198 Tait and Guiin 



On the other hand, when the successive muscle responses have become 

 less and less marked as a result of a steady diminution in the intervals of 

 time between the sets of stimulations, they gradually increase in height 

 again when the intervals of rest between sets of stimulations are made 

 longer and longer. (See the last three responses in fig. 1, tracing (2), or the 

 last five responses in fig. 3.) Provided the same short interval of time 

 elapses in each case between the successive sets of stimulations, the second, 

 third and fourth, etc., responses of the muscle may be all of about the same 

 magnitude, whereas the first response of any series— beginning after an 

 adequate interval of rest— is more marked. (See generally the tracings in 

 fio-s. 2 and 3.) At any given stage of anaesthesia, therefore, the efficiency 

 of the muscle response induced by stimulation of the proximal end of the 

 nerve is directly proportional to the interval of time during which the pre- 

 paration has rested from activity. This is clearly a fatigue phenomenon. 



During the later stages of anaesthesia with yohimbine the muscle 

 responses to rapid rhythmical stimulation tend to resemble simple muscle 

 twitches rather than tetani (see fig. 4), and this is the case whether strong 

 or weak stimulation is used. These seeming simple twitches are, however, 

 in reality summated muscle responses. This is readily seen when one 

 compares the height of the muscle contractions evoked on the one hand by 

 rhythmical stimulation, and on the other by single maximal break shocks 

 applied to the proximal end of the anaesthetised nerve (care being taken in 

 each case to examine the preparation after an adequate interval of rest). 

 In every instance the effect of rhythmical stimulation is to produce a much 

 higher muscle response than that produced by a single maximal excitation. 

 In this respect the action of yohimbine is once again different from that of 

 other anaesthetics, for in the later stages of anaesthesia with, say, ether or 

 cocaine, rapid stimulation, especially when strong, produces indeed a muscle 

 twitch, but this twitch is of the same height as the response to one single 

 maximal excitation of the nerve. 



If we come now to the interpretation of this phenomenon we must 

 conclude that when a series of excitatory processes are made to travel in 

 rapid succession from a normal portion of nerve into a portion deeply 

 anaesthetised with yohimbine, probably the first few excitatory processes 

 succeed in traversing the anaesthetised part, but the passage of these unfits 

 the affected portion of nerve for the immediate transmission of further 

 excitatory processes. Only after an adequate interval of rest is the nerve 

 able to function again, and an examination of the tracings in fig. 4 will show 

 that this interval must be spread over many seconds to restore the conduct- 

 ing mechanism to exactly the same degree of functional capacity as before. 

 From the fact that even w^ith a relatively rapid rate of stimulation 

 (between 100 and 200 per second) the deeply anaesthetised nerve is at the 

 start able to transmit more than the first excitatory process, while after 

 the passage of a few excitations it temporarily ceases to function, we infer 

 that the refractory period of yohimbinised nerve is dependent, not so much 



