RESPIRATORY MECHANISMS. 319 



way to and from the tissues and external air respectively. These 

 gas interchanges form the very important function of Respiration. 



Here, as in the case of the nutritive materials, the blood acts 

 as the carrier between the tissues and the outer world. The pul- 

 monary half of the circulation is devoted to the gas interchange 

 between the blood and the atmosphere, and is sometimes spoken 

 of as external respiration. The gas interchange between the blood 

 and the tissues goes on in the general systemic capillaries, and 

 has therefore been spoken of as the internal or tissue respiration. 



The special arrangement for the taking up of oxygen from the 

 air, and for the giving up of carbonic anhydride to the air is 

 named the pulmonary apparatus. In mammalia this is so far per- 

 fected that all the necessary gas interchange can be carried on 

 by the lungs, and the respiratory influence of the external skin 

 or the mucous passages may be regarded as insignificant. But it 

 should be remembered that whenever the blood is in close rela- 

 tion to oxygen, as in the case of swallowed air, the oxygen is soon 

 absorbed by the blood. 



In the lower animals the cutaneous surface aids very materially 

 in respiration, and thus frogs can live from this cutaneous respi- 

 ration alone for an almost indefinite time. 



In the lungs the change consists in oxygen being taken from 

 the atmospheric* air by the blood and carbonic anhydride being 

 given off from the blood to the air. In the capillaries, on the 

 other hand, the blood takes the carbonic anhydride from the tis- 

 sues, and yields to them a great portion of its oxygen. 



In the lowest class of animals (e.g., amoeba) we find no special 

 organs for the purpose of respiration, the gas interchange being 

 sufficiently provided for by the exposure of the general surface 

 of their bodies to the medium in which they live, namely, water. 



Other animals have some special apparatus for the purpose of 

 respiration. This apparatus has always the same essential object, 



* The composition of the atmosphere is everywhere remarkably constant, 

 in spite of its oxygen being used up by living beings. It consists of 



Oxygen, 21 vols. 



Nitrogen, 79 " 



Moisture (variable), ..... .8 per cent. 



Carbonic acid gas (also variable), , , .04 (( 



