144. The Coats of the Small Intestine. Like the stomach, the small intestine has four coats, the serous, muscular, sub-mucous, and mucous. The serous is the peritoneum.[22] The muscular consists of an outer layer of longitudinal, and an inner layer of circular fibers, by contraction of which the food is forced along the bowel. The sub-mucous coat is made up of a loose layer of tissue in which the blood-vessels and nerves are distributed. The inner, or mucous, surface has a fine, velvety feeling, due to a countless number of tiny, thread-like projections, called villi. They stand up somewhat like the "pile" of velvet. It is through these villi that the digested food pa.s.ses into the blood.

[Ill.u.s.tration: Fig. 56.--Sectional View of Intestinal Villi. (Black dots represent the glandular openings.)]

The inner coat of a large part of the small intestine is thrown into numerous transverse folds called _valvulae conniventes_. These seem to serve two purposes, to increase the extent of the surface of the bowels and to delay mechanically the progress of the intestinal contents. Buried in the mucous layer throughout the length, both of the small and large intestines, are other glands which secrete intestinal fluids. Thus, in the lower part of the ileum there are numerous glands in oval patches known as _Peyer's patches_. These are very p.r.o.ne to become inflamed and to ulcerate during the course of typhoid fever.

145. The Large Intestine. The large intestine begins in the right iliac region and is about five or six feet long. It is much larger than the small intestine, joining it obliquely at short distance from its end. A blind pouch, or dilated pocket is thus formed at the place of junction, called the caec.u.m. A valvular arrangement called the ileo-caecal valve, which is provided with a b.u.t.ton-hole slit, forms a kind of movable part.i.tion between this part of the large intestine and the small intestine.

[Ill.u.s.tration: Fig. 57.--Tubular Glands of the Small Intestines.

A, B, tubular glands seen in vertical section with their orifices at C, opening upon the membrane between the villi, D, villus (Magnified 40 diameters)]

Attached to the caec.u.m is a worm-shaped tube, about the size of a lead pencil, and from three to four inches long, called the _vermiform appendix_. Its use is unknown. This tube is of great surgical importance, from the fact that it is subject to severe inflammation, often resulting in an internal abscess, which is always dangerous and may prove fatal.

Inflammation of the appendix is known as _appendicitis_,--a name quite familiar on account of the many surgical operations performed of late years for its relief.

The large intestine pa.s.ses upwards on the right side as the ascending colon, until the under side of the liver is reached, where it pa.s.ses to the left side, as the transverse colon, below the stomach. It there turns downward, as the descending colon, and making an S-shaped curve, ends in the r.e.c.t.u.m. Thus the large intestine encircles, in the form of a horseshoe, the convoluted ma.s.s of small intestines.

Like the small intestine, the large has four coats. The mucous coat, however, has no folds, or villi, but numerous closely set glands, like some of those of the small intestine. The longitudinal muscular fibers of the large intestine are arranged in three bands, or bundles, which, being shorter than the ca.n.a.l itself, produce a series of bulgings or pouches in its walls. This sacculation of the large bowel is supposed to be designed for delaying the onward flow of its contents, thus allowing more time for the absorption of the liquid material. The blood-vessels and nerves of this part of the digestive ca.n.a.l are very numerous, and are derived from the same sources as those of the small intestine.

146. The Liver. The liver is a part of the digestive apparatus, since it forms the bile, one of the digestive fluids. It is a large reddish-brown organ, situated just below the diaphragm, and on the right side. The liver is the largest gland in the body, and weighs from 50 to 60 ounces. It consists of two lobes, the right and the left, the right being much the larger. The upper, convex surface of the liver is very smooth and even; but the under surface is irregular, broken by the entrance and exit of the various vessels which belong to the organ. It is held in its place by five ligaments, four of which are formed by double folds of the peritoneum.

The thin front edge of the liver reaches just below the bony edge of the ribs; but the dome-shaped diaphragm rises slightly in a horizontal position, and the liver pa.s.ses up and is almost wholly covered by the ribs. In tight lacing, the liver is often forced downward out from the cover of the ribs, and thus becomes permanently displaced. As a result, other organs in the abdomen and pelvis are crowded together, and also become displaced.

147. Minute Structure of the Liver. When a small piece of the liver is examined under a microscope it is found to be made up of ma.s.ses of many-sided cells, each about 1/1000 of an inch in diameter. Each group of cells is called a _lobule_. When a single lobule is examined under the microscope it appears to be of an irregular, circular shape, with its cells arranged in rows, radiating from the center to the circ.u.mference.

Minute, hair-like channels separate the cells one from another, and unite in one main duct leading from the lobule. It is the lobules which give to the liver its coa.r.s.e, granular appearance, when torn across.

[Ill.u.s.tration: Fig. 58.--Diagrammatic Section of a Villus

A, layer of columnar epithelium covering the villus; B, central lacteal of villus; C, unstriped muscular fibers; D, goblet cell ]

Now there is a large vessel called the portal vein that brings to the liver blood full of nourishing material obtained from the stomach and intestines. On entering the liver this great vein conducts itself as if it were an artery. It divides and subdivides into smaller and smaller branches, until, in the form of the tiniest vessels, called capillaries, it pa.s.ses inward among the cells to the very center of the hepatic lobules.

148. The Bile. We have in the liver, on a grand scale, exactly the same conditions as obtain in the smaller and simpler glands. The thin-walled liver cells take from the blood certain materials which they elaborate into an important digestive fluid, called the bile.[23]

This newly manufactured fluid is carried away in little ca.n.a.ls, called _bile ducts_. These minute ducts gradually unite and form at last one main duct, which carries the bile from the liver. This is known as the hepatic duct. It pa.s.ses out on the under side of the liver, and as it approaches the intestine, it meets at an acute angle the cystic duct which proceeds from the gall bladder and forms with it the common bile duct. The common duct opens obliquely into the horseshoe bend of the duodenum.

The cystic duct leads back to the under surface of the liver, where it expands into a sac capable of holding about two ounces of fluid, and is known as the gall bladder. Thus the bile, prepared in the depths of the liver by the liver cells, is carried away by the bile ducts, and may pa.s.s directly into the intestines to mix with the food. If, however, digestion is not going on, the mouth of the bile duct is closed, and in that case the bile is carried by the cystic duct to the gall bladder. Here it remains until such time as it is needed.

149. Blood Supply of the Liver. We must not forget that the liver itself, being a large and important organ, requires constant nourishment for the work a.s.signed to it. The blood which is brought to it by the portal vein, being venous, is not fit to nourish it. The work is done by the arterial blood brought to it by a great branch direct from the aorta, known as the hepatic artery, minute branches of which in the form of capillaries, spread themselves around the hepatic lobules.

The blood, having done its work and now laden with impurities, is picked up by minute veinlets, which unite again and again till they at last form one great trunk called the hepatic vein. This carries the impure blood from the liver, and finally empties it into one of the large veins of the body.

After the blood has been robbed of its bile-making materials, it is collected by the veinlets that surround the lobules, and finds its way with other venous blood into the hepatic vein. In brief, blood is brought to the liver and distributed through its substance by two distinct channels,--the portal vein and the hepatic artery, but it leaves the liver by one distinct channel,--the hepatic vein.

[Ill.u.s.tration: Fig. 59--Showing the Relations of the Duodenum and Other Intestinal Organs. (A portion of the stomach has been cut away.)]

150. Functions of the Liver. We have thus far studied the liver only as an organ of secretion, whose work is to elaborate bile for future use in the process of digestion. This is, however, only one of its functions, and perhaps not the most important. In fact, the functions of the liver are not single, but several. The bile is not wholly a digestive fluid, but it contains, also, materials which are separated from the blood to be cast out of the body before they work mischief. Thus, the liver ranks above all others as an organ of excretion, that is, it separates material of no further use to the body.

Of the various ingredients of the bile, only the bile salts are of use in the work of digestion, for they act upon the fats in the alimentary ca.n.a.l, and aid somehow in their emulsion and absorption. They appear to be themselves split up into other substances, and absorbed with the dissolved fats into the blood stream again.

The third function of the liver is very different from those already described. It is found that the liver of an animal well and regularly fed, when examined soon after death, contains a quant.i.ty of a carbohydrate substance not unlike starch. This substance, extracted in the form of a white powder, is really an animal starch. It is called glycogen, or liver sugar, and is easily converted into grape sugar.

The hepatic cells appear to manufacture this glycogen and to store it up from the food brought by the portal blood. It is also thought the glycogen thus deposited and stored up in the liver is little by little changed into sugar. Then, as it is wanted, the liver disposes of this stored-up material, by pouring it, in a state of solution, into the hepatic vein. It is thus steadily carried to the tissues, as their needs demand, to supply them with material to be transformed into heat and energy.

151. The Pancreas. The pancreas, or sweetbread, is much smaller than the liver. It is a tongue-like ma.s.s from six to eight inches long, weighing from three to four ounces, and is often compared in appearance to a dog's tongue. It is somewhat the shape of a hammer with the handle running to a point.

The pancreas lies behind the stomach, across the body, from right to left, with its large head embraced in the horseshoe bend of the duodenum. It closely resembles the salivary glands in structure, with its main duct running from one end to the other. This duct at last enters the duodenum in company with the common bile duct.

The pancreatic juice, the most powerful in the body, is clear, somewhat viscid, fluid. It has a decided alkaline reaction and is not unlike saliva in many respects. Combined with the bile, this juice acts upon the large drops of fat which pa.s.s from the stomach into the duodenum and emulsifies them. This process consists partly in producing a fine subdivision of the particles of fat, called an emulsion, and partly in a chemical decomposition by which a kind of soap is formed. In this way the oils and fats are divided into particles sufficiently minute to permit of their being absorbed into the blood.

Again, this most important digestive fluid produces on starch an action similar to that of saliva, but much more powerful. During its short stay in the mouth, very little starch is changed into sugar, and in the stomach, as we have seen, the action of the saliva is arrested. Now, the pancreatic juice takes up the work in the small intestine and changes the greater part of the starch into sugar. Nor is this all, for it also acts powerfully upon the proteids not acted upon in the stomach, and changes them into peptones that do not differ materially from those resulting from gastric digestion. The remarkable power which the pancreatic juice possesses of acting on all the food-stuffs appears to be due mainly to the presence of a specific element or ferment, known as _trypsin_.

Experiment 60. _To show the action of pancreatic juice upon oils or fats._ Put two grains of Fairchild's extract of pancreas into a four-ounce bottle. Add half a teaspoonful of warm water, and shake well for a few minutes; then add a tablespoonful of cod liver oil; shake vigorously.

A creamy, opaque mixture of the oil and water, called an emulsion, will result. This will gradually separate upon standing, the pancreatic extract settling in the water at the bottom. When shaken it will again form an emulsion.

Experiment 61. _To show the action of pancreatic juice on starch_.

Put two tablespoonfuls of _smooth_ starch paste into a goblet, and while still so warm as just to be borne by the mouth, stir into it two grains of the extract of pancreas. The starch paste will rapidly become thinner, and gradually change into soluble starch, in a perfectly fluid solution. Within a few minutes some of the starch is converted through intermediary stages into maltose. Use the Fehling test for sugar.

152. Digestion in the Small Intestines. After digestion in the stomach has been going on for some time, successive portions of the semi-digested food begin to pa.s.s into the duodenum. The pancreas now takes on new activity, and a copious flow of pancreatic juice is poured along its duct into the intestines. As the food is pushed along over the common opening of the bile and pancreatic ducts, a great quant.i.ty of bile from this reservoir, the gall bladder, is poured into the intestines. These two digestive fluids are now mixed with the chyme, and act upon it in the remarkable manner just described.

[Ill.u.s.tration: Fig. 60.--Diagrammatic Scheme of Intestinal Absorption.

A, mesentery; B, lacteals and mesentery glands; C, veins of intestines; R.C, receptacle of the chyle (receptaculum chyli); P V, portal vein; H V, hepatic veins; S.V.C, superior vena cava; R.A, right auricle of the heart; I.V.C, inferior vena cava.

The inner surface of the small intestine also secretes a liquid called intestinal juice, the precise functions of which are not known. The chyme, thus acted upon by the different digestive fluids, resembles a thick cream, and is now called chyle. The chyle is propelled along the intestine by the worm-like contractions of its muscular walls. A function of the bile, not yet mentioned, is to stimulate these movements, and at the same time by its antiseptic properties to prevent putrefaction of the contents of the intestine.

153. Digestion in the Large Intestines. Digestion does not occur to any great extent in the large intestines. The food enters this portion of the digestive ca.n.a.l through the ileo-caecal valve, and travels through it slowly. Time is thus given for the fluid materials to be taken up by the blood-vessels of the mucous membrane. The remains of the food now become less fluid, and consist of undigested matter which has escaped the action of the several digestive juices, or withstood their influence. Driven onward by the contractions of the muscular walls, the refuse materials at last reach the r.e.c.t.u.m, from which they are voluntarily expelled from the body.

Absorption.

154. Absorption. While food remains within the alimentary ca.n.a.l it is as much outside of the body, so far as nutrition is concerned, as if it had never been taken inside. To be of any service the food must enter the blood; it must be absorbed. The efficient agents in absorption are the blood-vessels, the lacteals, and the lymphatics. The process through which the nutritious material is fitted to enter the blood, is called absorption. It is a process not confined, as we shall see, simply to the alimentary ca.n.a.l, but one that is going on in every tissue.

The vessels by which the process of absorption is carried on are called absorbents. The story, briefly told, is this: certain food materials that have been prepared to enter the blood, filter through the mucous membrane of the intestinal ca.n.a.l, and also the thin walls of minute blood-vessels and lymphatics, and are carried by these to larger vessels, and at last reach the heart, thence to be distributed to the tissues.

155. Absorption from the Mouth and Stomach. The lining of the mouth and sophagus is not well adapted for absorption. That this does occur is shown by the fact that certain poisonous chemicals, like cyanide of potash, if kept in the mouth for a few moments will cause death. While we are chewing and swallowing our food, no doubt a certain amount of water and common salt, together with sugar which has been changed from starch by the action of the saliva, gains entrance to the blood.

In the stomach, however, absorption takes place with great activity. The semi-liquid food is separated from the enormous supply of blood-vessels in the mucous membrane only by a thin porous part.i.tion. There is, therefore, nothing to prevent the exchange taking place between the blood and the food. Water, along with any substances in the food that have become dissolved, will pa.s.s through the part.i.tion and enter the blood-current.

Thus it is that a certain amount of starch that has been changed into sugar, of salts in solution, of proteids converted into peptones, is taken up directly by the blood-vessels of the stomach.

156. Absorption by the Intestines. Absorption by the intestines is a most active and complicated process. The stomach is really an organ more for the digestion than the absorption of food, while the small intestines are especially constructed for absorption. In fact, the greatest part of absorption is accomplished by the small intestines. They have not only a very large area of absorbing surface, but also structures especially adapted to do this work.

157. The Lacteals. We have learned in Section 144 that the mucous lining of the small intestines is crowded with millions of little appendages called villi, meaning "tufts of hair." These are only about 1/30 of an inch long, and a dime will cover more than five hundred of them. Each villus contains a loop of blood-vessels, and another vessel, the lacteal, so called from the Latin word _lac_, milk, because of the milky appearance of the fluid it contains. The villi are adapted especially for the absorption of fat. They dip like the tiniest fingers into the chyle, and the minute particles of fat pa.s.s through their cellular covering and gain entrance to the lacteals. The milky material sucked up by the lacteals is not in a proper condition to be poured at once into the blood current. It is, as it were, in too crude a state, and needs some special preparation.

The intestines are suspended to the posterior wall of the abdomen by a double fold of peritoneum called the mesentery. In this membrane are some 150 glands about the size of an almond, called mesenteric glands. Now the lacteals join these glands and pour in their fluid contents to undergo some important changes. It is not unlikely that the mesenteric glands may intercept, like a filter, material which, if allowed to enter the blood, would disturb the whole body. Thus, while the glands might suffer, the rest of the body might escape. This may account for the fact that these glands and the lymphatics may be easily irritated and inflamed, thus becoming enlarged and sensitive, as often occurs in the axilla.