276 



FRANKLIN P. MALL 



union of the septum aorto pulmonale to the valve and in its sub- 

 sequent development is separated from the septum as the valve 

 enlarges. The beginning of this process is shown in fig. 19 which 

 is from a section of embryo 86 (30 mm.). It is here seen that the 

 valve mass is enlarging and that its lower border is being sepa- 

 rated from the muscle of the right ventricle as the tendinous cords 

 are forming. In so doing the medial papillary muscle is forming 

 and at the same time the trabeculae which encircle the base of the 



Fig. 19 Section through the conus and right ostium of the heart of an embryo 

 30 mm. long (No. 86). X 20. The attachment of the tricuspid valve to the sep- 

 tum aorto pulmonale and the ventricular septum is shown. 



right ventricle as shown in fig. 11, become fused to produce the 

 crista supra ventricularis. 21 This at first ends in the septum aorto 

 pulmonale but in the adult it passes around and over the medial 

 tendon and passes down the anterior inner wall of the right ven- 

 tricle. In the embryo the moderator band arises just below the 

 medial cusp of the tricuspid valve, but in its further development 

 is shifted towards the apex where it binds the large papillary mus- 



21 The 'crista' is correctly figured in Toldt's Atlas, fig. 946, while in Spalteholz, 

 fig. 424, it is pictured as extending down to the base of the large papilary muscle. 

 It is this extension which contains the right limb of the atrio-ventricular bundle 

 and therefore must represent the moderator band. A study of the development of 

 this portion of the heart shows that this is the case. 



