6o2 SCIENCE PROGRESS 



— a state of things of obviously exceptional occurrence under 

 natural conditions — or, there will persist during the new reflex 

 activities belonging to the old with, in result, confusion of the 

 two. For orderly sequence of reflex actions — also of willed 

 actions — inhibition is a necessary part of co-ordination. 



5. Reflex Inhibition and the Production of Rhythmic 

 Alternating Movements 



A yet further way whereby reflex inhibition contributes to 

 co-ordination is by assisting to convert actions which are 

 continuous into actions of rhythmic alternation. A stimulus 

 continuously applied to certain skin regions in the decapitate 

 mammal, or indeed to the transverse face of the cut spinal cord 

 itself, provokes from the limbs the rhythmic movement of step- 

 ping. The manner in which this rhythmic alternating movement 

 of extension and flexion of the limb results from the continued 

 stimulus appears to be as follows. The stimulus excites one 

 phase of the movement, e.g. extension ; this movement generates 

 in the musculo-articular structures of the limb a stimulus which 

 in its turn reflexly influences the nervous centres of the limb ; this 

 secondary stimulus inhibits the extensor centres and excites the 

 flexor, thus opposing the continuous stimulus S and temporarily 

 setting it aside. This secondary stimulus ^ lasts until, by its 

 own reflex movement, the limb is relieved from the posture 

 which generated X On Xs subsidence the original continuously 

 applied stimulus 5 again becomes effective. Thus reflex in- 

 hibition of the centres reacting to the continuous stimulus 5 

 recurs rhythmically by virtue of the secondarily generated 

 stimulus 5*. In other words, a " refractory period " in the S 

 reaction recurs rhythmically, the centres on which it plays 

 becoming rhythmically irresponsive to it. The reflex inhibition 

 by thus rhythmically introducing a refractory period in the S 

 reaction transmutes a continuous reaction into a rhythmic one ; 

 in the particular instance chosen it converts standing into walk- 

 ing or running. 



Co-ordinations of this kind seem of wide occurrence as the 

 basis of rhythmic alternating movements. The regulation 

 of the wing-beats in bird flight, and of the side-to-side strokes 

 of the tail in the swimming of fish, is probably of this nature. 

 Even in cases where the actual production of the rhythm does 

 not seem wholly explicable in this way the regulation of the 



