COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 257 



right side passing dorsal to the postcaval, and in the rabbit those of the left 

 side to the dorsal side of the left precaval. They convey the aerated blood from 

 the lungs into the left auricle. 



5. The pulmonary artery. The pulmonary artery is the conspicuous vessel 

 extending from the base of the right ventricle forward between the auricles; it 

 soon curves to the left. Its base is generally surrounded by fat, which should 

 be cleaned away. It divides in two at the turn into right and left pulmonary 

 arteries. The division may be found by dissecting along the pulmonary artery 

 immediately in front of the left auricle. Press the heart to the right and follow 

 the left pulmonary artery into the left lung. In the rabbit it passes to the dorsal 

 side of the left precaval vein which may now be severed. The left pulmonary 

 artery courses parallel to and anterior to the most anterior of the pulmonary 

 veins. Now turn the heart to the left and similarly find the right pulmo- 

 nary artery proceeding to the right lung; to trace it sever the precaval vein. It 

 lies immediately anterior to the foremost pulmonary vein. Dorsal to the right 

 pulmonary artery lies the trachea. 



6. The aorta and its branches. Springing from the base of the left ventricle 

 to the left of and dorsal to the pulmonary artery is a very large trunk, the aorta. 

 Right and left coronary arteries spring from the base of the aorta where it leaves 

 the ventricle. The left coronary artery lies between the pulmonary artery and 

 the left auricle, and branches over the ventral and left side of the heart. The 

 right coronary artery lies along the groove between the right auricle and right 

 ventricle and branches to the right and dorsal surfaces of the heart. 



Follow the aorta forward, cleaning away tissue from its surface. It soon 

 describes a curve, known as the arch of the aorta, to the left. From the arch of 

 the aorta spring the large arteries of the neck, head, and fore limbs. These are 

 two in number in the cat, three in the rabbit. Beginning at the right they are 

 in the rabbit: the brachiocephalic or innominate artery; the left common carotid; 

 and the left subclavian. In the cat the branches are the brachiocephalic or 

 innominate artery to the right and the left subclavian to the left. The difference 

 is due to the fact that in the cat, not in man, the left common carotid branches 

 from the brachiocephalic, and this may also occur in the rabbit as a variation. 



Trace the brachiocephalic artery forward. The precaval vein and its 

 branches may be removed. The artery gives off small branches into the thymus 

 gland and trachea lying dorsal to it and then divides into two branches in the 

 rabbit and three in the cat. These are : right subclavian and right common carotid 

 in the rabbit, and right subclavian, right and left common carotids in the cat. 

 Each of these will be traced separately. 



a) Subclavian artery: Trace the right subclavian; both have identical 

 branches. 



Rabbit: From the posterior surface of the subclavian arises the internal 

 mammary artery which follows the vein previously described along the ventral 



