76 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



system proper the brain and cord and the outlying sensory 

 ganglia of the cranial nerves and the posterior spinal roots are 

 medullated. 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 interfibrillar sub- 

 stance (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. It is further assumed that 

 the neurofibrils in the axis cylinder form the conducting mech- 

 anism rather than the interfibrillar substance. Surrounding the 

 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 

 necessary. Moreover, in the medullated fibers the myelin sheath 

 is lost toward its peripheral end after the nerve has entered the 

 tissue to which it is to be distributed, indicating that its function 

 is then no longer necessary. According to the older conceptions 

 of the process of conduction in nerve fibers, not only anatomical 

 but also physiological continuity is necessary. Mere contact of 

 living axis cylinders would not enable the nerve impulse to pass 



