CHAPTER V 



ON VITAL PRODUCTION 



We have now traced the working substance of life through the 

 transformations which it undergoes in the animal body. These 

 are as follows: 



Animal Metabolism. — (1) Certain food substances contain the 

 working substance in the form of a mixture of proteids, fats, and 

 carbohydrates, derived from the tissues of animals and plants. 

 This mixture is taken into the alimentary canal, where it is 

 digested and dissolved. As the result of the processes of diges- 

 tion and absorption the proteids, fats, and carbohydrates of the 

 food are made similar, or assimilated, to those substances such 

 as they occur in the body of the animal that eats them. 



(2) They are then distributed by the blood-stream and incor- 

 porated in the protoplasm of the muscles and other tissues. 



(3) These protoplasmic substances are disintegrated into 

 simpler chemical compounds, with the result that energy is 

 liberated. 



(4) The products of the disintegration of the substances of the 

 tissues are finally oxidised and excreted. 



The result of these metabolic processes is that the working 

 substance becomes degraded energetically and chemically. Let 

 us take the energy transformations first. An ordinary day's diet 

 is likely to contain about 125 grams of dry proteid, 125 grams 

 of dry fat, and 400 grams of dry carbohydrate,* and if the 

 heat value of these quantities of dry foodstuff be found it will 

 add up to about 3,500 calories. Now the result of the chemical 

 transformation undergone by this average quantity of food is 

 that a certain quantity of carbonic acid and water is excreted 

 by the lungs and kidneys, and about 42 grams of urea by the 

 kidneys. The water and carbonic acid contain no free (or 

 available) energy, but the urea contains about 105 calories. 

 Therefore about 3,390 calories of the energy contained in the 

 food is utilised by the body. About 12 to 30 per cent, of this 



* In British weights about 4J ounces of dry proteid, 4| ounces of dry 

 fat, and 14 ounces of dry carbohydrate. 



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