Touching. The whole surface of the skin is the organ of this sense, which gives us information concerning more properties of extraneous bodies than any other of our senses. The sight, hearing, smell, and taste, are confined to circum scribed limits; while the touch, distributed on the whole surface, effectually pro vides for our preservation, by giving us notice of the approach of external bodies, and informing us of their properties. Every thing that is not sound, light, odour, or savour, is appreciated by this sense, as the temperature, consistence, dryness, or moisture, magnitude, dis tance, &c. of objects. It corrects the er rors of sight and the other senses, of which it may indeed he justly termed the regulator; and, above all others, furnishes us with the most certain and exact ideas. Exercise and cultivation bring it to a wonderful degree of perfection, so that in blind persons it may almost be said to supply the loss of sight ; in some such in stances 'different colours and their various shades have been distinguished by its assistance.
Although the tangible qualities of bo dies can be perceived by every part of the cutaneous organ, it possesses in some situations a more delicate structure, con sisting of fine pointed prominences, called papilla, endued with greater sensibility and vascularity, and thereby constituting in a more especial manner organs of touch. This is the case with the hands. The number of bones that compose these organs render them susceptible of infi nitely varied motions, and enable them to explore with accuracy the surfaces of the most unequal bodies. The soft ends of the fingers receive large nerves and arteries, and possess very numerous and prominent papillx. This finely organized skin is formed into a gently convex pro tuberance, by an accumulation of a soft fat under it, and it is defended and sup ported by the nail ; and accordingly these pulpy extremities of the fingers are en dowed with the most refined sense of touch. The lips and the glans penis have a similar structure, and receive from this organization a very exquisite sensibility, Avhich is a modification of touch.
As the cutaneous papilla are covered by the epidermis, it follows that the very superficies of the body is insensible. The cuticle and its appendages, the nails and hhir, have neither vessels nor nerves, and possess no powers of life or growth in themselves. It forms a medium, mode rating the impressions which would be too vivid from an absolute contact of sub stances ; when preternaturally thickened, as in the hand of the labourer, it obstructs sensation ; and when entirely removed, as by a blister, the contact of bodies ex cites pain. It is also important in pre
venting the action which the atmosphere would otherwise exert in drying the sur face of the body ; when removed in the dead subject the skin immediately be comes horny, and the same effect extends more or less to the subjacent parts ; in the living body its removal is followed by incrustation or scabbing.
The skin is also to be considered as an organ of secretion, and perhaps of ab sorption. Under the former head we view it as the means of separating and expelling from the body extraneous mat ters, whose retention would be injurious to the system. This may be proved by eruptive disorders, by the odours of Musk, garlic, &c. which affect the per spiration ; by the phenomena of sweating, &c. by the injurious effects on the sys tem at large, which a suppression of the cutaneous secretion causes, and the relief experienced by sudorific remedies in various cases.
The secretion from the skin has been divided into the sensible and insensible. An abundant vapour continually exhales from the whole surface, and has the name of insensible transpiration, or perspira tion, when it is invisible to the naked eye, and passes off in the state of gas ; but it is called sweat, when, becoming more copious, it flows in form of a liquid. The innumerable arteries, which pervade every part of the skin, are the source of these secretions ; and their exhaling ori fices are supposed to penetrate the cuti cle in a state far too minute for any means of research that we possess. if the naked body be placed against a white wall in the sun during the summer season, a sha dow produced by the cutaneous exhala tion may be perceived ; and the following is also a decisive experiment to the same point. Apply the end of the finger near a glass or finely polished metallic instru ment, and the body will soon have its surface tarnished by a vapour, which is dissipated when the finger is removed.
A great resemblance exists between the cutaneous and pulmonary secretions ; both arc simple arterial exhalations, and the mucous membrane of the aerial pas sages is a continuation of the skin. The two secretions counterbalance each other; and the connection between them is evi denced by the remarkable distress of breathing attendant on extensive burns. There is a similar connection with the mucous exhalation of the intestinal canal ; and a still more remarkable one with the kidneys.