1862.] 



69 



of the axis-cylinder alone ; but it decidedly appears to me that, besides 

 the axis-cylinder, they are furnished with a prolongation of the 

 membranous sheath ; indeed I have seen this so clearly in a great 

 many favourable instances, that I can have no doubt on the point. 

 According to my view, therefore, the delicate membranous sheath 

 does not quit the nerve-fibre and coalesce with the sarcolemma, as 

 Kuhne believes, but continues to surround a pale prolongation of the 

 soft contents of the nerve- tube. As to the matter contained in the 

 membranous sheath, it no doubt always comprehends the axis- 

 cylinder, and is chiefly formed by a prolongation of that structure ; 

 but I have seen examples in which the contained matter showed slight 

 varicosities and a certain darkness of outline, from which I infer 

 that here and there at least a thin layer of the white substance 

 extends along the pale fibre. But whilst it is easy in most cases to 



Fig. 1*. 



perceive the membranous sheath and its enclosed matter distinct 

 from each other at the commencement of the pale fibres, yet in their 

 further progress these structures coalesce together, and the terminal 

 fibres then appear as uniform pale filaments. They are still, however, 

 to be regarded as tubes ; for, in the first place, they are prolonga- 

 tions of a decidedly tubular fibre, and, secondly, when treated with 



* Fig. 1. Termination of a dark-bordered tubular nerve-fibre on a muscular 

 fibre, /,/, of the cutaneous muscle of the frog, seen with Hartnach's Objective 

 "a immersion" No. 10, and Eyepiece No. 1. a, Sheath of the nerve-fibre con- 

 tinued at b upon the pale terminal fibre ; 6, axis-cylinder and contents of the 

 nerve-tube continued into the pale fibres; c, c, nuclei of the pale fibres; d, a 

 nucleus belonging to the muscular fibre/",/; e, e, e, e, ends of the pale fibres ; 

 g, g, nuclei of the dark-bordered fibres. [The figure shows the continuation of the 

 dark-bordered fibres into the pale filaments, but fails to exhibit the faint outline 

 and characteristic aspect of the latter.] 



