What admirable wisdom is displayed in the motion of animals, suited to their various occasions! Reptiles, to which a clod, a plant, a tree, or a hole, will afford the means of supporting life, and which protracted privations of food do not materially affect, require no legs to make extensive excursions, but their vermicular motion is adequate to every essential purpose. Beasts, whose necessities call for a larger sphere, possess accordingly a swifter motion; and this is imparted in various degrees, suitable to their range for food, and adapted to accelerate their speed in escaping from their enemies.
In the motion of animals, from the largest Elephant to the smallest Mite, the whole body is exactly balanced. The head is not too heavy, nor too light for its kindred parts, nor they for it. The bowels hang not loose, nor are so placed as to over-balance, or upset the system; but well-braced, and accurately distributed to maintain an equipoise. The most active members also are admirably well fixed, in respect to the centre of gravity, being placed in the very point which best serves to support and convey the body. Every leg bears its share of the weight.
The mouths of animals are nicely adapted to their different habits of life. The Ox, the Deer, the Horse, and the Sheep, have full lips, rough tongues, broad cutting teeth, corrugated cartilaginous palates, which qualify them for browsing, either by gathering large mouthfuls where the grass is long, or biting close where it is short. In those which subsist on flesh, the teeth are sharp, and calculated to hold and divide their food. The bore of the gullet in animals is answerable to their necessities. In a Fox, which feeds on bones, it is very large. But in a Squirrel it is exceedingly small, which prevents him from disgorging his meat in his descending leaps: and it is equally contracted in Rats and Mice, which run along walls with their heads downward.
In all animals, the strength and size of their stomachs are proportioned to the nature and quantity of their food. Those whose aliment is more tender and nutritive, have them smaller, thinner, and weaker: whereas they are large and strong in those whose food is less nutritive, and whose bodies require greater supplies. Carnivorous beasts have their stomachs small and glandular, as flesh is the most nutritious. Those that derive their support from fruits and roots have them of a middle size: while on the contrary, Sheep and Oxen, which feed on grass, have the largest stomachs; and those which ruminate have in general no less than four; in Africa, where the plants are nutritive, some of this class have only two. Yet the Horse, Hare, and Rabbit, though graminivorous, have comparatively small stomachs. The Horse is made for labor, and both he and the Hare are constructed for quick and continued motion; for these the most easy respiration, also the freest action of the diaphragm, is requisite. But this could not be, did the stomach lie heavy and cumbersome upon it, as in Sheep and Oxen.
Another very remarkable circumstance is, that those animals which have teeth on both jaws, possess but one stomach; whereas most of those which have no upper teeth, or no teeth at all, have three stomachs. For the meat which is first chewed, is easily digested; but that which it swallowed whole, requires a stronger concoctive power.
The Horse eats night and day, slowly, but almost continually: whereas the Ox eats quickly, and takes, in a short time, all the food nature requires; and then lies down to ruminate. This difference arises from the different conformation of these animals. The Ox, of whose stomachs the first two form but one capacious bag, can, at the same time, receive grass into both of them, without inconvenience, which he afterwards ruminates and digests at leisure. The Horse, whose stomach is small, and can receive but a small quantity of grass, is filled successively in proportion as he digests it; and it passes into the intestines, where is performed the principal decomposition of the food. Chewing the cud is but a vomiting without straining, occasioned by a re-action of the first stomach on the food which it contains. The Ox fills the first two stomachs, the paunch, and the bag, which is but a portion of the paunch. This membrane acts with force on the grass it contains; it is chewed but a little, and its quantity is greatly increased by fermentation. Were the food liquid, this force of contraction would occasion it to pass by the third stomach, which only communicates with the other by a narrow conveyance, and cannot admit such dry food, or, at least, can only admit the moistened parts. The food must, therefore, necessarily pass up again into the œsophagus, the orifice of which is larger than the orifice of the conduit, and the animal again chews and macerates it, and moistens it afresh with its saliva: he reduces it to a paste, sufficiently liquid to enter into this conduit, through which it passes into the third stomach, where it is again macerated before it goes into the fourth; and it is in this last receptacle that the decomposition of the hay is finished, which is reduced to a perfect mucilage. What chiefly confirms this explication is, that as long as the animals suck, and are fed with milk and other liquid aliments, they do not chew the cud; and that they chew the cud much more in winter, when they are fed with dry food, than in summer, when they eat tender grass.
All the parts of the same animal are adopted to each other. So, for instance, the length of the neck is always proportioned to that of the legs. Though the Elephant has a short neck, because the weight of his head and teeth would otherwise have been insupportable; but, then, he is provided with a trunk, which abundantly supplies the defect. In other beasts, the neck is always commensurate to the legs; so that they which have long legs have necks proportioned; and so vice versa, as is observable in Lizards of all kinds, even from the Eft to the Crocodile. And creatures that have no legs, as they want no necks, so they have none. This equality between the length of the neck and legs is peculiarly seen in beasts that feed on grass, in which these are very nearly equal; because the neck must necessarily have some advantage, for it cannot hang perpendicularly, but must incline a little.
These creatures, while feeding, bend their heads downward for a considerable time, which would be very laborious and painful to the muscles, were it not for a very stiff, strong cartilage, placed on each side of the neck, capable of stretching and shrinking again as need requires, which butchers call pax-wax. The one end of this is attached to the head, and the next vertebræ of the neck; and the other is knit to the middle vertebræ of the back: and by the assistance of this, animals are able to hold the head in that inclining posture all day long. The head being placed at the end of a long lever, in a direction nearly perpendicular to the joints of the neck, would be in constant danger of dislocation from its own weight, had not such a substance been added, which, by its great strength and toughness, retains the parts together, while, by its pliancy, it offers no obstruction to the free motion of the neck and head.
The members of animals are exactly adapted to their manner of living. A Swine, whose natural food is chiefly the roots of plants, is provided with a snout; long, that he may thrust it to a convenient depth in the ground without injuring his eyes; and strong and suitably formed, for rooting and turning up the earth: therefore the retiring under-jaw works after the manner of a plough-share, and makes its way to the food: and besides, his scent is extremely acute in discovering such roots as are fit for him. Hence in Italy, the usual way of finding truffles, or subterraneous mushrooms, is by tying a cord to the hind leg of a pig, and driving him into pastures. They who attend then mark where he stops and begins to root, and digging there, are sure to find a truffle. So in pastures where there are earth-nuts, though their roots are deep in the ground, and the leaves are quite gone, the Swine will find them by their scent, and root only in the places where they grow.[159]
In some animals the head is long, in order to give room for the olfactory nerves, as in Dogs, which hunt by scent. In others, it is short, as in the Lion, to give him the greater strength. In beasts of prey, as Lions, Tigers, Wolves, they have the trumpet-part or concavity of the ear standing forward, to meet the sound of the animals before them, which they pursue or watch. The ears of animals of flight are turned backward, to apprize them of the approach of the pursuing enemy, lest he should assail them unseen. Beasts of prey have their feet armed with claws, which some can sheath and unsheath at pleasure. The Babyrouessa, or Indian Stag, a species of Wild-Boar, found in the East Indies, has two bent teeth more than half a yard long, growing upward, and, which is very singular, from the upper jaw. These instruments are not wanted for defence, that service being provided for by two tusks issuing from the under jaw, and resembling those of the common Boar: nor does the animal thus use them. They might seem therefore both superfluous and cumbersome: however, they have their utility; for this animal sleeps standing, and, in order to support its head, hooks its upper tusks upon the branches of trees.