38* 



THE EVOLUTION OF MAN. 



FIG. 311. 



FIG. 313. 



FIG. 314. 



FIG. 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 ventricles ; 

 ta, artery-stalk ; of, 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 : 

 .', 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 i-ight ventricle) ; 

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

 V, lung-veins; P, lung-artery j 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 



