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 aortae, 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 Avhich 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 



