THE ROLE OF REFLEX INHIBITION 607 



restraint from discharge and excitatory activity of discharge are 

 demanded from the motor centre. And such alternating reflexes 

 are common in occurrence and important in function — for in- 

 stance, in locomotion, mastication, deglutition and respiration. 



A condition greatly favouring reflex rebound — at least in the 

 case of the extensor phase of the step — is the co-operation of 

 the afferent nerves of the muscles which exhibit the rebound 

 contraction, in the case in question the extensor muscles of the 

 limb. If these nerves are severed the rebound is much less 

 easy to evoke, less regular, and less pronounced. These same 

 nerves are responsible for the reflex tonus of their muscles ; this 

 tonus, which forms so marked a feature of the postural tonus 

 in the decerebrate animal, is lost on severance of these nerves. 

 The disease known as tabes dorsalis is characterised by de- 

 generation of afferent nerves, especially those of the muscles. 

 A symptom of this disease is a peculiar imperfection in the 

 management of the legs in walking ; and it may well be that 

 this is due to loss or grave defect of the post-inhibitory rebound 

 which seems to play so important a part in the reflex execution 

 of the step. 



7. Inhibition in "Conditioned" Reflexes 



There are other examples of inhibition accessible to physio- 

 logical study exhibiting it in the role of a co-ordinator of 

 nervous activities more complex than those above mentioned. 



The simultaneous and successive co-ordination of the in- 

 dividual muscular events composing such acts as respiration 

 and locomotion are innate and so fixed and rigid in character 

 as to be unalterable save in detail by will and wholly un- 

 conscious in their working. In these cases the inhibition, like 

 the rest of the reaction of which it is a part, lies in centres 

 phylogenetically among the oldest of the nervous axis. But in 

 the higher animals large masses of the central nervous S3'stem 

 belong to the very newest acquisitions of the organism. Part 

 of the utility of this new machinery lies in its plasticity ; the 

 comparative ease with which it responds to new needs by 

 adapting its reactions to new situations. Especially is this true 

 of the cerebral cortex and in greatest measure of its most recent 

 portions, such as the so-called association areas of the neo- 

 pallium. Their cortical reactions are to some extent subject to 

 an adaptation attainable in the course of the animal's individual 



