208 On the Construction of the 



it seems to me more practical to be guided only by the results of 

 ocular demonstration, which alone for the jDresent can be relied 

 upon. For this reason, we shall try to give from an anatomical 

 point of view, a reasonable explanation of those successive changes 

 occurring within the double-contoured nerve fibre corresponding 

 with our own observations. I presume, therefore, that the fibrils 

 of the fibrinous layer are formed of a consistent, though very soft 

 albuminous body, while the semi-fluid medullary layer consists of a 

 substance which is especially distinguished by its fat-like lustre and 

 by its partial solubility in water. As regards the organic consti- 

 tuents of these substances, it must be left to organic chemistry to 

 determine, for the above view is only based upon the behaviour of 

 the nerve-medulla to the action of various reagents, as seen under 

 the microscope. Now in examining a number of nerve fibres in 

 serum, no particular changes will be observed to take place within 

 them, because no considerable difference probably exists in the 

 density of this liquid and that of the semi-fluid medullary layer, and 

 also the intermediate ' substance of the fibrillous layer, in conse- 

 quence of which no endosmosis occurs. The changes, manifesting 

 themselves sometimes soon after, by the appearance of those inden- 

 tations or folds above mentioned, may be ascribed to an evaporation 

 of the serum, occurring at the margin of tlie covering glass, in- 

 creasing its density and giving rise to a slight exosmosis on those 

 points of the nerve fibre. 



The first change which is seen to occur on the nerve fibres, 

 prepared in serum, after the addition of water, is the appearance of 

 that fine line ah-eady mentioned (Fig. 2), dividing the double con- 

 tour into two parts.* It is produced by the action of the water, 

 which, obeying the law of endosmosis, penetrates through the outer 

 part of the double contour, representing the tubular membrane, 

 and, dissolving the connecting medium, causes this to be separated 

 from the inner, the fibrillous layer of the nerve-medulla. With the 

 continued advance of the water toward the axis of the nerve fibre, 

 that portion of intermediate substance which connects the indivi- 

 dual fibrils of the fibrillous layer to each other, is also dissolved, 

 in consequence of which they are rendered, either singly or still 

 holding together in the form of small bundles, visible to the eye of 

 the observer. Penetrating still farther into the interior of the 

 fibre, it now reaches the medullary layer, and dissolving the albu- 

 minous constituents of this body, finally causes those disturbances 

 and destructions, especially in the fibrillous layer, which have 

 already been partially described above. In order to explain these 



* This fine dark line, which divides the outer part of the nerve fibre, repre- 

 sented by the double contour, into two layers difiering in their refraction of light, 

 and which Stilling already described more than sixteen years ago, seems, as far 

 as I know, to have been overlooked or ignored by most histologists. 



