SWEAT. — The cutaneous secretion is formed by the spiral sudoriferous canals dis covered by Breschet and Purkinge. In con sidering this subject it is important to make the distinction between sweat and that ex halation which is constantly, and at the same time insensibly, excreting from the healthy skin. The latter is the manifestation of a function indispensably necessary for the con tinuance of life ; while sweat may be regarded merely as an occasional result, and as pro ducing on the surface certain excretive pro ducts which under ordinary conditions only appear in small proportion on the skin, and find their more natural exit from the organism through other channels. It is true that sweat contains among its constituents the ordinary products of the insensible perspiration, but it also contains other matters of a solid kind which do not appear in a fluid form unless cutaneous excitement be brought about, and which in cold weather and in the case of in dolent, inactive persons, adhere to the cuticle, and are gradually rubbed or washed off: These solid matters are not volatile at the ordinary temperature of the skin, and there fore will not pass off from the body in mate rial quantity by any amount of perspiration. They become constituents of sweat, however, inasmuch as they dissolve in the water which, under exercise or owing to excessive tempe rature, appears on the surface. Thus sweat is constituted of the ordinary aqueous pro ducts of the natural perspiration, which ap pearing in large quantity ceases to be va porised, and in addition it contains those cutaneous secretions which the water dis solves, but which, when an animal does not sweat, appear in less quantity, and adhere undissolved to the surface. As the ordinary cutaneous transpiration forms an important part of sweat, I shall proceed to consider it before entering on the more immediate subject of the present article.
In the healthy human subject, a portion of water is constantly finding its way to the surface, and under ordinary conditions is va porised nearly as fast as it is secreted. Some difference is observed in individuals in this respect, however, so that we observe some whose skins are constantly moist, while others, though in perfect health, have the skin almost always dry, yet doubtless performing its ordi nary function. Perhaps the most agreeable
proportion of perspiration, and that which we should regard as approaching more nearly to the normal quantity, is that which maintains a certain moisture on the surface sufficient to impart an agreeable softness to the touch, and scarcely to convey the idea of moisture. Many attempts have been made to determine the quantity of the cutaneous transpiration, and perhaps nothing has ever been attempted so unpromising in result. It is almost certain, indeed, that we do not transpire equally during any two minutes of the day, and on no two days alike.
Sanctorius, the Venetian physician, whose aphorisms contain much valuable matter, too much neglected by the physiologists of the present day, made lengthened experiments on perspiration. The insensible perspiration has, indeed, been termed " Sanctorian" in honour of him.
This subject, curiously enough, received some attention from the merry monarch Charles the Second, who instituted many ex periments on the subject. Among other less exalted, but probably more trustworthy ob servers, may be mentioned Dodart, Keil, Ro binson, Home, and Linings, and of later date Lavoisier and Seguin. According to an ex periment of Home, the perspiration during the twelve hours of night varies from 12 to 18 oz., and in 23i hours the discharge amounted to 3 lbs. 3i oz. On another occasion, however, the amount was only 2 lbs. 64 oz. Keil states the medium quantity of perspiration to be 31 oz. only. Robinson states the amount for summer to be 27 oz., and in winter 30 oz. daily. Hartmann, in Germany, makes the daily amount of perspiration 15 to 46 oz. Dodart, in France, gives 40 oz. 3 dr. 26 gr. for summer, and 26 oz. 46 gr. for winter. Sanctorius found that in the warm humid air of Venice, having taken 8 lbs. of ingesta, the perspiration in the 24 hours amounted to 5 lbs., the fasces to 4 oz., and the urine to 69 oz.