604 THE ENTRANCE OF OXYGEN. [BOOK n. 



pressure of oxygen; the oxygen seems to pass into the swim-bladder 

 by a process like that of secretion. Analogy suggests that the 

 entrance of oxygen into the blood in the lungs may be by a similar 

 process ; and arguments have been brought forward in favour of 

 such a view. These however do not seem conclusive ; the weight 

 of evidence is, at present at least, against this view that the film 

 in question exerts any influence of a secretory nature. Nor have 

 we any evidence that it exerts any marked influence as a mere 

 membrane or septum ; the oxygen appears to pass into the blood 

 in the same way that it would if the blood were freely exposed 

 without any intervening partition to the alveolar air. Further, 

 the evidence, so far as it goes, seems to shew that blood absorbs 

 oxygen in the same way as an aqueous solution of haemoglobin 

 of the same concentration ; the zone of plasma spoken of above as 

 surrounding each corpuscle behaves as far as regards the passage 

 of oxygen to and from the corpuscles in no essentially different 

 respect from the way the molecules of water, belonging to a 

 molecule of dissolved haemoglobin, behave in regard to the 

 absorption or the giving-off of oxygen by an aqueous solution 

 of haemoglobin. 



355. The evidence in favour of the entrance being a matter 

 of diffusion is as follows. In man, as we have seen, expired air 

 contains about 16 p.c. of oxygen. The air in the pulmonary 

 alveoli must contain less than this, since the expired air consists of 

 tidal air mixed by diffusion with the stationary air. How much 

 less it contains we do not exactly know, but probably the difference 

 is not very great. At the ordinary atmospheric pressure of 760 mm. 

 16 p.c. is equivalent to a partial pressure of 122 mm. The question 

 therefore stands thus, Will venous blood, exposed at the tempe- 

 rature of the body to a partial pressure of less than 122 mm. 

 (less than 16 p.c.) of oxygen take up sufficient oxygen (from 8 

 to 12 vols. p.c.) to convert it into arterial blood ? Numerous 

 experiments have been made (chiefly but riot exclusively on the 

 dog) to determine on the one hand the oxygen-pressure of both 

 arterial and venous blood (i.e. the partial pressure of oxygen in 

 an atmosphere exposed to which the arterial blood neither gives 

 up nor takes in oxygen, and the same for venous blood), and on 

 the other hand the behaviour at the temperature of the body or at 

 ordinary temperatures of blood or of solutions of haemoglobin (for 

 the two as we have just said behave in this respect very much 

 alike) towards an atmosphere in which the partial pressure of 

 oxygen is made to vary. 



The partial pressure of this or that gas in blood may be determined 

 by the aerotonometer. This consists essentially of a chamber (a glass 

 tube) into which the blood is allowed to flow from the living vessel 

 and in which it is freely exposed to the atmosphere contained in the 

 chamber, the whole being kept at a constant temperature, at the 



