DEVELOPMENT OF THE HUMAN HEART 



175 



In young hearts the septum aorto pulmonale encircles the right 

 ostium as shown in fig. 17 and from it the tricuspid valve arises. 

 In the adult the septum aorto pulmonale blends with the anterior 



RO 



A V. B. 



Fig. 17 The same as fig. 16 from the heart of an embryo 17 mm. long (No. 296) . 

 Most of the muscle wall of the atrial canal is absent . The atrio-ventricular bundle 

 is shown and thp semilunar valves of the aorta are indicated. 



A V.B. 



Fig. 18 Pketch of the base of an adult heart showing the valves. The atrio- 

 ventricular bundle is shown. The figure may be compared with figs. 15, 16 and 

 17. 



end of the medial and anterior cusp of the tricuspid valve, while 

 the same tip of the valve is tied to the con.us by the medial tendon 

 which frequently enlarges into a papillary muscle. In following 

 its development it is found that this tendon marks the primary 



