SEC. 5. THE MECHANISM OF THE SECRETION OF 



SWEAT. 



353. In dealing with the manner in which various circum- 

 stances affect the amount of sweat secreted we may, as we have 

 already said, consider the sweat as a whole to be supplied by 

 the sweat-glands alone. For though it seems evident that some 

 amount of fluid must pass by simple transudation through the 

 ordinary epidermis of the portions of skin intervening 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. We have as yet no evidence 

 that the sebaceous glands vary in activity ; their very peculiar 

 form of secretion, if we may speak of it as a secretion, is not 

 adapted to sudden changes, and at all events we have as yet no 

 evidence that circumstances rapidly and largely modify the 

 amount of sebum discharged by healthy sebaceous glands. 



The secreting activity of the skin, like that of the other 

 glands, is usually accompanied and aided by vascular dilation. 

 In one of the early experiments on division of the cervical sym- 

 pathetic, it was observed that in the case of the horse, the 

 vascular dilation of the face on the side operated on was ac- 

 companied 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 constricted, the secretion of the skin is 

 diminished ; when they are dilated, it becomes abundant. 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 evapora- 

 tion tends to cool down the body. On the other hand, when 

 the atmosphere is cold, the cutaneous vessels are constricted, 

 perspiration is scanty, and less heat is lost to the body by 

 evaporation. 



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