CHAPTER XVIII 



THE SKIN 



AND 

 KIDNEYS 



Excretory organ of Parame- 

 cium. 



Excretory organ of earth- 

 worm. 



What is the necessity for excretion ? How are wastes elim inated from 

 the body f What is thd relation of the skin and kidneys to excretion f 



Importance of excretion. The wastes formed in the various 

 tissues as a result of oxidation are carbon dioxide, water, organic 

 salts, and urea (nitrogenous compound) . These wastes would in- 

 terfere with the normal functions of the cells unless removed as 

 quickly as formed. The removal of these wastes is called excre- 

 tion, and the organs that eliminate the wastes are called the excre- 

 tory organs. The importance of an efficient system of elimination 

 can not be sufficiently emphasized. Carbon dioxide is eliminated 

 chiefly by the lungs, although some is dissolved in water and 

 is excreted by the skin and kidneys. Water and some soluble 

 salts are eliminated by the skin as perspiration, and by the kid- 

 neys as urine. Some water, however, passes out of the lungs in 

 the form of vapor. Most of the urea is excreted through the 

 kidneys as part of the urine, and a small amount through the skin 

 as part of the perspiration. 



Problem. Study of the skin. 



I. Touch the skin and describe how it feels. 



A. Explain the source of the heat in tlie skin. 



B. When one is exercising violently, why does the skin become much 

 warmer than it does at other times ? 



170 



