464 Animal Biology 



their excretions (sweat or perspiration) through pores on the skin sur- 

 face. Over two million sweat glands in the entire skin eliminate over a 

 quart of sweat per day under normal conditions. Under abnormal con- 

 ditions, more or less than this amount may be excreted. Perspiring 

 eliminates body wastes and regulates body heat through the evapora- 

 tion of water. Heat which is produced in various tissues, especially 

 muscles, is distributed b)- the blood throughout all parts of the body, 

 thus producing an average, normal body temperature of 98° to 99° F. 



Adamant 



or 

 enamel 



Crouin — 



Neck 



Root - 



Dentine 



or 

 Wory 



Dental pub 



Cement 

 Gum 



Alveolar 

 process 



Peridental 

 membrone 



Jont M.Hopf 



Fig. 231. — Human incisor tooth in vertical section. (From Francis 

 tion to Human Anatomy, The C. V. Mosby Co.) 



Introduc- 



Since the skin is well supplied with blood, it can efficiently act as a heat- 

 regulating and heat-eliminating mechanism. Dilation (enlargement) of 

 the blood vessels and relaxation of the muscle fibers of the dermis allow 

 more blood to lose more heat, while a contraction of these blood vessels 

 and muscles has the opposite effect. In addition to this loss of heat, the 

 skin may also be cooled by the evaporation of sweat from the skin surface. 

 The functions of the human skin and its accessory structures may be 

 summarized as follows: (1) regulation and elimination of heat; (2) 

 excretion of wastes; (3) protection against injury, harmful light rays, 



