CHAPTEK VI. 



RESPIRATION. 



THE maintenance of animal life necessitates the continual absorption 

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

 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 contain- 

 ing 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 practi- 

 cally 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 separation 

 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 diiference 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 

 diiference in the respiratory capacity of different animals. The number 

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

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

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

 in importance. 



In the heart of man and all other Mammalia, the right side from which 

 the blood is propelled into and through the lungs may be termed the 



