EXCITATION AND INHIBITION 



389 



contraction given by this experimenter show a number of steps of gradation 

 which do not sufficiently exceed the possible number of motor fibres in the nerve 

 to be satisfactory proof of the law failing to apply in this case. 



As far as the number of separate nerve fibres in the motor nerves to the eye-muscles is 

 concerned, it seems that they are fully sufficient to provide for all the degrees of contraction 

 required. In the sixth cranial nerve, which supplies the external rectus muscle, the number 

 of fibres is given by Zoth (1905) as 2,500 in man, and those in the nerve to the superior oblique 

 muscle as 2,150. This fact, in itself, obviously suggests that different degrees of contraction 

 are effected by changes in the number of muscle fibres stimulated. If there were a possibility 

 of different degrees of activity in the same nerve or muscle fibre, a very much smaller 

 number of individual fibres would be sufficient. 



Refractory State. It was first shown by Gotch and Burch (1899) that, if 

 a stimulus is followed by a second one at an interval less than about O'OOS second, 

 differing according to temperature, the second one does not give rise to a 



100- 



50 



<a ca 

 -.5 



J5 * 



CQ S^> 



01 -02 



Time since previous stimulus (seconds) 



03 



FIG. 108. CURVE OF RECOVERY OF EXCITABILITY OF NERVE AFTER A PREVIOUS 

 STIMULUS. The refractory state is, at first, absolute ; excitability returns gradually, 

 and becomes normal at about 0'012 second. It is followed by a brief stage of 

 supernormal excitability. 



(Adrian and Lucas, 1912, p. 114.) 



propagated disturbance, as indicated by an electrical change. This means 

 that the nerve is inexcitable immediately after a state of excitation. Further 

 investigation of the state of the nerve during the period succeeding the passage 

 of a disturbance was made by Adrian and Lucas (1912), by a method in which 

 contraction of the attached muscle was used as indication of the disturbance 

 in the nerve. Fig. 108 represents the percentage of normal excitability present 

 at various intervals of time after the excitation, at a temperature of H 0> 8 C. It 

 will be seen that for - 0025 second after a previous stimulus there is complete 

 inexcitability to any strength of stimulus (" absolute " refractory period). From 

 this time to about - 012 second the excitability is lower than normal, gradually 

 increasing ; this is the period of " relative " refractory state, in which a stimulus 

 stronger than normal is required to set up any propagated disturbance. Following 

 this, until 0'028 second, there is a period during which the nerve shows increased 

 excitability, to which reference will be made again presently. 



During the relative refractory period, the disturbance set up by a stimulus 

 which is just strong enough to excite is less than the normal one, as measured by 

 its ability to traverse a narcotised region. In the normal nerve, as we have seen, 



