For his complete nutrition man must have in his food the albuminoid materials of which his tissues are mainly composed, the iron and the salts contained in those tissues and in the blood, and fatty matter, or some substance which can readily be converted into fat, which enters into the composition of his body, and which serves to maintain the animal heat. (See Abstinence, Aliment, Animal Heat, and Digestion.) But food must not only contain all the principles necessary to nutrition, it must likewise be digestible and assimilable; it must be capable of being disintegrated and dissolved in the alimentary canal, so that it may be absorbed, and finally converted into blood from which the waste of the tissues may be supplied. An article may be highly nutritious, yet exceedingly indigestible; or it may be easily digestible, and afford little nutriment. While certain articles and classes of articles are in general more digestible, there is no rule of invariable application. There are differences in kind as well as in degree in the . digestive powers of different individuals; and what will offend the stomach of one man, another will digest with ease. But aside from individual peculiarities, there are more general causes of difference. 1. Habit has great influence.

What men have been accustomed to, they digest with greater facility. An American or Englishman visiting the continent of Europe is frequently attacked with diarrhoea, from an unaccustomed diet, which is in itself equally wholesome with his own. During the revolutionary war numbers of the troops from the southern states while on duty at the north became ill, and their health was only restored by an allowance of fat bacon. The ill-fed Irishman, on enlisting into the British army, frequently is affected with what is termed a "meat fever;" his new diet is so much superior to what he was accustomed to, that his organs do not readily adapt themselves to the change. 2. Circumstances have a great influence on the digestibility of food. A diet suited to Labrador would be oppressive and injurious in the West Indies. The season, amount of clothing, exposure, and exercise have an influence on the digestive capacity as well as on the requirements of the system. 3. The digestibility of food is much influenced by our liking for it; within certain limits, what we are fond of agrees with us, and what we dislike is not apt to digest well. The high flavor which excites the appetite of the epicure provokes nausea in a less cultivated stomach.

Still, despite the various sources of diversity, some articles are for the majority of men of comparatively easy digestion, while others are assimilated with greater difficulty. - Food is commonly classed as animal or vegetable. Animal food may be subdivided into the flesh of mammals, birds, fishes, reptiles, crustaceans, and mollusks. The flesh of the mammals, and indeed of the birds and fishes used for food, differs very little in chemical composition. The fibrine, albumen, and gelatine of which chiefly they are made up, may be considered as chemically identical, from whatever animal they may be derived. The fats differ in the relative proportions, and sometimes in the character, of the fatty acids which enter into their composition; the saline matters, varying in their proportions, are mainly of the same character; while the immense variety of flavors by which they are distinguished depend upon principles existing in exceedingly minute proportions, and for the most part soluble in water. The difference in meats arises from the varying proportions of fibrine, gelatine, and fat, and from variations in mechanical texture; and to these circumstances is due their difference in digestibility.

Whatever renders the animal fibre harder, makes the meat less digestible; whatever renders it more delicate and tender, more easily separated and disintegrated, makes it more easily soluble in the juices of the stomach. Provided an animal has reached maturity, the tenderness of its meat is increased by youth, by its not having been worked, by its being in good condition, the muscular fibres interpenetrated and separated by minute proportions of fatty tissue. Keeping tends very much to improve the tenderness of meat. Few animals are fit to be eaten the day they are killed; but when kept, long before the slightest taint can be detected, a change takes place that renders the fibres more easily separated and disintegrated, more readily broken down and comminuted during mastication, and more quickly reduced and assimilated by the stomach. Of the different meats, venison that has been well kept is, in-its season, perhaps the most tender and digestible. Dr. Beaumont found that in St. Martin a meal of broiled venison steak was completely digested and removed from the stomach in an hour and a half, a shorter time than was required by any other meat. Wether mutton of a proper age, that has hung for a sufficient length of time, is scarcely inferior in digestibility to venison.

Beef ranks next to mutton. Lamb and veal are less digestible than mutton or beef, and veal is less readily digested than lamb. Of all the meats in ordinary use, pork is most refrac-tory to the gastric juices; and, contrary to what holds with regard to beef and mutton, the sucking pig is more digestible than pork. The fat of meats generally, and all varieties of fatty matters, are difficult of assimilation; they are particularly offensive to weak stomachs, sometimes appearing to form an oily pellicle, which, floating on the partially chymified mass, becomes rancid and occasions distressing heartburn and nausea, or causes eructations of acrid matter which leave a peculiarly disagreeable taste upon the palate. The mode of dressing meat has a great influence upon its digestibility; that which agrees best with the majority of stomachs is broiling. The fire should be brisk, so that the albumen on the surface of the meat may be rapidly coagulated; this preserves the juices, and it is rendered at once more savory and more tender. The same rule applies to boiling and roasting.

When the meat is to be cooked, if boiled, it should be at once plunged into boiling water; while if soup is to be made, the meat should be put into cold water and the temperature slowly raised, thus extracting its nutritious fluids to the greatest possible extent. Of all methods of cooking, frying is the most objectionable; not only is the meat rendered harder than when boiled, and thus more indigestible, but it becomes imbued with boiling fat, and is thus rendered still more refractory to the gastric juice. Rich stews are objectionable on the same account. By the action of salt on muscular flesh, the juices of the meat are abstracted; in this manner not only is its nutritive value impaired, but it is rendered harder and drier and consequently more indigestible; the longer the flesh is exposed to the action of salt, the harder and drier it becomes. Perhaps all fats form an exception to the fact that meat is rendered more indigestible by salting; they have little water to lose, and their texture consequently cannot become consolidated; fat pork is even rendered more digestible by salting.

St. Martin, according to Dr. Beaumont's observations, digested recently salted pork when raw or broiled in from 3 to 3 1/4 hours; the same article fried occupied him 4 1/4 hours for its reduction; while fresh pork, fat and lean, roasted, required 5 1/4 hours. On the other hand, boiled fresh beef with a little salt was digested in 2 3/4 hours, while old salted beef required 4 1/4 hours when dressed in the same manner. All em-pyreumatic substances impair digestion by interfering with the action of the pepsin, which is the principal solvent agent of the gastric juice. In this manner smoking impairs the digestibility of meat; few things are more difficult of management by a feeble stomach than old and well smoked beef. St. Martin found fowls, roasted or boiled, of slower digestion than beef; ducks and geese, as might be supposed from the amount of fat they contain, are assimilated with difficulty. There is, however, so much variation in this respect in different individuals, that the absolute digestibility of an article of food can hardly be deduced from experiments on a single person. Fish furnishes an abundant and digestible variety of food.

The dry, white sorts, cod, haddock, bass, etc, are the most digestible; while the richer kinds, salmon, shad, mackerel, eels, etc, are less apt to agree with the stomach. St. Martin digested boiled or fried salmon trout in 1 1/2 hour, boiled dried cod in 2 hours, fried catfish in 3 hours 20 minutes, and boiled pickled salmon in 4 hours. Milk, the only food during the earlier months of infancy, contains from 12 to 13 per cent. of solid matter, about one half of what is contained in flesh; it is poorer in nitrogenous and richer in carbonaceous food; its ash furnishes but 0.47 per cent. of iron, while those of flesh and wheat flour yield 1 per cent. It is not digested so quickly as would be supposed, and in this respect boiled has the advantage of unboiled milk; the one took St. Martin 2 hours, the other 2 1/4, to convert into chyme. Milk disagrees with many persons; this is often connected with the readiness with which it undergoes change when exposed to the atmosphere, and this change commences long before it can be recognized by the taste. Milk just drawn from the cow agrees perfectly with some persons who are unable to take it a few hours later.

When cows are kept in an impure and confined atmosphere and badly fed, it has been conclusively shown that their milk produces disturbance of the digestive organs and diarrhoea in infants who are fed upon it. The caseine of milk, coagulated, generally mixed with more or less butter, and pressed so as to free it from the whey, constitutes cheese. Its richness varies with the quantity of butter it contains; some varieties, Stilton for instance, are made from milk to which an additional quantity of cream has been added. Salt is used to preserve it, and some kinds, as Dutch cheese, are very highly salted. When cheese is kept for a length of time, it undergoes a number of changes, partly dependent on the liberation of the volatile fatty acids existing in the butter, partly, in the richer varieties, on the commencement of putrefactive fermentation. The firm, close texture of cheese renders it always hard of digestion, and the rich and strong-smelling varieties are particularly to be avoided by delicate stomachs.

Fresh sweet butter is, perhaps, the most wholesome and digestible of fatty matters; by heating or rancidity its digestibility is greatly impaired. - Of farina-ceous articles, light well made wheaten bread, from 12 to 24 hours old, is the most generally digestible; warm bread is indigestible, because it forms a tough mass not readily penetrated by the saliva and rebellious to the gastric juices. Unleavened bread, maccaroni, and vermicelli are wholesome, and agree well with the stomach; on the other hand, flour combined with fatty matter, whether in the form of pastry, cake, or pudding, is more or less indigestible, according to its texture and richness. Next to wheat flour, rye affords the best and most wholesome bread. In various countries oatmeal, barley, and maize are used as substitutes for wheat; they form kinds of bread wholesome enough for those habituated to their use, but apt to disagree with strangers. In tropical countries rice to a great extent takes the place of the other cereals, and perhaps a larger population mainly subsist on it than on any other single article of food. It i affords very little of plastic or blood-making material, and hence when taken alone is consumed in enormous quantity; as an adjunct it forms an unstimulating and digestible article of food.

The leguminous seeds, peas and beans, afford a nutriment rich in plastic matter, but hard of digestion and predisposing to flatulence. The popular prejudice that sugar produces caries of the teeth has no good foundation. Closely allied to sugar are the various forms of fecula, arrowroot, tapioca, sago, potato starch, etc. They consist of minute granules composed of concentric layers, and termed the starch grains. These grains must be softened and hydrated by boiling, roasting, or panification before the starch is lit for use. It then forms an unstimulating and readily digestible ingredient of the food. Vegetables constitute an important part of our diet. With few exceptions their nutritive value is low; they consist largely of water holding organic-salts in solution, of starch granules, of small quantities of albuminous matter, and of cellulose and epidermis. The cellulose, though possessing a chemical constitution identical with that of starch, when at all firm, resists the action of the gastric juice, and passes unchanged through the intestinal canal. They are valuable on account of their large quantities of organic salts, of the bulk which they give to the food, and of their stimulating effect upon the peristaltic action of the intestines.

These latter qualities make them disagree where the digestive organs are feeble and irritable. They are digestible in proportion to their tenderness and the readiness with which they can be broken up into a pulp. The potato is one of the most valuable of the nutritious vegetables. St. Martin found potatoes roasted and baked disposed of more readily than when boiled, the one taking 2 1/2 hours to be converted into chyme, the other an hour longer. The same rule applies to fruits as to vegetables; they are digestible just in proportion to the readiness with which they can be completely reduced to a pulp. Ripe strawberries, peaches, oranges, and grapes rarely disagree, while cherries, apples, pears, etc, are more indigesti-ble; roasting improves the digestibility of apples and of most of the more solid fruits.