CHAPTER VI 



RESPIRATION 



THE maintenance of animal life necessitates the continual absorption of 

 oxygen and the excretion of carbon dioxide by the living tissues. The blood 

 is the medium, in all animals which possess a well-developed blood-vascular 

 system, by which these gases are carried. Oxygen is absorbed by the blood 

 from without and conveyed to all parts of the organism; and carbon dioxide 

 which comes from the cells within is carried by the blood to the surfaces, 

 from which it may escape from the body. The two processes absorption 

 of oxygen and excretion of carbon dioxide are complementary, and their 

 sum is termed the process of Respiration. 



In all Vertebrata, and in a large number of Invertebrata, certain parts, 

 either lungs or gills, are specially constructed for bringing the blood into 

 proximity with the aerating medium (atmospheric air, or water containing 

 air in solution). In some of the lower Vertebrata (frogs and other naked 

 Amphibia) the skin is important as a respiratory organ, and is capable of 

 supplementing to some extent the functions of the proper breathing ap- 

 paratus. 



A lung or a gill is constructed essentially of a fine transparent membrane, 

 one surface of which is exposed to the air or water, as the case may be, while 

 on the other surface is a network of blood-vessels. The only separation be- 

 tween the blood and aerating medium is the thin wall of the blood-vessels 

 and the fine membrane on which the vessels are distributed. The difference 

 between the simplest and the most complicated respiratory membrane is 

 one of degree only. 



In the mammals and the higher vertebrates the respiratory membrane 

 is included within a respiratory cavity, the chest or thorax, which carries on 

 regular movements, the respiratory movements, to bring changes of air into 

 close contact with the respiratory surface. 



The complexity of the respiratory membrane, the kind of aerating medium, 

 and the respiratory movements are not, however, the only conditions which 

 cause a difference in the respiratory capacity of different animals. The 

 quantity and composition of the blood, especially as regards the number 

 and size of the red corpuscles, and the vigor and efficiency of the circulatory 

 apparatus in driving the blood to and fro between the lungs and the active 

 tissues, these are conditions of equal, if not greater, importance. 



It may be as well to state here that the lungs are only the medium for the 



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