Letter C, and Has Its Convexity Turned Towards the Right, While the

pancreas or sweetbread (PP) lies in the space enclosed by its concavity. To enable the reader to form some notion of the relative position of these parts, I have introduced a wood-cut on the next page, shewing the situations and appearance of the different organs after the intestines, as in the figure on page 155, have been removed from the body. The letters LLLL point out the inferior surface of the liver, a little raised from its natural position to shew the gall-bladder G and the pancreas PP, round the right end of which the duodenum is curved. S indicates the spleen, with a vacant space over it, in which the stomach lies. The kidneys, KK, lie one on each side of the spine; and the two pipes UU are the ureters which convey their secreted fluid to the bladder B. The letters AA indicate the great artery (the aorta), through which the nutritive blood descends to supply the bowels and lower parts of the body; and VV mark the corresponding vein (the cava), by which the dark blood returns from the extremities towards the heart. R is the beginning of the rectum or straight gut seen at YY in the cut on page 155.

The duodenum, being thus in the immediate vicinity of the spine, is fixed firmly down in its position by the connecting membrane, and is not left to float loosely in the cavity of the abdomen or belly. Had it not been tied down in this way, it would not only have acted by its weight as a continual drag upon the stomach and disturbed its functions, but likewise have been constantly altering its own relation to the pancreatic and hepatic (or liver) ducts, and thereby affecting the flow of their respective fluids into its cavity, by which the chylification would have often been interrupted.

The duodenum is much smaller in diameter than the stomach, but larger than the jejunum or ileum; and its muscular coat is also thicker. From its size and the importance of the changes effected in it, it has been considered by some as a secondary stomach or ventriculus succenturiatus. Its mucous coat, which has a soft velvety feel, presents a greater multitude of the folds or plaits already described, and which have for their object to extend its surface and delay the passage of the chyme. Some notion of their appearance may be formed from the subjoined wood-cut. These folds or rugæ are called valvulæ conniventes or folding valves, and are inherent in the nature of the mucous coat, and not produced by mere folds of the whole thickness of the intestine, consequently they exist even when the latter is distended. They are comparatively few in number in the part of the duodenum near the stomach, and gradually multiply in the course downwards till they arrive at the maximum of development at its lower end and in the jejunum: they again diminish in the ileum, and disappear altogether in the large gut. The bloodvessels and nerves of the duodenum are extremely numerous, and indicate the importance of its functions.

The duodenum serves to receive the chyme as it issues from the stomach, and to prepare it for the farther changes which it is about to undergo. But as other organs, namely the liver and pancreas, are directly concerned along with the duodenum in effecting chylification, it will be proper to take a cursory view of them also before describing the rest of the intestine.

The liver (LLLL, p. 171) is a very large glandular body lying under the short ribs of the right side, immediately below the diaphragm or midriff, to which it is attached by strong ligamentary bands, which sustain its weight and keep it in its place. Its office is to secrete the bile, but it differs in one important particular from every other secreting organ. Its secretion is derived, not, as in other instances, from the arterial or nutritive blood, but from the venous or exhausted blood, which is collected from all the abdominal organs, and transmitted through it for this purpose on its way back to the heart. From this peculiarity, it is legitimately enough inferred that the liver serves the double purpose of providing a fluid indispensable for chylification and the proper action of the bowels as organs of excretion, and also of separating from the venous blood useless or spent materials, which require to be thrown out of the system. The influence of the bile as a stimulant to the bowels is proved by the fact that costiveness ensues when it is deficient in quantity, and an opposite condition when the secretion is redundant, as during the heat of autumn.

Bile is a bitter, viscid, greenish-yellow fluid, the taste and general appearance of which are familiar to most people, and the office of which in the animal economy must be one of no small moment, if it be justly chargeable with even a tenth part of the catalogue of human ills which are laid to its account. Its secretion seems to go on continuously; but the quantity produced depends much on the amount of venous blood which is circulating through the liver at the time. Hence, in health, it is always greatest soon after a meal; because, as we have already seen, the supply of blood in both the arteries and the veins connected with digestion is then at its maximum.

But as the secretion of the bile is constantly going on, and its presence in the duodenum is required only when the latter contains chyme, a contrivance becomes requisite for storing it up in the interval, to be ready for use when wanted. This is effected by the small shut sac or bag G, named the gall (or bile) bladder, which adheres to the lower surface of the liver, and is always most full after a fast of some duration. The bile contained in the gall-bladder is generally more viscid, dark, and bitter, than that which proceeds directly from the liver--apparently from the absorption of its more fluid parts. It is in this bag that gall-stones, as they are called, are sometimes formed, and it is their passage through the narrow tube in which it terminates that causes the acute pain so often complained of in that affliction.

From the liver the bile is conducted into the duodenum through a membranous tube, of about the diameter of a quill, and which is called the biliary or hepatic duct. In its course a similar pipe, called the cystic duct (from κυστις, kystis, a bladder), from the gall-bladder, unites with it into one trunk, like the two limbs of the letter Y; and this trunk enters the duodenum by an orifice common to it and the pancreatic duct.

In the healthy state bile is to be found only in the duodenum, and not in the stomach; although one would suppose the contrary from the familiar way in which we speak of the stomach being oppressed with bile, and of our being “very bilious.” When vomiting is excited, either artificially or by illness, we, no doubt, often bring up plenty of bile. Sometimes this bile has been existing in the stomach, and causing nausea; but very often it is brought into the stomach solely by the inverted action which constitutes vomiting, and was consequently placed there by the very remedy which is supposed to have cleared it away. The process of vomiting is accompanied by an inverted action of the intestinal canal whereby it propels its contents upwards instead of downwards, and thus the bile is, as it were, forced up from the duodenum into the stomach, instead of being propelled downwards and expelled in the usual way. Hence, in sea-sickness for example, the first fits of retching generally bring up nothing but food or mucus--the real contents of the stomach,--and it is only after continued straining that bitter bile makes its appearance. In the healthy state, fat or oily food often causes the presence of bile in the stomach, as if its aid were necessary there for the accomplishment of digestion.

The pancreas, or sweetbread, PP, is a large oblong gland, which lies across the spine, and secretes a fluid almost identical with the saliva. Its duct, as stated above, enters the duodenum along with the biliary duct, so that the two fluids meet at their entrance, which takes place at the first curvature of the intestine, at the distance of about one-third of its whole length from the stomach.

The bile and pancreatic juice thus poured out together are both requisite for the formation of chyle, and apparently modify the action of each other. The bile being somewhat of an unctuous nature, and the pancreatic juice somewhat alkaline, their union forms a kind of saponaceous compound, which is less irritating and more easily incorporated with the chyme than pure bile.

In proportion as the chyme is formed and expelled from the stomach, it is received into the duodenum, where it probably undergoes some farther change even before arriving at the entrance of the biliary and pancreatic ducts. From the numerous folds or wrinkles which line the inner surface of the duodenum and impede the motion of its contents, and also from the intestine itself being more fixed down, and less subjected to the influence of the movements of respiration, the progress of the chyme along its surface is very slow. Every particle of chyme is thus allowed to receive its share of the bile and pancreatic juice as it proceeds on its course, and time is afforded for the requisite changes taking place.

By simply stating that, in the duodenum, the chyme becomes mixed with the two fluids just mentioned, and that the result is its separation into a fluid, milky, and nutritive portion, named chyle, which is absorbed by the lacteals, and a darker yellow-coloured thickish mass which remains in the bowel, we communicate to the reader almost every thing that is known on the subject. Theories and conjectures could be added, but scarcely any facts of a very positive nature.

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The remainder of the small intestine, namely, the jejunum (from jejunus, fasting or hungry, because it is generally empty) and the ileum (from ειλεω, eilio, I twist or turn about), marked RSSSS in the cut on p. 155, are merely continuations of the duodenum, and have precisely the same number of coats and the same general structure. Their inner surface presents the same kind of folds or rugæ, whereby the extent of the mucous membrane is increased, and ample space given for the ramifications of bloodvessels and the origin of the absorbents by which the chyle is sucked up and carried into the system. Exhalation or exudation also goes on from their surface, and, in bowel-complaints, often becomes so excessive as in a few days to reduce the patient to the extremity of emaciation and weakness.

As the contents of the jejunum and ileum advance, the proportion of chyle in them becomes smaller and smaller, and the residual matter becomes more and more consistent, yellow, and fetid--approaching, in short, to the ordinary appearance of excrement when expelled from the body. In accordance with these changes, the number of absorbent vessels, and the distinctness of the villous folds, gradually diminish as we proceed downwards, till, on arriving at the termination of the ileum in the colon or great gut, they altogether disappear, and the contents assume the colour, smell, and appearance, by which excrement or feculent matter is characterized.

The division between the small and great intestines is indicated, not only by marked differences in their diameter and external appearance, but also by an internal valve placed between them, the object of which is to prevent the contents of the colon from following a retrograde course and returning to the ileum. It is also worthy of notice, that the colon is not a gradually enlarged continuation of the ileum. On the contrary, the latter enters the side of the colon almost at right angles to its course, at a little distance from its commencement. The small portion of the colon which thus lies at one side of the entrance of the ileum, and which has of course no opening at its extremity, is thence called the caput cœcum coli, or blind head of the colon, or simply the cœcum. Its position is at T in the figure on p. 155, but it is hidden by the folds of the ileum.

The colon (from χοιλον, coilon, hollow) or great gut UUU in the same figure, constitutes not more than one-fifth in length of the intestinal canal. It begins at the lower part of the right side of the belly, at T in the cut on page 155; rises upwards on the same side towards the liver; crosses over to the left side in contact with the stomach; descends along the left side of the abdomen; makes a turn at UWX like an italic s, (and hence called sigmoid flexure), while lying on the left haunch-bone; and lastly, terminates in the rectum or straight gut YY at the anus. Being fixed by local attachments, the colon remains always in the same situation, and thus describes a figure not unlike a square, in the centre of which lie the whole of the smaller intestines. In the cut referred to, the left portion is hidden behind them.

The diameter of the colon is about double that of the small intestines. In structure it is analogous to them, having three coats; but the valvulæ conniventes or folds of the mucous coat, are no longer to be seen, and with them all traces of chyle and chyle-bearing vessels also disappear. The colon serves more as a reservoir for waste or excrementitious matter than as a vital organ. Absorption is carried on from its inner surface, but through the medium of lymphatic absorbents and minute venous ramifications, and not of lacteals. Hence not only food, but medicine, are frequently administered by being injected into the rectum, and life has been saved in this way when nothing could be given by the mouth.

The passage of the intestinal contents from the stomach downwards, is affected chiefly by the peristaltic or vermicular motion, that is, the successive muscular contraction of the middle or fleshy coat, already frequently adverted to; and this, in its turn, is greatly aided by the constant but gentle agitation which the whole digestive apparatus receives during the act of breathing, and during exercise of every description. In inhaling air into the lungs, the diaphragm is depressed, the bowels are pushed down, the walls of the belly yield, and it becomes protuberant. When air is thrown out from the lungs, the diaphragm rises into the chest, the bowels follow, and the belly becomes flattened and drawn in. The stomach and bowels are thus placed between, and receive a never-ending impulse from, two bodies differently placed and in continual motion. During exercise, breathing is deeper, and muscular contraction greater in power and in extent; and hence the assistance afforded is also increased. Those who take no exercise, or who have the chest and bowels confined and bound down by tight stays and bandages, lose this natural stimulus, and have, in consequence, the bowels obstinate and troublesome.

The great extent and capacity of the intestinal canal in herbivorous animals, and others living on bulky and innutritious food, have been already noticed, and their reason explained. Perhaps it ought to be added, that an additional reason is the fact, that the digestion of vegetable nutriment is not, like that of animal food, completed in the stomach, but in the intestines. It is familiarly known, for example, that when digestion is weak, fruits and fresh vegetable aliments often pass through the bowels very little changed; and that, even at the best, they are digested more slowly than animal food. On examining the bodies of animals at different intervals from the time of feeding, the distinguishing fibrous structure of vegetable food is observed to diminish in proportion to its distance from the stomach, and it does not finally disappear till it is nearly arrived at the end of its course. From this it has been inferred, that the digestion of vegetable matter is only partially accomplished in the stomach, and that it requires the aid of the intestinal juices for its completion. Delabere Blaine arrives at the same conclusion, from considering the peculiar digestion of the horse. In the horse, the stomach is a simple bag, of very moderate size, and yet that animal not only can drink a gallon or two of water at a time, but can eat a much larger quantity of hay or grass than its stomach seems to be capable of containing. Blaine explains this, by stating that, in reality, oats and hay are not long retained in the stomach, and that after receiving the requisite supply of gastric juice, and undergoing its influence to some extent, they are gradually propelled towards the duodenum, where their digestion is continued, but not completed till long after being subjected to the action of the bile and pancreatic juice, and passing through the remainder of the small intestine. It is owing, he adds, to this speedy evacuation of the stomach, that the horse is less inclined to drowsiness, and less incommoded by active exercise, soon after meals, than almost any other animal.

The late Baron Dupuytren had several opportunities of observing something analogous to this in the human subject. He had, at various times, under his care, patients in whom an opening into the intestine had taken place at different distances from the stomach, and through which the intestinal contents readily escaped. On giving several kinds of food at one meal, he remarked that they presented themselves at the wound in the inverse order of their digestibility. Thus, fresh vegetables always made their appearance first, still retaining much of their peculiar structure; while animal substances either did not appear at all, or were so much altered in appearance as scarcely to be recognised. In the natural evacuations, however, the vegetable structure was generally imperceptible; so that a considerable change must have taken place on it as it advanced through the bowels, after passing the seat of the wound. Londe had occasion to remark the same thing in a lady in whom an opening into the intestine existed. When she ate cutlets or chicken, digestion was so far advanced before they reached the opening that their identity could not be recognised. But when she ate spinage, carrots, or vegetable soups, these articles arrived at the orifice scarcely at all changed. In this patient also, vegetable substances passed rapidly out of the stomach. Thus salads, prunes, apples, and spinage made their appearance at the opening in about an hour; while the remains of animal food never appeared in less than three hours. Londe indeed lays it down as a principle, that where the wants of the system are not great, the digestion or alteration of innutritious substances, such as stewed or raw fruits, carrots, &c., “begins only in the ileum.” He adds, that he “has always seen these substances resist the action of the acid and mucous juices of the stomach, as well as that of the pancreatic and biliary secretions.”

From the circumstance of vegetable aliment containing little nourishment and much indigestible matter, it naturally happens that a larger quantity of refuse remains to be thrown out of the bowels when it constitutes the chief part of the diet, than when animal or farinaceous food, which contains much nourishment and little indigestible matter, is used. Hence, as a general rule, the bowels act more freely, or are more open, in the former than in the latter case; and hence the common saying, that milk, eggs, jellies, and meat, are binding. They have the appearance of being so chiefly because they are almost wholly absorbed. But as neither the stomach nor the bowels are adapted in structure for very concentrated food, such articles cannot be long used with advantage. Brown and rye bread, and fruits, are in repute for relieving a costive habit of body, and their usefulness is explicable on the same principle. They leave a large residue to be thrown out of the system, and this residue forms the natural stimulus of the bowels, and consequently excites them to freer action. This effect is probably aided also by the stimulus which the indigestible refuse imparts to the mucous glands, increasing the lubricating secretion, and giving additional facility to the propelling powers.

In the preceding exposition of the structure and functions of the organs of digestion, many omissions necessarily occur, and many questions of much intrinsic interest are passed over with very little notice. But a minuter survey was incompatible with both the objects and the limits of the work. My great aim was, not so much to extend the bounds of physiology, but to turn to a useful purpose what is already known in regard to one of its most important departments, and to interest a larger class of people in its cultivation. If I have said enough to make the points of doctrine on which I have touched intelligible to the ordinary reader, and to impress him with a just sense of their practical value, I shall have accomplished the utmost I have sought to attain.