446 TEXT-BOOK OF PHYSIOLOGY 



physical values, and indicate the quantity of heat such quantities of foods 

 give rise to when completely oxidized to carbonic acid and water. In the 

 animal body the carbohydrates and the fats, with the exception of the small 

 portion which escapes digestion, are reduced to carbon dioxid and water, 

 and hence practically liberate as much heat as they do when oxidized outside 

 the body. The proteins are reduced to water, carbon dioxid and urea. As 

 urea is capable of further reduction in the calorimeter to carbon dioxid 

 and water, with the liberation of heat, the quantity of heat it contains 

 must therefore be deducted from the physical heat value of the protein. 

 According to Rubner, i gram of urea will yield 2.523 kilogram calories. 

 As about one-third of a gram of urea results from the oxidation of i gram 

 of protein, the amount of heat to be deducted from the heat value of the 

 protein is J of 2.523, or 0.841 Calories. It has also been shown by the same 

 investigator that some of the ingested protein is found in the feces, the heat 

 value of which must also be determined and deducted. This having been 

 done, the physiologic heat value of protein becomes 4.124 Calories. 



The following estimates give approximately the number of kilogram 

 calories which should be liberated within the body when the protein is burned 

 to the stage of urea, and the fat and carbohydrate to the stage of carbon dioxid 

 and water: 



i gram of protein. 4.124 Calories 



i gram of fat 9-353 Calories 



i gram of carbohydrate 4.116 Calories 



The total number of kilogram calories yielded by the various diet scales 

 can be readily determined by multiplying the quantities of the food prin- 

 ciples consumed by the foregoing factors. (See page 128.) 



2. Calorimetric Measurements. It has been determined from a long 

 series of experiments that the animal body dissipates a variable amount of 

 heat from day to day, an amount that can be collected and measured by 

 placing the animal in a suitable apparatus a calorimeter. A calorimeter 

 is therefore an apparatus designed for the direct determination of the quan- 

 tity of heat dissipated from the body in any given time. The amount 

 obtained by this method expressed in Calories is taken as a measure of the 

 heat liberated by the oxidation of the food, providing of course, the tempera- 

 ture of the animal remains unchanged. The substance employed for col- 

 lecting and measuring the heat is usually water. The calorimeters in 

 general use consist essentially of two metallic boxes placed one within 

 the other, though separated by a space sufficiently large to hold a defi- 

 nite amount of water (Fig. 200). The animal is placed in the inner box, 

 which is also provided with tubes for the entrance of fresh and the exit 

 of expired or vitiated air. The heat radiated is absorbed by the water and 

 its temperature raised. To prevent loss by radiation and to render it in- 

 dependent of changes in the surrounding temperature the calorimeter is 

 surrounded by a poorly conducting material, such as wool. The tempera- 

 ture of the animal is taken at the beginning and the end of the experiment. 

 If the temperature of the animal remains the same at the end of the experi- 

 ment, then the heat absorbed by the water represents the amount produced 

 by the animal. If, on the contrary, the temperature of the animal rises 

 or falls, the number of calories so retained or lost must be added to or sub- 

 tracted from the amount absorbed by the calorimeter. In the determination 



