352 
POPULAR SCIENCE REVIEW. 
This four-chambered heart is a muscle, and acts by means of 
muscular force. What the agent may be which irritates this 
muscle and so causes it to contract, has been a subject of much 
discussion ; but it is now generally considered that this agent 
is oxygen, which is received into the blood in the lungs, and 
stimulates the muscular contractility of the heart through the 
nerves of the organ. This muscular action of the heart is 
almost entirely beyond the control of the will, as indeed are all 
the vital actions of the body; yet instances have been related 
of persons who were able to stop the heart’s action at will, and 
in one case this experiment was carried too far, and the 
individual died by the mere act of his own will. On the other 
hand, the heart will continue its regular pulsations for a long 
time after its removal from the body, and of course the death 
of the animal; and the lower the creature in the scale of 
creation, the longer will this action continue ; so that the heart 
of a sturgeon will continue to beat as long as twenty-four hours 
after its removal from the creature. 
Through this four- chambered heart, then, the blood must 
pass in one direction only, and not indiscriminately backwards 
or forwards ; and to effect this there is a whole series of beautiful 
contrivances. To begin with the first chamber, namely, the 
right auricle. As this chamber is diluting after each contraction 
it receives the blood Avliich has been collected from all parts of 
the body into two enormous veins, and it also receives blood 
from some small veins which come from the substance of the 
heart itself, — for this organ has of course to supply blood to 
nourish its own muscular substance ; but some force is wanting 
besides the mere dilatation of the auricle in order to ensure its 
being properly filled with blood, especially in the case of the 
vein, which, coming from the lower parts of the body, has to 
send its blood against the force of gravity, and accordingly 
these two large veins are provided with muscular coats for a 
short distance back from the auricle. There is a large opening 
between this auricle and the second chamber, namely, the right 
ventricle, an opening large enough to admit the tops of three 
fingers, and some of the blood flows through this opening at 
once, but the greater part of it fills the auricle which will 
contain about two ounces. It fills slowly, but the moment it is 
full it makes an extremely quick contraction, by which it forces 
nearly the whole of the blood through the opening into the 
right ventricle. Quick as this contraction is, occupying about 
the eighth part of a second, it can be observed to begin where 
the great veins enter the auricle, and to extend gradually over to 
the opening into the ventricle ; and this is just what is necessary 
to ensure the driving of the blood gradually from behind 
forwards into the ventricle, and the blood cannot return up 
