Touch, the modification of the common sensibility of the body, especially seated in the skin, by which through physical contact we obtain an idea of resistance or weight, temperature, size, shape, smoothness or roughness, etc. It is most acute at the tips of the fingers, on the tongue, lips, portions of the mucous membrane, and the nipples, where the sensory papillae are the most numerous, each one receiving one or more nerve fibres. The nerve fibres appear to terminate in what has been called the tactile corpuscle in the interior of the papilla. All the afferent nerves of the general integument apparently minister to the sense of touch, by virtue of their connection with the seat of common sensation in the brain; those of the lower extremities are less concerned in conveying sensations than those of the upper, though they are far more efficient in exciting the reflex action of the spinal cord. The acuteness of touch differs in various parts of the' body, generally in proportion to their vascularity; the non-vascular parts, like the hair, nails, and teeth, have no sense of touch, while on the skin the nerves are spread in a minute network.
Its relative acuteness has been measured by Weber, by placing the legs of a pair of compasses on the skin, and approximating them until brought within the smallest distance at which they could be felt as distinct points, and with the following results: the point of the tongue, ½ line; palmar surface of third finger, 1 line; red surface of lips, 2 lines; tip of nose, 3 lines; edge of dorsum of tongue, 4 lines; skin of cheek, palm of hand, and end of great toe, 5 lines; back of hand, 8 to 14 lines; back of foot, 18 lines; over spine, and in middle of arm and thigh, 30 lines. There are considerable variations in this respect in different individuals. The feeling of tickling is most easily excited in parts having a feeble sense of touch, as the arm pits, sides below the ribs, palms, and soles, while the sensitive points of the fingers cannot thus be affected. This sense is exceedingly acute in the flying membrane of the bats and in the whiskers of the carnivora and rodents. It is combined with movement in the human hand, with its power of pronation and supination, opposability of the thumb, and great mobility of the fingers. The power of distinguishing the temperature of foreign bodies is restricted within certain rather narrow limits.
We can perceive the temperature of a substance which is moderately warm or cool; but if it be either above or below a certain limit, we fail to judge accurately of its temperature, and receive only a painful sensation. If the foreign body be excessively hot or cold, as in the case of boiling water or frozen mercury, the discrimination of temperature is lost altogether, and the painful sensation is the same in either instance. Thus the touch of a very cold conducting body may be said to burn the fingers, like that of a very hot one. Cold, by retarding the capillary circulation and by its direct sedative .influence, deadens the sense of touch; in like manner, pressure upon or disease of the nerve trunks, and various states of the brain receiving the sensory impressions, are accompanied by ob-tuseness of touch. Prominent among the causes acting on the nervous centres are the influence of toxic and anaesthetic agents, obstructed circulation, and chronic inflammations; on the other hand, irritation and acute inflammation in the course of the nerves, at their peripheral terminations, or in the centres, may be accompanied by hyperesthesia or excessive sensitiveness of the surface.
Subjective sensations, or those dependent on internal causes, are very common in the sense of touch; those of pleasure and pain, heat and cold, itching and creeping sensations, etc, are familiar examples. Touch may be greatly improved when the other senses are impaired or lost, partly from the greater attention given to the sensations, and the consequent increase of the power of discrimination. Instances of the education of this sense are very remarkable and well known in the blind. In the lower animals it is most acute in the hands, feet, and prehensile tail of monkeys; in the lips and tongue of herbivora; in the snout of the elephant, pig, tapir, and mole; in the flying membrane, ears, and nasal appendages of bats, which can perceive even the vibrations of air; in birds, in the under surface of the toes and their webs, and in the sensitive skin of the mandibles of the duck tribe and some waders; in the under surface of the toes in many lizards, in the extensile tongue of the chameleon and serpents, in the naked skin of batrachians, and in the thumbs of the males of the latter during the reproductive season; in the antennae and palpi of articulates, in the oral appendages of mollusks, and in the tentacles of radiates.