206 On the Construction of the 



Independent of the changes produced on the nerve fibre by the 

 points of the needles during the manipulation, others, which cannot 

 be ascribed to this cause, are observed to occur on its external 

 sm-face ; these consist of certain ' .'egular, more or less sack-hke 

 bulgings (Figs. 3 and 5), appearing in the course of the fibre. 

 They are particularly seen on fresh nerve fibres of man, of the ox, 

 and of other higher vertebrata when prepared in water, and they 

 always correspond, as is found by careful examination, to a loop or 

 wave formed by the fine fibrils of the nerve-medulla. On nerve 

 fibres of freshly-killed animals, on which the double contour has 

 remained unchanged, certain changes, frequently described, are 

 observed to occur, even when examined in serum. They consist in 

 the double contour breaking off in certain places, and terminating 

 in a sharp point in the direction of the axis of the fibre, while 

 another arises in the form of a point a short distance above the end 

 of the former (Fig. 1, c). These indentations, which are often 

 observed on both double contours, and in an almost regular distance 

 from and opposite to each other, represent folds of the tubular 

 membrane and the nerve-medulla, probably produced by certain 

 processes, to be directly explained. In the same manner, we 

 frequently observe on the surface of the nerve fibre, grotesque, 

 apparently double-bordered figures in the form of loops, hooks, &c., 

 &c., which have also frequently been described. 



All these changes, which the nerve-medulla often undergoes in 

 regular succession, have been hitherto described by the older, and 

 even some of the more recent investigators of the nervous tissues as 

 spherical, granulous, clodded masses, produced by coagulation ; and 

 as such they are still spoken of in recent works on histology. That 

 the true nature of these clods has not been recognized before this, 

 can hardly be explained otherwise than that they have been 

 examined with amplifications too low — 250 to 300 diameters — 

 and with insufficient illumination. 



As far as I am now able to judge of the anatomical composition 

 of the nerve-medulla by my numerous examinations relating to the 

 subject, it seems to consist of two layers, distinctly difiering from 

 each other. The outer one of these shows a structure composed of 

 very delicate and smooth fibrils, about tAt7 Dim. in thickness, and 

 arranged parallel and very close to each other ; they can be demon- 

 strated without difficulty and almost under all circumstances. The 

 inner one surrounds directly the axis cylinder, and consists of a 

 finely granular, amorphous, and semi-liquid substance. We will 

 designate the former as the fibrillous, and the latter as the medul- 

 lary layer. 



Although I doubt but little the pre-existence of these two layers 

 of the nerve-medulla, I do not venture to defend this view as the 

 only true one, but certain it is, that they manifest themselves 



