212 On the Construction of the 



nerve fibres, wlien prepared and examined in glycerine. As the 

 density of this hquid probably exceeds that of the liquid part of the 

 nerve-medulla, a current must take place in an opposite direction, 

 in consequence of which their diameter is considerably reduced, and 

 they furthermore assume a fat-like, opaque and greenish appearance 

 (Fig. 9). The double contour also loses in diameter. By a subse- 

 quent addition of water and the removal of the glycerine by means 

 of a small piece of blotting-paper, the endosmotic current goes again 

 from the exterior to the interior, the nerve fibre loses its fat-like 

 lustre by the entering water, swells even frequently beyond its 

 normal diameter, and appears finally in every respect as if it had 

 been treated with water from the beginning of the examination 

 (Figs. 8 and 10). 



The observations on the construction of the double-bordered 

 nerve fibre, described in the preceding pages, related mainly to those 

 of the periphery. The fibres of the central organs of the nervous 

 system possess the same structure as the latter, but differ from them 

 only in their smaller diameter, which, in the finest fibres of the 

 grey substance of the brain and spinal marrow, amounts to about 

 ^^, or sometimes to not more than t^Io mm., while in the larger 

 ones of the spinal marrow it only exceptionally exceeds y^^y nana. 

 Another peculiar trait of character of the central nerve fibres con- 

 sists in those well-known varicosities which appear in their course ; 

 these, however, occur principally on the finer fibres, on which they 

 are frequently observed at such a regular distance from each other, 

 that one might be tempted to regard them as natural products. 

 I have never succeeded in demonstrating a tubular membrane on 

 the nerve fibres of the brain and spinal marrow, but notwithstand- 

 ing, I would not venture to deny the existence of it entirely. Quite 

 different is it with the fine fibrils of the fibrillous layer of the nerve- 

 medulla, which, except on the finest fibres, can always be seen as 

 distinctly as on the double-bordered nerve fibres of the periphery 

 (Fig. 20). On the larger fibres, they are seen to escape from their 

 open ends in the same manner as on the peripheral nerve fibres, 

 that is, in the form of loops, coils, &c., &c. In the interior of the 

 nerve fibre, however, these loops are not observed so often, because, 

 as I imagine, the small diameter of the entire fibre does not allow 

 their formation. 



As regards now the formation of those peculiar varicosities or 

 dilatations of these nerve fibres, it is difficult to find a satisfactory 

 explanation for this phenomenon, especially as they do not appear as 

 irregular sack-hke bulgings, but — mostly in the finer fibres — in a 

 more or less regular form. In the larger fibres these varicosities 

 are observed more rarely, and when they do occur, it is more in an 

 indefinite irregular form. The most reasonable explanation seems 

 to be, that their formation is due to some decomposition taking 



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