Nature and Amount of Perspiration

The quantity of matter which leaves the human body by way of the skin is very considerable. Thus it has been estimated that while 0.5 gramme passes away through the lungs per minute, as much as 0.8 gramme passes through the skin. The amount, however, varies extremely; it has been calculated, from data gained by enclosing the arm in a caoutchouc bag, that the total amount of perspiration from the whole body in twenty-four hours might range from 2 to 20 kilos; but such a mode of calculation is obviously open to many sources of error.

Of the whole amount thus discharged part passes away at once as watery vapor mixed with volatile matters, while part may remain for a time as a fluid on the skin; the former is frequently spoken of as insensible, the latter as sensible perspiration or sweat. The proportion of the insensible to the sensible perspiration will depend on the rapidity of the secretion in reference to the dryness, temperature, and amount of movement of the surrounding atmosphere. Thus, supposing the rate of secretion to remain constant, the drier and hotter the air, and the more rapidly the strata of air in contact with the body are renewed, the greater is the amount of sensible perspiration which is by evaporation converted into the insensible condition; and conversely when the air is cool, moist, and stagnant, a large amount of the total perspiration may remain on the skin as sensible sweat. Since, as the name implies, we are ourselves aware of the sensible perspiration only, it may and frequently does happen that we seem to ourselves to be perspiring largely, when in reality it-is not so much the total perspiration which is being increased as the relative proportion of the sensible perspiration. The rate of secretion may, however, be so much increased that no amount of dryness or heat, or movement of the atmosphere, is sufficient to carry out the necessary evaporation, and thus the sensible perspiration may become abundant in a hot, dry air. And practically this is the usual occur­rence, since certainly a high temperature conduces, as we shall point out presently, to an increase of the secretion, and it is possible that mere dryness of the air has a similar effect.

The amount of perspiration given off is affected not only by the condition of the atmosphere, but also by the circumstances of the body. Thus it is influenced by the nature and quantity of food eaten, by the amount of fluid drunk, by the character of exercise taken, by the relative activity of the other excreting organs, more particularly of the kidney, by mental conditions and the like. Variations may also be induced by drugs and by diseased conditions. How these various influences produce their effects we shall study shortly.

The fluid perspiration, or sweat, when collected, is found to be a clear, colorless fluid of a distinctly salt taste, with a strong and distinctive odor, varying according to the part of the body from which it is taken. Besides accidental epidermic scales, it contains no structural elements.


Sweat, as a whole, is furnished partly by the sweat glands and partly by the sebaceous glands, for, as we shall see, the small amount which simply transudes through the epidermis, apart from the glands, may be neglected. Now, the secretions from these two kinds of glands differ widely in nature, and the characters of the sweat, as a whole, will vary according to the relative proportion of the two kinds of secretion. The secretion of the sebaceous glands appears to be fairly constant, the larger variations of the total sweat depending chiefly on the varying activity of the sweat glands. Hence, when sweat is scanty, the constituents of the sebum influence largely the character of the sweat; when, on the contrary, the sweat is very abundant, these may be disregarded, and the sweat may be considered as the product of the sweat glands.

We are not able at present to make a complete statement as to what bodies occur exclusively in the sebum and what in the secretion of the sweat glands. The former consists very largely of fats and fatty acids, and appear to contain some form or forms of proteids; but we have reason to think that the sweat glands secrete in small quantity some forms of fat, and especially volatile fatty acids.

When sweat is scanty, the reaction is generally acid, but when abundant, is alkaline; and when a portion of the skin is well washed the sweat which is collected immediately afterward is usually alkaline. From this we may infer that the secretion of the sweat glands is naturally alkaline, but that when mixed sweat is acid, the acidity is due to fatty (or other) acids of the sebum. In the horse, which is singu­lar among hair covered animals for its frequent profuse sweating, the sweat is said to be always acid and to contain a considerable quantity of some form of proteid. These fea­tures are probably due to the large admixture of sebum from the numerous sebaceous glands connected with the hairs.

Taking ordinary sweat, such as may be obtained by en­closing the arm in a bag, we may say that in man the ave­rage amount of solids is from 1 to 2 per cent., of which about two-thirds consist of organic substances. The chief normal constituents are: (1) Sodium chloride, with small quantities of other inorganic salts. (2) Various acids of the fatty series, such as formic, acetic, butyric, with probably propionic, caproic, and caprylic. The presence of these latter is inferred from the odor; it is probable that many various volatile acids are present in small quantities. Lactic

acid, which has been reckoned as a normal constituent, is stated not to be present in health. (3) Neutral fats and cholesterin; these have been detected even in places such as the palm of the hand, where sebaceous glands are present. (4) The evidence goes to show that neither urea nor any ammonia compound exists in the normal secretion to any extent, though some observers have found a consider­able quantity of urea (calculated at ten grams, in the twenty-four hours for the whole body). Apparently some small amount of nitrogen leaves the body or the skin, as a whole, but this is probably supplied by the sebum or by the epidermis.

In various forms of disease the sweat has been found to contain, sometimes in considerable quantities, blood, albu­min, urea (particularly in cholera), uric acid, calcium oxalate, sugar (in diabetic patients), lactic acid, indigo (or indigo-yielding bodies giving rise to "blue" sweat), bile, and other pigments. Iodine and potassium iodide, succinic, tartaric, and benzoic (partly as hippuric) acids have been found in the sweat when taken internally as medicines.