NER VE FIBRES— CRA YFISH 153 



Fih-es of the Nerve going to the Chela of Astacus 

 fluviatilis 



If fibres of this nerve are isolated by being teased 

 up after maceration in iodine alcohol, it is possible to de- 

 monstrate, especially in those of medium size, structural 

 relations which correspond completely to those of vertebrates. 

 In the first place, they also possess a sheath, which agrees 

 in all points with that of Schwann in the fibres of the 

 ischiadic nerve. In optical section the sheath appears rather 

 dark and glossy, permitting its composition out of a single 

 layer of alveoli to be seen very beautifully (Plate VII. Fig. 

 7, s). Also, by sharply focussing the surface, the somewhat 

 irregular net-like structure of the sheath can be recognised 

 very plainly (os) ; in places, however, its superficial structure 

 may also appear as a meshwork giving the appearance of 

 longitudinal fibres. In the sheath there are large elongate 

 nuclei (n), which project outwards only to a slight extent, 

 but more strongly into the interior of the fibres (to'). Just 

 as has been described in the case of the sheath of Schwann, 

 the layer of meshes composing the protoplasm of the sheath 

 divides in the vicinity of the ends of the nucleus into two, 

 so that the internal and external surfaces of the nucleus are 

 distinctly covered each by a layer of alveoli. In the thicker 

 fibres, however, the sheath also appears to become stronger, 

 sometimes to the extent of two or three layers of meshes^ 

 In such thicker fibres the axis-cylinder did not quite fill up 

 the sheath, while this was always the case in thinner ones. 



ISTow, although the structure of the axis-cylinder is very 

 pale, it can still be made out clearly that it agrees com- 

 pletely with that already described for the vertebrates. 

 The longitudinal fibrillse are here also distinctly linked with 

 one another by means of threads connecting them across 

 {Yig. 7,/), and they show the nodal points at the places of 

 union between the fibrillse and the cross threads very 

 beautifully. 



The examination of cross sections of such nerve fibres 

 proves that a distinct net-like meshwork exists in the axis- 



