Chapter VI: Digestion (1)

128. The Purpose of Digestion. As we have learned, our bodies are subject to continual waste, due both to the wear and tear of their substance, and to the consumption of material for the production of their heat and energy. The waste occurs in no one part alone, but in all the tissues.

Now, the blood comes into direct contact with every one of these tissues. The ultimate cells which form the tissues are constantly being bathed by the myriads of minute blood-vessels which bring to the cells the raw material needed for their continued renewal. These cells are able to select from the nutritive fluid whatever they require to repair their waste, and to provide for their renewed activity. At the same time, the blood, as it bathes the tissues, sweeps into its current and bears away the products of waste.

Thus the waste occurs in the tissues and the means of repair are obtained from the blood. The blood is thus continually being impoverished by having its nourishment drained away. How, then, is the efficiency of the blood maintained? The answer is that while the ultimate purpose of the food is for the repair of the waste, its immediate destination is the blood.[19]

129. Absorption of Food by the Blood. How does the food pass from the cavity of the stomach and intestinal canal into the blood-vessels? There are no visible openings which permit communication. It is done by what in physics is known as _endosmotic_ and _exosmotic_ action. That is, whenever there are two solutions of different densities, separated only by an animal membrane, an interchange will take place between them through the membrane.

To illustrate: in the walls of the stomach and intestines there is a network of minute vessels filled with blood,—a liquid containing many substances in solution. The stomach and intestinal canal also contain liquid food, holding many substances in solution. A membrane, made up of the extremely thin walls of the blood-vessels and intestines, separates the liquids. An exchange takes place between the blood and the contents of the stomach and bowels, by which the dissolved substances of food pass through the separating membranes into the blood.

Illustration: Fig. 46.—Cavities of the Mouth, Pharynx, etc. (Section in the middle line designed to show the mouth in its relations to the nasal fossæ, the pharynx, and the larynx.)

A, sphenoidal sinus;
B, internal orifice of Eustachian tube;
C, velum palati;
D, anterior pillar of soft palate;
E, posterior pillar of soft palate;
F, tonsil;
H, lingual portion of the pharynx;
K, lower portion of the pharynx;
L, larynx;
M, section of hyoid bone;
N, epiglottis;
O, palatine arch

This change, by which food is made ready to pass into the blood, constitutes food-digestion, and the organs concerned in bringing about this change in the food are the digestive organs.

130. The General Plan of Digestion. It is evident that the digestive organs will be simple or complex, according to the amount of change which is necessary to prepare the food to be taken up by the blood. If the requisite change is slight, the digestive organs will be few, and their structure simple. But if the food is varied and complex in composition, the digestive apparatus will be complex. This condition applies to the food and the digestion of man.

Illustration: Fig. 47.—Diagram of the Structure of Secreting Glands.

A, simple tubular gland;
B, gland with mouth shut and sac formed;
C, gland with a coiled tube;
D, plan of part of a racemose gland

The digestive apparatus of the human body consists of the alimentary canal and tributary organs which, although outside of this canal, communicate with it by ducts. The alimentary canal consists of the mouth, the pharynx, the œsophagus, the stomach, and the intestines. Other digestive organs which are tributary to this canal, and discharge their secretions into it, are the salivary glands,[20] the liver, and the pancreas.

The digestive process is subdivided into three steps, which take place in the mouth, in the stomach, and in the intestines.

131. The Mouth. The mouth is the cavity formed by the lips, the cheeks, the palate, and the tongue. Its bony roof is made up of the upper jawbone on each side, and the palate bones behind. This is the _hard palate_, and forms only the front portion of the roof. The continuation of the roof is called the _soft palate_, and is made up of muscular tissue covered with mucous membrane.

The mouth continues behind into the throat, the separation between the two being marked by fleshy pillars which arch up from the sides to form the soft palate. In the middle of this arch there hangs from its free edge a little lobe called the uvula. On each side where the pillars begin to arch is an almond-shaped body known as the tonsil. When we take cold, one or both of the tonsils may become inflamed, and so swollen as to obstruct the passage into the throat. The mouth is lined with mucous membrane, which is continuous with that of the throat, œsophagus, stomach, and intestines (Fig. 51).

132. Mastication, or Chewing. The first step of the process of digestion is mastication, the cutting and grinding of the food by the teeth, effected by the vertical and lateral movements of the lower jaw. While the food is thus being crushed, it is moved to and fro by the varied movements of the tongue, that every part of it may be acted upon by the teeth. The advantage of this is obvious. The more finely the food is divided, the more easily will the digestive fluids reach every part of it, and the more thoroughly and speedily will digestion ensue.

The act of chewing is simple and yet important, for if hurriedly or imperfectly done, the food is in a condition to cause disturbance in the digestive process. Thorough mastication is a necessary introduction to the more complicated changes which occur in the later digestion.

133. The Teeth. The teeth are attached to the upper and lower maxillary bones by roots which sink into the sockets of the jaws. Each tooth consists of a _crown_, the visible part, and one or more fangs, buried in the sockets. There are in adults 32 teeth, 16 in each jaw.

Teeth differ in name according to their form and the uses to which they are specially adapted. Thus, at the front of the jaws, the incisors, or cutting teeth, number eight, two on each side. They have a single root and the crown is beveled behind, presenting a chisel-like edge. The incisors divide the food, and are well developed in rodents, as squirrels, rats, and beavers.

Next come the canine teeth, or cuspids, two in each jaw, so called from their resemblance to the teeth of dogs and other flesh-eating animals. These teeth have single roots, but their crowns are more pointed than in the incisors. The upper two are often called eye teeth, and the lower two, stomach teeth. Next behind the canines follow, on each side, two bicuspids. Their crowns are broad, and they have two roots. The three hindmost teeth in each jaw are the molars, or grinders. These are broad teeth with four or five points on each, and usually each molar has three roots.

The last molars are known as the wisdom teeth, as they do not usually appear until the person has reached the “years of discretion.” All animals that live on grass, hay, corn, and the cereals generally, have large grinding teeth, as the horse, ox, sheep, and elephant.

The following table shows the teeth in their order:

Mo. Bi. Ca. In. In. Ca. Bi. Mo.
Upper 3 2 1 2 | 2 1 2 3 = 16
| } = 32
Lower 3 2 1 2 | 2 1 2 3 = 16

The vertical line indicates the middle of the jaw, and shows that on each side of each jaw there are eight teeth.

134. Development of the Teeth. The teeth just described are the permanent set, which succeeds the temporary or milk teeth. The latter are twenty in number, ten in each jaw, of which the four in the middle are incisors. The tooth beyond on each side is an eye tooth, and the next two on each side are bicuspids, or premolars.

The milk teeth appear during the first and second years, and last until about the sixth or seventh year, from which time until the twelfth or thirteenth year, they are gradually pushed out, one by one, by the permanent teeth. The roots of the milk teeth are much smaller than those of the second set.

Illustration: Fig. 48.—Temporary and Permanent Teeth together.

_Temporary teeth:_ A, central incisors;
B lateral incisors;
C, canines;
D, anterior molars;
E, posterior molars

_Permanent teeth:_
F, central incisors;
H, lateral incisors;
K, canines;
L, first bicuspids;
M, second biscuspids;
N, first molars

The plan of a gradual succession of teeth is a beautiful provision of nature, permitting the jaws to increase in size, and preserving the relative position and regularity of the successive teeth.

Illustration: Fig. 49.—Showing the Principal Organs of the Thorax and Abdomen _in situ_. (The principal muscles are seen on the left, and superficial veins on the right.)

135. Structure of the Teeth. If we should saw a tooth down through its center we would find in the interior a cavity. This is the pulp cavity, which is filled with the dental pulp, a delicate substance richly supplied with nerves and blood-vessels, which enter the tooth by small openings at the point of the root. The teeth are thus nourished like other parts of the body. The exposure of the delicate pulp to the air, due to the decay of the dentine, gives rise to the pain of toothache.

Surrounding the cavity on all sides is the hard substance known as the dentine, or tooth ivory. Outside the dentine of the root is a substance closely resembling bone, called cement. In fact, it is true bone, but lacks the Haversian canals. The root is held in its socket by a dense fibrous membrane which surrounds the cement as the periosteum does bone.

Illustration: Fig. 50.—Section of Face. (Showing the parotid and submaxillary glands.)

The crown of the tooth is not covered by cement, but by the hard enamel, which forms a strong protection for the exposed part. When the teeth are first “cut,” the surface of the enamel is coated with a delicate membrane which answers to the Scriptural phrase “the skin of the teeth.” This is worn off in adult life.

136. Insalivation. The thorough mixture of the saliva with the food is called insalivation. While the food is being chewed, it is moistened with a fluid called saliva, which flows into the mouth from six little glands. There are on each side of the mouth three salivary glands, which secrete the saliva from the blood. The parotid is situated on the side of the face in front of the ear. The disease, common in childhood, during which this gland becomes inflamed and swollen, is known as the “mumps.” The submaxillary gland is placed below and to the inner side of the lower jaw, and the sublingual is on the floor of the mouth, between the tongue and the gums. Each gland opens into the mouth by a little duct. These glands somewhat resemble a bunch of grapes with a tube for a stalk.

The saliva is a colorless liquid without taste or smell. Its principal element, besides water, is a ferment called _ptyalin_, which has the remarkable property of being able to change starch into a form of cane-sugar, known as maltose.

Thus, while the food is being chewed, another process is going on by which starch is changed into sugar. The saliva also moistens the food into a mass for swallowing, and aids in speech by keeping the mouth moist.

The activity of the salivary glands is largely regulated by their abundant supply of nerves. Thus, the saliva flows into the mouth, even at the sight, smell, or thought of food. This is popularly known as “making the mouth water.” The flow of saliva may be checked by nervous influences, as sudden terror and undue anxiety.

Experiment 56. _To show the action of saliva on starch_. Saliva for experiment may be obtained by chewing a piece of India rubber and collecting the saliva in a test tube. Observe that it is colorless and either transparent or translucent, and when poured from one vessel to another is glairy and more or less adhesive. Its reaction is alkaline to litmus paper.

Experiment 57.Make a thin paste from pure starch or arrowroot. Dilute a little of the saliva with five volumes of water, and filter it. This is best done through a filter perforated at its apex by a pin-hole. In this way all air-bubbles are avoided. Label three test tubes _A, B_, and _C_. In _A_, place starch paste; in _B_, saliva; and in _C_ one volume of saliva and three volumes of starch paste. Place them for ten minutes in a water bath at about 104° Fahrenheit. Test portions of all three for a reducing sugar, by means of Fehling’s solution or tablets.[21] _A_ and _B_ give no evidence of sugar, while _C_ reduces the Fehling, giving a yellow or red deposit of cuprous oxide. Therefore, starch is converted into a reducing sugar by the saliva. This is done by the ferment ptyalin contained in saliva.

137. The Pharynx and Œsophagus. The dilated upper part of the alimentary canal is called the pharynx. It forms a blind sac above the level of the mouth. The mouth opens directly into the pharynx, and just above it are two openings leading into the posterior passages of the nose. There are also little openings, one on each side, from which begin the Eustachian tubes, which lead upward to the ear cavities.

The windpipe opens downward from the pharynx, but this communication can be shut off by a little plate or lid of cartilage, the epiglottis. During the act of swallowing, this closes down over the entrance to the windpipe, like a lid, and prevents the food from passing into the air-passages. This tiny trap-door can be seen, by the aid of a mirror, if we open the mouth wide and press down the back of the tongue with the handle of a spoon (Figs. 46, 84, and 85).

Thus, there are six openings from the pharynx; the œsophagus being the direct continuation from it to the stomach. If we open the mouth before a mirror we see through the fauces the rear wall of the pharynx. In its lining membrane is a large number of glands, the secretion from which during a severe cold may be quite troublesome.

The œsophagus, or gullet, is a tube about nine inches long, reaching from the throat to the stomach. It lies behind the windpipe, pierces the diaphragm between the chest and abdomen, and opens into the stomach. It has in its walls muscular fibers, which, by their worm-like contractions, grasp the successive masses of food swallowed, and pass them along downwards into the stomach.

138. Deglutition, or Swallowing. The food, having been well chewed and mixed with saliva, is now ready to be swallowed as a soft, pasty mass. The tongue gathers it up and forces it backwards between the pillars of the fauces into the pharynx.

If we place the fingers on the “Adam’s apple,” and then pretend to swallow something, we can feel the upper part of the windpipe and the closing of its lid (epiglottis), so as to cover the entrance and prevent the passage of food into the trachea.

There is only one pathway for the food to travel, and that is down the œsophagus. The slow descent of the food may be seen if a horse or dog be watched while swallowing. Even liquids do not fall or flow down the food passage. Hence, acrobats can drink while standing on their heads, or a horse with its mouth below the level of the œsophagus. The food is under the control of the will until it has entered the pharynx; all the later movements are involuntary.

Illustration: Fig. 51.—A View into the Back Part of the Adult Mouth. (The head is represented as having been thrown back, and the tongue drawn forward.)

A, B, incisors;
C, canine;
D, E, bicuspids;
F, H, K, molars;
M, anterior pillar of the fauces;
N, tonsil;
L, uvula;
O, upper part of the pharynx;
P, tongue drawn forward;
R, linear ridge, or raphé.

139. The Stomach. The stomach is the most dilated portion of the alimentary canal and the principal organ of digestion. Its form is not easily described. It has been compared to a bagpipe, which it resembles somewhat, when moderately distended. When empty it is flattened, and in some parts its opposite walls are in contact.

We may describe the stomach as a pear-shaped bag, with the large end to the left and the small end to the right. It lies chiefly on the left side of the abdomen, under the diaphragm, and protected by the lower ribs. The fact that the large end of the stomach lies just beneath the diaphragm and the heart, and is sometimes greatly distended on account of indigestion or gas, may cause feelings of heaviness in the chest or palpitation of the heart. The stomach is subject to greater variations in size than any other organ of the body, depending on its contents. Just after a moderate meal it averages about twelve inches in length and four in diameter, with a capacity of about four pints.

Illustration: Fig. 52.—The Stomach.

A, cardiac end;
B, pyloric end,
C, lesser curvature,
D, greater curvature

The orifice by which the food enters is called the cardiac opening, because it is near the heart. The other opening, by which the food leaves the stomach, and where the small intestine begins, is the pyloric orifice, and is guarded by a kind of valve, known as the pylorus, or gatekeeper. The concave border between the two orifices is called the _small curvature_, and the convex as the _great curvature_, of the stomach.

140. Coats of Stomach. The walls of the stomach are formed by four coats, known successively from without as serous, muscular, sub-mucous, and mucous. The outer coat is the serous membrane which lines the abdomen,—the peritoneum (note, p. 135). The second coat is muscular, having three sets of involuntary muscular fibers. The outer set runs lengthwise from the cardiac orifice to the pylorus. The middle set encircles all parts of the stomach, while the inner set consists of oblique fibers. The third coat is the sub-mucous, made up of loose connective tissues, and binds the mucous to the muscular coat. Lastly there is the mucous coat, a moist, pink, inelastic membrane, which completely lines the stomach. When the stomach is not distended, the mucous layer is thrown into folds presenting a corrugated appearance.

Illustration: Fig. 53.—Pits in the Mucous Membrane of the Stomach, and Openings of the Gastric Glands. (Magnified 20 diameters.)

141. The Gastric Glands. If we were to examine with a hand lens the inner surface of the stomach, we would find it covered with little pits, or depressions, at the bottom of which would be seen dark dots. These dots are the openings of the gastric glands. In the form of fine, wavy tubes, the gastric glands are buried in the mucous membrane, their mouths opening on the surface. When the stomach is empty the mucous membrane is pale, but when food enters, it at once takes on a rosy tint. This is due to the influx of blood from the large number of very minute blood-vessels which are in the tissue between the rows of glands.

The cells of the gastric glands are thrown into a state of greater activity by the increased quantity of blood supply. As a result, soon after food enters the stomach, drops of fluid collect at the mouths of the glands and trickle down its walls to mix with the food. Thus these glands produce a large quantity of gastric juice, to aid in the digestion of food.

142. Digestion in the Stomach. When the food, thoroughly mixed with saliva, reaches the stomach, the cardiac end of that organ is closed as well as the pyloric valve, and the muscular walls contract on the contents. A spiral wave of motion begins, becoming more rapid as digestion goes on. Every particle of food is thus constantly churned about in the stomach and thoroughly mixed with the gastric juice. The action of the juice is aided by the heat of the parts, a temperature of about 99° Fahrenheit.

The gastric juice is a thin almost colorless fluid with a sour taste and odor. The reaction is distinctly acid, normally due to free hydrochloric acid. Its chief constituents are two ferments called pepsin and rennin, free hydrochloric acid, mineral salts, and 95 per cent of water.

Illustration: Fig. 54.—A highly magnified view of a peptic or gastric gland, which is represented as giving off branches. It shows the columnar epithelium of the surface dipping down into the duct D of the gland, from which two tubes branch off. Each tube is lined with columnar epithelial cells, and there is a minute central passage with the “neck” at N. Here and there are seen other special cells called parietal cells, P, which are supposed to produce the acid of the gastric juice. The principal cells are represented at C.

Pepsin the important constituent of the gastric juice, has the power, in the presence of an acid, of dissolving the proteid food-stuffs. Some of which is converted into what are called _peptones_, both soluble and capable of filtering through membranes. The gastric juice has no action on starchy foods, neither does it act on fats, except to dissolve the albuminous walls of the fat cells. The fat itself is thus set free in the form of minute globules. The whole contents of the stomach now assume the appearance and the consistency of a thick soup, usually of a grayish color, known as chyme.

It is well known that “rennet” prepared from the calf’s stomach has a remarkable effect in rapidly curdling milk, and this property is utilized in the manufacture of cheese. Now, a similar ferment is abundant in the gastric juice, and may be called _rennin_. It causes milk to clot, and does this by so acting on the casein as to make the milk set into a jelly. Mothers are sometimes frightened when their children, seemingly in perfect health, vomit masses of curdled milk. This curdling of the milk is, however, a normal process, and the only noteworthy thing is its rejection, usually due to overfeeding.

Experiment 58. _To show that pepsin and acid are necessary for gastric digestion._ Take three beakers, or large test tubes; label them _A_, _B_, _C_. Put into _A_ water and a few grains of powdered pepsin. Fill _B_ two-thirds full of dilute hydrochloric acid (one teaspoonful to a pint), and fill _C_ two-thirds full of hydrochloric acid and a few grains of pepsin. Put into each a small quantity of well-washed fibrin, and place them all in a water bath at 104° Fahrenheit for half an hour. Examine them. In _A_, the fibrin is unchanged; in _B_, the fibrin is clear and swollen up; in _C_, it has disappeared, having first become swollen and clear, and completely dissolved, being finally converted into peptones. Therefore, both acid and ferment are required for gastric digestion.

Experiment 59. Half fill with dilute hydrochloric acid three large test tubes, labelled _A_, _B_, _C_. Add to each a few grains of pepsin. Boil _B_, and make _C_ faintly alkaline with sodic carbonate. The alkalinity may be noted by adding previously some neutral litmus solution. Add to each an equal amount—a few threads—of well-washed fibrin which has been previously steeped for some time in dilute hydrochloric acid, so that it is swollen and transparent. Keep the tubes in a water-bath at about 104° Fahrenheit for an hour and examine them at intervals of twenty minutes. After five to ten minutes the fibrin in _A_ is dissolved and the fluid begins to be turbid. In _B_ and _C_ there is no change. Even after long exposure to 100° Fahrenheit there is no change in _B_ and _C_.

After a variable time, from one to four hours, the contents of the stomach, which are now called chyme, begin to move on in successive portions into the next part of the intestinal canal. The ring-like muscles of the pylorus relax at intervals to allow the muscles of the stomach to force the partly digested mass into the small intestines. This action is frequently repeated, until even the indigestible masses which the gastric juice cannot break down are crowded out of the stomach into the intestines. From three to four hours after a meal the stomach is again quite emptied.

A certain amount of this semi-liquid mass, especially the peptones, with any saccharine fluids, resulting from the partial conversion of starch or otherwise, is at once absorbed, making its way through the delicate vessels of the stomach into the blood current, which is flowing through the gastric veins to the portal vein of the liver.

Illustration: Fig. 55.—A Small Portion of the Mucous Membrane of the Small Intestine. (Villi are seen surrounded with the openings of the tubular glands.) [Magnified 20 diameters.]

143. The Small Intestine. At the pyloric end of the stomach the alimentary canal becomes again a slender tube called the small intestine. This is about twenty feet long and one inch in diameter, and is divided, for the convenience of description, into three parts.

The first 12 inches is called the duodenum. Into this portion opens the bile duct from the liver with the duct from the pancreas, these having been first united and then entering the intestine as a common duct.

The next portion of the intestine is called the jejunum, because it is usually empty after death.

The remaining portion is named the ileum, because of the many folds into which it is thrown. It is the longest part of the small intestine, and terminates in the right iliac region, opening into the large intestine. This opening is guarded by the folds of the membrane forming the ileo-cæcal valve, which permits the passage of material from the small to the large intestine, but prevents its backward movement.

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 passes into the blood.

Illustration: 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 _valvulæ 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 prone 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 cæcum. A valvular arrangement called the ileo-cæcal valve, which is provided with a button-hole slit, forms a kind of movable partition between this part of the large intestine and the small intestine.

Illustration: 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 cæcum 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 passes upwards on the right side as the ascending colon, until the under side of the liver is reached, where it passes 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 rectum. Thus the large intestine encircles, in the form of a horseshoe, the convoluted mass 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 canal 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 canal 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 passes 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 masses 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 circumference. 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 coarse, granular appearance, when torn across.

Illustration: 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 passes 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 canals, 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 passes 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 pass 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 assigned 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.

Illustration: 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 canal, 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 quantity 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 mass 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 pass 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 pass 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 quantity 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.

Illustration: 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 canal through the ileo-cæcal 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 rectum, from which they are voluntarily expelled from the body.

Absorption.

154. Absorption. While food remains within the alimentary canal 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 canal, 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 canal, 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 œsophagus 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 partition. 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 pass through the partition 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 pass 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.

Having been acted upon by the mesenteric glands, and passed through them, the chyle flows onward until it is poured into a dilated reservoir for the chyle, known as the receptaculum chyli. This is a sac-like expansion of the lower end of the thoracic duct. Into this receptacle, situated at the level of the upper lumbar vertebræ, in front of the spinal column, are poured, not only the contents of the lacteals, but also of the lymphatic vessels of the lower limbs.

158. The Thoracic Duct. This duct is a tube from fifteen to eighteen inches long, which passes upwards in front of the spine to reach the base of the neck, where it opens at the junction of the great veins of the left side of the head with those of the left arm. Thus the thoracic duct acts as a kind of feeding pipe to carry along the nutritive material obtained from the food and to pour it into the blood current. It is to be remembered that the lacteals are in reality lymphatics—the lymphatics of the intestines.

Illustration: Fig. 61.—Section of a Lymphatic Gland.

A, strong fibrous capsule sending partitions into the gland;
B, partitions between the follicles or pouches of the _cortical_ or
outer portion;
C, partitions of the _medullary_ or central portion;
D, E, masses of protoplasmic matter in the pouches of the gland;
F, lymph-vessels which bring lymph _to_ the gland, passing into its
center;
G, confluence of those leading to the efferent vessel;
H, vessel which carries the lymph away _from_ the gland.

159. The Lymphatics. In nearly every tissue and organ of the body there is a marvelous network of vessels, precisely like the lacteals, called the lymphatics. These are busily at work taking up and making over anew waste fluids or surplus materials derived from the blood and tissues generally. It is estimated that the quantity of fluid picked up from the tissues by the lymphatics and restored daily to the circulation is equal to the bulk of the blood in the body. The lymphatics seem to start out from the part in which they are found, like the rootlets of a plant in the soil. They carry a turbid, slightly yellowish fluid, called lymph, very much like blood without the red corpuscles.

Now, just as the chyle was not fit to be immediately taken up by the blood, but was passed through the mesenteric glands to be properly worked over, so the lymph is carried to the lymphatic glands, where it undergoes certain changes to fit it for being poured into the blood. Nature, like a careful housekeeper, allows nothing to be wasted that can be of any further service in the animal economy (Figs. 63 and 64).

The lymphatics unite to form larger and larger vessels, and at last join the thoracic duct, except the lymphatics of the right side of the head and chest and right arm. These open by the right lymphatic duct into the venous system on the right side of the neck.

The whole lymphatic system may be regarded as a necessary appendage to the vascular system (Chapter VII.). It is convenient, however, to treat it under the general topic of absorption, in order to complete the history of food digestion.

160. The Spleen and Other Ductless Glands. With the lymphatics may be classified, for convenience, a number of organs called ductless or blood glands. Although they apparently prepare materials for use in the body, they have no ducts or canals along which may be carried the result of their work. Again, they are called blood glands because it is supposed they serve some purpose in preparing material for the blood.

The spleen is the largest of these glands. It lies beneath the diaphragm, and upon the left side of the stomach. It is of a deep red color, full of blood, and is about the size and shape of the palm of the hand.

The spleen has a fibrous capsule from which partitions pass inwards, dividing it into spaces by a framework of elastic tissue, with plain muscular fibers. These spaces are filled with what is called the spleen pulp, through which the blood filters from its artery, just as a fluid would pass through a sponge. The functions of the spleen are not known. It appears to take some part in the formation of blood corpuscles. In certain diseases, like malarial fever, it may become remarkably enlarged. It may be wholly removed from an animal without apparent injury. During digestion it seems to act as a muscular pump, drawing the blood onwards with increased vigor along its large vein to the liver.

The thyroid is another ductless gland. It is situated beneath the muscles of the neck on the sides of “Adam’s apple” and below it. It undergoes great enlargement in the disease called goitre.

The thymus is also a blood gland. It is situated around the windpipe, behind the upper part of the breastbone. Until about the end of the second year it increases in size, and then it begins gradually to shrivel away. Like the spleen, the thyroid and thymus glands are supposed to work some change in the blood, but what is not clearly known.

The suprarenal capsules are two little bodies, one perched on the top of each kidney, in shape not unlike that of a conical hat. Of their functions nothing definite is known.

Experiments.

The action produced by the tendency of fluids to mix, or become equally diffused in contact with each other, is known as _osmosis_, a form of molecular attraction allied to that of adhesion. The various physical processes by which the products of digestion are transferred from the digestive canal to the blood may be illustrated in a general way by the following simple experiments.

The student must, however, understand that the necessarily crude experiments of the classroom may not conform in certain essentials to these great processes conducted in the living body, which they are intended to illustrate and explain.

Illustration: Fig. 62.