PHYSIOLOGY OF THE CIRCULATION. 119 



to be incapable of explaining the phenomena* : they 

 are therefore generally referred to the mutual pene- 

 trability of gases. 



That the extrication of carbonic acid in the lungs 

 is materially facilitated by this property of diffusion, 

 I can readily believe ; but are we to suppose that 

 this discharge of carbonic acid from the blood occurs 

 equally in every part of the pulmonary vessels, and 

 that the absorption of oxygen in like manner takes 

 place with equal activity in the minute arteries and 

 veins, and throughout the whole extent of the capil- 

 laries of the lung ? There are many recorded 

 observations which militate strongly against such a 

 supposition. And as many of these phenomena are 

 perfectly explicable by the general principles of 

 effusion and absorption, I may, perhaps, be excused 

 for now briefly noticing some of the more important 

 of them. It was shown by Lower, not long after 

 the discovery of the circulation, that the change 

 from venous to arterial blood takes place during the 

 passage of the former fluid through the small vessels 

 of the lungs. Subsequent experiments have proved 

 that this change consists essentially in the discharge 

 of carbonic acid from, and the absorption of oxygen 

 into, the mass of blood. And as the blood circu- 

 lating through the lungs does not present the bright 

 red colour of arterial blood till it has entered the 

 small pulmonary veins, this circumstance alone jus- 

 tifies the presumption that the absorption of oxygen 

 by the blood is effected in this part of its course. 



* Magendie, "Journal," t, i. p. 16. 

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