THE CIRCULATION OF THE BLOOD DEPENDS. 
491 
elasticity of the heart existed ; yet while artificial respiration was performed 
we could perceive no abatement in the vigour of the circulation. 
It is to be observed, that all these means can act only in one way in pro- 
moting the circulation, namely, by giving to the heart the power of suction ; 
that is, by producing a tendency to vacuum in its cavities, in consequence 
of which the pressure of the atmosphere propels the blood from the veins into 
them, that of the arteries being prevented from returning to the heart by the 
valves at their origins. But all, as far as I know, who have either made experi- 
ments with a view to prove the supposed effect of these means on the circulation, 
or who have sanctioned the inferences from such experiments, have overlooked 
the circumstance that the veins being tubes of so pliable a nature that when 
empty they collapse by their own weight, whatever may be said of the effect of 
such causes in favouring a horizontal or descending motion of the blood, it is 
impossible that an ascending motion could be produced in them on the prin- 
ciple of suction. As far as the heart may possess any such power, its tendency 
must be to cause the vessel to collapse, not to raise the fluid it contains. 
That the resilience of the lungs as far as they possess this property, and the 
act of inspiration, tend to dilate the heart and large vessels within the chest, is 
evident ; but the former is very trifling, if it exist at all, except as far as it 
depends on the mere weight of the lungs ; and the latter in common breathing 
is little more efficient, although the effect of respiration on the brain, when any 
part of the cranium is removed, sufficiently attests that it has a certain effect. 
When the breathing is so laborious as essentially to influence the circulation, 
it evidently tends to derange the regular flow of the blood towards the heart, 
inspiration of course acting interruptedly ; whereas it is only necessary to 
inspect the chest of any of the more perfect animals immediately after death, 
and while artificial respiration is being performed, provided death has not 
been caused by great loss of blood, or an extreme and instantaneous impression 
on the nervous system, to see that the blood flows uniformly towards the heart 
with no interruption but that which the contraction of the heart itself occasions. 
The elasticity of the heart is greater in some animals than in the rabbit; but 
it is in all cases very inconsiderable. The heart of the tortoise is the most 
elastic I have examined ; yet even it may be compressed during its diastole by 
a force not sensibly greater than is sufficient to compress other muscles in a 
