DEVELOPMENT OF THE HUMAN HEART 267 



shows that the cushions mark the borders of the medial sides of 

 the right and left ostia, these being notched to indicate the extent 

 of the anterior and posterior cushions. This is well shown in 

 Fig. 11 which was drawn from the model. On the dorsal side, 

 the posterior cushion extends well down the posterior border of 

 the interventricular foramen, that is it makes part of the border 

 of the septum of the ventricle. These are the primary connec- 

 tions of the two endocardial cushions. There are also secondary 

 connections which in a measure involve the valves lateral to the 

 ostia. 



On the lateral side of either ostium there is a rounded endo- 

 cardial thickening which marks the beginning of the lateral valves. 

 These are already observed in Embryo 422, 9 mm. long (fig. 7). 

 To anticipate the description I may state that the right cushion 

 marks the center of the anlage of the anterior and posterior cusps 

 of the tricuspid valve, and the left the anlage of the posterior 

 cusp of the mitral valve as seen in fig. 11. The septum aorto 

 pulmonale soon blends with the cushions* through a dorso-lateral 

 wing which is divided into two branches to encircle in part the right 

 venous ostium, one of which blends with the right lateral endo- 

 cardial thickening, and the other, the medial, blends with the 

 anterior process of the medial valves now represented by the right 

 lower wing of the anterior endocardial cushion. It is thus seen 

 that through the blending of the septum aorto pulmonale with the 

 right side of the anterior endocardial cushion, the right venous 

 ostium is nearly encircled by endothelial connective tissue. This 

 connection may still be seen in the adult heart where the septum 

 aorto-pulmonale (the tendon of the conus) is found to blend with 

 the fibrous ring of the right ostium at the anterior border of the 

 attachment of the medial cusp of the tricuspid valve. 



The large space marked by the interventricular foramen at 

 the root of the aorta remains constant in all subsequent stages of 

 development and is termed by Quain 15 the vestibule of the aorta. 

 This name, which is appropriate, I shall adopt and use in my de- 

 scription. The vestibule in fig. 11 is common to both ventricles, 



15 Quain's Anatomy, 10th Edition, vol. 2, fig. 317. 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 13, NO. 3 



