CLASSIFICATION OF NERVE-FIBRES, 185 



sations they are often called sensory fibres', and as many of 

 those which carry out impulses from the centres excite 

 movements, they are frequently called motor fibres; but 

 these last names are bad, since even excluding inhibi- 

 tory nerves, many afferent fibres are not sensory and many 

 efferent are not motor. 



We may distinguish as subdivisions of afferent fibres the 

 following groups. 1. Sensory fibres proper, the excitement 

 of which is followed by a sensation when they are con- 

 nected with their brain-centre, which sensation may or 

 may not give rise to a voluntary movement. 2. Refiex 

 fibres, the excitation of which may be attended with con- 

 sciousness but gives rise to involuntary efferent impulses. 

 Thus for example light falling on the eye causes not only 

 a sensation but also a narrowing of the pupil, which is en- 

 tirely independent of the control of the "Will. No absolute 

 line can, however, be drawn between these fibres and those 

 of the last group: any sudden excitation, as an unexpected 

 noise, will cause an involuntary movement, while the same 

 sound if expected would cause a movement or not accord- 

 ing as was willed. 3. Excito-motor fibres. The excitation 

 of these when reaching a nerve-centre causes the stimula- 

 tion of efferent fibres, but without the participation of 

 consciousness. During fasting for instance bile accumu- 

 lates in the gall-bladder and there remains until some 

 semi-digested food passes from the stomach into the intes- 

 tine. This is acid, and stimulates nerves in the mucous 

 membrane lining the intestine, and these convey an im- 

 pulse to a centre, which in consequence sends out impulses 

 to the muscular coat of the gall-bladder causing it to con- 

 tract and expel its contents into the intestine: but of all 

 this we are entirely unconscious. 4. Centro-inhibitory 

 fibres. Whether these exist as a distinct class is at present 

 doubtful. It may be that they are only ordinary sensory 

 or reflex fibres and that the inhibition is due only to the in- 

 terference of two impulses reaching a central organ at the 

 same time and impeding or hindering the full production 

 of the normal result of either. 



In efferent nerve-fibres physiologists also distinguish 



