Chapter VI: , in the great vein under the clavicle or collar-bone, hence (1)

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.

“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 muzzel 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 subsided, 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 prefixed figure 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 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 sound, 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 the most praiseworthy perseverance, and disinterestedness. 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 or 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 or superior termination of the great curvature. When the stomach was nearly empty, he was able to examine its 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 _rugæ_, villous membrane, and mucous coat, lining the gastric cavity. This appearance is almost invariably exhibited in the morning before rising from 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 removes any uncertainty which previously existed.

In treating of the properties of the gastric juice, I shall on all disputed points give a decided preference to the observations of Dr Beaumont over those of any other physiologist; because, although a few cases have occurred, 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 and essential result. In addition to these reasons, it ought to be added, in justice to the American physiologist, that, from the excellent judgment with which he carried on his investigations, and the scrupulous care with which he announces his results and separates 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 us with the strongest possible claims on our attention.[25]

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 with the opening 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 which 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 occurred, 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 appearance ensue whenever any aliment was brought into contact with it. The action of the neighbouring bloodvessels was instantly increased, and their branches dilated so 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 _worm-like_ motions of the stomach became excited, and innumerable minute lucid points and very fine nervous and vascular papillæ 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 point of the papillæ and trickled down the sides of the stomach till it mingled with the food. This afterwards 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 indigestible body, invariably gave rise to the exudation of the gastric fluid from these vascular papillæ; 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 which 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 procured under any other circumstances.

Various methods have been employed for procuring the gastric fluid in a state of purity. Pieces of dry sponge, inclosed in a small hollow perforated ball with a string attached to it, have been swallowed both by man and by inferior animals, and afterwards 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 is 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, as the gastric juice does not exist ready made in the stomach, either none but merely mucus will be procured, or it will be expelled mixed with the food or substance which had previously elicited its secretion.

Gastric juice, in its purest form, and unmixed with any thing 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 when diluted with saliva it becomes fetid in a few days. According to Professor Dunglison, to whom 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 constitute the chief part of the diet. Other chemists 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 and of health, but also the necessarily different degrees of purity of the fluid submitted to examination.