SEC. 7. THE MECHANISM OF THE SECRETION OF 



SWEAT. 



441. 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 sympathetic, it 

 was 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 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 evaporation tends to cool down the body. On the 

 other hand, when the atmosphere is cold, the cutaneous vessels 



