RESPIRATION 259 



consist of oxygen, nitrogen, and usually a small quantity of 

 carbon dioxide, but in very different proportions from those in 

 which they exist in the air or the water. Thus, as much as 87 per 

 cent, of oxygen has been found in the bladder of fishes taken at a 

 considerable depth, but a smaller amount in those captured near 

 the surface. When the gas is withdrawn by puncturing the 

 bladder with a trocar, the organ rapidly refills, and the percentage 

 of oxygen increases. Further, this process of gaseous secretion 

 is under the influence of nerves, for gas ceases to accumulate in 

 the organ when the branches of the vagi that supply it are cut. 

 In the tortoise stimulation of the peripheral end of the vagus 

 causes a fall of gaseous exchange in the corresponding lung, with 

 an accompanying rise in the other lung. That this is not the 

 consequence of an alteration in the pulmonary circulation is 

 indicated by the fact that the change is greater in the intake 

 of oxygen than in the output of carbon dioxide. In the mammal, 

 however, no such effect has been clearly demonstrated, and the 

 decisive proof that the lungs are gas-secreting glands which would 

 be afforded by the discovery of secretory nerves is still wanting. 



We have now completed the description of the phenomena 

 of external respiration, with the discussion of its central fact, 

 the exchange of gases between the blood and the air at the 

 surface of the lungs. It remains to trace the fate of the absorbed 

 oxygen, and to determine where and how the carbon dioxide 

 arises. 



Internal Respiration Seats of Oxidation. The suggestion 

 which lies nearest at hand, and which, as a matter of fact, was 

 first put forward, is that the oxygen does not leave the blood 

 at all, but that it meets with oxidizable substances in it, and 

 unites with their carbon to form carbon dioxide. While there is 

 a certain amount of truth in this view, oxygen, as already 

 mentioned, being to some extent taken up by freshly-shed blood, 

 and also by blood under other conditions, to oxidize bodies, 

 other than haemoglobin, either naturally contained in it or arti- 

 ficially added, there is no doubt that the cells of the body are the 

 busiest seats of oxidation. This is shown by the presence of 

 carbon dioxide in large amount in lymph and other liquids 

 which are, or have been, in intimate relation with tissue elements ; 

 by its presence, also in considerable amount, in the tissues them- 

 selves in muscle, for instance ; by its continued and scarcely 

 lessened production not only in a frog whose blood has been 

 replaced by physiological salt solution, and which continues to live 

 in an atmosphere of pure oxygen, but in excised muscles ; and by 

 the remarkable connection between the amount of this production 

 and the functional state of those tissues. In insects the finest twigs 

 of the tracheae, through which oxygen passes to the tissues, actually 



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