REMOVAL OF THE END-PRODUCTS OF METABOLISM 161 



more or less completely oxidized. Carbon dioxid is the 

 most conspicuous product. The respiratory exchanges 

 occur in the different tissues in a measure corresponding 

 with the extent to which they severally evolve energy. 

 The skeletal muscles lead in amount of respiration (and 

 of carbon dioxid set free), both because of their great mass 

 and their activity. The glands, especially the liver and 

 the kidneys, contribute largely to the total. So does the 



Cap 



Fig. 21. I is intended to suggest the form of an air-sac of the 

 lung overlaid with a network of capillaries belonging to the pul- 

 monary system. II is an imaginary section through such an air- 

 sac. B in both I and II is the minute bronchial tube through which 

 the air is renewed. Ill is a bit of detail from II, still more en- 

 larged, showing two capillaries (Cap.) conveying corpuscles (Corp.). 

 The air is close by (Al), yet two partitions intervene, the capillary 

 wall and the wall of the air-sac. 



heart. Other tissues have a minor part in the general res- 

 piration. Some which are passive and stable, like cartilage, 

 can have but little. 



The carbon dioxid formed by the cells is first transferred 

 to the lymph. The concentration of the gas in the lymph 

 leads to its passage into the blood. A gas will always pass 

 from a higher to a lower concentration when the two solu- 

 tions are placed in communication. The delicate capillary 

 11 



