RESPONSES OF HOMOIOTHERMS TO VARIATIONS 33 



subcutaneous fat is seen in the case of the homoiothermic 

 marine animals in the Arctic. Such animals live habit- 

 ually among the ice floes in a medium which conducts 

 heat at least twenty times better than air and yet they 

 are able to maintain a body temperature of between 35 

 and 40 deg. C. The outer layer of skin is subjected to 

 conditions that would lead to an enormous loss of heat 

 were it not for the layer of fat insulating the deeper 

 tissues from the skin. 



Insulation. — ^Homoiotherms living in the air are pro- 

 tected from heat loss by feathers and hair. These struc- 

 tures inclose a layer of air close to the body. By restrict- 

 ing the movement of this insulating air, much less heat 

 is carried away from the body. Rubner (1903) de- 

 termined the value of the hair of the guinea pig in saving 

 heat. He found that normally the average loss of heat 

 by radiation and conduction was 3.37 Cal. per hour. 

 After the guinea pig was shorn the hourly loss was 4.19 

 Cal., Le,, 33.3 per cent more. Hair or feathers have little 

 value to animals living in water, for when the air spaces 

 between these structures are filled with water heat is 

 readily absorbed by the surroundings. Water birds ob- 

 viate this difficulty by having oily feathers which do not 

 readily become wet and these obstruct the passage of 

 water to the air spaces next to the skin. They are often 

 also well insulated with subcutaneous fat. 



Resistance to Extreme Temperatures. — When a mam- 

 mal loses heat to excess it is no longer able to maintain 

 its temperature at a constant level. The lower the tem- 

 perature of the body falls, the greater will be the dis- 

 turbances thereby produced. The highest nerve centers 

 are the first to suffer from such cooling. The medulla, 

 where the important centers are located for the main- 

 tenance of various vital physiological processes, is not 



