*Drugs.*-Drugs act strongly upon the body and tend to bring about unusual and unnatural results. Their use should in no way be confused with that of foods. If taken in health, they tend to disturb the physiological balance of the body by unduly increasing or diminishing the action of the different organs. In disease where this balance is already disturbed, they may be administered for their counteractive effects, but always under the advice and direction of a physician. Knowing the nature of the disturbance which the drug produces, the physician can administer it to advantage, should the body be out of physiological balance, or diseased. Not only are drugs of no value in health, but their use is liable to do much harm.

NATURE OF DIGESTION

Before the nutrients can be oxidized at the cells, or built into the protoplasm, they undergo a number of changes. These are necessary for their entrance into the body, for their distribution by the blood and the lymph, and for the purposes which they finally serve. The first of these changes is preparatory to the entrance of the nutrients and is known as _digestion_. The organs which bring about this change, called digestive organs, have a special construction which adapts them to their work. It will a.s.sist materially in understanding these organs if we first learn something of the nature of the work which they have to perform.

*How the Nutrients get into the Body.*-The nature of digestion is determined by the conditions affecting the entrance of nutrients into the body. Food in the stomach and air in the lungs, although surrounded by the body, are still outside of what is called the _body proper_. To gain entrance into the body proper, a substance must pa.s.s through the body wall. This consists of the skin on the outside and of the mucous linings of the air pa.s.sages and other tubes and cavities which are connected with the external surface.

To get from the digestive organs into the blood, the nutrients must pa.s.s through the mucous membrane lining these organs and also the walls of blood or lymph vessels. Only _liquid materials_ can make this pa.s.sage. It is necessary, therefore, to reduce to the liquid state all nutrients not already in that condition. _This reduction to the liquid state const.i.tutes the digestive process_.

*How Substances are Liquefied.*-While the reduction of solids to the liquid state is accomplished in some instances by heating them until they melt, they are more frequently reduced to this state by subjecting them to the action of certain liquids, called _solvents_. Through the action of the solvent the minute particles of the solid separate from each other and disappear from view. (Shown in dropping salt in water.) At the same time they mix with the solvent, forming a _solution_, from which they separate only with great difficulty. For this reason solids in solution can diffuse through porous part.i.tions along with the solvents in which they are dissolved (page 73).

By digestion the nutrients are reduced to the form of a solution. _The process is_, simply speaking, _one of dissolving_. The liquid employed as _the digestive solvent is water_. The different nutrients dissolve in water, mixing with it to form a solution which is then pa.s.sed into the body proper.

*Digestion not a Simple Process.*-Digestion is by no means a simple process, such, for instance, as the dissolving of salt or sugar in water.

These, being soluble in water, dissolve at once on being mixed with a sufficient amount of this liquid. The majority of the nutrients, however, are insoluble in water and are unaffected by it when acting alone. Fats, starch, and most of the proteids do not dissolve in water. Before these can be dissolved they have to be changed chemically and converted into substances that are _soluble in water_. This complicates the process and _prevents the use of water alone_ as the digestive solvent.

*A Similar Case.*-If a piece of limestone be placed in water, it does not dissolve, because it is insoluble in water. If hydrochloric acid is now added to the water, the limestone is soon dissolved (Fig. 62). (See Practical Work.) It seems at first thought that the acid dissolves the limestone, but this is not the case. The acid produces a chemical change in the limestone (calcium carbonate) and converts it into a compound (calcium chloride) that is soluble in water. As fast as this is formed it is dissolved by the water, which is the real solvent in the case. The acid simply plays the part of a chemical converter.

[Fig. 62]

Fig. 62-The dissolving of limestone in water containing acid, suggesting the double action in the digestion of most foods.

*The Digestive Fluids.*-Several fluids-saliva, gastric juice, pancreatic juice, bile, and intestinal juice-are employed in the digestion of the food. The composition of these fluids is in keeping with the nature of the digestive process. While all of them have water for their most abundant const.i.tuent, there are dissolved in the water small amounts of active chemical agents. It is the work of these agents to convert the insoluble nutrients into substances that are soluble in water. The digestive fluids are thus able to act in a _double_ manner on the nutrients-to change them chemically and to dissolve them. The chemical agents which bring about the changes in the nutrients are called _enzymes_, or digestive ferments.

*Foods Cla.s.sed with Reference to Digestive Changes.*-With reference to the changes which they undergo during digestion, foods may be divided into three cla.s.ses as follows:

1. Substances already in the liquid state and requiring no digestive action. Water and solutions of simple foods in water belong to this cla.s.s.

Milk and liquid fats, or oils, do not belong to this cla.s.s.

2. Solid foods soluble in water. This cla.s.s includes common salt and sugar. These require no digestive action other than dissolving in water.

3. Foods that are insoluble in water. These have first to be changed into soluble substances, after which they are dissolved.

*Summary.*-Materials called foods are introduced into the body for rebuilding the tissues, supplying energy, and aiding in its general work.

Only a few cla.s.ses of substances, viz., proteids, carbohydrates, fats, water, and some mineral compounds have all the qualities of foods and are suitable for introduction into the body. Substances known as drugs, which may be used as medicines in disease, should be avoided in health. Before foods can be pa.s.sed into the body proper, they must be converted into the liquid form, or dissolved. In this process, known as digestion, water is the solvent; and certain chemical agents, called enzymes, convert the insoluble nutrients into substances that are soluble in water.

*Exercises.*-1. How does oxidation at the cells make necessary the introduction of new materials into the body?

2. What different purposes are served by the foods?

3. What is a nutrient? Name the important cla.s.ses.

4. What are food materials? From what sources are they obtained?

5. Name the different kinds of proteids; the different kinds of carbohydrates. Why are proteids called nitrogenous foods and fats and carbohydrates non-nitrogenous foods?

6. Show why life cannot be carried on without proteids; without water.

7. What per cents of proteid, fat, and carbohydrate are found in wheat flour, oatmeal, rice, b.u.t.ter, potatoes, round beef, eggs, and peanuts?

8. State the objection to a meal consisting of beef, eggs, beans, bread, and b.u.t.ter; to one consisting of potatoes, rice, bread, and b.u.t.ter. Which is the more objectionable of these meals and why?

9. State the general plan of digestion.

10. Show that digestion is not a simple process like that of dissolving salt in water.

PRACTICAL WORK

*Elements supplied by the Foods.*-The following brief study will enable the pupil to identify most of the elements present in the body and which have, therefore, to be supplied by the foods.

_Carbon._-Examine pieces of charred wood, c.o.ke, or coal, and also the "lead" in lead pencils. Show that the charred wood and the coal will burn.

Recall experiment (page 114) showing that carbon in burning forms carbon dioxide.

_Hydrogen._-Fill a test tube one third full of strong hydrochloric acid and drop into it several small sc.r.a.ps of zinc. The gas which is evolved is hydrogen. When the hydrogen is coming off rapidly, bring a lighted splinter to the mouth of the tube. The gas should burn. Hold a cold piece of gla.s.s over the flame and observe the deposit of moisture. Hydrogen in burning forms water. Extinguish the flame by covering the top of the tube with a piece of cardboard. Now let the escaping gas collect in a tumbler inverted over the tube. After holding the tumbler in this position for two or three minutes, remove and, keeping inverted, thrust a lighted splinter into it. (The gas should either burn or explode.) What does this experiment show relative to the weight of hydrogen as compared with that of air?

_Nitrogen._-Nitrogen forms about four fifths of the atmosphere, where, like oxygen, it exists in a free state. It may be separated from the oxygen of an inclosed portion of air by causing that gas to unite with phosphorus. Place a piece of phosphorus the size of a pea in a depression in a flat piece of cork. (Handle phosphorus with wet fingers or with forceps.) Place the cork on water and have ready a gla.s.s fruit jar holding not more than a quart. Ignite the phosphorus with a hot wire and invert the jar over it, pushing the mouth below the surface of the water. The phosphorus uniting with the oxygen fills the jar with white fumes of phosphoric oxide. These soon dissolve in the water, leaving a clear gas above. This is nitrogen. Place a cardboard under the mouth of the jar and turn it right side up, leaving in the water and keeping the top covered.

Light a splinter and, slipping the cover to one side, thrust the flame into the jar of nitrogen, noting the effect. (Flame is extinguished.) Compare nitrogen with oxygen in its relation to combustion. What purpose is served by each in the atmosphere?

_Oxygen._-Review experiments (page 114) showing the properties of oxygen.

_Phosphorus._-Examine a small piece of phosphorus, noting that it has to be kept under water. Lay a small piece on the table and observe the tiny stream of white smoke rising from it, formed by slow oxidation. Dissolve a piece as large as a pea in a teaspoonful of carbon disulphide in a test tube, pour this on a piece of porous paper, and lay the paper on an iron support. When the carbon disulphide evaporates the phosphorus takes fire spontaneously. (The heat from the slow oxidation is sufficient to ignite the phosphorus in the finely divided condition.) What is the most striking property of phosphorus? What purpose does it serve in the match?

_Sulphur._-Examine some sulphur, noting its color and the absence of odor or taste. (Impure sulphur may have an odor and a taste.) Burn a little sulphur in an iron spoon, noting that the compound which it forms with oxygen by burning has a decided odor.

_Other Elements._-_Magnesium._ Examine and burn a piece of magnesium ribbon, noting the white compound of magnesium oxide which is formed.

_Iron._ Examine pieces of the metal and also some of its compounds, as ferrous sulphate, ferric chloride, and ferric oxide or iron rust.

_Sodium._ Drop a piece of the metal on water and observe results. Sodium decomposes water. It has to be kept under some liquid, such as kerosene, which contains no oxygen. (It should not be touched except with the fingers wet with kerosene.) _Chlorine._ Pour strong hydrochloric acid on a little manganese dioxide in a test tube, and warm gently over a low flame.

The escaping gas is chlorine. Avoid breathing much of it.

*Composition of the Nutrients.*-The simplest way of determining what elements make up the different nutrients is by heating them and studying the products of decomposition, as follows:

_To show that Carbohydrates contain Carbon, Hydrogen, and Oxygen._-Place one half teaspoonful of powdered starch in a test tube and heat strongly.

Observe that _water_ condenses on the sides of the tube and that a black, charred ma.s.s remains behind. The black ma.s.s consists mainly of _carbon_.

The water is composed of hydrogen and oxygen. These three elements are thus shown to be present in the starch. The experiment may be repeated, using sugar instead of starch.

_To show that Proteids contain Carbon, Hydrogen, Oxygen, Nitrogen, and Sulphur._-Place in a test tube some finely divided proteid which has been thoroughly dried (dried beef or the lean of hard cured bacon). Heat strongly in the hood of a chemical laboratory or some other place where the odors do not get into the room. First hold in the escaping gases a wet strip of red litmus paper. This will be turned blue, showing _ammonia_ (NH3) to be escaping. Next hold in the mouth of the tube a strip of a paper wet with a solution of lead nitrate. This is turned black or brown on account of _hydrogen sulphide_(H2S) which is being driven off. Observe also that _water_ condenses in the upper part of the tube and that a black, charred ma.s.s remains behind. Since the products of decomposition (H2O, NH3, H2S, and the charred ma.s.s) contain hydrogen, oxygen, nitrogen, sulphur, and carbon, these elements are of course present in the proteid tested.

_To show the Presence of Mineral Matter._-Burn a piece of dry bread by holding it in a clear, hot flame, and observe the ash that is left behind.

This is the mineral matter present in the bread.