The sweat glands are distributed all over the cutaneous surface, but in some parts, such as the axilla, perineum, etc., they are both more abundant and larger than elsewhere. They are simple tubes extending in a more or less wavy manner through the skin, and ending in a rounded knot formed of several coils of the tube some way beneath the corium, where they are surrounded by a capillary plexus. The tube is lined with glandular epithelium, and its basement membrane is beset with longitudinally arranged smooth muscle fibres.

The secretion of sweat is always going on, though it does not constantly appear as a moisture on the surface, because the amount produced is only just equal to the amount of evaporation that takes place. In this case it is spoken of as insensible perspiration. Under certain circumstances the sweat collects on the surface and becomes obvious as liquid - sensible perspiration - which bathes the skin, being produced more rapidly than it can be evaporated. The quantity of secretion necessary to become sensible varies with the dryness and heat of the air, that is, with the rapidity with which evaporation takes place. It happens, however, that the very circumstances which tend to assist evaporation also promote the secretion of sweat. Indeed, the effect of great heat and dryness of the air is to increase the cutaneous secretion more rapidly than they increase the capability of evaporation, and therefore, when the air is hot and dry and evaporation is going on very actively, we have the secretion of sweat made sensible to our feelings. When dampness is associated with warmth of the atmosphere the sweat collects in large quantities on the skin, for the heat, as we shall see hereafter, aids the secretion, and the damp air impedes the evaporation.

The quantity of perspiration given off is considerable, but the wide limits within which the amount may vary render an attempt to express an average in numbers useless. The amount will depend on (i) the temperature of the air, (2) the quantity and quality of fluids imbibed, (3) the amount of heat generated in the body, and it therefore varies directly with muscular exercise. The amount that becomes perceptible to our senses depends on the impediments to evaporation that may exist, as well as on the amount of fluid produced.

The chemical composition of sweat varies with the amount secreted. When collected as a fluid by enclosing a part of the body in an impervious sac, it is found to have about two per cent, of solid matters, the greater quantity of which is made up of inorganic salts, sodium chloride being by far the most abundant. It also contains some epithelial debris, traces of neutral fats, and several volatile and fatty acids (butyric, proprionic, caproic), to which it owes its peculiar smell. It is said to contain urea, but this has been denied; and since all the nitrogenous income is accounted for in the urea excreted by the kidneys, it is probable that the cutaneous elimination of urea is minimal, if not exclusively pathological. It is also said to contain salts of ammonia, and it affords a means of escape to many drugs. In certain parts of the body, especially in some individuals, it contains a considerable amount of pigments, varying in color from brick-red to bluish-black, which need not be here further described.

The effect of nervous influence on the secretion of sweat is so associated with the nervous mechanisms of the cutaneous vessels that under ordinary circumstances it is a difficult matter to separate them. There can be no doubt, however, that a special nervous control is exerted over the production of sweat. This appears to be observable in some diseases, the poisons of which variously affect the two sets of nerves. Thus, in fever, we observe a dry red skin, accompanied by an increased supply of blood and a suppression of the secretion of the sweat glands; while in certain stages of acute rheumatism the exact opposite is seen, i. e., a profuse sweat drips from the pale, bloodless skin. It has, moreover, been recently shown that in some animals (cats) the stimulation of the sciatic nerve, causing contraction of the blood vessels, produces at the same time a copious secretion of sweat; and a warm atmosphere is said to have no effect on the secretion of a limb the nerve of which has been cut, although the warmth be so great as to make the rest of the animal's body sweat profusely.

The effect of drugs upon the cutaneous secretion is well known. There is a large group of medicines, especially pilocarpin, which produce an increased flow, while many others, notably atropin, have a contrary effect.