10 THE PRESIDENTIAL ADDRESS. 



How should inertia and momentum affect non-material reactions? 

 Quick though nervous reactions are, there is always easily observed delay 

 between delivery of stimulus and appearance of the nervous end effect; 

 and there is always the character that a reaction once set in motion 

 does not cease very promptly. Just the same order of lag and overran, 

 of want of dead-beat character, is met in sense-reactions. The sensa- 

 tion outlives the light which evoked it and for longer the stronger 

 the reaction. Just so the reflex after-discharge persists after the 

 stimulus is withdrawn, and subsides more slowly the stronger the 

 reaction. The times in both are oi the same order. Again, a reflex 

 act which contracts one muscle commonly relaxes another. Even so 

 along with rise of sensation in one part of the visual field commonly 

 occurs lapse of sensation in another. And the stoppage is in both by 

 inhibition, that is to say, active. Then, again, two lights of opposite 

 colour falling simultaneously and correspondingly on the two retinae 

 will, according to their balance, fuse to an intermediate tint or see-saw 

 back and forth between the one tint and the other. Just similarly a 

 muscle impelled by two reflexes, one tending to contract it, the other 

 to relax it, will according to the balance of these respond steadily with 

 an intensity, a compromise between the two, or see-saw rhythmically 

 from extreme to extreme of the two opposite influences. 



Reflex acts commonly predispose to their opposites. So, similarly, 

 the visual impression of one colour predisposes to that of its opposite. 

 Again, the 'position of the stimulated sensual point acts on the mind — 

 hence the light seen or the pain felt is referred to some locus in the 

 mind's space-system. Just similarly the reflex machinery directs, for 

 instance, the limb it moves towards the particular spot stimulated. And 

 such spots in the two processes, mental and non-mental, correspond. 



Characteristic of the nervous machinery is its arrangement in what 

 Hughlings Jackson called ' levels,' the higher levels standing to the 

 lower not only as drivers but also as restrainers. Hence in disease 

 underaction of one sort is accompanied by overaction of another. Thus 

 in the arm affected by a cerebral stroke, besides loss of willed — that 

 is higher level — power in the finger muscles, there is in other muscles 

 involuntary overaction owing to escape of lower centres from control 

 by the higher wliich have been destroyed. So, similarly, with the sensory 

 effects. Of skin sensations some are painful and some not, for instance 

 touch. The seat of the latter is of higher level, cortical; of the 

 former lower, sub-cortical. When cerebral disease breaks the path 

 between the higher and the underlying level a result is impairment of 

 touch sensation but heightening of pain sensation in the affected part. 

 The sensation oi touch, as Dr. Head says, restrains that of pain. 



Thus features of nervous working resemble over and over again 

 mental. Is it mere metaphor when we speak of mental attitudes as 



