CHEMICAL PHENOMENA OF RESPIRATION. 321 



blood, and only increases as we approach the summit of the 

 venous cone. The respiratory phenomenon of the lungs 

 simply consists in a gaseous exchange, resembling, more or 

 less, the phenomenon of diffusion, but which is not combus- 

 tion: combustion takes place at those points where the 

 tissues of the organism come in close contact with the blood, 

 and in the very structure of these tissues ; the arterial blood is 

 only the vehicle of the oxygen to these tissues, as the venous 

 blood is that which carries off the carbonic acid. 



C. Theory of respiration. 



Respiration, therefore, considered, not from the point of 

 view of the gaseous exchanges, but from that of the chemical 

 phenomena of combustion, of combination and separation, 

 respiration in its very essence, in short, takes place, not in 

 the lung, but in the most intimate portions of the tissues ; 

 thus the liver, in which extremely important, though not 

 well-defined, chemical phenomena take place, makes use of 

 all the oxygen contained in the blood of the portal vein, 

 while the blood which flows from the liver exhibits both the 

 highest temperature and the most decided features of the 

 typical venous blood. That the tissues themselves actually 

 breathe, in a chemical sense, is proved by placing them in 

 an oxygenized gaseous medium, 1 in which their respiration 

 may be seen directly : thus, if a muscle be detached from an 

 organism, and suspended in an oxygenized atmosphere, it 

 will consume oxygen, and exhale carbonic acid : this com- 

 bustion is still more intense if the muscle be made to contract, 

 the reason of which will be understood by referring to the 

 physiological study of the muscle. In its natural position in 

 the organism, the phenomena of the muscle are the same as 

 those of the other tissues ; with the exception that the blood 

 here performs the office of a medium from which the living 

 element borrows oxygen (arterial blood), and gives back car- 

 bonic acid (venous blood). Thus the blood of the veins of a 

 muscle is much darker, more venous, in short, when the 

 muscle is contracted than when it is in a state of entire 

 repose. 



Respiration consists, then, in man and the superior animals, 

 considered in a general way, of three principal parts, three 



1 See P. Bert, " Lemons sur la Respiration." Legons 3 and 4: 



Respiration den Tissus. 



21 



