Digestion (Lat. digerere, to dissolve or concoct), the liquefaction and preparation of the food in the alimentary canal. The organs by which this function is performed in the higher animals are the mouth, pharynx, oesophagus, stomach, and intestines, with their accessory salivary glands, pancreas, liver, and mucous follicles. The first act to which food is subjected is the mechanical division by the teeth. This process is important in order to reduce the food from a crude mass to a finely divided pasty condition, in which it is more readily and thoroughly affected by the digestive secretions of the alimentary canal. While some of the nutritive matters are dissolved in and absorbed directly from the stomach, others require further preparation, and are taken up by the vessels and absorbents of the intestines; by the time that the residue arrives in. the caecum, almost all the alimentary matter has been extracted, and the insoluble portions with the excess of biliary and mucous secretions are voided at the anal termination of the canal. The digestive process can hardly be separated from absorption, which takes up the nutritive materials, and assimilation, which converts them into a fluid resembling blood, poured into the circulation near the heart.

Though inorganic substances are necessary for the support of. the body, the organic alone are generally considered as food and as subjects for the digestive process. Organic substances used as food may be conveniently arranged under three heads: 1, the saccharine group, embracing substances composed of oxygen, hydrogen, and carbon, resembling sugar in composition, and readily convertible into it; such are starch, gum, woody fibre, and the cellulose of plants; 2, the oleaginous group, with a great preponderance of hydrogen and carbon, small proportion of oxygen, and absence of nitrogen, including vegetable oils and animal fats; 3, the albuminous group, containing a large proportion of nitrogen, comprising animal and vegetable substances allied in chemical composition to albumen and animal tissues. The saccharine substances taken as food do not directly form part of any animal tissue, but are decomposed in their passage through the circulation, and are thus employed in some unknown way in the nourishment of the body. Starch is converted into sugar during digestion, and the sugar thus formed, as well as that taken under its own form with the food, is decomposed and appropriated as above.

These substances, however, are not sufficient by themselves for the support or growth of the body, as is proved by the death from inanition of animals fed exclusively upon them. The articles of the albuminous group serve not only for nutrition, but for the maintenance of heat by their decomposition; the proportion of their four elements is the same in all, and they are all capable of reduction to a like condition by the digestive process, so that, as far as nutrition goes, the fibrine of animals, the albumen of eggs, the caseine of milk, and the gluten of wheat are equally acceptable to the organism. No one of these, however, is alone sufficient to support life. It is very remarkable, as Dr. Prout has observed, that milk, the only single article of food naturally provided for the continued growth of animals, contains albuminous caseine in its curd, a good deal of oily matter, and considerable sugar. - Supposing mastication to have been thoroughly performed, the food is first acted upon by the salivary fluid, which is secreted by the parotid, sublingual, and submaxillary glands, and the follicles of the mucous membrane of the mouth.

Saliva is but little heavier than water, contains minute corpuscles and epithelial scales, and in health has an alkaline reaction greatest during and after meals. It consists of about 995 parts of water in 1,000, and 5 parts of solid matters. Of the latter the most remarkable is ptyaline, to which the peculiar properties of the fluid are due; it closely resembles, but is not identical with, albumen and caseine; it acts the part of a ferment, and, according to Mialhe, 1 part is sufficient to convert 2,000 parts of starch into sugar; it also contains a compound of sulpho-cyanogen, not known to occur in any other animal product, and interesting in a medico-legal point of view. Its salts are nearly those of the blood, and its alkaline reaction seems to be due to the basic phosphate of soda. The "tartar" of the teeth and salivary concretions consist principally of earthy phosphates and animal matter. The limpid secretion of the parotid and sublingual glands saturates the food during proper mastication, while the viscid submaxillary fluid facilitates swallowing. The amount of saliva secreted daily by man will average, according to Bidder and Schmidt, 3 1/2 lbs., though it varies with the character and frequency of the meals.

Besides its mechanical action, the saliva, by its peculiar ferment, has the power of acting chemically upon the farinaceous elements of the food, leading to the conversion of the starch into sugar; but there is reason to believe that this action does not go on in presence of the acid of the stomach. There is no satisfactory evidence that saliva exerts any other than a physical action upon nitrogenized substances. When the food reaches the stomach the digestion is continued by the gastric juice, secreted by the numerous follicles of the mucous membrane, lined with glandular epithelium. The nature of the digestive process has been the subject of much speculation. It was at first supposed that the aliments underwent a coc-tion similar to that which they would experience in a vessel with hot water; to this succeeded the theory of acid fermentation, then of putrefaction, of trituration, and of maceration, till the present belief in the solvent action of the gastric juice was established. The gastric juice is transparent, nearly colorless, and without viscidity. Its most characteristic feature is acidity, which is even perceptible to the taste.

Many eminent chemists maintain that the real agent in the solvent process is free lactic acid, while others are in favor of free hydrochloric acid; the former opinion seems to be more fully borne out by the results of experiment. The peculiar organic ferment of the gastric juice is pepsin, which disposes albuminous matters to undergo solution by the contained acid, which they would otherwise only partially do unless exposed to a high temperature. The secretion of the empty stomach is neutral or alkaline, but it becomes acid on the introduction and during the digestion of food, resuming its neutral character when this process is finished. From the experiments of Dr. Dalton, it appears that an ounce of gastric juice of the dog will dissolve a little over 30 grains of fresh lean meat; at this rate the full digestion of a pound of raw-meat would require two gallons of gastric juice; and this apparently enormous quantity will not be considered incredible, if it be recollected that this fluid after it has done its work of solution is at once reabsorbed into the circulation, so that even this quantity might be secreted during the three or four hours of the digestive process, at an expense to the blood of not more than 2 or 3 oz. of fluid at any one time; the fluid does not accumulate in the stomach, but its watery portions are in continual process of secretion and reabsorption as long as any food remains undigested, within reasonable limits as to quantity ingested.

Many of the most important phenomena of gastric digestion were first demonstrated about 1830 by the experiments of Dr. Beaumont on Alexis St. Martin, through an opening in whose stomach the effect of food, stimulants, and sedatives could be seen. The color of the membrane was pale pink, its appearance velvet-like, and its surface lined with a transparent viscid mucus; the stimulus of food caused the gastric follicles to enter into activity, and to pour out the acid gastric juice; small quantities of very cold water, or ice, after the primary sedative effect, caused turgidity of the membrane and copious secretion, while ice in large amount and long continued retarded the process. The amount of gastric juice secreted depends on the requirements of the system, and not on the quantity of food taken into the stomach; this is most important to be remembered, since, after the fluid secreted has dissolved all it can, any excess of food must remain undigested, pass into the intestines in a crude state, and become a source of pain and irritation until it is expelled. When the system is diseased, there is no craving for food, which if taken would not cause the secretion of the gastric juice, but would remain undigested for an indefinite time, adding its irritation to the general diseased state.

The secretion of gastric juice is influenced by, though not dependent on, nervous agency; it is well known that strong mental disturbance will put a stop to the digestive process, and section of the pneumogastric nerves arrests for a time the elaboration of the gastric fluid. There can be no doubt that gastric digestion is essentially a process of chemical solution, the solvent fluid being prepared by the follicles of the stomach, and its action assisted by the peristaltic muscular movements of the organ; the experiments on St. Martin, and those subsequently performed on the lower animals, fully prove these facts, both in natural and artificial digestion. - Rapidity of digestion depends so much on the quantity and quality of the food, the state of health, the condition of the mind, and the habits of exercise, that it is difficult to determine the relative digestibility of different articles of diet. It appears from Dr. Beaumont's researches that, other things being equal, the flesh of wild animals is more easily digested than that of the allied domesticated races; in this respect venison stands first, then turkey, then beef, mutton, and veal, in the order mentioned.

A certain bulk of food is necessary for healthy digestion, as has long been practically known by uncivilized nations; soups and fluid aliment are not more readily chymified than solid substances, and cannot alone support the system in vigor. Moderate exercise before a meal facilitates digestion. A temperature of 98° to 100° F. is requisite for the perfect action of the gastric juice; hence the ingestion of cold and iced substances, if carried to such an extent as to depress the temperature of the stomach, must be very prejudicial to digestion. The most recent experiments go to show that the action of the gastric juice is confined to nitrogenized substances, and that it exerts no influence on starchy, saccharine, or oily matters. Its action on albuminous matters is to reduce them to a complete solution, alter their chemical properties, and convert them into albuminose (a kind of modified albumen), in which form they are readily assimilated. In this condition they form definite combinations with the solvent liquid, which have been called peptones. These are not mere solutions of the respective substances in acidulated fluids; for a converting power is exerted by the pepsin, the solvent power being due to the acid of the gastric juice.

The process of digestion is far from being completed in the stomach, but goes on in the intestine by the continued action of the gastric juice, as well as by that of the pancreatic and intestinal juices. Of these, the intestinal juice seems to have the power of rapidly transforming starchy matters into a form of sugar, while the pancreatic juice, on coming in contact with the fatty elements of the food, converts them into a finely divided milky-looking emulsion, known as chyle. All these substances, the fluid products of digestion, are then gradually taken up by the blood vessels and lac-teals of the alimentary canal, and mingled with the general mass of the circulating fluid; until in the lower part of the intestine there are left only the indigestible and refuse parts of the food, mingled with the excrementitious substances of the large intestine. (See Aliment, Alimentary Canal, and Dietetics.)