286 FRANKLIN P. MALL 



So far I have shown that there is a band of tissue apparently 

 muscular, which connects the sinus, or atria, with the ventricle. 

 At first it is broad but it gradually becomes constricted. In an 

 embryo 12.5 mm. long (317) the muscle fibers of the atria stream 

 towards the connecting muscle as two horns, somewhat later it 

 arises from a thickened mass in the neighborhood of the sinus and 

 finally from the mass which has become converted into a nodule. 

 About this time according to His, Jr., the nerves begin to invade 

 this region. However, I have been able to trace the ganglion 

 cells through the septum to the valves in but a single specimen 34 

 mm. long (No. 249) which shows that the nerve fibers really do 

 enter the bundle on its atrial side. To what extent they penetrate 

 the ventricular portion is still undecided. 



It is not easy to determine with certainty the destruction of the 

 lateral and anterior walls of the atrial canal, and my statements 

 rest upon repeated studies of this region in all of my embryos. 

 Not only does the endocardial thickening play a role in this proc- 

 ess but the connective tissue of the epicardium also plays a part, 

 as His, Sr., has shown. To what extent the outer connective tis- 

 sue plays a part is well shown in the reconstruction of an embryo 

 11 mm. long (353). The model shows that it forms a collar 

 encircling entirely the heart between the atria and ventricles and 

 also extends into the anterior and posterior longitudinal sulci. 

 On the two lateral sides the encircling connective tissue is drawn 

 into the valves as they become larger and this process completes 

 the separation of the atrial and ventricular musculature. Be- 

 tween the atria and the aorta it forms a large plug which soon 

 comes into apposition and blends with the anterior endocardial 

 cushion. Behind it encircles the atrio- ventricular bundle so that 

 this bundle becomes lodged between the posterior cushion and the 

 outer connective tissue. The relation here found corresponds 

 with the position of the bundle in the adult, for it then lies behind 

 the medial cusp of the tricuspid valve which arises from the endo- 

 cardial cushion, and the right annulus fibrosus, which arises from 

 the outer connective tissue. 



That the bundle differs in structure from the rest of the muscle 

 of the heart of the foetus has been shown by Monckeberg, who 



