HISTOLOGY 



99 



the fiber, it seems to be nearly or quite interrupted so that the neurilemma 

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

 neuraxon therefore presents a segmented appearance due to these nodes 

 of Ranvier. 



Nerves whose individual fibers possess the myelin sheath appear 

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

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

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

 stituents of "gray matter." 



The sheaths doubtless serve for the protection, insulation and nutrition 

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



Bundles of nerve fibers 



Epineurium. 



Perineurium. 



Endoneurium. 



Fig. 95. — 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. X20. 

 (From Bremer, "Text-book of Histology.") 



A nervous 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 

 tissue, the perineurium (Fig. 95) extensions of which (endoneurium) 

 may penetrate into the bundle. Larger nerves consist of several or many 

 bundles all tied together by connective tissue and enwrapped by a rela- 

 tively thick epineurium. Small blood-vessels traverse the connective- 

 tissue lavers of the nerve. 



