PASSAGE OF OXYGEN TO THE BLOOD. 163 



conclusions. It has "been taken for granted that the actual diffusion is 

 very rapid, perhaps approaching to the velocity with which gases rush 

 into a void. But I have shown* that this is altogether a misconception, 

 and that the transit of fresh air from the bronchi, exchanging with foul 

 air from the cells, if conducted on that principle alone, would require a 

 period greatly beyond the time occupied for one respiratory act, which is 

 about three seconds and a half. 



To an additional agent we must therefore look for a complete explana- 

 tion, and this, I think, is presented in the circular organic fibres of the 

 bronchial tubes and cells. It has long been understood that these pos- 

 sess the power of varying the capacity of the tubes. 



With this agency in view, this second stage of the process is accom- 

 plished as follows : The carbonic acid, vapor of water, and excess of ni- 

 trogen, if any, that have accumulated in the cells belonging to any given 

 bronchial tree, are expelled therefrom by the muscular contraction of the 

 circular organic fibres, and are delivered into the larger bronchial tubes, 

 in which diffusion at once takes place with the air just introduced. As 

 soon as the expiration is completed, relaxation of the muscular fibres oc- 

 curs, and the passages and cells dilating, both through their own elastic- 

 ity and the exhaustive effect arising from the simultaneous contraction of 

 other bronchial trees, fresh air is drawn into them, the alternate expulsion 

 and introduction being accomplished by muscular contraction and elas- 

 ticity, the different bronchial trees coming into action at different periods 

 of time, some being contracting while others are dilating. 



3d. The third stage is the passage of oxygen from the cells to the blood: 

 it is through the wall of the cell, the wall of the blood-vessel, Passage of ox- 

 and the sac of the blood disc. The carbonic acid issues from gen thr u s h 



the membranes 



the plasma, and passes through the wall of the blood-vessel to the blood. 

 and the wall of the cell. 



Many physiologists have supposed that this exchange of oxygen for 

 carbonic acid takes place on the principle of diffusion. On Exchan e of 

 the authority of Valentin and Brunner, it has been asserted carbonic acid 

 that the proportional exchange actually observed is 1174 of 

 oxygen for 1000 of carbonic acid, these being the theoretical quantities 

 under the law of diffusion ; but there is no difficulty in proving that this 

 is a physical impossibility, for the exchange is not merely that of oxy- 

 gen and carbonic acid; it is much more complicated. The lungs regu- 

 late the quantity of free nitrogen in the system, and there is a constant 

 escape of the vapor of water. These bodies, moreover, are not present- 

 ed in the gaseous state, but in that of liquid solution ; and the wall of 

 the cell, of the pulmonary capillary, and of the blood disc, by their con- 

 densing action, totally disturb the conditions of diffusion. 

 * American Journal of Med. Sciences, April, 1852. 



