74 



NERVE 



[cn. vn. 



spinal cord, 

 arrangement, 



The bodies of nerve-cells differ in size, shape, and 



and we shall discuss these fully when we get to the 



nerve-centres. For the present it will be convenient 



to confine ourselves to the nerve-fibres as they are 



found in a nerve. 



Nerve-fibres are of two histological kinds, medul- 

 lated and non-medullated. Medullated nerve-fibres 

 are found in the white matter of the nerve-centres 

 and in the nerves originating from the brain and 

 spinal cord. Non-medullated nerve-fibres occur in 

 the sympathetic nerves. 



The medullated or white fibres are characterised 



Fio. 89. Nerve- 

 fibre stained with 

 osmic acid. A, 

 node ; B, nucleus. 

 (Key and Ret- 

 zius.) 



Fio. 90. A node of Ranvier 

 in a medullated nerve-fibre, 

 viewed from above. The 

 medullary sheath is in- 

 terrupted, and the primi- 

 tive sheath thickened. 

 Copied from Axel Key and 

 Retzius. x 750. (Klein 

 and Noble Smith.) 



Fio. 91. Axis cylinder, 

 highly magnified, 

 showing its com- 

 ponent fibrils. (M. 

 Schultze.) 



by a sheath of white colour, fat-like in nature, and 

 stained black by osmic acid ; it is called the medullary 

 sheath or white substance of Schwann ; this sheathes the 

 essential part of the fibre which is a process from a 

 nerve-cell, and is called the axis cylinder. Outside the 

 medullary sheath is a thin homogeneous membrane of 

 elastic nature called the primitive sheath or neurilemma. 



