CH. VII. j 



MEDULLATED NERVE 



75 



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

 fibre; it is made up of exceedingly fine fibrils (fig. 91) 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 a nerve -fibre between two nodes is called an inter-node, 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 dividing the sheath into medullary seg- 

 ments (fig. 89) ; 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 fat-like matter consists largely of 

 cholesterin, a monatomic alcohol, and phosphorised fats, such as 

 lecithin. 



Near their terminations the nerve-fibres branch : the branching 

 occurs at a node (fig. 92). 



Fio. 92. 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. (Kiilliker.) 



Staining with silver nitrate produces a peculiar appearance at the 

 nodes, forming what is known as the crosses of Ranvier. 



One limb of the cross is produced by the dark staining of cement 

 substance which occurs between the segments of the neurilemma ; the 

 other limb of the cross is due to the staining of a number of minute 

 transverse bands in the axis cylinder (Fromann's lines), which is here 



