396 STUDIES IN ADVANCED PHYSIOLOGY. 



and to rid the body of the nitrogenous products resulting 

 from the metabolism of the tissues. The kidneys are able, 

 when necessary, to eliminate hurriedly accidentally received 

 substances, such, for instance, as poisons of one kind or 

 another, and so prevent the liability of injury from them. 

 This no doubt explains the power of the body to recover 

 from such poisonings. 



While most of the work of secretion thus devolves upon 

 the kidneys, they are able to be relieved in part by the skin, 

 which, in addition to its functions as a protection to the 

 body, serves quite materially in a manner similar to that of 

 the kidney, and in cases where the kidneys have been over- 

 worked and have lost some of their vitality, assumes this 

 extra duty. On the other hand, when the skin for some 

 reason is prevented from exercising this excretory function 

 an unnaturally increased amount of work is thrown upon 

 the kidneys. 



THE SKIN. 



The covering of the body, or the skin, consists of two 

 essentially different layers ; an outer layer made up of cells 

 entirely, called the cuticle of epidermis, and an inner layer 

 consisting almost wholly of connective tissue fibers, with 

 contained blood-vessels, glands, nerves and so on, called 

 the corium or cutis-vera, that is, true skin. 



1. Epidermis. The epidermis is a stratified epithelium 

 composed wholly of epithelial cells. These cells are, 

 however, not all alike, the cells of the deeper layers, next 

 to the cutis-vera, being more cuboidal in shape than the 

 flattened and dead cells forming the horny covering at the 

 surface. The gradation from these dead flattened cells on 

 the outside to the living cuboidal cells next to the corium is 

 tolerably gradual. The lowest layer of cells, that immedi- 

 ately in contact with the corium, is called the Malpighian 

 layer of the epidermis. It consists of well-marked nucle- 

 ated cells of pronounced cuboidal form. It is these cells 

 which by their division give rise to all of the remaining 

 cells of the epidermis. The cells of this Malpighian layer 



