THE NERVOUS SYSTEM 311 



feeble for such a mechanism. The short " reflex time " of 

 the knee-jerk points to the passage of the agitation up a 

 sensory root to the spinal cord, and through a non-resistant 

 medium to the environment of the motor cells which it dis- 

 charges, missing the neurone or neurones which intervene in 

 the case of ordinary reflex actions. This is an illustration of 

 the way in which tone-impulses, which we imagine as con- 

 ducted by the non-resistant medium, pass over into discharges 

 which produce visible effects. Again, the phenomena of in- 

 hibition appear to require the supposition of extra-neuronic 

 conduction. Whenever a reflex path is in use, all other paths in 

 its neighbourhood are closed. The passage of impulses leading 

 to a particular reflex action is favoured by the suppression of 

 conduction in its vicinity. When A is talking to D through the 

 nerve-telephone, B and C are compelled to hold their peace. 

 Inhibition is a phenomenon of universal occurrence. In re- 

 lation to various actions, it is sufficiently pronounced to be 

 visible in the effects which it produces. A simple experiment 

 will illustrate this. Holding water in the mouth has no effect 

 upon respiration, but during the act of swallowing respiratory 

 movements are suspended. Whilst the swallowing reflex is 

 occurring the respiratory reflex is inhibited. This might be 

 attributed to the volitional control of respiration, and cer- 

 tainly when attention is being directed to the process volition 

 plays a large part. But if a finger is placed on the pulse, it is 

 possible to detect that, during the act of swallowing, the pulse 

 quickens, owing to the suppression of the slowing action of 

 the vagus upon the heart. Here is a case in which inhibition 

 is in no degree a voluntary action. Nor is it of any value as 

 an adjunct to the particular reflex with which it is associated. 

 It is an illustration of the universal rule that activity of any 

 one spot in the nervous system is the cause of the quieting of 

 the surrounding area. Impulses which reflexly check the heart 

 cannot get through the medulla oblongata whilst the swallowing 

 impulses are traversing it. Inhibition has teen described as a 

 drainage of nerve-force into the active area. On the structural 

 side it seems to require the conception of an extra-neuronic 

 substance which, agitated in the vicinity of the cells which are to 

 be discharged, is brought to rest around neighbouring cells. The 

 promulgation through the nervous system of the state which, 



