THE MECHANISM OF THE SECRETION OF SWEAT. 435 



the body is largely regulated. When the surrounding atmosphere is warm, 

 the cutaneous vessels are dilated, the amount of sweat secreted is increased, 

 and the consequently augmented evaporation tends to cool down the body. 

 On the other hand, when the atmosphere is cold, the cutaneous vessels are con- 

 stricted, 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 varia- 

 tions in the vascular supply. And not only is this view suggested by many 

 facts, such as the profuse perspiration 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 

 hyperseniic, but we have direct experimental evidence of a nervous mechan- 

 ism of perspiration as complete as the vasomotor mechanism. 



If in the cat 1 the peripheral stump of the divided sciatic nerve be stimu- 

 lated 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 cuta- 

 neous 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 ampu- 

 tated leg. Moreover, when atropine has been injected, the stimulation pro- 

 duces no sweat, though vasomotor 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 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 or the other limbs sweat freely. 

 This result shows 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 tempera- 

 ture. A high temperature it is true up to a certain limit increases the irrita- 

 bility 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 stimulation of the sciatic in the cat produces a much more 

 abundant secretion in a limb exposed to a temperature of 35 or somewhat 

 above, than in one which has been exposed to a distinctly lower temperature, 

 and in a limb which has been placed in ice-cold water hardly any secretion 

 at all can be gained ; but apparently mere rise of temperature without nerve- 

 stimulation will not give rise to a secretory activity of the glands. The 

 sweating caused by a dyspnoBic condition of blood, and such appears to be 

 the sweat of the death-agony, is similarly brought about by the agency of 

 the central nervous system. When an animal with the sciatic nerve divided 

 on one side is made dyspnoeic, no sweat appears in the hind limb of that side 

 though abundance is seen in the other feet. 



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

 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 epi- 

 dermis. Rabbits 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. 



