176 FOUNDATIONS OF BIOLOGY 



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 warm-blooded animals (Birds 

 and Mammals) loses a large amount of water and heat; the 

 amount depending on the temperature and moisture of the air 

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

 ture is essentially that of the body and it is saturated with 

 water vapor. 



The SKIN in some of the lower Vertebrates, for instance the 

 Frog, is an exceedingly important excretory organ, because 

 more carbon dioxide is eliminated through the skin than 

 through the lungs ; but in higher forms, including Man, excre- 

 tion by the skin is confined to the SWEAT GLANDS. These 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 PERSPIRATION 

 evaporates. (Fig. 76.) 



The LIVER, in addition to its various other functions, aids 

 in no small way in excretion. On the one hand, the liver 

 removes deleterious compounds of ammonia from the blood 

 and transforms them into urea. Then it secretes the urea 

 into the blood from which it is later removed by the kidneys. 

 On the other hand, the liver collects other waste products etc., 

 from the blood, which form the bile. This passes to the GALL 

 BLADDER for temporary storage or directly to the intestine. 



The KIDNEYS are, in a way, the chief excretory organs of 

 Vertebrates, and any serious interference with their activity 

 rapidly leads to a poisoning of the body with its own waste 

 products. Certain cells of the kidneys remove the urea from 

 the blood stream which reaches them, while water and various 

 solutes are drained from the blood. Aside from their func- 

 tional importance, the kidneys are of considerable interest 

 to the comparative anatomist because of their complicated 



