276 THE NERVOUS SYSTEM 



conclusion that just as the junction between nerve and 

 muscle confers certain characteristics upon conduction, so 

 the synapse or junction between the neurones exerts, but 

 to a greater degree, its influence upon impulses passing 

 through it. As to the physical nature of the synapse we 

 have no definite information. Even if it be that the 

 synapse is composed of fibrils uniting neighbouring neurones, 

 conduction along these must be profoundly difierent from 

 conduction along the nerve-fibre. Moreover, it is only by 

 ascribing such an influence to the synapse that we can 

 explain how reflexes are, singly and in combination, 

 adapted so as to become purposive acts. This we shall 

 now consider. 



Inhibition — Reciprocal Innervation. — Among the visceral 

 nerves there are some, stimulation of which causes a 

 depression or even cessation of a pre-existing state of 

 activity. Stimulation of the peripheral end of the cut 

 vagus, for example, slows and even stops the heart. 

 Increased activity in a nerve leads to diminished activity 

 in the organ which the nerve supphes. Inhibition 

 is seated peripherally. Throughout the skeletal system 

 there is no instance of a peripheral nerve which, when 

 artificially stimulated, causes relaxation of a contracted 

 state previously existing. But when a muscle is made to 

 contract reflexly the act is always accompanied by active 

 relaxation of the antagonistic muscle. This is easily proved. 

 If an extensor muscle is severed from its distal bony con- 

 nection it undergoes lengthening when the flexion reflex 

 is stimulated. The relaxation of the one muscle is as 

 essential a part of the reflex as contraction of the other. 

 It has the same time relations, the same tendency to 

 after-discharge, and generally observes the same rules. 



Instances of reciprocal innervation are also seen, at any 

 rate in a crude form, in the visceral system. In the 

 intestine, stimulation at a certain point causes contraction 

 above and relaxation below. Here, as in the case of the 

 vagus nerve, the mechanism is purely peripheral. 



