456 GENERAL CONSIDERATIONS 



Warm-blooded animals, however, possess methods of heat regulation 

 which involve both structures and functions. Structures tending to 

 regulate the heat produced by the body are body coverings, such as 

 feathers and fur. 



Functionally the body regulates heat through the nervous system 

 by controlling the activities of the various parts, particularly the activity 

 of the circulatory system. By increasing the speed of circulation oxygen 

 may be more liberally supplied to the various parts of the body, and the 

 amount of heat developed correspondingly increased. The opposite 

 is true of a slowing up of the circulation. By changes in the caliber of 

 the blood vessels, the distribution of heat in the body may also be 

 modified; the blood may either be accumulated toward the center of the 

 body, where the heat is conserved, or be carried to the periphery, where 

 the heat is allowed to radiate freely. 



Another factor in heat regulation is evaporation from the surface 

 of the body. In many mammals the sweat glands provide a means by 

 which the body surface may be cooled when the temperature becomes too 

 high. Water, because of its high specific heat, absorbs a large amount 

 of heat in passing from a liquid to a gaseous state. This heat has to 

 come from the media with which the water is in contact, and so the 

 evaporation of perspiration cools the surface of the body. 



504. Warm-blooded and Cold-blooded Animals. — As a result of the 

 varying amounts of heat produced by different animals and the varying 

 efficiency of heat regulation, there is great diversity in the temperature 

 maintained in the bodies of different forms. In the case of all but birds 

 and mammals, however, this temperature approximates the temperature 

 of the air or water in which the animal lives, being sometimes a little 

 higher and sometimes a little lower but never departing far from it. In 

 the case of birds and mammals, because of the amount of heat they 

 produce, the presence of a heat-conserving body covering, and a heat 

 regulatory mechanism, the organism is able to maintain a temperature 

 independent of that of the surrounding medium and approximating a 

 constant under all conditions. What this constant temperature shall be 

 is determined by the individual type and varies considerably as between 

 different species. Animals which can maintain a constant temperature 

 are termed homoiothermous, though popularly they are known as warm- 

 blooded; animals unable to do so are termed poikilothermous, or cold- 

 blooded. In the case of mammals that go into true hibernation, which 

 involves a state of lethargy and practical cessation of all activity, a warm- 

 blooded animal may relinquish temporarily its power of heat regulation 

 and become actually cold-blooded. Thus an active ground squirrel, 

 which at different seasons and under different conditions possesses rectal 

 temperatures of 30° to 39°C. (86° to 102°F.), may in hibernation exhibit 

 a temperature of 5°C. (41°F.) or even less. The average temperature of 



