A STUDY OF THE NERVOUS SYSTEM 



259 



becomes surrounded by a thick covering, called the med'ul-la-ry 

 sheath. Nerve fibers outside the spinal cord are covered by 

 a very thin outer membrane, known as the prim'i-tive sheath. 

 (A common lead pencil 

 might be used to present 

 to the eye the structure 

 of such a nerve fiber. 

 The "lead" or graphite 

 in the center of the pen- 

 cil corresponds in posi- 

 tion and form to the axis 

 cylinder of the nerve 

 fiber ; the wood surround- 

 ing the lead answers to 

 the medullary sheath ; 

 and the thin layer of 

 paint or varnish on the 

 outside of the pencil rep- 

 resents the primitive 

 sheath of the nerve.) 



In all parts of the gray 

 matter we find nerve 

 cells. While they vary greatly in form and size, almost all 

 have a cell body with protoplasmic processes and a single axis 

 cylinder process. These axis cylinders, usually after a more 

 or less tortuous course, divide into very fine branches and 



FIG. 118. Branched Nerve Cell from 

 Spinal Cord. 



a = long axis cylinder. 

 ' 6 = branching protoplasmic processes, 

 c = supporting tissue. 



FIG. 119. Medullated Nerve Fiber. 



In the center is the axis cylinder. The black lines on either side repre- 

 sent the medullary sheath. On the outside represented in white is the 

 thin primitive sheath in which is a nucleus. 



thus form a so-called terminal brush. We shall find that 

 these cells and fibers perform the important function of 

 connecting or coordinating the various parts- of the cord. 



