RESPIRATION 243 



interchange of gases through the pulmonary membrane is 

 afforded by a study of the gases of the swim-bladder in 

 fishes. These consist of oxygen, nitrogen, and usually a 

 small quantity of carbon dioxide, but in very different pro- 

 portions from those in which they exist in the air or the 

 water. Thus, Biot found as much as 87 per cent, of oxygen 

 in the bladder of fishes taken at a considerable depth, but a 

 smaller amount in those captured near the surface. Moreau 

 observed that when the gas is withdrawn by puncturing 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 (Bohr\ 



We have now completed the description of the pheno- 

 mena 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 how and 

 where 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 artificially 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 nuscle, for instance; by its continued and 

 scarcely lessened production not only in a frog whose blood 

 has been replaced by normal saline solution, and which 

 continues to live in an atmosphere of pure oxygen, but in 



1 6 2 



