The Dejpressor Nerve of the Rabbit. 



233 



Microscopic Examination of the Nerve. 



For histological purposes a piece of the nerve was slightly stretched on a 

 card, fixed with osmic acid, washed and transferred to alcohol. Sections 

 were cut by the paraffin method (figs. 7 and 8). Teased preparations were 

 also examined. Microscopically, the nerve is found to consist of both 

 myelinated and non-myelinated fibres (fig. 9). The myelinated fibres are 

 both medium-sized and very fine. Most are between 4yx and (0 - 004- 

 - 006 mm.) in diameter, but in two nerves two or three fibres of about 8/a 

 (0 008 mm.) were found. The fine myelinated fibres have the character of 

 pre-ganglionic autonomic nerves. The non-myelinated resemble post- 

 ganglionic autonomic fibres. Both these kinds of fibres are presumably 

 efferent. 



The size of the whole nerve varies in different cases. The right nerve is 

 usually smaller than the left and contains fewer myelinated fibres. I have 

 made an attempt to count the myelinated fibres in six rabbits, but the small 

 diameter of the finest fibres renders the task a difficult one. And it is 

 impossible to see the non-myelinated fibres clearly enough in section to be 

 able to count them. Xor is it easy to obtain sections so exactly transverse 

 as to show each myelinated fibre distinctly. The following figures are 

 therefore given for what they are worth : — 



The total number of myelinated fibres of all sizes in the nerves of the two 

 sides showed an individual difference of from 375 to 496 fibres, the average 

 number being 433 in the two. The average number of myelinated fibres of 

 all sizes in the right nerve was 177 and in the left 256. The total number 

 of fibres contained in the branches when these remained separate fell within 

 these limits. 



Summary. 



1. The depressor nerve of the rabbit appears to be connected, at least in 

 part, with a special collection of ganglion-cells in the vagus, distinct from the 

 ganglion of the trunk. This collection may extend a certain distance into 

 the superior laryngeal, or may pass into the vagus trunk some distance below 

 the ganglion of the trunk, but in most it lies in close contiguity with and just 

 below that ganglion. The cells of the group in question probably give rise 

 to the afferent fibres of the depressor. 



The exact point of origin of the nerve is variable. It is usually formed by 

 two branches, one from the superior laryngeal and one from the vagus. In 

 some cases it is double throughout, in others single. It is connected below 

 by fine branches with the inferior cervical ganglion, and cam be traced to the 

 root of the aorta and base of the heart. 



vol. xciii. — B. s 



