The Action of Yohimbine on Medullated Nerve 199 



on mere degree of anaesthesia by itself as on the extent to which the 

 anaesthetised nerve is within any given short period of time thrown into 

 activity. It is activity during the anaesthesia rather than the anaesthesia 

 itself which causes the prolongation of the refractory period. 



The progressive impairment of function of the nerve with activity is 

 equally well shown if during this stage of deep anaesthesia the nerve is 

 stimulated at a slow rate (4 per second), with break shocks of a strength 

 that is just maximal. (See tig. 4, last two tracings.) Then the individual 

 twitches of the muscle consistently decline in height with each repetition 

 of the excitation and finally, after a certain small number of responses have 

 occurred, die away entirely. A rest of a considerable number of seconds 

 (not more than thirty) suffices to restore the nerve to its previous condition, 

 when the same process can be repeated again by rhythmical stimulation at 

 the same slow rate. On the other hand, if the interval of rest be not 

 sufficientlj^ long (say only two to five seconds), the process of recover}^ is 

 not so complete, and the next set of muscle responses are fewer in number 

 and of less height. Meanwhile, if the nerve is stimulated at a part distal 

 to the anaesthetised portion, the muscle responds by a continuous series of 

 maximal twitches. (See fig. 5.) 



Such experiments demonstrate in striking fashion not only the existence 

 of fatigue in yohimbinised nerve, but also the gradual nature of the 

 recovery from fatigue. In every case after a period of continuous activity 

 the nerve becomes exhausted and requires a rest of a considerable number 

 of seconds before it has regained its previous state of functional efficiency. 

 Nevertheless, by stimulating the nerve after a shorter period of rest it can 

 be shown that the recovery process, though incomplete, has still gone on to 

 a certain extent. Further, the fact that in every case recovery does occur 

 after fatigue indicates that nerve is characterised not so much by non- 

 fatigability as by the possession of an extremely efficient mechanism for 

 repair after fatigue. 



How far the refractory period of nerve may be prolonged when the 

 nerve is under the influence of yohimbine anaesthesia we have not deter- 

 mined exactly. Much depends on the signification in which the term 

 " refractory period " is used. If we take the term to mean that period of 

 time which elapses before one maximal stimulus following upon another of 

 equal intensity can be fully effective, then the refractor}^ period has been 

 prolonged to at least '25 second. If, however, we extend the definition to 

 include the period of time necessary for complete recovery of the nerve 

 after the application of a series of stimuli applied in rapid succession, then 

 the refractor}^ period has been prolonged to a number of seconds (more 

 than five). 



In the later stages of yohimbine anaesthesia when the proximal end of 

 the nerve is stimulated by isolated maximal shocks at long intervals, the 

 corresponding muscle responses are definitely lower in height than those pro- 

 duced by similar stimulation of the distal end. This points to a diminution 



