ANIMAL HEAT 181 



The blood streams from the tissues where most of the 

 heat is produced (muscles and large glands) to the skin, 

 where it becomes cooled. The temperature of the muscles 

 is therefore somewhat higher, and that of the skin lower, 

 than that of the blood. 



The body temperature shows some regular minor varia- 

 tions; shortly after midnight it is lowest (36.5), while in the 

 afternoon it is highest (37.5). It is somewhat increased by 

 the partaking of food and by muscle activity. 



Mammals have about the same body temperature as man ; in 

 birds it is higher (40-45). The body temperature of cold-blooded 

 or poikilothermic animals is a few degrees (1-4) higher than that 

 of the surrounding medium (provided they have not been placed 

 in a warmer or colder medium just previous to the measurement). 



Hibernating mammals are, during their winter sleep, like cold- 

 blooded animals. 



4. Regulation of body temperature. The body tempera- 

 ture remains constant when the production of heat Equals the 

 loss of heat. If changes occur in the production of heat 

 (e.g. by muscular activity) or in the loss of heat (e.g. in hot 

 or cold weather), the production and loss of heat must again 

 be regulated in order to keep the body temperature con- 

 stant. Concerning the nature of the regulation of tempera- 

 ture by the nervous system little is known. 



Some authors think that there are in the central nervous system 

 certain centres (heat centres) by which the mechanism for regula- 

 ting the body temperature is governed. But the account given of 

 these centres and their mode of action is not satisfactory. 



By the regulation of temperature both the production and 

 the loss of heat can be varied. 



Changes in the production of heat occur when the loss of 

 body heat is altered by variations in the temperature of the 

 surrounding medium. In cold weather the production of 

 heat is increased by the involuntary muscular contraction 

 that takes place (chattering of teeth, shivering). 



In small animals the proportion of the surface by which 

 heat can be lost to the heat-producing body mass is greater 

 than in larger animals. Therefore, in order to maintain a 



