ICp HISTOLOGY 



cell forms the myelin in its interior, thus leaving a thin layer of cytoplasm 

 between the myelin and the nerve process or neurite, is yet to be decided. 

 The first view is favored by the apparent absence of any structure (cyto- 

 plasm) lying between the myelin and the neurite. The second possibility 

 is advocated by some because of the presence of the nodes which separate 

 the myelin of the two consecutive sheath cells as if it were an internal 

 product. The usual presence of but one sheath-nucleus between any two 

 neighboring nodes heightens this latter probability. 



The myelin exists as a fine emulsion and is not homogeneous, but lies 

 in the interstices of a fine network of a substance that resembles keratin 

 or horn. This material gives off an odor like that of horn or feathers 

 when it is freed from the myelin and burnt. 



As has been intimated, the myelin sheath is not continuous. At 

 various distances, long when compared with the fiber's diameter, it is 

 constricted and separated by the substances of the sheath, thus forming 

 the nodes. There is discussion as to whether these nodes or, indeed, the 



FIG. 167. Portion of a medullated nerve fiber from a mammal. Node shown near middle. 

 Incisure shown near left end. X 1000. 



sheath itself is to be found in those medullated fibers that pass through 

 the brain and spinal cord of the mammals. They are probably provided 

 with a sheath, as this appears in their development. Its apparent ab- 

 sence in the adult form is probably due to its extreme delicacy, there being 

 no need of a strong and substantial sheath on the fiber in this position on 

 account of the strong and heavy surroundings. 



Where many fibers run in the same course they are usually found 

 together, forming a nerve. This collection of fibers is held together by 

 a connective-tissue covering, the endoneurium, and several of the smaller 

 bundles are often found to be surrounded by a still thicker and stronger 

 covering of the same kind of material. Such a composite bundle is also 

 called a nerve, and its covering a perineurium. 



Technic. Fibers of all kinds may be fixed and teased and examined 

 individually for both general features and to see the fibrillar nature of the 

 nerve process. A fiber cannot be traced for any distance in this way, 

 however, but must be stained by the Golgi method or the methylene-blue 

 method. As the first of these stains can be made to have a selective 

 action and to pick out only one or a few fibers from the great mass that 

 exist in most of the fiber paths or nerves, it is the most useful method 



