The Physiology of Digestion by Andrew Combe, M.B. (1837)
Part I. Chapter V. Theory and Laws of Digestion.
The Physiology of Digestion by Andrew Combe, M.D., Forth American Edition, was published By Marsch, Capen & Lyon in Boston in 1837. Andrew Combe was a physician in ordinary to their Majesties the King and Queen of the Belgians, and a Fellow of the Royal College of Physicians of Edinburgh. The third edition of this book was sold in 3000 copies. The text of this book of 310 pages will be found here, completely unchanged.
THEORY AND LAWS OF DIGESTION.
Different theories of Digestion. - Concoction. - Fermentation. - Putrefaction. - Trituration. - Chymical solution. - Conditions or laws of digestion. - Influence of gastric juice. - Experiments illustrative of its solvent power. - Iis mode of action on different kinds of aliment - beef, milk, eggs, soups, &c. - Influence of temperature. - Heat of about 100 degrees essential to digestion. - Gentle and continued agitation necessary. - Action of stomach in admitting food. - Uses of its muscular motion. - Gastric juice acts not only on the surface of the mass, but en every particle which it touches. - Digestibility of different kinds of food. - Table of results. - Animal food most digestible. - Farinaceous next. - Vegetables and soups least digestible. - Organs of digestion simple in proportion to concentration of nutriment. - Digestability depends on adaptation of food to gastric juice more than on analogy to composition. - Illustrations. - No increase of temperature during digestion. - Dr. Beaumontīs summary of inferences.
Before entering upon the consideration of the theory of digestion which naturally evolves itself from the facts expounded in the preceding chapter, it may be of advantage to turn for a moment to the various theories which have prevailed since the subject first attracted the attention of the learned.
Hippocrates regarded digestion as a kind of concoction, ro stewing; and many of his followers believed that it is effected in the stomach by the agency of heat alone, much in the same way as food is cooked over a fire. It is quite ascertained that heat favours the process, but it is pure absudity to maintain that that agent alone will accomplish digestion.
Others of the older physiologists contended that chymification results from simple fermentation of the alimentary mass, and referred to the gas disengaged during difficult digestion, as a proof that the process of fermentation was going on. But it is now demonstrated that the tendency of healthy digestion is rather to arrest than to induce fermentation, and that the latter takes place only when disease exists, or when more food has been swallowed that the quantity of gastric juice secreted by the stomach is able to dissolve. Moreover, the products of digestion and of fermentation are so extremely different, that it is impossible to believe them to originate from the same chymical action.
The next theory which prevailed considered digestion to be the result of the putrefactive process. The single fact that the gastric juice not only arrests putrefaction, but even restores to sweetness meat in which that process is begun, is sufficient to demonstrate the wildness of such a supposition.
Another set of physiologists imagined that trituration would account best for all the changes occuring in the food during digestion; and consequently regarded the chyme as a sort of emulsion formed byt the intimate mixture of the aliment with the juices of the stomach, just as an emulsion is formed by rubbing down almonds in a mortar. The advocates of this theory referred for proofs not only to the contractile motions of the stomach already noticed, but to the muscular apparatus for trituration which forms so remarkable a feature in the gizzard of granivorous birds. But, in adopting this conclusion, they forgot that in birds the triturating apparatus does not digest, but serves, like the organs of mastication in man and quadrupeds, merely to bruise the grain on which the animal lives. In birds, in fact, digestion begins only after the trituration is finished.
A more recent and much more accurate view of digestion is that which considers it as neither more nor less than a chymical solution of the food in the gastric juice. This theory is supported by a greater number of facts and experiments than any other: but, although substantially correct, it is, perhaps, too exclusive and limited to its principles. It is true, that by tha agency of gastric juice on food out of the body, a change very similar to chymification can be effected on it; but when we remember that chyme, or the result of real digestion, is essentially the same in its elementary or component principles, whatever be the kind of food from which it is formed, and that as yet we are acquainted with no purely chymical agent which, applied to different substances, gives rise to the same uniform product, we shall be more willing to believe that chymification is neither a purery mechanical nor a purely chymical operation; but the result of a vital process, to which both mechanical and chymical forces contribute, and which no action or combination of inaminate matter can either exactly imitate or supersede.
To enable ourselves to appreciate correctly the nature of digestion, we must begin by considering the conditions essential for its performance, or without which it cannot be carried out.
The FIRST indispensable requisite is an adequate supply of gastric juice, and its thorough admixture with every particel fo the food on which it is to operate. The SECOND is a steady temperature of about 98 or 100 degrees Fahr.; and the THIRD is the gentle and continued agitation of the alimentary mass in the stomach while digestion is going on.
In illustration of the influence of the FIRST condition, I may refer to the experiments already mentioned as having been made by Spallanzani, Stevens, and others, to show the solvent power of the gastric juice on food even out of the body. Spallanzani states, that when small portions of well-masticated beef or mutton are placed in a vial, with a dure proportion of gastric juice, and the requisite temperature and gentle agitation are secured by placing the vial in the armpit, the appearances presented at the end of a few hours are extremely analogous to those observed in the natural process of chymification; the meat being in both cases converted into the soft grayish mass of a pultaceous consistence called chyme.
William Beaumont, 1785-1853. This illustration is from a later source.
Dr. Beaumont, who was well aware of the importance of Spallanzaniīs researches, and of the almost universal adoptation of his views by succeeding physiologists, till confidence in their accuracy was for a time shaken by the bold and fallacious assertions of Montégre, felt that the opportunity afforded him by St. Martinīs wound for verifying or disproving the experiments on which these views were founded, was much too valuable to be lost. He therefore entered upon a long series of investigations, of which the following is an imperfect, though, I hope, instructive abstract.
To test the reality of the solvent powers ascribed to the gastric juice, Dr. Beaumont withdrew from St. Martinīs stomach about one ounce of it, obtained after a seventeen hours fast, by introducing first a thermometer to induce the secretion, and then a gum-elastic tube to carry it off. Into this quantity, placed in a vial, he introduced a piece of boiled, recently-salted beef, weighing three drachms. He then corked the vial thightly, and immersed it in water raised to the temperature of 100 degrees, which he had previously ascertained to be the heat of the stomach when the secretion was going on. In forty minutes, digestion had distinctly commenced on the surface of the beef. In fifty minutes, the fluid became quite opaque and cloudy, and the texture of the beef began to loosen and separate. In sixty minutes, chyme began to be formed. In one hour and a half, the muscular fibres hung loose and unconnected, and floated about in shreds. In three hours, they had diminished about one half. In five hours, only a few remained undissolved. In seven hours, the muscular texture was no longer apparent; and in nine hours, the solution was completed.
To compare the progress of digestion in the natural way with these results, Dr. Beaumont, at the time of commencing the experiment just described, suspended a piece of the same beef, of equal wight and size, within the stomach, by means of a string. At the end of the first half hour it presented the same appearances as the piece in the vial; but when Dr. Beaumont drew out the string at the end of an hour and a half, the beef had been completely digested and dissappeared, making a difference of result in point of time of nearly seven hours. In both, the solution began on the surface, and agitation accelerated its progress by removing the external coating og chyme as fast as it was formed. When the experiment was repeated with chicken instead of beef, the solution was slower, from the greater compactness of the chicken not allowing the gastric fluid to penetrate its substance so readily. Had the beef and chicken been masticated before subjected to experiment, the difference between them in the rapidity of digestion would probably have been less.
To ascertain still more accurately the difference between natural and artificial digestion (the on in and the other out of the stomach), Dr. Beaumont put twelve drachms of recently-salted boiled beef into a vial with the same number of drachms of fresh gastric juice obtained through the opening of the stomach after a fast of eighteen hours; and then placed it in a basin of water on a sand-bath, shere he kept it at the heat of 100 degrees Fahr., and continued to agitate it gently. Digestion soon commenced, and progressed uniformly for about six hours, when it ceased. One half of the meat was then dissolved, and the texture of the remainder loosened and tender - resembling the same kind of aliment when ejected from the stomach partly digested some hours after a meal, as frequently seen in cases of indigestion. On weighing the undissolved portion which remained after all action had ceased, six drachms and twelve grains of beef were found to have been digested by twelve drachms, or nearly double its weight, of gastric juice. It thus appears that a given quantity of gastric fluid can digest only a relative proportion of meat; so that, when more is eaten that what there is juice sufficient to dissolve, stomachie disorder must necessarily follow. In this latter case, Dr. Beaumont found that the addition of fresh juice causes digestion to be resumed.
Alexis St. Martin, 1794-1881. This illustration is from a later source.
To discover what influence would be exerted on food masticated, swallowed, and mixed with the gastric juice in the usual way, and then withdrawn from the stomach, Dr. Beaumont gave St. Martin an ordinary dinner of boiled salted beef, bread, potatoes, and turnips, with a gill of pure water for drink; and twenty minutes afterward drew off through the opening about a gill of the content of the stomach into an open-mouthed vial. In this short space of time digestion had already commenced, thus negativing the common notion that an hour elapses before it begins. The vial was now placed in an water-bath, at a temperature of 100 degrees, and continued there for five hours. Examined at the end of that time, the whole contents were found to be dissolved. On then extracting an equal quantity of chyme from the stomach, and comparing it with the solution in the vial, little difference was observable between them, except that the process had been somewhat more rapid in that out of the stomach. But this experiment is remarkable in another point of view, as showing that in the short space of twenty minutes enough of gastric juice had been secreted for the entire completion of digestion.
With a view to verify these results, and also to discover the comparative digestibility of different kinds of aliment, Dr. Beaumont gave St. Martin for dinner eight ounces of recently-salted lean beef, four ounces of potatoes, some bread, and four ounces of boiled turnips. After fifteen minutes he withdraw a portion of the contents of the stomach, and found that some of the meat had already been slightly digested. In a second portion, withdrawn at the end of forty-five minutes, fragments of the beef and bread were perceptible, and in a still more advanced state of digestion; the meat was in small shreds, soft and pulpy, and the fluid containing it had become more opaque and gruel-like in appearance. When two hours had elapsed, a third quantity was taken out, at which time nearly all the meat had become chymified and changed into a reddish-brown fluid; but small pieces of vegetable matter now presented themselves for the first time, but in a state of digestion so much less advanced than the meat, that their peculiar structure was still distinctly visible. Some of the second and third portions, put into a vial and treated in the usual way, advanced to complete digestion, as in the other experiment, except that the process was slower, and that a few vegetable fibres remained to the last undissolved; thus confirming the general opinion, that vegetables are more difficult of digestion than animal substances.
The mode of solution by the gastric juice varies according to the nature of the food on which it acts. We have seen that it gradually reduces solids to a soft and fluid state; but its effect on milk and albumen is different. It begins by coagulating them, so as to give them the requisite consistence for being affected by the muscular contractions of the stomach, and impregnated with the juice. Fifteen minutes after St. Martin had drunk half a pint of milk, a portion taken out of the stomach by Dr. Beaumont presented the appearance of a fine, loosely-coagulated substance, mixed wiht a semi-transparent why-coloured fluid. A drachm of warm gastric juice, poured into two drachms of milk, at a temperature of 100 degrees, produced a precisely similar appearance in twenty minutes. In another experiment, when four ounces of bread were given along with a pint of milk, and the contents were examined at the end of thirty minutes, the milk was coagulated, and the bread reduced to a soft pulp floating in a large proportion of fluid. In two hours the whole was competely digested.
When the white or albumen of two eggs was swallowed on an empty stomach, small white flakes beagn to be seen in about ten or fifteen minutes, and the mixture soon assumed an opaque whitish appearance. In an hour and a half the whole had dissappeared. Two drachms of albumen, mixed with two of gastric juice out of the stomach, underwent the same changes, but in a rather longer time.
When the food is chiefly liquid, as when soup is taken eihter alone or in large proportion, the more fluid part is speedily absorbed, to fit the remaining nutritious portion for being better acted on by the gastric juice and muscular power of the stomach. Fifty minutes after St. Martin had dined on vegetable soup, beef, and bread, Dr. Beaumont found the stomach to contain a pulpous mass, like thick gruel in consistence, and of a semi-gelatinous aspect. The fluid portion had been absorbed to such an extent, that the remainder was even thicker than is usual after eating more solid food. From many similar observations, Dr. Beaumont infers it to be a gereral law, that soup and liquids cannot be digested until they are formed into a thicker mass by the absorption of their watery part - as till then they are too liquid to be easily acted on by the gastric juice. Hence their unfitness for weak stomachs, and the impropriety of large libations of tea or coffee at breakfast by persons whose digestion is bad. During recovery from illness, chicken-tea, beaf-tea, and soups are often useful, simply because the system then requires the liquid to make up its lost blood.
Unfortunately, Dr. Beaumont made few experiments on the action of gastric juice upon vegetables; and, in the few recorded, he generally contents himself with noting the length of time required for their solution, which generally proved considerably longer than for animal substances. In one experiment, however, he states, that an hour after giving St. Martin nine ounces of raw, tipe, sour apples, the stomach was full of fluid and pulp, "quite acrid, and irritating the edges of the aperture, as is always the case when he eats acescent fruits or vegetables." In an hour and a half the contents were still more sharp and acrid, and the pulp of the apple visible. At the end of two hours the stomach was empty, but the mucous membrane exhibited an irritated appearance. With farinaceous vegetables, however, the results were different. Thus, when a pint of thick, rich, boiled sago, sweetened with sugar, was given, the whole was digested in less than two hours, and there was neither acrimony of the gastric contents, nor smarting of the edges of the wound; on the contrary, it seemed peculiarly grateful to the stomach, and rendered the mucous membrande soft, uniform, and healthy. The same results followed a repetition of the experiment, and also whe a pint of soft custard was taken. In some states of the stomach, it is true, even farinaceous food excites acrimony and irritation, but rarely in the same degree as the other forms of vegetable aliment.
Such being the influence of gastric juice on different aliments at the natural heat of the body, we have now to ascertain, in the SECOND place, what the high temperature has in result.
To determine this point, Dr. Beaumont took out two ounces of gastric juice, and divided it into two equal portions, in separate vials. He added to each an equal weight of masticated fresh beef; and placed the one in a bath, at the temperature of 99 degrees, and the other in the open air, at 34 degrees. As a contrast to these, he placed beside the latter a third vial, containing the same weight of masticated meat in an ounce of clear water.
In two hours the meat in the warm vial was partially digested; that in the cold gastric juice was scarcely changed; and the third portion, in the cold water, seemed only a little macerated. In six hours the meat in the warm vial was half digested, while that in the two others had undergone no farther alteration. The gastric juice in the first vial having by this time dissolved as much as it could of the beef, four drachms more were added from the stomach, and the vial was replaced in the bath. Digestion, which had previously ceased, was now resumed, and went on as steadily as if it had not been interrupted; thus showing, in a striking manner, the impropriety of exceeding in our meals the quantity for which alone a sufficiency of gastric juice can be provided.
At the end of twenty-four hours, the three portions were examined. That contained in the warm juice was completely dissolved, and presented the usual appearances. The portions contained in the cold juice and in the cold water very much resembled each other, and exhibited no appearance of chyme whatever. They vere macerated or softened, but not digested. These experiments, and others of a similar nature, show clearly that a temperature equal to ordinary blood-heat is requisite for chymification.
To make sure that it was the low temperature alone which prevented the occurence of digestion in the experiment delailed, Dr. Beaumont now placed the vial containing the meat which had been exposed without effect for twenty-four hours to tha action of the cold gastric juice, on a water-bath at the ordinary blood-heat. In a very short time "digestion commenced, adn advanced regularly as in the other parcels." The same results were always obtained from a repetition of these experiments, so that they may be held as perfectly conclusive in establishing the essentiality of heat to the digestive process.
THIRDLY. - The necessity of gentle and continued agitation for the accomplishment of digestion is so obvious from the preceding exposition, that it requires no direct experiments to establish it. When portions of meat were suspended in the stomach, by a string so short as to prevent them from being fully subjected to the motion already described as always going on during digestion, the action of the gastric juice was confined almost entirely to their surface, and a longer time was consequently required for their solution that when they were left at liberty. In like manner, when meat out of the stomach was placed in a vial containing gastric juice, its solution was uniformly accelerated by gentle agitaiton, which acted simply by removing the coating of chyme as it formed on the surface, and thus affording to the gastric fluid an easier access to the undigested portions below. Accordingly, when, in one of Dr. Beaumontīs experiments, two ounces of unmasticated roasted beef were introduced through the external aperture into the stomach, and held by a string, only one half of it was digested in four hours, evidently from the want of mastication confining the action of the gastric juice to the surface of the mass, and because the string prevented it from following the regular motions of the stomach.
Having now made the reader sufficiently acquainted with the agents concerned in, and the conditions essential to, the performance of digestion out of the body, we have next to exhibit the same agents and the same conditions in their ordinary operation in the living being, and to describe the beautiful arrangements by which they are respectively and unerringly regulated.
It has been already shown, that, in endowing us with appetite, nature has intended both to ensure by its means a timely provision for the wants of the system, and to guard against our eating nore than enough to supply them. We have also seen that, within certain limits, the quantity of gastric juice secreted bears a direct relation to the quantity of food consumed; that when the food exceeds considerably the real necessities of the system, a part of it remains undigested, because the stomach is unable to secrete a sufficiency of fluid for the solution of the whole; and that, as a necessary consequence, indigestion follows. This being the case, we may expect to find all the arrangements of nature made with a view to prevent us from hastily filling the stomach to repletion, without being fully warned beforehand of the error we are committing. And such accordingly is the fact.
Considered in this light, the processes of mastication, insalivation, and deglutition, are not only useful in preparing the morsel for the future action of the gastric juice, but, by transmitting the food to the stomach in small portions at a time, likewise serve the important secondary purpose of preventing its too rapid or excessive distension. To this good end, indeed, the stomach itsefl contributes, as has been distinctly shown by Dr. Beaumont. In the natural state of that organ, a regular and gentle contraction of its whole fibres and cavity follows the introduction of each individual morsel, and it is not till the relaxation consequent on that contraction takes place, that another is willingly admitted. This arrangement was more than suspected by other physiologists, but it remained for the American experimenter to demonstrate its existence and purposes. It is true that, during a hurried reapst for example, food may be rapidly introduced into the stomach by an active effort of the will, but it is precisely in such circumstances that we are apt to eat too much, and that indigestion follows; because, from no time being allowed for the secretion of the requisite quantity of gastric juice, and its proper mixture with each portion of the aliment, the stomach is placed in an unnatural situation, and its nerves cannot receive the same impression of "enoug eaten", which is designed by nature to arise only from the one being duly proportioned to and mixed with the other. The advantage of the natural arrangement is therefore confirmed rather than refuted by what may at first appear an exception.
When Dr. Beaumont depressed the valve in St. Martinīs stomach, and introduced a few spoonfuls of soup at the orifice, he observed the rugae or folds of the mucous membrane to close gently upon it, gradually diffusing it through the gastric cavity, and completely preventing the entrance of a second quantity till this diffusion was effected - when relaxation again took place, and admitted of a farther supply. When solid food was introduced in the same way, either in larger pieces or finely divided, the same gentle contraction and grasping motion were excited, and continued from fifty to eighty seconds, so ad to prevent more from being introduced without considerable force till the contraction was at an end. When St. Martin was so placed as to admit of the cardia or upper orifice of the stomach being brought into view, and he was then made to swallow a morsel of food in the natural way, a similar contraction of the stomach, and closing of its fibres upon the bolus, was invariably observed to take place; and till this was over, a second morsel could not be received without a considerable effort. And accordingly, when, either from haste or hunger, we disregard the order of nature, and hurriedly gulp down food without due mastication, and without allowing time for the regular contraction of the stomach, we necessarily expose ourselves to the risk both of overloading it, and of ultimately impairing its digestive power.
Such being the provision made for ensuring the gradual admission of food into the stomach, the next requisite is its proper admixture with the gastric juice.
Food being the appropriate stimulus of the secreting vessels of the stomach, the moment the alimentary morsel comes into contact with the mucous membrane, the action of the latter, as was formerly pointed out, becomes increased; its bloodvessels are distended, its colour deepens to a brighter red, and the gastric juice immediately begins to be poured out. The muscular fibres of the stomach, being acted upon by the same stimulus, next come into play, and execute their specific functions of alternate contraction and relaxation. By these means the aliment speedily becomes impregnated with the gastric fluid, and undergoes the influence of that continued gentle agitation already described as essential to digestion, and which seems to have for its chief object the careful admixture of every portion of the nutiment with the quantity of gastric juice necessary for its solution. The particles of food are thus continually changing place, and if the quantity taken be not too great for the power of the gastric juice which the stomach can supply, chymification goes on equally throughout; so that, if the contents of the stomach be withdrawn in from thirty minutes to an hour after a moderate meal, they will be found to consist of perfectly formed chyme and particles of food, intimately mixed and blended, in larger or smaller proportions, according to the vigorous or enfeebled state of the digestive organs, and the quality of the aliment itself. So effectually indeed has the admixture of food and solvent juice taken place in this short time, that, as already shown, when a portion is removed from the stomach and placed in an appropriate vessel, digestion will commonly continue in it, provided it be placed in a proper temperature, and subjected to gentle and continued agitation.
It is the impossibility of its being adequately acted upon by the muscular contractions of the stomach, which renders fluid and highly concentrated nourishment, when exclusively used, so difficult of digestion; and hence the reason why a certain bulk and consistence given to whale oil, for example, by the admixture of such innutritious substances as vegetable fibre, bran, or even sawdust, make it a more acceptable and digestible article of food to the inhabitants of the northern regions, than when it is consumed in its pure state. In like manner, in civilized society, bread, potatoes, and vegetables are useful, not less by giving the requisite bulk and consistence to the rest of the food, than by the nutriment which they contain. Soups, jellies, arrowroot, and similar substances, are, for the same reason, more easily digested when eaten along with bread or some bulkier aliment, than when taken alone, especially if used for some time.
The motion which we have seen to be excited in the stomach by the entrance of aliment, is at first very gentle and slight; but in proportion as digestion proceeds, or the organ is distended, it becomes more rapid and energetic; and then it serves the additional purpose of gradually propelling the chyme through the pylorus into the intestine, there to be farther prepared and converted into chyle. The necessary churning or agitation of the food is, from the peculiar situation of the stomach, greatly assisted by the play of the diaphragm and abdominal muscles during inspiration and expiration; and the diminution of the vivacity and extent of the respiratory movement which always attends despondency and grief, is one source of the enfeebled digestion which notoriously accompanies or follows depression of mind. The same cause alos leads necessarily to an unfavourable condition of the blood itself, which in its turn weakens digestion in common with every other function; but the muscular or mechanical influence is that which at present chiefly concern us. On the other hand, the active and energetic respiration attendant on cheerfulness and buoyancy of spirits adds to the power of digestion, both by aiding the motions of the stomach, and by imparting to it a more richly constituted blood. If to these causes be added the increase of nervous stimulus which pleasing emotions occasion in the stomach (as in the muscles, and organs of secretion generally), we shall have no difficulty in perceiving why digestion goes on so well in parties where there is much jocularity and mirth. "Laughter," says Professor Hufeland of Berlin, "is one of the greatest helps to digestion with which I am acquainted; and the custom prevalent among our forefathers, of exciting it at table by jesters and buffoons, was founded on true medical principles. In a word, endeavour to have cheerful and merry companions at your meals: what nourishment one receives amid mirth and jollity will certainly produce good and light blood." (Art of Prolonging Human Life, English edition, London, 1829, p. 282)
Exposed to the action of all the agents above enumerated, and to the increased nervous and vascular excitement which are necessary during digestion, a singular change soon commences in the food, and goes on till chymification is completed. After a little while the contents of the stomach, whatever their nature and variety, begin to be converted into a substance of a homogenous, soft, grayish, and viscid appearance, of a sweetish, fade, and slightly acid taste, but which still preserves some of the qualities of the food, and is called chyme. The chyme always forms on that part of the food with which the gastric juice is in immediate contact; and, in proportion as it is produced, it is carried gradually onwards by the gentle motion of the stomach towards the pylorus, where consequently it always exists in the greatest quantity. At the cardiac or left end of the stomach it is most sparingly found, both because digestion is there only beginning, and because the chyme is speedily removed from it and carried away towards the pylorus.
The doctrine hitherto generally received, and held by Dr. Wilson Philip and others as demonstrated, is, that "the layer of food lying next to the surface of the stomach is first digested, and in proportion as this undergoes the proper change, and is moved by the muscular action of the stomach, that next in turn succeeds, to undergo the same change." Dr. Beaumont however declares, that, whatever may be the case in rabbits and other animals on which Dr. Philipīs experiments were made, such was not the order observed in the stomach of Alexis St. Martin, - and such, he naturally infers, is not the order in the human stomach in general. Nor is such, I may add, the order shich either experience or a correct view of the antecedant circumstances and physiological conditions ought to lead us to anticipate. When vomiting, for instance, occurs an hour or two after a meal composed of different ingredients has been swallowed, no such separation into digested and undigested portions is to be found, but the whole are observed to have undergone changes proportioned to their degrees of digestibility, whether they were eaten first or last.
In conformity with this view, Dr. Beaumont mentions, that when St. Martin swallowed a mouthful of any tenacious food after the digestion of the preceding meal was considerably advanced, he always saw it first pass towards the great curvature of the stomach and then disappear. In a minute or two it reappeared, more or less broken down, and mixed with the general alimentary mass; and in a short time longer it was so much changed as entirely to lose its identity. From these and numerous other facts, Dr. Beaumont infers, that "there is a perfect admixture of the whole ingestae during the period of alimentation and chymification;" and that "the whole contents of the stomach, until chymification be nearly complete, exhibit a heterogeneous mass of solids and fluids - hard and soft, coarse and fine, crude and chymified - all intimately mixed, and circulating promiscuously through the gastric cavity, like the mixed contents of a close vessel, gently agitated or turned in the hand." - (P. 112.) As we proceed we shall meet with various indirect proofs of these statements being correct.
If Dr. Beaumontīs observations on this subject are accurate, we may expect to find that chymification commences on the surface of each individual fragment of the food, and is not confined to the outer surface of the entire alimentary mass, as stated by Dr. Philip. Such, accordingly, is the fact. When Dr. Beaumont extracted a portion of the food through the opening half an hour of an hour after eating, he invariably foudn it composed of perfectly formed chyme and particles of food intimately mixed and blended; and in these experiments every portion of the aliment was already so completely supplied with gastric juice, that its chymification proceeded till the whole was digested with no other aid than that of the requisite heat and agitaiton.
When a meal is taken before the preceding one is out of the stomach, digestion is observed to be disturbed. Dr. Wilson Philip explains this by stating, that the newly-swallowed food becomes imbedded in, and occupies the centre of, the older and half-digested mass, where it remains distinct and untouched till the former meal is entirely disposed of. A more logical explanation, however, and one more in harmony with facts, is offered by Dr. Beaumont, who ascribes the disturbed digestion to the supply of gastric juice having been entirely taken up by the first meal, and to the stomach, now comparatively exhausted, being unable to secrete a fresh supply for the second - seeing that, in ordinary circumstances, its vessels secrete only enough to meet the real wants of the system. Dr. Beaumont adduces, in confirmation of this view, the fact that many children, and most cooks, are in the habit of eating small quantities of food almost every hour or two without their digestion suffering materially, because the the amount of gastric juice secreted is quite equal to the chymification of the whole quantity taken.
Reasonable as this inference appears from the facts stated by Dr. Beaumont, I cannot help thinking that there is something more in the constitution of the stomach than the mere deficiency of gastric juice, which renders the too early intrusion of new food hurtful. We know, for example, that at the commencement of digestion, the muscular contractions of the stomach are comparatively slow and feeble, and that, as chymification advances, they become rapid and energetic, as if to remove the chyme as fast as it is formed. If, then, new food, for which the feebler movement is best adapted, be introduced when digestion is far advanced, and the energetic motion going on, may not this disproportion be itself an impediment, and co-operate with the deficiency of the gastric juice in disordering digestion? Moveover, as the stomach, in common with every other organ intended for alternate activity and repose, is always more or less fatigued by the active fulfilment of its function, its premature excitement by new food must add to its exhaustion, and weaken its power (in the same way in which fresh muscular exertion adds to the exhaustion of muscles already sufficiently exercised), and consequently lead to imperfect digestion.
The same principle which explains the necessity of repose for repairing the vascular and nervous energy of the stomach, when these have been excited and exhausted by the labour of digestion, also affords a solution ot the question why the appetite does not return as soon as the stomach is empty, but begins to be felt only after the stomach has enjoyed a period of repose, varying in duration with the mode of life, the state of health, and the nature of the previous meal.
If we regard chymification as going on at the surface of every individual portion of the food which has been sufficiently supplied with gastric juice, we at once see that the constant motion of the stomach is necessary, 1st, to remove the chyme from the surface of each little fragment of the alimentary mass; and, 2dly, by this very removal to expose a fresh surface of the fragment to the contact of the mucous membrane, thereby enabling it to excite such farther secretion of gastric juice as may be required to complete its digestion. In this way the formation and removal of chyme go on from the very first, although it is then necessarily produced and removed 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, grayish chyme, into which the aliment is converted, is gradually impelled towards the pyloric extremity of the stomach, in proportion as it is formed. 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 contractible 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 alos require to be noticed. Of these, a dure 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 producec 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 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 naturar state, and St. Martin himself subjected to ordinary exercise.
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 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 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 time required for the chymification of jelly, but in the 41st experiment, 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.
It may be said that, on the day of the 41st experiment, St. Martinīs digestion must have been particularly good - and, in truth, it seems to have been so; for at 9 oīclock A.M. he breakfested on soused tripe, pigīs 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 pigīs feet, instead of disproving my position, evidently strengthens it, by showing 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 show of reason, be considered as an accurate index to 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. Beaumontīs 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 woud be an 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 not only probable and approximative, and not strictly demonstrated or certain.
As a general rule, animal food is more easily and speedily digested, and contains a greater quantity of nutriment in a given bulk, 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 shown by Dr. Beaumontīs 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, and gruel, are also rapidly assimilated, and prove less stimulating to the system than concentrated animal food. Milk seems to rank in the same class, when the stomach is in a healthy state.
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 bowels, especially in persons of weak digestion, owing to the nerves of the intestines having a relation to digested food. 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 - are slow of digestion, and hence 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 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. When drink is swallowed, it is carried off in the same way 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 hervivorous animals, on the other hand, whose food is sparingly nutritious, and therefore requires to have a large bulk of 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 showing 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 way owing 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 chymical 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.
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 shows 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 showing 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 sawdust or other indigestible vegetable fibre.
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, ad 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," (Paris on Diet, p. 93.) - 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 supposet 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 tis 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 it 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 showing 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 occured during that process. By introducing a thermometer with a long stem at the external opening into St. Martinīs 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 appeared, however, that the variations of the atmosphere produced 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 degrees 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 degrees, and on another to 96 degrees; whereas next day, when the weather was clear and dry, it rose to 99 degrees, and on that following, when the weather was both clear and cold, to 100 degrees. On several occasions it rose as high as 102 degrees, and once to 103 degrees; 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 fiven without reference either to logical sequence or to time.
INFERENCES FROM DR. BEAUMONTīS EXPERIMENTS AND OBSERVATIONS (The inferences are given in Dr. Beaumontīs own words, and the italics also are his).
1. That hunger is the effect of distention of the vessels that secrete the gastric juice.
2. That the process of mastication, insalivation and deglutition, in an abstract point of view, do not in any way affect the digestion of the food; or, in other words, when food is introduced directly into the stomach in a finely divided state, without these previous steps, it is as readily and as perfectly digested as when they have been taken.
3. That saliva does not possess the properties of an alimentary solvent.
4. That the agent of chymification is the gastric juice.
5. That the pure gastric juice is a fluid, clear, and transparent; without odour; a little salt; and perceptidly acid.
6. That it contains free muriatic acid, and some other active chymical principles.
7. That it is never found free in the gastric cavity; but always excited to discharge itself by the introduction of food or other irritants.
8. That it is secreted from vessels distinct from the mucous follicles.
9. That it is seldom obtained pure, but is generally mixed with mucus, and sometimes with saliva. When pure, it is capable of being kept for months, and perhaps for years.
10. That it coagulates albumen, and afterward dissolves the coagulae.
11. That it checks the progress of putrefaction.
12. That it acts as a solvent of food, and alters its properties.
13. That, like other chymical agents, it commences its action of food as soon as it comes in contact with it.
14. That it is capable of combining with a certain and fixed quantity of food, and when more aliment is presented for its action than it will dissolve, disturbance of the stomach, or "indigestion", will ensue.
15. That its action is facilitated by the warmth and motions of the stomach.
16. That it becomes intimately mixed and blended with the ingestae in the stomach by the motions of that organ.
17. That it is invariably the same substance, modified only by admixture with other fluids.
18. That the motions of the stomach produce a constant churning of its contents. and admixture of food and gastric juice.
19. That these motions are in two directions, transversely and longitudinally.
20. That no other fluid produces the same effect on food that gastric juice does; and that it is the only solvent of aliment.
21. That the action of the stomach and its fluids is the same on all kinds of diet.
22. That solid food, of a certain texture, is easier of digestion than fluid.
23. That animal and farinaceous aliments are more easy of digestion than vegetable.
24. That the susceptibility of digestion does not, however, depend altogether upon natural or chymical distinctions.
25. That digestion is facilitated by minuteness of division and tenderness of fibre; and retarded by opposite qualities.
26. That the ultimate principles of aliment are always the same, from whatever food they may be obtained.
27 That chyme is homogeneous, but variable in its colour and consistence.
28. That, towards the latter stages of chymification, it becomes more acid and stimulating, and passes more rapidly from the stomach.
29. That the inner coat of the stomach is of a pale pink colour, varying in its hues according to its full or empty state.
30. That, in health, it is sheathed with mucus.
31. That the appearance of the interior of the stomach in disease is essentially different from that of its healthy state.
32. That stimulating condiments are injurious to the healthy stomach.
33. That the use of ardent spirits always produces disease of the stomach if persevered in.
34. that water, ardent spirits, and most other fluids, are not affected by the gastric juice, but pass from the stomach soon after they have been received.
35. That the quantity of food generally taken is more than the wants of the system require; and that such excess, if persevered in, generally produces not only functional aberration, but disease of the coats of the stomach.
36. That bulk as well as nutriment is necessary to the articles of diet.
37. That bile is not ordinarily found in the stomach, and is not commonly necessary for the digestion of the food; but,
38. That when oily food has been used, it assists its digestion
39. That oily food is difficult of digestion, though it contains a large proportion of the nutrient principles.
40. That the digestibility of aliment does not depend upon the quantity of nutrient principles that it contains.
41. That the natural temperature of the stomach is about 100 degrees Fahrenheit.
42. That the temperature is not elevated by the ingestion of food.
43. That exercise elevates the temperature; and that sleep or rest, in a recumbent position, depresses it.
44. That gentle exercise facilitates the digestion of food.
45. That the time required for that purpose is various, depending upon the quantity and quality of the food, state of the stomach, &c.; but that the time ordinarily required fot the disposal of a moderate meal of the fibrous parts of meat, with bread, &c., is from three to three and a half hours.
A few more inferences are given, but are here omitted, because they refer exclusively to the chyle, which has not yet been treated of. The second and seventeenth are, perhaps, to strongly expressed. A complete change of diet, for example, causes some variation in the gastric juice, although the latter inference, taken in a literal sence, affirms the contrary.