But these physical changes are not sufficient. That the food may be yet more completely simplified and dissolved, certain juices are poured out along the digestive tract which break up the food molecules into still more simple forms that the next important step may take place with the greatest ease and completeness. Thus chemical action in addition to mechanical action prepares the food for the process of absorption into the body proper, where it may repair and build the tissues and produce the necessary heat and energy for the efficient conduct of all body processes.

♦The chemical formula for starch is (C6H10O5)n. The formation of starch in the plant may be represented by the chemical equation: 6 CO2 (carbonic acid gas) + 5 H1O (water) = CeHioOs (starch) + O12 (oxygen).

The starch remains as a part of the plant - the oxygen returns to the air.

In this digestive process all starch is changed to sugar (See footnote page 72), all complex sugars to simple sugar or dextrose, fats are emulsified, and proteins are reduced to simpler forms called proteoses, peptones, and amino-acids. Many of these processes can be carried on to some extent outside of the digestive tract; thus we have certain predigested foods such as dextrinized cereals in which the starch has been largely changed to sugar by extreme heat, e. g. oven toast, shredded wheat, corn flakes, etc. Likewise fats may be eaten in an already emulsified form as in nut butter and cream. The sugar of fruit being in the form of dextrose and laevulose (See footnote page 74), needs but little digestion.

Starch digestion begins in the mouth and is completed, with the simplifying of the complex sugars, in the small intestine. The solution of protein and its change into proteoses and peptones begins in the stomach and is completed, with its final reduction to amino-acids, in the intestine. The preparation of fat for absorption is carried on entirely in the intestines; first by a process of saponification (soap formation) and emulsification, then by a splitting up into more simple parts (See page 68).

A Carburetor

Thus the digestive tract becomes a great preparation chamber, a carburetor, as it were, where the food is transformed into a simple state and put into solution so that it can easily be taken up by the blood and thus carried to and properly utilized by the body cells.

Strange as it may seem, the digestive tract must be considered physiologically as outside of the body proper. It is simply a tube extending through the body, but not connected with it except by an absorbing medium, the mucous membrane. It is continuous with the outside world from which it receives food substances and into which are discharged those parts which cannot be prepared for reception by the blood and tissues. The lining membrane of the digestive tract tries carefully to guard the body cavity against intrusion into it of substances which are undesirable or might do harm and, normally, after careful preparation only that part of the food which can serve as material to replace worn out tissue or as fuel to produce heat and energy is passed on into the blood.

A Sentinel

However, as the result of putrefactive processes going on in the intestine certain poisons are also carried through the mucous membrane. These would quickly prove fatal were it not for the faithful liver which stands as a sentinel to prevent these toxic materials from gaining entrance into the general circulation. The liquified and simplified food passing through the mucous membrane of the small intestine enters many tiny bloodvessels or capillaries which carry it to the portal vein through which it is carried to the liver. Here the poisonous substances are filtered out to be transformed by the liver cells into harmless materials and the purified food, passes on into the general blood stream.* The liver also acts as a great storehouse for sugar, this part of the food entering the system according to body demands.

If, as a result of dietetic errors, intestinal putrefaction is excessive, the amount of toxic material taken up by the great absorbing surface of the small intestine may be so great that even the ever vigilant liver is unable to filter out these poisons and to prevent their entrance into the blood where they are carried throughout the body causing various kinds and degrees of ill health.

♦The digested fat, called chyle, does not pass with the other food to the liver, but is absorbed directly into the lymphatic system.

The Cycle Of Life

Even though the food has been received by the blood, its analysis is not yet complete but is carried on still further in connection with a form of oxidation or combustion. During this process the nutritive substances are utilized by the tissues and finally, being reduced to a simple state, they are eliminated as waste by the lungs, skin and kidneys. Now they can again be taken up by the plant and combined into materials which may be used as food by animals and man. Thus the cycle of life goes on, nothing ever lost, each chemical element being used again and again.

Just how these final steps in the process of food analysis are carried on in the tissues, we will consider further in the next chapter, in connection with the study of metabolism.

"Down to the last detail the world is made for what is in it; and by whatever process things are as they are, all organisms find in surrounding Nature the ample complement of themselves." - Drummond.