4 i8 A MANUAL OF PHYSIOLOGY 



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 temperature and humidity of the air. 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 carefully 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 peri- 

 pheral end of the sciatic nerve causes a copious secretion of 

 sweat on the pad and toes of the corresponding leg of a 

 young cat, and this although the vessels are generally con- 

 stricted 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 atropia 

 abolishes the secretory power of the sciatic, while leaving 

 its vaso-motor influence untouched ; and pilocarpin increases 

 the flow of sweat by direct stimulation of the endings of the 

 secretory nerves in the glands. 



That the sweating caused by a high external temperature 

 is normally brought about by nervous influence, and not by 

 direct action on the secreting cells, is shown by the following 

 experiments. One sciatic nerve is divided in a cat, and the 

 animal is put into a hot-air chamber. No sweat appears 

 on the foot whose nerve has been cut, but the other feet are 

 bathed in perspiration. Similarly, a venous condition of 

 the blood (in dyspnoea) causes sweating in the feet whose 

 nerves have not been divided, but none in the other foot ; 

 and stimulation of the central end of the cut sciatic has 

 the same effect. All this points to the existence of a reflex 



