252 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



Fate of the Sinus Venosus and its Valves. The opening of the sinus 

 venosus into the dorsal- wall of 'the right atrium is guarded by two valves 

 (Fig. 260) . Along the dorsal and cephalic wall of the atrium these unite to 

 form the septum spurium. Caudally the valves flatten out on the floor 



V allies of sinus venosus 

 Septum I 

 Atria-ventricular canal 



Valves of sinus venosus 



^Septum I 



Foramen ovale 



Atria-ventric- 

 ular canal 



Interventricular 

 septum 



Sinus venosus 

 R. valve of sinus venosi 



Endocardial 

 Atria-ventricular groove' 

 R. ventricle 



R. common cardinal vein 



Septum II 



L. valve of sinus venosus 



L. atrio-ventricular opening 

 L. ventricle 



FIG. 260. -Horizontal sections through the chambers of the human heart: A, 6 mm.; B, 9 mm. 

 C, 12 mm. (A and B are based on figures of Tandler.) X about 50. 



of the atrium, but, as stated previously, the left valve later fuses with the 

 atrial septum II. In embryos of 10 to 20 mm. the atria increase rapidly 

 in size and the lagging right horn of the sinus venosus is taken up into 

 the wall of the right atrium. By this absorption the superior vena cava 



