THE tissues of which animals consist, like those of plants, are primarily derived from cells; in fact the essential part of the egg or óvum, from which all perfect animals originate consists at first only of a simple cell, with its nucleus and nucleolus.
The animal cell-wall differs from that of the vegetable cell in its softness and delicacy—also in its chemical composition,—the former consisting of albúminous (albúmen, white of egg) matter, while the latter is composed of cellular or vegetable-cell substance.
There is also a striking difference between vegetable and animal tissues, in the circumstance that, while the former retain their cellular condition to a very great extent, the cells of the latter are frequently so altered by compression and fusion together, or are obscured by the great development of the cell-contents, that the cell-form is obliterated, or can only be discovered by the application of chemical reagents; and in many instances, the relation of the tissues to the cell can only be discovered by tracing the growth or development of the latter from its earliest stages. Hence the examination of the elements and tissues of animals is not well adapted for those who are unpractised in the use of the microscope; and in treating of them, we shall simply notice a few which are most easily examined, beginning with those found in animals belonging to the subkingdom Vertebráta (ver´tebra, a spine-bone).
Mammália.—The animals belonging to this class suckle their young; and their blood-vessels contain red blood.
Blood.—This blood consists of a yellowish liquid, in which very numerous red blood-corpuscles or globules ([Pl. IX.] fig. 1) are suspended, and to which the red colour is owing. The blood-corpuscles are not globular, but discoidal, i. e. they are circular and flattened, the sides being slightly sunk in. Their form is best seen as they roll over on a slide, after the application of a glass cover. The coloured corpuscles are cells; they appear yellowish red under the microscope, the deep red colour of the blood depending upon the large number of them seen at once and crowded together. It need scarcely be stated that a drop of blood may easily be obtained, by puncturing the wrist with a clean needle. The blood is contained in the blood-vessels. These consist of the ar´teries, which convey the blood from the heart; the veins, which return it to the heart; and a very fine set of intermediate vessels, called the cap´illaries. If a little water be added to a drop of blood on a slide, colourless corpuscles, rather larger than the coloured disks, will be seen scattered among the latter. These are the colourless or lymph-corpuscles of the blood. They are truly spherical, and granular on the surface.
Bone.—In examining a transverse section of a bone, one or several very large cavities will be seen with the naked eye in the centre of the section; these contain the marrow, or medulla. In the long bones, the medullary cavity is single, and runs longitudinally down the bone; whilst in the flat bones the cavities are numerous, forming cancelli. Under the microscope, thin transverse sections of bone exhibit oval or rounded holes, or foramina ([Pl. IX.] fig. 15), which are sections of canals conveying blood-vessels through the bone; these are the Haver´sian canals. Around the sections of these canals are seen numerous concentric rings, indicating layers or lamellæ of bony matter. The substance of bone presents numerous black, somewhat elongated bodies (fig. 15 a), called the lacúnæ (lacúna, a little hollow) or bone-corpuscles, which are however hollow, therefore not truly corpuscles, as they were formerly considered. Between the adjacent lacunæ run numerous fine, dark, branched lines, consisting of very minute canals, or canalic´uli. If the section of bone be viewed by reflected light, the lacunæ and canaliculi will appear white. In the dried bone they contain air.
The structure of bone is best seen when viewed as a transparent object in the dry state; for when the section is immersed in liquid, the lacunæ and canaliculi become filled up.
The size and form of the bone-corpuscles and canaliculi vary in different animals, so much so that the Class or Order to which an animal belongs may be determined by reference to these particulars.
Bone consists of earthy salts deposited in a finely granular form throughout the substance of cartilage. By soaking a piece of bone in vinegar or other dilute acid, the earthy salts will be dissolved, the soft cartilage being left. But the structure of cartilage may be best observed by making thin sections of the gristle covering the ends of bones. It exhibits a bluish-white basis ([Pl. IX.] fig. 16), in which are imbedded numerous cells or cartilage-corpuscles, often undergoing cell-division. In some kinds of cartilage, the basis is composed of fibres.
Muscle.—On examining a piece of the red flesh of an animal under a low power, the mass will exhibit a number of coarse, parallel, longitudinal, dark lines ([Pl. IX.] fig. 20), the substance between these lines being marked with cross or transverse striæ, or lines, and with fine longitudinal lines. The coarse longitudinal lines indicate the intervals between bundles of slender fibres, or fibril´læ, of which muscle consists. The fibrillæ (fig. 20 b, c) are very difficult to separate; but when perfectly separated, they are seen to be exceedingly slender, and to consist of alternately light and dark portions in regular series. When the fibrillæ of the bundles are in close apposition, as in the natural muscle, the dark portions, being in the same lines, by their coincidence form the transverse striæ. The bundles into which they are combined are surrounded by a delicate skin or membrane, with a little cellular tissue.