CHAPTER XXI* 



THE NERVOUS SYSTEM 



Nerve Tissue exists in the form of cells and fibers, 

 the latter being prolongations of the bodies of the former. 

 Where the cells predominate, nerve tissue is grayish. 

 Such accumulations are called ganglia, or nerve centers, 

 and these alone originate nervous force ; the fibers 

 are generally white, and arranged in bundles, called 



FIG. 329. Nerve cell 

 from cerebral cortex : 

 , nerve process. 



FIG. 330. Diagram of Nervous Sys- 

 tem of Starfish: r, nervous ring 

 around mouth; n, radial nerves to 

 each arm, ending in the eye. 



nerves, which serve only as conductors. Most nerves 

 contain two kinds of fibers, like in structure, but 

 each having its distinct office : one carries impres- 

 sions received from the external world to the gray 

 centers, and hence is called an afferent, or sensory, 

 nerve ; the other conducts an influence generated in 

 the center to the muscles, in obedience to which they 



* See Appendix. 

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