EFFECTS OF RESPIRATION. 173 



Agents Concerned in the Circulation of the Blood. 



The agents concerned in the circulation of the blood 

 which have been now described, may be thus enume- 

 rated : 



1 . The action of the heart and of the arteries. 



2. The vital capillary force exercised in the capil- 

 laries. 



3. The possible slight action of the muscular coat of 

 veins ; and, much more, the contraction of muscles capable 

 of acting on veins provided with valves. 



It remains only to consider (4) the influence of the 

 respiratory movements on the circulation. 



Although the continuance of the respiratory movements 

 is essential to the circulation of the blood, and although 

 their cessation is followed, within a very few minutes, by 

 that of the heart's action also, yet their direct mechanical 

 influence on the movement of the current of blood is pro- 

 bably, under ordinary circumstances, but slight. The effect 

 of expiration in increasing the pressure of the blood in the 

 arteries is minutely illustrated by the experiments of Lud- 

 wig. It acts as the pressure of contracting muscles does 

 upon the veins, and is advantageous to the onward move- 

 ment of arterial blood, inasmuch as all movement backwards 

 into the heart, which would otherwise occur at the same 

 moment and from the same cause, is prevented by the force 

 of the onward stream of blood from the contracting ven- 

 tricle, and in the intervals of this contraction by the closure 

 of the semilunar valves. Under ordinary circumstances, 

 and with a free passage through the capillaries of the lungs, 

 the effect of expiration on the stream of blood in the veins 

 is also probably to assist, rather than retard its movement 

 in the proper direction. For, with no obstruction in front, 

 there is the force of the blood streaming into the heart from 

 behind, to prevent any tendency to a backward flow, even 



