HOUSING-SKIN AND ITS DERIVATIVES 



367 



-EPIDERMIS 



Sffboecous ^land ^ 

 arcctor muscle. 



bulb of hair 

 popillo 



DES^^mS 



sw«at 9lond 



Fig. 14-1. Human skin in cross-section. Gland ancJ hair development are shown in several stages (top left). 



much more complicated in higher forms; as, 

 for example, in the vertebrate skin with all 

 of its derivatives. 



In these animals the skin forms a contin- 

 uous unbroken sheath which protects the 

 internal structures from the environment. 

 In addition to being tough itself, it is often 



fortified with scales, feathers, or hair for 

 added protection against physical injury by 

 predaceous enemies. Moreover, it is refrac- 

 tive to bacteria and other microorganisms 

 that could be injurious if allowed to enter 

 the internal environment. It is impervious to 

 most harmful chemicals and is important in 



