54 HOMOIOTHERMISM 



tissue. The thickest furry and feathery coats are found 

 in the arctic and antarctic. Well-insulated homoiotherms 

 are often obliged to exercise some care to prevent their 

 bodies from becoming overheated and many have de- 

 veloped special cooling mechanisms, which usually pro- 

 duce their effects by evaporating water and permitting 

 the ready loss of heat from the skin, lungs, or alimentary 

 tract. Most of the large animals in the tropics avoid 

 the direct rays of the sun. 



Nervous Control.— The regulation of body tempera- 

 ture in man is largely under the control of the nervous 

 system, which contracts or expands appropriate blood 

 vessels so that liquid is sent to the skin or to the internal 

 organs, causes loss of liquid from the blood to the tissues, 

 stimulates glands to secrete water or substances which 

 facilitate the production of heat through chemical action, 

 and acts in other ways. A normal dog in an ice bath 

 shows a concentration of the blood, but a dog with a 

 transsected spinal cord does not. In mammals and birds 

 generally, temperature regulation is dominated by the 

 nervous system. In other vertebrates the nervous sys- 

 tem also exercises some control, but this control is not 

 complete enough to result in the maintenance of a con- 

 stant body temperature. In invertebrates the same type 

 of regulation appears to be present, but it takes the form 

 of slow acclimating changes which require days or weeks 

 for their completion (Behre, 1918; Hathaway, 1927). 

 Cooling mechanisms for the control of body temperatures 

 are present in insects, and probably in other terrestrial 

 arthropods, in birds, and in mammals. In homoiotherms 

 such mechanisms are concerned chiefly with peripheral 

 vasodilation and the evaporation of water from the skin, 

 lungs, or oral cavity. They are largely under the con- 

 trol of the nervous system. 



