44 MIND IN EVOLUTION CHAP. 



A second fallacious argument may be founded on the 

 disproportion in a reflex action between stimulus and 

 result. 



" When we stimulate the nerve of a muscle-nerve preparation 

 the result, though modified in part by the condition of the muscle 

 and nerve, whether fresh and irritable or exhausted, for instance, 

 is directly dependent on the nature and strength of the stimulus." 

 ... u In a reflex action, on the other hand, the movements 

 called forth by the same stimulus may be in one case insignificant, 

 and in another violent and excessive, the result depending on the 

 arrangements and condition of the central portion of the reflex 

 mechanism. Thus the mere contact of a hair with the mucous 

 membrane lining the larynx, a contact which can originate only 

 the very slightest afferent impulses, may call forth a convulsive 

 fit of coughing, in which a very large number of muscles are 

 thrown into violent contractions ; whereas the same contact or 

 the hair with other surfaces of the body may produce no obvious 

 effect at all. Similarly, while in the brainless but otherwise 

 normal frog a slight touch on the skin of the flank will produce 

 nothing but a faint flicker of the underlying muscles, the same 

 touch on the same part of a frog poisoned with strychnia will 

 produce violent lasting tetanic contractions of nearly all the 

 muscles of the body." l 



It might be inferred from this that the reflex is, in a 

 sense, originated by the organism, instead of being, as in 

 a machine, a transformation of energy from one shape to 

 another. But the truth is that the stimulus is not to 

 be compared to the fuel which supplies the engine with its 

 energy, but to the touch which turns the handle and sets 

 accumulated energy free to work. 



" The nerve centre may be regarded as a collection of explosive 

 charges ready to be discharged, and so to start efferent impulses 

 along certain efferent nerves, and these charges are so arranged 

 and so related to certain afferent nerves, that afferent impulses 

 reaching the centre along those nerves may in one case discharge 

 a few only of the charges and so give rise to feeble movements, 

 and in another case discharge a very large number and so give rise 

 to large and violent movements." : 



This fundamental feature of the reflex mechanism must, 

 however, be kept carefully in mind in judging the true 

 character of certain modifications of the reflex act to which 

 we shall shortly come. 



1 Foster, I. pp. 182, 183. * Ib. I. p. 183. 



