In the lower animals, as in man, it may be observed that the papillary structure is espe cially developed on those parts of the tegu mentary surface which are especially endowed with tactile sensibility, and the impressions received through which are of important use in finding the movements of the instruments of locomotion. Thus in the quadrumana generally, both the hands and feet are thickly set with papillm ; and in those which have a prehensile tail, the surface of this organ pos sesses them in abundance. In the carnivorous and herbivorous mammalia, whose extremi ties are furnished with claws or encased in hoofs, we find the lips and the parts sur rounding the nostrils to be the chief seat of tactile sensibility, • and to be copiously furnished with papillm ; this is especially the case with those which have the lips or nos trils prolonged into a snout or proboscis, as in the pig, the rhinoceros, the tapir, and the elephant. In the mole, too, the papillary structure is remarkably developed at the ex tremity of the snout. The only part of the skin of birds on which tactile papillm have been discovered is on the under surface of the toes, and on the web of the palmipedes, where they obviously receive impressions, which guide the prehensile and other move ments of the feet. It is probable, however, that the very sensitive skin which covers the greater part of the mandibles in the duck tribe, is furnished with papillm ; the tactile impressions received through this part, when the bill is plunged into mud &c., being the chief means by which the presence of food is discovered. In many lizards a papillary struc ture is found on the under surface of the toes ; and in the chameleon it exists, also, on the integuments of its prehensile tail. In the soft skinned batrachia, an imperfect papillary structure is more extensively diffused over the surface ; but on the thumb of the male frog, and probably on that of other batrachia, there is an extraordinary development of pa pillary tissue at the season of sexual excite ment, large papillm being formed all over it. This organisation is obviously connected with the extraordinary prehensile propensity which is then displayed by the animal, and which enables him to keep the female in his grasp during the whole period of the discharge of the ova, in a manner which no voluntary effort could effect. In serpents and chelonians, no papillary apparatus has yet been detected ; and in fishes and invertebrata its presence has not been ascertained, although it would ap pear that certain parts, especially the tenta cula around the oral orifice, are endowed with a high degree of tactile sensibility.* But it is not only on the tegumentary face of the exterior of the body, that tactile sensibility is particularly acute ; nor is the pillary apparatus restricted to it alone. In the tongue of man, we find the sense of touch remarkably developed, especially at its tip ; and of the papillm with which its surface is beset, it is probable that some are the ments of tactile sensibility ; whilst others minister to the gustative sense. (See TASTE.) So it is probable that in all animals which have a soft fleshy tongue, furnished with and serving as the organ of taste, this organ is the instrument of tactilesensibility also.
Besides the papillary apparatus, however, we find certain animals endowed with special organs of touch, which are constructed upon a very different plan ; consisting of a rod or filament, which is in itself insensible, but which is connected at its base with nervous fibres, in such a manner that any motion or vibration communicated to it must be transmitted to them. Such are the long stiff hairs which are known as the " whiskers" of the feline tribe, and which are so particularly large in the seal ; these are also highly deve loped in many of the rodentia, such as the hare and rabbit ; and it has been proved by experiment, that if they be cut off, the animal loses in great degree its power of guiding its movements in the dark. Thus Mr. Broughton
found that whilst a kitten whose whiskers were entire was capable of threading its way blindfold out of a labyrinth in which it was designedly placed, it was totally unable to do so when its whiskers were cut off; for it then struck its head repeatedly against the sides, ran against all the corners, and tumbled over steps placed in its way, instead of avoiding them as it did prior to the loss of its whis kers.* In animals whose hairs are their im portant instruments of touch, a true papilla, copiously furnished with nerves and blood vessels, is found to project into the bulb of each hair. The jointed appendages to the head, known as antenna and palpi, which are possessed by most articulated animals, are un doubtedly instruments of touch, whatever sensory impressions they may receive in addi tion. The antenna, when prolonged, serve to guide the movements of the animal ; the impressions which they receive at their ex tremities being communicated to the nerves at their base, just as a blind man judges by the stick held in his hand of the proximity of obstacles to his progress.t On the other hand, the palpi appear to minister to the cognisance of objects brought into the neighbourhood of the mouth, and to have for their chief office to guide in the selection of food. But while there are many facts which seem to indicate that the antenna; minister to the sense of hearing, there are others which appear to point to the palpi as the special instruments of that of smell.
Conditions of the Sense of Touch. — The sense of touch, strictly so called, is exercised under conditions essentially the same as those through which the general sensibility of the body is affected. It is requisite, in the first place, that the bodies, of whose presenceit takes cognisance, should be brought into actual contact with the tactile surface ; the only exception being in regard to the tempe rature of objects, the influence of which may be communicated by radiations from a distance. This difference, however, does not indicate any fundamental diversity such as some have imagined to exist, between the sense of tem perature and that of resistance ; for, in each case, that which is perceived by the mind is the impression made upon the sensory organ ; and the change in this is excited in the one case by pressure, and in the other by heat or cold. The same organ appears to be adapted to take cognisance of both classes of impres sions ; a feeling of one kind being excited, when its condition is altered by pressure; and a feeling of a different kind, when its tempe rature has undergone a change under the in fluence of calorific radiations. And the dif ference between these classes of sensations is not greater than that which exists among others, — whether of a general or a special kind, — which we know to be transmitted by the same nerve-fibres. Yet it would seem that, whilst there is no sufficient reason for supposing impressions of contact and of tem perature to be transmitted by different nerve fibres, we must admit that some of these fibres, either in virtue of their own constitu tion, of the locality of their central termina tion, or of the apparatus with which they are furnished at their peripheral origin, are en dowed with a greater readiness to receive and transmit one or the other class of impressions. For we find that the parts whose tactile sen sibility is the most discriminating, are not always those by which the keenest apprecia tion of changes of temperature is obtained. And, in like manner, the occasional occur rence of cases of paralysis, in which there is a total loss of one kind of sensation, whilst the other is preserved, or in which one is di minished beyond all proportion to the other, seems to show that such a change may take place in the nerve-fibres, as may indispose them to the reception and transmission of one class of impressions, whilst they are still capable of actively responding to the other.