344 THE NERVOUS SYSTEM. 



spheres, on entering the gray matter of the convolutions, are reduced 

 from 5 to 2 mmm. in diameter. 



The nerve fibre, in its most complete form, presents three distinct 

 structural elements, namely : an external tubular sheath, an interme- 

 diate medullary layer, and a central axis cylinder. 



The Tubular Sheath. The nerve fibre consists externally of a col- 

 orless, transparent, tubular membrane, known as the " sheath of 

 Schwa nn," which closely invests its remaining portions. This mem- 

 brane may often be distinguished at points where the fibre has been 

 accidentally compressed or indented ; or it may be brought into view 

 according to the method of Kolliker, by treating the fibres with a cold 

 solution of sodium hydrate, and afterward boiling them for an instant 

 in the same fluid. This extracts the greater part of their contents, and 

 leaves the sheath in the form of an empty cylindrical canal. In its 

 general character, the tubular sheath resembles the sarcolemma of 

 muscular fibre, its principal physical properties being its cohesion and 

 elasticity. Its office is no doubt that of a protecting envelope, by 

 which the internal portions are maintained in the cylindrical form. 



The Medullary Layer. Immediately within the tubular sheath is 

 a layer of transparent, highly refractive material, nearly oleaginous in 

 consistency, termed the "medullary layer," or myeline, which gives 

 to the nerve fibres, and the tracts composed of them, their white 

 glistening aspect. Owing to the presence of this substance, the nerve 

 fibre has, under the microscope, a characteristic double contour, pre- 

 senting two parallel outlines on each border ; indicating the external 

 and internal limits of the medullary layer. The fibres containing a 

 medullary layer, and exhibiting its characteristic double contour, are 

 called "medullated nerve fibres." 



The medullary layer is readily altered by the imbibition of water. 

 It swells up and exudes from the divided extremities of the nerve 

 fibres, in filamentous tufts and masses of irregular outline, which from 

 their peculiar appearance are known as " myeline forms." These masses 

 become mingled with each other when a number of divided or lacerated 

 nerve fibres have been placed in water ; and the myeline is after a time 

 so much altered and distorted, by the imbibition extending to the in- 

 terior of the fibre, as to obscure all its remaining anatomical characters. 

 Owing to this alterability of the nerve fibres it has been found of 

 advantage to study them with the aid of various staining and harden- 

 ing liquids ; one of the most useful of which is perosmic acid. Dilute 

 solutions of this substance fix the nerve fibres in their natural form 

 and position, so that they can afterward be manipulated with less 

 danger of injury ; and it moreover stains the medullary layer of a 

 blackish hue, without coloring the remaining elements. When a group 

 of nerve fibres, stained by perosmic acid, are viewed in transverse 

 section, each fibre appears as a dark zone enclosing a transparent, col- 

 orless space near its centre ; the dark exterior zone being the blackened 

 medullary layer, while the central space represents the uncolored axis 



