258 FRANKLIN P. MALL 



well marked in the human heart, but is indicated in figs. 3 and 

 8 (tip of TR) and is very pronounced in the pig. At this point 

 the fibers from the posterior papillary muscles are so intimately 

 blended with the deep bulbo-spiral band that often in peeling 

 off this band it breaks at the raphe and leaves the large ckcular 

 band around the left ostium as described by MacCallum and 

 pictured in figs. 19 and 20, BS'. If this bundle, as shown in 

 fig. 20, is pushed downward its cut end will come in contact with 

 the base of the posterior papillary muscle to which it was inti- 

 mately attached. 



What has just been said shows that the papillary muscles are 

 in direct continuation with all of the chief muscle bands of the 

 heart and the attachment of the atrio-ventricular system to them 

 gives meaning to this. An impulse or a wave coming through 

 the bundle of His is at once communicated to the entire muscula- 

 ture of the ventricles. 



In studying the musculature of the ventricles of the heart 

 one's attention is directed mostly towards that of the left ven- 

 tricle, and there is a tendency to neglect the right ventricle be- 

 cause it appears to be of simple construction. On the right side 

 the inner muscle bundles are directed towards the conus as they 

 are towards the aorta on the left side. 



In tearing ofif the muscle bundles from any portion of the ven- 

 tricle it is at once observed that the fibers always tend to pass 

 upwards, towards the base, as they are stripped off. That is, 

 they are constantly passing into the depth. Over the right 

 ventricle this is so marked that when bundle after bundle is 

 lifted up they are found to lie upon one another like the shingles 

 of a roof with the deeper longitudinal muscle bands below serving 

 as rafters (compare figs. 6 and 15). 



The superficial muscle bundles, the superficial sino-spiral, 

 come mostly from the left side of the heart, pass over the right 

 ventricle quite freely and then back to the left ventricle. Just 

 below this is a sheet which arises from the posterior part of the 

 left ostium and enters the right ventricle to end there. This, 

 the deep sino-spiral, is the sheet so well described by MacCallum. 

 When these two sheets are examined together it is seen that fiber 



