It may be said that, on the occasion just alluded to, St Martin’s digestion must have been particularly good—and, in truth, it seems to have been so; for at nine o’clock A. M. he breakfasted on soused tripe, pig’s feet, bread and coffee, and yet, only one hour later, no vestige of any of these savoury things remained in the stomach. What renders this result the more remarkable, is the fact, that, in another table, a simple breakfast of coffee and bread is set down as having required FOUR hours for its digestion. The rapid disposal of the same elements with the addition of soused tripe and pig’s feet, instead of disproving my position, evidently strengthens it, by shewing that, if from any cause the digesting power varies in intensity, the result obtained from the experiment on one kind of food, cannot, with any shew of reason, be considered as an accurate index of its rate of digestibility in comparison with that of other kinds.
This neglect of the other conditions is accordingly the circumstance which throws a doubt over the results not only of Dr Beaumont’s experiments, but of those of every other inquirer. Dr Beaumont indeed candidly admits, that his were performed for the purpose of demonstrating other important principles connected with digestion, and not at all with the view of determining the comparative rates of digestibility of different kinds of aliment; and in alluding to the various requisites for a satisfactory series of experiments, he himself justly states, that this would be a Herculean task which it would take years to accomplish. In considering the following general results, then, the reader ought to bear in mind that they are only probable and approximative, and not strictly demonstrated or certain.
As a general rule, animal food is more easily and speedily digested, contains a greater quantity of nutriment in a given bulk, and therefore satisfies hunger for a longer time, than either herbaceous or farinaceous food; but, apparently from the same cause, it is also more heating and stimulating. Minuteness of division, and tenderness of fibre, are shewn by Dr Beaumont’s experiments to be two grand essentials for the easy digestion of butcher-meat; and the different kinds of fish, flesh, fowl, and game, are found to vary in digestibility chiefly in proportion as they approach or depart from these two standard qualities.
Farinaceous food, such as rice, sago, arrow-root or gruel, is also rapidly assimilated, and proves less stimulating to the system than concentrated animal food; but as it affords no scope for the due action of the muscular coat of the stomach, its exclusive use for any length of time seldom fails to weaken that organ and impair digestion. For the same reason, however, it becomes a very appropriate aliment, where stomachic irritation already exists. When the stomach is in a healthy state, milk is digested almost as easily as farinaceous food and is equally unstimulating.
The other kinds of vegetable substance are the slowest of all in undergoing digestion, and very frequently pass out of the stomach and through the bowels comparatively little changed; and hence the uneasiness which their presence so often excites in the intestinal canal, especially in persons of weak digestion, owing to the nerves of the intestines having a relation to chyme or digested food, and not to substances which resist the action of the gastric juice. In a given bulk they contain less nutriment, and excite the system less, than any other kind of food; so that they are well adapted for the diet of those in whom it is necessary to avoid every kind of stimulus, and who are not subjected to great muscular exertion; but to a person undergoing hard labour, they afford inadequate support.
Liquids—soup, for example—do not call into play the muscular coat of the stomach, and are so slow of digestion where that organ is already weakened, that they often give rise to acidity; and hence they are unfit for most dyspeptic patients. Before the gastric juice can act upon them, the fluid part must be absorbed, and the mass thickened to a proper consistence for undergoing the usual churning motion. On examining the contents of the stomach an hour after St Martin had dined on beef-soup, Dr Beaumont found that on one occasion the absorption of the watery part had been carried so far as to leave the remainder of even a thicker consistence than after an ordinary solid meal, but a similar result follows only when the digestive powers are very vigorous. When drink is swallowed, it also is carried off by absorption, and is not digested or allowed to pass through the pylorus. One purpose of this provision seems to be to prevent the gastric juice from being rendered inefficient by too much dilution.
When the food on which an animal lives is of a highly concentrated kind, and contains much nourishment in a small bulk, the apparatus of organs provided for its digestion is on a correspondingly small scale in point of extent. Thus, in carnivorous animals, whose food is, bulk for bulk, the most nutritious of all, the stomach and intestines are simple and short, the latter not exceeding in length more than from one to four or five times that of the body. In herbivorous animals, on the other hand, whose food is sparingly nutritious, and therefore requires to have a large bulk or volume, the stomach, as we saw in a former chapter, is greatly more complicated, and the length of the intestines enormously increased. Man, being intended to feed on both animal and vegetable substances, possesses an organization which holds an intermediate place between the two extremes. In him neither are the intestines so short as in carnivorous animals, nor have they the complexity and length characteristic of the herbivorous—thus clearly shewing the intentions of Nature in regard to his food, and at the same time allowing him a considerable latitude of adaptation when the force of circumstances for a time denies him access to any variety.
Animal food being in general more quickly digested than vegetable, and a simpler organization being sufficient for its conversion into chyme, many physiologists have inferred that this is owing solely to its being already of an animal nature, and therefore requiring scarcely any change to fit it for becoming a constituent part of the living fibre. But I agree with Dr Beaumont in thinking that this explanation is more gratuitous than philosophical, and that the process of chymification implies almost as complete a change in the one instance as in the other. In both, the operation of the gastric juice seems to be entirely analogous. In both, a complete solution takes place, and the chyle into which animal food is ultimately converted bears no greater resemblance to the future animal fibre, than does that produced from vegetable aliment. Thus the chyle of a horse, which lives exclusively on vegetables, has quite as great a resemblance to its future muscle, as that of a tiger, a lion, or a fox has to its future produce. Besides, whether the food be animal or vegetable, the ultimate result of digestion is always the formation of new animal matter; but in the former case, the nutritive particles are mixed up with a smaller proportion of innutritious matter than in the latter, and consequently a larger quantity of them can be extracted from a given bulk in a shorter time, than in the case of vegetables. There are most probably also minute differences in the chemical composition of the chyle derived from different kinds of food; but its general nature—its fitness for forming new animal tissue—and that of the process by which it is produced, are always the same.
Animal food, it is true, affords a more stimulating nutriment than farinaceous and other kinds of vegetable aliment, and hence it is avoided in diseases of excitement. But it seems to me that this stimulus is owing not only to its own inherent properties, but also to its more highly concentrated state, and to the much greater quantity of chyle which is derived from it than from an equal bulk of vegetable aliment. From the numerous experiments of injecting water, poisons, and other substances into the veins, performed by Magendie and others, we have direct proofs that the same agent which, introduced rapidly into the system, will sometimes act so powerfully as to destroy life, will excite scarcely any perceptible disorder if introduced very slowly, Analogy, therefore, bears us out in believing that the rapid admixture of very nutritious chyle with the blood may over-stimulate the system, when its more gradual introduction would have produced no such effect. At the same time, there can be no doubt that there is also a greater inherent stimulus in animal than in vegetable aliment.