UNDER the vague term digestion all these processes may be included to which the food substances are subjected and the resultant changes which the various food constituents undergo in order to prepare them for assimilation by the tissues. It therefore comprises a series of changes all tending towards the same point, namely, the transformation of substances which in their original state could not be made use of by the cell into pabulum which can be easily taken up, assimilated, and used for the nutrition of the tissues. The changes which the food undergoes are brought about by secretions, which special glands elaborate, and are carried out mainly by the action of enzymes (ferments). The decomposition of the food principles into smaller molecules takes place gradually, the final products of metabolism being found in the tissue cells. The final transformation always results in the production of work and heat, and products which cannot be used further by the organism are excreted. The food principles are the following: - Proteins, tats, carbohydrates, mineral matter, water.
In order to prepare the food for absorption it is necessary to break it down and transform it into material which can pass into the blood or lymph. This is effected in the alimentary canal, where the food is subjected to processes of two great types - mechanical and chemical. A short summary of the changes which the food undergoes in the alimentary canal is here given; this is followed by an account of the manner in which the food is absorbed, and of its fate after absorption, those points only being considered which are of practical importance in dietetics.
In the mouth the food is broken up into small particles by mastication, and moistened with alkaline saliva, in order to fit it for deglutition. A small part of the starch is converted into dextrin or maltose under the influence of the ferment ptyalin in the salivary secretion. Thorough mastication of the food is of the first importance, not only for enabling this oral digestion to be normally carried out, but as an aid to healthy gastric and intestinal digestion. Defective mastication with impaired oral digestion from a septic condition of the teeth or gums, is frequently referred to in later chapters as an important contributory factor in the etiology of disease.
In the stomach the action of the saliva may go on for an hour. At the end of this time the secretion of gastric juice excited by the presence of food and of alkaline saliva in the stomach is sufficiently abundant to neutralise and render acid all the gastric contents, and so stop the action of the ptyalin. Under the action of the gastric juice the greater part of the protein is dissolved and converted into syntonin, albumoses, or peptones. The connective tissues are also dissolved, setting free the fat, which floats about in a free state. At the same time some of the salts and sugar which have been swallowed, and the peptones formed from the food, are being absorbed by the gastric mucous membrane. For the first half-hour after ingestion of a meal of solid food the pylorus is firmly closed. At the end of this time it relaxes at intervals to allow the passage of the fluid parts of the trie contents, which are spoken of at this period as chymehi the duodenum the chyme comes into contact with the bile, the flow of which is also quickened under the influence of the secretin which is being poured into the circulation. This causes a precipitate in the chyme consisting of bile acids, syntonin, and albumoses. This precipitate is dissolved later on by the further operation of the pancreatic juice. Here the remaining digestive processes take place; the undigested proteins are dissolved, and the acid albumin and albumoses resulting from gastric digestion are converted into peptone and partially into leucine, tyrosine, and similar nitrogenous bodies (amino-acids). Starches are changed into dextrin and maltose, and under the further agency of the intestinal juice into dextrose. The fats are partially split up and emulsified.
The passage of food through the pylorus goes on for seven or eight hours after the ingestion of food, and towards the end of this time larger lumps of undigested material are allowed to pass on into the duodenum.
As the acid chyme is squirted through the pylorus into the duodenum, it comes into contact with the cells of the mucous membrane, and in them causes a conversion of pro-secretin into secretin (Starling). The latter is absorbed by the blood-stream and carried to the pancreas and liver, causing a flow of pancreatic juice and an increased secretion of bile. The strongly alkaline pancreatic juice neutralises the acid chyme. So long as the duodenal contents are acid the pylorus remains closed, but opens as soon as sufficient pancreatic juice has been secreted to neutralise the chyme that has already passed through. When this happens the pylorus relaxes, a fresh quantity of acid gastric contents is forced through, more secretin is formed, and the pancreatic secretion is again excited until the second amount of chyme is neutralised; and this self-regulating mechanism of the activities of the stomach, pancreas, and intestines goes on until all the food has passed through the pylorus. At the same time the pancreatic juice poured into the gut evokes a secretion of intestinal juice (Sueeus enterieus), which supplies the entero-kinase necessary for the formation of the proteolytic ferment of the pancreatic juice.
Throughout the whole of the small intestines active secretion and absorption are taking place, so that the amount of water in the intestinal contents in the lower part of the small intestine is about the same as in the upper part. The contents of the lower part acquire a distinct fa:cal odour from the indol and skatol produced by the action of putrefactive bacteria on the proteins of the food. In this part of the intestine carbohydrates are also acted on by bacteria, which produce from them organic acids, which exercise a restraining influence on the putrefaction of the proteins. In the large intestine the processes of absorption predominate over those of secretion; hence that part of the intestinal contents which has not been absorbed becomes less and less watery, and acquires the character of faxes, in which form it is periodically expelled from the body.
The faces consist mainly of the indigestible residue of the food, or of substances which have been taken in too large quantities to be digested, and contain -
(a) Cellulose, woody fibre, elastic tissue, keratin, and remains of muscle fibre, starch grains, and fat.
(b) The unabsorbable part of the digestive juices, such as mucin, bile pigments, cholesterin.
(c) Disintegrated epithelial cells from the intestinal mucous membrane, and indol and skatol, decomposition products derived from the proteins of the food.
(d) Various foims of bacteria, and especially members of the Bacillus colt group. The influence of bacteria on the digestive process is further considered on p. 15; their importance becomes much more pronounced in various pathological conditions.
Valuable information as to the motor power of the stomach and bowels is given by the use of bismuth test meals. Precise information as to the existence of gastric stasis, and stasis in the small or large intestine, is afforded by this means.