230 THE CIRCULATION OF THE BLOOD. [CH. xix. 



next cavity, that of the ventricle, and last of all is the aortic bulb, 

 From the bulb, branches pass to the gills where they break up 

 into capillaries, and the blood is aerated : it then once more 

 enters larger vessels which unite to form the dorsal aorta, whence 

 the blood is distributed by arteries to all parts of the body ; here 

 it enters the systemic capillaries, then the veins which enter the 

 sinus (whence we started) by a few large trunks. 



Taking the frog as an instance of an amphibian, we find the 

 heart more complex still, and the simple peristaltic action of the 

 heart muscle as we have described it in the hearts of worm and 

 fish, becomes correspondingly modified. There is only one 

 ventricle, but there arc two auricles, right and left. 



-C.S.d. 



A.d: 



Fig. 233. The heart of a frog (Eana esculenta) from the back, s.v., sinus venosus opened ; 

 c.s.s., left vena cava superior; c.s.d., right vena cava superior ; c.i., vena cava inferior ; 

 v.p., vena pulmonalis ; A.d., right auricle ; A .,., left auricle ; A. p., opening of com- 

 munication between the right auricle and the sinus venosus. x 2^3. (Ecker.) 



The ventricle contains mixed blood, since it receives arterial 

 blood from the left auricle (which is the smaller of the two), and 

 venous blood from the right auricle ; the right auricle receives the 

 venous blood from the sinus, which in turn receives it from the 

 systemic veins. The left auricle, as in man, receives the blood 

 from the pulmonary veins. 



When the ventricle contracts, it forces the blood onward into 

 the aortic bulb which divides into branches on each side for the 

 supply of the head (fig. 232, i), lungs and skin (fig. 232, 3), and 

 the third branch (fig. 232, 2) unites with its fellow of the opposite 

 side to form the dorsal aorta for the supply of the rest of the 

 body. 



Passing from the amphibians to the reptiles, we find the 



