NEUROLOGY 277 



is pierced by a central canal which communicates with the fourth 

 ventricle at the calamus scriptorius. 



STRUCTURE OF THE NERVE TRUNKS AND GANGLIA 



The protoplasmic processes called dendrites have a similar struc- 

 ture to the cell body. The dendrites branch dichotomously, become 

 rapidly smaller, and usually end at no great distance from the cell 

 body. 



The axone or axis cylinder process, differs from the cell body and 

 from the dendrites. It does not contain chromophilic granules. 

 It consists entirely of neurofibrils and perifibrillar substance. It 

 emerges from the cell at an enlargement known as the axone hill. 

 This hill is free from chromophilic bodies. 



Nerves are divided into two kinds, medullated and non-medullated. 



Medullated nerves are divided into two kinds: medullated nerves 

 with a neurolemma, and medullated nerves without a neurolemma. 



Medullated nerves with a neurolemma consists of an axone, a 

 medullary sheath, and a neurolemma. A delicate membrane called 

 an axolemma, or periaxial sheath envelops the axone. The medullary 

 sheath called myelin, is semifluid, somewhat resembling fat. The 

 outer covering is the neurolemma, or sheath of Schwann. It is a 

 delicate, structureless membrane which incloses the myelin. There 

 is under this sheath an occasional oval nucleus. At intervals there 

 are constrictions, or nodes, called the constrictions of Ranvier. The 

 part between the nodes is called the internode. 



The medullated axones without a neurolemma are the medullated 

 nerve fibers of the central nervous system. 



The non-medullated axones, or non-medullated nerve fibers, are 

 divided into non-medullated axones with and those without, a neuro- 

 lemma. 



The non-medullated axone without a neurolemma is merely a naked 

 axone. They are confined to the gray matter and to the beginnings 

 and endings of sheath axones, all the latter being uncovered for a 

 short distance after leaving the nerve cell body and also just before 

 reaching their terminations. 



The long axones serve to make connections with the peripheral, 

 or distant, nerve cell, muscle cell, or gland cell; while the shorter 

 axones of certain neurones divide into terminal branches in the 

 immediate vicinity of its cell body, presumably to come into relation 

 with other nerve cells in the same or adjacent groups. 



