EXCRETION 



181 



tion UREA, URIC ACID, CREATININE, AMMONIA, etc., the products of 



nitrogenous metabolism. 



A. Gills and Lungs 



We have already emphasized the elimination of carbon dioxide 

 by the gills or the lungs. Here the cells of the respiratory mem- 

 branes play essentially a passive role in excretion, since the carbon 

 dioxide, which is under higher tension in the blood than in the 

 water or air, follows the physical laws of diffusion of gases and 

 passes from the blood. In addition to carbon dioxide, the blood 

 of homothermal animals loses a large amount of water and heat ; 

 the amount depending on the moisture and temperature of the 

 air which enters the lungs. When the air is exhaled, its tempera- 

 ture is very nearly that of the body and it is saturated with water 



vapor. In Man about one-third of the t 



water eliminated is excreted by the lungs. 



(Fig. 118.) ■=!&£- Epidermis 



B. Skin 



The skin of some of the lower Vertebrates, 

 for instance the Frog, is an exceedingly im- 

 portant excretory organ, because more car- 

 bon dioxide is eliminated through it than 

 through the lungs; but in most of the higher 

 forms, including Man, excretion by the skin 

 is confined to the sweat glands. There are 

 nearly three millions of them, with a total 

 length of several miles, opening on the 

 surface of the human body. They take from 

 the blood, in addition to large quantities of 

 water, traces of nitrogenous waste or urea, 

 fatty acids, and salts, which form a residue 

 on the surface of the skin when the per- 

 spiration evaporates. However, most of 

 the water may be regarded as a secretion 

 rather than an excretion because it is of use to the body, being 

 employed in regulating the body temperature. Everyone knows 

 that evaporation of perspiration accelerates the loss of heat by 

 the skin. In addition to sweat glands, the skin is provided with 

 seraceous glands which open, as a rule, at the base of hairs 



Secreting 

 Cells 



Capillaries 



Fig. 127. — Diagram 

 of a sweat gland. 



