124 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



places where it said "stimulated." At those places I simply poked the animal with a 

 pencil. The action potential continued for many seconds after the poking. As you 

 say, it continues for a long time. 



Dr. Burton: This question is prompted by Dr. Burch's question. Since the re- 

 markable x-ray pictures show that the blood doesn't go down to the femorals, 

 wouldn't it be worth making a careful examination — perhaps you have — of the 

 anatomy of the descending aorta to see if these animals don't have some specialized 

 structure which they can clamp off. The diving mammals have that, as you know. 



Dr. Lyman: I did that histologically, I am ashamed to say, in only one hamster. 

 It takes quite a few serial sections. I looked through the sections and there is no 

 evidence of a coarctation or anything like that in the one aorta. Of course, I could 

 have missed a few sections. 



Dr. R. K. And jus: I should like to discuss Dr. Lyman's definition of hiberna- 

 tion. Dr. Lyman, in your definition you incorporate the fact that a hibernating 

 animal is capable of spontaneous rewarming from hypothermia without the aid of 

 external heat. I think that this is not at all specific for a hibernating animal. If one 

 cools a non-hibernating mammal, a rat for instance, and leaves it at an ambient air 

 temperature equal to its body temperature, the animal will rewarm spontaneously 

 unless the body temperature was lowered below a critical level. If a rat is to be 

 maintained at body temperatures higher than 15°, its effort to actively rewarm must 

 be counteracted (by deep anesthesia for instance). Now the same is true to a 

 greater extent for a hibernating animal. It is capable of rewarming spontaneously 

 from much lower body temperatures. But this difference between the body tem- 

 peratures from which hibernating and non-hibernating animals can rewarm spon- 

 taneously is only a quantitative difference, so to speak. A phenomenon that is more 

 important for the characterization of hibernation and which represents a qualitative 

 difference is the capability of hibernating mammals to remain dormant and not to 

 start rewarming spontaneously in spite of the "aid of external heat." An unan- 

 esthetized rat will continue its efforts to rewarm spontaneously until the body 

 temperature is forced close to the level below which cardiac arrest occurs. By con- 

 trast the hibernating ground squirrel although capable of spontaneous rewarming 

 relaxes into hibernation and remains in hypothermia at body temperatures of 20° 

 or more below the active body temperature. 



Dr. Lyman: Unless it is disturbed, I think that is true. Possibly I should have 

 emphasized that the nonhibernators, as you say, can rewarm spontaneously from 

 16°, or something like that, in the case of the rat, but they can't rewarm from 5°. 

 I should also have said that my definition included the phrase, "a passive lowering 

 of body temperature to that of the environment." In rats and other nonhibernators 

 the only time they passively lower their body temperature to that of the environment 

 is under anesthesia, drugs, and so on. They then are unable to warm without heat 

 from external sources. As I said, it wasn't a very good definition. 



