NERVE 



[CH. VII. 



by a sheath of white colour, fatty 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. 



The axis cylinder is a soft transparent thread in the middle of the 

 fibre; it is made up of exceedingly fine fibrils (fig. 117) which stain 

 readily with gold chloride. The medullary sheath gives a character- 

 istic double contour and tubular appearance to the fibre. It is inter- 

 rupted at regular intervals known as the nodes of Ranvier. The 

 stretch of nerve between two nodes is called an inter-node, and in 

 the middle of each inter-node is a nucleus which belongs to the 



FIG. 120. Small branch of a muscular nerve of the frog, near its termination, showing division of the 

 fibres, a, into two ; b, into three, x 350. (Kolliker.) 



primitive sheath. Besides these interruptions, a variable number of 

 oblique clefts are also seen dividing the sheath into medullary seg- 

 ments (fig. 118); but most if not all of these are produced artificially 

 in the preparation of the specimen. 



The medullary sheath also contains a horny substance called 

 neurokeratin : the arrangement of this substance is in the form of a 

 network or reticulum holding the fatty matter of the sheath in its 

 meshes. The occurrence of horny matter in the epidermis, in the 

 development of the enamel of teeth and in nerve is an interesting 

 chemical reminder that all these tissues originate from the same 

 embryonic layer, the epiblast. The fatty matter consists largely of 

 lecithin, a phosphorised fat, and cholesterin, a monatomic alcohol. 



Near their terminations the nerve-fibres branch : the branching 

 occurs at a node (fig. 120). 



