Touch — Chambers's Encyclopaedia

Touch, the tactile sense, the least specialised of the senses. The sense-organs of the various groups of animals are discussed in the articles on the various groups. In the nervous system of man the organs of touch are (1) central end-organs (see BRAIN); (2) conducting nerve-fibres (see NERVOUS SYSTEM); and peripheral end-organs. Nerves end in the skin in two ways. In the simplest mode of termination they form a plexus in the dermis, pass outwards from this, lose their medulla, divide into fibrils, and are lost to view in or between the cells of the epidermis. Nerves end in this way in all parts of the skin, but the relative number of such terminations in different parts of the skin cannot be determined by present methods of research; they may be easily seen in the cornea. In the more complex mode of termination the nerves, retaining their medulla, end in the dermis in special structures of modified neurilemma and other cellular elements. These endings are of three sorts. (a) The end bulbs are round bodies $30 \mu$ to $100 \mu$ in diameter ( $\mu$ is the diameter of a red blood-corpuscle), and of limited distribution, found in the conjunctiva, the lips, also in mucous membranes—e.g. on the tongue, the palate and elsewhere, and, slightly modified, in the sensitive parts of the genital organs; (b) Touch corpuscles are oval bodies $60 \mu$ to $100 \mu$ in long diameter, also of limited distribution, but most numerous in the papillæ of the dermis of the palmar surface of the hand, less numerous in the nipple of the breast, very scarce on under surface of the forearm, absent from the greater part of the surface of the body. It is estimated that on the tip of the forefinger there are 100 touch corpuscles for two square millimetres. (c) Pacinian corpuscles are larger bodies, 1 millimetre and more in length, like end bulbs, found in the subcutaneous tissue. There are about 600 in the under surface of each hand, and a few may occur in the back of the hand; they are numerous at the joints, are found also on some periosteal nerves, and on the sympathetic nerves of the abdomen. From their distribution it is evident that they may or may not be essential to cutaneous sensibility. Indeed it seems that the touch corpuscles also are not essential to touch, for they are completely absent in certain parts of the body—e.g. the cornea—which are yet sensitive to pressure. Further, if those points of skin which are found by experiment to be peculiarly sensitive are cut out and examined, special nerve endings are not always to be found. Still the fact that as a general rule such endings are most frequent in areas of great sensibility compels the belief that they have some relation to that sensibility. The varying degrees of sensibility in the touch-organs are described at SENSATION, where also reasons are given for holding that the muscular sense is distinct from the tactile sense (see also SKIN, and TASTE). The functions of touch as a means of perception is dealt with at PSYCHOLOGY.—Touching for the king's evil is discussed at SCROFULA.