THE MECHANISM OF TRANSPORT 



121 



The invaginated portion has become greatly reduced in amount, 

 and instead of extending into the ventricles all round the A.-V. groove, 

 as it was in the primitive type of heart, it is represented by a small 

 muscular band of fibres arising from the A.-V. node, and passing into 

 the ventricles the auriculo-ventricular bundle. 



It consists essentially of four portions: The A.-V. node, thejmain 

 bundle, the septal divisions, the terminal ramifications. 



These parts can be made out more easily in some hearts than 

 other. In the hearts of the sheep and ox it is easy, owing to the 

 paleness of its fibres, to dissect out the whole bundle. In these hearts, 

 the fibres constituting the bundle present a greater contrast to the 

 musculature of the heart. 



FIG. 30. RIGHT AURICLE AND VENTSICLE OF CALF. (Keith and Flack.) 



1, Central cartilage; 2, main A.-V. bundle; 3, A.-V. node; 4, right septaljlivision ~of 

 A.-V. bundle; 5, moderator band; 8, orifice of coronary sinus. 



The auriculo-ventricular node lies at the base of the interauricular 

 septum on the right side, below and to the right of the coronary sinus. 

 It is in close muscular connection with the interauricular septum, 

 and thus indirectly with the sinu-auricular node. 



The main bundle, arising from the A.-V. node (Fig. 30), rides along 

 the top of the interventricular septum below the pars membranacea septi 

 a spot easily found in the human heart by holding the organ up to 

 the light after opening the chamber. It then divides into the right 

 and left septal divisions for the right and left ventricle respectively. 

 The right band is cord-like, and is somewhat embedded in the septum, 

 becoming superficial as it approaches the septal group of the musculi 

 papillares. The left bundle is subendocardial throughout its course to the 

 septal musculi papillares, and has the form of a delicate ribbon of fibres. 



The terminal ramifications may be said to arise from these groups 

 of septal musculi papillares. Starting from these, they run in the 

 " moderator band " on the right side, and in several small bands on 

 the left side, passing as delicate trabeculae to fuse with the ventricular 

 musculature. 



