*Preparation of Meals.*-The cooking of food serves three important purposes. It renders the food more digestible, relieving the organs of unnecessary work; it destroys bacteria that may be present in the food, diminishing the likelihood of introducing disease germs into the body; and it makes the food more palatable, thereby supplying a necessary stimulus to the digestive glands. While the methods employed in the preparation of the different foods have much to do with the ease with which they are digested and with their nourishing qualities, the scope of our subject does not permit of a consideration of these methods.

*Quant.i.ty of Food.*-Overeating and undereating are both objectionable from a hygienic standpoint. Overeating, by introducing an unnecessary amount of food into the body, overworks the organs of digestion and also the organs of excretion. It may also lead to the acc.u.mulation of burdensome fat and of harmful wastes. On the other hand, the taking of too little food impoverishes the blood and weakens the entire body. As a rule, however, more people eat too much than too little, and to quit eating before the appet.i.te is fully satisfied is with many persons a necessary precaution.

The power of self-control, valuable in all phases of life, is indispensable in the avoidance of overeating.

*Frequency of Taking Food.*-Eating between meals is manifestly an unhealthful practice. The question has also been raised as to whether the common habit of eating three times a day is best suited to all cla.s.ses of people. Many people of weak digestive organs have been benefited by the plan of two meals a day, while others adopt the plan of eating one heavy meal and two light ones. Either plan gives the organs of digestion more time to rest and diminishes the liability of overeating. On the other hand, those doing heavy muscular work can hardly derive the energy which they need from less than three good meals a day. Though no definite rule can be laid down, there is involved a hygienic principle which all should follow: _Meals should not overlap_. The stomach should be free from food taken at a previous meal before more is introduced into it. When this principle is not observed, material ferments in the stomach, causing indigestion and other disorders. It should be noted, however, that the overlapping may be due to overeating as well as to eating too frequently.

*Dangers from Impure Food.*-Food is frequently the carrier of disease germs and for this reason requires close inspection (page 128). Typhoid fever, a most dangerous disease, is usually contracted through either impure food or impure water (Chapter XXIII). One safeguard against disease germs, as stated above, is thorough cooking. Too much care cannot be exercised with reference to the water for drinking purposes. Water which is not perfectly clear, which smells of decaying material, or which forms a sediment on standing is usually not fit to drink. It can, however, be rendered comparatively harmless by boiling. The objections which many people have to drinking boiled water are removed when it is boiled the day before it is used, so as to give it time to cool, settle, and replace the air driven off by the boiling.

*Care of the Bowels.*-In considering the hygiene of the alimentary ca.n.a.l, the fact that it is used as a means of separating the impurities from the body must not be overlooked. Frequently, through lack of exercise, negligence in evacuating the bowels, or other causes, a weakened condition of the ca.n.a.l is induced which results in the retention of impurities beyond the time when they should be discharged. This is a great annoyance and at the same time a menace to the health.

In most cases this condition can be relieved, and prevented from recurring, by observing the following habits: 1. Have a regular time each day for evacuating the bowels. This is a most important factor in securing the necessary movements. 2. Drink a cup of cold water on rising in the morning and on retiring at night. 3. Eat generously of fruits and other coa.r.s.e foods, such as corn bread, oatmeal, hominy, cabbage, etc. 4.

Practice persistently such exercises as bring the abdominal muscles into play. These exercises strengthen indirectly the muscles of the ca.n.a.l. 5.

Avoid overwork, especially of the nervous system.

*Alcohol and Digestion.*-Though exciting temporarily a greater flow of the digestive fluids, alcoholic drinks taken in any but very small quant.i.ties are considered detrimental to the work of digestion. Large doses r.e.t.a.r.d the action of enzymes, inflame the mucous lining of the stomach,(65) and bring about a diseased condition of the liver. It may be noted, however, that the bad effects of alcoholic beverages upon the stomach, the liver, and the body in general are less p.r.o.nounced when these are taken as a part of the regular meals.

*Effects of Tea and Coffee.*-In addition to the stimulating agent caffeine, tea and coffee contain a bitter, astringent substance, known as tannin. On account of the tannin these beverages tend to r.e.t.a.r.d digestion and to irritate the lining of the stomach-effects that may be largely obviated by methods of preparing tea and coffee which dissolve little of the tannin. (They should be made without continued boiling or steeping.) The caffeine may do harm through its stimulating effect upon the nervous system (page 56) and through the introduction of a special waste into the body. In chemical composition caffeine closely resembles a waste, called uric acid, and in the body is converted into this substance. If one is in a weakened condition, the uric acid may fail to be oxidized to urea, as occurs normally, or to be thrown off as uric acid. In this case it acc.u.mulates in the body, causing rheumatism and related diseases. It thus happens that while some people may use tea and coffee without detriment, others are injured by them.

*Summary.*-The main structure in the digestive system is the alimentary ca.n.a.l. This provides cavities where important dissolving processes take place, and tubes for joining these cavities, while glands connecting with the ca.n.a.l supply the necessary liquids for changing and dissolving the foods. The general plan of digestion is that of pa.s.sing the food through the ca.n.a.l, beginning with the mouth, and of acting on it at various places, with the final result of reducing most of it to the liquid state.

The digestive fluids supply water which acts as a solvent and carries the active chemical agents, or enzymes, that convert the insoluble foods into substances that are soluble. The muscles in the walls of the ca.n.a.l perform the mechanical work of digestion, while the nervous system controls and regulates the activity of the various organs concerned in this work.

Exercises.-1. State the general purpose of digestion. How does digested food differ from that not digested?

2. Name all the divisions of the alimentary ca.n.a.l in the order in which the food pa.s.ses through them.

3. What other work besides digestion is carried on by the alimentary ca.n.a.l?

4. What is gained by the mastication of the food? Why should mastication precede the other processes of digestion?

5. What is the work of the tongue in digestion?

6. State the purposes served by the gastric juice.

7. Give reasons for regarding the small intestine as the most important division of the food ca.n.a.l.

8. At what places, and by the action of what liquids, are fats, proteids, and starch digested?

9. What enzymes are found in the pancreatic juice? What is the digestive action of each?

10. Describe the work performed by the muscles of the stomach, the mouth, the esophagus, and the small intestine.

11. What advantages are derived from the use of cooked food?

12. State the advantages of drinking pure water.

13. If all the food that one needs to take at a single meal can be thoroughly masticated in fifteen minutes, why is it better to spend a longer time at the table?

14. What is meant by the overlapping of meals? What bad results follow?

How avoided?

PRACTICAL WORK

Examine a dissectible model of the human abdomen (Fig. 75), noting the form, location, and connection of the different organs. Find the connection of the esophagus with the stomach, of the stomach with the small intestine, and of the small intestine with the large intestine.

Sketch a general outline of the cavity, and locate in this outline its chief organs.

Where it is desirable to learn something of the actual structure of the digestive organs, the dissection of the abdomen of some small animal is necessary. On account of unpleasant features likely to be a.s.sociated with such a dissection, however, this work is not recommended for immature pupils.

[Fig. 75]

Fig. 75-Model for demonstrating the abdomen and its contents.

*Dissection of the Abdomen.* (Optional)-For individual study, or for a small cla.s.s, a half-grown cat is perhaps the best available material. It should be killed with chloroform, and then stretched, back downward, on a board, the feet being secured to hold it in place.

The teacher should make a preliminary examination of the abdomen to see that it is in a fit condition for cla.s.s study. If the bladder is unnaturally distended, its contents may be forced out by slight pressure.

The following materials will be needed during the dissection, and should be kept near at hand: a sharp knife with a good point, a pair of heavy scissors, a vessel of water, some cotton or a damp sponge, and some fine cord. During the dissection the specimen should be kept as clean as possible, and any escaping blood should be mopped up with the cotton or the sponge. The dissection is best carried out by observing the following order:

1. Cut through the abdominal wall in the center of the triangular s.p.a.ce where the ribs converge. From here cut a slit downward to the lower portion of the abdomen, and sideward as far as convenient. Tack the loosened abdominal walls to the board, and proceed to study the exposed parts. Observe the muscles in the abdominal walls, and the fold of the _peritoneum_ which forms an ap.r.o.n-like covering over the intestines.

2. Observe the position of the stomach, liver, spleen, and intestines, and then, by pushing the intestines to one side, find the kidneys and the bladder.

3. Study the liver with reference to its location, size, shape, and color.

On the under side, find the gall bladder, from which a small tube leads to the small intestine. Observe the portal vein as it pa.s.ses into the liver.

As the liver is filled with blood, neither it nor its connecting blood vessels should be cut at this time.

4. Trace out the continuity of the ca.n.a.l. Find the esophagus where it penetrates the diaphragm and joins the stomach. Find next the union of the stomach with the small intestine. Then, by carefully following the coils of the small intestine, discover its union with the large intestine.

5. Within the first coil of the small intestine, as it leaves the stomach, find the _pancreas_. Note its color, size, and branches. Find its connection with the small intestine.

6. Beginning at the cut portion of the abdominal wall, lift the thin lining of the peritoneum and carefully follow it toward the back and central portion of the abdomen. Observe whether it extends back of or in front of the kidneys, the aorta, and the inferior vena cava. Find where it leaves the wall as a _double_ membrane, the _mesentery_, which surrounds and holds in place the large and small intestines. Sketch a coil of the intestine, showing the mesentery.

7. Find in the center of the coils of small intestine a long, slender body having the appearance of a gland. This is the beginning of the _thoracic duct_ and is called the _receptacle of the chyle_. From this the thoracic duct rapidly narrows until it forms a tiny tube difficult to trace in a small animal.

8. Cut away about two inches of the small intestine from the remainder, having first tied the tube on the two sides of the section removed. Split it open for a part of its length, and wash out its contents. Observe its coats. Place it in a shallow vessel containing water, and examine the mucous membrane with a lens to find the _villi_. Make a drawing of this section, showing the coats.

9. Study the connection of the small intestine with the large. Split them open at the place of union, wash out the contents, and examine the ileo-caecal valve.

10. Observe the size, shape, and position of the kidneys. Do they lie in front of or back of the peritoneum? Do they lie exactly opposite each other? Note the connection of each kidney with the aorta and the inferior vena cava by the renal artery and the renal vein. Find a slender tube, the _ureter_, running from each kidney to the bladder. Do the ureters connect with the top or with the base of the bladder? Show by a sketch the connection of the kidneys with the large blood vessels and the bladder.

*To demonstrate the Teeth.*-Procure from the dentist a collection of different kinds of teeth, both sound and decayed.

(_a_) Examine external surfaces of different kinds of teeth, noting general shape, cutting or grinding surfaces, etc. Make a drawing of an incisor and also of a molar.

(_b_) After soaking some of the teeth for a couple of days in warm water saw one of them in two lengthwise, and another in two crosswise, and smooth the cut surfaces with fine emery or sand paper. Examine both kinds of sections, noting arrangement and extent of dentine, enamel, and pulp.

Make drawings.