810 



AXIS-CYLINDER. 



But the latter is always present, and is indeed the chief functional constituent of 

 the nerve-fibre. The several parts of which the nerve-fibre is composed may now be 

 described in detail. 



Axis-cylinder. The essential part of every nerve-fibre is a pale and somewhat 

 indistinct strand, which runs in the axis of the fibre and is termed the axis-band, 

 axial fibre, or more commonly the axis-cylinder (fig. 355, 3 ; and 356, c). This 

 essential part is usually enclosed, as just mentioned, in one or more sheaths, but 

 these are not always present, especially at the origin and termination of a nerve- 

 fibre ; and even in the course of the fibre they may be interrupted at intervals. 

 The axis-cylinder, on the other hand, undergoes no interruption along the whole 

 course of the nerve, from the nerve-centre to the peripheral distribution. It 

 appears further to be clearly established that the axial fibre of a nerve is in every 

 case a direct prolongation of a branch of a nerve-cell. It is therefore to be looked 

 upon in the light of a far-extending cell-process ; and the study of the development 

 of the nerve-fibre affords a direct confirmation of this view of its nature. 



In the fresh state, and under high powers of the microscope, the axis-cylinder 

 presents an appearance of longitudinal striation, indicating a fibrillar structure (fig. 

 357) ; and at the termination of the nerves it may often be seen to separate into a 



^ jj Fig. 358. NERVE-FIBRES STAINED WITH NITRATE OF SILVER TO SHOW 



FROMMANN'S MARKINGS IN THE AXIS-CYLINDER. (Ranvier.) 



A, fibre showing a node, a, with the constricting ring. The axis- 

 cylinder has become shifted, and the part which was opposite the node 

 and which is stained by the silver, is now below it ; r, conical enlarge- 

 ment of the axis-cylinder. 



B, Isolated axis-cylinder. 



number of exquisitely fine filaments or fibrils. These, 

 the primitive fibrilke of Max Schultze, are embedded in a 

 homogeneous or finely-granular material, and may with 

 some reason be regarded as constituting the essential or 

 conducting part of the axis-cylinder, and therefore of the 

 nerve ; at least, it frequently happens that they form the 

 only visible portion of the nerve-fibre that is prolonged 

 to the ultimate termination. The fibrils often exhibit 

 minute varicosities which are highly characteristic in 

 appearance. They have a remarkable affinity for gold 

 salts, and when placed first in solutions of these and 

 afterwards in reducing agents (or merely exposed to the 

 light in water), the metal becomes deposited in the nerve-fibrils, giving them a dark 

 violet, almost black, appearance (see fig. 407, p. 346). They also become deeply 

 stained by methyl blue, especially when it is injected into the vascular system of the 

 living animal (Ehrlich). These methods are employed to trace the mode of ending 

 of the fibrils. 



The axis-cylinder is, by some authors, stated to be invested with a very delicate 

 structureless sheath. This sheath, which was first described by Mauthner, may 

 possibly be a protoplasmic inner layer of the medullary sheath ; but it is somewhat 

 doubtful whether the appearance is not due to a layer of fluid which has become 

 expressed by a shrinking of the axis-cylinder and coagulated under the influence 

 of the reagents employed. 



It was shown by Frommann that after treatment with nitrate of silver and subsequent 

 exposure to the light the axis-cylinder becomes stained in such a manner as to exhibit a distinct 

 cross-striated aspect (fig. 358), but it is not known whether this depends upon any structural 

 feature of the fibre or not. In addition to this cross-striated appearance, which may be seen 

 in any part of the axis-cylinders, they exhibit a peculiar biconical enlargement at each node 



