1909.] The Nerves of the Atrio-ventricular Bundle. 159 



tissue around it at various parts of its course, but chiefly prior to or near the 

 point of division into the right and left arms. Thus several strands appear 

 towards the coronary side of the division, corresponding to the position of 

 the ganglionic group referred to above. The question of the source of the 

 fibres lies outside the field of the present investigation, but their course 

 indicates an origin near the insertion of the aorta and the lower part of the 

 atrial septum. Sections of the bundle bring out more clearly the relation of 

 these strands to the ganglion cells. Throughout their course nerve cells are 

 scattered individually or in groups and the processes of these cells enter into 

 the nerve strand (fig. 4). The fibres of these strands are not in close 

 apposition. This is especially observed in the left septal part (fig. 4). They 

 are found in the connective tissue both around the bundle and between the 

 muscle cells. They have irregular connection with each other. Often one 

 can see a strand send off a single fibre or a group of three or four fibres 

 which pass across the intervening muscle cells to an adjacent strand and then 

 continue their course along this. Occasionally a fibre may be observed to 

 turn backwards ; however, the general tendency is for the strands to pass 

 downwards in the direction of the atrio-ventricular bundle. The fibres are 

 as a rule non-medullated, most of them with varicosities. These varicosities 

 occur at less frequent intervals than in group (b). The medullated nerves 

 seen have been chiefly in the cow. It is with fibres of this group that at 

 first the elastic tissue is apt to be confounded, but enough has been said to 

 show how they may be easily differentiated. 



It is difficult to tell what becomes of these strands. Many of them pass 

 through that part of the bundle I have examined. Some, however, may be 

 seen to break up into very fine varicose fibrils which enter into plexus (b) 

 (fig. 4, A.). 



(&) The nerve plexuses are composed of very fine fibrils with varicosities 

 at frequent intervals. They can be seen with the 8-mm. objective and 

 4 ocular, but require a higher power for distinct observation. The fine 

 branches lie in close apposition to the muscle cells and have absolutely no 

 resemblance to elastic tissue. In well-stained preparations they are so dense 

 and intricate from frequent branchings and anastomoses that it is impossible 

 to trace individual fibres for any distance. I have chiefly studied these 

 plexuses in the pig and sheep. Here they form a continuous network which 

 can be traced for long distances over a series of sections and appear to 

 extend the whole length of the atrio-ventricular bundle. Their general 

 characters can be well seen in figs. 7 and 8. As will be noted, it is not a case 

 of a nerve breaking up and surrounding individual muscle cells, but of 

 a complicated network lying around both single cells and groups of cells. 



