THE ROLE OF REFLEX INHIBITION 587 



pass in. They are afferent and conduct their impulses into the 

 central nervous system, passing either fairly directly to the motor 

 centre they influence or continued by secondary and relay paths 

 which ultimately reach it, running sometimes long distances 

 within the central nervous system from one part of it to another. 

 The central nervous system itself may be likened to a great 

 telephone exchange. Through it every entrant line can influence 

 practically any of all the outgoing lines of the system, establish- 

 ing temporary communication by sets of relay junctions often 

 very complicated. All the channels which play upon the motor 

 centre of the skeletal muscle excite or inhibit it through this 

 central exchange. They therefore obtain their effect over the 

 muscle reflexly. Hence the study of their influence on the 

 muscle falls wholly under the head of reflex action. Some ot 

 these reflex actions are comparatively simple, as in cases 

 where the afferent nerve impinges fairly directly upon the 

 motor centre itself; others are very complex, as where the 

 afferent channel exerts its influence on the motor centre indi- 

 rectly through some other centre or through a whole chain of 

 centres. The solidarity of the nervous system is so great that 

 the condition of higher nervous centres commonly affects the 

 condition of the lower centres and among these latter especially 

 the motor centres of the skeletal muscles. 



A question of importance and one to which we have at 

 present no complete answer is whether the motor centre of the 

 skeletal muscle when no afferent channel is actually at play 

 upon it, lies at functional rest or whether it then exhibits any 

 spontaneous discharge of motor impulses. Has it an autoch- 

 thonous activity like the heart or is it, when the centripetal 

 channels that influence it are inactive, itself quiescent, emitting 

 no discharge ? The view generally taken is that it exhibits 

 no spontaneous discharge of motor impulses. Certainly the 

 so-called tonus of skeletal muscles is due in the great majority 

 of cases to a reflex discharge from the motor centre, not to 

 spontaneous discharge. Thus the tonus which maintains steady 

 contraction of the anti-gravity muscles in the posture of 

 standing (dog, cat) is demonstrably reflex. On the other hand 

 careful experiments have failed to detect the reflex source of 

 the tonus which in the bird keeps the wings folded when 

 not in flight (W. Trendelenburg). 



On the motor centre of each muscle every influence which 



