THE GASES OF THE BLOOD 263 



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 the gas is withdrawn by puncturing the bladder with a trocar, the 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 pul- 

 monary 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 demon- 

 strated, 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. 



SECTION V. INTERNAL OR TISSUE 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 themselves 

 in muscle, for instance ; by its continued and scarcely lessened pro- 

 duction not only in a frog whose blood has been replaced by physio- 

 logical salt solution, and which continues to live in an atmosphere 

 of pure oxygen, but in excised muscles; and by the remarkable con- 

 nection 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 end in the cells; 



