THE GASES OF THE BLOOD 265 



in the tension of carbon dioxide in the alveolar air, so as always 

 to preserve a definite relation. It is quite generally assumed that 

 this relation is one of practical equality, and the classical method 

 of measuring the carbon dioxide tension in the arterial blood in man 

 is to determine it in the alveolar air. 



Evidence in favour of the view that there is, besides diffusion, 

 an element of selective secretion in the exchange of gases through 

 the pulmonary membrane has been found by some writers in the 

 results of a study of the gases of the swim-bladder in fishes. This 

 study has demonstrated the existence of animal cells which actually 

 secrete gases. This fact, however, even if it removes a presump- 

 tion against, does not establish a presumption in favour of, the 

 secretion theory of external respiration. These gases 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 the gas is withdrawn by puncturing the bladder with a trocar, the 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. But this is a consequence of an alteration in the pulmonary 

 circulation through the vasomotor fibres for the lungs which are 

 known to run in the vagus in this animal. In the mammal, no such 

 effect has been demonstrated, and the decisive proof that the lungs 

 are gas-secreting glands which would be afforded by the discovery 

 of secretory nerves is 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 



