EFFECTS OF RESPIRATION. 145 



Besides the assistance thus .afforded by muscular pressure 

 to the movement of blood along veins possessed of valves, it 

 has been discovered by Mr. Wharton Jones that, in the web 

 of the bat's wing, the veins are furnished with valves, and 

 possess the remarkable property of rhythmical contraction 

 and a dilatation, whereby the current of blood within them is 

 distinctly accelerated. The contraction occurred, on an aver- 

 age, about ten times in a minute ; the existence of valves pre- 

 venting regurgitation, the entire effect of the contractions was 

 auxiliary to the onward current of blood. Analogous phe- 

 nomena have been now frequently observed in other animals. 



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 enumerated : 



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



2. The vital capillary force exercised in the capillaries. 



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 respira- 

 tory 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 probably, under ordinary 

 circumstances, but slight. The effect of expiration in increas- 

 ing the pressure of the blood in the arteries is minutely illus- 

 trated by the experiments of Ludwig. It acts as the pressure 

 of contracting muscles does upon the veins, and is advantage- 

 ous to the onward movement 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 pre- 

 vented by the force of the onward stream of blood from the 

 contracting ventricle, and in the intervals of this contraction 

 by the closure of the semilunar valves. Under ordinary cir- 

 cumstances, 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 move- 

 ment 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 

 apart from what may be effected by the presence of the valves 

 of the venous system. 



