456 PRINCIPLES OF GENERAL PHYSIOLOGY 



vertebrates, birds and mammals, are " homoiothermic," that is, of uniform 

 temperature. 



Under certain circumstances, as in a hive of bees, the temperature of 

 poikilothermic animals may rise considerably. 



In a warm-blooded animal, even at rest, there is a considerable production 

 of heat by muscular contraction, which is always present in the form of reflex 

 tone. This is naturally less in sleep, and we can observe the care taken by 

 animals to avoid loss of heat when asleep. We know also how much more 

 rapidly we become cold when asleep than when awake. There are always 

 certain muscular movements going on, as those of the heart and muscles of 

 respiration. 



Calorimetry. The apparatus used for determining the output of heat is, 

 like that used for the same purpose in physics and chemistry, called a calori- 

 meter. It may be made on various principles, but the only satisfactory one 

 for use with large animals, such as man, is that described by Williams (1912), 

 as an improvement on that of Atwater, and by Macdonald (1913). The 

 principle on which this apparatus is constructed is to absorb the heat produced 

 by the animal by means of a current of water circulating through ribbed tubes 

 in the chamber. When the amount of water flowing and the temperature 

 difference between the inflow and outflow are known, the quantity of heat can 

 be calculated. A. V. and A. M. Hill (1913 and 1914) have arranged apparatus 

 of a similar kind for automatic registration over long periods of time and have 

 avoided the difficulty of loss of heat, by conduction and radiation, by the use 

 of large vacuum-jacketed flasks for small animals, and double-walled tanks 

 for larger animals, the heat insulation in the latter case being provided by 

 sawdust and "kapok wool." The heat produced is removed by a current of water 

 and the difference of temperature between thermo-electric junctions in inlet 

 and outlet is registered by a self-recording galvanometer. The micro-calorimeter 

 of A. V. Hill (1911 2,) is used for very small production of heat. 



The Normal Production of Heat. A. V. and A. M. Hill (1913) find that, 

 in adult or nearly full-grown rats, the fasting production is directly proportional 

 to their weight. In the case of young animals, the proportion is more nearly 

 to their surface, but is actually higher, compared with that of adults, than would 

 be due to their relatively greater surface. 



This non-proportion of heat-production to body surface in rats shows that 

 production is not regulated by actual loss alone, but that it is a consequence 

 of necessary tissue activity. Miss A. M. Hill (1913) points out that if the heat 

 production is determined by heat loss alone, the former should be proportional 

 to the difference between the animal's temperature and the external temperature. 

 If the difference is 22 (external temperature, 15) the mean production was 

 found to be 203 calories per gram per day. With a difference of 11 (external 

 temperature 26), instead of being half the previous one, the production was 

 found to be 166 calories, or about four-fifths. 



Macdonald (1913) finds in man at rest that the heat production is proportional 

 to the surface, that is, to the two-thirds power of the weight. In work, the effect of 

 weight decreases, or the heat production approaches more nearly to proportionality 

 to weight, as would be expected, since the muscles make up so large a proportion 

 of the weight. 



Effect of Food. Fed animals show considerably more evolution of heat than 

 fasting ones. 



The Regulation of Temperature. In warm-blooded animals, where a delicate 

 adjustment of rates of reactions to a particular temperature has been developed, 

 it is clearly of great importance that means should be adopted to maintain this 

 temperature at a constant level. 



Let us see what means are available for the purpose. The production may be 

 increased or decreased by muscular activity or rest ; but this, especially in high 

 external temperatures, is somewhat limited in range, since a certain degree of 

 muscular activity in respiration, heart beat and so on, must be continued. It is 

 particularly effective in counteracting fall of external temperature. Next, we 



