700 NERVOUS MECHANISM OF SWEATING. [BOOK n. 



are constricted, perspiration is scanty, and less heat is lost to the 

 body by evaporation. 



The analogy with the other secreting organs which we have 

 already studied leads us, however, to infer that there are special 

 nerves directly governing the activity of the sudoriparous glands, 

 independent of variations in the vascular supply. And not only 

 is this view suggested by many facts, such as the profuse perspira- 

 tion of the death agony, of various crises of disease, and of certain 

 mental emotions, and the cold sweats occurring in phthisis and 

 other maladies, in all of which the skin is anaemic rather than 

 hypersemic, but we have direct experimental evidence of a 

 nervous mechanism of perspiration as complete as the vasomotor 

 mechanism. 



If in the cat 1 the peripheral stump of the divided sciatic nerve 

 be stimulated with the interrupted current, drops of sweat may 

 readily be observed to gather on the hairless sole of the foot of 

 that side. The sweating is not due to any increase of blood-supply, 

 for it may be observed when the cutaneous vessels are thrown into 

 a state of constriction by the stimulus, or even when the aorta or 

 crural artery is clamped previous to the stimulation, and indeed 

 may be obtained by stimulating the sciatic nerve of a recently 

 amputated leg. Moreover when atropin has been injected, the 

 stimulation produces no sweat, though vaso-motor effects follow as 

 usual. The analogy between the sweat-glands of the foot and 

 such a gland as the submaxillary is in fact very close, and we are 

 justified in speaking of the sciatic nerve as containing secretory 

 fibres distributed to the sudoriparous glands of the foot. Similar 

 results may be obtained with the nerves of the fore limb. And 

 in ourselves a copious secretion of sweat may be induced by teta- 

 nizing through the skin the nerves of the limbs or the face. 



If a cat in which the sciatic nerve has been divided on one side 

 be exposed to a high temperature in a heated chamber, the limb 

 the nerve of which has been divided remains dry, while the feet of 

 the other limbs sweat freely. This result shews that the sweating 

 which is caused by exposure of the body to high temperatures is 

 brought about by the agency of the central nervous system, and 

 not by a local action on the sweat-glands ; for the foot of the limb 

 whose nerve has been divided is equally exposed to the high 

 temperature. A high temperature it is true up to a certain limit 

 increases the irritability of the epithelium of the sweat-glands and 

 predisposes it to secrete, just as it promotes action in the case of a 

 muscle or nerve or other forms of living substance. Thus stimu- 

 lation of the sciatic in the cat produces a much more abundant 



1 The cat sweats freely in the hairless soles of the feet but not on any part 

 of the body covered with hairs. The dog also sweats in the same regions but 

 not so freely as the cat ; indeed sweating is often absent, the ducts being stopped 

 by growth of the corneous epidermis. Babbits and other rodents appear not to 

 sweat at all. The snout of the pig sweats freely; and the often profuse sweating 

 of the horse, a singular event among hair-covered animals, is known to all. 



