TOUCH is the sense through which we take cognizance of the palpable properties of bodies. It is used in two senses. In its extended acceptation, it iMplies, says Dr. Car penter, "our consciousness of all those sensory impressions which are neither visual, auditory, nor gustative: and it is therefore designated as the general sense. in contradistinction to those which are considered as special senses. In its limited applica tion, on the other bend, it is used to designate that modification of the general sensibil ity which is restricted to the tegumentary surface, or to some special portion of it, and serves to excite definite ideas in our minds respecting the form, size, number, contigurstion, weight, temperature, hardness, softness, etc., of objects brought within its cognizance."—Article "Touch," in Cyclopedia of _Anatomy and Physiology, vol. iv, p. 1163. In t4 article SENSIBILITY, we have briefly noticed touch in its general sense; and we shall here confine ourselves to the investigation of the sense of touch in its limited application, as exercised by the organs specially adapted for the reception of tactile impressions.
The special organs of touch are the papillae, which are figured and very briefly noticed in the article SKIN. These papillae arc more elevated and numerous on the palmar sur face of the (Dias of the fingers than on any other part of the skin (although they are still .larger on the tongue). They have an average length in man of y&Ty of an inch. '1 heir surface, alter the temovat of the epidermis, appears, from the investigations of 'Fuld and Bowman, to be composed of the basement membrane of the culis itself; and their interior is composed of fibrous tissue, vessels, and nerves, as is seen in the figures of the lingual papilla3 given in the article TASTE, SENSE OP. In each papilla is a small arte rial twig, which, catering at the Lase, suLdivides into capillary vessels, which form loops, IN hose convexity lies in the papillary summit. The vascularity of the papilhe is so great, that their presence and relative size may be determined simply by the depth of the color imparted to the skin by a good injection of its vessels. Bence, as a general rule, time vascularity of the integument is proportioned to its perfection as an organ of touch. With regard to the mode in which the nerves terminate, there is still considera ble doubt. According to Tcdd and Bowman, it is often impossible to detect any nerves at all within the papilla!, when such were plainly visible at their base; and they incline to the belief, that the nervous tubules, either entirely, or in a great measure, lose the white sub-:home when within the papilla.
In the lower animals, as in man, the papilla3 are especially developed in those parts rf cuter surface which are especially endowed with tactile sensibility. For the fol illustrations of this statement, drawn from comparative anatomy, we are indebted to Dr. Carpenter: "In the quadrurnana generally, both the hands and feet are thickly set with papillae, and in those which have a prehensile tail, the surface of this organ possesses them in abundance. In the carnivorous and herbivorous mammalis, whose extremities are furnished with claws, or encased in hoofs, we find the lips and the parts surrounding the nostrils to be the chief scat of tactile sensibility, and lobe copiously fur nished with papilla; this is especially the case with those which have the lips or nostrils pa longed 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 extremity of the snout. The only part of the skin of birds on which tactile papilla have Leon 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. In many lizards, a papillary structure is found on the under surface of the toes; and in the chameleon, it exists also on the integument of its prehensile tail. . . . . In serpents and chelonians (tortoises), no papillary apparatus has as yet been detected ; and in fishes and invertebrata, its presence has not been ascertained, although it would appear that certain parts, especially the tentacles around the mouth, are endowed with a high degree of tactile sensibility." Op. cit., p. 1166. It is probable that in all animals which have a soft fleshy tongue furnished with papillae, this organ is an instrument of tactile as well as the organ of taste. Besides the papillary appanfim, certain animals have special organs of touch, constructed on a totally differ ent and "consisting of a rod or filament, which is in itself insensible, but which is connected at its base with nervous fibers in such a manner that any motion or vibration communicated to it must be transmitted to them." The so-called "whiskers " of the cats and certain rodents, as the hare and rabbit, belong to this class; and it has been proved, experimentally, that if they lie cut off, the animal loses, to a great extent, its power of guiding its movements in the dark.