GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 191 



nerve going to it be excited, the beat of the heart will be 

 stopped; it will cease to work and lie in a relaxed resting 

 condition : in this we have an instance of an inhibitory nerve 

 checking the activity of a peripheral organ. 



Classification of Nerve-Fibres. Nearly all the nerve- 

 fibres of the Body fall into one of two great groups corre- 

 sponding to those which carry impulses to the centres and 

 those which carry them out from the centres. The former 

 are called afferent or centripetal fibres, and the latter efferent 

 or centrifugal. Since the impulses reaching the centres 

 through the afferent fibres frequently cause sensations 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 inhibitory 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 connected 

 with their brain-centre, which sensation may or may not give 

 rise to a voluntary movement. 2. Reflex fibres, the excitation 

 of which may be attended with consciousness but gives rise 

 to involuntary efferent impulses. Thus for example light 

 falling on the eye causes not only a sensation, but also a nar- 

 rowing of the pupil, which is entirely 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 according as was willed. 3. Excito motor 

 fibres. The excitation of these when reaching a nerve-centre 

 causes the stimulation of efferent fibres, but without the par- 

 ticipation of consciousness. During fasting, for instance, bile 

 accumulates in the gall-bladder and remains there until some 

 semi-digested food passes from the stomach into the intestine. 

 This is acid and stimulates nerves in the mucous membrane 

 lining the intestine, and these convey an impulse to a centre, 

 which in consequence sends out impulses to the muscular coat 

 of the gall-bladder causing it to contract and expel its con- 

 tents into the intestine: but of all this we are errtirely un- 

 conscious. 4. Centro-inliibitory fibres. Whether these exist 

 as a distinct class is at present doubtful. It may be that 



