THE VASCULAR SYSTEM 



373 



like, one dorsal to the other. Both are known as endocardial cushions. 

 The extension of the interventricular septum described above results in 

 its fusion with the endocardial cushion and in the division of the single 

 atrio-ventricular foramen into right and left atrio-ventricular foramina. 

 The endocardial cushions guarding the right atrio-ventricular foramen 

 become the tricuspid valve, while those of the left foramen form the mitral 

 valve. The chordae tendineae and the papillary muscles which are con- 



sophagus 



pharynx 



epi-myo- 



cardium 



C. D. 



Fig. 311. — Schematic diagrams showing the manner in which the pleural and 

 pericardial region of the coelom become separated. (From Patten's "Embryology of 

 the Pig.") 



nected with the valves are derived from the inner walls of the ventricles, 

 which primarily are spongy masses of muscular trabeculae. As the walls 

 of the ventricles are converted into the compact muscle of the outer heart- 

 wall, the inner trabeculae become papillary muscles and chordae tendineae. 



Descensus Cordis. When the heart begins its development, it lies well 

 forward beneath the pharynx. As it takes on adult form and structure, 

 it shifts backward into the thorax. During its migration the atria which 

 were primarily posterior to the ventricles come to lie anterior to them. 



Development of Aorta and Pulmonary Artery. Extending anteriorly 

 from the ventricle, the bulbus and truncus arteriosus carry blood from 



