396 THE NERVES AND THE SENSE ORGANS 



ticular work. When the demand is over, the organ re- 

 ceives notice, and stops its work. This regulation of the 

 body by communication between its parts is the work of 

 the nervous system. 



The nervous system is studied in two divisions: 



(1) The central nervous system, consisting of the brain 

 and spinal cord, and the nerves that start from these 

 structures; 



(2) The sympathetic nervous system, which puts the 

 internal organs into communication with one another and 

 with the central system. 



399. Nerve Cells and Their Structure. The parts of 

 the nervous system are made up of nerve tissue, just as 

 muscles are made up of muscle tissue. The unit of nerve 

 tissue is the nerve cell, or neuron (Fig. 293). This con- 



Nucleus \ . 



Axon 



Fig. 293. 

 A Neuron, or Nerve Cell. 



sists of two parts, the cell body, or cell proper, and ex- 

 tensions of the cell body. The cell body looks much like 

 other cells, except that it has a grayish color. The ex- 

 tensions of the cell body are of two sorts. One of them 

 is the long, slender nerve fiber, or axon. Some cells have 

 one of these; others have two. Cells that have one axon 

 have also shorter extensions, or dendrites. Dendrites 



