120 PHYSIOLOGY OF THi: CIKCULATION. 



The property of mutual diffusion possessed l>y 

 gases, is useful, not only as facilitating the discharg.- 

 of carbonic acid gas through the lining membranes 

 of the lung, but also from its enabling us to explain 

 why that discharge is not general throughout the 

 body, and why it only takes place on a free surface ; 

 where the external pressure is inconsiderable, and 

 where the surrounding atmosphere presents a species 

 of vacuum into which the imprisoned and compressed 

 gases may freely expand. But it does not throw 

 any light upon the means by which gases are absorbed 

 into the blood. And I shall now mention some 

 reasons for thinking that the absorption of gases is 

 effected in the same manner as that of liquids ; viz., 

 by a preponderance of the pressure acting on the 

 exterior of the absorbing blood-vessels. 



1. That the absorption of oxygen in the lungs 

 takes place in the terminating portion of the capil- 

 laries and the small pulmonary veins, is, to say the 

 least, rendered highly probable by the above-men- 

 tioned observation of Lower. 



2. The general absorbing power of the healthy 

 lung is extremely active ; so that any poisonous sub- 

 stance brought in contact with its lining membrane 

 produces its effect upon the system almost instan- 

 taneously. 



3. Atmospheric air, or any other gas injected into 

 the cellular tissue of the body, is, if the quantity be 

 moderate, speedily absorbed. 



4. Jt is fully proved that streams of liquid po.s.^^s 

 the power of absorbing, through the lateral apertures 

 "f the containing tube, any gaseous lluids that mav 



