DEVELOPMENT OF THE HUMAN HEART 295 



in the posterior longitudinal furrow the fibers from the left side 

 enter the septum. There is every indication from the study of 

 older specimens that the 'vortex' formation is at first well up in 

 the middle of the heart and that only later it is pushed down to 

 the apex of the left ventricle. 37 I was unable to determine this 

 point with certainty from my sections. However, it is clear that 

 in an embryo 3.9 mm. (463) long (fig. 1) the posterior bundle, 

 that is the bulbo-spiral, enters the septum and forms its upper 

 border just below the inter-ventricular foramen. Below this the 

 sino-spiral should pass into the left ventricle in front, but sections 

 do not show that the cells in this region of the heart are passing in 

 any special direction. However, it is seen that the circular muscle 

 of the left ventricle appears at the base of the heart and not at its 

 apex. In this embryo the interventricular foramen is 0.15 mm. 

 in diameter; in one 8 mm. long (113) it is 0.3 mm. in diameter and 

 in one 11 mm. long (353) it is 0.4 mm. in diameter. This indicates 

 that the ventricles grow longer by a down growth of their walls 

 and not as an upgrowth of the inferior septum. 38 With this 

 in view it is clear that the bulbo-spiral should be located near the 

 base of the heart at the time of its earliest appearance. 



In an embryo 12 mm. long the superficial fibers on the dorsal 

 side of the heart, that is the bulbo-spiral, could be followed into 

 the ventricular septum and the sino-spiral around the heart into 

 the left ventricle. The structures showing the direction of the 

 muscle fibers could only be seen after the whole heart had been 

 stained with cochineal and methylene blue. In a specimen 14 

 mm. long (360) the heart was dissected from behind under the 

 binocular microscope and the sino-spiral bundle was followed. 

 These fibers passed on both sides of the ventricular septum in 

 front to the septum aorto pulmonale, that is they formed the longi- 

 tudinal bundles of the right ventricle ; the spreading out of the sino- 

 spiral in the left ventricle as well as the interpapillary muscle 



37 Mall, Bifid apex in the human heart. Anat. Rec, vol 6, 1912. 



38 Flack (Further advances in physiology, New York, 1909) says that the ventric- 

 ular septum grows downward as the ventricles expand and unite. The atrio-ven- 

 tricular bundle is thus associated from the first with the oldest part of the septum, 

 that is, its upper border which encircles the interventricular foramen. 



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



