110 REFLEX ACTIONS. [Boon i. 



protoplasm of the reflex centre. It is not that the afferent impulse 

 is simply reflected in the nerve-cell, and so becomes with but little 

 change an efferent impulse. On the contrary, an afferent impulse 

 passing along a single sensory fibre may give rise to efferent im- 

 pulses passing along many motor nerves, and call forth the most 

 complex movements. An instance of this disproportion of the 

 afferent and efferent impulses is seen in the case where the contact 

 with the glottis of a foreign body so insignificant as a hair causes a 

 violent fit of coughing. Under such circumstances a slight contact 

 with the mucous membrane, such as could not possibly give rise to 

 anything more than few and feeble impulses, may cause the 

 discharge of so many efferent impulses along so many motor 

 nerves, that not only all the respiratory muscles, but almost all the 

 muscles of the body, are brought into action. Similar though less 

 striking instances of how incommensurate are afferent and efferent 

 impulses may be seen in most reflex actions. In fact, the afferent 

 impulse when it reaches the protoplasm of the nerve produces there 

 a series of changes, of explosive disturbances, which, except that the 

 nerve-cell does not in any way change its form, may be likened to 

 the explosive changes in a muscle on the arrival of an impulse along 

 its motor nerve 1 . The changes in a nerve-cell during reflex action, 

 we might say during any form of activity, far more closely resemble 

 the changes during a muscular contraction than those which 

 accompany the passage along a nerve of either an afferent or 

 efferent impulse. The simple passage along a nerve is accompanied 

 by little expenditure of energy ; it neither gains nor loses force to 

 any great extent as it progresses. The transmutation in a nerve- 

 cell is most probably (though the direct proofs are perhaps wanting) 

 accompanied by a large expenditure of energy, and a simple nervous 

 impulse in suffering the transmutation in a central nervous organ 

 may accumulate in intensity to a very remarkable extent, as in the 

 case of strychnia poisoning. 



^ The nature of the efferent impulses is, however, determined also 

 ' by the nature of the afferent impulses. The nerve-centre remaining 

 in the same condition, the stronger or more numerous impulses will 

 give rise to the more forcible or more comprehensive movements. 

 Thus if the flank of a brainless frog be very lightly touched, 

 the only reflex movement which is visible is a slight twitching of 

 the muscles lying immediately underneath the spot of skin 

 stimulated. If the stimulus be increased, the movements will 

 spread to the hind-leg of the same side, which frequently will 

 execute a movement calculated to push or wipe away the stimulus. 

 By forcibly pinching the same spot of skin, or otherwise increasing 

 the stimulus, the resulting movements may be led to embrace the 

 fore-leg of the same side, then the opposite side, and finally, almost 

 all the muscles of the body. In other words, the disturbance 



1 The question as to how far these processes in the central cells are connected 

 with the development of consciousness is here purposely passed over. 



