434 THE ELIMINATION OF WASTE PRODUCTS. 



of epidermis, but it has been debated whether substances in aqueous solution 

 can be absorbed by the skin when the epidermis is intact, the evidence on 

 this point being contradictory. In the case of the skin of the frog an ab- 

 sorption of water and of various soluble substances certainly takes place. 

 In the case of the sound human skin there are no a priori reasons why water 

 carrying substances dissolved in it should not pass inward through the 

 corneous as well as the other layers of the epidermis, the amount so passing 

 depending, among other things, upon the condition of the skin ; and com- 

 mon experience seems to show that it does. Nevertheless, the results of 

 actual experiment are conflicting. Some observers maintain that soluble 

 non-volatile substances are not absorbed, and that volatile substances, such 

 as iodine, which may be detected in the system after a bath containing them, 

 are absorbed not by the skin, but by the mucous membrane of the respira- 

 tory organs, the substance making its way to the latter by volatilization from 

 the surface of the bath. Others, again, have found evidence of absorption, 

 especially with volatile substances, even when care has been taken to avoid 

 all errors ; and the greater weight may perhaps be given to these, since they 

 accord with common experience. The conflict of experimental results, how- 

 ever, at least shows that we do not fully understand the conditions under 

 which such absorption takes place. 



There is, moreover, evidence that even solid particles can pass through 

 an intact skin. The lymphatics in the skin of a newborn infant have been 

 found crowded with the particles of the peculiar fatty secretion which covers 

 the skin at birth ; and solid particles rubbed into even the sound skin may, 

 especially when applied in a fatty vehicle, as* e. g., in the well-known mer- 

 cury ointment, find their way into the underlying lymphatics. The wander- 

 ing leucocytes which are at times found among the epidermic cells may 

 perhaps take part in this transport. 



THE MECHANISM OF THE SECRETION OF SWEAT. 



369. In dealing with the manner in which various circumstances 

 affect the amount of sweat secreted we may, as we have already said, con- 

 sider 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 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 shows 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 dis- 

 charged by healthy sebaceous glands. 



The secreting activity of the skin, like that of the other glands, is usually 

 accompanied and aided by vascular dilatation. In one of the early experi- 

 ments on division of the cervical sympathetic it was observed that, in the 

 case of the horse, the vascular dilatation of the face on the side operated on 

 was accompanied by increased perspiration. Indeed, the connection between 

 the state of the cutaneous bloodvessels 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 



