116 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



hibernation in the ground squirrel ;^*'' in fact the presence of some adrenal tissue is 

 considered necessary for hibernation, ^^-' ^'^^ and hypophysectomy results in eventual 

 hypothermia and death. ^®' ^^'' 



On the other hand, though the endocrines are probably not the basic cause of 

 hibernation, they may well have profound influence in setting the stage for the final 

 act, and it would be folly to disregard their possible importance in such obvious 

 preparations as the building up of fat which takes place in many hibernators. 



The third concept of hibernation implicates the central nervous system as the basic 

 controlling mechanism. Prosser^'° has said that the outstanding fact of hibernation 

 is the "turning down of the thermostat" and this concept postulates that the "thermo- 

 stat" is in the central nervous system. It is well established that temperature control 

 in non-hibernating mammals is regulated in the region of the hypothalamus and 

 tegmentum and that chilling causes activity of the heat-conserving and heat-generat- 

 ing "centers." Assuming that hibernators have the same heat-regulating centers, 

 these centers must undergo some change in sensitivity when the animal starts to 

 hibernate. The nature or cause of this change is, of course, unknown, and there is 

 actually no direct evidence implicating these centers in the process of entering 

 hibernation. On the other hand, the central representation of the sympathetic nervous 

 system appears to be definitely involved in waking from hibernation, and it seems 

 reasonable to suspect that it is also of prime importance in the process of entering 

 the hibernating state. 



Throughout the preparation of this article we have constantly turned to Drs. 

 Helen W. Deane, Don W. Fawcett, A. Baird Hastings and George B. Wislocki for 

 advice, and we are most grateful to them. We also thank Mrs. Regina O'Brien for 

 checking and correcting the references. 



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