402 STUDIES IN ADVANCED PHYSIOLOGY. 



That the nail is an epidermal structure is not only 

 proved from its composition of epidermal cells, but also 

 proved by the source from which the root of the nail is de- 

 rived. In very early life, as early as the third month of 

 uterine life, the epidermis at the point where the root of 

 the nail is to form bends inward forming a groove, which 

 groove finally imbedded in the deeper layers of the skin be- 

 comes the matrix for the growing nail. It is said that the 

 continued rate of growth of a nail is about one-thirtieth of 

 an inch per week. As in case of epidermis in any part of 

 the skin a new nail is able to appear only when in the pull- 

 ing away of the old one the deeper layers, the Malpighian 

 layers, of the nail root are left intact. 



2. The Hair. A second modification of the epidermis 

 is the hairs. They appear about the third or fourth month 

 of embryonic life as growths from the Malpighian layer down 

 into the deeper parts of the corium. This ingrowth soon 

 divides into an outer wall of epidermal tissue which becomes 

 the wall of the hair follicle, and an inner portion which 

 appears as the hair. The growing portion of the hair is at 

 the bottom of the hair follicle where the Malpighian cells 

 are in constant process of division. This point of growth is 

 spoken of as the root of the hair, and on account of the 

 continued proliferation of new cells is highly vascular. 

 These blood-vessels really lie in the corium, but the corium 

 at this point usually extends some distance into the hair 

 root, like a papilla, on which and around which the dividing 

 cells of the hair root are placed. At the root the cells which 

 go to make up the shaft of the hair are more or less alike, 

 but a differentiation at once begins, and along the shaft 

 proper the following structure of the hair may be easily 

 made out with the microscope. 



Surrounding the hair is a single layer of flattened cells 

 forming a kind of scaly covering. This is called the hair 

 cuticle. Beneath this thin cuticle is the real fibrous por- 

 tion of the hair. In most hairs this fibrous portion consti- 



