HISTOLOGY. 73 



GANGLIA. 



These bodies are nerve centers, consisting of nerve 

 cells and nerve fibers. Some of the fibers end in' the 

 ganglion, while others pass on to more distant points. 

 The brain and spinal cord may be termed a group of 

 large ganglia. The whole ganglion is covered with a 

 fibrous tissue, called a sheath. 



NERVE 



The nerve fiber is composed of the axis cylinder of a 

 nerve cell, and three protecting membranes: the axi- 

 lemma, the medullary sheath and the neurilemma. 



In the medullary nerves, at regular intervals, there 

 appear constrictions, called the nodes of Ranvier. 

 These nodes are the result of an absence of the medul- 

 lary sheath at these points. The space between any 

 two nodes is called internode. In the center of an in- 

 ternode is found the nerve corpuscle, located directly 

 under the neurilemma. The non-medullated nerve 

 fibers are found in the sympathetic system. Each fiber 

 consists of an axis cylinder, neurilemma and an oval 

 nucleus upon the surface of the fiber. There are two 

 theories concerning the formation of the axis cylinder : 

 one is that the nerve cells fuse themselves together, 

 end to end, ancj form a long thread; the other is that 



