388 NERVOUS MECHANISM. [BOOK n. 



to those peculiar to the sweat of particular regions of the skin, 

 there can be no doubt that these are secreted by the epithelium of 

 the appropriate glands. There can be equally no doubt that the 

 fats which come to the surface of the skin from the sebaceous 

 glands arise from a metabolism of the cells of those glands. And 

 we shall probably not go far wrong in regarding the sweat as a 

 whole as supplied by the sweat-glands alone. For though it seems 

 evident that some amount of fluid must pass by simple transuda- 

 tion through the ordinary epidermis of the portions of skin inter- 

 vening between the mouths of the glands, yet on the whole it is 

 probable that the portion which so passes is a small fraction only 

 of the total quantity secreted by the skin ; and direct experiment 

 shews that even the simple evaporation of water is much greater 

 from those parts of the skin in which the glands are abundant than 

 from those in which they are scanty. 



The nervous mechanism of Perspiration. The secreting ac- 

 tivity of the skin, like that of other glands, is usually accompanied 

 and aided by vascular dilation. In one of Bernard's early experi- 

 ments on division of the cervical sympathetic, it w r as observed that 

 in the case of the horse, the vascular dilation of the face on the side 

 operated on was accompanied by increased perspiration. Indeed 

 the connection between the state of the cutaneous blood-vessels 

 and the amount of perspiration is a matter of daily observation. 

 When the vessels of the skin are contracted, the secretion of the 

 skin is diminished ; when they are dilated it becomes abundant. 

 And in this way, as we shall later on point out, the temperature of 

 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 constricted, perspira- 

 tion 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 supported by many pathological 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 hyperaemic ; but we have direct experimental evidence 

 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 



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. 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 is known 

 to all. 



