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 ca.n.a.ls. The root is held in its socket by a dense fibrous membrane which surrounds the cement as the periosteum does bone.

[Ill.u.s.tration: 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 princ.i.p.al 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 ma.s.s 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 sophagus. The dilated upper part of the alimentary ca.n.a.l 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 pa.s.sages 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 pa.s.sing into the air-pa.s.sages. 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 sophagus 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 sophagus, 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 ma.s.ses of food swallowed, and pa.s.s them along downwards into the stomach.

138. Deglut.i.tion, or Swallowing. The food, having been well chewed and mixed with saliva, is now ready to be swallowed as a soft, pasty ma.s.s.

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 pa.s.sage of food into the trachea.

There is only one pathway for the food to travel, and that is down the sophagus. 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 pa.s.sage. Hence, acrobats can drink while standing on their heads, or a horse with its mouth below the level of the sophagus. The food is under the control of the will until it has entered the pharynx; all the later movements are involuntary.

[Ill.u.s.tration: 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 raphe.

139. The Stomach. The stomach is the most dilated portion of the alimentary ca.n.a.l and the princ.i.p.al 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 quant.i.ty 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 quant.i.ty 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 const.i.tuents are two ferments called pepsin and rennin, free hydrochloric acid, mineral salts, and 95 per cent of water.

[Ill.u.s.tration: 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 pa.s.sage 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 princ.i.p.al cells are represented at C.]

Pepsin the important const.i.tuent 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 alb.u.minous 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 a.s.sume 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 ma.s.ses 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 quant.i.ty 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 ca.n.a.l. The ring-like muscles of the pylorus relax at intervals to allow the muscles of the stomach to force the partly digested ma.s.s into the small intestines.

This action is frequently repeated, until even the indigestible ma.s.ses 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 ma.s.s, 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.

[Ill.u.s.tration: 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 ca.n.a.l 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-caecal valve, which permits the pa.s.sage of material from the small to the large intestine, but prevents its backward movement.