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62 PHYSIOLOGY OF THE HUMAN BODY. 
the lower end, ventricles! (a the right, and’ m the left, fig. 54). And 
as the most important division of the heart is into two sides, there are 
a right and left auricle and a right and left ventricle. Between each 
auricle and the ventricle on the same side, there is a valve (c and J, in fig. 
54), like those in a force-pump [Puysics, page 17], through which the 
blood is forced, by the muscular contraction of the heart, from the auricle 
into the ventricle. The blood brought back 
from the body is poured, by the two veins f 
and 6, into the right auricle; the right, auricle 
contracting on this blood, drives it through 
the valve, ¢, into the right ventricle ; the right 
ventricle now contracting, drives the blood, 
which cannot pass back to the auricle, because 
the pressure on it closes the valve, along the 
pulmonary artery, branches of which, g and 4, 
go to the right and left lung respectively. 
The blood is brought back from the lungs by 
the pulmonary veins, two from each lung, 
e, e’, and poured into the left auricle; the 
contraction of the left auricle drives the blood 
through the valve, J, into the left ventricle ; 
the left ventricle contracts, and drives the blood into the large artery, h, 
called the aorta,? because it seems to suspend the heart between the 
lungs. : 
The whole of the arterial blood leaves the heart by the one large trunk 
of the aorta ; this trunk soon divides into branches ; these branches sub- 
divide into smaller branches, and these into smaller still, until every 
part of the body, especially at the surface, is intersected with such a net- 
work of small arteries, that the point of a needle cannot be inserted into 
the flesh without wounding one or more of them, and thus drawing 
blood. Toa certain extent, the veins might be said to run parallel with 
the arteries, branch for branch, with this difference, that they are carry- 
ing back to the heart the blood which has oozed into them through the 
capillaries from the arteries. It will easily be understood that the force 
of the blood in the arteries, as propelled direct from the heart, must be 
much greater than in the veins, in which the blood is found after it has 
passed through a succession of most minute tubes, where much of its. 
velocity is lost. Accordingly, the arteries are of a much stronger structure 
than the veins. In consequence of the strong flow of blood through the 
larger arteries, they are, for the most part, placed at a distance from the 
surface, so as to be protected from injury ; the veins, on the other hand, 

1 Latin ventriculus, diminutive of venter, the belly. 2 Greck aorté, from aeird, te suspend, 
