3 82 



THE EVOLUTION OF MAN. 



ao 



c J 



Fig. 311. 



Fig. 314. 



Ftg. 311.— Heart of a human embryo of four weeks ; 1, from the front; 

 2, from the back ; 3, open, and with the upper half of the auricle removed ; 

 a\ left auricular process ; a", right auricular process ; v, left ventricle ; 

 v", right ventricle ; ao, artery-stalk ; c, upper hollow vein (vena cava) (cd, 

 right, cs, left) ; s, rudiment of the partition, between the chambers. (After 

 Koelliker.) 



Fig. 312. — Heart of a human embryo of six weeks, from the front: 

 r, right ventricle ; t, left ventricle ; s, furrow between the two venti-icles ; 

 ta, artery-stalk ; af, furrow on its surface ; at the right and left are the 

 two large auricular processes of the heart. (After Ecker.) 



Fig. 313. — Heart of a human embryo of eight weeks, from behind : 

 a', left auricular process ; a", right auricular process ; v, left ventricle ; 

 v", right ventricle ; cd,', right upper vena cava ; cs, left upper vena cava ; 

 ci, lower vena cava. (After Koelliker.) 



Fig. 314. — Heart of human adult, perfectly developed, from the front, in 

 its natural position : a, right auricular process (below it, the right ventricle) ; 

 b, left auricular process (below it, the left ventricle) ; C, upper vena cava; 

 V, lung-veins ; P, lung-artery ; d, Botalli's duct ; A, aorta. (After Meyer.) 



right ventricle, and the aorta-trunk, which opens into the 

 left ventricle. Not until all these partitions are complete, 

 is the lesser, or lung-circulation, entirely distinct from the 



