54 THE INDIVIDUAL ORGANISM 



The skin and disease. One of the outstanding functions of the skin 

 is to repel the invasion of disease-producing organisms. In this respect 

 it plays a dual role. The first line of defense of the body is the skin surface, 

 both because the outer part of the epidermis is tough and relatively 

 impervious and impenetrable and because the normal skin secretions 

 have a considerable bactericidal action. If the normally intact epidermis 

 is injured and bacteria penetrate to the dermis, they encounter the 

 second line of defense, provided in part by tremendous numbers of white 

 blood cells that can be carried to the invaded dermis by its rich network 

 of capillaries and lymph vessels, and in part by chemical means. This will 

 be discussed more fully in the treatment of the circulatory system. 



In spite of the skin's protective ability, many plant and animal organ- 

 isms have developed the capacity to live in it (e.g., the fungi that cause 

 ringworm and athlete's foot) or to penetrate it and reach the internal - 

 organs (e.g., the larvae of hookworms). 



