THE CAUSES OF THE GASEOUS EXCHANGE 557 



It remains to touch briefly upon the question whether the 

 respiratory exchange is effected by the living cell through the 

 agency of ferments. The work of Schmiedeberg has already been 

 mentioned, and further research along the same lines has in recent 

 years demonstrated that different intracellular ferments with the 

 power of oxidising special substances are widely distributed in 

 animal and vegetable tissues. These ferments, which are called 

 oxydases, are present in different amounts in different tissues, 

 and Jacquet has found that extracts of organs free from cells can 

 still oxidise aldehydes. The chemical constitution of the oxydases 

 is unknown ; it is not apparently that of proteids or nucleo- 

 proteids. Future work must show whether respiratory exchange 

 is a complex process due to oxidising and reducing ferments. 



The Causes of the Gaseous Exchange between the Blood, Lymph, 

 and Tissues. The lymph acts as the means of transport between 

 the blood and the tissues ; it carries oxygen to, and carbon dioxide 

 from, the tissues. On the theory that this gaseous exchange is 

 a process of diffusion, it would be expected that the partial pressure 

 of oxygen should show a descending scale, that of carbon dioxide 

 an ascending scale, through the blood, lymph, and tissues. This 

 it is difficult to prove experimentally, for it is impossible to an- 

 alyse the lymph lying between the capillary blood-vessels and the 

 tissues, where the actual transference of gases occurs. The lymph 

 for analysis must be collected from some large lymphatic vessel, 

 such as the thoracic duct, and during its sluggish flow from the 

 lymph-spaces to such a large vessel the lymph will have been 

 exposed to the action of the blood circulating in the surrounding 

 vessels, and thus some gaseous interchange may occur. 



Lymph contains only traces of oxygen, but a large quantity 

 of carbon dioxide, according to Hammarsten's analyses Ol volume 

 per cent, of oxygen, 37-5 of carbon dioxide, and 1-6 of nitrogen. 

 The transference of gas depends not upon the quantity but the 

 partial pressure of the gas. Strassburg found that the partial 

 pressure of the carbon dioxide in the lymph was intermediate 

 between the values obtained for the arterial and venous blood ; 

 Gaule, on the other hand, obtained values higher than that for 

 the venous blood. 



In the case of oxygen there is a descending series from the 

 high pressure of the gas in the arterial blood down to the tissues 

 where the partial pressure on account of the great reducing activity 



