DEVELOPMENT OF THE HUMAN HEART 283 



dial cushion, there are several breaks on the right lateral side, a 

 large one just behind the aorta and one on the medial dorsal side 

 of the left ostium. In other words the muscular connections 

 between the atria and ventricles are as follows: A large one in 

 front of and a large one behind the left ostium, a number of small 

 ones lateral to the right ostium and a large one from the sinus to 

 the ventricle dorsally just over its inferior septum. In another 

 embryo of the same stage of development (No. 317, 12.5 mm.) 

 there are two marked muscular connections, one in front of each 

 of the ostia. There is a very marked one from the sinus to 

 the inferior septum below. This is composed of muscle fibers 

 which arise as two horns to which the dorsal muscle fibers of the 

 atria stream. This is clearly the atrio-ventricular muscle, as 

 shown in fig. 17. From now on it is the chief muscular connec- 

 tion but for some time additional connections are often seen. 

 For instance in embryos 14 mm. (No. 144) and 15.2 mm. (No. 

 423) there is an additional connection just in front of the right 

 ostium, in embryos 16 mm. (409), 18.5 mm. (431) and 20 mm. 

 (368) there is an additional connection just in front of the left 

 ostium. In a number of specimens the atrio-ventricular bundle 

 is not confined to the region immediately over the inferior septum 

 but is spread out over the dorsal side of the left ostium (432, 18 

 mm.; 431, 18.5 mm.; 368, 20 mm.), while in one specimen 21 mm. 

 long (460) there is a single additional strip on the left lateral side. 

 In general, however, the main bundle which is at first broad and 

 associated more with the left ventricle than the right, gradually 

 becomes constricted so that it is well formed and rounded by the 

 time the embryo is 20 mm. long. It is beginning to be isolated 

 at 11 mm. and well separated at 13 mm. 



The position of the bundle is shown in sagittal sections in figs. 

 9 (No. 113, 8 mm. long), 13 (144, 14 mm.), 20 (390, 15.5 mm.) ; 

 21 (432, 18 mm.), 22 (431, 18.5 mm.) and 23 (368, 20mm.). It is 

 clear in all of these hearts that the muscle connection is from the 

 sinus between the posterior cushion and the annulus fibrosus 

 into the septum of the ventricle. This is its position in the adult 

 heart. In transverse sections the bundle is shown in figs. 8 (No. 

 2, 7 mm.), 24 (353, 11mm.) and 26 (317, 16 mm.). Here again it 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 13, NO. 3 



