfaces or edges, without, on the external surfaces, the intervention of connective tissue.

There are four essential varieties:--1. Tesselated; 2. Columnar; 3. Spheroidal; 4. Ciliated; in all of which the nucleus remains remarkably uniform in its characters, is either round or oval, and flattened out, measuring 1/6000th to 1/4000th of an inch in diameter. They are insoluble in acetic acid, colourless, or slightly tinted by the structure with which they are in contact, and usually contain one or more nucleoli with a few minute irregular granules, as represented in Fig. 424.

The simplest and most commonly distributed variety is the tesselated, known also as the scaly, squamous, pavement, and flattened epithelium, always arranged in single layers, lining serous cavities, many parts of the mucous membrane, and the interior of ducts and blood vessels. Upon the external surface of the body it occurs in superimposed layers, forming the “stratified epidermis.” To obtain specimens of lamellar epithelium it is only necessary to collect a little saliva, or pass a glass slide over the lining membrane of the cheek, cover it with a thin cover glass, and examine it with a 1/4-inch objective. Pavement epithelium is the elementary structure of hair, nails, and horn.

ANIMAL TISSUES.]

Columnar epithelium exists upon the mucous membrane of the stomach, on the villi of the intestines, and in the several canals. It occupies either a vertical or horizontal position, and may be detached in rows, as shown in Plate XIX., No. 2, a section taken from the intestine of a rabbit. This variety, when more highly magnified, as in Fig. 424, is seen to consist of club-shaped nucleated cells, the thicker end being turned towards the surface. The protoplasm of the cell is granular, and the presence of minute vacuoles and fatty globules occupy a great part of the space. The nucleus is now seen to contain a fine network. At times the outer end of the cell is distended, as in Fig. 3. This form of columnar epithelium (known as the “goblet” cell) presents a close and remarkable resemblance to the cilio-flagellate “collared” infusorial monad in its extended “wine-glass” form.

No. 1. Pavement epithelium, taken from an internal membrane; 2. Columnar epithelium, from the intestine of a rabbit, showing central fat globules, and at str a fine ciliated border; 3. A so-called “goblet”-cell.]

Spheroidal epithelium is confined to the closed cavities of the body, and in the internal structure of the ducts of secreting glands. The cells are, for the most part, circular, although some are flattened out at the sides in which they are in contact with each other (Plate XIX., No. 1_a_). Specimens of this form may be taken from the internal surface of one of the lower animals with a scalpel. The collected matter must be placed in a drop of distilled water and examined with a high power.

Ciliated epithelium is characterised by the presence of those fine hair-like filaments (cilia) attached to the free surface of the cell. During life, and for some time after death, the cilia are seen to retain their constant waving motion. The cilia all move in one direction and rhythmically, thus giving rise to the appearance of a succession of undulations. Ciliated epithelium is found lining the mucous membrane of the air passages and nasal ducts, and wherever it is necessary to urge on a secretion by mechanical means, ciliated epithelium exists. Specimens for examination are easily obtained from the oyster, and with care will show the characteristic motion. A portion of a gill separated from the mollusc will live on for a considerable time if kept in a little of its natural secretion. The parameciæ, rotifera, and all the ciliata, are furnished with cilia as a means of locomotion and obtaining sustenance. By snipping off a small piece from the gills of the mussel, always accessible to the microscopist, and covering it over with thin glass to prevent evaporation of the animal juices, its cilia will continue to work for hours.

_Lymph and Blood_, Fig. 425 B, _a a_.--There are other cells in the animal body which possess a certain amount of resemblance to those confined to the more superficial structures--_i.e._, the lymph, chyle, and blood. These fluids present in one respect a physical uniformity of composition, and a resemblance in the size of their characteristic corpuscles. Chyle contains besides the corpuscles of lymph, a quantity of minute granules which imparts a white colour to the fluid. Intermixed are oil globules, free nuclei, and sometimes a few red blood discs. Chyle may be had for microscopic examination by squeezing a little juice from the lymphatic gland of a sheep just slaughtered.

A. _a a._ Red blood corpuscles lying flat on the warm stage; _b b._ in profile; _c c._ arranged in rouleaux; _d_. crenated; _e_. rendered spherical by water; I. leucocytes and white amœboid corpuscles; B. Blood discs of fowl, red and white, others seen in convexity and with a nucleus. Blood Crystals.--C. Hæmatin from human blood; D. Hæmatoidin; E. Hæmin; F. Tetrahedral; G. Pentagonal; H. Octahedral crystals from blood of mouse.]

_Blood Corpuscles_ or cells vary considerably in mammals, birds, reptiles, and fishes. Fig. 102 (page 143) is a microphotograph of a drop of blood magnified 3,500 times; and Fig. 425, A, shows both red and white discs drawn to scale, magnified 1,200 diameters. The red corpuscles of human blood are distinguished by their clearly defined outlines and dark centres. Each disc is biconcave in form, and hence the whole surface cannot be focussed at the same time. When the circumference is well illuminated the centre is dark, but by bringing the objective nearer to the object, the concavity of the disc is brought into focus. It generally happens that blood corpuscles, on being first drawn, run together, and present the appearance of rolls of coins; or they may be scattered about over the field. There is a considerable difference in the form of the discs; they are circular in all mammals, except the camel, dromedary, and llama, these being oval. In profile blood corpuscles are biconcave, their investing membrane is homogeneous and elastic, and will readily move along the smallest capillary vessels. There is no trace of a nucleus in the blood-discs of the adult Mammalia, while in size they bear no proportion to the bulk of the animal in whose blood-vessels they circulate. The corpuscles of Mammalia in general are like those of man in form and size, being either a little larger or smaller. The most marked exception is the blood of the musk-deer, in which the corpuscles are of extreme smallness, about the 1/12000th of an inch in diameter. In the elephant they are large, about 1/2700th of an inch in diameter. The goat, among common animals, has very small corpuscles, but they are, withal, twice as large as those of the musk-deer. In the _Menobranchus lateralis_ they are of a much larger size than in any animal, being the 1/350th of an inch; in the proteus, the 1/400th of an inch in the longest diameter; in the salamander, or water-newt, 1/600th; in the frog, 1/900th; lizards, 1/1400th; in birds, 1/1700th; and in man, 1/3200th of an inch. Of fishes, the cartilaginous have the largest corpuscles; in gold-fish, they are about the 1/1700th of an inch in their longest diameter.

The large size of the blood discs in reptiles, especially in the Batrachia, has been of great service to physiologists by enabling them to ascertain many particulars regarding structure which could not have been otherwise determined with certainty. The value of the spectroscope in the chemical examination of the blood has been already referred to. See page 252.

White corpuscles or leucocytes (Fig. 425, I) differ materially from the red. They are large, spheroidal, finely granular masses of about 1/2800th of an inch in diameter. In a cubic millimètre of human blood there are about 10,000 white corpuscles. They have a lower specific gravity than the red, have no cell-wall, and their substance mainly consists of protoplasm. The internal granular appearance is now believed to be due to a fine intercellular network having small dots at the intersections of the web. In the meshes of the net a hyaline substance is interspersed. They possess one or more nuclei; these are seen on the application of a few drops of acetic acid. When examined in a perfectly fresh state, especially if the glass slide be placed on the warm stage of the microscope, they exhibit a spontaneous change of shape, amœba-like, such movements being accordingly termed amœboid. The movements referred to consist in the protrusion of processes of protoplasm which are retracted and other processes protruded as represented (Fig. 425, I). Both in human blood and in newts there are colourless corpuscles which contain coarser granules than others; these are called granular corpuscles. Some are shown near the amœboid bodies. The white corpuscles are readily found in various tissues of the body, as in the lymphatic glands. In inflammatory diseases these _leucocytes_ pass through the walls of the capillaries into the tissues, and form morbid products, pus-cells.

Sections of blood discs are made by dipping a fine needle in a drop of blood as it exudes from a prick of the finger and drawing thin lines across the glass slip, allowing time to dry, and then cutting the lines across in all directions with a razor. The loosened portions should be removed with a camel’s-hair brush.

In birds, the blood discs are oval in shape and possess a nucleus, shown in Fig. 425 B, in the blood of the fowl; this is rendered more apparent on adding a drop of acetic acid. The blood of fishes is also oval and nucleated, rather more pointed than that of birds. In reptiles generally the red blood discs are large, oval, nucleated bodies, the white corpuscles still preserving their invariable circular form and granular appearance. In the salamander and proteus the discs attain to their greatest size. In the former they measure 1/700th of an inch, and in the latter 1/400th.

_Blood Crystals._--In addition to the elements described, the blood contains various crystalline forms, represented in Fig. 425, C to H. In connection with the micro-spectroscope (p. 253), the spectra of certain blood crystals are given; although varying in different animals, sufficient uniformity prevails as to render them characteristic. The crystals are formed when a little blood is mixed with water on the slide, allowing a short time for crystallisation. Near the edge of the cover-glass, where crystals begin to form, they are more distinct, but a high power is required for their examination. In human blood the crystals are prismatic; in that of the guinea-pig, tetrahedral; in the blood of the mouse, octahedral. Other forms may be obtained by the aid of chemical reagents.

In human blood there are at least three distinct forms of crystals: _Hæmatin_ is formed in normal blood, is made visible on the addition of a little water to blood, or by agitation with ether, so as to dissolve the cell-wall of the blood corpuscles, and allow the contents to escape. A drop of blood will furnish crystals large enough to be seen with a moderate power. _Hæmatoidin_ crystals are abnormal products, found in connection with certain diseased conditions. These crystals are seen as represented at D. _Hæmin_ crystals must be regarded as artificial chemical products, the result of treating blood with glacial acetic acid; the acicular crystals at E, reddish-brown in colour, are artificially produced.

1. White fibrous or non-elastic tissue; 2. Yellow fibrous elastic tissue.]

_Basement Membrane--Connective Tissue System._--Connective or areolar tissue is present almost throughout the whole of the human body, and serves to connect the various organs with one another, as well as to bind together the several parts. The muscles are surrounded by a connective tissue sheath; this penetrates into their substance, and binds together fasciculi and fibres. The same tissue is present in the skin and the mucous membranes; it also forms a sheath for the arteries, veins, and nerves. It is plentifully supplied by blood-vessels, and nerves pass through its substance. Microscopically, four different elements can be clearly made out:--1. Connective tissue cells or corpuscles; 2. White fibrous tissue; 3. Yellow fibrous tissue; 4. Ground substance.

On examining the connective tissue cells of young animals, various cells will be seen with fine granular contents, together with nuclei, lying in spaces in the ground substance, some branched, others flattened or rounded. Even tissues supposed to be homogeneous in structure, are on staining seen to have connective tissue cells, such as those represented in a section of the cornea of the eye (see p. 31). In this case the connective tissue cells are termed corneal corpuscles; the branched cells, it will be noticed, are united by branches.

The cells in the fibrous tissue of tendons are square or oblong, and form continuous rows. White fibrous tissue is distributed throughout the animal body, but in a variety of forms; it is found in the skin and other membranes, and in all parts where strength and flexibility are necessary. The structure of white and yellow fibrous tissues is shown in Figs. 426 and 427.

1. White fibrous tissue lining the interior of the egg shell, with the calcium carbonate removed by immersion in hydrochloric acid; 2. White fibrous tissue, from the sclerotic coat of the eye.]

White fibrous tissue presents silver-lustre bundles, running for the most part in parallel directions through and over the muscles and tendons. For examination under the microscope, obtain a fragment of fresh meat cut in the longitudinal direction; place it in water, and tease it out with needles as directed in a former chapter. The smallest fragment will suffice for examination under a quarter or one-sixth inch objective. These filaments are exceedingly minute, measuring 1/3000th to 1/2500th of an inch in diameter, and do not interlace through the bundles, although they intersect each other occasionally. Transverse sections may be made by drying a piece of tendon until it becomes sufficiently firm to cut with a razor or microtome, and mounted as a permanent specimen. From the cut ends of the fibres small dark points will be seen, especially in the denser structure of the tendons; these are termed “connective tissue corpuscles.”

Yellow elastic fibrous tissue is remarkable in contradistinction to the white for its elasticity and capability of extension. It is found on the coats of blood-vessels, between the vertebral arches, and in quadrupeds it forms a strong elastic band, extending from the occiput, throughout the spines of the vertebra, and enabling the animal to support the head in the pendent position, without muscular exertion. These fibres can only be separated from each other with difficulty, and their elasticity is shown by a tendency to curl up. These yellow fibres are somewhat coarser than the white, and they remain unaffected by acetic acid of the ordinary strength. Elastic tissue is a constituent of the skin, mucous, and serous membranes, and of the areolar or cellular tissue.

In order to microscopically examine this structure, take a small portion of the strong ligament of the neck of the ox, place it as before in water, and tease it out with needles; place a fragment on a glass slip, cover with a thin cover-glass, and submit it to a high magnifying power. Transverse sections made as directed in the case of white tissue will be seen to be hexagonal in form.

_Adipose Tissue._--Fat is found in many situations in the animal body, and on examination is seen to consist entirely of vesicles, distributed through a delicate membrane of connective tissue, shown in Plate XIX., Nos. 4 and 5. On pressure, the circular or oval form of the cells becomes polyhedral; occasionally the fatty acids in the interior of the vesicles crystallise, and give rise to a star-like appearance. For the examination of adipose tissue, take a portion of the mesentery of any small animal--a mouse, or rat.

_Retiform Tissue._--Adenoid, or retiform tissue, consists of a delicate network of connective tissue corpuscles, joining their branches together. This forms the stroma or framework of lymphoid tissue. It is found in connection with all the lymphatic glands, spleen, &c. Plate XIX., No. 3, _a b_, shows small sections of a lymphatic, together with capillary vessels.

_Muscular Fibre._--There are two varieties of muscular fibre in the body--_i.e._, striated, and non-striated. The striated is formed in muscles attached to bony structures, as those of the arm and leg, and in some of the soft structures, as the tongue, palate, œsophagus, in short, all muscles under the control of the will. Striped muscle is of a dull red colour and marked with peculiar longitudinal furrows on its surface. Voluntary muscle consists of:--1, a connective tissue sheath; 2, fasciculi; 3, fibres and sarcolemma; 4, discs, fibrilla and sarcous elements. These are shown in connection with other tissues in Plate XIX., Nos. 11 and 12, and also in Fig. 428 (1, 2, 3).

1. Muscular fibre broken across, the fragments connected by the connective tissue membrane × 100; 2. Fibre broken up into irregular distinct bands: a few blood corpuscles distributed about × 200; 3. A fasciculus of muscular fibre from leg of pig × 600.]

In Plate XIX., Fig. 11, the muscular fibre taken from the tongue of a lamb shows the continuity of the upper portion with the connective tissue membrane. In Fig. 12, a branching-out bundle of muscular fibre, taken from the upper lip of the rat, is seen to end in stellate connective cells. The delicate homogeneous sheath that binds the fibres together is termed _sarcolemma_. This is readily seen in prepared muscle of the frog and water-beetle, less plainly in man. Each muscle is provided with a sheath of connective tissue; this surrounds it, binds the fasciculi together, and supports the blood-vessels; it is called the _perimysium_, and sends fine prolongations in between the fibres, termed _endomysium_. The intervals seen on high amplification between the dark striæ are called Kruse’s membrane. On breaking up the striated structure it is resolvable into fibrillæ and furthermore into discs.

1. Vertical section of epidermis; 2. Pigment cells from a lower layer of cutis.]

Among mammalia the pig furnishes the best examples of muscle fibrillæ; among insects the water-beetle and the thorax of the housefly. A power of 600 or 800 diameters is required to separate the fibrillæ. Blood-vessels are well supplied with striated muscle, but none of their minuter branches penetrate the sarcolemma. The involuntary or non-striated variety of muscular fibre exists in all parts of the body where movements occur independently of the will, also in the ciliary muscle and the iris of the eye, as well as in the middle coats of the arteries. Non-striated fibres are pale in colour, prismatic in shape, and easily flattened by pressure. In size, they vary from 1/7000th to 1/3500th of an inch in diameter, and are marked at short intervals by oblong corpuscles.

_The Integument or Skin_ consists of epidermis or cuticle, dermis, corium or cutis vera, sweat-glands, nails, hairs, sebaceous glands, and numerous nerves and vessels. The epidermis forms a protective covering over the whole surface of the body, and is moulded on to the surface of the corium beneath, covering the ridges, depressions and papillæ. It is made up of three principal layers: the horny layer or _stratium corneum_, the most superficial, this consists of layers of flattened cells, which are without a nucleus; the _stratum lucidum_, composed of layers of nucleated cells, more or less indistinct in section; the _rete mucosum_ or malpighian layer; is composed in its upper part of layers of “prickle cells” and its inferior of a single stratum of columnar cells. Pigment is principally found in the lowest layer, Fig. 429.

The gradations of colour in the skin are due to the granular contents of the pigment cells. This is seen on steeping sections cut from the skin of a negro in chlorine; the colour is discharged. In Plate XIX., No. 13, the pigment cells of the choroid coat of eye are shown. Here the pigment is darker in colour, and its function is the absorption of light and the prevention of disturbing effects occasioned by circles of dispersion.

_The Dermis_, or true skin, consists of an interlacing network of connective tissue, yellow elastic tissue corpuscles, vessels, and nerves. There are also small muscular fibres in connection with the hair follicles, and beneath the subcutaneous tissues contain an abundant supply of fat adipose tissue. Numerous ridges are seen on the surface, especially on the palm of the hand and sole of the foot, caused by rows of little elevations of the cutis vera, termed papillæ. These are more or less conical, and contain a capillary loop, nerve, and touch corpuscle, which serve to increase the sensitiveness of the part, lodging a touch corpuscle in a favourable position for receiving sensations of touch, Fig. 430.

Sweat glands are situated in the subcutaneous tissue, and consist of fine tubes, which form the duct (seen in the section, Fig. 430); these are continuous with a blind extremity, coiled up into a ball one-sixtieth of an inch in diameter, and surrounded by a plexus of capillaries to form the gland (Fig. 431, No. 2). Between the layer of columnar cells and the limiting membrane is a layer of non-striated muscle, and beneath the rite mucosum there are several layers of polyhedral cells, and an external and internal limiting membrane; the epithelium of the duct is at its mouth continuous with the epithelium of the epidermis.

1. Blood vessels of papillæ supplied to cutis; 2. Perpendicular section through the scalp, with two hair-sacs; _a._ epidermis; _b._ cutis; _c._ muscles of the hair follicles.]

_Nails_ consist of a root and body, the lunular of which is the whitish portion of the body near the root, where the skin beneath is less vascular than any other portion of the finger. The nail closely resembles the epidermis, and consists of hard and thin layers of cells on the surface, and round, moist cells beneath. Posteriorly the nail fits into a groove which lodges its root. The part to which the nail is attached is known as the nail-bed. The stratified appearance produced by the coalescence of the cells, and their lying over each other, is shown in Plate VII., No. 149, the toe of the mouse; while the special arrangement of tissue is better seen under polarised light (Plate VIII., No. 174).

_Hairs_ consist of a shaft and root. The shaft is cylindrical, and covered with a layer of imbricated scales, arranged with their edges upwards. The substance of the hair consists of fibres, or elongated fusiform cells, in which nuclei are seen. There are present in some hairs (Fig. 432) small air spaces or lacunæ. In the coarser hair of the body there is a pith (medulla), occupied by small angular cells and fat granules.

1. Single Hair-root and Shaft; 2. Vertical section, showing fibrous character of the hair together with colouring matter, external edges serrated; 3. Transverse section of human hair, medullary substance, and central pith.]

The root of the hair is seen to dilate that it may fit more firmly into the skin hair-follicle. The latter consists of two coats, an outer and an inner, continuous with the epidermis, and this is called the root sheath. The outer portion consists of three layers, formed of connective tissue, blood-vessels, and nerves. The inner, or epidermic, coat comes away when the hair is pulled out, and hence is called the root sheath. This again is made up of two layers, the outer of which corresponds with the horny layer, and is composed of flattened cells. The bulbous root of the hair is connected with the papilla. In the cat the tactile nasal hairs are very large. Small bundles of involuntary muscular fibres connect the corium with the root, so that in contracting they elevate or expand the hair.

1. Jointed hairs of Indian bat; 2. Hair of flying-fox, showing imbricated scales; 3. Hair of mouse, showing pigment layers; 4. Hair of a small beetle (Dermestes). × 250.]

The hair of the lower animals presents a diversity of structure, especially on the outer surface, and with reference to the arrangement of the scales. The hair of the Indian bat, for instance, consists of a shaft invested with erectile scales, placed at regular intervals; these stand out from the shaft, as in Fig. 433, No. 1. This form of scale varies considerably in the different species of these animals, and a portion of hair near the root is nearly divested of scales. Many of the scales are not unlike those of certain of the insect tribe, seen in that of Dermestes, No. 3, while the hair of the mouse has a series of transverse imbricated scales arranged as tiles on a house, due to accumulated pigment. Hairs taken from various animals form interesting objects of study for the microscope, as already noticed. Other hairs are shown in Fig. 434. No. 1 is a transverse section of a hair from the ant-eater; the central part consists of air-cells, the outer of a granular pith. No. 2 is a transverse section of hair of peccary, with a diversified arrangement of the cortical envelope, sending outward a set of radial prolongations and air-cells; this kind of structure is also found in the quills of the porcupine. No. 3 is a transverse section of a hair of the elephant, which shows a combination of a number of tubes united together, somewhat resembling the arrangement of the hoof-horn of some of the ruminants, and the denser horny growth on the snout of the rhinoceros, No. 4. The curious modification of these horny structures is seen in the horns of other animals, and which may be likened to a bundle of hairs. On making a transverse section, as in Fig. 434, and submitting it to polarised light, on rotating the analyser, the dark central spot shown is replaced by a bright one with a play of colours due to the interference of light (Plate VIII., No. 178). The scales of fish are also of interest (Fig. 435). These have been shown to afford an unerring guide in the classification of fishes and in the examination of their fossil remains. As a class of objects for the microscope, they are found to be both curious and beautiful. Plate VIII., No. 176, is a scale of the grayling, seen under polarised light.

1. Hair of ant-eater; 2. Hair of peccary; 3. Hair of elephant; 4. Horn of Rhinoceros.]

Of the harder outgrowths of the dermal structures, the teeth afford the chief example among animals. The rough anatomy of the tooth in mankind consists of a crown, that projects from the gum; a root, or fangs, fixed in a socket of the jawbone, and a short intermediary neck. Each tooth is supplied with an artery and nerve, and has a central cavity filled with a soft, vascular, sensitive substance, the pulp. On making a vertical section of a tooth, we recognise the several structures in the order of, pulp, crusta, petrosa, dentine, and enamel. A section through a human molar tooth (shown in Fig. 436) will convey some idea of the arrangement of the denser structures referred to above.

Blandin was the first to demonstrate that teeth are developed in the mucous membrane, similar to that of hair and nails. Teeth are formed in grooves of the mucous membrane, and subsequently converted into closed sacs by a process of involution, and their final adhesion to the jaw is a later process. It is very generally conceded that teeth belong to the _muco-dermoid_, and not to the periosteal, series of tissues; that, instead of standing in close relation to the endo-skeleton, they are part of the dermal or exo-skeleton; their true analogues being the hair, and some other epidermic appendages. Huxley proved that, although teeth are developed in two ways, they are mere varieties of the usual mode in the animal kingdom. In the first, which is typified by the mackerel and the frog, the pulp is never free, but from the first is inclosed within the capsule, seeming to sink down as fast as it grows. In the other, the pulp projects freely at one period above the surface of the mucous membrane, becoming subsequently included within a capsule formed by the involution of the latter; this occurs in the human subject. The skate offers a sort of intermediate structure.

1. Section of a cusp of the posterior molar of a child. The inner outline represents it before the addition of acetic acid--the outer afterwards, when Nasmyth’s membrane _g_ is seen raised up in folds; _f._ the enamel organ; _c._ the dentine; the central portion being filled with pulp. 2. Edge of the pulp of a molar cusp, showing the first rudiment of the dentine, commencing in a perfectly transparent layer between the nuclei of the pulp and the _membrana preformativa_. 3. Nasmyth’s membrane detached from the subjacent enamel by acetic acid. 4. Stellate-cells of the enamel organ. 5. Tooth of frog, acted on by dilute hydrochloric acid, so as to dissolve out the enamel and free Nasmyth’s membrane. The structure of the dentine _e_ is rendered indistinct. At the base, Nasmyth’s membrane is continued over the bony substance at _z_, in which the nuclei of the lacunæ are visible. (After Huxley.) 6. Decalcified tooth-structure; _a._ the dentine; _b._ enamel organ; _c._ enamel; _d._ Nasmyth’s membrane.]

The _enamel_ forms a continuous layer, and invests the crown of the tooth; it is thickest upon the masticating surface, and decreases towards the neck, where it usually terminates. The external surface of the enamel appears smooth, but is always marked by delicate elevations and transverse ridges, and covered by a fine membrane (Nasmyth’s membrane), containing calcareous matter. This membrane is separable after being subjected to hydrochloric acid; it then appears like a network of areolar tissue, shown in Fig. 438, No. 6; Huxley’s “_calcified membrana_,” which commence at the pulp cavity, and pass up to the enamel.

1. Longitudinal section of superior canine tooth, exhibiting general arrangement, and contour markings, slightly magnified; 2 and 3. Portions from same, highly magnified, showing the relative position of bone-cells, cementum at 2, dentine fibres, and commencement of enamel at 3; 4. Dentine fibres decalcified; 5. Nasmyth’s membrane separated and the calcareous matter dissolved out with dilute acid; 6. Cells of the pulp lying between it and the ivory; 7. A transverse section of enamel, showing the sheaths of fibres, contents removed, and magnified 300 diameters.]

Czermak discovered that the curious appearances of globular conglomerate formations in the substance of dentine depend on its mode of _calcification_ and the presence of earthy material; and he attributed the contour lines to the same cause. Contour markings vary in intensity and number; they are most abundant in the root, and most marked in the crown. Vertical sections exhibit them the best; as Fig. 440, No. 1. In preparing a specimen, first make the section accurately, then decalcify it by submersion in dilute hydrochloric acid; dry it and mount in Canada balsam; place the specimen in the hot chamber for some time to soak in the fluid resin before it cools. The white opacity at the extremity of the contour markings gives the appearance of rings to the tooth-fang.

“The tooth-substance appears,” says Czermak, “on its inner surface, not as a symmetrical whole, but consisting of balls of various diameter, which are fused together into a mass with one another in different degrees, and in which the dentine tubes in contact with the germ cavity terminate. By reflected light, _dark-ground_ illumination, one perceives this stalactite-like condition of the inner surface of the tooth-substance very distinctly, by means of the varied illumination of the globular elevations, and by the shadows which they cast.” To see this structure to advantage the preparation should be made from a tooth root, the growth of which is not complete. With such preparations, the ground-substance of the last formed layer of the tooth-substance is seen to be, at least partly, in the form of globular masses, fused together with those of the penultimate layers.

The cementum is the cortical layer of osseous tissue, forming an outer coating to the fangs, which it sometimes cements together. Its internal surface is intimately united with the _dentine_, and in many teeth it would appear as if the earliest determined arrangement of the fibres of the dentine started from the _canaliculi_, as they radiate from the lacunæ in the cement. The inter-lacunar layer is often striated, and exhibits a laminated structure: sometimes it appears as if Haversian canals were running in a perpendicular direction to the pulp cavity. The _canaliculi_ frequently run out into numerous branches, connecting one with another, and anastomising with the ends of the dentine fibres. The thick layers of cement which occur in old teeth show immense quantities of aggregated lacunæ of an irregular and elongated form.

Fig. 439.--Transverse section of Tooth of Pristis, showing orifices of medullary canals, with systems of radiating fibres (_tubuli_) analogous to the Haversian canals in true bone.]

_Compact Tissues, Cartilage and Bone._--Cartilage is a bluish or yellowish-white, semi-transparent, elastic substance, without vessels or nerves, and surrounded by a membrane, termed pericondrium, of a dense fibrous nature. That kind, however, known as articular cartilage, receives a layer of epithelium from the synovial membrane, but this is confined to marginal portions, in consequence of the central wear which occurs as soon as the parts are subjected to friction, during the movement of the limbs. Cartilage covers the ends of all bones in apposition to form joints, and thus lessens the effects of concussion. Besides the ordinary kind of cartilage, temporary and permanent, there are two modifications of the tissue, confined to certain portions of the body: cellular cartilage, composed of cells lying close together, in a mesh formed of fine fibres; and fibro-cartilage, cells distributed in a matrix of fibrous tissue.

Examined with a low power, cartilage appears to be homogeneous in structure, studded over with numerous round, oval, oblong, semilunar, and irregular-shaped corpuscles, as seen in Plate XIX., No. 8, a vertical section of animal cartilage, arranged in columns, and condensed at the lower surface previous to its conversion into bone. The greater opacity of this portion is owing to the increase of osseous fibres, and the multiplication of oil globules, and the intercellular spaces becoming filled with vessels. No. 9 shows a small transverse section of the same, with a further change of the cartilage cells at _a_ into bone cells, and at _b_ with the characteristic canaliculi and lacunæ. No. 7 further shows a section of the large tendon fixed to the back of the heel of the foot, near the juncture of the tendo-Archillis with the cartilage. For the examination of these several changes a high power is necessary, and for the purpose pieces taken from the ox may be easily obtained from the butcher, and fine sections cut with a razor parallel to the surface.

1. Cartilage from a mouse’s ear closely resembling vegetable tissue ×200; 2. Cartilage from rabbit’s ear, with nucleated cells embedded in matrix; 3. Cartilage from the end of a human rib ×300.]

The better specimens for microscopical examination are those taken from very young animals, in whom the ossific process is still incomplete. In order to examine cellular cartilage, the ear of the mouse should be taken and just dried sufficiently to enable fine sections to be cut by the microtome transversely (Fig. 440).

Cartilage forms the entire skeleton of a certain number of fishes, as the skate, lamprey, ray, shark, &c., the cells of which are embedded in a matrix of granular matter, which has been properly termed intercellular. The nearest approach to ossification of cartilage in fishes is that of the cuttle-fish; in this stellate cells are freely distributed, as shown in Fig. 441, No. 3.

1. Cartilage from the head of the skate, cells filled with nuclei; 2. Cartilage from frog, oblong cells with nuclei; 3. Cartilage from cuttle-fish, with stellate cells, × 200.]

White fibro-cartilage occurs between the bodies of the vertebræ as a connecting medium. In this kind the cells are more widely distributed, specimens of which may be taken from the central portion of an interarticular disc of any animal. The oval or circular corpuscles will be seen surrounded by an abundance of fibrous tissue.

An acquaintance with the degeneration of the textures with which we have been dealing may be of service to the student, as he may, in the course of his examination, meet with an abnormal condition altogether different to those described. The process of degeneration is usually a slow one, except in the case of fatty infiltration, an example of which is furnished by the fatty degeneration of the liver in Strasburg geese. Muscular tissue is very prone to fatty degeneration, and fatty heart is often met with. Calcareous degeneration of the muscles, ligaments, and cartilages, as well as morbid deposits, are not at all uncommon in these structures. In Plate XIX., No. 9, a small section is given of an enchondroma, and in which the round or ovoid cells of the cartilage are seen degenerated and converted into granular masses of a calcareous nature. Fig. 442 is a somewhat more highly magnified section of a calcareous or morbid growth, taken from a human subject in which a morbid growth was seen to be gradually destroying the bone and cartilage cells.

Fig. 442.--Cartilage taken from a diseased finger, in which both cartilage and bone were in a state of degeneration.]

_Bone._--Bone is a hard unyielding structure, and which in the vertebrata forms the skeleton of the adult. It is the framework for the support of the soft tissues of the body, and forms various cavities for the reception of important organs, as the brain, spinal cord, eyes, heart and lungs, and acts as levers for the action of the muscles and joints. The partial elasticity of bone is seen in the ribs, and the rebound when the skull is dropped on the ground. Bone consists of earthy and animal matters intimately combined; the removal of either, however, does not destroy the form of the bone, if the process of separation be carefully conducted. The earthy constituents may all be dissolved out by hydrochloric acid, but the form of the bone is preserved in its minute particular, and in this state sections may be cut for microscopical examination. If allowed to become dry it shrivels, and assumes the density of horn. The interior of a bone is of a spongy or cancellated structure, particularly at the ends. The outer portion of the bone is more dense than the internal part. The study of bone should commence with sections of the softened structure. Directions for making sections of bone are given in the chapter on Practical Microscopy.

VERTEBRATA, BONE STRUCTURE.]

The intimate structure of bone will be studied in connection with Plate XX. Two series of lamellæ may be demonstrated in bone after maceration in acid, a larger system surrounding the medullary canal, and a smaller surrounding the Haversian canals, both of which are seen in Nos. 1 and 2. In macerating bones, the lamellæ of the layer concentric system may be peeled off in layers; these are seen to be pierced with fine apertures, caused by the canaliculi. In some parts larger apertures are seen through which bundles of fibres pass, pinning, as it were, the several layers together; these are the perforating fibres. The outermost of the layers, being near the periosteum, the membrane covering the bone, are termed periosteal layers; the innermost, being close to the canal, are called medullary layers. No. 1 is a transverse section of a flat bone, the clavicle, and it shows the Haversian canals, varying in size from 1/2000th to 1/200th of an inch in diameter, the largest being near the medullary canal. In shape they are round, oval, or oblong, according to the line of section. Each canal is surrounded by rings, none of which are complete, and running one into the other at various parts. Under a higher power, those irregular shaped bodies termed lacunæ, with fine radiating fibres, are seen to be smaller canals, canaliculi.

By means of this complete and intricate distribution of the canals of the Haversian system, the nutritive fluids pass into the most compact parts of the osseous tissue. Longitudinal sections of the long bones show these canals as continuous branching-out cells.

In many of the lower animals the bony structure differs from those of man, as will be seen in Plate XX. No. 3 shows a transverse section of the femur, or leg-bone of an ostrich, magnified ninety-five times, in which the Haversian canals are much smaller and more numerous, and many of them run in the transverse direction. No. 4, again, is a transverse section of the humerus, or fore-arm bone of a turtle (_Chelonia mydas_). This exhibits traces of Haversian canals, with a slight tendency to a concentric arrangement of bone-cells around them, the bone-cells being large and numerous, and occur, for the most part, in parallel rows. In No. 5, a horizontal section of the lower jaw-bone of a conger-eel exhibits a single plane of bone-cells arranged in parallel lines. There are no Haversian canals present, and when this specimen is contrasted with that of No. 4, it will be noticed that the canaliculi given off from each of the bone-cells of this fish are very few in number in comparison with that of the reptile. No. 6 is a section of a portion of the cranium of a siren (_Siren lacertina_), remarkable for the large size of the bone-cells, and of the canaliculi, which are larger in this animal than in any other yet examined; and as in the preceding specimen, no Haversian canals are present. No. 7 is a section of bone taken from the exterior of the shaft of the humerus of a Pterodactyle; this exhibits the elongated bone-cells characteristic of the order Reptilia. No. 8 is a horizontal section of a scale, or flattened spine, from the skin of a Trygon, or sting ray; this exhibits large Haversian canals, with numerous wavy parallel tubes, like those of dentine, communicating with them. This specimen shows, besides wavy tubes, numerous bone-cells, whose canaliculi communicate with the tubes, as in dentine.

The following points may be noted with regard to the several sections of bone described. That of the bird, for instance, contrasted with that of the mammal, exhibits the following peculiarities: the Haversian canals are more abundant, much smaller, and often run in a direction at right angles to that of the shaft, by which means the concentric laminated arrangement is in some cases lost; the direction of the canals follows the curve of the bone; the bone-cells are much smaller and more numerous; while the number of canaliculi sent off from the cells is less than in those of mammals. No. 3 is the average length of a bone-cell of the ostrich, 1/2000th of an inch, in breadth 1/6000th.

In the Reptilia, the bones may be either hollow, cancellated, or solid; and their specific gravity is less than that of birds or mammals. The short bones of most of the chelonian reptiles are solid, and the long bones are either hollow or cancellated; the ribs of the serpent-tribe are hollow, the medullary cavity performing the office of a Haversian canal; the bone-cells are accordingly arranged in concentric circles around their canals. The vertebræ of these animals are solid; and the bone, like that of certain birds, is remarkable for density and whiteness. When a transverse section is taken from one of the long bones, and contrasted with that of a mammal or bird, the difference will be noticed; there are very few, if any, Haversian canals, and these are large; and at one view, in the section, No. 7, the canals and bone-cells are arranged both vertically and longitudinally. The bone-cells are remarkable for the great size to which they attain; in the turtle they are 1/375th of an inch in length, the canaliculi are extremely numerous, and are of a size proportionate to that of the bone-cell.

In fishes a greater variation occurs in the minute structure of the skeleton than in either of the three preceding classes. A rare structure is that of the sword of the sword-fish (Istiophorus). In this, Haversian canals and a concentric laminated arrangement of the bone are found, but no bone-cells. The Haversian canals, when they are present, are of large size, and very numerous, and then the bone-cells are, generally speaking, either absent or but few in number, their place being occupied by tubes or canaliculi, which are often of a very large size. The bone-cells are remarkable for their graduate figure, and the canaliculi derived from them are comparatively few in number. In a thin section of the scale of an osseous fish, the cells lie nearly all in one plane, and the anastomoses of the canaliculi are more distinctly seen; in the hard scales of many, as the Lepidosteus and Calicthys, and in spines of the Siluridæ, the bone-cells are well differentiated. In the true bony scales comprising the exo-skeleton of cartilaginous fishes the bone-cells are seen in great numbers.