556 NERVOUS MECHANISM OF SWEATING. [Book ii. 



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 per- 

 spiration 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 hyperaemic, but we have direct experimental evi- 

 dence of a nervous mechanism of perspiration as complete as 

 the vaso-motor 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 submax- 

 illary 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 ob- 

 tained with the nerves of the fore limb. And in ourselves a 

 copious secretion of sweat may be induced by tetanizing 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 

 increases up to a certain limit the irritability of the epithelium 

 of the sweat-glands and predisposes it to secrete, just as it pro- 

 motes action in the case of a muscle or nerve or other forms of 

 living substance. Thus stimulation of the sciatic in the cat 



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. Rabbits and other rodents appear not to 

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

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



