The Physiology of Digestion by Andrew Combe, M.B. (1837)
Part I. Chapter VI. Chylification, and the Organs Concened in it.
The Physiology of Digestion by Andrew Combe, M.D., Forth American Edition, was published By Marsch, Capen & Lyon in Boston in 1837. Andrew Combe was a physician in ordinary to their Majesties the King and Queen of the Belgians, and a Fellow of the Royal College of Physicians of Edinburgh. The third edition of this book was sold in 3000 copies. The text of this book of 310 pages will be found here, completely unchanged.
CHYLIFICATION, AND THE ORGANS CONCERNED IN IT.
Chylification. - Not well known. - Organs concerned in it. - The intestinal canal. - Its general structure. - Peritoneal coat. - Mesentery. - Muscular coat. - Analogous to skin. - The seat of excretion and absorption. - Mucous glands. - Absorbent vessels. - Course of chyle towards the heart. - Nerves of the mucous coat. - Action of bowels explained. - Individual structure of intestines. - The Duodenum-Jejunum- and Ileum. - Liver and pancreas concerned in chylification. - Their situation and uses. - Bile, its origin and uses. - The pancreas. - Its juice. - The jejunum described. - The Ileum. - Coecum. - Colon - and Rectum. - Peristaltic motion of bowels. - Aids to it. - Digestion of vegetables begins in stomach, but often finished in the bowels. Illustration from the horse. - Confirmation by Dupuytren.
The conversion of food into chyme, an operation which, as we have seen, takes place in the stomach, is only one of the series of changes which aliment undergoes before becoming fit to be assimilated with the living body; and the next process which we have to notice is chylification, or that by which chyme is converted into chyle.
In proportion as chyme is formed from the food, it is gradually propelled, as already shown, through the pyloric orifice of the stomach into the duodenum, or beginning of the small intestine. On its arrival there, it is acted upon by the bile from the liver, and the pancreatic juice from the pancreas; and the result is the separation of the chyme into two distinct substances, - the one a milky-white fluid called chyle, which is absorbed into the system, and forms nutriment, - and the other a yellowish and more consistent mass, which is the indigestible remains of the food, and which, after traversing the whole length of the intestinal canal, and being there mixed with the waste matter separated from the blood in order to be thrown out of the system through the same channel, is at last expelled in the form of faeces or excrement.
If physiologists experience much difficulty in satisfactorily explaining all the phenomena of chymification, or stomach-digestion, the reflecting reader will not be surprised to learn that they are still more puzzled to account for those of chylification, or intestinal digestion. The organs concerned in the latter are so deep-seated and inaccessible during life, that wery few opportunities occur of obtaining accurate information on the subject; and, therefore, in what follows, I shall not enter into disputed or intricate details, but confine myself to such general views as are not contested, and as the reader may easily understand. Fortunately, ignorance of this branch of the inquiry is of less practical importance than if it extended to stomachic digestion also; because such is the harmony between all the parts of the system, that whatever conduces to the perfect accomplishment of the first stage of the process, chymification, is in so far equally conducive to the proper fulfilment of chylification, or intestinal digestion.
The simple fact, indeed, of our having no direct control over the process of chylification, and of our being able to modify it only by varying, through the medium of the stomach, the elements out of which chyle is to be formed and the mode in which they shall be digested, is a proof that, practically speaking, it is chiefly the laws or conditions of stomachic digestion which are intended to regulate our conduct; and that, in obeying them, we in reality obey also those of intestinal digestion.
The organs concerned in chylification are the duodenum, the liver, and the pancreas; but in order to avoid repetition, I shall, in describing the first, notice also the remainder of the intestine.
The intestine, or intestinal canal, as represented in the subjoined figure, begins at the pyloric orifice of the stomach, P, and after many windings and turnings, callled convolutions (from the Latin word convolutus, rolled or folded together), terminates in the rectum or straight gut, Y, at the external orifice called the anus. Although continuous throughout its whole extent, the intestinal tube is nevertheless divided by anatomists into six portions, to each of which a different name is assigned; the distinction between some of these is more nominal than real, but it still continues to be made on account of its convenience.
The first grand division is into the small and great intestines; the former beginning at the stomach, including all the convolutions marked RSSSS, - and the latter beginning at T, where the small intestine terminates, and including the large gut UUUUXYY, which surrounds, and is partly hidden by, the other bowels.
The small intestines, again, are subdivided into three portions, - the duodenum, the jejunum, and the ileum; and the larger, in like manner, into three portions - the caput caecum, or simply the caecum, the colon, and the rectum. Of the whole length, the small intestines constitute by much the greater part, and they differ somewhat from the larger in function as well as in magnitude.
In structure, the intestines exhibit a great analogy with the stomach. They consist, in common with it, of three coats or layers of membrane; the outer, or peritoneal, - the middle, or muscular, - and the internal, mucous, or villous.
The peritoneal coat is the white, firm, smooth shining, and moist membrane, seen on the outside of the intestine on opening the cavity of the of the abdomen. It serves both as a support and as a medium of attachment to fix the intestine in its place. By its smooth, soft, and lubricated surface, it admits readily of the change of place among the bowels necessarily produced by respiration, exercise, and even by different degrees of distention of the bowels themselves. Every time we breathe a undulating motion is communicated to the whole intestines, which facilitates their action, but which could not take place unless they were capable of gliding easily and freely over each other. The peritoneal coat, being strong, extensible, and elastic, is very useful also as a support to the other coats.
The peritoneal coat, after forming the outer covering of the intestine, rerpesented by the dotted line round the circle I in the subjoined picture, is continued from it in the form of a double membrane (represented by the two dotted lines) towards the spine S, to which it is first firmly attached by cellular substance; after which the folds again separate, each being continued, or reflected, as it is called, over the whole inner substance of the cavity of the abdomen in the course shown by the dotted line, the figure itself representing a transverse section of the abdomen. By this arrangement two important objects are attained. First, the abdominal peritoneum AP forms a soft lubricated surface, corresponding to that of the bowels themselves; and, secondly, a firm point of attachment for the bowels is secured, by which they may be supported in their proper places, and at the same time admit of some change of position. The floating portion of the peritoneum, M, by which the attachment is effected, is called the mesentery (from mesos, the middle, and enteron, intestine). But the intestinal canal being so much longer than the portion of the spine to which the mesentery is attached, the latter is necessarily disposed in folds, converging towards the spine, someting like the folds of a fan converging towards its narrow end.
In this way the mesentery, besides serving as a support to the gut, serves also to receive and afford protection to the numerous vessels, nerves, and lacteals, which are copiously ramified on every portion, particularly of the small intestines. This feature, however, will be better understood by inspecting the next wood-cut, representing a portion of the bowell II, as attached to the spine by the mesentery MM, along which the absorbent vessels or lacteals LL are seen to pass from the gut towards the thoracic duct TD.
The portion of peritoneum by which tha small intestines are fixed to the spine, constitutes what is properly called the mesentery. That portion by which the larger bowel is attached is called the mesocolon, from its enclosing the colon; but in other respects the membrane presents no difference.
The muscular coat is composed principally of transverse and longitudinal fibres, and its sole object here, as elsewhere, is to effect motion. By the alternate contraction of the two kinds of fibres, the contents of the gut are gradually propelled in a downward direction, just as we see a motion propagated from one end of a worm to the other; and hence it is sometimes called the vermicular motion (from vermis, a worm). Some nauseating substances, such as emetics, have the power of inverting the order of the muscular contractions, and directing the contents upwards instead of downwards - whence vomiting ultimately arises. Other substances, again, have the property of exciting the natural action to a higher degree, and consequently propelling the contents faster downwards - in other words, of purging. Rhubarb, aloes, and similar laxatives, especially when combined with tonics, act in this way, and are consequently best adapted for obviating the kind of costiveness which arises from imperfect intestinal contraction. In a natural mode of life, the muscular coat is greatly aided in its operation by the large abdominal and thoracic muscles, brought powerfully and frequently into play during active exercise and employments. When this aid is withdrawn, as it is in sedentary people, the intestinal action often proves insufficient for the purpose; and hence the costiveness which is so invariable an attendant on most females, literary men, and others whose occupations deprive them of active muscular exercise in the open air. In females, the use of tight stays renders the free expansion of the chest, and corresponding motion of the abdomen, altogether impossible, and thus aggravates the evils of their sedentary mode of life. Hence also the peculiar fitness, in such cases, of the class of purgatives above alluded to, in preference to those of a saline nature, which act chiefly by stimulating the mucous surface to farther secretion.
In addition to the ordinary longitudinal and transverse fibres, the colon presents three remarkable muscular bands running along its whole length, and one of which is represented on the colon in a woodcut earlier in this text. On the rectum all the three bands are seen. It is in the colon and rectum tht the feculent matter accumulates before it is thrown out of the bowels, and these bands are useful chiefly by adding to their propelling power.
The natural tendency of muscular fibre to contract, it may naturally be supposed that, after the intestine is emptied, its opposite sides will come into contact, and. by thus obliterating the cavity altogether, present an obstacle to the subsequent passage of any solid matter. But, on inspecting the abdomen after death, we rarely meet with any considerable portion thus contracted; and in general, the whole intestines are distended to a greater or less degree, according to circumstances. The agent by which this effect is brought about, is one known more familiarly by the inconveniences and pain to which it gives rise when in excess, than by its proper uses, which are nevertheless important. I allude to the presence of air in the bowels, which is as necessary to their healthy action as their muscular contractions itself. Air, in fact, by its expansive energy, forms the antagonist power to the muscular coat, and serves to dilate the bowel after the requisite contraction has propelled its contents. A certain degree of distention, indeed, not only is a stimulus to farther muscular contraction, but is useful in facilitating the passage of the subsequent portions of the feculent matter; and hence the injection of air into the bowels in large quantity, has lately been employed successfully in overcoming obstinate constipation.
The mucus, internal, or villous coat of the intestine, also resembles in many respects that of the stomach. It is a soft, velvety membrane, full of wrinkles or folds through the greater part of its course, by means of which its surface is greatly increased in extent, so as to afford ample space for the ramification of the bloodvessels, nerves, and absorbents, with which it is very plentifully supplied. A later woodcut will convey some idea of its appearance, as seen in the smaller intestine. So far as nutrition is concerned, the mucous coat is the truly essential part of the bowel. It alone is in direct contact with the chyme, and in its cavity the bile and pancreatic juice perform their respective parts, and give rise to the formation of chyle, which is afterward transmitted from its surface into the general system. The peritoneal and muscular coats are useful only in affording protection, and communicating the power of propelling its contents.
The mucous coat appears, on examination, to be so entirely continuous with the skin, that no line of demarcation can be detected between them either at the mouth or at the anus. In structure they greatly resemble each other, and the sympathy between them is well known to be very rapid and intimate. Eruptions on the skin, for example, are almost always owing to disorder of the digestive organs; and bowel-complaint, on the other hand, is often produced by a sudden chill on the surface. In like manner, in enormous eaters like those formerly mentioned, an immense exhalation takes place from both the skin and the bowels, and in many instances the one supplies the place of the other in a considerable degree. We have seen, moreover, that in the lowest tribes of animals, the digesting surfaces and skin are not only undistinguishable, but actually convertible into each other by the simple process of turning the animal inside out, when each will perform the function of the other as well as if it had never done any thing else.
In common with the skin, too, the mucous coat is charged with the double function of excretion and absorption. For the former, it is eminently fitted by its plentiful supply of blood, and by the great number of minute vessels ramified on its surface, from the extremities of which the excretion takes place. It is by this channel that much of the vaste matter requiring to be removed from the body is thrown out. Being poured into the cavity of the intestine from the small arterial branches, it mixes there with the indigestible residuum of the food and bile, and, united with them, forms the common feces or excrement. When the blood is suddenly repelled from the surface by a chill, and thrown in upon these vessels in large quantity, the natural excretion is sometimes increased to such an extent as to constitute bowel-complaint; while at other times, that peculiar form of action is induced which constitutes inflammation. The local stimulus of some kinds of food, and of many medical substances, also excites the secretion to unusual activity. Salts, for instance, have this effect, and thus often produce numerous fluid evacuations, the substance or materials of which did not before exist in the bowels; and hence the mistanke into which many fall, of taking more medicine on the ground of this effect proving that much stuff was lodging in the bowels - when, in fact, it was not only removed, but created by the physic. It is from exciting a fluid discharge of this description, that saline purgatives are so useful for lowering the tone of the system when that is required; but, for the same reason, they are most improper where relaxation and debility already exist. In the cholera, almost the whole fluids of the body are carried off by this channel, leaving the blood too thick in consistence to circulate longer through the smaller vessels.
The excretions from the minute arterial branches ramified on the internal coat are mingled with a bland fluid from the mucous follicles, the evident use of which is to protect from injury the sensitive surface of the intestine. Occasionally, however, the mucous secretion becomes so abundant and viscid as to adhere with unusual force, and to impede the formation and absorption of the chyle, and even the action of the usual purgatives. Worms are then common, and cannot be expelled except gy remedies which tend to remove the mucus in which they live imbedded.
To fit the mucous coat for its office of absorption, an immense number of minute vessels, called absorbents, are ramified on its internal surface, the nature and purposes of which are analogous to those mentioned in the former volume when describing the functions of the skin (Principles of Physiology, &c., chap. ii.). In both structures the absorbents are small capillary or hair-sized vessels, so infinite in number that at least one goes to every little point or papilla. Those which open upon the inner surface of the smaller intestines, and which suck in or absorp the chyle, are called lacteal absorbents, or simply the lacteals, or milk-vessels (marked LL in the subjoined wood-cut), from white colour of the chyle shining through them, and giving them the appearance of vessels full of milk.
In that part of the gut they are so numerous, that every minute point of the villous coat may be seen by the aid of a microscope to contain one, with its mouth open to receive the chyle as fast as it is formed. Even in the colon the absorbents are numerous; but, as all traces of chyle have there disappeared, they are much fewer than in the smaller intestines. In the colon they serve chiefly to remove the more watery portions of the intestinal contents, by which means the feces are rendered more solid and less bulky, and therefore better adapted for being retained for a time without inconvenience. It sometimes happens that, when food or medicine cannot be swallowed in the usual way, life is preserved by injecting it into the bowels; in which case the absorbents of the large gut become active, and carry it into the system. Opium laxatives and other remedies are often administered in the same way, when any reason exists against giving them by the mouth.
There are absorbents in every structure of the body, because there are everywhere waste particles to be taken up and removed; but, except in the case of the lacteals, their contents are limpid, or colourless, and hence in other places they are called lymphatics; in almost every other respect, however, the two classes of vessels are analogous to each other.
The peculiar property by which the minute lacteal vessels take up the white chyle is not well understood. From the fact in physics that liquids rise in capillary tubes, the inference has been drawn that absorption in living vessels also takes place from capillary attraction. But in the animal body the application of the principle is undoubtedly modified by the properties peculiar to organization, and one of the most remarkable proofs to this is the circumstance of the absorbent vessels in different situations having to some extent a specific adaptation to the qualities of the substances upon which they are severally destined to act. At one time, indeed, it was supposed that the principle of exclusive adaptation was so complete that every absorbent vessel was permanently shut to every thing except its own peculiar object, and that, from amid many elements, each selected its own with unerring tact. But of late it has been prooved that the absorbents are less rigidly discriminating that was previously supposed, and that substances are readily taken up by them whcih nature never intended them to receive. In mixing madder with the food of fowls, for example, for the purpose of dying their bones, the colouring matter of the root is taken up without difficulty by the absorbents along with the chyle, although madder was certainly never intended to be their natural stimulus. but even admitting this latitude in its fullest extent, there still exists a fitness and peculiarity of relation between the absorbents and their proper objects, which renders the latter more accessible to them than any foreign body.
The lacteal vessels are most easily seen an hour or two after a meal; because they are then fully distended with chyle, even in their smaller branches. The latter, indeed, may then be distinctly traced proceeding from the different portions of the intestine, and gradually coalescing into larger trunks, as seen at the LL in the wood-cut above. These, again, terminate in the vessel called the thoracic duct (the beginning of which is seen at TD in the same figure), by which the chyle is conveyed almost in a direct dourse along the spine, and which is represented ad DDDD in the cut below. On its arrival at the upper part of the chest, the thoracic duct crosses over and opens into the vein S, just before the latter reaches the right side of the heart, so that the chyle is there poured into the circulating current of the venous blood.
Such is the course of the chyle. But the lacteal absorbents, in their progress from the intestine to the thoracic duct, pass through the small glandular bodies called the mesenteric glands, where some changes, the nature of which is not at all understood, is produced upon the chyle, but which seems nevertheless to be of importance to its constitution. Where these glands are hardened and enlarged, as they often are in scrofulous children with large prominent bellies and thin bodies, nutrition is greatly impaired, although the appetite and stomachic digestion remain comparatively unaffected.
The reason why the chyle is carried so far to be poured into the current of the venous blood, just before the latter reaches the right side of the heart, is, on consideration, not less obvious than cogent. Chyle itself is not fitted to become a constituent part of the animal frame. Before it can become so, it must be converted into blood; and this can be effected only by exposing it to the action of the air, in the air-cells of the lungs, in a state of intimate mixture with the venous blood. This admixture, again, is ensured by the gradual way in which the chyle advances along the thoracic duct, and falls into the circulating current almost drop by drop; and it takes palce just before the dark blood has finished its course, and is again subjected to complete a๋ration in its passage through the lungs. As explained in the former volume, this a๋ration is so indispensable to the renovation of the old and the formation of new blood, that whenever it is rendered imperfect, either by obstruction in the lungs themselves, of by the absence of a sufficiently pure air without, the result is invariably injurious to health; because the blood, being no longer properly constituted, becomes incapable of furnishing a healthy stimulus and nourishment to all the parts of the body. Hence the rapid "decline" which follows the appearance of pulmonary consumption, and other diseases affecting the structure, and interrupting the functions, of the lungs.
Everybody knows as a fact that bad air is hurtful, and that wasting disease of the lungs is attended with rapid loss of flesh and strength; but the manner in which these effects are produced is not so familiarly known. Yet, in a practical point of view, a knowledge of the principle is highly important. Properly considered, respiration is in reality the completion of digestion. The stomach may convert the food into chyme, the small intestines may convert the chyme into chyle, and the absorbents may take up the latter, and duly convey it into the circulating system; but, unless it undergo the necessary change in the air-cells of the lungs, it will not constitute good blood, or afford due nourishment to the body. Hence it is that those among the working classes who are much confined in an impure and insalubrious atmosphere, even when plentifully supplied with food, are generally thin and ill-nourished; and hence those who, along with good digestion, have small narrow chests, and very limited respiration, are commonly found to be constitutionally lean, - while those who, along with good digestion, have amply-developed lungs, and free and powerful respiration, are at the same time remarkable for proportional vigour of nutrition and stoutness of body. It is on this account that in chronic pulmonary disease recovery is always to be distrusted, unless, along with the disappearance of the proment symptoms, restoration of the lost flesh occurs. If nutrition remains impaired, however great the relief may be in other respects, there is reason to believe that the lungs are still so extensively diseased as to injure their functions, and that, on the application of any fresh exciting cause, the dormant mischief will resume its activity. In such cases, when stomachic digestion is sound, a full diet generally over-stimulates the system, by pouring into the blood more chyle than the lungs are able to assimilate; in consequence of which it is diffused over the whole body in an imperfect state of preparation.
The mucous membrane is, like the skin, well provided with nerves, and has a mode of sensation peculiar to itself. Every villous point, indeed, has a nervous fibre ramified on it, to give it the necessary sensibility to its own stimuli. It is true, we are not so conscious in health of the impressions mad on the intestinal nerves; but this, as already shown in describing the stomach, is a privilege and not a defect. They recognise their appropriate stimuli, and cause the necessary actions to follow without requiring aid from the will. But when they meet with substances which ought not to be there, such as pieces of indigested food, or foreign bodies which have no natural relation to their constitution, they immediately indicate uneasiness, and excite the muscular contractions to rid them of the offending cause.
To secure full and natural action in the intestinal canal, several principal conditions are thus necessary, failure of any of which may impair their activity. The first condition is well-digested chyme and chyle; the second, a due quantity and quality of mucous and vascular secretions from the villous coat; the third, full contractile power in the muscular fibres of the intestine, and free action of the abdominal and respiratory muscles; and the last, a due nervous sensibility to receive impressions, and communicate with necessary stimulus. And hence, when the bowels act imperfectly, it is of importance to ascertain to what cause the inability is to be ascribed, that an appropriate treatment may be devised.
Such are the general structure and uses of the intestinal canal; but there are modifications in its individual portions on which it may be right to offer a few additional remarks. We shall begin with the duodenum.
The duodenum (from duodeni, twelve, being considered equal in length to twelve finger-breadths) commences at the pyloric orifice of the stomach, from which it crosses over, under the lower surface of the liver, towards the right side; it then descends in front of the right kidney, and there forming a second curve, it proceeds again towards the left, and a little beyond the spinal column, terminates in the jejunum. It thus describes a course like the 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 beneath, showing the situations and appearance of the different organs after the intestines 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 show 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 urethers, which convey their secreted fluid th 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 an earlier wood-cut.
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 in 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 chylification would have been often 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. It is more firmly fixed to the body than the other intestines, and does not, like them, float loosely in the abdomen. In its course, until its termination in the jejunum, it describes a kind of C, the concavity of which looks to the left. From this shape it has been separated into three portions - the first situated horizontally beneath the liver; the second descending vertically in front of the right kidney; and the third in the transverse mesocolon.
Its mucous membrane presents a number of circular folds, as shown in the subjoined wood-cut. These folds or rugae are called valvulae 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 action of the agents of digestion. The biliary and the pancreatic juices contribute most to chylification. The pancreas is the largest of the salivary glands. A considerable excretory duct passes entirely through this organ, and opens into the duodenum. The excretory portion of the biliary system includes the proper excretory duct of the liver, and a cul-de-sac of this canal termed the gall-bladder. The bile is generally distinguished into cystic and hepatic. From this peculiarity it is legitimely 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 thown 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 the autumn.
Bile is a bitter, viscid, greenish-yellow fluid, the taste and general appearance of which are familiar to most people; and the offfice 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 of 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 that 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 affection.
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 cystis, 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 voimiting is excited, either artificially of 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 or 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.
The remainder of the small intestine, namely, the jejunum (from jejunus, fasting or hungry, because it is generally empty), and the ileum (from eileo, I twist or turn about), marked RSSSS in the earlier wood-cut, are merely continuations of the duodenum, and have precisely the same number of coats and the same general structure. The inner surface presents of the same kind of folds or rugae, 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 downward, 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 caecum coli, or blind head of the colon, or simply the caecum. Its position is at T in an earlier wood-cut, but it is hidden by the folds of the ileum.
The colon (from 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 above mentioned wood-cut; rises upwards on the same side towards the liver; crosses ovet 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 the 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 valvulae conniventes, or folds of the mucous coat, are not longer to be seen, and with them all traces of chyle and chyle-bearing vessels also dissappears. 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 effected 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 adn 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 in 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.
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 exclipable 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.
I 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 to extend the bound 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.