THE CONTROL OF RESPIRATION 87 



involuntary mechanisms involving an increase in muscular 

 activity which may be asynchronous in individual muscle fibres 

 are also important. The most notable method of course is that 

 of shivering which occurs as a reflex when the temperature of 

 the body falls by 0-5-0-7° C. and which is capable of more 

 than doubling the basal heat production. In addition to these 

 mechanisms involving muscular activity there are certainly 

 mechanisms for increasing the basal metabolic heat production 

 by direct effects upon the chemical mechanisms of the body. 

 One obvious means of increase would be to make some of the 

 metabolic pathways less efficient, so that more energy is lost as 

 heat and not transferred from one system to another. Various 

 suggestions have been made which include those of enzymatic 

 shunts in phosphorylation (see p. 130) whereby less ATP is 

 produced and a greater amount of heat liberated. 



A notable example of the importance of increased heat pro- 

 duction through raising the basal metabolism is that of accli- 

 mated white men when sleeping in the cold. Before acclimation 

 they were restless and did not raise their basal metabolism and 

 in consequence the temperature of their extremities fell during 

 the night. Following acclimation the foot temperature remained 

 fairly constant but the basal metabolism was nearly doubled 

 and they were able to sleep quite well. In contrast, Australian 

 aborigines slept well at a normal level of basal metabolism 

 despite a marked fall in their foot temperatures. 



Many of the adaptations for heat conservation involve struc- 

 tural features which insulate the body against rapid changes of 

 external temperature. The insulation provided by fur is pro- 

 portional to its thickness and during winter many arctic 

 mammals increase the thickness of their coats. A large volume 

 of still air enclosed by the fur decreases the heat lost by con- 

 duction and convection. The increases in insulation are greater 

 for large mammals (black bear, 52 %) than for small mammals 

 (deer mouse, 21 %). Fur is also a barrier to the gain of heat, 

 e.g., the camel increases in temperature considerably following 

 shearing. This animal is able to tolerate wide fluctuations in body 

 temperature (34°-40° C.) and this must be regarded as a genetic 



