AN ENQUIRY INTO THE TOKMATION OF HABIT IN MAN. 139 



matter. At the same time, tlierefore, that an impulse floAvs 

 across the spinal cord, as a simple direct reflex action, a certain 

 part of this impulse is also diverted to the brain along fibres 

 which ascend in the outer part of the spinal cord ; and from 

 the brain descending fibres carry the impulse back again to 

 the lower arc. Accurate measurements of the time taken by 

 impulses in travelling through the grey matter have done 

 much to throw fight upon the route they follow ; but we do 

 not yet know whether we ought to speak of the conversion 

 of a sensory into a motor impulse, as its passage through the 

 lower network under the direction of nerve currents which 

 originate in the higher ; or whether the impulse when it 

 reaches the lower grey matter takes in some cases a direct 

 cross path, while in others it makes its transit through a longer 

 loop. One thing is quite certain, namely, that the routes 

 which are most frequently used are the most open, and there- 

 fore the most easily traversed." 



The functions of the nerve-cells are various and must be con- 

 sidered in detail ; the molecules, or particles, of which a nerve- 

 cell is built up, are in such an unstable condition that any 

 stimulus readily excites them to change; this molecular change 

 is believed to constitute a nerve-cell action; it may be of very 

 various degrees of violence ; it may exhaust the nerve-cell in 

 proportion to its violence (and when exhausted the cell cannot 

 act again until restored by nutrition from the blood) ; it may 

 affect the substance of the cell, and especially of young grow- 

 ing cells, so as to leave an impression on the cell, permanent 

 in proportion to the violence of the action and the number of 

 its repetitions. When a nerve-cell acts (whatever this may 

 mean), impulses tend to pass off from it along its various con- 

 necting fibres; the force and number of these impulses depends 

 on the violence of the cell action ; if this is gentle there may be 

 only aslight impulse passing off through the largest connecting 

 fibre (the freest channel) ; if the action is violent it may over- 

 flow through the various connecting fibres in impulses increas- 

 ing in force and number with the violence of the cell action. 



If the foot of a sleeping (or deeply thinking) person is 

 tickled it is qiuetly withdrawn ; that is to say, the gentle skin 

 irritation sends a gentle impulse to the sensory cells, which are 

 gently excited, and send gentle impulses to a few motor cells ; 

 but if the foot be suddenly burnt, the sensory cell action, 

 excited by the powerful impulse from the severely irritated 

 skin, will be so violent that it will overflow through many 

 more connecting fibres, andalmoet every muscle in the body 



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