HISTOLOGY 149 



The neuraxon is a delicate tliread consisting of a probably modified 

 protoplasm in which, as just mentioned, neurofibrils may be demonstrated. 

 It may be surrounded by one or even two special ensheathing layers. 

 The medullary or myelin sheath is a relatively thick layer of fat-like 

 substance, myeHn, fitting the neuraxon closely. The neurolemma or 

 sheath of Schwann is a cellular layer wrapped around the neuraxon. Its 

 cells are so exceedingly thin that the layer could hardly be detected were it 

 not for the nuclei which at intervals occasion roundish bulges in the con- 

 tour of the nerve fiber. 



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

 When both are present the myelin sheath is always next 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 neurolemma 

 there comes into close relation with the nerve fiber (Fig. loSB). The 

 neuraxon therefore presents a segmented appearance due to these nodes 

 of Ranvier. Usually only one neurolemma nucleus can be found between 

 successive nodes. A single internodal segment therefore probably repre- 

 sents the territory covered by one sheath cell. 



Nerves whose individual fibers possess the myehn sheath appear more 

 nearly white than do non-medullated nerves. The so-called "white" 

 parts of the brain and spinal cord consist mainly of meduUated nerves. 

 Non-medullated fibers and the cell-bodies of neurons are the chief con- 

 stituents of "gray matter." 



On meduUated nerves within the brain and spinal cord no neurolemma 

 can ordinarily be found, but meduUated fibers in nerves external to the 

 brain and cord commonly have both sheaths except in the vicinity of the 

 terminal arborization of a neuraxon. The myelin sheath stops short of 

 the arborization. The neurolemma may continue alone somewhat further, 

 but the terminal twigs of the neuraxon are always bare. 



Most autonomic fibers and the fibers of the olfactory nerve are non- 

 medullated. A cellular neurolemma may occur on a non-medullated fiber. 



The sheaths doubtless serve for the protection, insulation and nutrition 

 of the nerve fiber. The source of the myehn is not definitely known. 



A n-ervous organ is constituted of neurons supported by connective 

 tissues accompanied by vascular tissues. In the brain and spinal cord 

 of vertebrates occurs not only the usual mesenchymal connective tissue 

 but another which is unique in that its cells have ectodermal origin in 

 common with the nerve cells. Some of the cells of this neuroglia possess 

 branched processes which make them confusingly similar in appearance to 

 nerve cells. The neuroglia cells form, by means of their processes, a 

 supporting network for the nerve cells. 



A nerve is a bundle of neuraxons, each of which may be ensheathed as 

 described above, and all wrapped together within a sheet of connective 



