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.
Now, if we proceed at once to the application of the facts which have been laid down, and make a fragment of bone of an extinct animal the subject of investigation, it will be found that the bone-cells in Mammalia are tolerably uniform in size; and if we take 1⁄2000th of an inch as a standard, the bone-cells of birds fall below that standard; but the bone-cells of reptiles are much above either of the two preceding, while those of fishes are essentially different, both in size and shape, and are not likely to be mistaken for one or the other; so that the determination of a minute yet characteristic fragment of fishes’ bone is a task easily performed. If the portion of bone does not exhibit bone-cells, but presents either one or other of the characters indicated, the task of discrimination is equally easy. We have now the mammal, the bird, and the reptile to deal with. In consequence of the very great size of the cells and their canaliculi in the reptile, a portion of bone of one of these animals can readily be distinguished from that of a bird, or a mammal. The only difficulty lies between these two last; but, notwithstanding that on a cursory glance the bone of a bird appears very like that of a mammal, there are certain points in their minute structure in which they differ; and one is the difference in size of the bone-cells. To determine accurately, therefore, between the two, we must, if the section be a transverse one, also note the comparative sizes of the Haversian canals, and the tortuosity of their course; for the diameter of the canal bears a certain proportion to the size of the bone-cells, and after close examination the eye will readily detect differences.
Fig. 443.
1. A portion of the web of frog’s foot, spread out and slightly magnified to show distribution of blood-vessels; 2. Is a portion magnified 250 diameters to show the ovoid form of the blood discs in a vessel, beneath which hexagonal nucleated epithelium cells appear.
Arteries and Veins.—The circulation of the animal frame is maintained by arteries, veins, and capillaries. The arteries are elastic and contractile tubes; these convey the blood from the heart to the capillaries. The larger arteries are exceedingly elastic, but feebly contractile on account of the muscular tissue in their walls. The veins ramify throughout the body, are more numerous than the arteries, and of greater capacity. They usually accompany the arteries and correspond to them in structure, the larger veins possessing semi-lunar valves; these project into their interiors, and thus prevent the regurgitation of the blood. They have four coats, consisting of areolar tissue, yellow fibres combined with muscular fibres, and white fibrous tissue, two layers of yellow fibres arranged longitudinally, and a single layer of epithelial cells. Intermediate between the arteries and veins there are exceedingly fine tubes, termed capillaries, in which the arteries terminate, and from which the veins arise. These are composed of a fine homogeneous membrane, with here and there a nucleus. The capillary circulation of the blood is readily seen in the tail of the newt and the foot of the frog, [Fig. 443].
Fig. 444.—A network of capillaries.
A network of capillaries conveying blood to the lungs, and ramifying throughout the structure, is shown in [Fig. 444], and in [Plate XIX]., No. 6, the termination of a capillary of a blood-vessel in the fat-cells of the human body. [Plate VII]. illustrates the distribution of the arteries and veins to various parts of the animal body. This coloured plate, however, is designed to show the value of injected preparations in the delineation of animal structures. By thus artificially restoring the blood and distending the tissues, a much better idea is obtained of the relative condition of parts, the appearance presented by the erectile papillæ, &c. In the section of foot of mouse (No. 149), the bone is seen surrounded by its vascular supply, arterial and venous; in No. 150, the papillæ of the tongue are distended and seen erect; in No. 152, a vertical section of the fungi-form papillæ on the tongue of cat, with capillary loops passing into them, is demonstrated; in No. 151, the vertical section of brain of a rat, the vascular supply is shown; No. 153, the malpighian tufts (circular bodies) and arteries ramifying about the structure; in No. 154, the vertical section through the intestine of the rat, shows villi (arteries and veins) surmounted by epithelium, and supported on a layer of the mucous membrane; in No. 155, the vascular supply sent to the roots of the whisker of the nose of the mouse; in No. 157, a tangential section cut through the several textures, the sclerotic coat and retina of the eye of a cat is clearly made out although not highly magnified; again, in No. 156, the beautiful vascular arrangement of the internal gill of the tadpole could scarcely be so strikingly illustrated in any other way; while in the central, No. 158, the vascular system throughout the whole of the body of a fully developed tadpole, with the way in which the blood is carried from the remotest part of the tail to the heart, and sent to the gills, the brain, &c., it is quite unnecessary to enlarge upon. These are seen under a low power, but for the purpose of studying the basement membrane, together with the intimate association and termination of the nerves accompanying the arteries and veins, it is absolutely necessary to resort to a staining process, and cutting fine sections with the microtome. Small portions of a nerve may be cut off with fine scissors, teased out with needles, and a drop of acetic acid added to render the sheath more transparent; in a few seconds the connective tissue corpuscles will be brought into view. For the microscopical examination of nerve-fibrillæ take a small section from the leg of a frog, and tease it out in blood serum or white of egg. In size the fibrillæ vary, even in the same nerve, from the 1⁄12000th to the 1⁄1500th of an inch in diameter.
