224 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



cardinal vein, which drains the posterior half of the body, receive 

 a tributary (segmental vein) from each body segment. The 

 cardinal veins lie in the mesoblast on the dorsal side of the 

 coelom at the junction of the splanchnopleure and somatopleure 

 (Fig. 183). By their union they form on each side the 

 Duct of Cuvier which conveys the blood to the sinus venosus 

 — a contractile chamber opening into the primitive auricle. The 

 sinus venosus remains as a separate chamber of the heart in lower 

 vertebrates, but in the course of mammalian development it 

 becomes partly merged in the right auricle of the heart. 



stomodaeum 

 aorta 



jug. vein 



card. vein, 

 somatopleure 4k~W/^l'\W\j:)lst 



,prim. auricle 

 sinus uenosus 

 pericardium- 

 'uct of Cuuier 



pleura. 



^m^-A — peritoneum 



splanchnopleure ^^ 



Fig. 183.— Diagram to show the maimer in which the Ducts of Cuvier encircle the 

 Coelom at the junction of the Pericardial and Pleural Parts (Iter venosum). 

 (After His.) ' 



It is important to notice how each duct of Cuvier reaches the 

 sinus venosus (see Fig. 183). They pass from the dorsal to 

 the ventral surface of the body in the somatopleure and thus 

 encircle the coelom. The right and left ducts of Cuvier lead to 

 a constriction of the coelom, the fold in which each descends 

 being known as the lateral or venous meso-cardium (Fig. 205, 

 p. 251). "Ultimately, by the end of the 4th week, the part of 

 the coelom lying in front of the] ducts of Cuvier is cut off from 



