16 THE LAWS OF ORGANIC 



tion, therefore, will stop the easy passage of the 

 blood through the lungs."* 



XXIII. The above description is sufficiently 

 correct and explicit to shew, that the circulation 

 of the blood is impeded in expiration ; and, 

 therefore, if that which is returned by the veins 

 is partly obstructed in its passage, the obvious 

 consequence is, that the quantity in the lungs 

 will be diminishing more rapidly than the quan- 

 tity of the venous blood necessary to maintain the 

 ordinary balance between the two systems can be 

 supplied. If we suppose that the lessened capa- 

 city of the chest simply retards the flow of blood 

 from the veins, without in the least considering 

 that this diminution facilitates arterial circula- 

 tion, the lungs will contain less blood than is 

 found to exist in the well-poised states of respi- 

 ration. The lungs, as stated in XVIII., always 

 contain a considerable quantity of air ; and as this 

 is that portion which immediately acts upon the 

 blood, if we diminish this blood by a series of 

 expiratory actions, it is philosophical to suppose 

 that it will be more perfectly oxygenated, be- 

 cause a smaller quantity is submitted to the in- 

 fluence of that important agent. 



XXIV. I have observed, in speaking of the 

 quantity of air contained within the chest, that 

 expiration can lessen this only to a determinate 



* HALLER'S First Lines of Physiology, Vol, I. Sect. CCLXXII. 



