78 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



system proper — the brain and cord and the outlying sensory 

 gangha of the cranial nerves and the posterior spinal roots — are 

 meduUated (Fig. 28a). These fibers contain a central core, the axis 

 cylinder, which is usually regarded as an enormously elongated 

 process of the nerve cell with which it is connected. The axis 

 cylinder shows a differentiation into fibrils (neurofibrils) and inter- 

 fibrillar substance (neuroplasm) . All of our evidence goes to show 

 that the axis cylinder is the essential part of the nerve fiber so far 

 as its property of conduction is concerned. Surrounding the 



Kg. 28o. — Diagram of structure of a medullated nerve fiber: A, Nucleus of the internodal seg- 

 ment; B, neurilemma; C, myelin sheath; D, axis cylinder. 



axis cylinder we have the medullary or myelin sheath, varying 

 much in thickness in different fibers. This sheath is composed of 

 peculiar material and is interrupted or divided into segments at cer- 

 tain intervals, the so-called nodes of Ranvier. Outside the myelin 

 there is a delicate elastic sheath comparable to the sarcolemma of 

 the muscle fiber and designated as the neurilemma. Lying under 

 the neurilemma are found nuclei, one for each internodal segment 

 of the myelin, surrounded by a small amount of granular proto- 

 plasm. The non-medullated fibers have no myelin sheath. They 

 are to be considered as an axis cylinder process from a nerve cell, 

 surrounded by or inclosed in a neurilemmal sheath. These fibers 

 arise histologically from the nerve cells found in the outlying 

 ganglia of the body, the ganglia of the sympathetic system and 

 its appendages. 



The Function of the Myelin Sheath. — The myelin sheath of 

 the cerebrospinal nerve fibers is a structure that is interesting and 

 peculiar, both as regards its origin and its composition. Much, 

 speculation has been indulged in with regard to its function, but 

 practically nothing that is certain can be said upon this point. It 

 has been supposed by some to act as a sort of insulator, preventing 

 contact between neighboring axis cylinders and thus insuring 

 better conduction. But against this view, it may be urged that 

 we have no proof that the non-medullated fibers do not conduct 

 equally as well. The view has some probability to it, however, 

 for we must remember that the non-medullated fibers do not run 

 in large nerve trunks that supply a number of different organs, 

 and therefore in them a provision for isolated conduction is not so 



