86 Tait 



ive 



not disappear until some little time after the convuls 

 twitchings of the muscle accompanying freezing of the nerve 

 have ceased. These facts indicate that the absence of con- 

 ductivity induced by freezing does not come on abruptly, but 

 gradually and progressively. 



During the frozen condition the nerve, while very susceptible 

 to mechanical injury, is not necessarily dead. 



II. Changes ox Subsequent Thawing. 



The process of thawing of the nerve, unlike the process of freezing, is 

 not accompanied b}^ any muscular twitchings. In the case of a nerve 

 which has been frozen until all convulsive twitchings of the muscle have 

 stopped, the only external sign of subsequent thawing is the disappearance 

 of the hoar-frost round the frozen part of the nerve. If the thawing take 

 place while the muscle is still contracting vigorously as a result of the 

 freezing process, the muscle contractions cease almost abruptly, and the 

 lever, after one or two diminished excursions, drops back to the base line, 

 to remain there till external stimulation of the nerve be applied. The 

 melting of the ice in the nerve, and the presumable re-dilution of its fluid 

 contents, consequently sets up no marked excitation of the structure. 



Nor is the vitality of the nerve in any way prejudiced by the rapidity 

 with which thawing occurs. Pictet (4) found that, when the living animal 

 is frozen, the thawing process must be gradual if life is to continue. 

 Although I have thawed nerves at very different rates, in no case have I 

 seen the slightest ultimate harm ensue from rapid thawing. 



As was said before, the conductivity of the nerve does not necessarily 

 return at once on thawing. The time taken for recovery varies consider- 

 ably in different experiments. According to the time taken and the mode 

 of recovery, we might distinguish three different types of cases : — A. In 

 some cases recovery is rapid, what is apparently full conduction being 

 restored within the space of a minute after thawing. B. In other cases 

 (the majority in my experiments) recovery is delayed for a period varying 

 from a few minutes to an hour or even longer. C. In a few cases a partial 

 recovery of conduction occurs after a minute or two, this is succeeded again 

 by absence of conductivity, and finally, after a more or less prolonged period, 

 full recovery occurs. 



As to the conditions under which these various eventualities occur, I 

 have no information to offer. In any one experiment there are a number 

 of variable factors, e.g. the original state of the nerve itself as regards 

 nutrition, thickness, temperature at which cold rigor occurs, etc. ; the length 

 of nerve frozen ; the rate at which freezing occurs ; the ultimate temperature 

 to which the frozen nerve is lowered; the rate of thawing and of sub- 

 sequent rise of temperature ; the degree to which the temperature is 

 ultimately raised; and finally, the extent to which the nerve has been 



