OF THE ORGANS OF CIRCULATION. 
61 
heart, and losing itself in the fleshy fibres of that organ. 
The walls of the auricle, although fleshy, are thin ; those 
of the ventricle are of considerable thickness, especially on 
the left side of the portion which extends, in the form of a 
conical appendage, under the left auricle. Each auricle 
communicates with the ventricle by an opening of some 
width, but capable of being closed by a valve. The right 
auricle receives all the veins of the body, with the exception 
of the left jugular ; which, before it perforates the walls of 
the auricle, forms a sac of variable length, and has, besides 
the ordinary tunics, a well-defined muscular coat : two great 
valves serve to close the common entrance of the veins into 
this auricle. The blood delivered into the right chamber 
of the ventricle is propelled into the pulmonary artery, the 
entrance to which is shut by two valves, included, at its 
origin, in the common aortal trunk ; this artery bends below 
the left aorta, and receding from it, approaches the lungs, 
passing along their posterior surface, before penetrating into 
that organ. A single pulmonary vein, proceeding from 
the lung behind the artery of the same name, carries the 
oxygenated blood into the left auricle, which is of a conical 
form, and less capacious than the right auricle. The oxy- 
genated blood, after having passed into the left cavity, is 
propelled towards the right side, where it encounters the 
orifices of the two aortse, each provided with two semicir- 
cular valves, even when these two openings are united into 
one ; as I have repeatedly observed. We have already 
stated that all the arteries arise from the same trunk, the 
interior of which they may be said to penetrate ; the arte- 
ries, however, are most frequently separated from each 
other by partitions, which, although each at the same time 
serves for the walls of two adjacent vessels, prevent the 
blood they carry from mingling before it enters the heart. 
The right artery is considerably less capacious than the 
left, and is divided within a short distance of its exit from 
the common arterial trunk into several branches, which are 
the sole arteries of the neck and head ; it then bends back- 
wards and descends behind the heart to unite with the left 
aorta, which has followed a similar course on the other side. 
This great artery then descends along the body, and only 
