This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The Perspiration is the combined product of both sets of glands, but principally is derived from the sudoriparous. The sebaceous glands secrete nothing but oil, and they are not the exclusive source of the oil of the skin; for not only may particles of oil be detected in the interior of large sweat glands, but oil is secreted by the palms of the hands, which have no sebaceous glands. In connection with this it may be mentioned that the ceruminous glands, which secrete the wax of the ears, are simply large sweat glands. The most abundant solid constituent of the perspiration is common salt, chloride of sodium; and besides other salts, there is always a small amount of urea in it. Carbonic acid is likewise given off by the skin, although the amount of it is insignificant compared with what escapes by the lungs. The total amount of perspiration is obviously exceedingly variable; but in experiments made by enveloping the body in a water-tight bag with apertures for breathing, it has been found to be little short of two pounds per diem. One obvious use of the perspiration, and probably the principal purpose which it serves, is the protection of the body from too great heat, whether of external or internal origin, by the cooling effect of its evaporation from the surface, as will be further referred to (p. 149). Its flow, like that of saliva, is probably under the control of the nervous system; certainly it is no mere fitration dependent only on the amount of blood sent to the surface, for the skin may be hot and dry, particularly in fevers, and a cold sweat may burst out when the surface is pale from deficient flow of blood to the surface, as in the recovery from fainting.
Considering the function of the perspiration as a moderator of the temperature of the body, the results which are obtained by varnishing the bodies of Animals with an impermeable coating are not only interesting, but exceedingly difficult to explain. In such experiments, when the varnishing is complete, the temperature rapidly falls, and the animals die after periods varying from a few hours to days; the smaller animals, those in which the total surface bears the largest proportion to the body, being those which succumb soonest. Why the temperature falls is not understood; but whatever the cause of death, such experiments show the importance, in a sanitary point of view, of removing accidental accumulations of all kinds from the surface of the body.