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.”[35]
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.