The intestinal stretch of the digestive tract is a long lane with many turnings. In higher vertebrates it is differentiated into the small intestine, or degressive zone, in which food substances passing through are diverted into the blood stream, and a shortened part, usually of somewhat greater diameter, known as the large intestine, constituting the egressive zone from which the unutilized residue of the food-mass is ejected.
The Small Intestine
All other regions of the digestive tract are subsidiary in function to the small intestine. It is here that the food-mass, which has undergone chemical and mechanical modification on the way, is finally converted into sufficiently soluble form to be passed over into the blood stream by diffusion, whence it is finally distributed to the uttermost needy cells of the body.
The straight, comparatively short intestine of cyclostomes, undifferentiated into small and large regions, has its absorbing surface increased by a typhlosole, an internal fold including mucosa and submucosa, which makes only a few spiral turns in the entire length of the intestine.
In elasmobranch fishes the typhlosole, making numerous spiral turns, is much longer than the intestine, with the result that it becomes twisted into a spiral valve. This makes an enlarged surface within a very compact space for the diversion of food, since the intestine is no longer than the J-shaped stomach and is not bent.
A spiral valve is also present in the intestine of dipnoans, certain ganoids, and at least one exceptional teleost (Chenocentrus). Twisted coprolites (fossil feces), found with the bones of ichthyosaurs, indicate that these extinct reptiles might also have been equipped with a spiral valve device that moulded the feces into a twisted shape.
The ganoids show a different method of increasing the intestinal surface, by means of pyloric caeca which are saclike diverticula at the beginning of the small intestine. Both spiral valve and pyloric caeca are present in the sturgeon, Acipenser, although poorly developed.
The next step in the evolution of the small intestine is found in the teleost fishes, which have given up the spiral valve idea and gone over entirely to the elaboration of pyloric caeca. In some of the bony fishes these structures form a large tuft of tubules, occupying considerable space within the constricted body cavity. They vary in number from one in the ganoid Polypterus, and the sand-lance, Ammodytes, to over two hundred in the mackerel, Scomber.
The distinction between the small and large intestine begins with amphibians, also the diversification of the inner surface of the small intestine by villosities, which reach their greatest differentiation in the small intestine of mammals. In amphibians the entire lining of the digestive tract is composed of potentially absorbing cells, corresponding in function to the small intestine of higher forms.
The sluggish reptiles as a class have a definite large intestine marked off from the small intestine that joins the stomach. At the junction between the small and large intestines a new diverticulum, the colic caecum, appears. The colic caecum of a turtle is hardly more than a slight enlargement, but higher up among rabbits and some other rodents it may become an enormously enlarged tube with an internal capacity nearly equal to that of the rest of the digestive canal to which it is attached.
Birds, which have evolved a long way from their reptilian forebears, have a much coiled small intestine, two colic caeca, and a large intestine that is decidedly foreshortened, since it is incompatible with their strenuous aerial life to carry about the ballast of unnecessarily retained feces. The colic caeca are short in pigeons and comparatively long in owls and turkeys. In ostriches they are sometimes reduced to a single caecum, which is as capacious as all the rest of the small intestine and is made even more effective by the presence of a spiral valve.
The small intestine of mammals is usually easily distinguishable from the large intestine, a single colic caecum marking the transition from one region to the other. Exceptions are Trichechus, Hyrax, and the edentates Dasypus and Myrmecophaga, which have two colic caeca. Monotremes, flesh-eating marsupials, edentates, insectivores, bats, carnivores, and toothed whales, either lack or have only a small colic caecum, but in herbivores it is so large that it may even exceed the body in length. Herbivores also have a noticeably longer intestine than carnivores.
The degenerate free end of the colic caecum forms the processus vermiformis in certain rodents, civets, monkeys, and man. According to Wiedersheim the processus vermiformis, or vermiform appendix, in man, which has outlived its usefulness and bears an unsavory reputation, varies in length from two to twenty-five centimeters, with an average of about eight and one half centimeters. It tends to shorten with age and to become closed in later life. Statistics on the closure in 1005 observed cases are given by Muller in percentages as shown in Table IV.
The small intestine is divided more or lens arbitrarily into duodenum, jejunum, and ileum, a distinction which, though first made out in man, applies to most other mammals.
The duodenum, or part next to the stomach, is comparatively short. The jejunum, which follows, and the more posteriorly located ileum, forming about equal lengths of the remainder of the small intestine, are not distinctly marked off. The jejunum is richer in blood vessels as well as having a somewhat thicker wall and wider lumen than the ileum.
The characteristic modification of the lining of the mammalian small intestine is the presence of innumerable tiny thickset velvety projections, or villi, which enormously increase, in a minimum of space, the absorbing surface exposed to the dissolved food. In fact these are the definite organs of absorption.
Each villus consists of a thimble-like projection whose thin wall of cells encloses a capillary loop and a microscopic lacteal, or terminal element of the intestinal lymphatics (Fig. 260). Fat that has been reduced to lower terms in the intestine passes into the lacteals and thence to the lymphatics, eventually emptying into the venous system by way of the thoracic duct. Digested proteins and carbohydrates are collected by the venous capillaries of the villi and carried to the liver by way of the portal vein.
In the ileum particularly, the forest of microscopic villi is frequently interrupted by irregular bare patches from half an inch to three or four inches in extent, which show like worn places in the nap of a Brussels carpet. These “intestinal tonsils,” or Peyer’s patches (Fig. 261), are lymphoid in character. It is important to remember that in typhoid fever the chief lesions occur in these areas. Smaller lymph nodes are also interspersed among the villi.
Two conspicuous glands differing greatly in appearance, intimate structure, and function, but which are alike in being endodermal derivatives of the mesodaeum, are connected with the duodenum just posterior to the pylorus. These are the liver and the pancreas. They are so large that, although they arise from the lining of the small intestine, they push through to extend entirely outside the digestive tube itself and come to occupy space within the body cavity.
The liver is an older organ, both ontogenetically and phylogenetically, than the pancreas. It should not be confused with the so-called “liver” of starfishes, crabs, mollusks, or other invertebrates, since it is in no way either homologous with or analogous to these structures.
Contrary to the popular impression, the vertebrate liver is not primarily an organ of enzymatic digestion, although the bile which it provides aids materially in the digestive processes by the stimulative action of the bile salts upon the enzymes of the pancreatic juice, by the emulsifying action of these salts, and by furnishing a favorable alkaline medium without which the digestive enzymes produced by the pancreas fail to act. In the bile itself no enzymes are present.
The liver, which has been characterized as the “busiest port on the whole river of life,” is so voluminous that in man it may easily contain one fifth of all the blood, while several times an hour the entire blood supply of the body passes through its myriad capillaries, undergoing there profound modifications by way of additions and subtractions of constituent substances in the blood. Thus it acts like a strainer of the blood, storing sugar-fuel (glycogen) and then restoring it to the circulation when the muscles need it; balancing the circulating food ration generally by withdrawing and restoring certain constituents; eliminating bacteria; turning poisons into harmless wastes; abstracting nitrogenous by-products from protein compounds to be disposed of through the kidneys; and by drenching the intestine with the indispensable alkaline bile. No wonder Dr. Woods Hutchinson said of the liver: “It is all together the most useful and desirable citizen, and withall a cheerful and even convivial one, mixing our drinks, putting the stick into our vitamin cocktails, and the sugar and cream into life’s coffee.”
The bile is a bitter alkaline fluid, about ninety per cent water, tinged with pigments from the wreckage of red blood corpuscles and containing salts, both organic and inorganic, besides waste materials of different kinds. It is formed continuously in the liver and is poured at food-taking intervals into the duodenum where it mingles with the food-mass upon the escape of the latter through the pylorus into the intestine. The bile, which may amount to a pint or a pint and a half daily, contributes materially to the excreta that pass out of the alimentary canal. When an excess of bile is produced it may temporarily be stored between periods of digestive activity, by backing up into the gall bladder, a reservoir-like enlargement of the bile duct (Fig. 266). That part of the bile duct that drains the liver is called the hepatic duct as distinguished from the cystic duct that comes from the gall bladder (Fig. 262). Whenever these two ducts join to empty into the intestine the common bile duct thus formed is called the ductus choledochus, a name that has the same root as “melancholy” and “choleric,” words descriptive of conditions for which the misunderstood liver has been held responsible by the popular mind in the past.
The choledochal duct and the duct of the pancreas open together into the duodenum of the horse, dog, cat, ape, and man, but separately in the pig, ox, rabbit, and guinea pig.
The gall bladder, which seems to be an emergency device for animals that digest a considerable amount of fatty food, is absent in many plant-eaters. Its total absence, as well as the abnormal presence of two gall bladders, has been noted in man. Its normal capacity in man is about an ounce and a half.
“Gall stones,” which are found in the gall bladder in about 6 to 12 per cent of autopsies, are concretions composed chiefly of cholesterol but usually including bile pigments and calcium salts. Gall stones containing limy deposits may be detected by the X-ray. If large they are comparatively harmless, but while small they may block the ducts, thus causing the bile to be resorbed and passed back by way of the lymphatics into the blood stream, a condition that results in jaundice. The cystic duct especially lends itself to such obstruction, since it is modified within by mucous folds forming the Heisterian valve, which makes blocking the passage-way easier than would be possible if the lumen were entirely open.
The liver is an adaptive space-filler, weighing about one fortieth of the total body weight, and consisting usually of two or more lobes. Its shape and size are conditioned first by the abundant blood vessels, nerves and ligaments, or connective tissue attachments; second, by the neighboring organs which crowd it; and third, by the confining walls of the body cavity. With every breath that expands the neighboring lungs, and because of the uneasy peristalsis of the stomach and intestine pressing upon it, the flexible lobes of the liver are constantly slipping slightly over each other, changing meanwhile somewhat in shape in accommodation and adjustment to the varying conditions of available space.
In mammals the two larger lobes are separated by the umbilical fissure, as determined by the round ligament representing the atrophied remains of the umbilical vein. A large portal fissure marks the gateway for the blood vessels, ducts, and nerves that pass to and from the liver in adult life.
In histological structure the mammalian liver is made up of cords of glandular cells in close contact with capillaries (Fig. 263) and with an elaborate system of drainage ducts and an adequate nerve supply. The whole mass of cellular cords and capillaries is embedded and encapsuled in a supporting network of connective tissue.
Along the approximated sides of neighboring polyhedral gland cells that form the cords, are tiny intercellular spaces or grooves (Fig. 264), like the spaces between the fingers and knuckles when two fists are placed together. These bile canaliculi, formed as indenting intercellular grooves permeating the entire liver mass, empty into drainage ducts which compound with others in an ever enlarging array until they finally emerge as the single large outlet of the hepatic duct. The system of capillaries that enmeshes the gland cells is independent of the mesh work of canaliculi and drainage ducts.
The blood supply of the liver is unlike that of most other organs in that there are two sources from which it is derived, namely, the hepatic artery which brings blood from the heart in the same way as all organs of the body are supplied, and the portal vein, that comes freshly laden with dissolved food from the intestine. The capillaries derived from both of these sources hopelessly lose their identity as they gradually anastomose within the liver to form the hepatic veins, which drain the blood of the liver into the heart.
Embryonically the liver is a hollow ventral outgrowth near the beginning of the intestine just anterior to the attachment of the embryonic yolk sac (Fig. 265). It lies at first between two layers of ventral mesentery in the transverse septum, but eventually it becomes so large that it projects some distance into the body cavity, pushing a covering of serosa, or visceral peritoneum, with it. The endodermal outgrowth from the gut itself becomes the secretory glandular part of the liver, and this soon becomes enmeshed with the vascular mesenchyme from the transverse septum and neighboring blood vessels, including the vitelline veins leading into the body from the yolk sac, to form the liver tissue. As the mass of this tissue grows into the transverse septum and the ventral mesentery, the material which is at the primary point of origin develops into the ducts and gall bladder.
In amphioxus the liver remains a simple, single sac projecting forward, that is beset with capillaries which bring food from the intestine, much as in the early embryonic phase of higher forms. It is lined with ciliated glandular epithelium and probably secretes a digestive fluid.
Typically the liver of most vertebrates has two lobes, although lampreys and snakes, perhaps on account of their elongated body form, have only one lobe. The liver is relatively larger in carnivores than in herbivores. In Anamnia, or fishes and amphibians, it is larger than in Amniota, or reptiles, birds, and mammals. This fact no doubt is connected with the presence of more fat in the diet of the former in each case. In certain carnivores, dogs and weasels for example, there are as many as seven lobes present.
In man four lobes are described (Fig. 266). The right lobe is the largest, constituting about four-fifths of the entire mass, while the wedge-shaped left lobe is next in size. These lobes are separated from one another by the falciform ligament. On the posterior aspect between these two major lobes, an oblong quadrate lobe lies near the gall bladder, while the small caudate lobe spreads between the postcaval and portal fissures.
The second largest vertebrate gland, the pancreas, is a compound alveolar gland of irregular shape lying in the fold between the stomach and the duodenum, and projecting into the body cavity from the point of its embryonic connection with the digestive tube, although in the lamprey, Petromyzon, and in certain teleosts, it may remain embedded in the wall of the intestine.
It arises as one or more endodermal outgrowths from the embryonic gut just posterior to the liver. These outgrowths are ordinarily three in number, of which one is dorsal and two are ventral in position (Fig. 267). The two ventral parts fuse together into a common gland, while the ducts formed at each point of outgrowth may either persist, or as is more often the case disappear with the exception of one (Fig. 268). The ducts of the ventral components are called Wirsung’s ducts, while that of the dorsal pancreas is named the duct of Santorini. In some forms they unite either with each other to make a common duct, or with the bile duct. In lampreys all of the ducts are lost, the secretion of the pancreas consequently becoming entirely endocrine, that is, distributed by the blood rather than poured directly to the outside or into some passage-way leading to the outside.
A curious modification of the pancreatic apparatus which not infrequently appears, particularly in the cat, is a pancreatic bladder, or reservoirlike enlargement of the pancreatic duct for the temporary storage of excessive secretion, an emergency organ quite cojnparable with the gall bladder that serves similarly as a storage reservoir for the bile of the liver (Fig. 269).
In general it may be said that the pancreas is a gland of dual character since, in addition to its production of pancreatic “juice” which is poured into the intestine through ducts, there are present in the pancreatic tissue certain distinct interlobular cell aggregates, also endodermal in origin, called the islands of Langerhans (Fig. 270). These secrete substances (hormones) of a character quite different from the pancreatic juice itself, which are carried to all parts of the body through the circulating blood. The islands of Langerhans vary from single interstitial cells to masses of several hundred. They are more abundant late in life than in youth, appearing in man when the human embryo is only about 54 mm. in length. It is estimated that in the pancreas of the guinea pig there may be present as many as 25,000 islands of Langerhans.
Which of these two kinds of secreting glandular cells, the endodermal outgrowths from the embryonic gut or the islands of Langerhans, represents the original pancreatic tissue, and whether one has or has not been derived from the other, is still a matter of controversy. It may be pointed out, however, that the islands of Langerhans are invariably present in all true vertebrates, and are undoubtedly early settlers if not the first inhabitants, whereas enzymatic secreting cells are wanting in certain vertebrates.
There is no pancreas in amphioxus. In sturgeons the pancreas is made up of two dorsal and two ventral components with only the right ventral ductus Wirsungianus remaining. The pancreas of bony fishes and lower vertebrates generally is primitive in character, being widely diffused and irregular in form. Among mammals there is also a great variety in the form, position and size of this important gland. A single surviving embryonic pancreatic duct is found in man, Wirsung’s duct, connecting with one of the ventral embryonic components.
The activating substance, or hormone, that is diverted from the islands of Langerhans into the blood has to do with the utilization of sugar in the tissues. Its failure to be produced in sufficient quantity results in diabetes, or the condition in which sugar is eliminated unused through the kidneys.
The substances secreted by the pancreas proper and eliminated through the ducts into the intestine are enzymes, which are essential in speeding up the chemical action begun in the intestine.
The principal enzymes in the pancreatic juice that aid in the digestive process are three, namely, amylopsin, which like the ptyalin of the saliva acts in making carbohydrates soluble; the inactive precursor of trypsin, which modifies proteins; and steapsin, which breaks down fats into simpler fatty acids and glycerine. These three important digestive enzymes, therefore, are prepared not only to transform chemically the three fundamental kinds of foods, carbohydrates, proteins, and fats, but also to render them fit for transference through the blood to all parts of the body.
In man about a pint and a half of digestive pancreatic juice is poured daily into the intestine.
In addition to the liver and pancreas, the secretions of which are mixed with food materials in the duodenum, there are numerous smaller glands, occupying the walls of the intestinal tract, that likewise make chemical contributions essential to digestion. An intestinal juice that has been given the blanket name of succus entericus combines the products of these small, intestinal glands. Among mammals there are at least two kinds of intestinal glands that contribute to this juice, namely, Brunner’s glands in the anterior end of the duodenum, and the glands of Lieberkilhn, which are vastly more numerous, being embedded throughout the entire length of the small intestine in its walls. The multitudinous glands of Lieberkiihn are in the form of sunken pits, or crypts, which are interspersed among the villi like deep gorges among steep mountains (Fig. 260).
Upon chemical stimulation by the chyme, which is the acidulated food mass that passes through the pylorus from the stomach into the duodenum, the epithelium of the small intestine produces a hormone called pancreatic secretin, which is carried by the blood to the pancreas where it excites that gland into activity. This reaction occurs only when chyme enters the intestine; otherwise, if continuously produced, the pancreatic juice much of the time would be wasted, not having any food upon which to act.
The succus entericus also includes several enzymes. Enterokinase converts the inactive trypsinogen of the pancreatic juice into the active trypsin. Erepsin completes the work of protein digestion begun by pepsin and trypsin. Other enzymes aid in carbohydrate digestion. Thus the intestinal contents include a variety of enzymes, derived from both pancreatic juice and succus entericus, which play a part in the complicated chemical preparation of the chyme for its transfer to the blood.
Most food material owing to its colloidal nature is inert and insoluble when taken into the digestive tract and cannot be directly transferred by osmosis to the circulating blood. Even water and inorganic soluble salts, that undergo no metabolic alteration in passing through the body, temporarily join hands with other substances in various combinations.
The army of enzymes, which are “substances of indefinite composition whose existence is known to us only by their action on other substances, play an indispensable role in making foods diffusible and available for use. Energy-producing carbohydrates, for example, if not taken in primarily in the form of soluble monosaccharids, must be made soluble by enzymatic action. Such enzymes as ptyalin in saliva and amylopsin in the pancreatic juice change insoluble starches into soluble sugars.
The complex proteins have two fundamental uses, namely, production of energy and restoration of tissues. As with carbohydrates, proteins must first be reduced to soluble form by the action of enzymes before they can be oxidized to produce energy, while to restore worn-out tissues the diverse complex molecules that make up proteins must be broken down and reassembled into the specific kind of molecules that are the building blocks which go to form the different body tissues. This process, like wrecking an old house to build a new one, calls for a succession of breaking-down operations, each dependent on a different enzyme.
In the disposition of insoluble fats two things happen: they are emulsified and then broken up into soluble fractions. Fats are said to be emulsified when mechanically broken up into particles of oil so small that each hangs suspended in a watery medium. Fats are chemically broken down into soluble fatty acids and glycerine, by such enzymes as steapsin in the pancreatic juice. These products are picked up by the epithelial cells of the intestinal lining in which they are reconverted into fats, which then pass into the lymphatic cores of the villi in emulsified condition, to arrive eventually in the blood stream. The emulsified fat, known as chyle, is milky in appearance as it is carried away by the lymphatics, which are therefore known as lacteals (lact-, milk). Of course milk itself is such an emulsion, too.
Enzymes and what they do, a subject so appalling to the layman, furnishes the happy hunting ground for the physiologist. A list of the principal enzymes concerned with digestion, prepared by Burton-Opitz, is shown in Table V.