546 THE PHYSIOLOGY OF ALIMENTATION. 



researches of C. von Noorden have directly shown that alcohol can not 

 be substituted in a ration in the place of an isodynamic quantity of 

 carbohydrates. If this substitution be made, a ration before just capa- 

 ble of maintaining an organism in equilibrium becomes insufficient; 

 the body loses weight; the nitrogenous materials which enter into its 

 constitution are broken up, and the animal declines. 



In the preceding we have accustomed ourselves to look upon a single 

 characteristic of an aliment (though its first essential, to be sure) as 

 its energetic character. It is necessary that it furnish energy to the 

 organism, and in order to do so it must be decomposed or broken up 

 into simpler substances. Thus fat, which has rather a complicated 

 molecular structure from a chemical point of view, is given off as car- 

 bonic acid and water. The same is true for the carbohydrates, such as 

 saccharine and amylaceous substances. It is because of the simplifica- 

 tion in structure attending the passage of these substances through 

 the organism that they give up the chemical energy which they have 

 stored up in the potential form. Thermo-chemistry enables us to deter- 

 mine from the initial and final states the amount of energy given over 

 to the living being in the interim. This energetic value, dynamogenic 

 or thermogenic, gives thus a measure of the alimentary capacity of the 

 substance. A gram of fat, for example, gives up a quantity of energy 

 equal to 9.4 calories; the thermogenic value of the carbohydrates is 

 about half as great, or 4.2 calories; and the thermogenic value of 

 albuminoids is 4.8 calories. This being so we see that the animal is 

 best nourished by aliments which are of very complicated chemical 

 structure. 



IV. 



In addition to the energetic theory which we have always discussed 

 there is another way of conceiving the role of the aliment. It consists 

 in the consideration of the aliment as a source of heat. We have seen 

 that an aliment is a source of thermal energy for the organism. 

 Inversely, can it be said that all substances which give out heat when 

 brought into the organism are aliments ? This is a much controverted 

 question at present. Most physiologists admit that it is so. Their 

 notion of an aliment is interchangeable with that which produces heat; 

 in their idea everything is nutritive which disengages- heat within the 

 body. The most imperative need of the living body is to be kept warm. 

 Even cold-blooded animals have a constant internal temperature which 

 must be maintained for the preservation of their lives. On the other 

 hand the animal heat of the body is continually dissipated in the colder 

 surrounding medium. Hence a continuous supply of thermal energy 

 is necessary for the preservation of life. Hence the necessity for ali- 

 mentation is mixed up in the necessity of a vehicle for heat to cover the 

 deficit due to the inevitable cooling off of tbe organism. As a rule the 



