GENERAL STRUCTURE OF THE NERVOUS SYSTEM. 345 



FIG. 85. 



cylinder. When viewed in profile, such fibres exhibit a dark colored 

 double border, formed by the medullary layer, surrounding the longi- 

 tudinal axis cylinder. 



In regard to its physiological function, the 

 medullary layer is considered by some writers 1 

 as an isolating substance, like the gutta-percha 

 envelope of a submarine telegraph wire, to con- \ 

 fine the transmission of nerve force within proper 

 limits, and prevent its diffusion to neighboring- 

 parts. It certainly does not act directly in this 

 transmission ; since, as hereafter shown, it is 

 interrupted at numerous points in the course of 

 the fibres ; and it is always wanting for some 

 distance in the neighborhood of both their 

 origin and their termination. These facts are 

 also at variance with its supposed character as 

 an isolating material ; since any discontinuity of 

 its substance would seem to destroy its efficiency 

 for that purpose. It is sometimes regarded, with 

 perhaps greater plausibility, as affording, by its 

 consistency, a physical protection to the axis 

 cylinder; securing it from local injury, in flexions 

 or indentations, by the uniform support which a 

 fluid envelope would give. Its interruptions 

 during the course of the nerve fibres are not 

 sufficient to interfere with its usefulness in this 

 respect. 



The Axis Cylinder. The central part of the 

 nerve fibre consists of a pale, homogeneous, or finely granular cord, of 

 nearly cylindrical form, situated in its longitudinal axis. From these 

 characters it has received the name of the " axis cylinder." In consist- 

 ency the axis cylinder is a soft solid, and, though very delicate, it has 

 a certain degree of elasticity. By some observers (Schultze, Gerlach) 

 it is regarded as composed of minute fibrillae, united into a uniform 

 bundle; by others of equal authority (Kolliker) the indications of its 

 fibrillated constitution are considered as uncertain. 



The axis cylinder consists of an albumenoid substance, insoluble in 

 water, alcohol, and ether. It becomes pale and swollen by the action 

 of concentrated acetic acid, and is readily dissolved by a boiling solution 

 of sodium hydrate. It is stained red by solutions of carmine, which, on 

 the other hand, produce no effect on the medullary layer ; and after the 

 use of this agent, the transverse section of a nerve shows in the interior 

 of each fibre a red or pinkish spot in the place of the axis cylinder r 

 surrounded by a colorless zone representing the medullary layer. In 

 nerve fibres treated with a solution of gold chloride and subsequently 

 exposed to light, the axis cylinder is stained of a dark purple, nearly 

 black color ; and by this mode of preparation nerve fibres of extreme 



NERVE FIBRKS, fixed and 

 stained by perosmic acid ; 

 from the posterior wall of 

 dorsal lymph-sac of Frog. 

 1, 1, Medullary layer. 

 2, Axis Cylinder. 3, 3, Con- 

 strictions of Ranvier. 4, 4, 

 Incisions of Schmidt. 



