THE NERVOUS TISSUES. 



lOOI 



Fig. 842. 



Axis-cylinders 



Axolemma 



Medullary sheath 



Node of Ranvier 



Neurilemma 



Medullated nerve-fibres, as' seen in longi- 

 tudinal sections of spinal nerve. X 500. 



with neurones. Recognizing, therefore, that the nerve-fibres are only processes of 

 neurones, their separate description is justified only as a matter of convenience. 

 The fundamental part of every nerve-fibre is the central cord, commonly known 

 as the axis-cylinder, which is composed of threads of great delicacy, the axis- 

 fibrillce, jifolonged from the nerve-cell and embedded within a semifluid interfibrillar 

 substance, the neuroplasfu, the entire cord per- 

 haps being enclosed by a delicate structureless 

 sheath, the axolemma. The existence of the 

 axolemma as a distinct sheath, however, is ques- 

 tionable, the appearance of such investment not 

 improbably being due to a local condensation of 

 the framework of the medullary coat immediately 

 around the axis-cylinder. 



In the case of the typical fibres, such as form 

 the chief constituents of the peripheral nerves 

 distributed to various parts of the body, the axis- 

 cylinder is surrounded by a relatively thick coat, 

 known as the medullary sheath, outside of which 

 lies a thin structureless envelope, the neurilemma 

 or sheath of Schwami, that invests the entire 

 nerve-fibre. In the case of fibres proceeding from 

 neurones composing the sensory ganglia, the 

 neurilemma is continuous with the nucleated 

 sheath enclosing the individual ganglion-cells. 

 The medullary sheath consists of two parts, 

 a delicate reticular framezvork and a fatty substance, the myelin, that fills the meshes 

 of the supporting reticulum. The latter, arranged for the most part as anastomosing 

 membranous lamellae, that in transverse sections of the nerve-fibre appear as faint 

 concentric lines, resists pancreatic digestion and fat-dissolving reagents, and was 

 regarded by Ewald and Kiihne as possessing properties similar to the keratin of 

 horny substances and, hence, was named by them neurokeratin. The blackening 

 after treatment with osmic acid and other reactions exhibited by myelin indicate its 

 fatty nature, and it is probable that this substance exists during life in the form of a 

 fine emulsion supported by the framework. When fresh, myelin appears highly 

 refracting and homogeneous, and confers upon the medullated nerve-fibres their 

 characteristic whitish color. It is, howev^er, prone to post-mortem changes, so that 

 after death it loses its former uniformity and presents irregular contractions and 

 collections, or at the broken end of the fibre extrudes in irregular globules, due 

 probably to fusion of the normal individual minute droplets into larger masses. 



The medullary sheath is not uniformly continuous, but almost completely inter- 

 rupted at regular, although in different fibres variable, intervals marked by annular 



constrictions. These constrictions, the nodes 

 of Ranvier^ correspond to narrow zones at 

 which the medullary sheath is practically 

 wanting and the neurilemma dips in and, some- 

 what thickened, lies in close relation with the 

 axis- cylinder. According to Hardesty ^ the 

 medullary sheath does not suffer complete 

 suppression at the nodes, but is represented 

 by part of its reduced framework which trans- 

 verses the constriction, a conclusion which 

 we can confirm. The nodes occur at regular 

 intervals along the fibre, which they thus divide 

 into a series of internodal segments. In general, the latter are longer in large fibres, 

 where they have a length of about i mm., and shorter in those of small diameter, in 

 which they may measure . i mm. or less in length. The axis-cylinder passes uninter- 

 ruptedly across the nodes, although it often presents a slight fusiform enlargement 



Fig. 843. 



Axis-cylinder 



Neurilemma 

 Medullary sheath 



Medullated nerve-fibres in transverse 

 section. X 550. 



'Amer. Journal of Anatomy, vol. iv., 1905. 



