OHAPTEE VII. 



RESPIRATION. 



THE maintenance of animal life necessitates the continual absorption 

 of oxygen and excretion of carbonic acid; the blood being, in all ani- 

 mals which possess a well-developed blood-vascular system, the medium 

 by which these gases are carried. By the blood, oxygen is absorbed 

 from without and conveyed to all parts of the organism; and, by the 

 blood, carbonic acid, which comes from within, is carried to those parts 

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

 of oxygen and excretion of carbonic acid, 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 con- 

 taining 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 apparatus; but in all the higher animals, including 

 man, the respiratory capacity of the skin is so infinitesimal that it may 

 be practically disregarded. 



Essentially a lung or gill is constructed of a fine transparent mem- 

 brane, one surface of which is exposed to the air or water, as the case 

 may be, while, on the other, is a network of blood-vessels, the only sep- 

 aration between the blood and aerating medium being the thin wall of 

 the blood-vessels, and the fine membrane on one side of which vessels 

 are distributed. The difference between the simplest and the most 

 complicated respiratory membrane is one of degree only. 



The various complexity of the respiratory membrane, and the kind 

 of aerating medium, are not, however, the only conditions which cause 

 a difference in the respiratory capacity of different animals. The num- 

 ber and size of the red blood-corpuscles, the mechanism of the breathing 

 apparatus, the presence or absence of a pulmonary heart, physiologically 

 distinct from the systemic, are, all of them, conditions scarcely second 

 in importance. 



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

 for the exchange, on the part of the blood, of carbonic acid for oxygen. 

 They are not the seat, in any special manner, of those combustion-pro- 



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