THE STRUCTUEE OF THE XERVE-FIBEES SYSTEM. 201 



however, soon bifurcates ; whether any of the fibres so formed pass 

 as far as the cord or "beyond its blood-vessels is doubted by most ob- 

 servers, and denied by Gaskell. These remarks and the description 

 of the cells of the spinal g-anglia (p. 176) hold good for the lateral 

 or vertebral ganglia and the prevertebral g-anglia (solar gan- 

 g-lion, etc.) of the sympathetic system ; the terminal ganglia will 

 be described with the organs in which they are found. 



H. Histological Notes on the Nekvous System. 



(In order to render the foregoing description of the nervous system more complete, 

 the following notes have been added by the translator.) 



[The histological elements of the nervous system are nerve-cells 

 and nerve-fibres ; of these the nerve-cells have been described with 

 the parts in which they occur ; it may simply be remarked that 

 later observers have been unable to discover the ' spiral cells ' de^ 

 scribed by Beale, Arnold, and others. The nerve-fibres, as in most 

 other animals, are of two kinds, medullated and non-medullated. 



1. Medullated nerve-fibres or white fibres are found in all cranial 

 and spinal nerves, with the exception of the olfactory nerves, and 

 in many of the sympathetic nerves (see Sympathetic System) ; 

 also in the white matter of the brain and spinal cord ; examined 

 microscopically the fibres are seen to consist of an external sheath or 

 neurilemma, a medullary sheath, and an axis-cylinder : — 



a. The neurilemma (Sheath of Schwann, Outer or Primitive 

 Sheath) is a nucleated endothelial layer covering the nerve-fibre ; 

 it is continuous with the corresponding coat of the nerve-cells, and 

 is uninterrupted throughout the length of the nerve ; at the nodes, 

 however, it dips down towards the axis-cylinder, the circular groove 

 so formed being filled with cement substance. 



b. The medullary sheath (AVhite substance of Schwann) ; the 

 presence of this sheath is the chief cause of the whiteness of these 

 nerves ; the thickness of the sheath varies considerably, and towards 

 the ultimate distribution of the nerve it is entirely lost. At more 

 or less regular intervals along the com'se of the nerve-fibre the 

 continuity of the medullary sheath is broken, and gives the 

 fibres the appearance of being constricted at these places ; such 

 constrictions are known as nodes of Ranvier ; the portion between 

 two such nodes being termed an internode. Each internode 

 possesses an oval, flattened, granular nucleus at about its middle 



