EVOLUTION OF THE NEUROMUSCULAR MECHANISM 



831 



sequently acquires a certain individuality, and can become specialized for 

 special kinds of activity, and is called a neuron. The neuron is the fund- 

 amental unit of structure and function in the central nervous system. It 

 consists typically of a nerve cell body containing the nucleus, from which 

 extend numerous short fibers, or dendrites, and one long fiber, or axon. 

 The axon may be branched, such branches being called collaterals. 



Fig. 204. Normal cell from the anterior horn, stained to show Nissl's granules. a, the axon. 



(From Howell.) 



The arrangement of neurons in the central nervous system of the 

 worms is characteristic. Most of the nerve cell bodies are collected to- 

 gether in centrally located masses or ganglia. Fibers pass from the 

 receptors in the epithelium to the ganglion of the same segment in defi- 

 nite nerve trunks, while other fibers originating from nerve cell bodies 

 within the ganglion pass in other trunks to the muscles. In addition 



