3 88 TEXT-BOOK OF PHYSIOLOGY. 



temperature of the air, that of the body remains almost constant. 

 The same is true for the seasonal variations in the temperature of 

 the temperate regions. 



THE SOURCE AND TOTAL QUANTITY OF HEAT PRODUCED. 



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^xEach contraction of a muscl&^each act of 

 secretion^each exhibition of nerve-force^is accompanied by the 

 evolution of heajt^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 determining experimentally the heat values of different food 

 principles by direct oxidation; (2) by collecting and measuring with 

 a suitable apparatus, a calorimeter, 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 foocl 

 principle 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 kilogram degrees or calories; a gram Calorie 

 or kilogram calorie being the amount of heat required to 

 ..-temperature of a gram nr a Ininprram ^loopWanU of 

 rETapparatus employed for this r purpose Is termed a 



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

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

 of the water indicates the amount of heat produced. 



The results obtained by investigators employing different calor- 

 imeters 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. 

 icse results are, however, physical values, and indicate the quan- 



