96 NOTES AND MEMORANDA, 



furcated in optical section), in which the edge of the neigh- 

 bouring segment is locked as in a vice. The free edges of the 

 segments next to Ranvier's nodes show a more or less marked 

 curvature. Changes in the appearance of the preparation 

 soon take place, and these are always most marked near 

 the cut ends of the fibres and near Ranvier's nodes. Four 

 stages may be noticed. Fh'st stage. The fibre looks like a 

 band of uniform width, bounded laterally by sharp and shin- 

 ing double contours, which are parallel and rectilinear. It 

 requires close attention to make out the separate medullary 

 segments, as these are in close contact, and, so to speak, 

 soldered to each other. Second stage. The compound nature 

 of the medullary sheath becomes more evident as the seg- 

 ments become slightly detached from each other. The con- 

 tours do not remain quite rectilinear, but show commencing 

 undulation. Third stage. The sharp edges of the segments 

 become split up into fibres, or swell into globose and irregular 

 masses, which mask the discontinuity of the medullary sheath. 

 Fourth stage. The whole medullary segments lose their recti- 

 linear arrangement, and appear swollen, coagulated, and 

 contorted. In salt-solution preparations the axis-cylinder is 

 at first indistinctly seen, appearing perfectly clear and homo- 

 geneous ; but afterwards a nebulous turbidity, at first very 

 pale and indistinct, begins to appear, and finally an irregular 

 coagulum, more visible, but still rather pale, forms within 

 the medullary sheath. This observation, which has hitherto 

 escaped the attention of histologists, demonstrates that the 

 axis-cylinder is originally liquid, and confirms another proof 

 of the same fact, viz. molecular movement within the fresh 

 electric nerve of the torpedo, previously described by Boll. 

 This, however, was not seen in the frog. The nerve nviclei 

 also are satisfactorily seen in these preparations ; they occupy 

 alveolar depressions on the surface of one, or sometimes two, 

 medullary segments. They are surrounded by a zone of 

 protoplasm, which, however, never extends over two neigh- 

 bouring medullary segments, so that in the adult frog there 

 is no delicate protoplasmic layer between the sheath of 

 Schwann and the medullary sheath. The sheath of Schwann 

 can hardly be seen in salt-solution preparations. 



B. — Nerve Fibre in Distilled Water. 



In distilled water rapid changes take place. At first the 

 medullary segments lose the appearance of shining bands, 

 becoming wider and duller ; they also lose their homogeneity, 

 splitting into a number of concentric tubes and layers, which 

 appear as longitudinal striae in optical section. This change 



