Whatever importance may be attached to skin as an organ of absorption, its activity as a secreting organ is altogether beyond question. In the first place, the cuticular cells are constantly being thrown off under the influence of attrition, and the vascular surface of the true skin is actively engaged in furnishing material for the reproduction of new cells. Further, the sudoriparous and sebiparous glands discharge both fluid and solid substances, in the form of secretions, which, as will presently be seen, perform important functions. As has already been stated, the sudoriparous or sweat glands, which are extremely numerous, separate a fluid from the blood, a considerable quantity of which is constantly being given off as invisible vapour, or insensible perspiration; while under conditions which excite the circulation and still further increase the body temperature, a large quantity of liquid is poured out, carrying with it a considerable amount of solid material, in which is contained various salts of the blood, as well as a large number of acids, including butyric, formic, propionic, and an acid peculiar to the sweat, which is called sudoric. This fluid perspiration is distinguished as sensible perspiration. The reaction of the sweat is generally acid. From some parts, perspiration contains a larger proportion of fat than from others, in association with certain nitrogenous matters, urea among them. It is understood by physiologists that the nervous system has considerable influence in regulating the function of perspiration.

One very important purpose which the perspiration serves, is that of lowering the temperature of the body by evaporation, and another is the removal of a quantity of waste material, or excreta, from the system. Suppression of the function permits an accumulation of effete products in the body, to the detriment of the general health. One of the first effects which follow is the lowering of the temperature, and certain changes in the fibrin of the blood, which is bound to be increased in quantity, and the occurrence of glandular swellings. In the case of animals which have died after being covered with a coat of varnish, it has been found that crystals of the triple phosphate of lime and magnesia are deposited in the cellular tissue and the muscles. Some of the effete products, however, which accumulate under these conditions are eliminated by the kidneys, and these organs consequently become overtaxed and congested, leading to impairment of their function and consequent retention of the urea, which it is their special province to excrete.