THE NEURON AND ITS CONDUCTING PATHS 113 



and may or may not be enveloped by neurolemma. A typical nerve-fiber consists 

 of the following parts : 



1. The axis-cylinder forms the central core of the fiber and about one-half of 

 its total thickness. It appears as a dim or faintly granular thread which, under 

 the influence of certain reagents, may become fibrillated. This peculiarity, as 

 will be shown later on, is one of the important contentions of the fibrillar theory 

 of nerve action. Each axis-cylinder pursues an unbroken course to the end-organ 

 where it divides into a number of fibrUlse which may at times be closely interwoven 

 with one another. 



2. The medullary substance forms a close-fitting jacket around the axis-cylinder 

 and consists of a network of neurokeratine, the meshes of which contain a fatty 

 material. Under normal conditions it appears as a continuous layer of homogeneous 

 substance which, after fixation or even while still in the body, splits up into seg- 

 ments possessing a length of about 1 mm. The indentations between these differ- 



FIG. 65. TRANSVERSE SECTION OF A NERVE (MEDIAN). 

 ep, epineurium; pe, perineurium ; ed, endoneurium. (Landois and Stirling.) 



ent segments are known as the nodes of Ranvier. They do not implicate the axis- 

 cylinder. About midway between two neighboring indentations lies the nucleus, 

 its long axis being directed parallel to that of the fiber. Immediately surrounding 

 it is a layer of undiff erentiated protoplasm which thus appears as small islands 

 directly underneath the neurolemma. 



3. The neurolemma is a transparent sheath of homogeneous material which 

 retains a uniform thickness throughout, with the exception of the aforesaid nodes 

 where it is augmented by cement substance and lies in direct contact with the 

 axis-cylinder. Staining reagents are prone to enter these indentations and to 

 progress from here along the axis-cylinder. As far as the relative amounts of these 

 substances are concerned, it might be mentioned that the median nerve contains 

 63 per cent, of connective tissue, 28 per cent, of myelin and 9 per cent, of axis-cylin- 

 der (Ellison). 



4. The end-organs to which the axis-cylinders are distributed, vary greatly in 

 their structure as well as in their chemical composition. They are divided first of 

 all into receptors and effectors. Among the former might be mentioned the retina 

 of the eye, the organ of Corti of the internal ear, the olfactory cells, the taste buds, 

 the cutaneous corpuscles for pressure, pain and temperature and the sensory 

 spindles of striated muscle tissue. Probably the best known motor end-organ is 

 the so-called end-plate of striated muscle. It appears as a low, conical or rounded 



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