268 FRANKLIN P. MALL 



but as the septum of the ventricles and the septum aorto pul- 

 monale approach each other more and more to form the perma- 

 nent membranous septum, the vestibule becomes transferred to the 

 left ventricle as may be seen in figs. 16 and 17. An open inter- 

 ventricular foramen in the adult always communicates between 

 the aortic vestibule and the space below the medial cusp of the 

 tricuspid valve, as is clear by observing Spalteholz's figure. 16 



The topography in the wall of the left ventricle is much easier 

 to define. The common endocardial mass borders the left venous 

 ostium and each of its two horns are continuous with pro- 

 nounced muscular bands, the papillary muscles, which extend to 

 the more solid muscular wall of the heart. In their course from 

 the valve to the outer wall of the heart muscle the papillary mus- 

 cles communicate continuously with the trabecular system. Both 

 the anterior and posterior papillary muscles connect with the 

 lateral valve which is being extended around the left ostium by 

 an "undermining" process, already well described by His. So in 

 this early stage of development the anterior papillary muscle 

 unites the anterior tip of the medial and lateral valves (anterior 

 and posterior B.N.A.) with the anterior wall of the left ventricle, 

 and the posterior muscle unites the posterior tip with the posterior 

 wall of the heart as seen in fig. 11. The vestibule of the aorta 

 connects the aorta with the left ventricle; it is encircled by the 

 border of the ventricular septum. In the course of time the 

 border of the ventricular septum unites with the septum aorto 

 pulmonale and thus finally separates the two ventricles of the 

 heart. When viewed through the aorta the muscular interven- 

 tricular septum usually makes in the adult the right border of the 

 vestibule but often it projects into the vestibule as is normally 

 the case in the pig and the ox. In such specimens, as well as in 

 the pig and the ox, the right semilunar valve arises directly from 

 the interventricular muscular septum. This shows to what ex- 

 tent the valves must 'sink into' the bulbus in passing from the 

 stage represented in No. 353 to the adult form. 17 



16 Spalteholz, Hand atlas, vol. 2, fig. 420. 



17 The normal position of the membranous septum is described in an article on 

 aneurysms arising from it, in the Anatomical Record, vol. 6, 1912. 



