40 THIRD LECTURE. 



Its smooth or frizzled condition depends on the cross section 

 In the former hair it is round, in the latter oval or reniform. 

 The growth of the hair takes place by a cell increase from the 

 lower portion of the hair bulb. So long as the sac with its 

 papilla remains uninjured, it regenerates the lost hairs ; that is, 

 such as are stunted in their hair bulbs and are separated from 

 the papillae. This power of reproduction is tolerably ener- 

 getic, for the physiological loss of hair is not inconsiderable. 

 The origin of the embryonic hair commences at the end of 

 the third or the beginning of the fourth month (Fig. 41). 

 The epidermis forms with its deeper cells (b) a knobby down- 

 ward growth. A structureless boundary layer, furnished by 

 the impressed corium (/), leads to the formation of the hair 

 sac. From the cell aggregations (m, m), are developed both 

 the root-sheaths and the entire true hair with its cuticula. 

 The hairs, like the nails, are, therefore, secondary epidermoi- 

 dal structures. 



