CM. VII.] 



NERVE. 



97 



peripheral nervous system consists of the nerves, which conduct 

 the impulses to and from 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 



Fig. 116. Two nerve-fibres of 

 sciatic nerve. A. Node of 

 llanvier. u. Axis-cylinder, 

 c. Sheath of Schwann, with 

 nuclei. Medullary sheath is 

 not stained, x 300. (Klein 

 and Noble Smith.) 



Fig. 117. Axis 

 cylinder, high- 

 ly magnified, 

 showing its 

 component 

 fibrils. 

 (M. Schultze.) 



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 one feels pain in the hand, 

 afferent or sensory nerve-tracts convey an impulse to the brain 

 which is there interpreted as a sensation. If all the nerves 

 going to the hand are cut through, all communication with 

 the nerve-centres is destroyed, and the hand loses the power of 

 moving under the influence of the will, and the brain receives 

 no impulses from the hand, or as we say the hand has lost 

 sensibility. 



This distinction between efferent and afferent nerves is a 

 physiological one, which we shall work out more thoroughly later 

 on. No histological distinction can be made out between motor 

 and sensory nerves, and it is histological structure which we 



K.P. H 



