474 A MANUAL OF PHYSIOLOGY 



relative proportions of visible and invisible perspiration, vary 

 greatly. A dry and warm atmosphere increases, and a moist 

 and cold atmosphere diminishes the total, and, within limits, 

 the invisible perspiration. Visible sweat given the condition 

 of rapid heat-production in the body as in muscular labour- 

 is more readily deposited on freely exposed surfaces when the 

 air is moist than when it is dry. The air in contact with surfaces 

 covered by clothing is never far from being saturated with 

 watery vapour. Here, accordingly, a comparatively slight 

 increase in the activity of the sweat-glands suffices to produce 

 more water than can be at once evaporated ; and the excess 

 appears as sweat on the skin, to be absorbed by the clothing 

 without evaporation, or to be evaporated slowly, as the pressure 

 of the aqueous vapour gradually diminishes in consequence of 

 diffusion. The power of imbibition (p. 398) of water by the 

 various layers of the skin diminishes as we pass outwards, and 

 the cells of the epidermis are characterized by the rapidity with 

 which they return from a condition of excessive imbibition to 

 their normal state. This constitutes a protective mechanism 

 against excessive loss of water. When the skin is thoroughly 

 moistened its degree of imbibition is three times the 

 normal. 



The quantity of sweat given off by a man in twenty-four hours 

 varies so much that it would not be profitable to quote here the 

 numerical results obtained under different conditions of tem- 

 perature and humidity of the air (but see p. 588). It is enough to 

 say that the excretion of water from the skin is of the same order 

 of magnitude as that from the kidneys : a man loses upon the 

 whole as much water in sweat as in urine. But it is to be care- 

 fully noted that these two channels of outflow are complementary 

 to each other ; when the loss of water by the skin is increased, the 

 loss by the kidneys is diminished, and vice versa. 



The Influence of Nerves on the Secretion of Sweat. The 

 sweat-glands are governed directly by the nervous system ; 

 and though an actively perspiring skin is, in health, a flushed 

 skin, the vascular dilatation is a condition, and not the chief 

 cause of the secretion. Stimulation of the peripheral end of the 

 sciatic nerve causes a copious secretion of sweat on the pad and 

 toes of the corresponding foot of a young cat, and this although 

 the vessels are generally constricted by excitation of the vaso- 

 motor nerves. Not only so, but when the circulation in the foot 

 is entirely cut off by compression of the crural artery or by 

 amputation of the limb, stimulation of the sciatic still calls forth 

 some secretion. As in the case of the salivary glands, injection 

 of atropine abolishes the secretory power of the sciatic, while 

 leaving its vaso-motor influence untouched ; and pilocarpine 



