The processes of nutrition are:

1. Ingestion, for which may be substituted more or less satisfactorily, rectal injection, inunction of fats, subcutaneous injection of salts, dextrose, etc., in solution, as well as gav-age, or feeding through a fistula;

2. Digestion, which does not apply to inorganic matters nor to organic foods already in the state suitable for absorption. Digestion is commonly divided into mechanic, including the comminution and softening of food, and chemic;

3. Absorption, through animal membrane, into the blood or lymph vessels;

4. Assimilation, which literally implies that the food is rendered like the body itself, and which, therefore, refers strictly only to the utilization of certain foods to repair or form tissue; the term is, however, usually extended to indicate the final utilization of any organic, or even inorganic, substance. Following assimilation, after a shorter or longer period, there is 5. Catabolism, which, like digestion, applies only to certain organic substances and which usually occurs in successive steps, often involving several organs:

6. Elimination of ultimate waste products, by the skin, lungs, kidneys and, to some extent, through the liver directly or even through the alimentary mucous membrane, independently of the liver. Substances which escape absorption in the alimentary canal are wasted rather than waste products.

Chemic digestion occurs as follows:

1. Proteins. In the stomach, a variable fraction is converted into acid albumin and then into albumoses and peptones, and part into amido-acids under the joint action of hydrochloric acid and pepsin. When the meal is very small and liquid or pultaceous, as after unusually careful mastication, probably nine-tenths of the protein is digested in the stomach. On the other hand, when the meal is large, and the protein is contained in large masses as of meat, milk curds, bread, etc., probably not more than a tenth is digested.

The pancreatic ferment, trypsin, when activated by a duodenal secretion (not a ferment) digests most of the remainder of the albumin, into substances similarly named, except that alkali-albumin is substituted for acid albumin, but which are not exactly identical with those produced by peptic digestion.

2. Albuminoids, etc., seem to be digested analogously to proteins, but the exact nature of the process is not so well understood.

3. Carbohydrates. Cellulose is not digested at all by human ferments, but is decomposed into methane and hydrogen, carbon dioxid and water, by bacteria, in the intestine. Being indigestible and innutritious, it is commonly excluded in dietetics, when the term carbohydrate is used without quali-cation.

Starch, when cooked so as to disintegrate the cellulose envelopes of the granules, is digested by ptyalin of the saliva, into maltose, a disaccharid, through intermediate stages of dextrins. Dextrins, as in bread crust, are, of course, also digested into maltose. With ordinary, imperfect mastication and insalivation, ptyalin digestion does not amount to much, although it proceeds in the stomach for an hour or so, till a considerable degree of hydrochloric acidity stops it. Certain rodents have a very powerful salivary ferment which will digest the raw starch of nuts, grains, etc.

Whatever cooked starch and dextrin is left after salivary digestion - usually a large majority of what has been taken - as well as any raw starch - particularly in bananas - is similarly transformed by amylopsin of the pancreas, which is about 40 times as strong (or possibly as concentrated) as ptyalin.

The disaccharids, saccharose (cane, maple and beet sugar), lactose (milk sugar) and maltose, as ingested or prepared by the salivary and pancreatic ferments, are split into monosaccharids by an invert ferment of both the stomach and intestine.

The monosaccharids, dextrose, levulose and galactose are utilizable as such, though, apparently, large amounts only of the first can be absorbed without producing a waste through the kidneys.

4. Fats are split to a very slight degree in the stomach. In the intestine they are broken up into fatty acids and glycerole, under the action of steapsin of the pancreas. The former unite with the alkaline carbonates of the bile, pancreatic and intestinal juices, to form soaps, the latter with water to form glycerin.

Absorption: Proteins and carbohydrates are absorbed mainly into blood capillaries, whence they reach the liver by the portal vein; fats by lacteals, whence they reach the innominate vein through the thoracic duct. Part of each takes the opposite course. Peptones are apparently reconverted into an albumin during absorption; at any rate, artificial peptones directly introduced into the circulation are toxic. Fats also seem to be re-combined during absorption and, possibly, are further elaborated in the mesenteric lymph nodes. Carbohydrates, absorbed as dextrose and to some extent as the other monosaccharids, are converted by the liver into glycogen and thus stored in the liver, to be reconverted into dextrose (apparently even if the glycogen has been derived from one of the two other monosaccharids) and are fed into the blood so as to maintain as nearly as possible the proportion of 9/10 per mille. Secondary minor reserve stores of glycogen are also found in glands and muscle.

Soluble substances generally are absorbed without digestion, through the alimentary mucous membrane, but raw albumin and sugars not taken in too great amount, are changed to the appropriate forms. If absorbed in too great quantity, they are eliminated in the urine. So, too, when dextrose accumulates in the blood in too great quantity, as in diabetes, the excess is eliminated.

The stomach is not adapted to absorption, even of water, though doubtless some absorption occurs through it. Owing to secretions, the alimentary contents become more watery in passing through the stomach and upper small intestine, so that no great net amount of absorption of water occurs till the caecum and ascending colon are reached.

In addition to digestive changes, all kinds of organic foods are more or less attacked by bacteria and yeasts, especially in the bowel. The present opinion is that bacterial digestion does not assist in the preparation of nutriment for the body, and that it is rather a waste.

Under normal conditions, no appreciable amounts of pro-teid or carbohydrate should appear in the faeces, except in insoluble masses such as unchewed peas, beans, corn, dense tendon ends, lumps of vegetable tissue, etc. Fat, on the other hand, is always wasted in appreciable amounts, about 10% on a normal diet containing 50 - 100 grams, relatively more for larger or for very small amounts.

Practically nothing is known of the digestion, absorption and assimilation of lecithin, iodothyrin, etc.

Catabolism and Excretion. Dextrose, representing carbohydrates, is oxidized in the blood (or cells) mainly under the influence of a glycolytic ferment whose source, at least for the most part, is probably an internal secretion of the islands of Langerhans in the pancreas. At least, many cases of diabetes are definitely shown to have a lesion of these islands. The products are carbon dioxid and water. Oxidation without a ferment is too slow to account for the physiologic process.

The exact catabolism of proteins is not known. The nitrogen is mainly eliminated as urea, formed mainly, at least, in the liver. In grave forms of diabetes, part of the protein molecule is transformed into sugar, and since fat may be deposited on a protein or carbohydrate diet, it is obvious that part of the protein molecule may be either indirectly or directly transformed into fat. Creatinin is undoubtedly derived from creatin which is formed in muscles from protein, either of the food itself directly, or more probably of the muscle, indirectly derived from food.

Nucleins yield various purin bodies, of which uric acid is the best known. Whether, as was formerly taught, uric acid is also derived from proteins directly, as the result of iucomplete oxidation into urea, is not positively known.

Fate, like carbohydrates, yield carbon dioxid and water as end products but also intermediate products consisting of fatty acids, and, under some conditions, acetone. The oxidation of fats unquestionably requires a ferment whose source is unknown, though the obesity of eunuchs and of women after the menopause or oophorectomy, suggests an internal secretion of the sexual glands.

Of the waste products of the body, the water is eliminated by the skin, kidneys and lungs, and also a small amount by the bowel; the normal quantitative relations are disturbed by diaphoresis and, to a less degree, by diuresis and diarrhoea. Nearly all of the carbon dioxid is eliminated by the lungs, but small amounts are found in all excretions. The saline material that has been absorbed is eliminated mainly through the urine, but considerable is also eliminated by the skin. Nitrogenous waste is eliminated to the extent of about 6/7 through the kidneys, most of the remainder and the most toxic portion, is eliminated through the liver and bowel, minute quantities through the lungs and skin, through the latter mainly in cases in which the kidney function is disturbed.

Under ordinary diet, about half of the faeces consists of undigested and unutilized ingesta, including strictly indigestible material, and about half of shed epithelium and mucous, more or less altered, and of waste matter in the strict sense.