93 



any other part of the auricular muscle. It has no fixed attachment 

 to the central fibrous body or cartilage or to the annular fibrous ring. 

 Therefore we cannot consider it as an insertion or a muscle in the 

 ordinary sense. In the sheep's, and in some human hearts, it would 

 seem that a pull on the reticulum would be transferred to the main 

 bundle of His and thence down along its branches. This would be 

 purposeless so far as auricular efficiency is concerned. The chief 

 function of muscle is contractility, and any other function or quality 

 is subservient to this. It is usual, however, for any muscle to have 

 two attachments, or to be arranged in a circular form in order to 

 gain any advantage by contraction. Here we have a muscle which 

 has practically no attachment at one end. It has been suggested that 

 the system is conductive only, and this would coincide well with 

 anatomical findings. 



The auricular parts of this system, that is the reticulum and its 

 branches first lie between the auricles as they turn in to join the ven- 

 tricle at the central fibrous body, and if we wished to dissect these from 

 either auricle, we must cut through the auricular layer of muscle from 

 the inside of the heart and we find them lying between the two auricles 

 as between two leaves of a book. If we trace the auricular septal 

 branch, we find that for a short distance, two or three centimetres, it 

 still lies between the auricles, distributing itself as it proceeds along 

 the septum and giving a large branch to the sinus venosus. Its ter- 

 minals become intimately mixed with the auricular musculature. The 

 other branches, as I have previously noted, come to the pericardial 

 surfaces (immediately under which they lie, especially in the right 

 auricle) as they proceed to the auricular appendages. We thus have 

 well defined branches coming from or going to the reticulum from the 

 great systems of muscle in the auricles, viz: — the septum, and the 

 two auricular appendages. If this is a conducting system at all it is 

 clear from the anatomy of the structure that the auricular impulse 

 must begin in the reticulum and radiate to the auricles through the 

 auricular branches, or that it begins simultaneously in both auricles 

 and is carried through its branches to the reticulum. If this is the 

 case it is within the bounds of possibility that the left auricular 

 branches control the left ventricle, and that the right branches control 

 the right ventricle, and that their meeting at the reticulum and 

 continuing in one main bundle to the ventricles is only an economy 

 of space. 



Fig. 1 is a drawing of the subaortic region in the left ven- 

 tricle, showing the bursa on this side. This is a common variation 



