Contrary to the lay impression, it is exceedingly difficult to make a general comparison of foods as regards digestibility. Certain ingredients of diet require no digestion at all, as water, salines, etc. Regarding the digestion of haemoglobin, nucleins, lecithin, organic combinations of iodine as in thyroid, we have no definite knowledge. In other cases mechanic and chemic digestion are of very different relative importance. Carbohydrates and pro-teids pass through different stages of digestion. For example, a cooked starch is partially digested, both in the mechanic and the chemic sense, dextrin, as in bread crust, is still further digested; the double hexoses, cane sugar, maltose - the next step beyond colorless dextrin in the digestion of starches - and lactose, require inversion into single hexoses and of the latter, dextrose is the ultimate one ready for oxidation, while levulose and galactose require change into dextrose, probably effected by the liver.

While fats are split into glycerole and fatty acids, which combine, respectively, with water to form glycerine and with alkaline bases to form soaps, it does not appear that glycerine and soaps can be considered in any practical sense as nutrients. Indeed, there remains considerable mystery as to the exact nature of fat digestion.

The coagulation of protein constitutes a preliminary step in digestion. Especially important, because unique, is the provision for the coagulation of caseinogen of milk by rennet. The pancreatic and intestinal juices, as well as the gastric, coagulate caseinogen, although the pathologic instances in which the latter fails to coagulate milk are so rare and are so nearly confined to adults that it is difficult to explain the development of this factor of safety. It is still in dispute whether there is a separate rennet ferment or ferments, or whether pepsin, trypsin and the intestinal activator for tryptic digestion, also coagulate milk. While caseinogen is the only instance of a protein normally coagulated by the body, egg-albumin is a somewhat analogous, unique instance of a substance which, if not previously coagulated and if given in considerable quantities - 5 or 6 egg whites or more - is eliminated to some degree, as a foreign substance, by the kidneys. While the coagulation or analogous change of all proteins by cooking may be considered a step in digestion, it does not appear to be necessary - the principal value of cooking being to kill parasites including bacteria and to render the food more tasty and more amenable to mechanic comminution and softening. Paradoxically, while the artificial coagulation of milk by rennet may be considered a digestive process, it does not seem to add materially to the ease of further digestion in the alimentary canal, while the curd, if allowed to become tough, even if grated into fine particles, is less digestible than raw milk, although not to the degree ordinarily taught.

For some reason, it seems impossible to imitate normal digestion of proteins outside the body. So-called predigested protein occurs mainly as albumose, and if the process is allowed to proceed to the stage chemically recognized as peptones, the product is not only disagreeably bitter, but is actually toxic. When carried to the stage of amido-acid, it has commonly been stated that the nutritive value has been lost, though this has recently been denied. If so, it would seem that there is truth in the old theory that this stage in protein digestion represented a natural safe-guard against excessive protein nourishment, though, of course, not against the toxic effect of the waste products.

Any vegetable food that is rich in cellulose is relatively indigestible, not only in the sense that cellulose itself is not a nutrient, but that many inclosures of starch and other, nutrients escape digestion, with variable but considerable aggregate waste. The value of cellulose, especially that occurring in soft, thread-like or netted masses, in stimulating peristalsis must not be forgotten. Similarly, any dense animal structure, such as cartilage and tendon or fibrous tissue, is largely indigestible, and considerable waste also occurs when muscle is not cut, chopped or chewed into fragments at most half a centimeter in thickness. (See discussion of waste of nutriment.)

The older physiologies published elaborate tables of digestibility according to the time required for the stomach to empty itself. Modern research shows that, in the first place, this is not a test of digestion itself - gastric digestion being always incomplete - since the emptying of the stomach depends upon various and exceedingly variable factors, and since, in general, the more quickly food leaves the stomach, the less has it been digested.

E. Jessen found that the muscular fibre had disappeared microscopically 2 hours after the introduction of 6 grams of raw beef or mutton, 2 1/2 hours after the similar introduction of half-done beef or fully-cooked veal, 3 hours after thoroughly cooked beef or pork, 4 hours after beef overdone. Other writers have given shorter times.

W. G. Thompson has compiled the following table of "digestibility," based mainly on the average sojourn in the stomach, those most digestible being given first; the least digestible last. Oysters, soft-cooked eggs, sweetbread, boiled or broiled whitefish, blue fish, shad, red snapper, weakfish, smelt; boiled or broiled chicken, lean roast beef or beefsteak, scrambled eggs or omelette, roast or boiled mutton, squab or partridge, bacon, roast fowl, tripe, brains or liver, roast lamb, mutton or lamb chops, corned beef, veal, ham, duck, snipe, venison, rabbit or other game, salmon, mackerel or herring, roast goose, lobster or crabs, pork, smoked, dried or pickled fish and meats.

Providing that there is not undue stagnation in the stomach, digestive disturbance, and that the faeces contain no excess of undigested muscle, the exact time of digestion makes no difference and it is better to have the patient eat with relish a slowly digestible meat, such as ham, than to swallow beefsteak as a matter of duty.

It is quite impossible to compare the digestibility of different kinds of foods, for example, meats and fruit. Indeed, it is difficult to express the digestibility of fruits accurately. If not well masticated or otherwise comminuted, there is much waste. If thoroughly broken up, the water and salts are ready for absorption and the digestion of the sugars is a simple matter. There is very little fat present, and it is doubtful whether much of the protein is assimilated. Bananas must be considered as a class by themselves, since they are the only food in which notable quantities of raw starch are introduced. Obviously, they cannot be digested to any extent until they have passed out of the stomach, but if no fermentation occurs or interference with gastric motility, no harm results from the slow digestion.