THE MECHANISM OF CO-ORDINATED MOVEMENTS 383 



minimal, involves the simultaneous action of more than one anterior 

 root. We may say that the motor response is determined to a certain 

 extent by the spatial proximity of the afferent to the efferent tracts, 

 but that it is always pluri-segmental, the most important deter- 

 mining factor being the adaptation of the movement to the stimulus 

 which is applied. 



The gradual spread of the response with increasing strength of 

 stimulus is spoken of as ' irradiation* The nature of the response 

 is determined by the locus or place of application of the stimulus 

 and by the quality of the latter. While a painful stimulus causes 

 flexion of the leg, deep pressure on the plantar surface of the paw 

 causes extension the ' stepping ' reflex. However extensive the 

 irradiation, the muscles which are set into action are always such 

 that their actions co-operate towards a given end. Thus, when the 

 impulse spreads to the opposite limb and produces an extension, the 

 reaction is such as would ensue when the dog steps on a sharp point 

 and immediately retracts the irritated limb away from the injurious 

 agent while it extends the other limb in the first act of progression 

 or movement away from the dangerous spot. 



A superficial study of this reflex would therefore lead us to the 

 conclusion that, by the varying resistance in the different synapses 

 on the course of the connections of the stimulated afferent nerves, 

 the impulses are directed so as to affect solely and exclusively the 

 muscles whose activity will co-operate and aid the primary reflex. 

 Such a description would, however, only represent one half of the 

 process. Every muscle in the body is in a state of tone varying 

 with its extension. If this tone is not to interfere with the carrying 

 out of a reflex movement, there must be some me-ans by which it can 

 be inhibited. Such an inhibition we have seen occur as the result 

 of contraction of antagonistic muscles ; but the remarkable fact 

 has been brought out by Sherrington that the impulses, which start 

 on the surface of the body and set loose a chain of motor impulses 

 resulting in the co-ordinated contraction of certain muscles, spread 

 at the same time to the motor mechanisms governing the muscles 

 antagonistic to the movement, and exercise on these an inhibitory 

 effect. 



This inhibition can be easily shown in a spinal animal in the 

 following way. The anterior thigh muscles are cut away from their 

 attachments to the tibia and the patellar tendon connected by a 

 thread with a recording lever. On then exciting the flexor reflex 

 by nocuous stimulation of the foot, the lever attached to the patellar 

 tendon falls (Fig. 170B), showing that the extensor muscles have 

 undergone actual elongation. The same effect is observed even 

 when the hamstrings, the flexors of the knee, have been divided. The 



