ANIMAL HISTOLOGY (from tστos, a web or tissue, and λόγος, discourse) is the study of the minute structure of the tissues of animals. By a tissue is meant any part of an organism which has undergone special changes in structure in adaptation to the performance of special functions. These special changes are expressed by the general term “differentiation.” In the lowest animal organisms, the whole of whose bodies are composed of the undifferentiated living substance termed “protoplasm,” we find all its functions shared by every part of the organism. An amoeba, for example, it is well known, is capable of finding, seizing, devouring, digesting, and assimilating food, has a special provision for collecting fluid and pumping it out of its body, respires by its whole surface, moves about apparently where it will, exhibits a sensibility to tactile impressions, and reacts in all probability to smell if not to sound and light,—in short, is capable of performing, although with the lowest possible amount of activity, almost every function which animals vastly higher in the scale of organization exhibit. But even in the amoeba we cannot say certainly that there is no differentiation of its protoplasm. For a condensed portion—the nucleus—is set aside to initiate the reproductive function, and it is by means of the external and firmer layer (ectoplasm) that its movements are effected and its relations with the external world maintained, while the internal more fluid protoplasm (endoplasm) is concerned with the digestion of the food. Still there are simple organisms whose protoplasm is probably absolutely undifferentiated. On the other hand, there are other organisms which are also regarded as composed of simple protoplasm, and are constituted by a single cell, which nevertheless show a marked progress in the differentiation of portions of their substance apart altogether from the presence of a nucleus. Such differentiation in unicellular organisms generally takes the form of the production of a shell or “test,” as in the Foraminifera and in Nodiluca, which subserves purely passive functions of sustentation or defence. It is not certain in such cases whether the structure thus produced is formed by the direct conversion of the protoplasm or by an exudation on the surface which subsequently hardens. But portions of the protoplasm may be set aside for the performance of active functions. We see this in its production in the form of locomotory organs, either temporary (pseudopodia) or permanent (cilia). But in neither of these can any actual change in the minute structure of the protoplasm be observed. A differentiation does, however, occur in one remarkable instance—the flagellum, namely, of the Noc- tilucidae (fig. 1), which exhibits as definite a transverse striation as does the cross-striated or voluntary muscular tissue of the higher animals, in which structural peculiarity it is impossible not to infer a relation to its contractile functions; and similarly, in the VorticeUidoe, there is a differentiation of the protoplasm of the rapidly contractile stalk.

Whereas in the more highly organized unicellular animals portions of the single cell are thus set aside for the performance of special functions, and modified in structure accordingly, in multicellular animals, on the other hand, we find whole cells and sets of cells set apart and differentiated. It is to such modifications in sets of cells in multicellular organisms, rather than in portions of the protoplasm of a unicellular organism, that “histological differentiation” is commonly restricted; and each such set of cells, destined for the performance of a special function, and modified accordingly in structure, is denominated a “tissue.”

The animal tissues may be classed under the four heads of Epithelium, Connective Tissue, Muscular Tissue, and Nervous Tissue. Of these four classes of tissue the epithelium is the most primitive and least altered. In the development of the Metazoa the numerous embryonic cells which result from the division of the single cell—the ovum —tend in nearly every case to arrange themselves as a single layer surrounding a central cavity (unilaminar condition of the blastoderm), (fig. 2, A). Presently a part of the wall of the hollow sphere becomes invaginated, so that, instead of a vesicle enclosed by but a single layer of cells, a cup (Gastrula, Haeckel), is produced (fig. 2, B), the wall of which is formed by two layers derived from the original single layer, and separated from one another by a narrow interval (which is all that remains of the original cavity of the vesicle) except at the orifice of the cup, where they are continuous (bilaminar condition of the blastoderm). At this part some cells become separated from one or both of these two primary layers, and, extending in and occupying the cleft-like space which separates them, become a third layer of cells, which differs from those of the other two in not being arranged into a continuous membrane, and not, therefore, forming an epithelium (trilaminar condition of the blastoderm), (fig. 2, C). Now, of these three layers, the outer one, or ectoderm, and the inner one, or entoderm, give rise to all the epithelial tissues of the body. The nervous tissues are also derived from the ectoderm; whereas the connective and muscular tissues originate in the mesoderm or middle layer. In most of the Coelenterata, however, the mesoderm is not developed at one part only of the embryo as in the higher Metazoa. In the hydroid polyps and Medusae it never becomes completely distinct from the ectoderm and entoderm, although a jelly-like sustentacular substance may be formed to a greater or less extent between the two primary layers, and cells may pass into it from one of them, so that a kind of mesoderm is thus produced. In the Medusae, also, the muscular function is performed by cells which either still have their place in the general layer of the ectoderm or are but imperfectly separated from it; and here, again, the commencing separation does not occur at one part only, but over extensive tracts of the surface. Nevertheless these cells are modified in structure precisely in the same way as those which in higher animals are derived from the mesoderm. The nervous functions are also performed by cells and fibres, which, although they show those modifications of structure which in the higher animals are characteristic of nervous tissue, yet remain strictly confined to the ectoderm, and do not, as in the higher animals, penetrate into the mesoderm.

The Epithelial Tissues.— Although, as we have seen (see p. 4, note 2), the layers of cells which are first formed are layers of epithelium, and, therefore, the epithelial tissues are the first to be produced, nevertheless we find that they undergo less modification in structure than any of the other three classes of tissue. As before said, they invariably consist merely of cells cemented together by an imperceptible amount of intercellular substance, and the cells themselves only show minor degrees of modification in shape and structure, at least as compared with the other tissues constituted mainly of cells, namely, the muscular and nervous.

Modifications in Shape of Epithelial Cells.— The cells of this tissue may be either elongated and set like palisades over the surface which they cover, in which case they are termed “columnar” (fig. 3), or they may be flattened out over the surface, and they then appear as thin “scales”; and every variation in shape is met with between these two extremes. In any case where they form a single layer, since the cells are set closely together, the mutual apposition of neighbouring cells produces a flattening of the opposed sides, so that, when the epithelium is looked at from the surface, the cells have a polygonal outline, and collectively present the appearance of a mosaic pavement (fig. 4). In certain cases, especially where there is liability to abrasion of the surface which they cover, the epithelial cells are disposed in two, three, or more superimposed layers (fig. 5), and then the cells of the different layers may vary much in size, shape, and consistence. Such an epithelium is termed “stratified.”

It frequently happens that the layer of epithelium which covers a surface is prolonged into depressions, which may be quite simple or may be ramified either slightly or in the most complex manner. The epithelial cells which line such depressions may resemble those of the surface upon which the depression opens, or they may become more or less modified in size, shape, and other particulars, and constitute themselves into a distinct variety of epithelial tissue. Since depressions like those just mentioned are generally for the purpose of forming some special secretion, and are termed glands , and since this secretion is elaborated by the agency of the epithelial cells which line the gland, any such special variety of epithelium is termed a “glandular” or “secreting” epithelium.

Modifications in Structure.— The modifications in structure which the cells of epithelial tissue undergo are comparatively slight. One of the most common is the conversion of the external layer of the protoplasm of the cell into a firm membrane, generally of a horny nature, but this membrane is seldom sharply marked off from the substance of the cell, as is the case with the cellulose membrane of the vegetable cell. It becomes formed, moreover, to a very different extent in different cells, according to the function which the particular epithelium has to perform; where, for example, the epithelium is almost purely a protective covering, as in the stratified scaly epithelia, a considerable part, or even the whole thickness of many of the epithelial cells, is thus transformed; but where, on the other hand, the cells have to play an active part in yielding a secretion to moisten the surface, or in protruding a portion of their protoplasm in the form of vibratile cilia to produce currents over the surface, or to move the organism through the water, we find little, if any, of such conversion of the superficial cell-substance. What little there may be is confined to the attached surfaces of the cell, or if there is any such covering on the free surface, it is penetrated by pores which allow of a communication between the protoplasm of the cell and the external medium.

Another common modification of structure which epithelial cells exhibit is the existence of vibratile cilia at the free surface (fig. 6). This, again, is especially frequent with cells of a columnar shape, but it may occur in any. The cilia appear to be protrusions of the more active external protoplasm of the cell, which are in most cases incapable of being again withdrawn, and are in all probability modified in minute structure, although they are always so small that such modification, if it exists, escapes detection even with the employment of the highest powers of the microscope. At their base, however, the cilia are certainly continuous with the unaltered protoplasm of the cell. This may be seen even where the cilia are small and spring in a bunch from the free surface of the cell, but much better in those kinds of ciliated epithelium in which but a single large cilium is connected to each columnar cell (fig. 8).

Many epithelial cells, especially those of secreting glands, show a differentiation of their protoplasm in the form of fine striae or rods which pass from the attached border of the cell towards the free end (fig. 7). Cells thus modified are found in the ducts of the salivary glands, in the alveoli of the pancreas, and in the convoluted tubules of the kidney in Vertebrata.

One of the most remarkable modifications which epithelium cells exhibit is found in the organs of special sense. This is the presence of a fine filamentous process or processes springing from the free surface of the epithelium cells, and resembling in their appearance long cilia, but not spontaneously vibratile (fig. 9). Moreover, the cells in question, which are generally of an elongated columnar form, commonly branch out at their detached end into fine processes which appear to become connected with nerve-fibres. Cells of this character occur even so low in the Metazoa as the Medusae, in connexion with the nerve-epithelium to be afterwards mentioned. And, indeed, in many cases where cells of this character enter into the constitution of the sense organs, it is probably most consistent with their true nature to regard them as detached portions of nervous tissue, which also in every case is originally of an epithelial nature.

Modifications in the Cell Contents.— Another chief modification which the cells of an epithelial tissue may undergo consists in the accumulation within the cells of various chemical substances, which may Be either taken in bodily as such, or may be formed in the cell from other substances which are supplied to it by the blood. The substances that are thus accumulated and formed within the cells of an epithelium are of very various nature, as, for example, the constituents of special secretions (fig. 10), mucin, pigment, fatty globules, uric acid, &c., &c. These several substances are tolerably constant in an epithelium of the same kind—thus, mucin is a very frequent constituent of columnar epithelium, and in glands which have the same function in different animals, the same substances are found in the epithelium cells of the gland.

Exudation from Epithelium Cells. Formation of Cuticular Structures.— In many invertebrates the epithelium which covers the surface of the body, and sometimes also that which lines a part of the alimentary canal, forms an exudation which is generally soft at first, but may afterwards harden into a horny consistency, or may be rendered still harder and at the same time more brittle by impregnation with earthy salts. Any such structure is termed a cuticular formation. It may be composed of a single thin layer, or a number of layers may be superimposed, so that a “shell” of considerable thickness is thus formed. ^ The chitinous oi∙ calcareous covering which forms the exoskeleton in many molluscs, arthropods, annelids, and Hydrozoa is of this nature. On the other hand, the firm skeletons of sponges, Actinozoa, and Echinodermαta are formed by deposition in the connective tissue.

The Connective Tissues.— The connective tissues are characterized by the great development of intercellular substance in comparison with the cells; indeed in those animals in which connective tissue may first of all be said to appear, there is an entire absence of cellular elements properly belonging to the tissue. This is the case in many of the Coelenterata, in which the connective tissue is represented merely by a layer, more or less thick, of hyaline substance, which undoubtedly performs a sustentacular function, in addition to connecting together the epithelial layers of the ectoderm and entoderm.

The intercellular or ground substance almost invariably takes a prominent part in the formation of connective tissue. It is of a semi-fluid nature, and often contains in addition to albumen a certain amount of mucin. In most cases the cells of the connective tissue separate themselves from the primary layers before the formation of this ground substance; indeed the mesoderm is at first chiefly formed of these cells. The stages of development are as follows. The mesodermic cells, which are at first in apposition, become separated from one another by the accumulation of intercellular substance, but at the same time maintain a connexion with one another throughout the tissue by their branching cell-processes (see fig. 2, C, c). Presently, in the production of ordinary connective tissue, fibres of two kinds make their appearance in the intercellular substance, and to all appearance independent of the cells. Those of the one kind (fig. 11, A) are highly elastic and refracting, not easily affected by reagents, stain deeply with magenta, run singly, always branch, and become united with neighbouring fibres so as to form a network throughout the tissue; those of the other kind (fig. 11, B) are excessively fine and indistinct, never run singly but always in bundles, and generally with a wavy course, are readily affected byreagents, and, in vertebrates, yield gelatin on boiling. In the various kinds of connective tissue the relative proportion of these two kinds of fibres to one another and to the cellular elements of the tissue varies. Thus in the so-called elastic tissue of the Vertebrata the elastic fibres greatly preponderate; in tendinous tissue, on the other hand, they are scarcely to be found, and the ground is almost wholly occupied by the white fibres. It may happen that the intercellular substance is so completely occupied by the fibres as to be entirely obscured, but its presence may be always recognized in consequence of the property which it possesses of reducing silver from its salts when exposed to the light. In certain cases the intercellular substance becomes hardened by the deposit within it either of a substance termed chondrin, which confers upon it the well-known toughness and elasticity of cartilage, or by a deposit of earthy salts imparting to it the firmness of bone. These several changes in the intercellular substance are accompanied by special modifications in the form and relations of the cells (by whose agency they are in all probability effected). In comparatively rare cases the intercellular substance which is found occupying the meshes of the network formed by the branched cells of the developing connective tissue may disappear entirely, and the meshes may be occupied either by blood or by the lymph or plasma of the blood (spleen and lymphatic glands of vertebrates).

It frequently happens that the connective tissue presents the consistence of jelly, and this is generally ascribed to the characters of the intercellular substance. It may, however, be due in many cases to the entanglement of fluid in the meshes of the fibres, and not to a gelatinization of the ground substance. This is shown by the fact that the fluid may be drained from out the meshes by means of filter paper. And the possibility of the formation of a jelly in this manner is evidenced in the coagulation of lymph, where the apparently solid gelatinous clot is a tangled meshwork of fine filaments enclosing fluid.

The connective tissues of invertebrates are, on the whole, similar to those of the vertebrate; at the same time it must be admitted that there are not unimportant differences in chemical constitution, such as the absence of a substance yielding gelatin, and the absence for the most part of mucin, both of which are characteristic constituents of vertebrate connective tissue. On the other hand the anatomical characters of the elements, both cells and fibres, are in most cases sufficiently well marked to be recognizable.

In the sponges the bulk of the animal is made up of a jelly which, when examined under the microscope, is found to consist of large branched cells (fig. 12) connected together by their processes into a network. The meshes of this are occupied by clear intercellular substance within which the calcareous or horny matter which forms the skeleton is deposited.

When the development of the sponge is traced it is found that the first part of the tissue to be seen is the clear intermediate substance, and the skeletal spicules begin to appear in this before the cellular elements. These wander subsequently into it from one of the primary layers. There can be no doubt that this jelly-like tissue of the sponge represents a primitive form of connective tissue, although, so far as has at present been ascertained, no fibres are developed in it.

In the Coelenterata, as in the sponges, the connective tissue makes its first appearance in the form of a clear intermediate substance, which may be so small in amount as to be almost imperceptible, or so large in amount as to form the main bulk of the organism. In the former case, as in the developing sponge, there is an entire absence of both fibres and cells, whereas in the latter case both kinds of elements are found. The fibres are the more constant, and are of the elastic kind (fig. 13); they have for the most part a direction across the thickness of the tissue stretching from entoderm to ectoderm, branching and uniting with their neighbours to form the characteristic network which, enclosing watery fluid in its meshes, produces the jelly-like consistence of the tissue. Fibres of the white variety are also found as low down in the Metazoa as the Coelenterata. In some of the acraspedote Medusae they occur in the form of bundles of indistinct wavy fibres situated near the surface of the jelly, and in the Actiniae similar fibres are found forming membranes which bear a strong resemblance to some of the forms of membranous connective tissue of the Vertebrata (fig. 14). As before mentioned, in the lower forms of Coelenterata cells are entirely absent from their jelly-like connective tissue, but in the higher forms scattered cells (fig. 13) of indeterminate shape and position, but generally in the neighbourhood of the entoderm, begin to make their appearance. Some of these cells...