78 THE FROG 



low, which are not properly parts of the nerve cell but are 

 added to the axis cylinder from without. 



i. Nerve fibers. Cut out a piece of nerve from a freshly- 

 killed frog and with needles tease it apart in normal salt 

 solution. The nerve will be seen to consist of a large num- 

 ber of straight, unbranched nerve fibers, bound together by 

 connective tissue. Examine with the high power. In a fiber 

 make out : 



(a) The axis cylinder, forming an apparently homo- 

 geneous central axis. 



(b) The medullary sheath, or white substance of 

 Schwann, forming a thick, highly refracting coat around 

 the axis cylinder. The medullary sheath is composed largely 

 of a fatty substance called rnyelin, which often becomes ag- 

 gregated into irregular masses. 



(c) The neurilemma, or sheath of Schwann, a very 

 thin membrane outside the white substance. It is best seen 

 at the nodes (described below). 



(d) Nodes of Ranvier, constrictions where the medul- 

 lary sheath is interrupted. The segments of the nerve be- 

 tween the nodes are called internodes. The nodes occur 

 only at considerable intervals. 



(e) The incisures of Schmidt, oblique markings at 

 rather close intervals, across the medullary sheath. These 

 are sometimes difficult to make out. 



(/) In each internode is a nucleus, lying just beneath 

 the neurilemma and surrounded by a small amount of cyto- 

 plasm. 



In the development of a nerve fiber the axis cylinder ap- 

 pears first; as it grows out from the cell body, making its 

 way through the other tissues, it becomes surrounded by 



