It seems to be partly for the purpose of obviating the evil of the too rapid introduction of nutriment, and partly for that of varying the stimulus, that Nature has rendered a certain bulk of food advantageous to digestion, and decreed that no animal can long retain its health if fed on highly concentrated aliment alone. Dogs fed on oil or sugar, which are almost wholly converted into chyle, become diseased and die in a few weeks; and, as Dr Paris has acutely remarked, the very capacity of our digestive organs is a proof that Nature never intended them for the exclusive reception of highly concentrated food. Dr Paris refers to post-horses fed chiefly on beans and corn, as instances among the lower animals of the insalubrity of too condensed nutriment, and shews that they live constantly on the brink of active disease, and every now and then require bleeding, laxatives, and emollients, to keep them in condition. Sportsmen, boxers, and others, who train themselves for severe exertion, are additional examples shewing that a similar mode of living induces a morbid tension of the system which cannot be long kept up without danger. The Kamtschatdales sometimes live with impunity for months on fish-oil, by wisely mixing it up with saw-dust or other indigestible vegetable fibre, which has the double advantage of diluting the food, and of affording due scope for the action of the muscular coat, and thus placing the food in more perfect harmony with the constitution of the stomach.
If the preceding explanation of the more rapid digestion of animal than of vegetable substances, and the higher stimulus which they afford be correct, the common notion of the former being more digestible than the latter solely because there is a greater analogy between animal food and the system which it goes to nourish, and therefore a smaller change to be undergone, necessarily falls to the ground. If it be true—which it seems to be—that, in the natural state, in a temperate climate, animal food is more easily digested than vegetable, the fair inference ought rather to be that the system requires the former in larger proportion than the latter, and that the gastric juice is purposely constituted with reference to this circumstance. Accordingly, in the Arctic Regions, where the climate renders great stimulus necessary, animal food, of to us the most indigestible kind—that consisting of pure fat and oil—is eaten in immense quantities and digested with enviable facility; while in India and other tropical climates, where much less stimulus is required, the natives digest vegetable aliment with at least equal ease and satisfaction.
If, as Dr Paris imagines, animal food owes its digestibility simply to its possessing “a composition analogous to that of the structure which it is designed to supply,” and therefore requiring “little more than division and depuration,” instead of the alleged “complicated series of decompositions and recompositions, which must be effected before vegetable matter can be animalized or assimilated to the body,”[32]—it follows that butcher-meat must in all climates and situations be more digestible than vegetables; and that raw meat, which has the greatest analogy of all to the structure of the body, must require still less digestive power for its solution and assimilation than cooked meat. These propositions, however, are wholly at variance with experience: in particular, the effect of cooking is unquestionably to induce a change of composition subversive of the analogy on which Dr Paris rests his opinion.
That the easier digestibility of animal food in man arises chiefly from its greater adaptation to the qualities of the gastric juice, and not from any such analogy as that now alluded to, is rendered still more probable by the fact, that in him the gastric fluid contains scarcely any free acid, except where the diet has consisted for some time principally of vegetables; whereas it always contains a considerable proportion of acid in herbivorous creatures. In the latter, moreover, the analogy is quite as great between animal substances and their own structure as in man, and yet to a cow, beef is much more indigestible than grass, notwithstanding the “decompositions and recompositions” which the latter is supposed to require before becoming animalized. Dr Beaumont is therefore quite justified in maintaining, that the process of digestion implies as complete a solution and recomposition in the case of animal as of vegetable substances; and that the rapidity with which the chymification of either is effected depends more on its adaptation to the properties of the gastric juice provided by Nature for its solution, than on the closeness of resemblance of its own composition to that of the body of which it is to become a part.
Another prevalent notion—that the digestive apparatus is simpler and shorter in carnivorous than in herbivorous animals, merely because their food is more analogous in composition to their own bodies, and therefore requires less perfect digestion—seems to me equally unfounded, and to be negatived by the fact, that, in the grain-eating birds, in the constituent elements of whose food there is no such analogy, the intestines nevertheless scarcely exceed in length those of carnivorous birds—a circumstance at variance with the notion of length being necessary solely on account of the great elaboration required for the conversion of vegetable into animal substance. The true principle—and it is important to notice it, as the error is generally adopted—appears to be, that where the food of the animal contains much nutriment in a small bulk, there the stomach and intestinal canal are simple and short; but where, on the contrary, it contains little nutriment in a large bulk, there great capacity, complexity, and length become requisite to enable the animal to elaborate a sufficiency of nourishment for its subsistence, by taking in the requisite quantity from which it is to be derived. Accordingly, in the elephant and some other herbivorous animals we find the capacity to depend not on the length, but on the width and increased surface of the intestine, or, in other words, on the greater calibre of its cavity; whereas, in some fishes which live on very concentrated aliment, the intestinal canal is not much more than the length of the body—thus shewing that the common opinion on the subject is utterly untenable.
Before concluding his experiments on the agents employed in digestion, Dr Beaumont made many observations with a view to ascertain whether any increase of temperature occurs during that process. By introducing a thermometer with a long stem at the external opening into St Martin’s stomach, both before and during chymification, he succeeded in obtaining very accurate information on this point. In two or three of the experiments the heat of the stomach seemed to be increased after taking food, but in by far the greater number the temperature remained the same. It appears, however, that the variations of the atmosphere produce a sensible change on the heat of the stomach—a dry air increasing and a moist air diminishing it. The ordinary temperature may be estimated at 100° Fahr., and in several instances it was higher at the pyloric than at the cardiac end. On one cloudy, damp, and rainy day, the thermometer rose only to 94°, and on another to 96°; whereas next day, when the weather was clear and dry, it rose to 99°, and on that following, when the weather was both clear and cold, to 100°. On several occasions it rose as high as 102°, and once to 103°; but these were after exercise, which was always observed to cause an increase of two or three degrees. We have already seen that artificial digestion is entirely arrested by cold, and is resumed on raising the temperature to ordinary blood heat.
Such, then, are the phenomena and conditions of healthy digestion, and such is the light thrown upon them both by the valuable publication of the American physiologist. Before leaving this branch of the subject, however, it may be useful to lay before the reader, as a kind of summary, the principal inferences deduced by Dr Beaumont from his numerous experiments and observations. But in doing so I shall attempt to arrange the results in their natural order; for in the original work they are given without reference either to logical sequence or to time.
INFERENCES FROM DR BEAUMONT’S EXPERIMENTS AND OBSERVATIONS.[33]