154 



Mr. J. G. Wilson. 



[Jan. 21, 



(2) This muscular system has a uniform arrangement in all mammals, 

 though slight individual differences appear. It runs from the atrial wall 

 through the atrio-ventricular fibrous septum to the point where it spreads 

 out in the ventricular wall, at first as a bundle enclosed in connective tissue, 

 then spreading out into tree-like end branches. The closed strand never 

 enters into relation with the ventricular muscle, but the end branches fuse 

 witli the usual ventricular muscle. 



(3) This uniting system is early developed in the embryo. From this 

 time on, leaving growth out of consideration, it remains unchanged during 

 life. It is not affected by hypertrophic and atrophic processes in the heart in 

 the same way as the ordinary cardiac muscle. 



(4) The topographical, histological, and biological peculiarities of this 

 system are opposed to the suggestion that its function is that of a heart 

 pump, like the ordinary cardiac muscle. Moreover, the physiological 

 experiments of Gaskell, Engelmann, Herring, and others suggest that in this 

 system can be found a conducting path for the co-ordination of the heart 

 miTScle. 



Macroscopic Description of the Bundle. — To dissect out rapidly the atrio- 

 ventricular bundle, I have found it best first to identify the pale pink muscle 

 fibres of the right and left branches. These are more or less subendocardial. 

 The left is readily observed at a varying distance beneath the junction of the 

 posterior semilunar (non-coronary) with the right semilunar valve. The 

 right branch, less readily found in some mammals because covered by a thin 

 layer of ventricular muscle, can be found lying near the line which joins the 

 moderator band to a point under the medial (septal) cusp, adjacent to its 

 junction with the anterior (infundibular) cusp of the tricuspid valve. From 

 either of these points the entire atrio-ventricular band can be dissected out. 



As seen in the calf, where it is very large, the bundle originates in the 

 posterior wall of the right auricle in the region of the sinus coronarius (Plate 4, 

 figs. 1 and 2). At this point there is a mass of pink fibres — not red like the 

 ordinary cardiac fibres, and less pale than the main continuation of the band. 

 It is cone-shaped in appearance, with an ill-defined base merging into the 

 auricular muscle and a well-marked apex passing into the narrow band of 

 white tissue which goes forward into the atrio-ventricular septum in a groove 

 on the under surface of the cardiac cartilage. Eeaching the upper part of 

 the interventricular septum, it divides into a right and a left branch. The 

 right branch passes downward on the septal wall of the right ventricle more 

 or less superficial towards the septal attachment of the moderator band. 

 Here it begins to separate out and send branches into the septum. If a 

 transverse section be made of the moderator band, Purkinje fibres are seen in 



