Freezing of Nerve, with Special Reference to Fatigability 83 



testing the conductivity at every stage, a sudden marked improvement in 

 conduction occurs just before freezing sets in. Before we discuss this 

 question, however, a word or two is necessary as to the method adopted to 

 test the conductivity. 



The work of Wedensky (9) and of Frohlich (10) on anassthetised nerve 

 has shown the value of rapid rhythmical stimulation at varying intensities 

 for detecting slight changes in conductivity at a time when, by ordinarily 

 employed methods, such changes are not discoverable. By the method of 

 applying single maximal shocks to the nerve, depreciation of conductivity 

 is indicated only by a falling off in the height of the muscle twitch follow- 

 ing stimulation of the nerve. Apart from this change, conductivity seems 

 to remain unaltered almost up to the point at which it suddenly disappears 

 for good. By the method of rhythmical stimulation, on the other hand, as 

 Frohlich showed, one is .able not only to determine roughly the degree to 

 which the amplitude of the excitation is cut down, but also to show changes 

 in the refractory period. 



With progressive anaesthesia the refractory period of the nerve becomes 

 longer and longer ; consequently, when one stimulates at such a rate that 

 the interval between the individual excitations is less than the refractory 

 period, or about equal to the refractory period, the muscle response becomes 

 abnormal. It may be simply an initial twitch of the same height as that 

 evoked by one maximal excitation ; it may be a tetanus which attains its 

 maximal height at the very start, to fall off very rapidly thereafter, and 

 finally to drop to the base line again, thus showing that the conductivity 

 of the nerve has temporarily ceased. Indeed, Frohlich proved that this 

 latter form of tetanus was an expression of fatigue on the part of the 

 nerve, and a tetanus of this special form he names a " fatigue tetanus." 



Such effects are more readily obtained with strong than with weak 

 stimulation, for the duration of the refractory period seems to vary with 

 the intensity of the corresponding stimulation. As previously mentioned, 

 they are to be detected at a time when the conductivity, as tested by the 

 method of isolated maximal break shocks, is apparently present in un- 

 diminished degree. Consequently, in testing the conductivity of cooled or 

 of frozen nerve, I availed myself of this method. The rate of rhythmical 

 stimulation used was such as could be obtained by means of the springs 

 supplied with the Kronecker coil, and varied with occasion from 30 to 256 

 stimulations per second. 



Fig. 1 is a record of an experiment in which a portion of nerve was 

 gradually cooled from 0^ C. to — 3"5° C, the state of the conductivity being 

 meantime tested every few seconds by short rhythmical stimulation of 

 constant intensity (30 Kronecker units) and rate (144 per second). In 

 this case the cooling was not effected by means of the thin glass tube, but 

 about 7 mm. of the nerve was cooled in a cold chamber. The first six 

 stimulations produce a kind of tetanic response of the muscle, each succes- 

 sive response, apart from the preliminary uprise, being less marked than the 



