EXCRETORY SYSTEM 



359 



Corte 



Pelvis 



products which they excrete, but only remove them 

 from the blood. 



Sometimes extra waste products are sent to the kid- 

 neys for removal. 

 They become over- 

 w or k e d , poisonous 

 substances collect, 

 and severe illness en- 

 sues. Unless the de- 

 fect is remedied death 

 may result, for the 

 work of the kidneys 

 is indispensable to 

 the body. 



Theskin. The 

 whole outer surface 

 of our bodies is cov- 

 ered with a flexible, elastic tissue, the skin. This 

 varies in thickness and in texture. On the palm of 

 the hand and the sole of the foot it is thick and 

 tough ; the covering of the lips is extremely thin. The 

 hair and nails are modifications of it. 



Ordinarily we do not appreciate the great importance 

 of the skin. It has at least four important uses. First, 

 its most obvious use is that of protection for the muscles 

 and other organs which lie beneath it. It keeps them 

 from drying; prevents the irritation which would 

 result from contact with clothing; and also keeps 

 bacteria from entering the body. Second, it contains 



Medulla 



Fig. 168. Longitudinal section of a 

 kidney. 



