The Physiology of Digestion by Andrew Combe, M.B. (1837)
Part I. Chapter IV. Organs of Digestion - the Stomach - the Gastric Juice
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.
ORGANS OF DIGESTION - THE STOMACH - THE GASTRIC JUICE
Surprising power of digestion. - Variety of sources of food. - All structures, however different, formed from the same blood. - General view of digestion, chymification, chylification, sanguification, nutrition. - The stomach in polypes, in quadrupeds, and in man. - Its position, size, and complexity, in different animals. - Its structure; its peritoneal, muscular, and villous coats; and uses of each. - Its nerves and bloodvessels, their nature, origins and uses. - The former the medium of communication between the brain and stomach. - Their relation to undigested food. - Animals not conscious of what goes on in the stomach. - Advantages of this arrangement. - The gastric juice the grand agent in digestion. - Its origin and nature. - Singular case of gunshot wound making a permanent opening into the stomach. -- Instructive experiments made by Dr. Beaumont. - Important results.
If, in the whole animal economy, where all is admirable, there be one operation which, on reflection, appears more wonderful than another, and which evinces in a higher degree the prodigious resources and power of the Creator in fashioning every thing to his own will, it is perhaps that by which the same kind of nutriment is extracted from the most opposite varieties of food consumed by living beings. For, singular as it may appear, recent researches tend to establish the fact, that, even in animals differing so widely in their aliment as the herbivorous and carnivorous quadrupeds, the ultimate products of digestion in both - the chyle and the blood - are identical in composition, in so far at least as can be determined by their chymical analysis. (See Roget´s Bridgewater Treatise, note at p. 58, vol ii.)
Remarkable, however, as this uniformity of result undoubtedly is, it becomes still more striking when we contemplate the variety of sources from which the food is derived for the support of animal life. To use the words of an able writer already quoted, "There is no part of the organized structure of an animal or vegetable, however dense its texture or acrid its qualities, that may not, under certain circumstances, become the food of some species of insect, or contribute in some mode to the support of animal life. The more succulent parts of plants, such as the leaves or softer stems, are the principal sources of nourishment to the greater number of larger quadrupeds, to multitudes of insects, as well as to numerous tribes of other animals. Some plants are more particularly assigned as the appropriate nutriment of particular species, which would perish if these ceased to grow; thus, the silkworm subsists almost exclusively upon the leaves of the mulberry-tree; and many species of caterpillars are attached each to a particular plant, which they prefer to all others. There are at least fifty different species of insects that feed upon the common nettle; and plants of which the juices are most acrid and poisonous to the generality of animals, such as euphorbium, henbane, and nightshade, afford a wholesome and delicious food to others." (See Roget´s Bridgewater Treatise, vol ii, p. 59.) Nor are the precision and accuracy with which the same fluid - the blood - affords to every structure of the body the precise species of nourishment or secretion which its elementary composition requires, however different each may be from the rest in chymical qualities, less admirable and extraordinary than its own original formation from such a variety of materials. To bone, the blood furnishes the elements of bone with unerring accuracy; to muscle the same blood furnishes the elements of muscle, - to nerves the elements of nerve, - to skin the elements of skin, - and to vessels the elements of vessels; - and yet, while each of these differs somewhat in composition from the others, the constituent element of the blood by which they are furnished are everywhere the same.
Similar phenomena, indeed, occur in the vegetable world; but this, instead of diminishing our wonder, rather tends to augment it. The same elements, extracter from the same soil, are converted into every variety of vegetable product - into leaves of every shade of green, flowers of every form and teint, and juices of every quality, from the deadly poison up to bland and life-supporting milk. Nay, even in the same plant - as in the poppy - we sometimes find the seeds and the capsule which covers them endowed wiht the most opposite properties.
It would be very interesting to discover by what resources nature thus effects the production of the same kind of nutritive fluid, or chyle, from so great a variety of substances, and apportions to every part the precise elements of which it stands in need; but it is doubtful whether the human faculties were ever designed to penetrate so far into the modes of vital action, and, in the meantime, it will be better for us to confine our attention to that branch of the inquiry which bears a direct reference to our own welfare. We know already, that certain organs are concerned in the processes above mentioned, and that these organs act under the regulation of certain general laws. If we make ourselves acquained with, and carefully fulfil these laws, we reap a rich reward in the enjoyment of sound and vigorous digestion. Whereas, if, either from ignorance or from carelessness, we neglect their fulfilment, we bring upon ourselves a severe punishment in the form of dyspeptic or nervous disease. Assuredly, then, alternatives like these ought to excite som desire for information in the minds even of the most indifferent.
Before, however, commencing a description of the organs concerned in digestion, it will be useful to take a general view of the different stages of preparation through which the food passes, between its reception into the stomach and its assimilation, or ultimate conversion into a constituent part of the animal body, and becoming endowed with the properties of life. The reader will thus be better enabled to understand the meaning of various terms and expressions, the frequent use of which it is almost impossible to avoid, even in the beginning of our exposition.
When the food is received into the stomach, it is there subjected to the action of a solvent fluid, called the gastric or stomach juice (from gaster, stomach), by which it is gradually converted into a soft grayish and pultaceous mass, called chyme (from chymos, humour of juice); whence the process is called chymification, or chyme-making. The chyme, as fast as it is formed, is expelled by the contractile power of the stomach into the duodenum (from duodenus, consisting of twelve, because it is supposed to be about twelve inches long), or first portion of the instestines. It there meets with the bile from the liver, and with the pancreatic juice, which very much resembles the saliva, from the pancreas, or sweet-bread (pas, all, and kreas, flesh, it being of a fleshy consistence), a large gland which lies across the spine a little below the stomach, and is marked P in the wood-cut given in the chapter on Chylification. By the action of these two fluids, the chyme is converted into two distinct portions, - a milky white fluid named chyle (from chylos, chyle), and a thick yellow residue. This process is called chylification, or chyle-making. The chyle is then sucked in by aborbent vessels, extensively ramified on the inner membrane of lining of the bowels, and sometimes named, from the white colour of their contents, lacteals, or milk-bearers (from lac, milk). These lacteals ultimately converge into one trunk, named the thoracic duct, or chest-pipe (from its course lying through the throrax or chest), and which terminates, as will be seen in a subsequent cut, in the great vein under the clavicle, or collar-bone, hende called subclavian vein, just before the latter reaches the right side of the heart; and there the chyle is poured into the general current of the venous blood.
But although thus mingled with the blood, the chyle is not yet sufficiently capacitated for its duties in the system. To complete its preparation, it still requires to be exposed to the action of the air during respiration (For a full explanation of the nature of respiration, see the author´s Principles of Physiology, &c., chap. vii.). This is accordingly done by its passing through the lungs along with the dark or venous blood, which stands in need of the same change. In the course of this process, both the chyle and the venous blood are converted into red, arterial, or nutritive blood, which is afterward distributed by the heart through the arteries, to supply nourishment and support to every part of the body. Hence the change which takes place in the lungs is properly enough named sanguification, or blood-making.
The thickish yellow residue left in the duodenum after the separation of the chyle from the chyme, is that portion of the food which affords no nourishment, and which, after traversing the whole length of the intestinal canal, and undergoing still farther change, is thrown out of the body in the shape of faeces or excrement. But in this course its bulk is increased, and its appearence changed, by the addition of much waste matter, which, having already served its purposes in the system, is at last, as will be afterward shown, thrown otu by the same channel
With this general view of the nature of Digestion before us, we can now examine more satisfactorily the structure and mode of action of the organs concerned in effecting it. Chymification being the first step in the complicated process, we shall begin wiht the organ by which it is performed, namely the STOMACH.
In the lowes class of animals, - the hydra, for example, which belongs to the order of gelatinous polypi, and abounds in stagnant pools, - the stomach is like a simple bag, devoid of any peculiar organization; or, more properly speaking, the animal itself is nothing more than a living stomach; for the minutest inspection can discover in it no trace of any thing like vessels, nerves, brain, lungs, heart or other known organ. Even the experienced eye of Cuvier, aided by a powerful microscope, could detect in their structure nothing more than a transparent parenchyma, full of darkish grains or points, and offering no trace of any distinguishable organs (Cuvier´s Régen Animal, vol. iii., p. 295.). In form, the animal somewhat resembles the finger of a glove, the hollow in the centre being appropriated for the reception of its food; and yet with all this simplicity of structure, it not only moves and swims, but seizes its prey by means of its tentacula, thrusts it into its cavity, and digests it visibly - "á vue d´aeil." And what is still more strange, when it is turned inside out, the surface, which was formerly the exterior of the body. now digests as actively and efficiently as if it had never served any other purpose.
Animals of this very simple description have obviously no need of vessels of any kind, because every part of the internal surface of the body is in immediate contact with its food, and seems to imbibe or absorb directly all the nourishment which it requires; for vessels are wanted only where digestion goes on in one part, and the nutriment requires to be carried to another - a vessel being neither more nor less a tube for conveying the requisite nourishment from the place in which it is prepared to parts where its presence is necessary. Owing to this extreme simplicity of organization, a hydra may be cut into pieces, each of which will become a perfect animal. But, in proportion as we ascend in the scale of creation, the organization becomes more complicated, and the functions more numerous, and also more dependant on each other; so that, when we arrive at man - the highest of all - we find that the loss of injury of any vital organ puts a stop to or impairs the action of all the rest. It is impossible, however, in an elementary work like this, to trace the graduation through the different series of animals. We must confine our examination to man, and only borrow from other creatures such illustrations as may be necessary for throwing light upon the human structure.
In man, then, the stomach is a large membranous and muscular bag, lying under the convexity of the lower ribs of the left side, and stretching towards the right a little beyond the hollow commonly called the pit of the stomach. In shape it somewhat resembles the bag of a bagpipe, as will be seen from the picture beyond; its left or larger extremity being in contact with the ribs, and its right or narrow extremity situated under the pit of the stomach.
Its position relatively to the chest, bowels, and liver, will be understood by inspecting the subjoined figure, already given in the former volume. It is separated from the cavity of the chest by the diaphragm of midriff D D, with which its upper surface is in immediate contact, and throuht which the gullet passes to enter its left extremity. Its right of pyloric extremity, marked P in the cut below, lies close to the lower surface of the liver, the latter being a little diplaced to show its situation. On the lower surface it has the appearance of resting on the intestines, as if imbedded among their folds.
The parts of the stomach which have received names and require to be noticed, are, - the cardiac orifice (marked C in the next figure, and so named from being near the kear, or heart), in which the gullet terminates, and through which food and drink are introduced; P, the pylorus, or pyloric orifice (from pylorus, a gate-keeper, because it allows none but digested food to pass out), where the intestine called the duodenum begins, and through which the chyme passes after digestion in completed, and which, when the stomach is full, is nearly on a level with the cardia, although when empty it is lower; SS, the smaller arch or curvature; and GGG, the greater arch or curvature. The spleen is attached to that part of the larger arch marked with dotted points. From the situation of the cardia C, and its connexion with the gullet, it will be at once perceived that this forms one of the points of attachment by which the stomach is retained in its place.
In size, the stomach varies much in different individuals as well as in different animals, according to the bulk and quality of their food. As a general rule, it is larger among the labouring poor than among the rich, as the former require a larger quantity of their inferior food to obtain from it an equal amont of nourishment. For the same reason, animals which subsist on vegetable substances hav a very capacious stomach, while those subsisting on animal or concentrated food have it simple and small. In man, its capacity may be diminished or augmented within certain limits by corresponding modifications of diet. In some gluttons, and in cases of diseased appetite, it has been found of enormous dimensions; but this rule does not always hold, for the stomach is sometimes smaller than usual in immoderate eaters, and then its contents pass through only partially digested.
In accordance with this relation between the capacity and structure of the organs of digestion and the quality of the food, the stomach and intestines are found to be very small and short in carnivorous quadrupeds and birds of prey, which are intended to live on concentrated aliment. The same is the case with the granivorous or grain-eating birds, as their food also is contained in a small bulk. But in herbivorous animals - in the food of which the nutritive principle forms a very small portion of the whole, perhaps not one twentieth, and which consequently require a large bulk of it for their sustenance - the digestive apparatus is on a large scale, as any one may conceive in a moment, by comparing the portly protuberance of the cow with the lank paunch of the greyhound. The cow, in fact, is little else but a living laboratory for the conversion of vegetable into animal matter; and accordingly, not only is its stomach large and complex, but its intestinal canal is nearly twenty-four times the length of its body; whereas, in some carnivorous animals, the whole intestine does not exceed once their own length.
In ruminating animals, such as the sheep and ox, the stomach, as will be seen from the annexed figure, not only is large, to adapt to the bulky nature of their food, but is complicated in its structure, to fit it for effecting the great changes which vegetable aliment requires to undergo before it can be converted into blood. It may, indeed, be said to consist of four distinct stomachs conjoined.
In the first of these, AA, termed the paunch, the herbage is deposited when first swallowed, after hasty and ineffectual mastication. It there undergoes a kind of maceration, or steeping, in a fluid provided for the purpose; after which it passes from the paunch into a smaller bag, called the reticulum, or bonnet, B, which in some animals, such as the camel and dromedary, is designed exclusively as a reservoir for water, which, being there stored up in large quantities, ready for use when wanted, fits them in a wonderful manner for travelling through the arid deserts where no water is to be obtained, and where, without such provision, they would of course soon perish. So admirably is the reticulum adapted for this special purpose, that the water contained in it undergoes little or no change either in quality or quantity, although, if it were collected in the ordinary digesting stomach, it would be entirely absorbed in the course of a few minutes. It is not even mixed with the food which is swallowed after it, as the animal has the power of directing solids at once into the other cavities. From the reticulum, the alimentary mass is again returned to the mouth, there to be thoroughly masticated and mixed with the saliva; after which it descends a second time through the gullet; but instead of passing, as before, into the paunch, it enters the third bag, omasum, or many-plies, C, where it undergoes farther changes, and is then transmitted to the fourth portion, D, adjoining the pylorus, and named ab-omasum, or redbag. The last portion is exactly similar in structure and in function to the simple stomach of man and the other mammalia, and is, in fact, the true stomach, the other three being merely preparatory organs
The first part of the process, by which the food is taken hastily into the paunch, and afterward sent back to the mouth in detached portions, for farther mastication, is called rumination, or chewing the cud, and those species which perform it are thence called ruminating animals. Sheep and cows may be seen lying ruminating in pasture-fields after having cropped ad much herbage as fills the paunch; and feeding is thus rendered to them a source of prolonged enjoyment.
In those birds, again, which live on hard grain and seed, and possess no organs of mastication wherewith to bruise or grind them down, another modification of the digestive apparatus is found. Nature has furnished them with a membranous bag, called a crop, or craw, into which the food is received, and where it is slightly softended by a mucous fluid secreted from the surface of the bag. Thus prepared, it is transmitted into an organ analogous to the stomach of other animals, and called the gizzard, which has a very singular structure. Its walls are composed of four distinct portions of thich, tough, muscular substance, a large one at each side of the cavity, and a small one at each end. The inner surface of the muscle is lined with an extremely callous cuticle, approaching in hardness to cartilage or horn. When the moistened grain is introduced into the gizzard from the crop, the muscular walls of the gizzard enter into powerful action, and, by their alternate contraction and relaxation, bruise the grain as between two grindstones. In some birds their action is assisted by a quantity of small gravel, purposely swallowed along with the food; and it is well known to seamen that poultry never thrive on a voyage, hosever well they may be fed, if gravel or coarse sand, as well as food, is not placed within their reach. Mr. Hunter has counted as many as a thousand small stones in the gizzard of a common goose (The above description and figure are taken, with slight alterations, from the Treatise on Animal Physiology in the Library of Useful Knowledge.).
The astonishing force with which the muscles of the gizzard act, and the resistance of its lining membrane, may be conceived from the experiments of Spallanzani and Reaumur, who compelled geese and other birds to swallow needles, lancets, and other sharp metallic bodies, and, on afterward killing them, regularly found the points broken off, and the edges blunted, without any injury having been sustained by the gizzard itself.
In STRUCTURE, the stomach of both man and animals consists of three membranous layers or coats, of follicles or glands, and of numerous bloodvessels and nerves.
The first or external layer is the smooth, glistening, whitish membrane, which is familiar to all who have ever seen an animal opened, or hanging at a butcher´s door. It si a fold of the tough shining membrane, called peritoneum (from periteino, I extend round), which lines the abdomen, and constitutes the outer covering of all the abdominal organs. Its use is obviously to strengthen the substance of the stomach, to assist in binding down this and the other organs in their respective situations, and, by the smoothness and constant moisture of their surfaces, to enable them to move upon each other, and adapt themselves freely to their different states of emptiness and distention.
The second, middle or muscular coat consists of fleshy fibres, one layer of which, running longitudinally from the cardia to the pylorus, seems to be a continuation of teh longitudinal muscular fibres of the gullet; another runs in a circular direction, embracing, as it were, the stomach from one curvature to the other, and constituting what are called the transverse fibres. A third and more internal layer of the coat is spoken of by Sir Charles Bell, as a continuation of the circular fibres of the gullet, which divide into two parcels, the one distributed over the left or larger end, and the other over the pyloric or narrower end.
The uses of the muscular coat have, as we shall afterward see, a direct reference to the special function of digestion. By the joint action of its longitudinal and circular fibres, the stomach is enabled to contract, and shorten its diameter in every direction, so as to adapt its capacity to the volume of its contents; while, by their successive action, or alternate contraction and relaxation, a kind of churning motion is produced, which contributes greatly to digestion by the motion which it imparts to the food, and the consequent exposure which it effects of every portion of it in its turn to the contact of the gastric juice
The force and rapidity of these muscular contractions are modified by the more or less stimulant nature of the food, the state of health, exercise, and other circumstances; but, according to Dr. Beaumont, the ordinary direction in which they take place, and the course which they impart to the food, are as follows:-
William Beaumont, 1785-1853. This illustration is from a later source.
The alimentary bolus or morsel, on entering the cardiac orifice, turns to the left, follows the line of the great curvature of the stomach towards the pylorus, returns in the line of the smaller curvature, makes it appearence again at the cardia, and then descends as before to the great curvature, to undergo similar revolutions till digestion be completed. Each revolution occupies about from one to three minutes, and its rapidity increases as chymification advances.
In treating of muscular action in the former volume, I pointed otu the necessity of the cooperation of a nervous stimulus to produce the result; and remarked that there are tow kinds of muscles, one called the voluntary, which contract at the command of the will, and the other the involuntry, over which the will has no control, and which act only in obedience to their own peculiar stimuli. Of the latter description are the muscular fibres of the stomch. They contract when the stiulus of food is applied to them; but we can neither contract nor relax them by an effort of the will, nor are we even concious of their existence.
The third and innermost coat, called the mucous, or villous, is that smooth, unequal, velvety membrane, of a reddish-white or pale pink colour, which lines the internal surface of the stomach. From being of much greater extent than the other two coats, its surface is thrown into rugae, plicae, folds or wrinkles, which are simple in man, but very marked in some animals, as seen familiarly in tripe. The subjoined wood-cut, from the Library of Useful Knowledge, will give some notion of their appearence.
Near the pyloric orifice the villous coat is doubled on itself, so as to form a ring, called the valve of the pylorus, the object of which is to prevent the too early exit of food; this object however, it accomplishes not by any contractile power of its own, but by the aid of a layer of muscular fibres lying behind it. The villous coat is constantly covered with a very thin, transparent, viscid mucus, and its folds are always best seen in those who die suddenly. After disease, when the stomach is relaxed, they frequently disappear.
In addition to the folds just described, the mucous coat contains a great number of spheroidal glandular bodies or follicles, some of them scarcely largen than pinheads, which lie immediately beneath and almost incorporated with it, and which are most numerous near the pylorus. Physiologists are not entirely agreed, whether the fluid secreted by these follicles be the gastric juice, or merely the mucus already referred to as lubricating the internal surface of the stomach. The latter, however, is the opinion generally entertined, and the one which is supported, as we shall afterward see, by the stongest evidence; the gastric juice being, in fact, secreted directly from the capillary or hair-sized vessels in which the minute branches fo the arteries terminate.
Of the nerves and bloodvessels supplying the stomach it is unnecessary to say much. We shall afterward have occasion to notice the former at some length, and to the general reader the origin and distribution of the bloodvessels are as unimportant as they would be difficult of comprehension; for the nature of the red blood is the same by whatever artery is supplied, and that of the dark blood the same by whatever vein it is returned to the heart. All that it is important to know is, that the stomach receives a large supply of blood by means of numerous bloodvessels, the principal of which, as represented on an earlier wood-cut, follow the course of the greater and smaller curvatures GG and SS, and send off innumerable small branches as they proceed to every part of the stomach. The coronary artery, and the pyloric branche of the hepatic, or liver artery, go to the smaller curvature, while another branche of the hepatic, and one from the splenic, or spleen artery, are ramified on the larger curvature.
In determining the uses of the internal or villous coat of the stomach, we must begin by considering separately that of each of the elementary structures of which it is composed - its follicles, bloodvessels, and nerves - and the nature of the peculiar secretion, the gastric juice, to which it gives rise.
The FOLLICLES pour out the bland viscid mucus which lubricates the internal coat, and protects it in some degree from sustaining injury by the immediate contact of irritationg bodies. When the follicles are diseased, as in what is called water-brach, they sometimes throw out a large quantity of a ropy transparent fluid, which oppresses the stomach and impais digestion.
The BLOODVESSELS of the stomach, like those of every other part, are more or less active, according to the energy of tis functions at the time. In treating of the laws of exercise as applicable to al living parts (Principles of Physiology, &c.), I took considerable pains to point out the relation which the Creator has established between the activity of every orgn and the energy of its vital functions. When the brain is exercised and the mind active, an augmented flow of blood takes place towards it to support its increased action. When it is inactive and the mind indolent, a diminished flow of blood occurs. In like manner, when the muscles are called into vigorous action, the circulation of the blood through them is quickened, and their nerves are more than usually excited; greater waste of material is caused by the increase of activity, and more blood, consequently, is required to repair the waste and sustain their tone. This law was so well known to the older writers, that is was announced by them as an axiom in the very comprehensive phrase, Ubi stimulus, ibi affluxus - "Whenever a stimulus is, there is also an afflux."
The stomach forms no exception to this general law of animal economy. When it is empty and idle, it is contracted upon itself into comparatively small bulk; and its bloodvessels become shortened and tortous in a corresponding degree. The result is both a diminution of their calibre and a slower circulation through their branches. But when the stomach is full and active, the bloodvessels have free ccope, their tortuosity disappears, their diameter enlarges, and the circulation through them becomes quicker, and fit for the rapid secretion of the mucous and gastric fluids in the quantities which we have seen to be required for the fulfilment of digestion. Accordingly, when the latter process is going on, the small arterial branches ramified on the mucous coat of the stomach become so multiplied and distended, as to impart to it a deeper red colour than it has when the stomach is empty. The increased afflux of red or arterial blood to the stomach during digestion, is not merely inferred from the analogy of other organs. Many opportunities have occured of ascertaining the fact; and, as I shall have occasion to mention, Dr. Beaumont very often saw it take place.
A corresponding change occurs in the veins of the stomach during digestion. Their diameter becomes enlarged, their course more straight, and the current of blood through them more rapid. As the minute or capillary extremities of the arteries open upon the inner surface of the stomach, and there exhale a fluid secretion, so the corresponding venous capillaries likewise open upon the same surface, and inhale or absorb fluid, which they carry into the general circulation. The rapidity with which this absorption sometimes takes place is almost incredible; for a large draught of water may be thus taken up in a few minutes. Fluids mixed with camphire or other strong-scented substance have been given to animals as an experiment, and, on killing them shortly afterward, the peculiar smell has been detected in the blood. Liquids are thus not digested, but simply absorbed.
Rapid, however, as the process is, poisons which enter the system by absorption do not by any means act so instantaneously as those which directly affect the nervous system.
In regard to the peculiar influence which each of the NERVES ramified on the stomach exercises on its functions, much difference of opinion still prevails. We may, however, gather some useful notions by adverting to the different sources whence they are derived, and comparing these with the purposes for which we know from analogy that different kinds of nerves are required.
Strictly speaking, the nervous filaments supplied to the stomach proceed from three distinct sources, and may be held to fulfil as many distinct uses. In apparent accordance with this, we observe three, if not four, distinct classes of operations going on in that organ, each of which may, from analogy, be presumed to require a distinct nerve for its performance. These are, first, the pleasurable consciousness attendant on the presence fo wholesome food in a healthy stomach, and which becomes painful and disagreeable when the stomach is diseased or the food of improper quality; secondly, the peristaltic or muscular motion which commences the moment food is swallowed, and continues till digestion is completed; and, lastly, the different processes of circulation, nutrition, secretion, and absorption, which go on in the component tissues of the stomach and support its life. To these ought perhaps to be added the sensation in which the feeling of appetite originatesm and which we have already seen to be connected wiht the pneumogastric nerve. But as it is still uncertain whether it and the first of the three now named may not be modifications of the same thing, I shall not insist on considering them.
Although we cannot state positively what particular nerve presides over each of these functions, it may be mentioned that strong presumptive evidence has been adduced, particularly by Brachet, to show that the pneumogastric (from pneumon, a lung, and gaster, the stomach, or lung and stomach nerve) nerve is charged with the involuntary motions of the stomach, as well as with the sense of its condition. Food being the natural stimulus of that organ, as light is of the eye, its presence alone, without and even against the will, suffices to produce the contraction of its muscular coat; and accordingly, the more stimulating the food, the more rapid and vigorous is the muscular contraction which it excites. So far, indeed, do the stomachic nerves respond to their own stimuli, that, if nauseous or other irritant and indigestible substances be swallowed, the action of the muscular coat becomes so violent as to excite sympathetically the simultaneous contraction of the diaphragm and abdominal muscles, to aid in their immediate expulsion by vomiting; and this is the reason why such substances are in common use as emetics.
Magendie doubts whether these movements are in any measure dependant upon nervous influence; but the fact of their being so seems to be proved by the experiments of Gmelin and Tiedemann, who found them constantly produced when the pneumogastric nerve was irritated either by the scalpel or by the contact of alcohol. Brachet, also, who examined the subject with great care, obtained similar results; and the only plausible argument against their conclusiveness consists in the double function which seems thus to be assigned to a single nerve - that of conveying to the brain a sense of the state of the stomach, and that of imparting motion to its muscular fibres. Brachet, however, turns this charge into an additional proof; for, on careful dissection, it appears that the pneumogastric nerve is really a compound of two distinct sets of fibres, intimately connected, no doublt, in structure and in function, but each essentially distinct in its origin, and so far fitted for a peculiar office.
When the pneumogastric or chief nerve of the stomach is tied or cut through, and its ends separated so as to interrrupt the flow of nervous energy towards that organ, digestion is either entirely arrested or greatly impaired. By the greater number of physiologists this result is considered to arise from the consequent stoppage of that gentle and continued agitation of the alimentary mass in the stomach which is necessary for its thorough impregnation with the gastric juice, and which we have seen to depend on a stimulus communicated to its muscular coat by that nerve. By some, however, this explanation is regarded as incorrect. Magendie and Dr. Holland, for example, say that they have sometimes observed digestion continue even after the division of the nerve; and that, when it is retarded or impaired, the result arises only from the troubled state of respiration which the cutting of the nerve induces at the same time. To this, again, it is answered, that Dupuytren has divided the nerve below the part where the pulmonary branches are given off, and consequently left respiration unimpaired; but that still digestion was arrested, provided a portion of it was cut out so as not to allow the current of nervous influence to continue: for if the two ends of the nerve be left nearly in contact, it appears that little interruption of its action takes place.
Here, however, I cannot help observing, that, in drawing conclusions from experiments of this nature, the constitutional disturbance inseparable from the infliction of extensive wounds on living animals is seldom taken sufficiently into account. As regards digestion, for example, it is not to be expected that that function can be carried on with all the regularity of health when the animal is suffering severe pain, even although the stomach be left untouched. Brachet indeed has shown, by direct experiment, that digestion may be interrupted as effectually by making an incision on the side or thigh (provided it be sufficiently deep and painful to excite constitutional disturbance), as by cutting the pneumogastric nerve itself. This being the case, we must not be too hasty in considering every apparent result as inseparably and exclusively connected with the individual injury under our notice at the time; we must repeat our observations under every variety of circumstances, and be careful to separate the accidental from the essential, before admitting the inference to be correct. It is, in fact this unavoidable source of vagueness which so often renders experiments on living animals as inconclusive as they are inherently cruel.
But after making every allowance on this account, the experiments on digestion have been so frequently repeated, and so extensively varied, that the general results already noticed may safely be regarded as demonstrated. On all hands, accordingly, the necessity of the co-operation of the nervous energy in effecting it is admitted; for no one seriously denies the fact, that retardation or total cessation of digestion ensues, when the flow of the nervous fluid towards the stomach is prevented by the division and separation of the cut ends of the pneumogastric nerve, or by the operation of narcotics and the other disturbing causes already alluded to. It is true that the mode in which the nerve acts is not yet ascertained, although the fact of its necessary co-operation is rarely disputed. As, however, the direction of a current of galvanism to the cut end of the nerve next the stomach suffices to re-establish digestion after that process has been suspended by the interruption of the nervous influence consequent on its division, we may reasonably infer that, in the healthy state, the nerve merely transmits to the stomach a stimulus or energy generated for the purpose either in the brain or in the spinal marrow and ganglia - that the nerve, in short, acts only as a conductor, and does not originate the influence which it evidently imparts. In several of Brachet´s cases, indeed, as well as in those of Tiedemann, the continued irritation of the cut end of the nerve proved sufficient to carry on digestion to a certain point, by affording, in another way, the necessary stimulus to the muscular contractions of the stomach: for in all these experiments, digestion was found to have advanced almost in exact proportion to the degree of admixture which had been effected of the food with the gastric juice, - an admixture now ascertained to be produced chiefly by the contractile power of the stomach itself.
The muscular contractions of the stomach being thus under guidance of the pneumogastric nerve, what are called its vital functions - those by which its life is sustained - viz., circulation, nutrition, secretion, and absorption, are generally considered to be carried on under the influence of the great symphatetic or ganglionic nerve, so called from its very extensive ramifications being supposed to bring the different parts of the system into relation with each other, and which, accordingly, is found to exist in animals that have neither brain nor spinal marrow, nor nerves of voluntary motion. In man, however, the symphatetic nerve receives filaments from the sentient or feeling part to the spinal marrow, probably for the purpose of connecting more intimately the organic with the higher functions of animal life. But as much obscurity still prevails on this subject, and moreover whe have no direct control over the action of the ganglionic nerves, I shall not detain the reader with any conjectural discussion, but rather request hist attention for a moment to the circumstance that it is through the medium of the nervous communications above hinted at, that the very remarkable influence which all must have been experienced and observed as constantly exerted by the mind and stomach on each other, is kept up - an influence so powerful in its effects on both bodily and mental health, as to require special notice when we shall treat of the practical application of the present exposition.
The importance of the nervous agency in effecting digestion has been denied, because we are not conscious of the presence of food in the stomach. But in health the want of such consciousness is a privilege and not a defect; and it has been admirably pointed out by Dr. Southwood Smith (Philosophy of Health, vol. i., p. 80), that in possessing, as we do, the distinct consciousness of a pleasurable feeling in the stomach after indulging in a suitable meal, we have all that is desirable for either utility or enjoyment. If we were aware of the presence of every portion of food which the stomach contained, and of the changes occuring in each, our attention would be so disagreeably and unprofitably taken up that we would pray to be delivered from the annoyance. Where, however, from disease or the food being inappropriate, the stomach is injured byt what is eaten, consciousness then becomes painful, for the express purpose of warning us that mischiefs has been done, and that we must take means for its removal.
The nerves of the stomach, it ought to be remarked, have a direct relation to undigested food; or, in other words, undigested food forms their natural and appropriate stimulus. In consequence of this arrangement, shen any body incapable of digestion is introduced into its cavity, distinct uneasiness is speedily excited, and an effort is soon made to expel it either upwards by the mouth or downwards by the bowel. It is in this way that bile in the stomach excites nausea, and that tartar emetic produces vomiting. The nerves of the bowels, on the other hand, are constituted with relation to the presence of digested food, and consequently, when any thing escapes into them from the stomach in an undigested state, it becomes to them a source of irritative excitement; and hence the colicky pains and bowel-complaints, which so commonly attend the passage of such undigestible substances as fat, husks of fruits, berries, and cherry-stones, through the intestinal canal.
Such, then, are the component parts of the stomach, and such the uses which they individually fulfil; but before we can consider them in their combined form, there is still another agent, and an important one, in digestion, which has already been repeatedly named, and which, though not a portion of the stomach, yet plays too conspicious a art in its operations not to require some separate notice - the GASTRIC or STOMACH JUICE.
The existence of a solvent fluid in the stomach has long been known, and its uses suspected; but for our first accurate acquaintance with its properties and mode of action, we are indebted chiefly to the sagacity and persevering zeal of Spallanzani, who investigated the subject with great care and success about the middle of last century. Considering the peculiar difficulties by which the inquiry is surrounded, it is offering no trifling homage to that distinguished observer to say, that by means of numerous, varied, and well-devised experiments on man and animals, he succeeded in overcoming most of the obstacles which had baffled the ingenuity of his predecessors, and in obtaining results, the general accuracy and importance of which are now appreciated more and more highly, in proportion as our knowledge advances, and opportunities present themselves or bringing them to the test of experience.
It is rarely, indeed, that we can actually see what is going on in a healthy stomach; but in a few instances this advantage has been enjoyed, and turned to account in investigating the phenomena of digestion. By far the most instructive example of this kind which has ever occured, has lately come under the observation of Dr. Beaumont of the American army; and, as that gentleman eagerly embraced the opportunity so unexpectedly afforded him, of testing the prevailing doctrines by a series of experiments, continued during a period of several years, and under various conditions of health and external circumstances, I shall so frequently have occasion to refer to his observations, that it will be useful to give a brief outline of the case before entering farther upon the subject, in order that the reader may be enabled to judge for himself what weight is due to Dr. Beaumont´s evidence on any disputed point.
Dr. Beaumont, while stationed at Michilimackinac, in the Michigan territory, in 1822, in the military service of the United States, was called upon to take charge of ALEXIS ST. MARTIN, a young Canadian of eighteen years of age, good constitution, and robust health, who was accidentally wounded by the dischard of a musket on 6th June, 1822.
Alexis St. Martin, 1794-1881. This illustration is from a later source.
"The charge," says Dr. Beaumont, "consisting of powder and duck-shot, was received in the left side, at the distance of one yard from the muzzle of the gun. The contents entered posteriorly, and in an oblique direction, forward and inward; literally blowing off integuments and muscles to the size of a man´s hand, fracturing and carrying away the anterior half of the sixth rib, fracturing the fifth, lacerating the lower portion of the left lobe of the lungs, the diaphragm, and PERFORATING THE STOMACH."
On the fifth day, sloughing took place; lacerated portions of the lung and stomach separated, and left a perforation into the latter "large enough to admit the whole length of the middle finger into its cavity; and also a passage into the chest half as large as his fist." Violent fever and farther sloughing ensued; and for seventeen days every thing swallowed passed out through the wound, and the patient was kept alive chiefly by nourishing injections. By-and-by the fever subsied, the wound improved in appearance, and after the fourth week the appetite became good, digestion regular, the evacuations natural and the health of the system complete. The orifice, however, never closed; and at every dressing the contents of the stomach flowed out, and its coats frequently became everted or protruded so far as to equal in size a hen´s egg, but they were always easily returned. The figure below exhibits the appearance of the wound after it was healed. The circumference of the wound EEEL, extended to about twelve inches; and the opening into the stomach AAA, nearly in its centre, was about two inches long below the left nipple F. The folds of the villous coat are visible at BC.
Some months after, St. Martin suffered extremely from the death and exfoliation of portions of the injured ribs and their cartilages, and his life was often in jeopardy; but, through the skill and unremitting care with which he was treated by Dr. Beaumont, he ultimately recovered, and in April, 1823, was going about, doing light work, and rapidly regaining strength.
On 6th June, 1823, a year from the date of the accident, the injured parts were all sounds except the perforation into the stomach, which was now two and a half inches in circumference. For some months thereafter the food could be retained only by constantly wearing a compress and bandage; but early in winter, a small fold or doubling of the villous coat began to appear, which gradually increased till it filled the aperture, and acted as a valve, so as completely to prevent any efflux from within, but to admit of being easily pushed back by the finger from without.
Here, then, was an admirable opportunity for experimenting on the subject of digestion, and for observing the healthy and undisturbed operations of nature free from the agony of vivisections, and from the sources of fallacy inseparable from operating on animals. Dr. Beaumont was sensible of its value, and accordingly pursued his inquiries with a zeal, perseverance, and disinterestedness highly creditable to his character both as a man and as a philosopher. Having been fortunate enough to obtain a copy of his work, I shall not hesitate to make free use of its contents.
Dr. Beaumont began his experiments in May, 1825, and continued them for four to five months, St. Martin being then in high health. In the autumn, St. Martin returned to Canada, married, had a family, worked hard, engaged as a voyageur with the Hudson´s Bay Fur Company, remained there four years, and was then engaged at a great expense by Dr. Beaumont to come and reside near him on the Mississippi, for the purpose of enabling him to complete his investigations. He came accordingly in August, 1829, and remained till March, 1831. He then went a second time to Canada, but returned to Dr. Beaumont in November, 1832, when the experiments were once more resumed, and continued till March, 1833, at which time he finally left Dr. Beaumont. He now enjoys perfect health, but the orifice made by the wound remains in the same state as in 1824.
Dr. Beaumont describes the aperture in St. Martin´s stomach as being situated about three inches to the left of the cardia, near the left superior termination of the great curvature. When the stomach was nearly empty, he was able to examine is cavity to the depth of five or six inches by artificial distention. When it was entirely empty, the stomach was always contracted on itself, and the valve generally forced through the orifice, together with a portion of the mucous membrane equal in bulk to a hen´s egg. After sleeping for a few hours on the left side, the protruded portion became so much larger as to spread "over the neighbouring integuments five or six inches in circumference, fairly exhibiting the natural rugae, villous membrane, and mucous coat, lining the gastric cavity. This appearance is almost invariably exhibited in the morning before rising fomr bed." Such was the very favourable subject on whom Dr. Beaumont´s observations and experiments were made, and such were the numerous opportunities which he enjoyed for repeating them, and verifying their accuracy. Having given this outline, we now return to the consideration of the gastric juice, on the origin and qualities of which it throws so much light.
In treating of the properties of the gastric juice, I shall on all disputed points five a decided preference to the observations of Dr. Beaumont over those of other physiologists; because, although a few cases have occured, in which, from external wounds, direct access has been given to the interior of the stomach, and Richerand and others have availed themselves of the opportunities thus afforded of adding to our knowledge of the digestive process, still, in most of them which have been recorded, the patients were a comparatively short time under observation, and were not sufficiently re-established in health to admit of either extensive or conclusive experiments being made. Whereas, in the case which Dr. Beaumont had the good fortune to meet with, the patient remained under his eye for several years, and in the enjoyment of the most robust health; so that ample time and opportunity were afforded for every variety of experiments which reflection could suggest, and for their subsequent repetition under such modifications as seemed to be requisite for clearly distinguishing the accidental from the constant ahd essential health. In addition to these reasons, it ought to be added, in justice to the American physiologist, that, from the excellent judgement with which he carried on his investigations, and the scrupulous care with which he announces his results and separate facts from theory, it is impossible not to place great confidence both in his personal qualifications as an observer, and in the general accuracy of his statements. Moreover, as he enjoyed the rare advantage of seeing what he describes to have taken place in the stomach during healthy digestion, his evidence comes before during healthy digestion, his evidence comes before us with the strongest possible claims on our attention.
The first disputed point which is conclusively settled by Dr. Beaumont is, that the gastric juice does not continue to be secreted between the intervals of digestion, and does not accumulate to be ready for acting upon the next meal. By inducing St. Martin to fast for some hours, and then placing him wiht the opering in the left side exposed to a strong light, so as to give a distinct view of the cavity of the stomach, Dr. Beaumont found its only contents to consist of a little viscid and occasionally slightly acidulated mucus mixed with saliva, and in no instance did he perceive any accumulation of the proper gastric juice. The same results had indeed been obtained by Tiedemann and other physiologists before the publication of Dr. Beaumont´s memoir; but the evidence of the latter is so much more direct and incontrovertible, that it may justly be regarded as setting the question for ever at rest.
Having proceeded so far, Dr. Beaumont next endeavoured to discover at what time the gastric juice begins to be poured out, and under what conditions its secretion is carried on; and here again ocular inspection afforded him satisfactory results.
It has already been remarked, that, on pushing back the valve filled up the opening into the stomach, the cavity within became visible to a considerable extent; and that when St. Martin lay over for a time on the left side, a portion of the villous coat, large enough to exhibit several inches of its surface, generally protruded. Owing to these circumstances, Dr. Beaumont could easily observe what changes occured, both when food was swallowed in the usual way, and when it was introduced at the opening left by the wound. Accordingly, on examining the surface of the villous coat with a magnifying-glass, he perceived an immediate change of appearence ensue whenever any aliment was brought into contact with it. The action of the neighbouring bloodvessels was instantly increased, and their branches dilated as to admit the red blood much more freely than before. The colour of the membrane consequently changed from a pale pink to a deeper red, the vermicular or wormlike motions of the stomach became excited, and innumerable minute lucid points and very fine nervous and vascular papillae could be seen arising from the villous coat, from which distilled a pure, colourless, and slightly viscid fluid, which collected in drops on the very points of the papillae, and trickled down the sides of the stomach till it mingled with the food. This afterward proved to be the secretion peculiar to that organ, or, in other words, the true gastric juice; the mucous fluid secreted by the follicles, which some have mistaken for it, is not only more viscid, but wants altogether the acid character by which it is generally distinguished.
Pursuing his experiments, Dr. Beaumont then found that the contact, not only of food, but of any mechanical irritant, such as the bulb of a thermometer, or other indigistible body, invariably gave rise to the exudation of the gastric fluid from these vascular papillae; but that, in the latter cases, the secretion always ceased in a short time, as soon apparently as the organ could ascertain that the foreign body was one over shich the gastric juice had no power. But the small quantity obtainable in this way is perhaps more pure and free from admixture, and therefore better adapted for examination, than any which can be produced under any other circumstances.
Various methods have been employed for producing the gastric fluid in a state of purity. Pieces of dry spongen, enclosed in a dry, hollow, perforated ball, with a string attached to it, have been swallowed both by man and by inferior animals, and afterward withdrawn to have the juice expressed from them. In some instances the stomachs of criminals and animals killed after fasting have been opened, and the secretion collected. At other times the juice has been procured by voluntary or artificial vomiting. None of these methods are equal to that employed by Dr. Beaumont; but of the three the first is unquestionably the best, because, although no gastric juice previously exists, the very contact of the ball excites the secretion of a quantity sufficient to moisten the sponge. In the second mode of proceeding, any portion of juice secreted in consequence of a stimulus applied after the stomach is opened, must necessarily be very small, and rendered impure by the large admixture of mucus which it will contain; while, by the third method, either no gastric juice but merely mcus will be procured, or it will be expelled mixed with the food which had previously elicidated its secretion.
Gastric juice, in its purest form, and unmixed with anything except the small portion of mucus from which it can never be obtained entirely free, is described by Dr. Beaumont to be a clear transparent fluid, without smell, slightly saltish (probably from the admixture of mucus), and very perceptibly acid. Its taste, he says, resembles that of thin mucilaginous water, slightly acidulated with muriatic acid. It is readily diffusible in water, wine, or spirits, and effervesces slightly with alkalis - a direct proof of its acid nature. It coagulates albumen, and is powerfully antiseptic, checking the progress of putrefaction in meat. When pure, it will keep for many months; but shen diluted with saliva, it becomes fetid in a few days. According to Professor Dunglison, to whome some was submitted by Dr. Beaumont for analysis, it contains free muriatic and acetic acids, - phosphates and muriates with bases of potassa, soda, magnesia, and lime, - together with an animal matter soluble in cold, but insoluble in hot water. Tiedemann and Gmelin, again, describe it as composed principally of muriatic and acetic acids, mucus, saliva, osmazome, muriate and sulphate of soda, with little or no albumen; and, according to the same physiologists, the proportion of acid is always greatest when vegetables or other substances of difficult digestion constutute the chief part of the diet. Other chymists give an analysis somewhat different from either of these; a circumstance which was indeed to be expected, considering not only the differences caused by variations of diet, but also the necessarily different degrees of purity of the fluid submitted to examination.
The most remarkable property of the gastric juice is unquestionably the power which it possesses of dissolving and reducing to the appearance of a soft, thickish fluid mass every thing in the shape of food which is submitted to its action, - while it exerts no perceptible influence on living or inorganic matter; for, so far as is yet known, nothing which is not organized, or which is still alive, can serve as nutriment for the animal frame. Water in the only inorganic body which is taken into the system for its own sake, and all mineral and other inorganic productions enter it as component parts of previously organized substances of either an animal or a vegetable nature. To a great extent, indeed, vegetation seems to be merely a process for the conversion of inorganic matter into a proper nutriment for the support of animal life; and many species of animals seem in their turn to be little else than living machines for the conversion of vegetable substances into a nutriment fit for other species by which they are intended to be devoured. It is true that, in some parts of South America, the natives, pressed by want, consume quantities of a soft unctuous clay, which is of course destitute for organization; but as there is every reason to believe that no nourishment is derived from it, and that it merely serves to allay tha pangs of hunger, such instances form no exception to the general rule.
It would have been easy for the Creator to bestow such a structure on all animals, as to make them subsist entirely on vegetable aliment. But the arrangement which he has seen fit to adopt, is the source of an infinitely greater amount of active enjoyment than what could otherwise have existed. Had there been no beasts to prey, the world would soon have been overrun with hervivorous creatures to such an extent, that their numbers would speedily have become excessive in reference to the possible supply of food, and there would have been infinitely more suffering from starvation and disease, than what actually arises out of their existing relation to each other. On the present plan, there is ample food and enjoyment for all; and when the time does arrive when one animal must become the prey of another, the deprivation of life is in most cases unforeseen, and the suffering which attends it is in general only momentary in duration. There is thus both complete enjoyment of life while it lasts, and a great additional field opened for the support of an immense class of animals, which, with their present constitution, could not otherwise have existed at all.
The gastric juice, as already remarked, has no power over living animal matter - a most wise and admirable provision, since otherwise it would at once have attacked and destroyed the very organ which produces it. This is the very reason why worms are able to exist in the stomach of man and other animals; so that, if it were possible for an oyster swallowed directly from the shell to continue to live, it would effectually resist every attempt at digestion. But it, in common with most other beings, cannot sustain life under such circumstances; and as soon as it dies, the gastric juice assumes the mastery, and speedily converts it into chyme.
If any thing could have opened Montégre´s eyes to the fallacy under which he laboured in considering the gastric juice as almost identical with saliva, the circumstance we are now to mention would have sufficed. When a person, previously in good health, dies by a violent death, or when an animal is killed soon after a meal, it very often happens that, on opening the body after an interval of some hours, the stomach is found to be eroded, and its contents poured into the cavity of the abdomen, precisely as if a hole had been formed in it by ulceration. It was long before the reason of this was discovered; but at length it was ascertained to arise from the action of gastric juice (the abundant secretion of which was provoked by the immediately preceding meal) upon the substance of the stomach, now subjected to its power from being deprived of life. This fact has been so often verified, that it is by all admitted as incontrovertibly true. If, therefore, the gastric juice be merely saliva and mucus, we might expect to find after death som traces of similar results from the contact of saliva with the mouth or gullet; but there no such erosion is ever witnessed, nor, as Montégre himself admits, does saliva exert any solvent power whatever over dead animal matter out of the body. These facts appear quite sufficient to convince any unprejudicated mind.
The power of coagulating milk, and albumen ot the white of eggs, is another remarkable property of the gastric juice, - and one so familiarly known, that in dairies an infusion of the stomach of the calf is in common use, under the name of runnet or rennet, for curdling milk. In infants, also, we know that the nurse´s milk has scarcely reached the stomach before coagulation takes place; a fact which leads many inexperienced mothers to infer that the infant is already suffering from acidity, and to counteract the supposed evil by repeated doses of magnesia - which, of course, do more harm than good. The coagulation of milk in the stomach is so far from being a morbid process, that milk cannot be properly digested without it. By the separation and absorption of the fluid whey, the curd is reduced to a proper consistence for being acted upon, both by the gastric juice and by the contractions of the muscular coat.
The gastric juice is also powerfully antiseptic; that is to say, it prevents animals substances from being putrid, and even renders sweet such as have advanced a considerable way towards putrefaction. Dr. Beaumont mentions that the pure juice will keep unchanged for almost any lenght of time; and, according to Spallanzani, meat may be preserved in it without taint for five or six weeks, or even longer. This antiseptic tendency of the gastric fluid accounts for the circumstance that little or no mischief results from the common practice among epicures, of not making use of game till the putrefactive process is advanced farther than is agreeable to tha palates of the uninitiated.
The qualities of the gastric juice are so directly adapted to the natural food of the animal, that flesh introduced into the stomach of an ox or a sheep, for example, undrgoes scarcely any change; while vegetable food, on the other hand, remains equally undigested in that of a beast of prey. Thus, "when a hawk or an owl has swallowed a small bird, in the stomach of which have been seeds, these bodies are not dissolved by the gastric fluid," (Macgillivray´s Description of the Rapacious Birds of Great Britain, p. 24) but pass through the intestines unaltered. Man, the dog, and some other creatures, possess the power of digesting all sorts of aliment, whether vegetable or animal, and are hence called omnivorous; but even in them, the relation which the properties of the gastric juice bear to the qualities of the food chiefly or exclusively used is so close, that, when a widely different kind is suddenly resorted to, indigestion is the almost inevitable consequence, because then the gastic juice has not had time to acquire its requisite adaptation to the new materials on which it has to act.
Even in its chymical constitution, the gastric juice of carnivorous animals differs from that of the herbivorous; a circumstance which accounts for the difference of effect. In the former, such as that of birds of prey, serpents, and fishes, no free or uncombined acid can be detected, although it is invariably found in the gastric fluid of vegetable-eaters. In crows and dogs, on the other hand, and such animals as can live on either kind of aliment, it is never acid except when they have been fed chiefly on grain or plants. In man, the same relation has, by numerous experiments, been ascertained to exist.
But although, in every class of living beings, the gastric juice is constituted with a direct relation to its natural food, still its qualities may be som much modified by a very gradual change of diet, as to fit it for digesting aliment of a very dissimilar or opposite kind. Thus, in the natural state, the stomach of a sheep exerts scarcely any action on beef or mutton; but if the change from the one kind of food to the other be made by slow degrees, the gastric juice will in the end become so essentially altered as to enable it to digest them. In this way, as is mentioned by Delabere Blaine, a horse at the Veterinary College was supported for sime time by animal matter alone; while others have subsisted on dried fish or on milk (Blaine´s Veterinary Art, 3d edit. p., 274). It has been shown, also, by John Hunter, Spallanzani, and others, that eagles, falcons, owls, pigeons, and domestic fowls, may for a time be fed on aliments altogether foreign to their natural habits. But these facts only show the extent to which nature will go, on an emergency, for the preservation of life; and no more indicate the equal fitness of both kinds of aliment, than the fact of some men being able to stand for a few minutes on their heads proves an inverted position to be the natural attitude of the human race.
In consequence of this adaptation of the gastric juice to the nature of the food, it is obvious that sudden and extreme changes from one kind of diet to another must be inurious, because the stomach has not time to modify its secretions sufficiently to meet the altered demand made upon its powers. This, accordingly, is one of the reasons why so much caution is used in bringing horses into condition after having been for some time in the pasture-field. When they have previously been on dry foot in the straw-yard, corn may be given with greater safety; so that it is the change, not so much in quantity as in kind of aliment, which causes the risk. And, on this account, when a horse is to be put upon hard food, after having been fed on grass or other succulent vegetables, Blaine recommends, not only that hay and corn should be given in very small quantities at first, but that the hay should be moistened, and the corn mixed with bran and mashed; by which means, having acquired a greater analogy to grass, it will be more easily acted upon by the gastric juice, which has been previously adapted for green food.
Even in man, the gastric juice undergoes considerable modifications, not merely according to the kind of aliment habitually used, but also according to the wants of the system, the season of the year, and the state of the health; so that, while sudden and great changes from one kind or diet to another are positively hurtful on the one hand, absolute uniformity is not less objectionable on the other.
Many attempts have been made to ascertain to which of the elements of the gastric juice its power is chiefly to be ascribed, and experiments have been instituted on them individually to discover which of them is most nearly analogous to it in effect. From the general results, it appears that acetic acid (vinegar) and muriatic acid have a wider range of influence, and produce solvent effects more closely resembling those of gastric juice, than any other known substances. Both of these acids, it will be recollected, are constituent elements of the gastric fluid; and it has in consequence been argued, that to them it is indebted for all its energy. And, indeed, without laying too much stress on this real or supposed analogy, it is impossible to overlook the well-known fact, that scurvy, and a highly alkaline state of the system, are generally induced by a diet restricted for a long time to animal food alone, and are prevented or cured most easily by a free use of lemonjuice or of vegetable matter, either fresh or fermented. In these circumstances, the vegetable acid is probably efficacious both by directly improving digestion, and by combining with the excess of the alkaline salts already existing in the system. It is worthy of remark, too, that in weak stomachs acidity is almost invariably induced by the use of vegetable food, possibly to some extent for the very purpose of effecting its digestion; for it has been ascertained beyond a doubt, that in hervivorous animals the gastric juice always contains some free or uncombined acid - and in man also, after living much on vegetables for some time.
The necessity of acid for the chymification of vegetable food, affords an explanation of the fondness with which the Germans and Dutch display for saurkraut - a cabbage in a state of acetous fermentation - and of its alleged easy digestibility. It explains, also, the general use of vinegar along with salads, cucumbers, oysters, salmon, and other substances of difficult digestion, and shows that its utility is not imaginary, but loudly proclaimed by nature´s own acts.
Another important principle, which Dr. Beaumont conceives to be established by his numerous experiments, and which forced itself upon him by degrees, is, that in health THE GASTRIC SECRETION ALWAYS BEARS A DIRECT RELATION TO THE QUANTITY OF ALIMENT NATURALLY REQUIRED BY THE SYSTEM; so that if more than this be taken, there will necessarily be too small a supply of the juice for the digestion of the whole. The principle here laid down is in perfect harmony with the sympathy which we have seen to exist between the stomach and the rest of the body, and therefore not only is highly probable in itself, but, if sound, will prove a most valuable guide in the practical regulation of diet. The number of phenomena which it explains, and its general applicability to daily use, afford no small presumption of its truth. When, for example, we eat more the the wants of the system require, indigestion will follow, because there will be more food in the stomach than what the quantity of gastric juice provided is able to dissolve; the proportion of the juice secreted being in relation, not to what we eat, but to the actual wants of the system, which, in the case supposed, we have greatly exceeded. Here a remarkable harmony will be perceived between the quantity of the secretion and the true implications and uses of appetite as a guide to diet, explained in a preceding chapter.
The gastric secretion, and the appearance of the villous coat, undergo great modifications during disease, and on this subject also Dr. Beaumont´s observations are highly valuable; because, instead of merely inferring, as others are obliged to do, he enjoyed the privilege of SEEING with his eyes what was actually going on. In the course of his attendance on St. Martin, he found that, whenever a feverish state was induced, whether from obstructed perspiration, from undue excitement by stimulating liquors, from overloading the stomach, or from fear, anger, or other mental emotion depressing or disturbing the nervous system, the villous coat became sometimes red and dry, and at other times pale and moist, and lost altogether its smooth and healthy appearance. As a necessary consequence, the usual secretions became vitiated, impaired, or entirely suppressed; and the follicles from which, in health, the mucus which protects the tender surface or the villous coat is poured out, became flat and flaccid, and no longer yielded their usual bland secretion. The nervous and vascular papillae, thus deprived of their defensive shield, were then subjected to undue irritation. When these diseased appearences were considerable, the system sympathized, and dryness of the mouth, thirst, quickened pulse, and other symtoms, showed themselves; and NO GASTRIC JUICE COULD BE PROCURED OR EXTRACTED EVEN ON THE APPLICATION OF THE USUAL STIMULUS OF FOOD.
These facts, if correctly observed, are of extreme importance; and from the care with which Dr. Beaumont pursued his investigations, I do not think their general accuracy can be called in question. The dry, irritated appearance of the villous coat, and the absence of the healthy gastric secretion in the febrile state, not only explain at once the want of appetite, nausea, and uneasiness generally felt in the region of the stomach, but show the folly of attempting to sustain strength by forcing the patient to eat when food cannot be digested, and when nature instinctively refuses to receive it.
Before dismissing this part of the subject, it may be remarked, that the alleged sympathy of the stomach with the wants of the body has been denied, because the sense of hunger disappears the moment food is swallowed, or the stomach is distended even with clay or sawdust, although the actual wants of the system cannot by possibility have been supplied in either case. But these facts seem to me rather to justify the inference that a sympathy does exist. Hunger ceases when food is taken, simply because now the condition of the stomach is in the desired relation to the state of the body, and the nerves consequently feel and transmit this impression to the more distant parts. In the other case, again, it ceases because the stomach cannot at first distinguish what is food from what is not; and therefore, when distended, expresses content, because it feels satisfied that it has been honestly dealt with, and got what it wanted. But whenever it discovers the cheat, which it does in no long time, hunger returns, and can be properly appeased only by digestible substances. Dr. Beaumont indeed expressly mentions, that, although the gastric secretion commences the moment any indigestible body touches the mucous surface of the stomach, it invariably ceases soon after discovering that the substance is one over which it has no power - thus strongly confirming the existence of the sympathy.