DISSIMILATION 93 



regions of the capillary networks, free oxygen leaves the blood 

 and passes into the cells. This assimilation of oxygen is designated 

 internal respiration. 



The bodily processes preUminary to assimilation have now been 

 discussed. These include ingestion, digestion, and absorption of 

 the foods that enter the blood by way of the digestive tract, as 

 well as external respiration, which suppKes oxygen to the blood 

 and is dependent, in terrestrial animals, upon breathing. In addi- 

 tion to the incorporation and synthetic processes that maintain 

 the protoplasmic system, assimilation, in a wide sense, includes 

 also the utihzation of reserves or stored materials, such as glj^cogen 

 and neutral fat, and storage products which may be more tempo- 

 rary and present in all cells. It is possible, also, that the produc- 

 tion of certain secretions should be classed under assimilative 

 metabolism. 



Dissimilation. — Oxidation. — Dissimilative metabohc activ- 

 ities include those reactions by which protoplasmic constitu- 

 ents are chemically decomposed for the transformation of 

 energy and production of heat, and probably, in some cases, 

 for the elaboration of certain secretions. The reactions that 

 transform energy and produce heat are in the nature of oxi- 

 dations, that is, reactions in which oxygen unites with com- 

 pounds of the protoplasmic system. Oxidation is commonly 

 known as combustion, or burning. When fuel burns, oxygen 

 from the air is combined with the chemical compounds that 

 make up the fuel, producing heat and hberating energy which can 

 be made to do work. It was Lavoisier, in 1777, who first recog- 

 nized the exact similarity between the combustion of fuels and oxi- 

 dation as it occurs in the bodies of animals. The statement is 

 sometimes made that " food " is oxidized in our bodies to produce 

 heat and energy. It is well to keep in mind that this is true only in 

 the sense that our organic foods, broken down into their simple 

 structural units, are assimilated by the protoplasm of our cells. 

 The protoplasmic constituents, synthesized from nutrients deliv- 

 ered to the cells, are the compounds that unite with the oxygen. 

 Oxidation of carbohydrates and fats goes to completion in the 

 body and results in the liberation of energy, some of which is con- 

 verted into heat. The combustion of these classes of compounds 

 is the chief source of animal energy. Proteins are also oxidized 

 to some extent, but the incompleteness of the reaction makes it 



