ANIMAL HEAT 445 



the remaining 28 kilograms consist of tissues, the specific heat of which is 

 but 0.8 that of water, hence the 28 kilograms of tissue will contain 28 X 0.8 

 Calories, the equivalent of 22.4 kilograms of water. Since the temperature 

 of the body is 37C. the additional number of Calories will be 22.4 X 37 

 or 828, making a total of 2382 Calories an amount of heat absolutely necessary 

 to maintain the body-temperature at the physiological level. Notwith- 

 standing the constant liberation of large amounts of heat each day, it is 

 dissipated more or less rapidly in accordance with variations in temperature, 

 character of clothing and a variety of other conditions, and so accurately 

 is this done, that at the end of the twenty-four hours the body possesses 

 its customary quantity of heat and its physiologic temperature. 



HEAT-PRODUCTION. THERMOGENESIS 



The Source of Heat. The immediate source of the body-heat is to be 

 found in the chemic changes which take place in all the tissues and organs 

 of the body. Each contraction of a muscle, each act of secretion, each 

 exhibition of nerve-force, is accompanied by the evolution of heat. The 

 chemic changes are for the most part of the nature of oxidations, the union 

 of oxygen with the elements, carbon and hydrogen, of the food principles 

 either before or after they have become constituents of the tissues. The 

 ultimate source of the body-heat is the latent or potential energy in the food 

 principles, which was absorbed from the sun's energy and stored up during 

 the growth of the vegetable world. In the metabolism of the animal body 

 the food principles are again reduced through oxidation, directly or 

 indirectly, to relatively simple bodies, such as urea, carbon dioxid, and 

 water, with a liberation of a large portion of their contained energy which 

 manifests itself as heat and mechanic motion. 



The Total Quantity. The total quantity of heat liberated in the body 

 daily may be approximately determined in at least two ways: (i) By deter- 

 mining experimentally the heat values of different food principles by direct 

 oxidation; (2) by collecting and measuring with a suitable apparatus, a cal- 

 orimeter, the heat evolved by the oxidation of the food within, and dissipated 

 from, the body daily. 



i. Direct Oxidation. The amount of heat which any given food prin- 

 ciple will yield can be determined by burning a definite amount e.g., i 

 gram to carbon dioxid and water and ascertaining the extent to which 

 the heat thus liberated will raise the temperature of a given amount of water, 

 e.g., i kilogram. The amount of heat may be expressed in gram or kilo- 

 gram degrees or calories; a gram calorie or kilogram calorie being the amount 

 of heat required to raise the temperature of a gram or a kilogram (1000 

 grams) of water iC. The apparatus employed for this purpose is termed 

 a calorimeter, which consists essentially of a closed chamber, in which the 

 oxidation takes place, surrounded by a water-jacket. The rise in tempera- 

 ture of the water indicates the amount of heat produced. 



The results obtained by investigators employing different calorimeters 

 and different food principles of the same class vary, though within narrow 

 limits: e.g., i gram casein yields 5.867 kilogram calories; i gram of lean 

 beef, 5.656; i gram of fat, 9.353, 9.423, 9.686 Calories; i gram of starch or 

 sugar, 4.116, 4.182, 4.479, etc -> Calories. These results are, however, 



