Histology 



319 



vitally dependent, throughout its entire length, on substances derived 

 from the cell-body. Any part of it whose connection with the cell-body 

 is severed promptly degenerates. 



The neuraxon may be surrounded by one or two special ensheathing 

 layers. The medullary or myelin sheath is a relatively thick layer of 

 fatlike substance, myelin, fitting the neuraxon closely. The neuri- 

 lemma or sheath of Schwann is an exceedingly thin cellular layer 

 wrapped around the neuraxon (Figs. 132, 258). 



A neuraxon may possess either, both, or neither of these two 

 sheaths. When both are present, the myelin sheath is always next to 

 the nerve-fiber and, at fairly regular intervals (in man averaging about 

 0.5 mm.) along the fiber, it seems to be nearly or quite interrupted so 

 that the neurilemma there comes into close relation with the nerve- 

 fiber (Fig. 258B). The neuraxon therefore presents a segmented appear- 

 ance due to these nodes of Ranvier. 



Nerves whose individual fibers possess the myelin sheath appear 

 more nearly white than do nonmedullated nerves. The so-called 

 "white" parts of the brain and spinal cord consist mainly of medul- 

 lated nerves. Nonmedullated fibers and the cell-bodies of neurons are 

 the chief constituents of "gray matter." Medullated nerves inside the 

 brain and spinal cord usually have no neurilemma. Most autonomic 

 nerves and the olfactory nerves are nonmedullated. The terminal twigs 

 of a neuraxon are always bare. 



The sheaths doubtless serve for the protection, insulation, and 

 nutrition of the nerve-fiber. The source of the myelin is not definitely 

 known. 



A nervous organ is constituted of neurons supported by connective 



Epineurium. 



Artery. 



Bundles of nerve fibers 



loneunum. 



Fig. 262. Structure of a nerve. The figure represents a small part of a transverse 

 section of a large nerve constituted of many bundles of medullated fibers. (X 20.") 

 (Courtesy, Bremer: "Text-Book of Histology,*' Philadelphia, The Blakiston 

 Company.) 



