ORGANS OF SENSE.
Few subjects in comparative anatomy and physiology have given rise to more various and contradictory opinions, than the organs of sense in some classes of ani mals. Much misunderstanding on this point has clearly arisen from the incon siderate and unconditional application of inferences drawn from the human sub ject to animals. Thus it has been sup posed that those which possess a tongue must have it for the purpose of tasting, and that the sense of smell must be wanting, where we are unable to ascer tain the existence of a nose. Observa tion and reflection will soon convince us, that the tongue, in many cases (in the ant-eaters among the mammalia, and almost universally in birds,) cannot, from its substance and mechanism, be consider ed as an organ of taste ; but must be merely subservient to the ingestion and deglutition of the food. Again, in several animals, particularly among insects, an acute sense of smell seems to exist, al though no part can be pointed out in the head, which analogy would justify us in describing as a nose.
However universally animals may pos sess that feeling which makes them sensible to the impressions of warmth and cold, very few possess, like the human subject, organs exclusively ap propriated to the sense of touch, and ex pressly constructed for the purpose of feeling, examining, and exploring the qualities of external objects.
This sense appears, according to our present state of knowledge, to exist only in three classes of the animal kingdom ; siz. in most of the mammalia, in a few birds, and probably in The structure of the organ of touch is the most perfect, and similar to that of the human subject in the quadrumana. The ends of their fingers, 'particularly of the posterior extremities, are coveredwith as soft and delicately organized a skin as that which belongs to the corresponding parts of man.
Several of the digitata are probably pro vided with this sense ; the organization of the under surface of the front toes of the raccoon (ursus lotor,) and the use which the animal makes of those parts, prove this assertion.
It is not so clear that we are author ised in considering the snout, of the mole and pig, not to mention the tongue of thy solidungula and bisulca, or the snout of these and other animals, as true organs of touch, according to the explanation above laid down.
Much less can we suppose the long bristly hairs,which constitute the whiskers of the cat-kind, and other mammalia, to be organs of touch, in the sense we are now considering, although they may be serviceable, when they come in contact with any object, in warning, and making the animal attentive. Bats have been supposed to possess a peculiar power of perceiving external objects without com ing actually into contact with then. In their rapid and irregular flight amidst various surrounding bodies, they never fly against them ; yet it does not seem that the senses of hearing, seeing, or smelling, serve them on these occasions ; for thPy avoid any obstacles with equal certainty when the ear, eye, and nose, are closed. Hence naturalists have ascribed sixth sense to these animals. It is pro bably analogous to that of touch. The nerves of the wing are large and nume rous, and distributed in a minute plexus between the integuments. The impulse of the air against this part may possibly be so modified by the objects near which the animal passes, as to indicate their situa tion and nature.
In geese and ducks the bill is covered with a very sensible skin, supplied with an abundance of nerves from all the three branches of the fifth pair. This appara tus enables them to feel about for their food in mud, where they can neither see nor smell it. None of the amphibia or fishes seem to possess the sense of touch, according to the acceptation stated a bove.
All the observations and investigations of the structure of the antennae, those peculiar organs which exist universally in the more perfect insects, and of the use which these animals generally apply them to, lead us inevitably to the con clusion, that they really are proper or gans of touch, by which the animal ex amines and explores surrounding ob jects. Such organs are particularly ne cessary to insects, on account of the in sensibility of their external coat, which is generally of a horny consistence, and also from their eyes being destitute in most in stances of the power of motion.