40 THE HEAKT 



who first described it, the bundle of His ; but it is perhaps simpler 

 to term it the auriculo- ventricular or A-V bundle. The bundle, 

 as first shown by Tawara, consists of four portions : 



(1) The auriculo- ventricular or A-V node. 



(2) The main bundle. 



(3) The septal divisions, right and left. 



(4) The terminal ramifications. 



These divisions can be followed in some hearts better than in others ; 

 for instance in the hearts of the sheep and ox it is a matter of great 

 ease, since the fibres constituting the bundle are much paler than 

 those of the surrounding musculature, and therefore easier to dissect 

 out. These hearts, therefore, are recommended for preliminary 

 dissections of the bundle. Microscopically also the fibres of the 

 bundle present a greater contrast to the rest of the musculature in 

 these hearts, so that the course and structure of the bundle are 

 more easily followed through a series of sections, a matter of con- 

 siderable difficulty at first in the human heart where the fibres 

 are less differentiated. 



Taking the human heart, however, as a type, it may be said 

 that the auriculo-ventricular node lies, as described above, at the 

 base of the interauricular septum on the right side (3, Fig. 2). It 

 is in close connection (1) with the fibres of the interauricular septum 

 and thus indirectly with the sino -auricular node ; (2) with the 

 right auricle proper by means of the circular fibres of the A-V 

 groove. A good guide to its position is the coronary sinus (8, 

 Fig. 2). The node lies below and to the right. 



Arising from the A-V node is the main bundle (2, Fig. 2). 

 This rides along the top of the interventricular septum below 

 the pars membransea septi a spot easily found in the human 

 heart by holding the organ up to the light. The knife may be 

 safely entered through this spot, and the isolation of the bundle 

 thereby facilitated. At this point the main bundle divides into 

 its right and left septal divisions (Fig. 2). These divisions turn 

 downwards on the interventricular septum, and make for the 

 septal groups of musculi papillares. On the right side the 

 division is in the form of a fairly fine cord, and may run part 

 of its course embedded in the tissue of the septum ; but it 

 usually becomes superficial as it approaches the septal group of 

 musculi papillares, and it can be invariably found in that position 

 (Fig. 2, 4). On the left side close inspection of the septum will 



