296 FRANKLIN P. MALL 



bundles were also seen. A large band of fibers crossing the right 

 ventricle was incorporated with this system; no doubt it is the 

 moderator band, which is very pronounced in the embryo. 



The two main bundles could be followed with precision in 

 the heart of an embryo 17 mm. long, the bulbo-spiral passing into 

 the upper part of the septum and the sino-spiral to the anterior 

 wall of the left ventricle. 



By the time the embryo reaches 25 mm. in length it is easy to 

 demonstrate that the strands of muscle which were observed in 

 younger specimens are destined to become the main muscle bun- 

 dles of the heart in the adult. First of all there are fibers on the 

 dorsal side of the heart which cross diagonally the posterior longi- 

 tudinal sulcus to reach the apex of the right ventricle. From 

 here they pass to the left ventricle and pierce it near the apex 

 anteriorly to form the anterior horn of the vortex. 39 From this 

 horn fibers pass on the right side of the ventricular septum to the 

 septum aorto pulmonale, that is they are the longitudinal fibers 

 of the right ventricle. Under the sino-spiral band loops of fibers 

 are seen which encircle the left ventricle and end in the posterior 

 triangular field. All of these loops enter the septum from behind 

 and form the bulbo-spiral fibers. This arrangement was clearly 

 demonstrated in a heart from an embryo 17 mm. long, as well as 

 in specimens 25 and 27 mm. long (figs. 34 to 37). In one of these 

 (No. 33) the sino-spiral turns upward at the apex of the right 

 ventricle along the lower part of the anterior longitudinal sulcus 

 and then enters the left ventricle. At this point the fibers blend 

 with the trabeculae of the right ventricle and then give rise to 

 the longitudinal fibers of the septum of the right ventricle. The 

 bulbo-spiral bundles form loops reaching towards the apex of the 

 left ventricle, that is, they are only an extension of the posterior 

 triangular field. This, as has been shown in the previous study, 

 remains in the adult as the circular fiber of the left venous ostium. 

 At the extreme posterior border of the left ventricle there is a 

 marked raphe to correspond with the base of the posterior papil- 

 lary muscle much as in the adult pig's heart. The sino-spiral 



39 Mall, Amer. Jour. Anat., vol. 11, 1911. 



