DEVELOPMENT OF THE VASCULAR SYSTEM 



511 



septum, the sinus septum, grows from the posterior wall of the sinus venosus to fuse 

 with the valve and divide it into two parts an upper, the valve of the inferior 

 vena cava, and a lower, the valve of the coronary sinus (Fig. 468) . The extreme 

 upper portion of the right venous valve, together with the septum spurium, form 



Eight atrium 

 Bulbiis cordis 



Left atrium 

 Atrial canal 



Ventricle 



FIG. 466. Heart showing expansion of the atria. (Drawn from Ecker-Zeigler model.) 



the crista terminalis already mentioned. The upper and middle thirds of the left 

 venous valve disappear; the low r er third is continued into the spina vestibuli, 

 ind later fuses with the septum secundum of the atria and takes part in the forma- 

 tion of the limbus fossae ovalis. 



Septum secundum 

 Opening of coronary sinus 



Septum spurium 



Right venous val 



Right atrioventricular 

 opening 



Left atrioventricular 



opening 

 Septum intermedium 



Septum inferius 

 FIG. 467. Interior of dorsal half of heart of human embryo of about thirty-five days. (From model by His.) 



The atrial canal is at first a short straight tube connecting the atrial with the 

 ventricular portion of the heart, but its growth is relatively slow, and it becomes 

 overlapped by the atria and ventricles so that its position on the surface of the heart 

 is indicated only by an annular constriction (Fig. 466). Its lumen is reduced to a 





