i8 



HUMAN BIOLOGY 



ridge in the skin above each row of papillae (Fig. 24). In 

 the papillae are small loops of blood vessels and sometimes 

 a nerve fiber (colored Fig. i). 



The epidermis is composed of a mass of cells held to- 

 gether by a cement resembling the white of an egg. The 

 cells near the surface are hard and flattened ; those deeper 

 down near the dermis are round and soft (see Fig. 21). 



These cells are liv- 

 ing cells. They are 

 kept alive by the 

 nourishment in the 

 watery portion of 

 the blood which 

 soaks through from 

 the blood tubes in 

 the neighboring pa- 

 pillae. Hence these 

 cells are growing 

 cells; they subdivide 

 when they reach a 

 certain size, and re- 

 place those wearing 

 away at the surface, thus constantly repairing the epider- 

 mis. The dry outer cells wear away rapidly. They have 

 no nuclei and are dead cells. The new cells forming be- 

 neath push them so far away from the dermis that nour- 

 ishment no longer reaches them, and they die. 



Pigment. The cells in the lower layers of the epidermis 

 contain grains of coloring matter, or pigment. All other 

 cells of the epidermis are transparent ; the pigment has the 

 function of absorbing and arresting light. Albinos or 

 animals entirely without pigment have pallid skins and 

 pink eyes (Exp. i). 



FIG. 20. EPIDERMIS 

 OF ETHIOPIAN. 



FIG. 21. EPIDERMIS 

 OF CAUCASIAN. 



