264 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



as much blood as the right atrium, the septum I is pressed against the limbus of 

 septum II, and soon fuses with it. The depression formed by the thinner walled 

 septum I is the fossa ovalis. 



The foramen ovale may fail to close soon after birth and the mixed blood produces a 

 purplish hue in the child which is known popularly as a "blue baby." This condition may 

 be persistent in adult life. 



Pulmonary Veins. In embryos of 6 to 7 mm. a single vein grows out from 

 the caudal wall of the left atrium to the left of the septum I. This vein bifurcates 



Sept I 



Lv.c. 



Bic. va. 



Sept jr. 



Lvent. 



A B 



FIG. 258. Dissections from the left side of human hearts: A , from a 22 mm. embryo; B, from a 105 

 mm. embryo. Cor. sin., coronary sinus; For. ov., foramen ovale; I.v.c., inferior vena cava; L. atr. 

 vent, c., left atrio-ventricular canal; L. vent., left ventricle; Pul. a., pulmonary artery; Sept. I, Sept. II, 

 septum primum and septum secundum; Bic. va., bicuspid valve. 



into right and left pulmonary veins which divide again before entering the lungs. 

 As the atrium grows, the proximal portion of the pulmonary vein is taken up into 

 the atrial wall. As a result, at first two, then four, pulmonary veins open into 

 the left atrium. 



Origin of the Right and Left Ventricles. In embryos of 5 to 6 mm. there 

 appears at the base of the primitive ventricular cavity a sagittally placed eleva- 

 tion, the interventricular septum (Fig. 253, B}. It later grows cephalad and dorsad 

 toward the endocardial cushions, and forms an incomplete partition between the 

 right and left ventricles, which still communicate through the persisting inter- 



