THE MECHANISM OF CO-ORDINATED MOVEMENTS 385 



must be the policy of the body as a whole. Yet the surface of the 

 body is being continually played upon by ever-changing stimuli, 

 tending to excite first one reflex and then another, and the activities 

 so excited would produce confusion in the conduct of the animal, if 

 there were not some means by which at any one time only one reaction 

 should be in the act of being carried out. The imperative stimulus 

 should dominate the actions of the 

 body as a whole. Just as, in the 

 mental world, attention must be un- 

 divided if we are to avoid confusion 

 of judgment, so in the lower nervous 

 activities there must always be con- 

 centration on one act or another. 

 There may be a struggle of different 

 stimuli, but one must finally -be pre- 

 potent and annul altogether the in- 

 fluence of the others. The study of 

 the spinal animal shows that this 

 concentration of energy is obtained by 

 the process of inhibition. Every suc- 

 cessful reflex, i.e. one which actually 

 occurs, inhibits all other reflexes which 

 are not co-operative with the one 

 which is taking place. We may, for 

 instance, stimulate the area of skin 

 which gives rise to the scratch reflex, 

 and at the same time apply a painful 

 stimulus to the foot. The result is not 

 a movement compounded of the two 

 reflexes, but, as a rule, the flexor 

 reflex preponderates. If, for instance, 

 the scratch reflex be proceeding and 

 then the foot be pricked, the scratch 

 reflex immediately comes to an end, 

 and the flexor reflex occurs. When 

 this in its turn has come to an end, the scratch reflex may be once 

 more resumed (Fig. 171). 



One stimulus may reinforce another if the reactions ensuing on the 

 two stimuli are allied i.e. tend to co-operate one with another. In 

 every other case, however, an afferent impulse entering the cord and 

 spreading to a motor mechanism, so as to produce a co-ordinate con- 

 traction of various muscles, causes at the same time inhibition of the 

 muscles antagonistic to the movement, and a block, or inhibition, in 



all other reflex arcs of the cord. 



25 



FIG. 171. Scratch reflex tempo- 

 rarily inhibited by application 

 of a pathic stimulus to foot. 



Signal A, stimulation of scratch 

 area. Signal B, stimulation of paw 

 by strong induction shock. 



