THE STRUCTURE OF THE NERVE SYSTEM 335 



an intervening semifluid substance, the medulla or 

 myelin; and an internal dark thread, the axis-cylinder 

 or axone. 



The neurilemma is a delicate, transparent membrane 

 investing the myelin and axone, and occurs wherever 

 the meylin sheath is absent. It possesses a nucleus, 

 which may be seen between the nodes of Ranvier along 

 the course of the nerve. It acts as a protective mem- 

 brane to the nerve-fiber.* 



The medulla, myelin, or white substance of Schwann 

 lies between the neurilemma and invests the axone. 

 This is the substance of the nerve fiber, the presence 

 of which imparts to the nerve tissue its white appear- 

 ance, and gives rise to the term white fibers in speaking 

 of nerve tissue, to differentiate them from gray fibers, 

 the latter having no myelin sheath. 



Along the course of nerve fibers may be seen a 

 diminution or shrinkage/ in its caliber, due to an 

 absence of the myelin Aeath, permitting the neuri- 

 lemma to be in direct Imposition to the axis-cylinder. 

 ^TKese narrowed parts are called the nodes of Ranvier, 

 named after their discoverer. The portion of the 

 nerve fiber between these interruptions is termed the 

 internodal segment. Axones give off their collateral 

 branches at the nodes of Ranvier. 



Non-medullated or Amyelinic Nerve Fibers. These are 

 devoid of a myelin salath, or the white substance of 

 Schwann, thus printing a gray appearance, and 

 giving origin to Kfye term gray fibers. Some non- 

 medullated nerve fibers possess only an axis-cylinder, 

 or axone. These are found in the central ganglia. 

 Others possess a neurilemma investing the single 

 axis-cylinder, and are the main variety of nerve fiber 

 constituting the sympathetic system. Non-medullated 

 nerve fibers are not so abundant throughout the 

 nerve system as the medullated variety. 



Nerves. Nerves are round, flattened bundles of 

 axones, held firmly together by investing connective 



