150 THE SINO-VENTRICULAR BUNDLE. 



As that material was at one time most available, I used the pig heart for study. 

 For comparison, many other hearts were studied, including the human. Various 

 methods were used, such as the maceration, the injection, the dissection, and the 

 use of other reagents to bring about a clear picture of the system in gross, but, as 

 stated before, it meant merely a repetition of what is already known. I was, 

 therefore, forced to depend entirely upon histological technique. At first I was 

 content with the use of such fixatives as Zenker's, formaldehyde, mercuric bichloride, 

 and other combinations, and with such stains as would differentiate between mus- 

 cular tissue and connective tissue. It was soon seen that these methods would not 

 lead me beyond the confirmation of previous work. As stated before, as long as 

 we have an investment of the bundle there is no difficulty in tracing it, but I can 

 state from broad experience that no one who is not thoroughly conversant with the 

 finer structures of cardiac musculature can distinguish between a few cells of the 

 bundle and those of ordinary cardiac musculature. This is especially true of the 

 human heart. 



With Zenker's fixation and hematoxylin and eosin stain, or hematoxylin and 

 van Gieson, there is one characteristic common to the entire system, and that is a 

 clear perinuclear space. The nature of this perinuclear space will be discussed later. 



Let us look more closely into the histology and cytology of the conductive 

 system in the pig's heart. For descriptive purpose we divide the system into five 

 parts: the atrial part, the junctional part, the ventricular stems, the preterminal 

 part, and the terminal part or Purkinje fibers. There is a gradual transition from 

 one part to the other, but the difference between the first and last is very great. 



The first part begins in the region of the great veins and ends in the node of 

 Tawara. It is most difficult to tell where this musculature begins and where the 

 atrial musculature ends, but if one looks underneath the endocardium or underneath 

 the intima of the great veins one will see the clearly defined cells with a perinuclear 

 space. The fibrils are not as numerous nor is the striation of them as marked as in 

 the atrial musculature. 



It is a matter of opinion whether these cells around the great veins belong to 

 the sino-ventricular bundle or to the system belonging to the Keith-Flack node. 

 It makes but little difference, because one system is connected with the other and 

 it becomes more a question as to where one ends and the other begins. It is my 

 opinion that we can not make any anatomical distinction between the Keith-Flack 

 system on the one hand and the sino-ventricular on the other. Physiologists may 

 find an easier explanation of cardiac phenomena by considering these systems 

 separate and distinct, but we have no anatomical proof that such a condition 

 exists. 



The second or junctional part is what is known as Tawara's node. In most 

 animals this node is situated in a restricted area between the trigonum fibrosum 

 and the endocardium in the right side of the interatrial septum immediately above 

 the interventricular septum. In most mammals (including man) the trigone 

 above mentioned contains either bone or cartilage. It is owing to this fact that 

 thin coronal sections of this region are most difficult to obtain. The necessity of 



