FUNCTIONS OF NERVE-FIBRES. 477 



from the periphery to the centre are classed together as 

 centripetal or afferent nerves, or, when speaking exclusively 

 of cerebro- spinal nerves, nerves of sensation, or sensitive 

 nerves. Those fibres, on the other hand, which are em- 

 ployed to transmit central impulses to the muscles, are 

 classed as centrifugal, efferent, or motor nerves, or nerves of 

 motion. 



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 than 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 motor nerves. Neither can the difference in 

 function be due to the kind of tissue to which a nerve is 

 distributed; for although the nerves supplying muscles 

 are principally motor, yet the muscular tissue contains 

 sensitive fibres also, for pain is felt when it is injured, and, 

 as will be hereafter shown, much of the exactness and 

 precision of muscular action is determined by the power 

 which the muscular tissue has of communicating to the 

 mind the sensation of its own contraction, and of the 

 effects produced by it. 



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. Under ordinary circumstances, nerves of 

 sensation are stimulated by external objects acting upon 

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

 by some force generated in the nervous centres. But 

 almost all things that can disturb the nerves from their 

 passive state act as stimuli, and agents the most dissimilar 



