NERVE FIBERS. 99 



Myelin is a mixture of complex fats and lipoid substances, some of 

 which are combined with sugar. Like fat it is dissolved by ether and 

 blackens with osmic acid. It exists as an emulsion, and appears very 

 white macroscopically. Between the myelin globules there is a network 

 of neurokeratin, a substance unstained by osmic acid and not dissolved 

 by ether. Fig. 115, A and B, show the neurokeratin network after treat- 

 ment with ether, surrounding the axis cylinder, a.c. The meshes vary 

 greatly in diameter, becoming coarse with the rapid post mortem coales- 

 cence of myelin droplets. Fig. 115, C, shows a heavier framework which 

 toward the right of the figure tends to form conical layers, the axis cylinder 

 penetrating their apices; in E a cross section of C is drawn showing a myelin 

 vacuole, my, encircling the fiber. In specimens stained with osmic acid 

 (D), the myelin is very dark and the framework light. The latter is 

 prominent only in oblique lines called in- 

 cisures [or Lantermann's segments]. The 

 lines seen on the opposite sides of the 

 fiber are interpreted as optical sections of 

 a cone of neurokeratin. A cross section 

 of D through an incisure would appear 

 as in I. Successive incisures may point 

 in opposite directions. They do not all 

 represent perfect cones, but in that form 

 they are characteristic post mortem fig- no 1 D 



-r^. T^ /-i FlG - II6 - NODES. 



UrCS. Fig. 115, t, G, and H, Show Other A , Diagram of the intracellular explanation 



A' f i 11 > i n i i of myelin; B, the cross obtained with 



CrOSS Sections Of medullated fibers in Which silver nitrate; C, the biconical enlarge- 



, , . , i- n ment (after Gedoelst) ; D, intercellular 



the neurokeratin is arranged radially or myelin (after Hardesty); a. c., axis 



cylinder; ax., axolemma; my., myelin; 

 in COncentriC layers. ne., neurolemma; no., node. 



At regular intervals the myelin sheath is more or less interrupted by 

 nodes [of Ranvier]. The intervals vary from 80 // to a millimeter, being 

 shorter in growing fibers and in the distal portions of adult fibers. The 

 branching of medullated fibers occurs at these nodes. Fig. 116, A, an 

 osmic acid preparation, illustrates one interpretation of the myelin and 

 nodes, according to which the sheath cells are thought to be wrapped 

 around the axis cylinders, and to contain within them the myelin which 

 develops like fat in the mesenchymal cells. The nodes (A, no) are at 

 the junction of two sheath cells, and there the outer cell membrane or 

 neurolemma is continuous with the axolemma or inner cell membrane, 

 the latter being in contact with the axis cylinder. It accords with this 

 view that the neurolemma usually has but a single nucleus, found midway 

 between two nodes. Surrounded by very little protoplasm it occupies 

 a depression in the outer surface of the myelin. 



