TII STRUCTURE AND ORGANISATION OF NERVE 49 



apparently gelatinous consistency (the " axoplasma "), and the 

 fibrils embedded in the same. But while in the above examples 

 the fibrils constitute a central bundle, more or less thickly 

 invested with axoplasma, in medullated fibres they are distributed 

 equally over the entire section of the axis-cylinder, so that the 

 investing layer fails to appear, and forms an insignificant marginal 

 zone. With appropriate reagents (acid fuchsin, bismarck brown, 

 etc.) the fibrils are distinct upon the longitudinal aspect of the 

 fibres as well as at the cross-section. It then appears that the 

 fibrils lie closer together and are united by less cement-substance 

 at the constrictions, so that the axis-cylinder is most slender at 

 these points (Fig. 157). Engelmann (8) argues in favour of a pre- 



FIG. 157. L. and T.S. of medullatecl nerve-fibre from frog's sciatic nerve (osmic acid, acid fuchsin). 

 Nodes of Rauvier and two Lantermann's notches, fibrillar structure of axis-cylinder. 



formed discontinuity of fibrils at the nodes, chiefly on the ground 

 that the axis-cylinder under certain conditions exhibits a break 

 in continuity at the point of constriction, " corresponding with 

 the black cross-lines of the silver reaction " (Engelmann's Quer- 

 schcibe). Engelmann's reasoning was subsequently rejected, and 

 cannot therefore be cited in support of the view that the medul- 

 lated nerve-fibre consists of single juxtaposed cells (cf. Jacobi, 

 Boveri) an assumption that is disproved by recent embryo- 

 logical observations. It may be remarked that Ehrlich's 

 " intravital " methylene-blue method has never (in Biedermann's 

 experience) brought out distinct traces of fibrillar structure in the 

 axis-cylinder of fresh preparations, either in medullated or in 

 non-medullated fibres ; on the other hand, certain splanchnic 

 fibres of Hirudo medicinalis stained by this method show, after 

 treatment with picrate of ammonia, an unmistakable construction 

 from single fibrils. It is still uncertain whether the axis-cylinder 

 as a whole (fibrils + neuroplasm) may not, in addition to its 



VOL. II E 



