DEVELOPMENT OF THE CIRCULATORY SYSTEM. 237 



the superior to the inferior vena cava, and indicates the junction 

 of the primitive auricle with the sinus venosus. 



left sup t intercost 

 vein of Marshal/ 



sup. uen. cau 

 vena, azyg. 



sap. uen. eav. 



part of rt. aur. 

 left venous value 



Eustach. value 

 (right venous 



valve) , consinus 



open, ofoor. sinus 

 thebesian valve 

 mf. ven. cava 



Fig. 196.— Showing the Structures formed from the Sinus Venosus. 



^sup. uen. cau. 



right uen. value 

 right aur. 



secundum 

 sept primum 

 left uenous ualue 



left aur. 



open, sinus, uenosus. 



post 



endocard. cushion 



aur. uent. canal 

 -left uent. 



inter-uent sept. 



Fig. 197.— Section of the Heart of a 5th week human foetus showing the Right and 

 Left Venous Valves which guard the entrance of the Sinus Venosus into the 

 Primitive Auricle. (After His.) 



The Valves of the Sinus Venosus. — Eight and left lateral valves 

 (venous valves) guard the entrance of the sinus to the primitive 



