[cameron-brownlee] low temperatures ON THE FROO 113 



Our experiments have dealt with the intact frog, and the exsected 

 frog's heart. The species used was R. pipiens, obtained from a Chicago 

 dealer, who informs us that the frogs were obtained in the neighbourhood 

 of Chicago, in part caught in running water, and in part dug from sandy- 

 soil. It will be convenient to consider the results for the frog's heart 

 first. These show clearly that the hearts will recover after exposure 

 for two hours to a temperature varying between —2° and — 2-5°C. 

 (experiment 12). They will not recover exposure to a temperature of 



— 3-0°C. for one hour (experiments 9 and 10). The death-point there- 

 fore lies between —2-5° and — 3'0°C. At the higher of these tempe- 

 ratures the hearts are frozen completely hard. Reference to experiment 

 14 shows that prolonged cooling to — 2 «75° kills the normal frog's heart, 

 while in experiment 19 it survived in vivo a temperature of — 2-4°C. 

 This suggests that here also the death-point lies between —2-5° and 



— 3'0°C., and that unlike the gastrocnemus muscle there is no difference 

 between the exsected and perfused heart (unless, indeed, Brunow's 

 results are to be explained as simply due to the necessity of a longer 

 freezing period for equilibrium, and hence a pseudo-lowering of the 

 temperature, due to the circulating blood bringing heat to the limb). 



These results for R. pipiens are in good agreement with Heubel's 

 for European frogs, (p. 110). 



The experiments on the intact frog show that it survives subjection 

 to its freezing-point temperature for such a long period that it seems 

 certain that all the ''free-water" has been frozen (see especially experi- 

 ment 22). Further that after two hours cooling, exposure for a shorter 

 period to a temperature between — 0'8° and — 1°C. does not kill (ex- 

 periments 18 and 21). And finally that after exposure to a temperature 

 of between — 1 -5° and — 1 «8° for two hours, the frog no longer regains 

 coordinative movement, although heart, muscle, and peripheral nerves 

 are apparently undamaged (experiment 20). 



The fact that a distinctly less low temperature suffices to kill the 

 frog than is necessary to kill its heart, its striped muscular tissue, 

 and especially its peripheral nervous system, suggests that the death 

 of the frog is really due to a specific temperature effect either on the 

 brain or on the cord. This does not altogether agree with Gadow's 

 hypothesis* that the heart is the essential organ concerned, especially 

 since our experiments have demonstrated that the heart can be frozen 

 stiff for long periods and survive. It would be very difficult to test 

 our hypothesis directly. It has no direct bearing on our initial prob- 

 lem. We have carried out no direct experiments on exsected muscle. 

 Our experiments show no results disagreeing with Brunow's except 

 perhaps experiment 14. 



*See page 109 



