178 COMPARATIVE ANATOMY ous appendages in the shape of combs, wattles, bristly hairs, and caruncles ; in the swimmers and waders the bills are very sensitive ; in the snipe and woodcock, for example, the fifth pair of nerves is freely distributed to the long bill, making it an exceedingly delicate organ for the explorations necessary in their search for food. In mammals, most of which are cov- ered with hair or thick hides, the sense of touch is most acute in the neighborhood of the mouth ; the lips of the horse are very sensi- tive ; those of the carnivora and rodents are provided with long whiskers, into which nerves enter, forming exquisite organs of touch ; the complicated appendages to the nose of bats, and also their delicate wings, are very sensitive tactile or'gans. With the exception of the tongue, which is sensitive in all, there is no approach to the perfect development of touch in the human subject until we reach the quad- rumana, in which the hand is the chief tactile organ. The fore foot of the ruminant, ending in a hoof, serves only for support and locomo- tion ; in the feline tribes it is in addition pre- hensile, and to a certain extent tactile ; in the monkey the palmar cuticle is thin and sensitive, though the hand is still used in locomotion ; in man the sense of touch is very acute in the tips of the fingers, though all parts of the skin are more or less sensitive. The sense of touch may be educated to a degree almost substitu- tive for the sense of sight; what the bat's wing is naturally, the blind man's fingers may become by education. The sense of taste, even in man, must be considered as principally a modification of the sense of touch ; as it is impossible to draw the line between the mu- cous membrane and the skin, which are also composed of the same elements, and as the sensory branches of the same fifth nerve are distributed to the face, lips, mouth, and tongue, it is naturally inferred that a great part of what is called taste is really touch, applied to a special locality for special purposes, as in the wing of the bat and the tips of the fingers ; and some are of opinion that the sense of smell supplies all of taste which is not derived from touch. The papilla) of the tongue are more highly developed than those of the skin, and are scattered over its surface in all parts likely to come in contact with matters in a state of solution. The tongue of fishes can rarely be an organ of taste, being of a hard consistence, and frequently armed with teeth ; their voracity and the element in which they live are also against their possessing in the tongue anything more than an organ of touch. In all the vertebrates the tongue answers for other purposes than that of taste ; in the toad it is darted with great quickness and usually un- erring aim against the insects upon which it feeds ; in the chameleon it is capable of con- siderable protrusion, enlarged at the end, and covered with a glutinous secretion which en- traps its insect prey when projected against them. In birds it is chiefly an organ of pre- hension, rarely possessing papillae, and gener- ally sheathed in front with horn ; the tongue of the woodpecker is a kind of barbed spear, pro- truded by a muscle and long bony tendon from the top of the head, for the transfixion of grubs and insects. In mammals generally the tongue is only an organ of taste ; but in the giraffe it forms a long flexible process, which can take up small bodies almost like a hand, and by which the leaves of high trees are stripped off into the mouth ; the tongue of the ant-eater is covered with a viscid secretion, to which the ants adhere in great numbers ; in the cats it is beset with strong spines for pur- poses of prehension and of tearing the small fibres of the flesh on which they feed ; in man it is also of the first importance in the articula- tion of words. The sense of smell doubtless exists in many invertebrates, at the entrance of the respiratory passages. In fishes the organ of smell consists of a sac containing a folded membrane largely supplied with nerves, and not communicating with the mouth or pharynx ; that this sense is acute in the cod and other deep swimmers is probable from the fact, well known to fishermen, that these may be attracted from great distances by peculiar kinds of bait, while the top-water and mid- water species select their food by the sight, also often showing what seems to be a decided sense of taste ; the sharks, from their large olfactory nervous centres, are believed to have an acute sense of smell. In the higher amphibia the nostrils become reptilian, being partly osseous and opening into the anterior part of the mouth ; in saurians and chelonians the nostrils open posteriorly into the pharynx. In birds the external nostrils vary much in size, shape, and situation, but are generally freely open; their sense of smell is not so acute as had been supposed from some of the habits of the vul- tures, as Audubon has satisfactorily shown that these birds detect their carrion prey from great heights by the sense of sight and not by that of smell. In mammals the olfactory lobes are much more fully developed than in man, as also are the nasal cavities in which the sensitive membrane is spread out, and the ex- ternal nose; the nostrils are valvular in the cetaceans, beavers, seals, &c. ; in the hog the nose is enlarged into a gristly ring, in the tapir into a short proboscis, and in the elephant into a long prehensile trunk. The sense of smell is most acute in the carnivora and rumi- nants, as would be expected from the extent and convolutions of the turbinated bones. From the communication of the nose with the pharynx, mammals are able to breathe with the mouth shut. An organ of hearing may be detected in many invertebrates, as in the lob- ster, insects, and cephalopods. Its simplest form in the vertebrates is in fishes, where it is a sac filled with fluid, in which are distributed the nervous filaments on small particles of cal- careous matter; in connection with this sac are two or three semicircular canals opening
