CH. XX.] 



HEART OF FROG 



231 



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 



an Tck/->o-iircc! ^.o.o. <^^^ 



v.p. 



c ' 



A.S:- 



s.v: 



--A.Z. 



auricle, as in man, receives 

 the blood from the pulmon- 

 ary veins. 



When the ventricle con- 

 tracts, it forces the blood 

 onward into the aortic bulb 

 which divides into branches 

 on each side for the supply 

 of the head (fig. 212, 1), 

 lungs and skin (fig. 212, 3), 

 and the third branch (fig. 

 212, 2), unites with its 

 fellow of the opposite side 

 to form the dorsal aorta for 



thp Qnrvnlv nf tbp rp<af nf 

 supply i 



the body. 



Passing from the amphi- 

 bians to the reptiles, we 

 find the division of the 

 ventricle into two beginning, but it is not complete till we reach 

 the birds. The heart reaches its fullest development in mammals, 

 and we have already described the human as an example of the 

 mammalian heart. The sinus venosus is not present as a distinct 

 chamber in the mammalian heart (except in a very early foetal stage), 

 but is represented by that portion of the right auricle at which the 

 large veins enter. 



FIG. 213. The heart of a frog (Rana 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.s., left auricle ; A.p., opening of communi- 

 cation between the right auricle and the sinus venosus. 

 X 2-3. (Ecker.) 



