The following passage from Lehmann's "Physiological Chemistry," vol. ii, page 389, shows that the skin picks up plenty of nitrogen from somewhere: "It has been shown by the experiments of Milly, Jurine, Ingenhouss, Spallanzani, Abernethy, Barruel, and Collard di Martigny, that gases, and especially carbonic acid and nitrogen, are likewise exhaled with the liquid secretion of the sudoriparous glands. According to the last-named experimentalist, the ratio between these two gases is very variable; thus, in the gas developed after vegetable food, there is a preponderance of carbonic acid, and, after animal food, there is an excess of nitrogen. Abernethy found that on an average the collective gas contained rather more than two thirds of carbonic acid and rather less than one third of nitrogen." But it appears that less gas is exhaled when there is much liquid perspiration.
Lehmann's summary of the experiments of Abernethy, Brunner, and Valentin (vol. ii, page 391), gives the amount of hourly exudation, under ordinary circumstances, as 50·71 grammes of water, 0·25 of a gramme of carbon, and 0·92 of a gramme of nitrogen. This amounts to twenty-one and a half grammes of nitrogen per day in the insensible perspiration; three quarters of an ounce avoirdupois, or as much nitrogen as is contained in four and a half ounces of dried muscle, or more than one pound of natural living muscle.
That the liquid perspiration contains compounds of nitrogen, and just such compounds as would result from the degradation of nitrogenous tissue, is unquestionable. As Lehmann says (vol. ii, page 389), "the sweat very easily decomposes, and gives rise to the secondary formation of ammonia." Simon and Berzelius found salts of ammonia in the sweat; that the ammonia is combined both with hydrochloric acid and with organic acids; that it probably exists as carbonate of ammonia in alkaline sweat.
The existence of urea in sweat appears to be uncertain; some chemists assert its presence, others deny it. Favre and Schottin, for example, who have both studied the subject very carefully, are at direct variance. I suspect that both are right, as its presence or absence is variable, and appears to depend on the condition of the subject of the experiment.
Favre describes a special nitrogenous acid which he discovered in sweat, and names it hydrotic or sudoric acid. Its composition corresponds, according to his analysis, to the formula C10H8NO13.
I have summarized these facts, as they show clearly enough that conclusions based on an examination of the quantity of nitrogen excreted by the kidneys alone (and such is the sole basis of the modern theories) are of little or no value in determining whether or not muscular work is accompanied with degradation of muscular tissue. The well-known fact that the total quantity of excretory work done by the skin increases with muscular work, while that from the kidneys rather diminishes, indicates in the plainest possible manner that an examina-
