CH. vil.] NERVE. 99 



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 characteristic double contour and tubular appearance to 

 the fibre. It is interrupted at regular intervals known as the 

 nodes of Ranvier. The stretch of nerve between two nodes is 

 called an inter^tiode, and in the middle of each inter-node is a 

 nucleus which belongs to the primitive sheath. Besides these 

 interruptions, a variable number of oblique clefts are also seen 



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

 divisions of the fibres, a, into two ; b, into three. X 350. (Kolliker.) 



dividing the sheath into medullary segments (fig. 1 1 8) ; 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. We shall make a more intimate acquaint- 

 ance with these chemical materials at a later stage in our 

 studies. 



Near their terminations the nerve-fibres branch : the branching 

 occurs at a node (fig. 120). 



H 2 



