THE CONTROL OF RESPIRATION 93 



conditions of lowered external temperatures have not been eluci- 

 dated with the same precision, partly because localised cooling 

 is not so easy to achieve experimentally. 



Acclimation (i.e., short-term effects, as distinct from accli- 

 matisation) to sudden lowering of temperature is mainly pro- 

 duced by increases in body metabolism but these are usually 

 preceded by responses which restrict heat loss. The increased 

 metabolism is only a temporary solution found in animals not 

 normally subjected to long-term lowering of the external tem- 

 perature. The more immediate responses probably result from 

 stimulation of cold receptors in the skin. Reflexes take place 

 which tend to conserve heat, e.g., the constriction of the skin 

 capillaries, raising the hair or feathers which increases the thick- 

 ness of the insulating layers, and the animal may move its limbs 

 about or change its position in order to reduce the heat loss. 

 Only at a later stage does the increased heat production within 

 the animal take place, mainly by increases in muscle activity 

 which eventually lead to shivering. The adrenal cortex and 

 thyroid may also become involved because of their effect on 

 metabolism. A rat regulates its body temperature down to 

 environmental temperatures of - 10° C, but in the absence of 

 the thyroid or adrenal it can only regulate down to — 2° C. ; in 

 the absence of both glands the temperature is only adjusted 

 when the environment falls to 10° C. 



