224 
STRUCTURE OF THE HEART. 
of which, the upper one is termed the auricle , and the lower 
the ventricle . Thus we have the right and left auricles, and 
the right and left ventricles. Each auricle communicates 
with its corresponding ventricle, by an aperture in the 
Superior Pulm. Pulmonary 
vena cava art. Aorta artery 
—y Pulmonary veins 
Pulmonary veins , 
Right auricle- 
Tricuspid valves- ~J 
Inferior vena cava *—~l 
Right ventricle 
Left ventricle 
Partition Aorta 
Fig. 123.—Ideal Section op the Human Heart. 
transverse partition, which is guarded by a valve. The walls 
of the ventricles are much thicker than those of the auricles; 
and for this evident reason,—that the ventricles have to 
propel the blood, by their contraction, through a system of 
remote vessels; whilst the auricles have only to transmit the 
fluid that has been poured into them by the veins, into the 
ventricles, which dilate themselves to receive it. The difference 
in the thickness of the walls of the left and the right ventricles 
is explainable on the same principle; for the left ventricle 
has to send the blood, by its contractile power, through the 
remotest parts of the body; whilst the right has only to 
transmit it through the lungs, which, being much nearer, 
require a far less amount of force for the circulation of the 
blood through them. 
258. The arterial system of the greater circulation entirely 
springs from one large trunk, which is called the aorta (see 
figs. 122-124); this originates in the left ventricle, and is 
the only vessel which passes forth from that cavity. It first 
ascends towards the bottom of the neck; then forms what is 
termed the arch , a sudden curve, which gives it a downward 
