THE CIRCULATION OF THE BLOOD DEPENDS. 
495 
and blood-vessels themselves, and that no auxiliary power is necessary for its 
perfect performance. Here, as in other cases, the more we study the opera- 
tions of nature, the more direct and simple we find them. The resilient power 
of the lung-s and elasticity of the ventricles of the heart, as far as they exist, 
favour the free entrance of the blood into these cavities, an office adapted to the 
feebleness of such powers, which, in many animals we have seen, have no exist- 
ence. Their operation is similar, but probably much inferior, to the elastic power 
of the arteries, by which the ingress of the blood suddenly impelled into them by 
the svstole of the heart, is rendered more free than it would have been had these 
vessels tended to collapse in the intervals of its contractions. Had the blood 
flowed into them in a continued stream, and been carried through them by their 
own powers alone, their elasticity would evidently have impeded, not pro- 
moted, the circulation through them. Thus the veins, where these conditions 
obtain, are so pliable that they collapse by their own weight, and hence it 
was that in the preceding experiments the vein carried on its blood so much 
more rapidly and completely than the artery, which felt the want of the 
impulse it receives from the heart, that at once assists in propelling its blood, 
and through the blood stimulates the vessel itself. The action of the vein 
was perfect ; it possessed all its usual powers, which reside in itself alone. 
It only remains for us to inquire into the nature of the power by which the 
heart and blood-vessels maintain the circulation. Respecting the nature of 
the power of the heart there cannot be two opinions. It is evidently a mus- 
cular power. The structure of its parietes is similar to that of other muscles, 
and they obey all the usual laws of the muscular fibre. 
Is the power of the vessels of the same nature ? This is a question which 
has frequently been discussed. The chief arguments which have been ad- 
duced in favour of the affirmative are, the nature of their function ; the fibrous 
appearance observed in some of the vessels, which is more evident in some 
other animals than in man ; and the minuteness of most of the vessels, which, 
if they are muscular, accounts for the difficulty with which the muscular 
structure is detected in them. The chief arguments against the muscularity 
of the vessels have been, that they could not be made to obey an artificial 
stimulus in the way that the heart and other muscles are found to do, and that 
their chemical analysis gives no evidence of fibrin. Of the latter of these ob- 
3 s 
MDCCCXXXI. 
