156 



HISTOLOGY 



Fig. 143, C, the neurokeratin framework is so arranged as to correspond with these 

 intervals. In transverse sections, incisures are included in Fig. 143, E and I; the 

 concentric, vacuolated and radial appearances of the myelin are represented in F-H. 



The nodes of Ranvier, shown in the diagram, Fig. 131, are conspicuous in isolated 

 nerve fibers stained with osmic acid. Various interpretations of their structure are 

 represented in Fig. 144. According to the first (Fig. 144, A) the myelin occurs 

 like fat, within distinct cells wrapped around the nerve fibers; the node is the interval 

 between successive cells. The nucleus, which is flattened by the myelin against 

 the outer cell wall, mid-way between the nodes, is not shown. Corresponding with 

 the neurolemma on the outside, there is an "axolemma" next the axis cylinder; 

 neurolemma and axolemma come together at the node. If .the nerve fibers are treated 

 with silver nitrate, a black precipitate is produced at the nodes, as if an intercellular 

 substance were present; the blackening may extend up the axis cylinder producing 

 cross-shaped figures (Fig. 144, B). 



my 



FIG. 143. MEDULLATED NERVE FIBERS. 

 A-D, Longitudinal sections; E-I, cross sections. 

 (A-B, after Gedoelst; C, E, F, after Hardesty; D 

 and I, osmic acid preparations, after Prenant 

 and Scymonowicz; G, alcoholic preservation, 

 after Koelliker H, picric acid preservation, after 

 Schafer.) a. c., Axis cylinder; in., incisure; my., 

 myelin; nu., nucleus of the neurolemma. 



no D 



FIG. 144. NODES. 



A, Diagram of the intracellular explanation 

 of myelin; B, the cross obtained with 

 silver nitrate; C, the biconical enlarge- 

 ment (after Gedoelst); D, intercellular 

 myelin (after Hardesty); a. c., axis cylin- 

 der; ax., axolemma; my., myelin; ne., 

 neurolemma; no., node. 



As the axis cylinder traverses the node, its fibrils may spread apart, forming a 

 "biconical enlargement." The fibrils in the midst of the enlargement have been 

 described as thickened (Fig. 144, C). The same figure shows no axolemma and 

 suggests that the neurolemma passes across the node without interruption. This 

 is clearly shown in D, where the myelin layer also, though constricted, is not com- 

 pletely divided. The myelin has accordingly been regarded as an exoplasmic part 

 of the axis cylinder, and chemically it is said to be related to the interfibrillar substance 

 or neuroplasm. Bardeen (Amer. Journ. Anat., 1903, vol. 2, pp. 231-257) considers 

 that the myelin is derived from the intercellular substance between the fiber and the 

 sheath, and is "due to influences exerted by the axis cylinder fibrils." That the 

 axis cylinder plays the chief part in its production is indicated by the fact that the 

 myelin breaks down when the fiber degenerates, and that it forms around fibers in the 

 central nervous system where there are no continuous sheaths. 



The production of myelin is said to begin at about the fourth month, at the central 

 ends of the nerves. It begins at different times in different tracts and systems, and 



