762 



THE VASCULAR SYSTEMS 



unite to form the septum intermedium which divides the canal into two channels, the future 

 right and left auriculoventricular orifices. 



The primitive auricular cavity becomes subdivided into right and left auricles by an incom- 

 plete septum, the septum primum (Fig. 542), which grows downward into the auricular cavity. 

 For a time the two auricles communicate with each other by an opening, the ostium primum of 

 Born, below the free margin of the septum. This opening is, however, closed by the union of the 

 septum primum with the septum intermedium, and the communication between the auricles 

 is reestablished through an opening which is developed in the upper part of the septum primum; 



Aortic septum. 



Common auriculo- 

 i ventricular orifice. 



Pulmonary 



artery. 



Right 



auriculo- 



ventricular^ 



orifice. 



Aortic septum. 



,Aorta. 



Eight 

 ventricle. 



Septum Left 

 inferius. ventricle. 



Left 



auriculo- 

 ventricular 

 orifice. 



Right 



ventricle. Ke P fwm inferius. ventricle. 

 FIG. 539. Diagrams to show the development of the septum of the aortic bulb and of the ventricles. (Born.) 



this opening is known as the foramen ovale (ostium secundum of Born), and persists until birth. 

 A second septum, the septum secundum, semilunar in shape, grows downward from the upper 

 wall of the auricle to the right of the primary septum and foramen ovale. Shortly after birth 

 it fuses with the primary septum, and by this means the foramen ovale is closed, but sometimes 

 the fusion is incomplete and the upper part of the foramen remains patent. The annulus ovalis 

 denotes the free margin of the septum secundum. 



The primitive ventricle becomes divided by a septum, the septum inferius or ventricular 

 septum (Figs. 539 and 542), which grows upward from the lower part of the ventricle, its position 

 being indicated on the surface of the heart by a furrow. Its dorsal part grows more rapidly 



FIG. 540. Diagrams to illustrate the transformation of the bulbus cordis. Ao. Primitive aortic stem. Au. 

 Auricle. B. Bulbus cordis. RV. Right ventricle. LV. Left ventricle. P. Pulmonary artery. (Keith.) 



than its ventral portion, and fuses with the dorsal part of the septum intermedium. For a time 

 an interventricular foramen exists above its ventral portion, but this foramen is ultimately closed 

 by the fusion of the aortic septum with the ventricular septum. 



As already stated, the aortic bulb consists of a proximal muscular portion, the bulbus cordis, 

 and a distal portion, the primitive aortic stem. When the heart assumes its S-shaped form the 

 bulbus cordis lies ventral to and in front of the primitive ventricle. The adjacent walls of the 

 bulbus cordis and ventricle approximate, fuse, and finally disappear, and the bulbus cordis now 

 communicates freely with the right ventricle, while the junction of the bulbus with the primitive 



