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. 1). 



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. 1). 



Fig. 20. — Epidermis 

 of Ethiopian. 



Fig. 21. —Epidermis 

 of Caucasian. 



