THE MECHANISM OF CO-ORDINATED MOVEMENTS 341 



shows that this concentration of energy is obtained by the process of inhibi- 

 tion. Every successful 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 re- 

 sumed (Fig. 170). 



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 contraction 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. 



The anatomical basis of the various 

 events involved in the carrying out of such 

 a reflex as that just studied is shown in the 

 diagram (Fig. 171). In this diagram the 

 nerve fibre represents the pain-receiving or 

 nociceptive nerve from the skin of the 

 foot. This passes by a posterior root into 

 the spinal cord, where it divides and gives 

 off a number of collaterals. These collaterals, 

 as we have already seen, pass in various 

 directions ; some to^the neighbouring grey 

 matter, some to the centres in the higher 

 parts of the nervous system. Neglecting 

 the latter and any intermediate neuron 

 which may be intercalated between the afferent fibre and the motor cell, 

 we see that those collaterals which affect the motor cells of the muscles 

 of the two hind limbs can be divided into two sets, one of which always 

 produces during activity excitation in certain efferent neurons, whilst the 

 other produces inhibition of the efferent neurons of the antagonistic muscles. 

 The single afferent nerve fibre is therefore, with regard to one set of its 

 central terminal branches, specifically excitor and, in regard to another 

 set of its central endings, specifically inhibitor. In the case in point the 

 central terminal branches of the nerve a are excitor for the flexor muscles 



FIG. 170. 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. 



