CHAPTER IV.. Chylification, and the Organs Concerned in It.

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--Uses of these.--Air in intestines--Uses of.--Mucous coat--Analogous to skin--The seat of excretion and absorption--Mucous glands--Absorbent vessels--Course of chyle towards the heart.--Nerves of 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--Cœcum--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 shewn, 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 fæces 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 very 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.

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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, called 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 cœcum or simply the cœcum, 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 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, an 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, represented by the dotted line round the circle I in the figure on next page, 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 shewn 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 μεσος, mesos, the middle, and εντερον, 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, something 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 wood-cut on page 163, representing a portion of the bowel II, as attached to the spines 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 the 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 or worm-like 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 the figure on page 155. On the rectum all the three bands are seen. It is in the colon and rectum that 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 being 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 contraction 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 mucous, 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. The cut on p. 172 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 afterwards 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 waste 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 fæces 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 mistake into which many fall, of taking more medicine on the ground of this effect proving that much stuff was lodged 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 Asiatic 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 by 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. 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 absorb the chyle, are called lacteal absorbents, or simply the lacteals or milk-vessels (marked LLLL in the subjoined wood-cut), from the 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 fæces 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. Strong soups, milk, 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 imbibe or 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 of 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 amidst many elements, each selected its own with unerring tact. But of late it has been proved that the absorbents are less rigidly discriminating than was previously supposed, and that substances are readily taken up by them which Nature never intended them to receive. In mixing madder with the food of fowls, for example, for the purpose of dyeing 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 exist a fitness and peculiarity of relation between the absorbents and their proper objects, which renders the latter more accessible to them than to 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 intestine, and gradually coalescing into larger trunks, as seen at LL in the figure on p. 163. 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 course along the spine, and which is represented at DDDD in the annexed cut. On its arrival at the upper part of the chest, the thoracic duct crosses over and opens into the subclavian 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 (MG, p. 163), where some change, 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 insured 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 place just before the dark blood has finished its course, and is again subjected to complete aëration in its passage through the lungs. As explained in the former volume, this aëration is so indispensable to the renovation of the old and the formation of new blood, that whenever it is rendered imperfect, either by obstructions in the lungs themselves or 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.

Every body 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 prominent 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 conscious in health of the impressions made on the intestinal nerves; but this, as already shewn 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 undigested 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 the 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.

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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