FUNCTIONS OF NERVE-FIBRES. 475 



from the periphery to the centre are classed together as 

 centripetal or afferent nerves. Those fibres, on the other 

 hand, which are employed to transmit central impulses 

 to the periphery are classed as centrifugal or efferent 

 nerves. 



Centripetal or afferent nerve-fibres may (a) convey to 

 the nerve-centres with which they are connected impres- 

 sions which will give rise to sensation (sensitive nerves), or 

 (6) they may convey an impression which travels out again 

 from the nerve-centre by an efferent nerve-fibre, and pro- 

 duces some effect where the latter is distributed, (see 

 Section on He flex Action), or (c) they may convey an im- 

 pression which will produce a restraining or inhibitory 

 action in the nerve-centre, (inJiibitory nerves, p. 131). 



Centrifugal or efferent nerves may be (a) for the con- 

 veyance of impulses to the voluntary and involuntary 

 muscles, (motor nerves,) or ([>) they may influence nutrition 

 (trophic nerves), (p. 388,) or (c) they may influence secre- 

 tion (sometimes called secretory nerves) (p. 409). 



With this difference in the functions of nerves, there is 

 no apparent difference in the structure of the nerve-fibres 

 by which it might be explained. Among the cerebro- 

 spinal nerves, the fibres of the optic and auditory nerves 

 are finer that those of the nerves of common sensation ; 

 but, with these exceptions, no centripetal fibres can be dis- 

 tinguished in their microscopic or general characters from 

 those of centrifugal nerves. 



Nerve-fibres possess no power of generating force in 

 themselves, or of originating impulses to action : for the 

 manifestation of their peculiar endowments they require 

 to be stimulated. They possess a certain property of con- 

 ducting impressions, a property which has been named 

 excitability ; but this is never manifested till some stimulus 

 is applied. Thus, under ordinary circumstances, nerves of 

 sensation are stimulated by external objects acting upon 

 their extremities ; and nerves of motion by the will, or 



