9 8 



HISTOLOGY. 



FlG. 114. NON-MEDULLATED NERVE FIBERS. 



(After Schafer.) 



X 400. 



corresponding groups. They are considered to be the essential conducting 

 element of nerves, but it is known that conduction occurs in protoplasm 

 in which fibrils cannot be demonstrated. 



As the fibers in the embryo grow out from the pentral nervous system 

 they form bundles, in and around which there are numerous nuclei. 

 Opinions differ as to whether these nuclei belong with the mesenchymal 



cells through the meshes of 

 which the nerve is growing, 

 or with ectodermal cells car- 

 ried along from the spinal 

 ganglia or cord. In either 

 case they are called sheath cells, and are "so closely applied to the fibers 

 that it becomes a matter of judgment to decide whether the fibrils are sur- 

 rounded by or imbedded in the sheath cells." Therefore some writers 

 have thought that the nerve fiber was not the outgrowth of a single cell, 

 but was produced by the end to end anastomosis of many sheath cells, 

 each of which formed that portion of the nerve fiber which it enclosed. 

 Since the fiber may be a meter long 

 and perhaps ten thousand times the 

 diameter of the cell body from which 

 it comes, such an assumption seems 

 plausible; nevertheless it is not sus- 

 tained by recent embryological in- 

 vestigations. 



The cells applied to the nerve 

 fiber may unite and thus surround it 

 with a delicate homogeneous sheath 

 called the neurolemma [sheath of 

 Schwann. Some fibers in the adult, 

 especially in the sympathetic system, 



pOSSeSS Only a sheath Of this SOrt, FIG. 115. MEDULLATED NERVE FIBERS. 



and they are called non-medullated 

 fibers (Fig. 114). Other fibers in the 

 sympathetic system and near the 

 nerve terminations may be sur- 

 rounded only by ordinary connective tissue; these are non-medullated fibers 

 without a neurolemma [naked axis cylinders]. (Non-medullated fibers of 

 the sympathetic system are often called Remak's fibers.) The fibers of the 

 spinal nerves are generally characterized by a deposit of myelin, found 

 between them and the neurolemma. The fibers with a myelin sheath are 

 called medullated, and the fibers themselves within the myelin sheath, 

 whether they are dendrites or neuraxons, are called axis cylinders. 



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

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

 D and I, osrnic 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. 



