160 



Mr. J. G. Wilson. 



[Jan. 21 



The source of the plexuses is not easy to determine. The difficulty is due 

 partly to the length of the varicose fibrils before they enter intimately into 

 the plexus, and partly to the difficulty of staining at the same time 

 sufficiently well ganglion cells and muscle plexuses. Occasionally, however, 

 a non-medullated fibre can be seen to pass out from the strand and break up 

 into fine twigs, which ultimately become very fine varicose fibrils and enter 

 into direct relation with the muscle plexus. (See Plate 6.) 



III. Nerves related directly to the blood-vessels of the atrio-ventricular 

 bundle. These have no special significance in this locality and differ in no 

 way from nerves found in arteries elsewhere, so well described by Dogiel and 

 others. Two distinct varieties present themselves :— 



(1) A vasomotor plexus of fine non-medullated varicose fibrils (fig. 9). 

 These are most abundant in the large vessels and gradually get fewer as the 

 smaller arterioles are reached. At the point where a vessel divides or 

 a branch is given off, the plexus becomes more dense ; it is as if the fibres 

 became concentrated at the point of division. Then the plexus divides or 

 sends offshoots along the arterial ramus, the amount sent off varying with 

 the size of the branch. The main plexus runs in the adventitia, but a part of 

 it passes into the tunica media to form anastomosing branches directly 

 related to the muscle fibres. These do not appear to me to form definite 

 endings ; the knob-like endings on the muscle cell sometimes described 

 appear to be artefacts due to a stoppage of the dye. This appears to be 

 confirmed by relatively few appearing in well-stained preparations and their 

 abundance in badly-stained tissue. 



(2) Distinct from these are the so-called sensory endings. These are 

 definite end organs situated in the fibrous coat around the vessel, and differ 

 from the above described anastomosing plexus formations. They are so 

 called because of their resemblance to sensory endings elsewhere. A nerve 

 fibre thicker and less varicose than the ordinary vasomotor nerve and at 

 times faintly medullated is seen to break up in the tunica adventitia into 

 a more or less complex arborisation which is non-capsulated. In the smaller 

 arterioles they are very simple (fig. 9, S.), in the larger arteries of the bundle 

 they become extremely complex (fig. 10). 



Co'iiclusions. 



I. Anatomically the atrio-ventricular bundle contains not only a special 

 form of muscle fibre distinct from the ordinary muscle of the atrium or the 

 ventricle, but, as I have shown, is an important and intricate nerve pathway 

 in which we find : — 



