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weight; but a dog, in order to keep in condition on an exclusive flesh diet-and this is its natural food-should take 50 parts of flesh, containing 10 parts of dry albuminates, for every 1000 parts of its weight. Hence its gastric juice only suffices for the digestion of half the albuminates necessary for nutritiona result which is in accordance with the observed fact, that a considerable portion of the albuminates enters the duodenum in an undissolved state, and which will be explained when we consider the part which the intestimal juice—the fluid secreted by the various glands lying in the mucous membrane of the small intestine—takes in the digestive process. On comparing the experiments made on dogs with those made on Catharine Kütt, it appears
FIG. 4. that in the human subject the gastric digestion of the albuminates is much more imperfect
The under surface of the stomach and liver,
which are raised to show the duodenum than even in the dog.
and pancreas. The process of gastric digestion is slow. st, stomach; p, its pyloric end; 1, liver; 9, According to Beaumont's researches on Alexis gall-bladder; d, duodenum, extending from
the pyloric end of the stomach to the front, St. Martin, the mean time required for the
where the superior mesenteric artery (sm)
crosses the intestines; pa, pancreas; sp, butcher's meat, fowl, and game, was from two spleen; a, abdominal aorta. hours and three quarters to four hours.
The next point to be considered is: What becomes of the matters that are thoroughly dissolved in the stomach? Are they absorbed, without passing further down the canal? or do they pass through the pyloric valve into the duodenum, and are they finally taken up by the lacteals? Two of our highest authorities in physiological chemistry, Frerichs and Donders, maintain that the absorption of the peptones commences in the stomach; but the view generally adopted is, that the albuminates, etc., which are converted into peptones, are for the most part taken up by the lacteals. The rapidity with which
d do Fig. 5.
FIG. 6. Vertical and longitudinal section of the small integ- Two villi, denuded of epithelium, with the lao
tine in the lower part of the jejunum, showing teal vessels in their interior. the general arrangement of its coats.
a, limitary membrane of the villus; b, basis of a, villi; b, intestinal tubes or follicles of Lieberkuhn; the sanie; c, dilated blind extremity of the
C, submucous areolar tissue; d, circular muscular central lacteal; d, trunk of the same.
fibers; e, longitudinal muscular fibers. aqueous solutions of iodide of potassium, the alkaline carbonates, lactates, citrates, etc., pass into the blood, and thence into the urine, saliva, etc., shows that the absorption of fluids must take place very shortly after they are swallowed, and there is little doubt that the blood vessels (capillaries) of the stomach constitute the principal channel through which they pass out of the intestinal tract into the blood. As the veins of the stomach, which are formed by the union of these capillaries, contribute to form the portal vein (see CIRCULATION, ORGANS OF), the absorbed matters pass directly to the liver, and probably stimulate it to increased secretion (fig. 4).
6. We must now follow the progress of the semi-fluid mass known as the chyme, from the stomach into the small intestine, and notice the changes which are collectively impressed upon it, and are known as chylification or intestinal digestion. But before we can satisfactorily do this, we must say a few words regarding the intestinal mucous membrane, with its various glands, etc., and on the changes which take place in it during digestion.
The mucous membrane of the small intestine resembles that of the stomach in so far as it is of considerable thickness, and consists in a great measure of laterally grouped tubes. The reader is referred to fig. 5, which exhibits a section of the mucous membrane of the small intestine in the dog. These tubes, which form the great mass of the middle portion of the section marked b, are commonly called the follicles of Lieberkuhn, although they were first described by Brunner. They are straight, nearly uniform in diameter throughout their entire length, and are parallel to one another, and perpendicular to the inner surface of the small intestine on which they open. Nothing is known of the exact nature of their secretion; but in association with the secretions of other glands, they combine to yield the intestinal juice whose characters and uses will shortly come under our notice.
The projecting bodies marked a in the figure are termed the villi; they are minute processes of the mucous membrane of the smali intestine, and obviously serve to increase to a great extent the amount of absorbing mucous membrane. They first appear in the duodenum, where they seem to develop themselves as elongations of the partitions between the cells or pits into which the tubes open. Comparatively scanty in number at first, they become very numerous (covering the whole surface) in the further part of the duodenum and the rest of the small intestines, giving to the mucous membrane a velvet-like or pilous appearance; they finally cease at the ileo-cæcal valve, which forms the boundary between the small and large intestine. In man, they are conical in shape, and measure from Ath to oth of an inch in length. They vary much in shape and size in the lower mammals and in birds. (In carnivorous animals, as the dog, they are longer and more filiform than in man.) The structure of a villus (fig. 6) is somewhat complicated, but we must endeavor to
explain it, because, without tolerably accurate knowledge on this point, no one can understand how most of the essential elements of food (the albuminates and fatty matters) make their way from the intestine to the blood. Each villus is provided with an abundant set of capillaries, which doubtless absorb fluid matters, which thus find their way directly from the bowels into the blood (fig. 7). A single artery enters its base, and passing up its center, divided into a capillary plexus, which almost surrounds the villus immediately beneath the mucous membrane. From these arise small veins, which usually pass out of the villus in two, three, or more trunks, and contribute to form the portal vein. See CIRCULATION.
The villus also contains in its interior one or zyc
more lacteals, which are vessels with club-shaped closed extremities, which absorb the chyle from
the intestine. Their milk-white appearance, FIG. 7.
when they are filled with chyle, suggests the Vertical section of the coats of the small in
in- origin of their name. The tissue which occupies testine, showing the capillaries and the beginnings of the portal vein. The arter. the cavity of the villus, in which the lacteals are ies are not seen, not having been pene- imbedded, and which supports the capillary trated by the injection which has been plexus, is in a great measure made up of nuclei thrown into the portal vein, a. vessels of the villi: b. those of the tubes and granules, except at the free extremity, where
or follicles of Lieberkuhn; c, those of the a vesicular structure, resembling very minute fat muscular coat.
globules, is apparent. There is abundant evidence that the function of the villi is connected with absorption, and mainly with the absorption of chyle. 1. The villi exist only in the small intestine, where the absorption of food goes chiefly on. 2. They are most developed in that part of the intestine where chyle is first formed. 3. They are turgid, enlarged, and opaque during the process of chylification, and small and shrunken in animals that have been kept fasting for some time before death.
In addition to the villi, the mucous membrane of the small intestine presents numerous transverse folds, which are termed the valvulæ conniventes, from their valvular form and from their movements under water resembling the winking motion of the eyelids (fig. 8). Each fold passes round three fourths or more of the gut; and in the lower part of the duodenum, and in the jejunum (the parts in which they are most fully developed) they are often more than half an in, in depth; further on, they diminish in depth, length, and number, and in the lowest part of the ileum they can scarcely be traced. Their object clearly is to increase the extent of the absorbent mucous membrane.
In addition to Lieberkuhn's follicles or tubes, which exist in the whole of the smaller intestine, there are other glandular or secreting structures, imbedded in the submucous tissue of certain portions of the intestinal tract, which require consideration. These
are: 1. Brunner's glands, which occur only in the duodenum; 2. Solitary glands, which seem to occur in all parts of the intestine, both small and large; and 3. Peyer's glands, which are usually confined to the ileum.
Brunner's glands are most abundant at the pyloric end of the duodenum. In struct ure, they resemble the pancreas, their ultimate elements being bunches of vesicles, from which minute ducts arise, which coalesce and form larger ducts, through which the secretion is poured into the duodenum. It is believed that they secrete a fluid similar to the pancreatic juice. The solitary glands occur in all parts of the intestine, but are perhaps more numerous in the jejunum than elsewhere. Each gland is a simple membranous flask-shaped vesicle, the neck corresponding to the surface of the intestine, while the rounded base lies in the submucous tissue. The neck presents no opening, and how the contents, which consist of nuclei and granular particles, are discharged into the intestine, is not clearly known. As we never see them larger than a mustardseed, we may presume that, on attaining that size, they burst. Peyer's glands (fig. 9) are apparently mere aggregations of solitary glands, forming oval patches in the ileum. These patches vary in size and number, being largest towards the cæcum, where their long diameter sometimes measures 3 or 4 in., and smallest towards the jejunum; while their number varies from 15 to 20, or even more. Nothing certain is known regarding the uses of these solitary or aggregated glands; but as they are largest during the digestive process, we must infer that they are in some way connected with that function. Possibly the peculiar odor of the fæces may be due to their secretion. In typhoid or enteric fever, and in phthisis, these glands become ulcerated, which probably occasions the diarrhea so common in these diseases.
Brunner's glands are much more developed in the herbivora than in the carnivora; Peyer's, on the other hand, are most developed in the latter.
We have endeavored, in the preceding sentences, to give the reader some idea of the complicated structure of the mucous and submucous coat of the small intestines; we now proceed to notice the chief uses of the muscular coat of the intestine. This coat,
FIG. 9. Small intestine distended and Vertical section through a patch of Peyer's glands in the dog.
hardened by alcohol, and laid a, villi; b, tubes of Lieberkuhn; c submucous tissue, with the open to show the valvulæ con glands of Peyer imbedded in it; d, muscular and peritoneal niventes.
coats; e, apex of one of the glands projecting among the tubes. as has been already mentioned, consists of two layers of muscular fibers-namely, circular and longitudinal fibers, of which the former lie next to the submucous coat. The peristaltic or vermicular action by which the substances which enter the duodenum from the stomach are moved onwards, is due to this muscular coat. A person who has once seen the abdomen of an animal laid open immediately after death, will have a better idea of the nature of this movement than can be afforded by any description. It commences about the pyloric third of the stomach, from whence successive wave-like movements are propagated through the entire length of the intestinal canal. It is the rapid succession of these alternate contractions and relaxations that impels the intestinal contents onwards, and occasion those movements which, from their resemblance to the writhings of a worm, have been termed vermicular. It is very probable that the rapidity of this movement varies in different individuals—those persons, for example, whose bowels act twice daily having a more rapid vermicular motion than those in whom the act of defecation occurs only once in the twenty-four hours.
We have now to consider the effects produced on the chyme by the different fluids with wbich it becomes mixed in the small intestine. These fluids are: 1. The bile; 2. The pancreatic juice; and, 3. The intestinal juice.
The bile (see BILE) is a faintly alkaline or neutral fluid, containing two essential con. stituents, one of which is of a resinous nature, while the other is a pigment. The resin. ous constituent is not precisely identical in all kinds of bile, but it generally consists of a soda-salt whose acid is either glyco-cholic or tauro-cholic acia (q.v.), or of a mixture of these salts. Strecker, to whom we are mainly indebted for our knowledge of the chemistry of the bile, states that in most mammals the resinous constituent merely differs in the varying proportions in which the taurocholates and glycocholates are intermixed, the former usually preponderating. According to Lehmann, the resinous constituent amounts to at least 75 per cent of the solid residue. The bile-pigment occurs in the bile of different animals under two forms-namely, as a brown and as a green pigment, the latter probably only differing from the former in being mere highly oxidized. There has never been a case in which physiologists have had an opportunity of directly observing the quantity of bile that is secreted by the human subject, and all our information on this subject is derived from observations on animals, in which the ductus choledochus communis (see LIVER) has been tied, and a fistulous opening established into the gall-bladder. If the same proportion of bile to bodily weight holds good in man as in the dog, a man weighing ten stone would secrete daily about five pounds of bile. All observers agree, that the amount of the biliary secretion varies directly with the quantity of food; and as animals with biliary fistulæ (in whom all the bile escapes externally, instead of making its way into the duodenum) usually have voracious appetites, experiments on this point are easily made. There is great discrepancy of opinion as to how soon after a meal the bile flows most abundantly into the intestine. According to Kölliker and Müller, whose experiments were made on dogs fed only once a day, very little bile is secreted in the first and second hour after a meal, more in the third, fourth, and fifth; the maximum being sometimes attained in the fifth, sometimes not till the eighth hour.
Numerous and somewhat discrepant views have at different times been advanced regarding the functions of this fluid; we shall here only notice those functions which are connected with digestion. One use that has been ascribed to it, is to neutralize in the small intestine the acid chyme which emerges from the stomach. But the bile can contribute little or nothing to the neutralization of the free acid, because, in the first place, the bile is very slightly alkaline, and often perfectly neutral; and secondly, because the chyme in the intestime is still acid after the admixture of the bile. Again, the bile has been asserted to possess a special solvent action on the chyme; but none of the ordinary constituents of the latter seem to be essentially changed, even when digested for a long time with fresh bile. Again, much importance has been attached to the anti. septic action of the bile on the contents of the intestinal canal, in favor of which view it is alleged that when no bile is poured into the intestine, the fæces have a putrid odor, as is sometimes observed in patients with jaundice, and as was noticed by Frerichs in animals in which the ductus choledochus had been tied. Another use that has been assigned to the bile is, that it exerts a stimulating action on the intestinal walls, and thus acts as a natural purgative; and in support of this view, it may be mentioned that jaundice (in which the bile does not flow into the intestine) is often accompanied by extreme constipation, and that purified ox-gall, taken either in the form of pill or enema, produces an undoubted purgative action. But the main use of the bile seems to be to promote the digestion of fatty matters, and it accomplishes this end not so much by any solvent chemical action on the fats (which at most is extremely slight), as by a peculiar physical action both on the fats and on the intestinal walls, disintegrating the former, and impressing on the latter (by moistening the villi) a peculiar condition which singularly facilitates the absorption of fatty matters. This view is fully confirmed both by direct experiments out of the body, and by comparing the relative qualities of fat that are retained in the body and applied to the purposes of life by animals with biliary fistulous openings, and by healthy animals.
The pancreatic fluid which is poured into the duodenum at the same spot with the bile. (see fig. 1), is a colorless, clear, somewhat viscid and ropy fluid, devoid of any special odor, and exhibiting a strong alkaline reaction. This fluid, as yielded by different dogs with permanent fistulous openings, varies considerably in chemical composition; the collective solid constituents ranging from 1.5 to 2.3 per cent, the organic matters from 0.9 to 1.6, and the mineral matters from 0.62 to 0.75.
The most abundant and important of the solid constituents is a peculiar substance termed pancreatine, or pancreatic diastase or ferment, in combination with soda, to which this fluid owes its principal chemical and physiological properties. Calculating from the quantity of pancreatic juice secreted by dogs of known weight, we may infer that a man weighing ten stone secretes daily about ten ounces of this fluid..
One of the chief uses of the pancreatic juice in relation to digestion, is to convert into sugar the amylaceous or starchy matters which have escaped the action of the saliva, and have passed unchanged into the duodenum. It possesses this property in a far higher degree than the saliva; and, as might be expected in reference to this use, the pancreas is found to be much more developed in herbivorous than in carnivorous animals. Bernard, the representative of the modern school of physiology in France, claims for this fluid another important function; he believes that he has proved that it is solely by the action of this secretion that the fat is reduced to a condition in which it can be absorbed and digested; that is to say, that it is decomposed into glycerine and a fatty acid. See Fats. This view, has, however, not been generally accepted, and it seems probable that although the chauge described by Bernard takes place when fat and pancreatic juice are simply mixed together in a test-tube, it does not actually take place in the intestine, the acid gastric juice probably acting as an interfering agent. An attempt has lately been made by Corvisart and Meissner to prove that, like the gastric juice, this fluid can dissolve albuminous matters; but this view cannot be substantiated. Considering the large quantity of pancreatic fuid which is yielded in 24 hours, Schmidt, who has made the digestive juices the subject of his special study, is of opinion that the function of this fluid is not so much to promote the conversion of starch into sugar, as for the purpose of diluting the chyme, and for reconverting the soda (which in the pancreas has been separated from the chlorine of the chloride of sodium, and has combined with the pancreatine) into chloride of sodium. He shows, from numerical calculations, that more than half of the chloride of sodium existing in the blood which circulates through the pancreas, is broken up into hydrochloric acid and soda, of which the former is separated by the gastric glands, while the latter unites with the pancreatine. Meeting again in the duodenum, the hydrochloric acid and the soda reunite, and re-form chloride of sodium, which is again absorbed, and re-enters the circulation. This is perhaps one of the most singular decompositions and reunions occurring in the animal body.
Of the last of the fluids poured into the intestine, and co-operating in the digestive process, the intestinal juice, we know comparatively little. It is the aggregate secretion of the various glands which we have described as occurring in ihe walls of the small intestine. It is a colorless, or sometimes yellowish, ropy, viscid fluid, which is invariably alkaline. We are not aware of any special or characteristic constituent in it, such as occurs in the other chylopoietic fluids. Its daily quantity is probably nine or ten ounces. It seems to unite in itself the leading properties of the pancreatic and gastric juices; that is to say, it resembles the former in converting starch into sugar, and the latter in dissolving flesh and other albuminous bodies.
We shall conclude this part of the subject with a few remarks on the chemical composition of the contents of the small intestine. On laying open the gut, we usually find a semi-solid admixture of imperfectly digested and indigestible substances and of tbe constituents of the digestive fluids in a more or less changed condition. The reaction of this mass varies in different parts of the canal, and in some measure with the nature of the food. Thus, the contents of the stomach always redden litmus paper, whatever kind of food has been taken; the duodenal contents are also always acid, but in a far less intense degree; in the jejunum we meet with only a faint acid reaction, which altogether disappears in the ileum; while in the cæcum, and sometimes in the lower part of the ileum, an alkaline reaction occurs. After a purely flesh diet the acid reaction disappears shortly below the duodenum, while, after the sole use of vegetable food, it may sometimes be traced even to the cæcum. As a general rule, the contents of the large intestine are alkaline.
In consequence of the rapid absorption that goes on along the intestinal surface, we meet with a comparatively small amount of soluble matters in these contents. Among these soluble matters we often find glycose (or grape-sugar), which seems to owe its origin to the metamorphosis of starch, and not to sugar having been present in the food; for after saccharine food has been taken, we rarely meet with it in any quantity in the small intestine, its absorption taking place with great rapidity. In the alcoholic extract of these contents we can almost always find evidence of the presence of biliary constituents. In the duodenum, and for a little way beyond it, we find glycocholic and tauro-cholic acid; descending a little further, they rapidly diminish, till we find the products of their disintegration; while in the large intestine, little more than a trace of these products can be detected. These
FIG. 10. chemical observations confirm the experiments of Schmidt, Cæcum inflated, dried, and which show that nearly half the bile which is poured into opened to show the arrangethe duodenum is decomposed before it reaches the middle ment of the valve. of the small intestine.
a, termination of the ileum; b, 7. We have now arrived at the seventh stage of the diges
ascending colon; c, cæcum; d,
a transverse construction protive process, that of defecation. The line of demarcation jecting into the cæcum ef, between the small and large intestine is very obvious, and lips of the valve separating
the small from the large interby the peculiar arrangement of the ileo-cæcal valve (see fig. tine: g, the vermiform appen. 10), matters are allowed to pass forward with facility, dix of the cæcum. while regurgitation is impossible. For anatomical details regarding the large intestine, we may refer to the articles ALIMENTARY CANAL, CÆCUM, and Colon. The contents of the large intestine differ very materially from those which we have been considering in the last paragraph, and constitute the fæces. They are more solid and homogeneous, and are often molded into a definite shape by the cells of the colon. The only essential change which the contents undergo in this part of their course is, that they increase as they pass onward in solidity, in consequence of the absorption of fluid from them by the mucous membrane. They are propelled forward