CHAPTEE VII 



NERVE 



NERVOUS tissue is the material of which the nervous system is com- 

 posed. The nervous system is composed of two parts, the central 

 nervous system, and theperipheral nervous system. The central nervous 

 system consists of the brain and spinal cord ; the peripheral nervous 

 system consists of the nerves, which conduct the impulses to and from 



FIG. 116. Two nerve-fibres of sciatic 

 nerve. A. Node of Ranvier. 

 B. Axis-cylinder, c. Sheath of 

 Schwann, with nuclei. Medul- 

 lary sheath is not stained, x 300. 

 (Klein and Noble Smith.) 



Fw. 117. Axis cylinder, 

 highly magnified, 

 showing its com- 

 ponent fibrils. (M. 

 Schultze.) 



the central nervous system, and thus bring the nerve centres into 

 relationship with other parts of the body. 



Some of the nerves conduct impulses from the nerve-centres and 

 are called efferent ; those which conduct impulses in the opposite 

 direction are called afferent. When one wishes to move the hand, the 

 nervous impulse starts in the brain and passes down the efferent or 

 motor nerve-tracts to the muscles of the hand, which contract; when 



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