No. 598] THEORIES OF HIBERNATION 



613 



in connection with the marmot, as indicating I he existence 

 of a heat center in the brain through whose influence on 

 the various organs of the body, metabolism and heat regu- 

 lation are so affected as to produce winter-sleep. The 

 altered respiration and circulation are secondary results. 

 Dutto 39 (1896) is also inclined to believe that hibernation 

 strictly depends upon the regulative influence of the nerv- 

 ous system upon metabolism and thermogenosis. lie 

 further considers that the marmot lias the power to emit 

 more heat than has the rabbit, so that torpor may be 

 based upon the difference in the power of the integuments 

 of hibernating and non-hibernating animals to lose heat. 

 Merzbaeher 40 (1904), after reviewing much of the liter- 

 ature dealing particularly with the role of the external 

 temperature, food and the nervous system in the produc- 

 tion of winter-sleep, concludes that the external cold is 

 only a secondary aid. Cold, like abstinence from food, 

 immobility, slower respiration and lack of oxygen, simply 

 makes it easier for the animal to cool off and remain cold, 

 and tends to make the sleep more profound. The essen- 

 tial characteristic of the hibernating animal, as compared 

 with the non-hibernating animal, according to him, is its 

 ability to change at a rather definite period and in a com- 

 paratively short time from the homoiothermal to the 

 poikilothermal type and again at the end of hibernation 

 to return rather abruptly to the former condition. The 

 explanation of both of these alterations, he thinks, is prob- 

 ably to be found in a certain nervous mechanism in the 

 mid-brain and medulla which is capable of influencing res- 

 piration, circulation and metabolism, and, in short, the 

 production and loss of heat. The other changes charac- 

 teristic of the lethargy are natural consequences of and 

 adaptations to the hypothermic and hypofunctional con- 

 dition. 



In addition to other internal factors there is, accord- 

 ing to Barkow 15 (1846), a special susceptibility to the ex- 

 ternal cold due to a rather primitive organization of hi- 

 bernating animals. Xoe 41 (1903) thinks that a primitive 



