INTEGRATION OF ACTION WITHIN THE REFLEX ARC 937 



period lasting nearly a whole second, during which a second stimulus 

 elicits no response. The object of this long refractory period is no doubt 

 that opportunity may be given for the flexor muscles to perform the 

 contraction that would naturally ensue during the normal occurrence 

 of the extensor thrust, as in the act of walking. When the animal 

 places his foot on the ground, the sudden pressure exerted on the pad 

 of the foot immediately calls forth the extensor thrust, by means of 

 which the weight of the body is temporarily removed from the ground, 

 and the muscles perform the contractions necessary to produce flexion 

 of the limb. Although the refractory period is unaffected by the strength 

 of the stimulus it is very dependent upon the internal condition of the 

 reflex arc, such as that caused by changes in blood supply or by narcosis. 



Reciprocal Inhibition 



It might appear that to bend a joint or to move the eyeball the only 

 muscular action required would be contraction of the muscles which 

 flex the joint or rotate the eyeball, and that the antagonistic muscles 

 would merely become passively elongated. When we remember, how- 

 ever, that all the muscles of the body are ordinarily in a condition 

 of slight tonic contraction, and that this tends to become increased 

 when the muscles are passively stretched, then we see that for effi- 

 cient movement there must be inhibition of the tone of the muscles 

 which oppose those that are contracting. This reciprocal inhibition, 

 as it is called, is a very widespread function throughout the animal 

 body. Sometimes it is purely peripheral in origin, as in the claw 

 of the crayfish, where stimulation of the nerve causes an opening of the 

 claw due to the contraction of one set of muscles and the simultaneous 

 inhibition of their antagonists. Instances of peripheral reciprocal in- 

 hibition in the higher animals are not so common, but are illustrated in 

 the case of the myenteric reflex, where it will be remembered a contraction of 

 the intestine over a bolus of food is accompanied by inhibition in front of 

 the bolus. The reciprocal action in this case is probably dependent on 

 the myenteric plexus. 



On the other hand, reciprocal inhibition of central origin is very com- 

 mon in the higher mammalia. Thus, in the case of the lateral movement 

 of the eyes, if we cut the third and fourth nerves to one eye, say, the 

 left, the external rectus of that eye will alone be under the control 

 of the nervous system, through the sixth nerve; nevertheless, if we after- 

 ward cause the animal to look toward the right, as by holding some ob- 

 ject in that direction, it will be found that the left eye as well as the 

 right folloAvs the object. Obviously there must be an inhibition of the 



