September 29, 1922] 



SCIENCE 



351 



I do not want, and do not need, to stress oiir 

 inability at present to deal with mental actions 

 in terms of nervous actions, or vice versa. 

 Facing the relation borne in upon us as ex- 

 istent between them, however, may we not gain 

 some further appreciation of it by reminding 

 ourselves even briefly of certain points of con- 

 tact between the two ? Familiar as such points 

 are, I will mention rather than dwell upon 

 them. 



One is the so-called expression of the emo- 

 tions. The mental reaction of an emotion is 

 accompanied by a nervous discharge which is 

 more or less characteristic for each several type 

 of emotion, so that the emotion can be read 

 from its bodily expression. This nervous dis- 

 charge is involuntary, and can affect organs, 

 such as the heart, which the will can not reach. 

 Then there is the circumstance that the peculiar 

 ways and triets of the nervous machinery as 

 revealed to us in the study of mere reflex reac- 

 tions repeat themselves obviously in the work- 

 ing of the machinery to which mental actions 

 are adjunct. The phenomenon of fatigue is 

 common to both, and .imposses similar disa- 

 bilities on both. Nervous exhaustion and 

 mental exhaustion mingle. Then, as offset 

 against this disability, there exists in both the 

 amenability to habit formation," mere repetition 

 within limits rendering a reaction easier and 

 readier. Then, and akin to this, is the oft- 

 remarked trend in both for a reaction to leave 

 behind itself a trace, an engram, a memory, the 

 reflex engram, and the mental memory. 



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 appear- 

 ance 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 overrun, of want of 

 dead-beat character, is met in sense-reactions. 

 The sensation outlives the light which evoked 

 it, and the stronger the reaction the longer the 

 sensation persists. Similarly the reflex after- 

 discharge persists after the stimulus is with- 

 drawn and subsides more slowly the stronger 

 the reaction. The times in both are of the 

 same order. Again, a reflex act which con- 

 tracts one muscle commonly relaxes another. 



Even so, with rise of sensation in one part of 

 the visual field commonly occurs lapse of sensa- 

 tion in another. The stoppage is in both by 

 inhibition, that is to say, active. Then again, 

 two lights of opposite color falling simultane- 

 ously and correspondingly on the two retinas 

 will, according to their balance, fuse to an in- 

 termediate tint or see-saw back and forth be- 

 tween the one tint and the other. Similarly a 

 muscle impelled by two reflexes, one tending to 

 contract it, the other to relax it, will, accord- 

 ing to the balance of the reflexes, respond 

 steadily with an intensity which is a com- 

 promise between the two, or see-saw rhyth- 

 mically from extreme to extreme of the two 

 opposite influences. 



Eeflex acts commonly predispose to their op- 

 posites; thus the visual impression of one color 

 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. Similarly the reflex machinery 

 directs, for example, the limb it moves towards 

 the particular spot stimulated. Such spots in 

 the two processes, mental and non-mental, cor- 

 respond. 



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 ac- 

 companied by overaotion 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 centers from control by the higher which 

 have been destroyed. Similarly with the sen- 

 sory effects; of skin sensations some are pain- 

 ful and some not, for example, 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 sensa- 

 tion in the affected part. The sensation of 

 touch, as Dr. Head says, restrains that of pain. 



Thus features of nervous working resemble 

 over and over again mental activities. Is it 

 mere metaphor, then, when we speak of mental 



