(_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 ill.u.s.trating pa.s.sage of oxygen through the body.

(_e_) Review experiments (page 101) showing the presence of carbon dioxide in the breath.

*To ill.u.s.trate the General Movement of Oxygen through the Body.*-Into a gla.s.s 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 gla.s.s tube which is bent at right angles and which is made to pa.s.s 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 pa.s.sing through the tube, unites with the carbon, forms carbon dioxide, and liberates energy? Compare the changes which the oxygen undergoes while pa.s.sing through the tube with the changes which it undergoes in pa.s.sing 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 quant.i.ty 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 a.s.sistance 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 a.n.a.lyzed, 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 cla.s.ses of nutrients are as follows:

Nutrients: Proteids (Alb.u.minoids) 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 _alb.u.min_, 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 cla.s.s 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.

*Alb.u.minoids* form a small cla.s.s 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 const.i.tuent of soup and obtained from bones and connective tissue by boiling, is the best known of the alb.u.minoid foods. On account of the nitrogen which they contain, proteids and alb.u.minoids are often cla.s.sed 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 cla.s.ses, known as _double sugars_ (disaccharides) and _single sugars_ (monosaccharides). To the first cla.s.s 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 cla.s.s 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 cla.s.ses, known as solid fats and oils. The solid fats are derived chiefly from animals, and the oils are obtained mostly from plants. b.u.t.ter, 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 cla.s.sed together as _non-nitrogenous_ foods.

*Purpose Served by Carbohydrates, Fats, and Alb.u.minoids.*-These cla.s.ses 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 quant.i.ties 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 quant.i.ties and serve a variety of purposes.(51) Calcium phosphate and calcium carbonate are important const.i.tuents 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 const.i.tuent 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 const.i.tuents 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 quant.i.ties 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 const.i.tuent of all our solid foods. (See table of foods, page 126.) It is also formed in the body by the oxidation of hydrogen. It pa.s.ses 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 Quant.i.ty 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 quant.i.ty 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 cla.s.ses of nutrients are necessary.

If one could subsist on any single cla.s.s, 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 cla.s.ses 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 quant.i.ty of the nutrients present in the different materials used as food. This information is supplied by the chemist, who has made extensive a.n.a.lyses for this purpose. Results of such a.n.a.lyses 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, b.u.t.ter, and nuts (Fig.

61). Carbohydrates are supplied in abundance by potatoes, rice, corn, sugar, and mola.s.ses. 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 b.u.t.ter 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 appet.i.te, 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 pa.s.sage 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 quant.i.ties 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 cla.s.sed as a food. But alcohol is known to be harmful to the body. When used in large quant.i.ties, 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 cla.s.sed as a food.(56) Instead of cla.s.sing 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