CHAP, in.] GENERAL FEATURES OF NERVOUS TISSUES. 181 



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 arid violent movements. In a reflex action 

 then the number, intensity, character and distribution of the efferent 

 impulses, and so the kind and amount of movement, will depend 

 chiefly on what takes place in the centre, and this will in turn 

 depend on the one hand on the condition of the centre and, on 

 the other, on the special relations of the centre to the afferent im- 

 pulses. 



At the same time we are able to recognize in most reflex actions 

 a certain relation between the strength of the stimulus, or the 

 magnitude of the afferent impulses and the extent of the move- 

 ment or the magnitude of the efferent impulses. The nerve-centre 

 remaining in the same condition, the stronger or more numerous 

 afferent impulses will give rise to the more forcible or more 

 comprehensive movements. Thus if a 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 fre- 

 quently will execute a movement calculated to push or wipe away 

 the stimulus. By forcibly pinching the same spot of skin, or other- 

 wise 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 set going in the centre, confined when the stimulus 

 is slight to a small part of the centre, overflows, so to speak, 

 when the stimulus is increased, to other parts of the centre, and 

 thus throws impulses into a larger and larger number of efferent 

 nerves. 



-We may add, without going more fully into the subject here, 

 that in most reflex actions a special relation may be observed 

 between the part stimulated and the resulting movement. In the 

 simplest cases of reflex action this relation is merely of such a 

 kind that the muscles thrown into action are those governed by a 

 motor nerve which is the fellow of the sensory nerve, the stimula- 

 tion of which calls forth the movement. In the more complex 

 reflex actions of the brainless frog, and in other cases, the relation 

 is of such a kind that the resulting movement bears an adaptation 

 to the stimulus : the foot is withdrawn from the stimulus, or 

 the movement is calculated to push or wipe away the stimulus. 

 In other words, a certain purpose is evident in the reflex action. 



Thus in all cases, except perhaps the very simplest, the move- 

 ments called forth by a reflex action are exceedingly complex com- 

 pared with those which result from the direct stimulation of a 



