280 FRANKLIN P. MALL 



On the right side of the heart the septum aorto pulmonale 

 soon blends with the lateral tip of the anterior cushion and by 

 the time the septum is complete, as in No. 353, it divides, one 

 portion of which is attached directly to the left anterior tip of 

 the common cushion and the other encircles the right venous 

 ostium and blends with the right lateral cushion. The posterior 

 side of the ostium is formed by trabeculae which pass from the 

 lateral cushion mainly to the muscular septum and others which 

 course symmetrically into the trabecular system of the ventricle. 

 It is by no means easy to determine three systems to correspond 

 with the three cusps of this valve. Neither is it easy to recognize 

 the three cusps in the adult unless we associate the medial papil- 

 lary muscle, which is constant, with the anterior cusp and the 

 large papillary muscle with the posterior cusp. If this is done it 

 is possible to name the valves in the heart of embryo No. 356 as 

 follows (figs. 15 to 18) : 



The median cusp is attached in this embryo to the septum aorto 

 pulmonale in front and to the muscular septum behind. It has cor- 

 responding attachments in the adult. The anterior cusp is attached 

 partly "to the septum aorto pulmonale and to the large papillary 

 muscle. The posterior cusp is attached to the large papillary 

 muscle. If this division is correct the lateral cushion belongs to 

 the anterior cusp while the posterior cusp is developed entirely 

 from the lateral side of the ventricle. The medial papillary mus- 

 cle is constant and always extends as a muscle, or as a tendon, 

 from the region of the septum aorto pulmonale (tendon of the 

 conus) to the anterior cusp which is marked by a similar strand 

 of tissue from the septum to the lateral cushion in the embryo. 

 That this is the case is shown in sections of older hearts, one of 

 which is given in fig. 19. The posterior cusp, over the large papil- 

 lary muscle, is of irregular formation and by a process of exclu- 

 sion the muscle strands in the embryo which encircle the right 

 ostium behind the right cushion, should give rise to it. At any 

 rate the right and left ostia of numerous hearts of embryos less 

 than 20 mm. long / are slit-like and similar, so that it is impossible 

 to state that one is encircled by a tricuspid valve. Each is 

 bordered by medial and lateral valves, and each medial and lateral 



