THE ROLE OF REFLEX INHIBITION 585 



are excited and maintained by impulses issuing from the nervous 

 centres and descending to the skeletal muscle via its motor 

 nerve. So long as the motor nerve is not active the skeletal 

 muscle does not contract and the one function of these efferent 

 nerves is to activate their muscles. 



But in the case of muscles other than the skeletal this does 

 not hold good. The contraction of these other muscles is not 

 so dependent on the activity of the central nervous system. 

 The activity of the heart muscle for instance is largely inde- 

 pendent of the central nervous system. The beat continues 

 when the heart is completely separated from all connection 

 with the central nervous system, e.g. when altogether removed 

 from the body. Similarly, the intestinal and other visceral 

 muscles continue their rhythmic contractions after complete 

 severance from the central nervous system. The heart and 

 these visceral muscles are it is true supplied with nerve- 

 trunks from the central nervous system, and through those 

 trunks the central nervous system can and does influence 

 them. It can augment their contractions, and in so far its 

 influence upon them resembles its influence upon the skeletal 

 muscles. But to the cardiac and visceral muscles — and this is a 

 significant point of difference from skeletal muscles — the central 

 nervous system supplies efferent nerves not merely of one kind, 

 but of two. To these muscles with autochthonous and quasi- 

 autonomous power of contraction the central nervous system 

 supplies not only driving, i.e. excitatory, nerves, but checking, 

 i.e. inhibitory, nerves. It was on one of these latter, namely 

 the check-nerve of the heart, the vagus, that the exercise of 

 direct inhibitory power by nerve was first discovered. The 

 vagus has power to slacken or even temporarily suppress the 

 heart's beat. Knowledge of inhibition as a physiological phe- 

 nomenon dates substantially from the discovery of this fact by 

 E. H. and W. Weber, in the year 1846. 



Search for inhibitory nerves similarly controlling the con- 

 traction of the skeletal muscles has naturally been prosecuted 

 often. Invertebrate instances of such nerves have been found 

 in the claw-muscles of Arthropods. But in Vertebrates none 

 such have been discovered. Yet it were strange did that 

 marvel of physiological mechanism, the Vertebrate nervous 

 system, have no means of restraining the contractions of its 

 skeletal muscles. The system of the skeletal musculature is 



