?S2 



THE EVOLUTION OF MAN. 



Fig. .311. 



Fig. 311. 



Fig. 311. — Heart of fi liiiman 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. — Ileart of a human embryo of six weeks, from the front: 

 r, right ventricle ; t, left ventricle ; s, furrow between the two ventricles ; 

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

 two large auricular processes of the heaii:. (After Ecker.) 



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

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

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

 ci, lower vena cava. (After Koelliker.) 



Fig. 314. — Heart of human adult, perfectly dcvchiped, from the front, iu 

 its natm-al position : o, right auricular process (below it, tlie right ventricle) ; 

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

 F, lung. veins; P, lung-artery ; tl, Botalli's duct ; ^, aorta. (After Meyer.) 



riLjIifc ventricle, and tlic aorta-tnmk, wliich opens into the 

 left ventricle. Not until all these partitions are complete, 

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



