If we regard chymification as going on at the surface of every individual portion of the food, we at once see that the constant motion of the stomach serves, not only to affect the original mixture of the food with the gastric juice, but to remove the chyme from the surface of each little fragment of the alimentary mass in proportion as it is formed, and by this very removal to expose a fresh surface of the fragment to the contact of the mucous membrane, thereby enabling it to excite a farther secretion of the gastric juice, where, as sometimes happens, the stomach is unable to provide a sufficient quantity from the beginning. In this way the formation and removal of chyme go on from the very first, although, of course, more slowly than after the gastric solvent has had time to act. It is generally said that an hour elapses before any chyme is formed; but Dr Beaumont has detected its existence at a much earlier period, and is of opinion that, from the uniform and constant decrease in the contents of the stomach, which begins as soon as the food is swallowed, chymification commences almost immediately. This decrease, though slow at first, becomes gradually accelerated, till the whole mass is converted into chyme. Apparently in harmony with this more energetic action, the acidity of the gastric fluid also becomes greater, and affords a greater stimulus, in proportion as digestion advances.
As formerly explained, the thickish, semi-fluid, greyish, chyme, into which the aliment is converted, is gradually impelled towards the pyloric extremity of the stomach. On its arrival there, the pylorus, or valve between the stomach and the intestine called the duodenum, opens and allows the chyme to pass into the intestine. But, by a curious mode of sensibility, if any portion of undigested food be mixed with it, the pylorus contracts upon it, refuses it egress, and throws it back into the stomach for farther digestion.
If, however, any thing really indigestible finds its way into the stomach, and presents itself at the pylorus—or if the stomach has temporarily lost its digestive power, and the food remains in it for many hours unchanged—then the pylorus, after repeatedly refusing egress, at last opens and allows it to pass into the gut. So marked is the contractile impulse towards the pylorus when digestion is going on, that Dr Beaumont found even the bulb of his thermometer carried down with a steady and considerable force.
Such are the direct conditions requisite for the fulfilment of digestion: but there remain others, of an indirect kind, which also require to be noticed. Of these, a due supply of arterial blood and nervous energy is the most remarkable; but as both produce their effect by modifying the secretions and motions of the stomach, already described as the direct requisites of digestion, it will save a good deal of repetition if, for the present, we take their influence for granted, and reserve their further elucidation till we come to treat of the practical applications of the preceding exposition.
Having thus obtained a comprehensive view of the agents employed in effecting digestion, and of the changes produced by it on different kinds of food, we find another important subject of investigation, immediately connected with the process, presenting itself—the comparative digestibility of different kinds of food. Dr Beaumont did not neglect this branch of the inquiry; but the experiments which he performed for its elucidation are, like those of most of his predecessors, deprived of great part of their value by the vague way in which they seem to have been conducted, and the common omission of all particulars in regard to those conditions which are known to exert a powerful influence on the progress of digestion.
The following Table, which I have arranged in a more lucid order than Dr Beaumont has done, exhibits the general results of all the experiments made upon St Martin posterior to 1825; and the average is deduced from those which were performed when the stomach was considered by Dr Beaumont to be in its natural state, and St Martin himself subjected to ordinary exercise.
TABLE shewing the Mean Time of Digestion of the different Articles of Diet.
| Articles of Diet. | Mode of Preparation. | Time required for Digestion. | |
| H. | M. | ||
| Rice | Boiled | 1 | |
| Pig’s feet, soused | Boiled | 1 | |
| Tripe, soused | Boiled | 1 | |
| Eggs, whipped | Raw | 1 | 30 |
| Trout, Salmon, fresh | Boiled | 1 | 30 |
| Trout, Salmon, fresh | Fried | 1 | 30 |
| Soup, barley | Boiled | 1 | 30 |
| Apples, sweet and mellow | Raw | 1 | 30 |
| Venison steak | Broiled | 1 | 35 |
| Brains | Boiled | 1 | 45 |
| Sago | Boiled | 1 | 45 |
| Tapioca | Boiled | 2 | |
| Barley | Boiled | 2 | |
| Milk | Boiled | 2 | |
| Liver, beef’s, fresh | Broiled | 2 | |
| Eggs, fresh | Raw | 2 | |
| Codfish, cured, dry | Boiled | 2 | |
| Apples, sour and mellow | Raw | 2 | |
| Cabbage, with vinegar | Raw | 2 | |
| Milk | Raw | 2 | 15 |
| Eggs, fresh | Roasted | 2 | 15 |
| Turkey, wild | Roasted | 2 | 18 |
| Turkey, domestic | Boiled | 2 | 25 |
| Gelatine | Boiled | 2 | 30 |
| Turkey, domestic | Roasted | 2 | 30 |
| Goose | Roasted | 2 | 30 |
| Pig, suckling | Roasted | 2 | 30 |
| Lamb, fresh | Broiled | 2 | 30 |
| Hash, meat and vegetables | Warmed | 2 | 30 |
| Beans, pod | Boiled | 2 | 30 |
| Cake, sponge | Baked | 2 | 30 |
| Parsnips | Boiled | 2 | 30 |
| Potatoes, Irish | Roasted | 2 | 30 |
| Potatoes, Irish | Baked | 2 | 30 |
| Cabbage, head | Raw | 2 | 30 |
| Spinal marrow | Boiled | 2 | 40 |
| Chicken, full grown | Fricassee | 2 | 45 |
| Custard | Baked | 2 | 45 |
| Beef, with salt only | Boiled | 2 | 45 |
| Apples, sour and hard | Raw | 2 | 50 |
| Oysters, fresh | Raw | 2 | 55 |
| Eggs, fresh | Soft boiled | 3 | |
| Bass, striped, fresh | Broiled | 3 | |
| Beef, fresh, lean, rare | Roasted | 3 | |
| Beef, steak | Broiled | 3 | |
| Pork, recently salted | Raw | 3 | |
| Pork, recently salted | Stewed | 3 | |
| Mutton, fresh | Broiled | 3 | |
| Mutton, fresh | Boiled | 3 | |
| Soup, beans | Boiled | 3 | |
| Chicken soup | Boiled | 3 | |
| Aponeurosis | Boiled | 3 | |
| Cake, corn | Baked | 3 | |
| Dumpling, apple | Boiled | 3 | |
| Oysters, fresh | Roasted | 3 | 15 |
| Pork-steak | Broiled | 3 | 15 |
| Pork, recently salted | Broiled | 3 | 15 |
| Mutton, fresh | Roasted | 3 | 15 |
| Bread, corn | Baked | 3 | 15 |
| Carrot, orange | Boiled | 3 | 15 |
| Sausage, fresh | Broiled | 3 | 20 |
| Flounder, fresh | Fried | 3 | 30 |
| Catfish, fresh | Fried | 3 | 30 |
| Oysters, fresh | Stewed | 3 | 30 |
| Beef, fresh, dry | Roasted | 3 | 30 |
| Beef, with mustard, &c. | Boiled | 3 | 30 |
| Butter | Melted | 3 | 30 |
| Cheese, old, strong | Raw | 3 | 30 |
| Soup, mutton | Boiled | 3 | 30 |
| Oyster soup | Boiled | 3 | 30 |
| Bread, wheaten, fresh | Baked | 3 | 30 |
| Turnips, flat | Boiled | 3 | 30 |
| Potatoes, Irish | Boiled | 3 | 30 |
| Eggs, fresh | Hard boiled | 3 | 30 |
| Eggs, fresh | Fried | 3 | 30 |
| Green corn and beans | Boiled | 3 | 45 |
| Beet | Boiled | 3 | 45 |
| Salmon, salted | Boiled | 4 | |
| Beef, fresh, lean, | Fried | 4 | |
| Veal, fresh | Broiled | 4 | |
| Fowls, domestic | Boiled | 4 | |
| Fowls, domestic | Roasted | 4 | |
| Ducks, domestic | Roasted | 4 | |
| Soup, beef, vegetables, and bread | Boiled | 4 | |
| Heart, animal | Fried | 4 | |
| Beef, old, hard, salted | Boiled | 4 | 15 |
| Pork, recently salted | Fried | 4 | 15 |
| Soup, marrow bones | Boiled | 4 | 15 |
| Cartilage | Boiled | 4 | 15 |
| Pork, recently salted | Boiled | 4 | 30 |
| Veal, fresh | Fried | 4 | 30 |
| Ducks, wild | Roasted | 4 | 30 |
| Suet, mutton | Boiled | 4 | 30 |
| Cabbage, with vinegar | Boiled | 4 | 30 |
| Suet, beef, fresh | Boiled | 5 | 30 |
| Pork, fat and lean | Roasted | 5 | 15 |
| Tendon | Boiled | 5 | 30 |
This table is very interesting, but the results must not be too much relied upon, or regarded as representing the uniform rate of digestibility. We have already seen that chymification is greatly influenced by the interval which has elapsed since the preceding meal, the amount of exercise taken, the keenness of the appetite, the state of the health and mind, the completeness of the mastication, the state of rest or exercise after eating, and various other circumstances; and, above all, the quantity swallowed in proportion to the gastric juice secreted. And consequently, if an experiment be made without regard to these conditions, and without any thing being recorded except the time occupied in digestion in the individual case, the conclusions deduced from it may be most fallacious. The very aliment which, taken in full quantity, remains on the stomach for hours, may, in a smaller quantity, be entirely digested in one-third of the time. Thus, in the foregoing table, two and a half hours are set down as the average period required for the chymification of jelly; and yet, in one of Dr Beaumont’s experiments, we find that eight ounces of that substance were entirely digested in ONE hour. So that, if all the other conditions are not carefully kept in view at each trial, the results cannot possibly be held as conclusive.