THE RABBIT 449 



auricle into the right ventricle is guarded by a threefold 

 tricuspid valve (Fig. 420), with chorda? tendineas, and a similar 

 twofold mitral valve guards the opening between the 

 chambers of the left side. The two sides do not communi- 

 cate with one another. From the front end of the right 

 ventricle arises the pulmonary artery, and from the left 

 ventricle the aortic arch arises in a similar position, but 

 behind the pulmonary artery. The opening of each of 

 these vessels is provided with three semilunar valves. The 

 pulmonary artery divides to supply the two lungs, and the 

 arteries to the head and arms arise from the arch of the 

 aorta, which afterwards supplies the trunk. In the beating 

 of the heart, the auricles contract simultaneously, and the 

 ventricles follow immediately afterwards ; then after a short 

 pause the auricles start another contraction. The venous 

 blood which reaches the right auricle from the capillaries 

 of the body is driven by the auricular contraction into 

 the right ventricle and thence in turn through the pulmon- 

 ary artery to the lungs. Returning oxygenated to the left 

 auricle it is driven into the left ventricle, and thence 

 through the aorta to all parts of the body. There is thus 

 a double circulation, as in the frog, but the separation of 

 the ventricles and connection of the pulmonary artery with 

 one of them and the aorta with the other dispenses with 

 the elaborate apparatus of the truncus arteriosus. 



The aortic arch bends over to the left and, as the dorsal 

 aorta, passes backwards under the backbone through the 

 chest and abdomen, till it becomes the small 

 caudal artery. A ligamentous band, known as 

 the ductus arteriosus, connects the aortic arch with the 

 pulmonary artery, just before the bifurcation of the latter. 

 At one stage in development this band is represented by an 

 open tube (p. 502). In its course the aorta gives off numerous 

 arteries, of which the following are the most important : 

 (1) The innominate, arising from the top of the aortic 

 arch and dividing into the right subclavian and right 

 common carotid, the latter passing up the neck and 

 forking opposite the angle of the jaw into external and in- 

 ternal branches, (2) the left common carotid, arising from the 

 aortic arch immediately beyond the innominate, 1 (3) the left 



1 Sometimes from the innominate itself. 



