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 chemical analysis.[16]

Remarkable, however, as this uniformity of result undoubtedly is, it becomes still more striking when we contemplate the variety of sources from which 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 silk-worm 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.”[17] 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 chemical 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 elements 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, tends rather to augment it. The same elements, extracted from the same soil, are converted into every variety of vegetable product—into leaves of every shade of green, flowers of every form and tint, 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 with 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 mean time, it will be better for us to confine our attention to that branch of the inquiry which we can easily comprehend, and 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 acquainted 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 some 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 γαστηξ, gaster, stomach), by which it is gradually converted into a soft greyish and pultaceous mass, called chyme, (from χυμος, chymos, humour or 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 intestines. 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 (πας, pas, all, and κρεας, 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 χυλος, chylos, chyle), and a thick yellow residue. This process is called chylification or chyle-making. The chyle is then sucked in by absorbent vessels, extensively ramified on the inner membrane or 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 thorax or chest), and which terminates, as will be seen in a cut in Chapter VI., in the great vein under the clavicle or collar-bone, hence 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 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. This is accordingly done by its passing through the lungs along with the dark and 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 afterwards 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.[18]

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 fæces or excrement. But in this course its bulk is increased, and its appearance changed, by the addition of much waste matter, which, having already served its purposes in the system, is at last, as will be afterwards shewn, thrown out 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 each of the organs concerned in effecting it. Chymification being the first step in the complicated process, we shall begin with the organ by which it is performed, namely, the Stomach.

In the lowest 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.[19] 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—“à vue d’œil.” 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.

Bloodvessels being merely pipes serving for the conveyance of nourishment from the place where it is stored up and prepared, to those more distant parts which stand in need of it, it is obvious that in animals of this very simple description, where every part of the internal surface of the body is already in immediate contact with its food, and itself imbibes or absorbs directly all the nourishment which it requires, they would be entirely superfluous, and accordingly none are to be found.

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 dependent on each other; so that, when we arrive at man—the highest of all—we find that the loss or 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 gradation 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 figure on page 67; its left or larger extremity being in contact with the ribs, and its right or narrow extremity situated under the pit of the stomach.

The position of the stomach (St^m.) relatively to the chest, bowels, and liver, will be understood by inspecting the figure on the next page. It is separated from the cavity of the chest by the diaphragm or midriff DD, with which its upper surface is in immediate contact, and through which the gullet passes to enter its left extremity. Its right or pyloric extremity, marked P in the cut on the opposite page, lies close to the lower surface of the liver (Liv^r.), the latter being a little displaced to shew 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 κεαρ, kear, or heart), in which the gullet terminates, and through which food and drink are introduced; P, the pylorus, or pyloric orifice (from πυλωρος, pyloros, 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 is 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 connection 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 amount of nourishment. For the same reason, animals which subsist on vegetable substances have 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 proportion 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 it 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 some 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 red-bag. 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 afterwards 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 as 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 seeds, 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 softened 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 thick 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 grains 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, however well they may be fed, if gravel or coarse sand, as well as food, be not placed within their reach. Mr Hunter has counted as many as a thousand small stones in the gizzard of a common goose.[20]

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 afterwards 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 a fowl drawn for cooking. It is a fold of the tough shining membrane called peritoneum (from περιτεινω, 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 the 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 this 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 afterwards 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.

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 its appearance 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 out (p. 122) the necessity of the co-operation of a nervous stimulus to produce the result; and remarked that there are two kinds of muscles, one called the voluntary, which contract at the command of the will, and the other the involuntary, 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 stomach. They contract when the stimulus of food is applied to them, but we can neither contract nor relax them by an effort of the will, nor are we even conscious of their existence.

It is, indeed, fortunate for us that the necessary motions of the stomach are not entrusted to our guidance, like those of the hand or foot. Supposing that we were to eat three meals a-day, the digestion of each requiring three or four hours,—and that its management depended entirely upon our superintendence,—our whole attention would be required to the process, to the exclusion of every other duty, for ten or twelve hours a-day; and every time that our thoughts wandered for a few minutes, digestion would stand still, and the stomach be disordered by the chemical decomposition of the food which would ensue, so that it would be impossible for us to dedicate any time either to business or to social enjoyment. But from all these inconveniences we are entirely freed by the stomach being placed under the dominion of the involuntary nerves, and so constituted as to perform its functions without any aid from our will.

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 rugæ, plicæ, 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 appearance. 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 the 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 larger than pin-heads, 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 entertained, and the one which is supported, as we shall afterwards see, by the strongest evidence; the gastric juice being, in fact, secreted directly from the capillary or hair-sized vessels in which the minute branches of the arteries terminate.

Of the nerves and bloodvessels supplying the stomach it is unnecessary to say much. We shall afterwards 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 it 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 the wood-cut at page 67, follow the course of GG the greater and SS the smaller curvatures, and send off innumerable small branches as they proceed to every part of the stomach. The coronary artery and the pyloric branch of the hepatic or liver artery go to the smaller curvature, while another branch 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 irritating bodies. When the follicles are diseased, as in what is called water-brash, they sometimes throw out a large quantity of a ropy transparent fluid, which oppresses the stomach and impairs digestion.

The BLOODVESSELS of the stomach, like those of every other part, are more or less active according to the energy of its functions at the time. In treating of the laws of exercise as applicable to all living parts,[21] I took considerable pains to point out the relation which the Creator has established between the activity of every organ 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, of which the throbbing temples and fiery complexion of a man in a paroxysm of rage are familiar examples. 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 it was announced by them as an axiom in the very comprehensive phrase, Ubi stimulus, ibi affluxus—“Wherever a stimulus is, there is also an afflux.”

The stomach forms no exception to this general law of the animal economy. When it is empty and idle, it is contracted upon itself into comparatively small bulk; and its bloodvessels become shortened and tortuous 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 scope, 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 occurred 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 camphor or other strong-scented substance have been given to animals as an experiment, and, on killing them shortly afterwards, the peculiar smell has been detected in the blood. Most liquids are thus not digested, but simply absorbed.

Rapid, however, as the process is, poisons which enter the system by absorption do not act by any means 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 of 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 originates, and which we have already seen to be connected with 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 as different.

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 shew that the pneumogastric[22] 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 dependent 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. Bracket, 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 doubt 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 interrupt 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 shewn, by direct experiment, that digestion may be interrupted almost 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 the 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 sympathetic 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 sympathetic nerve receives filaments from the sentient or feeling part of 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 we have no direct control over the action of the ganglionic nerves, I shall not detain the reader with any conjectural discussion, but rather request his 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 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,[23] 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 contains, and of the changes occurring 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 by what is eaten, consciousness then becomes painful for the express purpose of warning us that mischief has been done, and that we must take means for its removal. In some kinds of dyspepsia indeed, the sensibility becomes exalted to an extraordinary degree. Barras who suffered intensely from this cause, says of himself “the sensibility of the stomach increased to a surprising extent; instead of organic it became animal, to use the expression of Bichat. Every thing which took place in the principal organ of digestion became as palpable to sense as if it had taken place on the organ of touch, and the presence of aliment was perceived as clearly as if it been under my hand.”[24]

The nerves of the stomach, it ought to be remarked, have a direct relation to undigested but digestible substances; or in other words, undigested food forms their natural and appropriate stimulus. In consequence of this arrangement, when 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 bowels. 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 through the intestinal canal of such undigestible substances as fat, husks of fruits, berries, and cherry-stones.

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 conspicuous a part 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 of 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 occurred, came 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 Michillimackinac 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 discharge of a musket on 6th June 1822.