xra PHYLUM CHORDATA 93 



branches of the ventral aorta do not break up into capillaries, but 

 pass directly into the dorsal aorta, forming the aortic arches (Fig. 783, 

 Ab.). With the appearance of the lungs, however, a very funda- 

 mental change occurs in the blood-system. The last aortic arch of 

 each side gives off a pulmonary artery (Fig. 784, Ap.) to the corres- 

 ponding lung, and the blood, after circulating through the capil- 

 laries of that organ, is returned by a pulmonary vein (lr.), not 

 into an ordinary systemic vein of higher order, but into the heart 

 directly : there it enters the left side of the auricle, in which 

 a vertical partition is developed, separating a left auricle (A 1 ), 

 which receives the aerated blood from the lungs, from a right 

 auricle (A), into which is poured the impure blood of the sinus 

 venosus. Lastly, in Crocodiles, Birds, and Mammals (B) the 

 ventricle also becomes divided into right and left chambers, and we 

 get a four-chambered heart, having right and left auricles and right 

 and left ventricles : at the same time the conus arteriosus and sinus 



FIG. 781. 



venosus cease to exist as distinct chambers. The left auricle receives 

 aerated blood from the lungs and passes it into the left ventricle, 

 whence it is propelled through the system : the right auricle receives 

 impure blood from the system, and passes it into the right ventricle 

 to be pumped into the lungs for aeration. Thus the four-chambered 

 heart of the higher Vertebrata is quite a different thing from that 

 of a Fish : in the latter the four chambers sinus venosus, auricle, 

 ventricle, and conus arteriosus form a single longitudinal series, 

 whereas in a Mammal, for instance, the four chambers constitute 

 practically a double heart, there being no direct communication 

 between the auricle and ventricle of the right side, or respiratory 

 heart, and those of the left side, or systemic heart. The modifications 

 undergone by the arteries and veins in the higher Vertebrata will be 

 best considered under the various classes. 



It will be noticed that there is a sort of rough correspondence 

 VOL. IT G** 



