ANATOMY, PHYSIOLOGY, OF NERVOUS SYSTEM. 435 



Two opposing views are prevalent as to the nature of 

 the axis-cylinder. According to one view this cylinder 

 consists of a bundle of still finer fibrillae, too small to be 

 resolved by the microscope, and along these fine fibrillae the 

 nervous impulses are supposed to pass. The presence of 

 many such fibrillae in a single axis-cylinder would explain 

 how a single nerve fiber might branch near its termination, 

 as many do. By others the axis-cylinder is looked upon as 

 a tube of nervous plasm, through which the nervous impulse 

 finds its way, and the branching of the nerve fibers is ex- 

 plained in the same way as the terminal branchings of a 

 water-main. 



The medullary sheath forms soon after the appearance 

 of the axis-cylinder and arises from certain cells which sur- 

 round the axis-cylinder much like a string of spools would 

 surround a wire passing through them. The substance of 

 these enclosing cells after they have elongated is changed 

 into myelin, retaining, however, its nucleus. The divisions 

 between contiguous cells is still indicated by the nodes of 

 Ranvier. 



While the matter of origin of the medullary sheath is, 

 therefore, fairly clear we are almost entirely at sea for its 

 physiological explanation. That it serves as a kind of insu- 

 lation for the nervous impulse somewhat like the silk around 

 a copper wire seems hardly true. There is nothing to 

 warrant such a belief. That it may serve as a protection 

 against changes of temperature, and possibly even in a 

 mechanical way, may be true in some cases but would hardly 

 explain the existence of a medullary sheath in the brain or 

 spinal cord itself where fluctuations of temperature certainly 

 do not occur. The statement that nerves do not become 

 functional until the medullary coat is developed does not 

 necessarily mean that the function depends upon this coat. 

 It may simply mean that the axis-cylinder becomes func- 

 tional about the same period that the medullary sheath 



