EXCRETION By THE SKIN 513 



ture and humidity of the air (but see p. 691). 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 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 vasomotor nerves. Not only so, 

 but when the circulation in the foot is entirely cut off by compres- 

 sion 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 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 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 asphyxia) 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 mechanism ; 

 and it is certain that asphyxia acts by direct stimulation of the 

 centre or centres. The vaso-motor centre is at the same time 

 stimulated, and the bloodvessels constricted, as in the cold sweat 

 of the death agony. Fear may also cause a cold sweat, impulses 

 passing from the cerebral cortex to the vaso-motor and sweat 

 centres. 



It is probable that a general sweat - centre exists in the medulla 

 oblongata, but its position has not been exactly determined nor even 

 its existence definitely proved. On the other hand, it is known that 

 in the cat there are at least two spinal centres, one for the fore-limbs 

 in the lower part of the cervical cord, and another for the hind-limbs 

 where the dorsal portion of the cord passes into the lumbar. That this 

 latter centre does not exist or is comparatively inactive in man is 

 indicated by the following case : A man fell from a window and fractured 

 his backbone at the fifth dorsal vertebra. The lower half of the body 

 was paralyzed for a time, but recovered. Ultimately, however, the 



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