148 THE SINO-VENTRICULAR BUNDLE. 



nection extending from the region of the coronary sinus and the great veins to the 

 ventricles, including the Purkinj e fibers . The nervous mechanism is without a name. 



ANATOMICAL DESCRIPTION. 



The general gross anatomy of the muscular connections has been so ably 

 described and so well illustrated by Aagaard and Hall (1912) that it would seem 

 unnecessary to describe it again were it not that slight differences of opinion con- 

 cerning the extent of these connections necessitate a definite statement upon which 

 I base my later discussion. We generally find the descriptions and figures begin 

 with the node of Tawara and end with the ramification of the Purkinj e fibers. 

 Such descriptions are quite correct as far as one can recognize these structures in 

 the gross. Injection methods which give most satisfactory results for demon- 

 strating the system are useless beyond these areas, and it is doubtful whether any 

 other gross method will reveal anything further that can be considered conclusive. 

 The use of the Zeiss-Greenough binocular in dissecting out the sinus portion will 

 show fibers leading from the node to the coronary sinus and inferior vena cava, 

 but whether these fibers are continuous with the bundle or not is a question that 

 needs close histological examination. Both with the gross and with the micro- 

 scopic investigation there seems no difficulty, as long as the bundle is invested by 

 fibrous sheath. It is this fibrous sheath which is injected, and it is this sheath 

 also which stands out most prominently when stained with dyes that differentiate 

 between musculature and fibrous tissue. As soon as we reach the node of Tawara, 

 when tracing the bundle to its origin, the fibrous sheath disappears, and 

 from here on the tracing is difficult, if not impossible, by any gross methods. Sim- 

 ilarly, at the other end of the bundle, we can not demonstrate in the gross the 

 transition between it and the cardiac musculature. 



The description of the gross, therefore, will closely correspond to the descrip- 

 tion given by most writers of recent years, but to make it complete we must add 

 to it what the microscope reveals. The bundle takes its origin in the musculature 

 of the right side of the interatrial septum immediately in front of the coronary 

 sinus or of the left superior vena cava in those mammals that lack a coronary 

 sinus that is, in those that have a persistent left superior vena cava. It passes 

 forward and downward* in the septum and comes to lie beneath the insertion of 

 the septal leaflet of the tricuspid valve. This part of the sino-ventricular bundle 

 is so delicate and diffuse that it can not always be distinguished from the sur- 

 rounding atrial musculature, even after the removal of the endocardium. Here 

 the diffuse strands are collected into a bundle which can readily be seen as a light- 

 colored strand having a diameter of 2 to 3 mm., running perpendicularly to the 

 direction of the fibers of the ventricular septum and between them and the pars 

 membranacea septi. At a point that lies in a line drawn from the apex of the left 

 ventricle to the junction of the right and of the posterior cusps of the aortic valve, 

 it divides into a right and a left branch that straddle the muscular interventricular 

 septum. These two main branches pass slightly forward and downward toward 



*The directions refer to the excised heart with base upward and septum antero-posterior, 



