462 RESPIRATION. 



sary to resort to some other process, in addition to this, to 

 disengage the other gases contained in the blood. It is only 

 necessary to arrest the action of the corpuscles upon the oxy- 

 gen, and then the gases may be set free by the air-pump, or 

 any method which may be convenient. The method adopted 

 by Bernard for the disengagement of all the gases contained 

 in the blood is first to displace the oxygen by carbonic oxide, 

 using about two-thirds of gas by volume to one-third of 

 blood, then to attach the tube to a tube of mercury, and sub- 

 ject the blood to the barometric vacuum, which sets free the 

 carbonic acid and the nitrogen. The results obtained by 

 this method correspond with our ideas concerning the nature 

 of the respiratory process ; and analyses of the blood taken 

 at different periods show variations in the quantities of oxy- 

 gen in the arterial, and carbonic acid in the venous blood, 

 corresponding with some of the variations which we have 

 noted in the loss of oxygen and gain of carbonic acid in the 

 air, in respiration. 



The analyses of Bernard, who obtained from fifteen to 

 twenty per cent, of oxygen in volume from the arterial blood, 

 show the great imperfection of the process employed by 

 Magnus, who obtained from the arterial blood of horses and 

 calves a mean of but 2*44 per cent, of oxygen. It does 

 not seem necessary, therefore, to discuss the criticisms of the 

 results obtained by Magnus which were made by Gay-Lussac 

 and Magendie, soon after their publication, and more recent- 

 ly by Harley and others. 1 



1 To Magnus belongs the credit of demonstrating the important fact that oxy- 

 gen, carbonic acid, and nitrogen can be extracted from the blood by removing the 

 atmospheric pressure. Before his observations, Gmelin, Mitscherlich, and Tiede- 

 manu placed venous blood in a tube over mercury, in the receiver of an air-pump, 

 and by removing the pressure as far as possible, caused the mercury to descend. 

 On admitting air into the receiver and restoring the pressure, the mercury 

 ascended, with the blood, again filling the tube completely. From this they 

 reasoned that there was no free carbonic acid in the blood. By passing up a 

 little acetic acid, carbonic acid was set free, which led them to believe that all 

 the carbonic acid was in combination. Magnus showed that the reason why 

 other observers had failed to extract gas by means of the air-pump was, that the 



