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2. Name the divisions of the lungs.
3. Trace air from the outside atmosphere into the alveoli. Trace the blood from the right ventricle to the alveoli and back again to the left auricle.
4. How does the movement of air into and from the lungs differ from that of the blood through the lungs with respect to (a) the direction of the motion. (b) the causes of the motion, and (c) the tubes through which the motion takes place?
5. How are the air passages kept clean and open?
6. Describe the pleura. Into what divisions does it separate the thoracic cavity?
7. Describe and name uses of the diaphragm.
8. If 30 cubic inches of air are passed into the lungs at each inspiration and .05 of this is retained as oxygen, calculate the number of cubic feet of oxygen consumed each day, if the number of inspirations be 18 per minute.
9. Find the weight of a day's supply of oxygen, as found in the above problem, allowing 1.3 ounces as the weight of a cubic foot.
10. Make a study of the hygienic ventilation of the schoolroom.
11. Give advantages of full breathing over shallow breathing.
12. How may a flat chest and round shoulders be a cause of consumption? How may these deformities be corrected?
13. Give general directions for applying artificial respiration.
PRACTICAL WORK
Examine a dissectible model of the chest and its contents (Fig. 49). Note the relative size of the two lungs and their position with reference to the heart and diaphragm. Compare the side to side and vertical diameters of the cavity. Trace the air tubes from the trachea to their smallest divisions.
*Observation of Lungs* (Optional).—Secure from a butcher the lungs of a sheep, calf, or hog. The windpipe and heart should be left attached and the specimen kept in a moist condition until used. Demonstrate the trachea, bronchi, and the bronchial tubes, and the general arrangement of pulmonary arteries and veins. Examine the pleura and show lightness of lung tissue by floating a piece on water.
*To show the Changes that Air undergoes in the Lungs.*—1. Fill a quart jar even full of water. Place a piece of cardboard over its mouth and invert, without spilling, in a pan of water. Inserting a tube under the jar, blow into it air that has been held as long as possible in the lungs. When filled with air, remove the jar from the pan, keeping the top well covered. Slipping the cover slightly to one side, insert a burning splinter and observe that the flame is extinguished. This proves the absence of sufficient oxygen to support combustion. Pour in a little limewater(43) and shake to mix with the air. The change of the limewater to a milky white color proves the presence of carbon dioxide.
[Fig. 50]
Fig. 50—*Apparatus* for showing changes which air undergoes while in the lungs.
2. The effects illustrated in experiment 1 may be shown in a somewhat more striking manner as follows: Fill two bottles of the same size each one fourth full of limewater and fit each with a two-holed rubber stopper (Fig. 50). Fit into each stopper one short and one long glass tube, the long tube extending below the limewater. Connect the short tube of one bottle and the long tube of the other bottle with a Y-tube. Now breathe slowly three or four times through the Y-tube. It will be found that the inspired air passes through one bottle and the expired air through the other. Compare the effect upon the limewater in the two bottles. Insert a small burning splinter into the top of each bottle and note result. What differences between inspired and expired air are thus shown?
3. Blow the breath against a cold window pane. Note and account for the collection of moisture.
4. Note the temperature of the room as shown by a thermometer. Now breathe several times upon the bulb, noting the rise in the mercury. What does this experiment show the body to be losing through the breath?
*To show Changes in the Thoracic Cavity.*—1. To a yard- or meter-stick, attach two vertical strips, each about eight inches long, as shown in Fig. 51. The piece at the end should be secured firmly in place by screws or nails. The other should be movable. With this contrivance measure the sideward and forward expansion of a boy's thorax. Take the diameter first during a complete inspiration and then during a complete expiration, reading the difference. Compare the forward with the sideward expansion.
[Fig. 51]
Fig. 51—*Apparatus* for measuring chest expansion.
2. With a tape-line take the circumference of the chest when all the air possible has been expelled from the lungs. Take it again when the lungs have been fully inflated. The difference is now read as the chest expansion.
[Fig. 52]
Fig. 52—*Simple apparatus* for illustrating the action of the diaphragm.
*To illustrate the Action of the Diaphragm.*—Remove the bottom from a large bottle having a small neck. (Scratch a deep mark with a file and hold on the end of this mark a hot poker. When the glass cracks, lead the crack around the bottle by heating about one half inch in advance of it.) Place the bottle in a large glass jar filled two thirds full of water (Fig. 52). Let the space above the water represent the chest cavity and the water surface represent the diaphragm. Raise the bottle, noting that the water falls, thereby increasing the space and causing air to enter. Then lower the bottle, noting the opposite effect. To show the movement of the air in and out of the bottle, hold with the hand (or arrange a support for) a burning splinter over the mouth of the bottle.
*To estimate the Capacity of the Lungs.*—Breathing as naturally as possible, expel the air into a spirometer (lung tester) during a period, say of ten respirations (Fig. 53). Note the total amount of air exhaled and the number of "breaths" and calculate the amount of air exhaled at each breath. This is called the tidal air.
[Fig. 53]
Fig. 53—*Apparatus* (spirometer) for measuring the capacity of the lungs.
2. After an ordinary inspiration empty the lungs as completely as possible into the spirometer, noting the quantity exhaled. This amount, less the tidal air, is known as the reserve air. The air which is now left in the lungs is called the residual air. On the theory that this is equal in amount to the reserve air, calculate the capacity of the lungs in an ordinary inspiration.
3. Now fill the lungs to the full expansion of the chest and empty them as completely as possible into the spirometer, noting the amount expelled. This, less the tidal air and the reserve air, is called the complemental air. Now calculate the total capacity of the lungs.
CHAPTER VIII - PASSAGE OF OXYGEN THROUGH THE BODY
What is the nature of oxygen? What is its purpose in the body and how does it serve this purpose? How is the blood able to take it up at the lungs and give it off at the cells? What becomes of it after being used? These are questions touching the maintenance of life and they deserve careful consideration.
*Nature of Oxygen.*—To understand the relation which oxygen sustains to the body we must acquaint ourselves with certain of its chemical properties. It is an element(44) of intense affinity, or combining power, and is one of the most active of all chemical agents. It is able to combine with most of the other elements to form chemical compounds. A familiar example of its combining action is found in ordinary combustion, or burning. On account of the part it plays in this process, oxygen is called the supporter of combustion; but it supports combustion by the simple method of uniting. The ashes that are left and the invisible gases that escape into the atmosphere are the compounds formed by the uniting process. It thus appears that oxygen, in common with the other elements, may exist in either of two forms:
1. That in which it is in a free, or uncombined, condition—the form in which it exists in the atmosphere.
2. That in which it is a part of compounds, such as the compounds formed in combustion.
Oxygen manifests its activity to the best advantage when it is in a free state, or, more accurately speaking, when it is passing from the free state into one of combination. It is separated from its compounds and brought again into a free state by overcoming with heat, or some other force, the affinity which causes it to unite.
*How Oxygen unites.*—The chemist believes oxygen, as well as all other substances, to be made up of exceedingly small particles, called atoms. The atoms do not exist singly in either elements or compounds, but are united with each other to form groups of atoms that are called molecules. In an element the molecules are made up of one kind of atoms, but in a compound the molecules are made up of as many kinds of atoms as there are elements in the compound. Changes in the composition of substances (called chemical changes) are due to rearrangements of the atoms and the formation of new molecules. The atoms, therefore, are the units of chemical combination. In the formation of new compounds they unite, and in the breaking up of existing compounds they separate.
The uniting of oxygen is no exception to this general law. All of its combinations are brought about by the uniting of its atoms. In the burning of carbon, for example, the atoms of oxygen and the atoms of carbon unite, forming molecules of the compound known as carbon dioxide. The chemical formula of this compound, which is CO_2, shows the proportion in which the atoms unite—one atom of carbon uniting with two atoms of oxygen in each of the molecules. The affinity of oxygen for other elements, and the affinity of other elements for oxygen, and for each other, resides in their atoms.
*Oxidation.*—The uniting of oxygen with other elements is termed oxidation. This may take place slowly or rapidly, the two rates being designated as slow oxidation and rapid oxidation. Examples of slow oxidation are found in certain kinds of decay and in the rusting of iron. Combustion is an example of rapid oxidation. Slow and rapid oxidation, while differing widely in their effects upon surrounding objects, are alike in that both produce heat and form compounds of oxygen. In slow oxidation, however, the heat may come off so gradually that it is not observed.
*Movement of Oxygen through the Body.*—Oxygen has been shown in the preceding chapters to pass from the lungs into the blood and later to leave the blood and, passing through the lymph, to enter the cells. That oxygen does not become a permanent constituent of the cells is shown by the constancy of the body weight. Nearly two pounds of oxygen per day are known to enter the cells of the average-sized person. If this became a permanent part of the cells, the body would increase in weight from day to day. Since the body weight remains constant, or nearly so, we must conclude that oxygen leaves the body about as fast as it enters. Oxygen enters the body as a free element. The form in which it leaves the body will be understood when we realize the purpose which it serves and the method by which it serves this purpose.
*Purpose of Oxygen in the Body.*—The question may be raised: Is it possible for oxygen to serve a purpose in the body without remaining in it? This, of course, depends upon what the purpose is. That it is possible for oxygen to serve a purpose and at the same time pass on through the place where it serves that purpose, is seen by studying the combustion in an ordinary stove (Fig. 54). Oxygen enters at the draft and for the most part passes out at the flue, but in passing through the stove it unites with, or oxidizes, the fuel, causing the combustion which produces the heat.
[Fig. 54]
Fig. 54—*Coal stove* illustrating rapid oxidation.
Now it is found that certain chemical processes, mainly oxidations, are taking place in the body. These produce the heat for keeping it warm and also supply other forms of energy,(45) including motion. It is the purpose of oxygen to keep up these oxidations and, by so doing, to aid in supplying the body with energy. It serves this purpose in much the same way that it supports combustion, i.e., by uniting with, or oxidizing, materials derived from foods that are present in the cells.
*Does Oxygen serve Other Purposes?*—It has been suggested that oxygen may serve the purpose of oxidizing, or destroying, substances that are injurious and of acting, in this way, as a purifying agent in the body. In support of this view is the natural tendency of oxygen to unite with substances and the well-known fact that oxygen is an important natural agent in purifying water. It seems probable, therefore, that it may to a slight extent serve this purpose in the body. It is probable also that oxygen aids through its chemical activity in the formation of compounds which are to become a part of the cells. Both of these uses, however, are of minor importance when compared with the main use of oxygen, which is that of an aid in supplying energy to the body.
*Oxygen and the Maintenance of Life.*—In the supplying of energy to the body, one of the conditions necessary to the maintenance of life is provided. Because oxygen is necessary to this process, and because death quickly results when the supply of it is cut off, oxygen is frequently called the supporter of life. This idea is misleading, for oxygen has no more to do with the maintenance of life than have the food materials with which it unites. Life appears to be more dependent upon oxygen than upon food, simply because the supply of it in the body at any time is exceedingly small. Being continually surrounded by an atmosphere containing free oxygen, the body depends upon this as a constant source of supply, and does not store it up. Food, on the other hand, is taken in excess of the body's needs and stored in the various tissues, the supply being sufficient to last for several days. When the supply of either oxygen or food is exhausted in the body, life must cease.
*The Oxygen Movement a Necessity.*—Since free oxygen is required for keeping up the chemical changes in the cells, and since it ceases to be free as soon as it goes into combination, its continuous movement through the body is a necessity. The oxygen compounds must be removed as fast as formed in order to make room for more free oxygen. This movement has already been studied in connection with the blood and the organs of respiration, but the consideration of certain details has been deferred till now. By what means and in what form is the oxygen passed to and from the cells?
*Passage of Oxygen through the Blood.*—In serving its purpose at the cells, the oxygen passes twice through the blood—once as it goes toward the cells and again as it passes from the cells to the exterior of the body:
Passage toward the Cells.—This is effected mainly through the hemoglobin of the red corpuscles. At the lungs the oxygen and the hemoglobin form a weak chemical compound that breaks up and liberates the oxygen when it reaches the capillaries in the tissues. The separation of the oxygen from the hemoglobin at the tissues appears to be due to two causes: first, to the weakness of the chemical attraction between the atoms of oxygen and the atoms that make up the hemoglobin molecule; and second, to a difference in the so-called oxygen pressure at the lungs and at the tissues.(46)
The attraction of the oxygen and the hemoglobin is sufficient to cause them to unite where the oxygen pressure is more than one half pound to the square inch, but it is not sufficiently strong to cause them to unite or to prevent their separation, if already united, where the oxygen pressure is less than one half pound to the square inch. The oxygen pressure at the lungs, which amounts to nearly three pounds to the square inch, easily causes the oxygen and the hemoglobin to unite, while the almost complete absence of any oxygen pressure at the tissues, permits their separation. The blood in its circulation constantly flows from the place of high oxygen pressure at the lungs to the place of low oxygen pressure at the tissues and, in so doing, loads up with oxygen at one place and unloads it at the other (Fig. 55).
Passage from the Cells.—Since oxygen leaves the free state at the cells and becomes a part of compounds, we are able to trace it from the body only by following the course of these compounds. Three waste compounds of importance are formed at the cells—carbon dioxide (CO2), water (H2O), and urea (N2H4CO). The first is formed by the union of oxygen with carbon, the second by its union with hydrogen, and the third by its union with nitrogen, hydrogen, and carbon. These compounds are carried by the blood to the organs of excretion, where they are removed from the body. The water leaves the body chiefly as a liquid, the urea as a solid dissolved in water, and the carbon dioxide as a gas. The passage of carbon dioxide through the blood requires special consideration.
[Fig. 55]
Fig. 55—*Diagram illustrating movement, of oxygen and carbon dioxide through the body* (S.D. Magers). Each moves from a place of relatively high to a place of relatively low pressure. (See text.)
*Passage of Carbon Dioxide through the Blood.*—Part of the carbon dioxide is dissolved in the plasma of the blood, and part of it is in weak chemical combination with substances found in the plasma and in the corpuscles. Its passage through the blood is accounted for in the same way as the passage of the oxygen. Its ability to dissolve in liquids and to enter into chemical combination varies as the carbon dioxide pressure(47) This in turn varies with the amount of the carbon dioxide, which is greatest at the cells (where it is formed), less in the blood, and still less in the lungs. Because of these differences, the blood is able to take it up at the cells and release it at the lungs (Fig. 55).
[Fig. 56]
Fig. 56—*Soap bubble* floating in a vessel of carbon dioxide, illustrating the difference in weight between air and carbon dioxide gas.
*Properties of Carbon Dioxide.*—Carbon dioxide is a colorless gas with little or no odor. It is classed as a heavy gas, being about one third heavier than air(48) (Fig. 56). It does not support combustion, but on the contrary is used to some extent to extinguish fires. It is formed by the oxidation of carbon in the body, and by the combustion of carbon outside of the body. It is also formed by the decay of animal and vegetable matter. From these sources it is continually finding its way into the atmosphere. Although not a poisonous gas, carbon dioxide may, if it surround the body, shut out the supply of oxygen and cause death.(49)
*Final Disposition of Carbon Dioxide.*—It is readily seen that the union of carbon and oxygen, which is continually removing oxygen from the air and replacing it with carbon dioxide, tends to make the whole atmosphere deficient in the one and to have an excess of the other. This tendency is counteracted through the agency of vegetation. Green plants absorb the carbon dioxide from the air, decompose it, build the carbon into compounds (starch, etc.) that become a part of the plant, and return the free oxygen to the air (Fig. 57). In doing this, they not only preserve the necessary proportion of oxygen and carbon dioxide in the atmosphere, but also put the carbon and oxygen in such a condition that they can again unite. The force which enables the plant cells to decompose the carbon dioxide is supplied by the sunlight (Chapter XII).
[Fig. 57]
Fig. 57—*Under surface* of a geranium leaf showing breathing pores, highly magnified (O.H.).
*Summary.*—Oxygen, by uniting with materials at the cells, keeps up a condition of chemical activity (oxidation) in the body. This supplies heat and the other forms of bodily energy. Entering as a free element, oxygen leaves the body as a part of the waste compounds which it helps to form. The free oxygen is transported from the lungs to the cells by means of the hemoglobin of the red corpuscles, while the combined oxygen in carbon dioxide and other compounds from the cells is carried mainly by the plasma. The limited supply of free oxygen in the body at any time makes necessary its continuous introduction into the body.
*Exercises.*—1. Describe the properties of oxygen. How does it unite with other elements? How does it support combustion?
2. State the purpose of oxygen in the body. What properties enable it to fulfill this purpose?
3. What is the proof that oxygen does not remain permanently in the body? How does the oxygen entering the body differ from the same oxygen as it leaves the body?
4. What is the necessity for the continuous introduction of oxygen into the body, while food is introduced only at intervals?
5. How are the red corpuscles able to take up and give off oxygen? How is the plasma able to take up and give off carbon dioxide?
6. If thirty cubic inches of air pass from the lungs at each expiration and 4.5 per cent of this is carbon dioxide, calculate the number of cubic feet of the gas expelled in twenty-four hours, estimating the number of respirations at eighteen per minute.
7. What is the weight of this volume of carbon dioxide, if one cubic foot weigh 1.79 ounces?
8. What portion of this weight is oxygen and what carbon, the ratio by weight of carbon to oxygen in carbon dioxide being twelve to thirty-two?
9. What is the final disposition of carbon dioxide in the atmosphere?
PRACTICAL WORK
*To show the Difference between Free Oxygen and Oxygen in Combination.*—Examine some crystals of potassium chlorate (KClO3). They contain oxygen in combination with potassium and chlorine. Place a few of these in a small test tube and heat strongly in a gas or alcohol flame. The crystals first melt, and the liquid which they form soon appears to boil. If a splinter, having a spark on the end, is now inserted in the tube, it is kindled into a flame. This shows the presence of free oxygen, the heat having caused the potassium chlorate to decompose. The difference between free and combined oxygen may also be shown by decomposing other compounds of oxygen, such as water and mercuric oxide.
*Preparation and Properties of Oxygen.*—Intimately mix 3 grams (1/2 teaspoonful) of potassium chlorate with half its bulk of manganese dioxide, and place the mixture in a large test tube. Close the test tube with a tight-fitting stopper which bears a glass tube of sufficient length and of the right shape to convey the escaping gas to a small trough or pan partly filled with water, on the table. Fill four large-mouthed bottles with water and, by covering with cardboard, invert each in the trough of water. Arrange the test tube conveniently for heating, letting the end of the glass tube terminate under the mouth of one of the bottles (Fig. 58). Using an alcohol lamp or a Bunsen burner, heat over the greater portion of the tube at first, but gradually concentrate the flame upon the mixture. Do not heat too strongly, and when the gas is coming off rapidly, remove the flame entirely, putting it back as the action slows down. After all the bottles have been filled, remove the end of the glass tube from the water, but leave the bottles of oxygen inverted in the trough until they are to be used. On removing the bottles from the trough, keep the tops covered with wet cardboard.
[Fig. 58]
Fig. 58—*Apparatus* for generating oxygen.
1. Examine a bottle of oxygen, noting its lack of color. Insert a small burning splinter in the upper part of the bottle and observe the change in the rate of burning. The air contains free oxygen, but it is diluted with nitrogen. Compare this with the undiluted oxygen in the bottle as to effect in causing the splinter to burn.
2. In a second bottle of oxygen insert a splinter without the flame, but having a small spark on the end. As soon as the oxygen kindles the spark into a flame, withdraw from the bottle and blow out the flame, but again insert the spark. Repeat the experiment as long as the spark is kindled by the oxygen into a flame. This experiment is usually performed as a test for undiluted oxygen.
3. Make a hollow cavity in the end of a short piece of crayon. Fasten a wire to the crayon, and fill the cavity with powdered sulphur. Ignite the sulphur in the flame of an alcohol lamp or Bunsen burner, and lower it into a bottle of oxygen. Observe the change in the rate of burning, the color of the flame, and the material formed in the bottle by the burning. The gas remaining in the bottle is sulphur dioxide (SO2), formed by the uniting of the sulphur and the oxygen.
4. Bend a small loop on the end of a piece of picture wire. Heat the loop in a flame and insert it in some powdered sulphur. Ignite the melted sulphur which adheres, and insert it quickly in a bottle of oxygen. Observe the dark, brittle material which is formed by the burning of the iron. It is a compound of the iron with oxygen, similar to iron rust, and formed by their uniting.
*Preparation and Properties of Carbon Dioxide.*—1. (a) Attach a piece of carbon (charcoal) no larger than the end of the thumb to a piece of wire. Ignite the charcoal in a hot flame and lower it into a vessel of oxygen. Observe its combustion, letting it remain in the bottle until it ceases to burn. Note that the burning has consumed a part of the carbon and has used up the free oxygen. Has anything been formed in their stead?
(b) Remove the charcoal and add a little limewater. Cover the bottle with a piece of cardboard, and bring the gas and the limewater in contact by shaking. Note any change in the color of the limewater. If it turns white, the presence of carbon dioxide is proved.
2. Burn a splinter in a large vessel of air, keeping the top covered. Add limewater and shake. Note and account for the result.
3. Place several pieces of marble (limestone) in a jar holding at least half a gallon. Barely cover the marble with water, and then add hydrochloric acid until a gas is rapidly evolved. This gas is carbon dioxide.
(a) Does it possess color?
(b) Insert a burning splinter to see if it supports combustion.
(c) Place a bottle of oxygen by the side of the vessel of carbon dioxide. Light a splinter and extinguish the flame by lowering it into the vessel of carbon dioxide. Withdraw immediately, and if a spark remains on the splinter, thrust it into the bottle of oxygen. Then insert the relighted splinter into the carbon dioxide. Repeat several times, kindling the flame in one gas and extinguishing it in the other. Finally show that the spark also may be extinguished by holding the splinter a little longer in the carbon dioxide.
(d) Tip the jar containing the carbon dioxide over the mouth of a tumbler, as in pouring water, though not far enough to spill the acid, and then insert a burning splinter in the tumbler. Account for the result. Inference as to the weight of carbon dioxide.
[Fig. 59]
Fig. 59—*Simple apparatus* for illustrating passage of oxygen through the body.
(e) Review experiments (page 101) showing the presence of carbon dioxide in the breath.
*To illustrate the General Movement of Oxygen through the Body.*—Into a glass tube, six inches in length and open at both ends, place several small lumps of charcoal (Fig. 59). Fit into one end of this tube, by means of a stopper, a smaller glass tube which is bent at right angles and which is made to pass through a close-fitting stopper to the bottom of a small bottle. Another small tube is fitted into a second hole in this stopper, but terminating near the top of the bottle, and to this is connected a rubber tube about eighteen inches in length. The arrangement is now such that by sucking air from the top of the bottle, it is made to enter at the distant end of the tube containing the charcoal. After filling the bottle one third full of limewater, heat the tube containing the charcoal until it begins to glow. Then suck the air through the apparatus (as in smoking, without drawing it into the lungs), observing what happens both in the tube and in the bottle. What are the proofs that the oxygen, in passing through the tube, unites with the carbon, forms carbon dioxide, and liberates energy? Compare the changes which the oxygen undergoes while passing through the tube with the changes which it undergoes in passing through the body.
CHAPTER IX - FOODS AND THE THEORY OF DIGESTION
The body is constantly in need of new material. Oxidation, as shown in the preceding chapter, rapidly destroys substances at the cells, and these have to be replaced. Upon this renewal depends the supply of energy. Moreover, there is found to be an actual breaking down of the living material, or protoplasm, in the body. While this does not destroy the cells, as is sometimes erroneously stated, it reduces the quantity of the protoplasm and makes necessary a process of repair, or rebuilding, of the tissues. This also requires new material. Finally, substances, such as water and common salt, are required for the aid which they render in the general work of the body. Since these are constantly being lost in one way or another, they also must be replaced. These different needs of the body for new materials are supplied through
*The Foods.*—Foods are substances that, on being taken into the healthy body, are of assistance in carrying on its work. This definition properly includes oxygen, but the term is usually limited to substances introduced through the digestive organs. As suggested above, foods serve at least three purposes:
1. They, with oxygen, supply the body with energy.
2. They provide materials for rebuilding the tissues.
3. They supply materials that aid directly or indirectly in the general work of the body.
*The Simple Foods, or Nutrients.*—From the great variety of things that are eaten, it might appear that many different kinds of substances are suitable for food. When our various animal and vegetable foods are analyzed, however, they are found to be similar in composition and to contain only some five or six kinds of materials that are essentially different. While certain foods may contain only a single one of these, most of the foods are mixtures of two or more. These few common materials which, in different proportions, form the different things that are eaten, are variously referred to as simple foods, food-stuffs, and nutrients, the last name being the one generally preferred. The different classes of nutrients are as follows:
Nutrients: Proteids (Albuminoids) Carbohydrates Fats Mineral salts Water
It is now necessary to become somewhat familiar with the different nutrients and the purposes which they serve in the body.
*Proteids.*—The proteids are obtained in part from the animal and in part from the plant kingdom, there being several varieties. A well-known variety, called albumin, is found in the white of eggs and in the plasma of the blood, while the muscles contain an abundance of another variety, known as myosin. Cheese consists largely of a kind of proteid, called casein, which is also present in milk, but in a more diluted form. If a mouthful of wheat is chewed for some time, most of it is dissolved and swallowed, but there remains in the mouth a sticky, gum-like substance. This is gluten, a form of proteid which occurs in different grains. Again, certain vegetables, as beans, peas, and peanuts, are rich in a kind of proteid which is called legumen.
Proteids are compounds of carbon, hydrogen, oxygen, nitrogen, and a small per cent of sulphur. Certain ones (the nucleo-proteids from grains) also contain phosphorus. All of the proteids are highly complex compounds and form a most important class of nutrients.
*Purposes of Proteids.*—The chief purpose of proteids in the body is to rebuild the tissues. Not only do they supply all of the main elements in the tissues, but they are of such a nature chemically that they are readily built into the protoplasm. They are absolutely essential to life, no other nutrients being able to take their place. An animal deprived of them exhausts the proteids in its body and then dies. In addition to rebuilding the tissues, proteids may also be oxidized to supply the body with energy.
*Albuminoids* form a small class of foods, of minor importance, which are similar to proteids in composition, but differ from them in being unable to rebuild the tissues. Gelatin, a constituent of soup and obtained from bones and connective tissue by boiling, is the best known of the albuminoid foods. On account of the nitrogen which they contain, proteids and albuminoids are often classed together as nitrogenous foods.
*Carbohydrates.*—While the carbohydrates are not so essential to life as are the proteids, they are of very great value in the body. They are composed of carbon, hydrogen, and oxygen, and are obtained mainly from plants. There are several varieties of carbohydrates, but they are similar in composition. All of those used as food to any great extent are starch and certain kinds of sugar.
*Starch* is the carbohydrate of greatest importance as a food, and it is also the one found in the greatest abundance. All green plants form more or less starch, and many of them store it in their leaves, seeds, or roots (Fig. 60). From these sources it is obtained as food. Glycogen, a substance closely resembling starch, is found in the body of the oyster. It is also formed in the liver and muscles of the higher animals, being prepared from the sugar of the blood, and is stored by them as reserve food (Chapter XI). Glycogen is, on this account, called animal starch. Starch on being eaten is first changed to sugar, after which it may be converted into glycogen in the liver and in the muscles.
[Fig. 60]
Fig. 60—*Starch grains* in cells of potato as they appear under the microscope. (See practical work.)
*Sugars.*—There are several varieties of sugar, but the important ones used as foods fall into one or the other of two classes, known as double sugars (disaccharides) and single sugars (monosaccharides). To the first class belong cane sugar, found in sugar cane and beets, milk sugar, found in sweet milk, and maltose, a kind of sugar which is made from starch by the action of malt. The important members of the second class are grape sugar, or dextrose, and fruit sugar, or levulose, both of which are found in fruits and in honey.
The most important of all sugars, so far as its use in the body is concerned, is dextrose. To this form all the other sugars, and starch also, are converted before they are finally used in the body. The close chemical relation between the different carbohydrates makes such a conversion easily possible.
*Fats.*—The fats used as foods belong to one or the other of two classes, known as solid fats and oils. The solid fats are derived chiefly from animals, and the oils are obtained mostly from plants. Butter, the fat of meats, olive oil, and the oil of nuts are the fats of greatest importance as foods. Fats, like the carbohydrates, are composed of carbon, hydrogen, and oxygen. They are rather complex chemical compounds, though not so complex as proteids. Since neither fats nor carbohydrates contain nitrogen, they are frequently classed together as non-nitrogenous foods.
*Purpose Served by Carbohydrates, Fats, and Albuminoids.*—These classes of nutrients all serve the common purpose of supplying energy. By uniting with oxygen at the cells, they supply heat and the other forms of bodily force. This is perhaps their only purpose.(50) Proteids also serve this purpose, but they are not so well adapted to supplying energy as are the carbohydrates and the fats. In the first place they do not completely oxidize and therefore do not supply so much energy; and, in the second place, they form waste products that are removed with difficulty from the body.
*Mineral Salts and their Uses.*—Mineral salts are found in small quantities in all of the more common food materials, and, as a rule, find their way into the body unnoticed. They supply the elements which are found in the body in small quantities and serve a variety of purposes.(51) Calcium phosphate and calcium carbonate are important constituents of the bones and teeth; and the salts containing iron renew the hemoglobin of the blood. Others perform important functions in the vital processes. The mineral compound of greatest importance perhaps is sodium chloride, or common salt.(52) This is a natural constituent of most of our foods, and is also added to food in its preparation for the table. When it is withheld from animals for a considerable length of time, they suffer intensely and finally die. It is necessary in the blood and lymph to keep their constituents in solution, and is thought to play an important role in the chemical changes of the cells. It is constantly leaving the body as a waste product and must be constantly supplied in small quantities in the foods.
*Importance of Water.*—Water finds its way into the body as a pure liquid, as a part of such mixtures as coffee, chocolate, and milk, and as a constituent of all our solid foods. (See table of foods, page 126.) It is also formed in the body by the oxidation of hydrogen. It passes through the body unchanged, and is constantly being removed by all the organs of excretion. Though water does not liberate energy in the body nor build up the tissues in the sense that other foods do, it is as necessary to the maintenance of life as oxygen or proteids. It occurs in all the tissues, and forms about 70 per cent of the entire weight of the body. Its presence is necessary for the interchange of materials at the cells and for keeping the tissues soft and pliable. As it enters the body, it carries digested food substances with it, and as it leaves it is loaded with wastes. Its chief physiological work, which is that of a transporter of material, depends upon its ability to dissolve substances and to flow readily from place to place.
*Relative Quantity of Nutrients Needed.*—Proteids, carbohydrates, and fats are the nutrients that supply most of the body's nourishment. The most hygienic diet is the one which supplies the proteids in sufficient quantity to rebuild the tissues and the carbohydrates and fats in the right amounts to supply the body with energy. Much experimenting has been done with a view to determining these proportions, but the results so far are not entirely satisfactory. According to some of the older estimates, a person of average size requires for his daily use five ounces of proteid, two and one half ounces of fat, and fifteen ounces of carbohydrate. Recent investigations of this problem seem to show that the body is as well, if not better, nourished by a much smaller amount of proteid—not more than two and one half ounces (60 grams) daily.(53)
While there is probably no necessity for the healthy individual's taking his proteid, fat, and carbohydrate in exact proportions (if the proportions best suited to his body were known), the fact needs to be emphasized that proteids, although absolutely necessary, should form but a small part (not over one fifth) of the daily bill of fare. In recognition of this fact is involved a principle of health and also one of economy. The proteids, especially those in meats, are the most expensive of the nutrients, whereas the carbohydrates, which should form the greater bulk of one's food, are the least expensive.
*Effects of a One-sided Diet.*—The plan of the body is such as to require a mixed diet, and all of the great classes of nutrients are necessary. If one could subsist on any single class, it would be proteids, for proteids are able both to rebuild tissue and to supply energy. But if proteids are eaten much in excess of the body's need for rebuilding the tissues, and this excess is oxidized for supplying energy, a strain is thrown upon the organs of excretion, because of the increase in the wastes. Not only is there danger of overworking certain of these organs (the liver and kidneys), but the wastes may linger too long in the body, causing disorder and laying the foundation for disease. On the other hand, if an insufficient amount of proteid is taken, the tissues are improperly nourished, and one is unable to exert his usual strength. What is true of the proteids is true, though in a different way, of the other great classes of foods. A diet which is lacking in proteid, carbohydrate, or fat, or which has any one of them in excess, is not adapted to the requirements of the body.
*Composition of the Food Materials.*—One who intelligently provides the daily bill of fare must have some knowledge of the nature and quantity of the nutrients present in the different materials used as food. This information is supplied by the chemist, who has made extensive analyses for this purpose. Results of such analyses are shown in Table 1 (page 126), which gives the percentage of proteids, fats, carbohydrates, water, and mineral salts in the edible portions of the more common of our foods.
[Fig. 61]
Fig. 61—Relative proportions of different nutrients in well-known foods.
*Food Supply to the Table.*—The main problem in supplying the daily bill of fare is that of securing through the different food materials the requisite amounts of proteids, carbohydrates, and fats. In this matter a table showing the composition of foods can be used to great advantage. Consulting the table on page 126, it is seen that large per cents of proteids are supplied by lean meat, eggs, cheese, beans, peas, peanuts, and oatmeal, while fat is in excess in fat meat, butter, and nuts (Fig. 61). Carbohydrates are supplied in abundance by potatoes, rice, corn, sugar, and molasses. The different cereals also contain a large percentage of carbohydrates in the form of starch.
TABLE I. THE COMPOSITION OF FOOD MATERIALS(54) Food Water Solids Proteid Fat Carbohydrates Mineral Heat Materials Matter Value of One Pound Animal Per cent Per cent Per cent Per cent Per cent Per cent Calories(55) foods, edible portion Beef: 63.9 36.1 19.5 15.6 ... 1 1020 Shoulder Rib 48.1 51.9 15.4 35.6 ... .9 1790 Sirloin 60 40 18.5 20.5 ... 1 1210 Round 68.2 31.8 20.5 10.1 ... 1.2 805 Veal: 68.8 31.2 20.2 9.8 ... ... 790 Shoulder Mutton: 61.8 38.2 18.3 19 ... .9 1140 Leg Loin 49.3 50.7 15 35 ... .7 1755 Pork: 50.3 49.7 16 32.8 ... .9 1680 Shoulder Ham, 41.5 58.5 16.7 39.1 ... 2.7 1960 salted, smoked Fat, 12.1 87.9 .9 82.8 ... 4.2 3510 salted Sausage: 41.5 58.8 13.8 42.8 ... 2.2 2065 Pork Bologna 62.4 37.6 18.8 42.8 ... 3 1015 Chicken 72.2 27.8 24.4 1 ... 1.4 540 Eggs 73.8 26.2 14.9 10.5 ... .8 721 Milk 87 13 3.6 4 4.7 .7 325 Butter 10.5 89 .6 85 .5 .3 3515 Cheese: 30.2 69.8 28.3 35.5 1.8 4.2 2070 Full cream Skim milk 41.3 58.7 38.4 6.8 6.9 4.6 1165 Fish: 82.6 17.4 15.8 .5 ... 1.2 310 Codfish Salmon 63.6 36.4 21.6 13.4 ... 1.4 965 Oysters 87.1 12.9 6 1.2 3.7 2 230 Vegetable foods Wheat 12.5 87.5 11 1.1 74.9 .5 1645 flour Graham 13.1 86.9 11.7 1.7 71.7 1.8 1635 flour (wheat) Rye flour 13.1 86.9 6.7 .8 78.7 .7 1625 Buckwheat 14.6 85.4 6.9 1.4 76.1 1 1605 flour Oatmeal 7.6 92.4 15.1 7.1 68.2 2 1850 Cornmeal 15 85 9.2 3.8 70.6 1.4 1645 Rice 12.4 87.6 7.4 .4 79.4 .4 1630 Peas 12.3 87.7 26.7 1.7 56.4 2.9 1565 Beans 12.6 87.4 23.1 2 59.2 3.1 1615 Potatoes 78.9 21.1 2.1 .1 17.9 1 375 Tomatoes 95.3 4.7 .8 .4 3.2 .3 80 Apples 83.2 16.8 .2 .4 15.9 .3 315 Sugar, 2 98 ... ... 97.8 .3 1820 granulated White 32.3 67.7 8.2 1.7 56.3 .0 1280 bread (wheat) Peanuts 9.2 90.8 25.8 24.4 38.6 2 2560 Almonds 4.8 95.2 21 17.3 54.9 2 3030 Walnuts 2.5 97.5 16.6 16.1 63.4 1.4 3285 (English)
Variety in the selection of foods for the table is an essential feature, but this should not increase either the work or the expense of supplying the meals. Each single meal can, and should, be simple in itself and, at the same time, differ sufficiently from the meal preceding and the one following to give the necessary variety in the course of the day. The bill of fare should, of course, include fruits (for their tonic effects) and very small amounts perhaps of substances which stimulate the appetite, such as pepper, mustard, etc., known as condiments.
*Purity of Food.*—The fact that many of the food substances are perishable makes it possible for them to be eaten in a slightly decayed condition. Such substances are decidedly unwholesome (some containing poisons) and should be promptly rejected. Not only do fresh meats, fruits, and vegetables need careful inspection, but canned and preserved goods as well. If canned foods are imperfectly sealed or if not thoroughly cooked in the canning process, they decay and the acids which they generate act on the metals lining the cans, forming poisonous compounds. The contents of "tin" cans should for this reason be transferred to other vessels as soon as opened.
Foods are also rendered impure or weakened through adulteration, the watering of milk being a familiar example. The manufacture of jellies, preserves, sirups, and various kinds of pickles and condiments has perhaps afforded the largest field for adulterations, although it is possible to adulterate nearly all of the leading articles of food. A long step in the prevention of food and drug adulteration was taken in this country by the passage of the Pure Food Law. By forcing manufacturers of foods and medicines to state on printed labels the composition of their products, this law has made it possible for the consumer to know what he is purchasing and putting into his body.
*Alcohol not a Food.*—Many people in this and other countries drink in different beverages, such as whisky, beer, wine, etc., a varying amount of alcohol. This substance has a temporary stimulating or exciting effect, and the claim has been made that it serves as a food. Recently it has been shown that alcohol when introduced into the body in small quantities and in a greatly diluted form, is nearly all oxidized, yielding energy as does fat or sugar. If no harmful effects attended the use of alcohol, it might on this account be classed as a food. But alcohol is known to be harmful to the body. When used in large quantities, it injures nearly all of the tissues, and when taken habitually, even in small doses, it leads to the formation of the alcohol habit which is now recognized and treated as a disease. This and other facts show that alcohol is not adapted to the body plan of taking on and using new material (Chapter XI), and no substance lacking in this respect can properly be classed as a food.(56) Instead of classing alcohol as a food, it should be placed in that long list of substances which are introduced into the body for special purposes and which are known by the general name of
*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 assist 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 pass through the body wall. This consists of the skin on the outside and of the mucous linings of the air passages and other tubes and cavities which are connected with the external surface.
To get from the digestive organs into the blood, the nutrients must pass through the mucous membrane lining these organs and also the walls of blood or lymph vessels. Only liquid materials can make this passage. It is necessary, therefore, to reduce to the liquid state all nutrients not already in that condition. This reduction to the liquid state constitutes 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 partitions 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 passed 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 constituent, 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 Classed with Reference to Digestive Changes.*—With reference to the changes which they undergo during digestion, foods may be divided into three classes 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 class. Milk and liquid fats, or oils, do not belong to this class.
2. Solid foods soluble in water. This class 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 classes 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 passed 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 classes.
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, butter, potatoes, round beef, eggs, and peanuts?
8. State the objection to a meal consisting of beef, eggs, beans, bread, and butter; to one consisting of potatoes, rice, bread, and butter. 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, coke, 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 scraps 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 glass 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 glass 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 mass remains behind. The black mass 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 mass remains behind. Since the products of decomposition (H2O, NH3, H2S, and the charred mass) 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.
*Tests for Nutrients.* Proteids.—Cover the substance to be tested with strong nitric acid and heat gradually to boiling. If proteid is present it turns yellow and partly dissolves in the acid, forming a yellow solution. Let cool and then add ammonia. The yellow solid and the solution are turned a deep orange color. Apply this test to foods containing proteid such as white of egg, cheese, lean meat, etc.
Starch.—(a) Place a small lump of starch in one fourth of a pint of water and heat gradually to boiling, stirring well. Then add enough water to form a thin liquid and fill a test tube half full. Add to this a few drops of a solution of iodine. (Prepare by dissolving a crystal of iodine in 25 cubic centimeters (1/20 pint) of a solution of potassium iodide in water and add water to this until it is a light amber color.) The starch solution is turned blue, (b) Cut with a razor a thin slice from a potato. Place this in a weak solution of iodine for a few minutes and then examine with the microscope, using first a low and then a high power. Numerous starch grains inclosed in cellulose walls will be seen (Fig. 60).
Dextrose, or Grape Sugar.—Place a solution of the substance supposed to contain grape sugar in a test tube and add a few drops of a dilute solution of copper sulphate. Then add sodium hydroxide solution until the precipitate which first forms is redissolved and a clear blue liquid obtained. Heat the upper portion of the liquid slowly to near the boiling point. A little below the boiling point the blue color disappears and a yellow-red precipitate is formed. If the upper layer of the liquid is now boiled, the color deepens and this may be contrasted with the blue color below. Apply this test to the sugar in raisins and in honey.
Fat.—Fat is recognized by its effect on paper, making a greasy stain which does not disappear on heating and which renders the paper translucent. Try butter, lard, or olive oil. Also show the presence of fat in peanuts by crushing them in a mortar and rubbing the powder on thin paper. If the substance to be tested contains but little fat, this may be dissolved out with ether. If a drop of ether containing the fat is placed on paper, it evaporates, leaving the fat, which then forms the stain.
*To show the Effect of Alcohol upon Proteid.*—Place some of the white of a raw egg in a glass vessel and cover it with a small amount of alcohol. As the albumin (proteid) hardens, or coagulates, observe that the quantity of clear liquid increases. This is due to the withdrawal of water from the albumin by the alcohol. Since the tissues are made up chiefly of proteids, a piece of muscle or of liver may be used in the experiment, instead of the egg, with similar results.
*To illustrate the Digestive Process.*—To a tumbler two thirds full of water add a little salt. Stir and observe that the salt is dissolved. Taste the solution to see that the salt has not been changed chemically. Now add a little powdered limestone to the water and stir as before. Observe that the limestone does not dissolve. Then add some hydrochloric acid and observe the result. State the part played by the acid and by the water in dissolving the limestone. Apply to the digestion of the different classes of foods.
CHAPTER X - ORGANS AND PROCESSES OF DIGESTION
The organs of digestion are adapted to the work of dissolving the foods by both their structure and arrangement. Most of them consist either of tubes or cavities and these are so connected, one with the other, as to form a continuous passageway entirely through the body. This passageway is known as
*The Alimentary Canal. *—The alimentary canal has a length of about thirty feet and, while it begins at the mouth, all but about eighteen inches of it is found in the abdominal cavity. On account of its length it lies for the most part in coils, the two largest ones being known as the small intestine and the large intestine. Connected with the alimentary canal are the glands that supply the liquids for acting on the food. The divisions of the canal and most of the glands that empty liquids into it are shown in Fig. 63 and named in the table below:
[Table]
*Coats of the Alimentary Canal.*—The walls of the alimentary canal, except at the mouth, are distinct from the surrounding tissues and consist in most places of at least three layers, or coats, as follows:
[Fig. 63]
Fig. 63—*Diagram of the digestive system.* 1. Mouth. 2. Soft palate. 3. Pharynx. 4. Parotid gland. 5. Sublingual gland. 6. Submaxillary gland. 7. Esophagus. 8. Stomach. 9. Pancreas. 10. Vermiform appendix. 11. Caecum. 12. Ascending colon. 13. Transverse colon. 14. Descending colon. 15. Sigmoid flexure. 16. Rectum. 17. Ileo-caecal valve. 18. Duct from liver and pancreas. 19. Liver.
Diagram does not show comparative length of the small intestine.
1. An inner coat, or lining, known as the mucous membrane. This membrane is not confined to the alimentary canal, but lines, as we have seen, the different air passages. It covers, in fact, all those internal surfaces of the body that connect with the external surface. It derives its name from the substance which it secretes, called mucus. In structure it resembles the skin, being continuous with the skin where cavities open to the surface. It is made up of two layers—a thick underlayer which contains blood vessels, nerves, and glands, and a thin surface layer, called the epithelium. The epithelium, like the cuticle, is without blood vessels, nerves, or glands.
2. A middle coat, which is muscular and which forms a continuous layer throughout the canal, except at the mouth. (Here its place is taken by the strong muscles of mastication which are separate and distinct from each other.) As a rule the muscles of this coat are involuntary. They surround the canal as thin sheets and at most places form two distinct layers. In the inner layer the fibers encircle the canal, but in the outer layer they run longitudinally, or lengthwise, along the canal.(57)
3. An outer or serous coat, which is limited to those portions of the canal that occupy the abdominal cavity. This coat is not found above the diaphragm. It is a part of the lining membrane of the cavity of the abdomen, called
[Fig. 64]
Fig. 64—*Diagram of the peritoneum.* 1. Transverse colon. 2. Duodenum. 3. Small intestine. 4. Pancreas.
*The Peritoneum.*—The peritoneum is to the abdominal cavity what the pleura is to the thoracic cavity. It forms the outer covering for the alimentary canal and other abdominal organs and supplies the inner lining of the cavity itself. It is also the means of holding these organs in place, some of them being suspended by it from the abdominal walls (Fig. 64). By the secretion of a small amount of liquid, it prevents friction of the parts upon one another.
*Digestive Glands.*—The glands which provide the different fluids for acting on the foods derive their constituents from the blood. They are situated either in the mucous membrane or at convenient places outside of the canal and pass their liquids into it by means of small tubes, called ducts. In the canal the food and the digestive fluids come in direct contact—a condition which the dissolving processes require. Each kind of fluid is secreted by a special kind of gland and is emptied into the canal at the place where it is needed.
*The Digestive Processes.*—Digestion is accomplished by acting upon the food in different ways, as it is passed along the canal, with the final result of reducing it to the form of a solution. Several distinct processes are necessary and they occur in such an order that those preceding are preparatory to those that follow. These processes are known as mastication, insalivation, deglutition, stomach digestion, and intestinal digestion. As the different materials become liquefied they are transferred to the blood, and substances not reduced to the liquid state are passed on through the canal as waste. The first two of the digestive processes occur in
*The Mouth.*—This is an oval-shaped cavity situated at the very beginning of the canal. It is surrounded by the lips in front, by the cheeks on the sides, by the hard palate above and the soft palate behind, and by the tissues of the lower jaw below. The mucous membrane lining the mouth is, soft and smooth, being covered with flat epithelial cells. The external opening of the mouth is guarded by the lips, and the soft palate forms a movable partition between the mouth and the pharynx. In a condition of repose the mouth space is practically filled by the teeth and the tongue, but the cavity may be enlarged and room provided for food by depressing the lower jaw.
The mouth by its construction is well adapted to carrying on the processes of mastication and insalivation. By the first process the solid food is reduced, by the cutting and grinding action of the teeth, to a finely divided condition. By the second, the saliva becomes mixed with the food and is made to act upon it.
[Fig. 65]
Fig. 65—*The teeth.* A. Section of a single molar. 1. Pulp. 2. Dentine. 3. Enamel. 4. Crown. 5. Neck. 6. Root. B. Teeth in position in lower jaw. 1. Incisors. 2. Canine. 3. Biscuspids. 4. Molars. C. Upper and lower teeth on one side. 1. Incisors. 2. Canines. 3. Biscuspids. 4. Molars. 5. Wisdom. D. Upper and lower incisor, to show gliding contact.
*Accessory Organs of the Mouth.*—The work of mastication and insalivation is accomplished through organs situated in and around the mouth cavity. These comprise:
1. The Teeth.—The teeth are set in the upper and lower jaws, one row directly over the other, with their hardened surfaces facing. In reducing the food, the teeth of the lower jaw move against those of the upper, while the food is held by the tongue and cheeks between the grinding surfaces. The front teeth are thin and chisel-shaped. They do not meet so squarely as do the back ones, but their edges glide over each other, like the blades of scissors—a condition that adapts them to cutting off and separating the food (D, Fig. 65). The back teeth are broad and irregular, having surfaces that are adapted to crushing and grinding.
Each tooth is composed mainly of a bone-like substance, called dentine, which surrounds a central space, containing blood vessels and nerves, known as the pulp cavity. It is set in a depression in the jaw where it is held firmly in place by a bony substance, known as cement. The part of the tooth exposed above the gum is the crown, the part surrounded by the gum is the neck, and the part which penetrates into the jaw is the root (A, Fig. 65). A hard, protective material, called enamel, covers the exposed surface of the tooth.
The teeth which first appear are known as the temporary, or milk, teeth and are twenty in number, ten in each jaw. They usually begin to appear about the sixth month, and they disappear from the mouth at intervals from the sixth to the thirteenth year. As they leave, teeth of the second, or permanent, set take their place. This set has thirty-two teeth of four different kinds arranged in the two jaws as follows:
In front, above and below, are four chisel-shaped teeth, known as the incisors. Next to these on either side is a tooth longer and thicker than the incisors, called the canine. Back of these are two short, rounded and double pointed teeth, the bicuspids, and back of the bicuspids are three heavy teeth with irregular grinding surfaces, called the molars (B and C, Fig. 65). Since the molar farthest back in each jaw is usually not cut until maturity, it is called a wisdom tooth. The molars are known as the superadded permanent teeth because they do not take the place of milk teeth, but form farther back as the jaw grows in length.
[Fig. 66]
Fig. 66—*Diagram* showing directions of muscular fibers in tongue.
2. The Tongue.—The tongue is a muscular organ whose fibers extend through it in several directions (Fig. 66). Its structure adapts it to a variety of movements. During mastication the tongue transfers the food from one part of the mouth to another, and, with the aid of the cheeks, holds the food between the rows of teeth. (By an outward pressure from the tongue and an inward pressure from the cheek the food is kept between the grinding surfaces.) The tongue has functions in addition to these and is a most useful organ.
3. The Muscles of Mastication.—These are attached to the lower jaw and bring about its different movements. The masseter muscles, which are the heavy muscles in the cheeks, and the temporal muscles, located in the region of the temples, raise the lower jaw and supply the force for grinding the food. Small muscles situated below the chin depress the jaw and open the mouth.
[Fig. 67]
Fig. 67—*Salivary glands* and the ducts connecting them with the mouth.
4. The Salivary Glands.—These glands are situated in the tissues surrounding the mouth, and communicate with it by means of ducts (Fig. 67). They secrete the saliva. The salivary glands are six in number and are arranged in three pairs. The largest, called the parotid glands, lie, one on either side, in front of and below the ears. A duct from each gland passes forward along the cheek until it opens in the interior of the mouth, opposite the second molar tooth in the upper jaw. Next in size to the parotids are the submaxillary glands. These are located, one on either side, just below and in front of the triangular bend in the lower jaw. The smallest of the salivary glands are the sublingual. They are situated in the floor of the mouth, on either side, at the front and base of the tongue. Ducts from the submaxillary and sublingual glands open into the mouth below the tip of the tongue.
*The Saliva and its Uses.*—The saliva is a transparent and somewhat slimy liquid which is slightly alkaline. It consists chiefly of water (about 99 per cent), but in this are dissolved certain salts and an active chemical agent, or enzyme, called ptyalin, which acts on the starch. The ptyalin changes starch into a form of sugar (maltose), while the water in the saliva dissolves the soluble portions of the food. In addition to this the saliva moistens and lubricates the food which it does not dissolve, and prepares it in this way for its passage to the stomach. The last is considered the most important use of the saliva, and dry substances, such as crackers, which require a considerable amount of this liquid, cannot be eaten rapidly without choking. Slow mastication favors the secretion and action of the saliva.
*Deglutition.*—Deglutition, or swallowing, is the process by which food is transferred from the mouth to the stomach. Though this is not, strictly speaking, a digestive process, it is, nevertheless, necessary for the further digestion of the food. Mastication and insalivation, which are largely mechanical, prepare the food for certain chemical processes by which it is dissolved. The first of these occurs in the stomach and to this organ the food is transferred from the mouth. The chief organs concerned in deglutition are the tongue, the pharynx, and the esophagus.
*The Pharynx* is a round and somewhat cone-shaped cavity, about four and one half inches in length, which lies just back of the nostrils, mouth, and larynx. It is remarkable for its openings, seven in number, by means of which it communicates with other cavities and tubes of the body. One of these openings is into the mouth, one into the esophagus, one into the larynx, and one into each of the nostrils, while two small tubes (the eustachian) pass from the upper part of the pharynx to the middle ears.
The pharynx is the part of the food canal that is crossed by the passageway for the air. To keep the food from passing out of its natural channel, the openings into the air passages have to be carefully guarded. This is accomplished through the soft palate and epiglottis, which are operated somewhat as valves. The muscular coat of the pharynx is made up of a series of overlapping muscles which, by their contractions, draw the sides together and diminish the cavity. The mucous membrane lining the pharynx is smooth, like that of the mouth, being covered with a layer of flat epithelial cells.
*The Esophagus*, or gullet, is a tube eight or nine inches long, connecting the pharynx with the stomach. It lies for the most part in the thoracic cavity and consists chiefly of a thick mucous lining surrounded by a heavy coat of muscle. The muscular coat is composed of two layers—an inner layer whose fibers encircle the tube and an outer layer whose fibers run lengthwise.
*Steps in Deglutition.*—The process of deglutition varies with the kind of food. With bulky food it consists of three steps, or stages, as follows: 1. By the contraction of the muscles of the cheeks, the food ball, or bolus, is pressed into the center of the mouth and upon the upper surface of the tongue. Then the tongue, by an upward and backward movement, pushes the food under the soft palate and into the pharynx.
2. As the food passes from the mouth, the pharynx is drawn up to receive it. At the same time the soft palate is pushed upward and backward, closing the opening into the upper pharynx, while the epiglottis is made to close the opening into the larynx. By this means all communication between the food canal and the air passages is temporarily closed. The upper muscles of the pharynx now contract upon the food, forcing it downward and into the esophagus.
3. In the esophagus the food is forced along by the successive contractions of muscles, starting at the upper end of the tube, until the stomach is reached.
Swallowing is doubtless aided to some extent by the force of gravity. That it is independent of this force, however, is shown by the fact that one may swallow with the esophagus in a horizontal position, as in lying down.
[Fig. 68]
Fig. 68—*Gastric Glands.* A. Single gland showing the two kinds of secreting cells and the duct where the gland opens on to the surface. B. Inner surface of stomach magnified. The small pits are the openings from the glands.
*The Stomach.*—The stomach is the largest dilatation of the alimentary canal. It is situated in the abdominal cavity, immediately below the diaphragm, with the larger portion toward the left side. Its connection with the esophagus is known as the cardiac orifice and its opening into the small intestine is called the pyloric orifice. It varies greatly in size in different individuals, being on the average from ten to twelve inches at its greatest length, from four to five inches at its greatest width, and holding from three to five pints. It has the coats common to the canal, but these are modified somewhat to adapt them to its work.
The mucous membrane of the stomach is thick and highly developed. It contains great numbers of minute tube-shaped bodies, known as the gastric glands (Fig. 68). These are of two general kinds and secrete large quantities of a liquid called the gastric juice. When the stomach is empty, the mucous membrane is thrown into folds which run lengthwise over the inner surface. These disappear, however, when the walls of the stomach are distended with food.
The muscular coat consists of three separate layers which are named, from the direction of the fibers, the circular layer, the longitudinal layer, and the oblique layer (Fig. 69). The circular layer becomes quite thick at the pyloric orifice, forming a distinct band which serves as a valve.
[Fig. 69]
Fig. 69—*Muscles of the stomach* (from Morris' Human Anatomy). The layer of Longitudinal fibers removed.
The outer coat of the stomach, called the serous coat, is a continuation of the peritoneum, the membrane lining the abdominal cavity.
*Stomach Digestion.*—In the stomach begins the definite work of dissolving those foods which are insoluble in water. This, as already stated, is a double process. There is first a chemical action in which the insoluble are changed into soluble substances, and this is followed immediately by the dissolving action of water. The chief substances digested in the stomach are the proteids. These, in dissolving, are changed into two soluble substances, known as peptones and proteoses. The digestion of the proteids is, of course, due to the
*Gastric Juice.*—The gastric juice is a thin, colorless liquid composed of about 99 per cent of water and about 1 per cent of other substances. The latter are dissolved in the water and include, besides several salts, three active chemical agents—hydrochloric acid, pepsin, and rennin. Pepsin is the enzyme which acts upon proteids, but it is able to act only in an acid medium—a condition which is supplied by the hydrochloric acid. Mixed with the hydrochloric acid it converts the proteids into peptones and proteoses.
*Other Effects of the Gastric Juice.*—In addition to digesting proteids, the gastric juice brings about several minor effects, as follows:
1. It checks, after a time, the digestion of the starch which was begun in the mouth by the saliva.(58) This is due to the presence of the hydrochloric acid, the ptyalin being unable to act in an acid medium.
2. While there is no appreciable action on the fat itself, the proteid layers that inclose the fat particles are dissolved away (Fig. 79), and the fat is set free. By this means the fat is broken up and prepared for a special digestive action in the small intestine.
3. Dissolved albumin, like that in milk, is curded, or coagulated, in the stomach. This action is due to the rennin. The curded mass is then acted upon by the pepsin and hydrochloric acid in the same manner as the other proteids.
4. The hydrochloric acid acts on certain of the insoluble mineral salts found in the foods and reduces them to a soluble condition.
5. It is also the opinion of certain physiologists that cane sugar and maltose (double sugars) are converted by the hydrochloric acid into dextrose and levulose (single sugars).
After a variable length of time, the contents of the stomach is reduced to a rather uniform and pulpy mass which is called chyme. Portions of this are now passed at intervals into the small intestine.
*Muscular Action of the Stomach.*—The muscles in the walls of the stomach have for one of their functions the mixing of the food with the gastric juice. By alternately contracting and relaxing, the different layers of muscle keep the form of the stomach changing—a result which agitates and mixes its contents. This action varies in different parts of the organ, being slight or entirely absent at the cardiac end, but quite marked at the pyloric end.
Another purpose of the muscular coat is to empty the stomach into the small intestine. During the greater part of the digestive period the muscular band at the pyloric orifice is contracted. At intervals, however, this band relaxes, permitting a part of the contents of the stomach to be forced into the small intestine. After the discharge the pyloric muscle again contracts, and so remains until the time arrives for another discharge.
In addition to emptying the stomach into the small intestine, these muscles also aid in emptying the organ upward and through the esophagus and mouth, should occasion require. Vomiting in case of poisoning, or if the food for some reason fails to digest, is a necessary though unpleasant operation. It is accomplished by the contraction of all the muscles of the stomach, together with the contraction of the walls of the abdomen. During these contractions the pyloric valve is closed, and the muscles of the esophagus and pharynx are in a relaxed condition.(59)
[Fig. 70]
Fig. 70—*Passage from stomach* into small intestine. Illustration also shows arrangement of mucous membrane in the two organs. D. Bile duct.
*The Small Intestine.*—This division of the alimentary canal consists of a coiled tube, about twenty-two feet in length, which occupies the central, lower portion of the abdominal cavity (Fig. 71). At its upper extremity it connects with the pyloric end of the stomach (Fig. 70), and at its lower end it joins the large intestine. It averages a little over an inch in diameter, and gradually diminishes in size from the stomach to the large intestine. The first eight or ten inches form a short curve, known as the duodenum. The upper two fifths of the remainder is called the jejunum, and the lower three fifths is known as the ileum. The ileum joins that part of the large intestine known as the caecum, and at their place of union is a marked constriction which prevents material from passing from the large into the small intestine (Fig. 73). This is known as the ileo-caecal valve.
The mucous membrane of the small intestine is richly supplied with blood vessels and contains glands that secrete a digestive fluid known as the intestinal juice. The membrane is thrown into many transverse, or circular, folds which increase its surface and also prevent materials from passing too rapidly through the intestine. One important respect in which the small intestine differs from all other portions of the food canal is that its surface is covered with great numbers of minute elevations known as the villi. The purpose of these is to aid in the absorption of the nutrients as they become dissolved (Chapter XI).
The muscular coat of the small intestine is made up of two distinct layers—the inner layer consisting of circular fibers and the outer of longitudinal fibers. These muscles keep the food materials mixed with the juices of the small intestine, but their main purpose is to force the materials undergoing digestion through this long and much-coiled tube.
The outer, or serous, coat of the small intestine, like that of the stomach, is an extension from the general lining of the abdominal cavity, or peritoneum. In fact, the intestine lies in a fold of the peritoneum, somewhat as an arm in a sling, while the peritoneum, by connecting with the back wall of the abdominal cavity, holds this great coil of digestive tubing in place (Fig. 64). The portion of the peritoneum which attaches the intestine to the wall of the abdomen is called the mesentery.
Most of the liquid acting on the food in the small intestine is supplied by two large glands, the liver and the pancreas, that connect with it by ducts.
[Fig. 71]
Fig. 71—*Abdominal cavity* with organs of digestion in position.
*The Liver* is situated immediately below the diaphragm, on the right side (Figs. 71 and 72), and is the largest gland in the body. It weighs about four pounds and is separated into two main divisions, or lobes. It is complex in structure and differs from the other glands in several particulars. It receives blood from two distinct sources—the portal vein and the hepatic artery. The portal vein collects the blood from the stomach, intestines, and spleen, and passes it to the liver. This blood is loaded with food materials, but contains little or no oxygen. The hepatic artery, which branches from the aorta, carries to the liver blood rich in oxygen. In the liver the portal vein and the hepatic artery divide and subdivide, and finally empty their blood into a single system of capillaries surrounding the liver cells. These capillaries in turn empty into a single system of veins which, uniting to form the hepatic veins (two or three in number), pass the blood into the inferior vena cava (Fig. 72).
[Fig. 72]
Fig. 72—*Relations of the liver.* Diagram showing the connection of the liver with the large blood vessels and the food canal.
The liver secretes daily from one to two pounds of a liquid called bile. A reservoir for the bile is provided by a small, membranous sack, called the gall bladder, located on the underside of the liver. The bile passes from the gall bladder, and from the right and left lobes of the liver, by three separate ducts. These unite to form a common tube which, uniting with the duct from the pancreas, empties into the duodenum. Though usually described as a digestive gland, the liver has other functions of equal or greater importance (Chapter XIII).
*The Bile* is a golden yellow liquid, having a slightly alkaline reaction and a very bitter taste. It consists, on the average, of about 97 per cent of water and 3 per cent of solids.(60) The solids include bile pigments, bile salts, a substance called cholesterine, and mineral salts. The pigments (coloring matter) of the bile are derived from the hemoglobin of broken-down red corpuscles (page 27).
Much about the composition of the bile is not understood. It is known, however, to be necessary to digestion, its chief use being to aid in the digestion and absorption of fats. It is claimed also that the bile aids the digestive processes in some general ways—counteracting the acid of the gastric juice, preventing the decomposition of food in the intestines, and stimulating muscular action in the intestinal walls. No enzymes have been discovered in the bile.
*The Pancreas* is a tapering and somewhat wedge-shaped gland, and is so situated that its larger extremity, or head, is encircled by the duodenum. From here the more slender portion extends across the abdominal cavity nearly parallel to and behind the lower part of the stomach. It has a length of six or eight inches and weighs from two to three and one half ounces. Its secretion, the pancreatic juice, is emptied into the duodenum by a duct which, as a rule, unites with the duct from the liver.
*The Pancreatic Juice* is a colorless and rather viscid liquid, having an alkaline reaction. It consists of about 97.6 per cent of water and 2.4 per cent of solids. The solids include mineral salts (the chief of which is sodium carbonate) and four different chemical agents, or enzymes,—trypsin, amylopsin, steapsin, and a milk-curding enzyme. These active constituents make of the pancreatic juice the most important of the digestive fluids. It acts with vigor on all of the nutrients insoluble in water, producing the following changes:
1. It converts the starch into maltose, completing the work begun by the saliva. This action is due to the amylopsin,(61) which is similar to ptyalin but is more vigorous.
2. It changes proteids into peptones and proteoses, completing the work begun by the gastric juice. This is accomplished by the trypsin, which is similar to, but more active than, the pepsin.
3. It digests fat. In this work the active agent is the steapsin.
The necessity of a milk-curding enzyme, somewhat similar to the rennin of the gastric juice, is not understood.
*Digestion of Fat.*—Several theories have been proposed at different times regarding the digestion and absorption of fat. Among these, what is known as the "solution theory" seems to have the greatest amount of evidence in its favor. According to this theory, the fat, under the influence of the steapsin, absorbs water and splits into two substances, recognized as glycerine and fatty acid. This finishes the process so far as the glycerine is concerned, as this is soluble in water; but the fatty acid, which (from certain fats) is insoluble in water,(62) requires further treatment. The fatty acid is now supposed to be acted on in one, or both, of the following ways: 1. To be dissolved as fatty acid by the action of the bile (since bile is capable of dissolving it under certain conditions). 2. To be converted by the sodium carbonate into a form of soap which is soluble in water.
The emulsification of fat is known to occur in the small intestine. By this process the fat is separated into minute particles which are suspended in water, but not changed chemically, the mixture being known as an emulsion. While this is believed by some to be an actual process of digestion, the advocates of the solution theory claim that it is a process accompanying and aiding the conversion of fat into fatty acid and glycerine.(63)
*The Intestinal Juice* is a clear liquid with an alkaline reaction, containing water, mineral salts, and certain proteid substances that may act as enzymes. It assists in bringing about an alkaline condition in the small intestine and aids in the reduction of cane sugar and maltose to the simple sugars, dextrose and levulose. Since it is difficult to obtain this liquid in sufficient quantities for experimenting, its uses have not been fully determined. Recent investigators, however, assign to it an important place in the work of digestion.
*Work of the Small Intestine.*—The small intestine is the most important division of the alimentary canal. It serves as a receptacle for holding the food while it is being acted upon; it secretes the intestinal juice and mixes the food with the digestive fluids; it propels the food toward the large intestine; and, in addition to all this, serves as an organ of absorption.
Digestion is practically finished in the small intestine, and a large portion of the reduced food is here absorbed. There is always present, however, a variable amount of material that is not digested. This, together with a considerable volume of liquid, is passed into
*The Large Intestine.*—The large intestine is a tube from five to six feet in length and averaging about one and one half inches in diameter. It begins at the lower right side of the abdominal cavity, forms a coil which almost completely surrounds the coil of small intestine, and finally terminates at the surface of the body (Figs. 2, 71 and 73). It has three divisions, known as the caecum, the colon, and the rectum.
[Fig. 73]
Fig. 73—*Passage from small into large intestine.* At the ileo-caecal valve is the narrowest constriction of the food canal.
The caecum is the pouch-like dilatation of the large intestine which receives the lower end of the small intestine. It measures about two and one half inches in diameter and has extending from one side a short, slender, and blind tube, called the vermiform appendix. This structure serves no purpose in digestion, but appears to be the rudiment of an organ which may have served a purpose at some remote period in the history of the human race. The caecum gradually blends into the second division of the large intestine, called the colon. |
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