298 FRANKLIN P. MALL 



turn behind quite high up so that they form its border just below 

 the interventricular foramen. Those that arise behind encircle 

 the right ventricle and enter the heart in front of through the an- 

 terior longitudinal sulcus. Now as the heart grows by an extension 

 of the ventricles downward these two encircling bands also grow 

 and ultimately become interlocked at the apex of the left ventricle, 

 the fibers from the bulb forming the bulbo-spiral band and the 

 posterior horn of the vortex; the fibers from the sinus side, the 

 sino-spiral band form the anterior horn of the vortex. As the 

 sino-spiral band becomes larger and larger it is shifted over the 

 greater part of the bulbo-spiral band on the posterior side of the 

 heart. On the anterior side of the heart with the growth of the 

 anterior longitudinal sulcus, the sino-spiral band gradually extends 

 sending bundles to both sides of the septum, that is it constantly 

 straddles the septum from its lower and under side. By this 

 extension it binds together the inner walls, especially the papil- 

 lary muscles, of the two ventricles with each other. These are 

 the two strands shown in figs. 35 to 37. 



The bulbo-spiral band first extends from the bulb over the left 

 ventricle, and with its growth penetrates its wall, ultimately 

 including its apex to end in the posterior horn of the vortex (figs. 

 31 to 33). In so doing it produces loops which constantly turn upon 

 themselves to return to the base so that when viewed together 

 the bulbo-spiral band appears as a series of telescoped loops of 

 sheet fibers as is now well known. At the base of the left ven- 

 tricle the fibers are more circular forming a thick fleshy mass on 

 its dorsal side, the triangular field, which in the adult still marks 

 the embryonic conditions; all loops of the bulbo-spiral band are 

 but an extension of this field as is shown by the development of 

 the fibers as well as by the architecture of this muscle in the adult. 



