OCR Text |
Show ANIMALIA VERTEBRATA. t~e sides of the cavity of the trunk, they are articulated at on~ extremity with the vertebrre, and most generally at the other with the sternum; sometimes, however, they do not encircle the trunk, and there are genera in which they are hardly visible. There are never more than two pairs of members, but some-times one or the other is wanting, or even both. Their forms vary according to the movements they have to execute. The superior members are converted into hands, feet, wings or fins, and the inferior into feet or fins. The blood is always red, and appears to be so composed as to sustain a peculiar energy of sentiment and muscular strength, but in various degrees, corresponding to their quality of respiration: from which originates the subdivision of the vertebrata into four classes. The ext~rnal senses are always five in number, and reside in two eyes, two ears, two nostrils, the teguments of the tongue, and those of the body, general1y. In some species, however, tHe eyes are obliterated. The nerv¢s reach the medulla through the foramina of the vertebrre or \hose of the cranium; they all seem to unite with this medulla, which, after crossing its filaments, spreads out to form the various lobes of which the brain is composed, and terminates in the two medullary arches called hemispheres, whose volume is in proportion to the extent of the intelligence. There are always two jaws, the greatest motion is in the lower one, which rises and falls; the upper jaw is sometimes immovable. Both of these are almost always armed with teeth, excrescencb of a peculiar nature, which in their chemi· cal composition are very similar to that of bone, but which grow by .layers an~ransudation; one whole class, however, that of buds, hast e jaws invested with horn, and the genus Testudo, in the cia of reptiles,, is in the same case. The intestinal can: I traverses the body from the mouth to the anus, experienci~g various enlargements and contractions, having appendages a~d receiving solvent fluids, one of which, the saliva, is discharged into the mouth. The others, which are poured into the intestine only, have various names: the ANIMAL! A VERTEBRATA. 35 two principal ones are the juices of the gland called the pan· creas, and bile, a product of another very large gland named the liver. While the digested aliment is traversing its canal, that portion of it which is fitted for nutrition, called the chyle, is ab· sorbed by particular vessels styled lacteals, and carried into the veins; the residue of the nourishment of the parts is also carried into the veins by vessels analogous to these lacteals, and forming with them one same system called the lymphatic system. ~he blood which has served to nourish the parts, and which has JUSt been renewed by the chyle and lymph, is returned to the heart by the veins-but this blood is obliged, either wholly or in part, to pass into the organ of respiration, in order to regain its arterial nature, previous to being again sent through the system by the arteries. In the three first classes this respiratory organ consists of lungs, that is, a collection of cells into which air penetrates. In fish only, and in some reptiles, while young, it consists of branchire or a series of laminre, between which water passes. In all the vertebrata, the blood which furnishes the liver with the materials of the bile is venous blood, which has cir· culated . partly in the parietes of the intestines, and partly in a pecu~mr body called the s~n, and which, after being umted In a trunk called the vena porta, is again subdivided at the liver. All these animals have a particular secretion· the urine wh 1' c h I.S produced in two large glands, attached ' to the sides' of the SJ>ine of the back, called ~-the liquid they secrete IS most commonly poured into a reservoir, named bladder ~exes are separate, and the female has always one or two ovaries, from which the eggs are detached at the instant of conception. The male fecundities them with the seminal fluid, but the mode varies greatly. In most of the genera of ~he three first classes, it requires an intromission of the fluid • lll some repti.l es, and in most of the fishes, it takes place after' the exit of the egg. |