306 



MANUAL OF HISTOLOGY. 



called "axis cylinder;" and of another portion situated between the 

 envelope and the latter, a mixture of albuminous substances, cerebral 

 matters, and (?) fats, the "medullary sheath" or neural medulla" 

 (Nervenmark). Of these three, which cannot be demonstrated on the 

 perfectly fresh fibre, but only by round-about modes' of treatment, the 

 axis cylinder must be looked upon as the most essential and only indis- 

 pensable structural constituent. 



The appearance of broad nerve fibres in a recent state is that of threads 



formed of some completely homo- 

 geneous transparent or milky mass. 

 It is rare, however, that we obtain a 

 view of them in this state, owing to 

 the exceedingly rapid changes which 

 take place in the contained matter. 

 All the more customary modes of 

 preparation (if we desire to isolate 

 the fibres) bring the latter before us 

 in a form which has already under- 

 gone change, or has " coagulated," as 

 the saying is. This congelation, 

 however, is met with in various stages 

 of completeness (fig. 296, a, b ; fig. 

 297). 



When isolated with as great rapidity 

 and care as possible, the nerve fibre 

 presents to our view a dark border, 

 and closely applied to this internally 

 a second and finer bounding line 

 (fig. 296, a, b- } fig. 297, 6, above). 



Later on, these two lines or 

 "double contours" are not quite par- 

 allel, and the internal one is no longer 

 continuous throughout. The thin 

 layer interposed between the two 

 lines on each side of the fibre appears 

 homogeneous (fig. 296, a, ft) or granular. 

 At this stage of transformation the nervous fibre may remain stationary, 

 the outer coagulated layer acting to a certain extent as a protecting 

 covering for the portion situated more internally, or, the congelation may 

 advance further at points, and the nerve fibre may frequently present a com- 

 pletely different appearance at various parts of its course (fig. 297, b). 



After this the internal line becomes separated more and more from 

 the outer one, while between the two, and also in the central part of 

 the fibre, lumpy, granular, or globular masses are formed (a, b), until 

 eventually the whole appears transformed into a sometimes coarse and 

 sometimes finely granular substance (c), and the entire nerve tube has 

 become dark (2). 



REMARKS. 1. Literature is very rich in information on this subject. 2. Neural 

 medulla which has escaped from its sheath displays precisely the same changes 

 ("Myelin"ofVirchow.) 



175. 

 The existence of an envelope on the nervous tube is easily inferred 



Fig. 297. Human nerve fibres at an advanced 

 stage of coagulation. 



