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HUMAN EMBRYOLOGY AND MORPHOLOGY. 



Through the ventral part of the mesocardiurn the ducts of 

 Cuvier reach the sinus from the somatopleure (Fig. 183). In 

 fishes and in the human embryo the sinus pumps the blood 

 into the primitive auricle, and its orifice into the auricle is 

 protected by two valves, right and left (Fig. 197). 



vent, bend; 



stomodaeum 



conus arteriosus 

 ^r-aur. vent bend 



pericardium 

 prim. aur. 

 sinus venosus 

 sept, trans 

 . duct of Cuvier 

 liver bud 



pleura. 



gut. 

 vit. vein 



umb. vein 



Fig. ] 95. — Showing the two chief Bends which occur in the Heart during the 3rd week. 



Fate of the Sinus Venosus (Fig. 196). — The sinus venosus 

 shifts towards the right side of the primitive auricle and ulti- 

 mately forms part of the right auricle and the coronary sinus. 

 The part which it forms of the right auricle is indicated by the 

 entrance of the following vessels which primarily terminate in 

 the sinus : 



(1) The superior vena cava (the right duct of Cuvier); 



(2) The inferior vena cava, which also opens into the sinus ; 



(3) The oblique vein of Marshall (left duct of Cuvier), which 

 opens into the left horn of the sinus venosus. The left horn of 

 the sinus becomes the coronary sinus. A groove, the sulcus 

 terminalis, which is marked on the interior of the right auricle by 

 a crest, runs down on the anterior wall of the right auricle from 



