306 THE CIKCULATION. 



the lungs, exhales the carbonic acid which it has accumulated else- 

 where, and absorbs the oxygen which is afterward transported to 

 distant tissues by the current of arterial blood. The phenomena of 

 the circulation are even liable, as we have already seen, to periodical 

 variations in the same organ ; increasing or diminishing in intensity 

 with the condition of rest or activity of the whole body, or of the 

 particular organ which is the subject of observation. 



