96 Journal of Comparative Neurology. 



spindle shaped muscle cells are usually found between two such 

 fibers. It is of course impossible to say whether, in the pre- 

 ceding or succeeding sections, terminal fibers which held a 

 very close relation to the muscle cells found between two par- 

 allel fibers, might not exist. In thicker sections, there always 

 seem to be some muscle cells that are not touched by a nerve 

 fiber or an ending of such fibers. I am inclined to think there- 

 fore that not all the cells in involuntary muscle have a nerve 

 fibril ending on them. 



The innervation of the involuntary muscle of vessels is 

 best studied in relatively small vessels. In such vessels, sym- 

 pathetic nerve fibers, often very varicose, form a plexus in or 

 outside of the adventitia. Such a plexus is well shown in Fig. 3, 

 which shows a portion of a small vessel from the pharynx of 

 a frog, injected with methylen-blue. From this peri-vascular 

 plexus, terminal fibers or small bundles of fibers enter the 

 muscular media, where in thicker vessels they form an intra- 

 muscular plexus; from the intra-muscular fibers, terminal 

 branches arise which end on the muscle cells. 



The account here given is very similar to one given by 

 Retzius (44), who has described the innervation of the vessels 

 in the choroid of white rabbits, also vessels of the frog's pha- 

 rynx, and corroborates observations made by Dogiel (45) on 

 the nerve supply of the vessels of the human eye-lid, to some 

 extent also the accounts given by His and Kolliker of an 

 earlier date. 



2. Ending of sympathetic neurons in heart muscle. — It is 

 not my purpose to give here the older literature bearing on this 

 portion of my subject ; for this the reader is referred to articles 

 by Retzius (44), Berkley (46) and Jacques (47). The first com- 

 munication dealing with the innervation of heart muscle, giv- 

 ing observations obtained either with the Golgi or methylen- 

 blue method, we have from Arnstein (48), who describes a 

 loose plexus around the bundles of the heart muscle ceMs. 

 From it small varicose branches are given off, which may often 

 be traced for long distances. These end on the cells without 

 forming bulbar enlargements. In the account given by Retzius 



