no 



ELEMENTS OF HISTOLOGY. [Chap. xiv. 



when pressure or reagents are applied to the fresh 

 nerve-fibre. 



144. Medullated nerve-fibres without any neuri- 

 lemma, and consequently without any nodes of Ranvier, 

 with a thick more or less distinctly laminated medul- 

 lary sheath, form the 

 white substance of the 

 brain and spinal cord. 

 In these organs, in the 

 hardened and fresh state, 

 numerous nerve - fibres 

 may be noticed, which 

 show more or less regu- 

 lar varroosities, owing to 

 local accumulations of 

 fluid between the axis 

 cylinder and medullary 

 sheath. These are called 

 varicose nerve-fibres. 

 They occur also in the 

 branches of the sympa- 

 thetic nerve. 



The nerve-fibres of 

 the optic and acoustic 

 nerve are medullated, 

 but without any neuri- 

 lemma ; varicose fibres 

 are common in them. 



145. Medullated 

 nerve-fibres occasionally 



in their course divide into two medullated fibres. Such 

 division is very common in medullated nerve-fibres 

 supplying striped muscular fibres, especially at or near 

 thepointof entrance into the muscular fibres (see below). 

 But also in other localities division of nerve-fibres 

 may be met with. The electric nerve of the electric 

 fishes (malapterurus, gymnotus, silurus, etc., elec- 



Fig. 67.-Medullated Nerve-fibres. 



A, B, showing on a surface view tbe 

 reticulated nature of the medullary 

 sheath ; c, two nerve-fibres showing 

 the axis cylinder, the medullary sheath 

 with their vertically-arranged minute 

 rods, and the delicate neurilemma or 

 outer hyaline eheath. (Atlas.) 



