No. 582] SIGNIFICANCE OF INTERNAL CONDITIONS 335 



Animals unable to maintain this constant body temper- 

 ature throughout the year may maintain a constant tem- 

 perature during the warmer seasons of the year and hiber- 

 nate during the winter. This is particularly the case with 

 small mammals, whose relatively large ratio of surface to 

 mass may greatly facilitate heat loss, and with animals 

 whose supply of food is difficult or impossible to obtain 

 during the colder season. The body temperature of the 

 animal falls greatly during hibernation. The importance 

 of size in its relation to metabolism may be shown from 

 the specific energy requirements of various animals. In 

 general, the heat requirement of all well-nourished warm- 

 blooded animals is about 35 calories per hour for each 

 square meter of body surface. But since the surface va : 

 ries as the square of the dimensions of the body, and the 

 mass varies as the cube of these dimensions, the ratio of 

 surface to mass is much greater in small animals than in 

 large. A mouse requires 452 calories per kilogram of 

 body weight in twenty-four hours, while a horse requires 

 but 14.5 calories and a man about 24 calories in the same 

 time. 24 To sustain a number of mice equal in weight to 

 a man would require more than eighteen times as much 

 food, measured in calories, as a man would need; and 

 more than thirty horses could subsist upon the same 

 amount of food that would be necessary to sustain a num- 

 ber of mice whose aggregate weight was equal to that of 

 one horse. This is very different from saying how much 

 food one mouse as large as a man or a horse would need, 

 and should not, under any conditions, be confused with 

 such a statement. 



Milne-Edwards observed that in a small bird such as 

 the sparrow, the body temperature might be lower in 

 winter— 40.8° C— than in summer,— 43.77° C.— the dif- 

 ference in temperature amounting to about 3° C. 



The heat regulating mechanism of the body is, then, 

 not a simple one but a complex one, involving muscle and 

 gland, food supply and distribution of the blood, the nerv- 



