THE SINO- VENTRICULAR BUNDLE. 149 



the base of the papillary muscles. In the left ventricle, there being two papillary 

 muscles, the bundle must necessarily divide, while in the right ventricle it 

 remains single, reaching the large papillary muscle by way of the moderator band 

 if there is one, or in relatively the same position if it is lacking. In the left ventricle 

 the main branches are visible without dissection, appearing as light strands under- 

 neath the endocardium, sweeping downwards towards the base of the papillary 

 muscles making an inverted V. In the right ventricle the bundle is frequently 

 beneath a millimeter or more of musculature, but becomes visible at the base of 

 the papillary muscle. The end-ramifications or the Purkinje fibers are quite visible 

 in the fresh heart, but in many of the preserved hearts they can not be defined. 

 They spread out underneath the endocardium in a complex network, usually 

 bridging over the recesses between the trabeculse carnese. 



This description is applicable to all mammalian hearts. In the human heart 

 as well as in the dog's, it should be noted that the strands are exceedingly delicate 

 and tear easily when dissected out. In the Herbivora, however, the entire system 

 is ensheathed by collagenic fibers that give the bundle a very much lighter color 

 than the surrounding musculature and also make dissection easy. In these species, 

 sheep especially, the Purkinje fibers are accompanied by fat-cells that stand out 

 gray on the dark background. In these hearts, furthermore, the right branch lies 

 immediately beneath the endocardium, while in the human and dog it must be 

 dissected out of a depth of a millimeter or more of musculature. 



The microscopic anatomy is far more complex than the gross. It is due to 

 inadequate histological work that our present conception of the nature and function 

 of the sino-ventricular bundle is somewhat erroneous. The microscope must 

 settle two questions one, the origin of the bundle where we lose trace of it by 

 dissection, and the other, the nature of the material that constitutes the bundle. 

 In reference to the first question I must confess that we have not advanced 

 far. As previously stated, it is my opinion that the bundle does not originate in 

 Tawara's node but in the musculature that represents the primitive sinus region 

 of the heart. I [base this opinion upon the examination of the embryonic heart, 

 where the musculature of the various parts presents greater cytological differ- 

 ences than it does in the adult. In the latter the problem is similar to tracing 

 finer nerve tracts in the medulla without the use of pathological material which 

 singles out these tracts by degeneration. Nerves, connective tissues of various 

 kinds, and poorly striated musculature interwoven in a most complex manner 

 present almost insurmountable difficulties. In reference to the second question 

 we realize that our histological knowledge is based almost entirely upon Tawara's 

 work. His illustrations are more or less diagrammatic and for the sake of clearness 

 such structures as seemed to him unessential were omitted. He showed that 

 there were histological differences between various mammals and in various parts 

 of the same heart. But as to why we have these differences he does not even spec- 

 ulate. I think that we can show the reason of these differences and what their 

 significance is. 



