ix THE SKIN AND CUTANEOUS GLANDS 487 



In the upper arm and leg . . . 55-70 



,, palm of hand .... 310 



back of hand .... 170 



sole of foot .... 300 



dorsum of foot . ,. . 100 



They are more numerous in the axilla than elsewhere, but hard to 

 count, because the excretory tubes of several glands converge into 

 one single excreting duct. 



Aubert (Lyons) invented a very elegant method for the exact calculation 

 of the enormous number of sweat glands in the skin. A sheet of ordinary 

 white paper is closely applied to any area of perfectly dry skin surface, for a 

 period of 30-80 minutes. On then plunging it into a 0'25 per cent silver 

 nitrate solution, and exposing it to sunshine, the ground of the paper turns 

 black, but every point at which it came in contact with the mouth of a sweat 

 gland remains white, so that a photograph is obtained in which the orifices 

 of the glands appear as ducts (Fig. 136). The explanation is very simple. 

 Sweat is continually given off from the mouths of the sudoriferous glands in 

 the form of invisible perspiration. The sweat contains chlorides which are 

 deposited on the paper only at the points corresponding with the glandular 

 orifices, so that these points only react to the silver nitrate by forming silver 

 chloride. If the paper is applied, on the contrary, to a surface that is visibly 

 perspiring, the sweat with its chlorides will cover the whole surface of the 

 papillary ridges, and instead of dots, an exact impression of the outline of the 

 ridges is seen as a white line, the furrows that bound them coming out 

 black (Fig. 133). 



So long as the secretion of sweat from the coiled glands is 

 below a certain narrow limit, the water excreted (with the volatile 

 substances of the sweat) evaporates from the surface of the skin, 

 which remains dry (perspiratio insensibilis) ; but when the 

 secretion increases, or evaporation is hindered by the state of the 

 atmosphere, sweat appears on the surface of the skin. First 

 minute droplets form at the orifices of the coiled glands, next 

 these run together and form larger drops, finally by gravity they 

 flow over the skin, or saturate the clothing (perspiratio sensibilis). 



The loss of substance, particularly of water, from the human 

 body by the skin is undoubtedly enormous, but it varies consider- 

 ably with different circumstances. The first experimental attempts 

 to determine this loss were made by Santorio (1614). He weighed 

 the intake of food and drink, as well as the ponderable excreta of 

 his own body, and found (after remaining whole days on the 

 balance) that f of the weight of the ingesta are eliminated by 

 the skin and lungs. Dodart (1725) made similar experiments. 

 The question was subsequently taken up by Lavoisier and Seguin 

 (1790), who estimated the water excreted from the human skin at 

 one litre in 24 hours. In his later work on the total excretion by 

 the skin and lungs, W. F. Edwards (1824) found that it varied 

 quantitatively in different animals in ratio with their weight. 

 But he erred in differentiating between sudoratio and perspiratio, 

 as if there were two different secretions, instead of two different 



