3. THE CHEMICAL COMPOSITION OF THE HUMAN BODY

We are now to consider the application of some of these principles of chemistry to the study of the human body. Chemists have accurately determined the chemical composition of the various parts of the body, and we will discuss some of the most important substances that have been shown by analysis to be present.

Water. The great importance of water in the composition of living substance is evident from the fact that it forms about 62% of the weight of an adult man. Hence, if all the water were removed from the body of a man weighing one hundred and sixty-five pounds, the solids that remained would weigh but a little over sixty pounds. The different organs vary greatly in their percentage of water: bones contain about 22%, muscles have 75%, and the kidneys 82 %. Mineral Matters. - Mineral matters are found in greatest quantity in the bones. When we burn bones, about one third of the weight disappears, the remaining two thirds being bone ash, which is the mineral matter. Every part of the body, however, contains some mineral ingredients, for when muscle, liver, brain, or blood is burned, in each case there remain some traces of ash.

Gases. A large amount of the gas oxygen is taken into the lungs, whence it is distributed to all the organs by the blood. We shall find that in our bodies, as well as in the experiments with the match this oxygen performs the all-important function of causing oxidation. One of the products of this oxidation is the gas we have already considered; namely, carbon dioxid. If we blow through a tube or a straw into a glass of clear limewater, the liquid becomes milky. Our breath is therefore constantly removing from our bodies a gas exactly like one of those formed by burning the match.

Fats. The amount of fat in the body varies greatly in

different individuals, but it is always present in some quantity. Muscle, however lean, contains particles of fat; fat constitutes a small percentage of the blood; it fills the spaces in the interior of bones; and it is often deposited in considerable quantity in the deeper layers of the skin. When fat is heated, it first melts to a liquid. At a higher temperature it will scorch, and the black residue shows the presence of carbon. In the body this fat is burned by combining with oxygen, and this is one of the ways in which we are kept warm. If we were to eat nothing for several days, we could still be kept warm and be able to do a certain amount of work, a result due largely to the slow oxidation of the fat stored in various parts of the body.

Carbohydrates.

The substances we know as starches and sugars are made up of the three chemical elements, carbon, hydrogen, and oxygen, and the hydrogen and oxygen of these compounds are always in the same proportion in which they occur in water (that is, H2O). Hence these compounds are called car-bo-hy' drates (carbon+hydor = water). In the blood and other animal tissues we find some of the carbohydrate called grape sugar.1 Another carbohydrate, known as animal starch or glycogen, is found in the liver. Carbohydrates, like fats, contain a large amount of carbon, which also unites with oxygen. A second ingredient of both of these classes of compounds is hydrogen; it readily combines with oxygen to form water. The fats and carbohydrates found in the composition of the body may be regarded as stored-up fuel which can be drawn. upon in case of need. Like the engine, we are kept warm and enabled to do work by the oxidation of fuel.

1 The chemical composition of grape sugar is represented by the chemical symbol C6H12O6, which means that every molecule of grape sugar contains six atoms of carbon, twelve atoms of hydrogen, and six atoms of oxygen.

2 Glycogen is composed of six atoms of carbon, ten atoms of hydrogen, and five atoms of oxygen, its chemical symbol being CH1005.

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Nitrogenous Substances.

The most important substances in the living body are the ni-trog'e-nous compounds. They are all characterized, as the name implies, by the presence of the element nitrogen (symbol N). Some of these compounds which are present in all living substance are known. Without exception,

as pro'te-ids or albuminous substances. proteids consist chiefly of the elements carbon, hydrogen, oxygen, nitrogen, and sulphur, and often other elements are present in their composition. They are the most complex substances in the body.!

From the carbon of the proteids also there is formed by oxidation the waste gas carbon dioxid, which, as we have demonstrated, is thrown off from the lungs. Another important waste compound that comes from the oxidation of proteids is a substance known as u're-a. Urea contains most of the nitrogen that cannot be further used by the body. It is taken from the blood by the kidneys and forms the principal solid constituent dissolved in the urine.

The summary on the following page contains in brief the principal facts we have learned in regard to the chemical composition of the body.

1 The composition, for example, of the proteid hem-o-glo'bin (Greek hai'ma=blood) which gives the red color to the blood, is said by one chemist to be C600 H960O179N154S3 Fe (Fe being the symbol for iron). This symbol means that a molecule of the compound hemoglobin consists of over eighteen hundred atoms, six hundred of which are carbon, nine hundred and sixty, hydrogen, etc.

C. Products of oxidation of the body (= waste substances).

9. Urea.

Thrown off from body by lungs, skin, kid

neys. Taken by blood from all parts of body to lungs and skin, whence it is given off.

Solid (in so- C, H, O, and Taken by blood from

lution).

N.

all parts of body to kidneys and skin, whence it is given off.

CHAPTER III

A STUDY OF LIVING SUBSTANCE

1. THE GENERAL STRUCTURE OF ANIMAL BODIES

Vertebrates and Invertebrates. All animals may be divided into two great classes, known respectively as the ver'te-brates and the in-ver'te-brates. To the first group belong the animals that have a backbone. We are all familiar with common vertebrates like fishes, frogs, snakes, birds, and cats. Insects, worms, and clams, on the other hand, have no backbone; hence they are called invertebrates (ie. animals without vertebræ).

Regions of the Body. In man and in most other vertebrates we can distinguish the head and neck region, the trunk, and the four ap-pend'ag-es or limbs which are attached to the trunk, namely, two arms and two legs, or, as they are more often called in descriptions of the lower animals, the four legs. Since frequent reference will be made in the following pages to different vertebrates and invertebrates, we must become familiar at the outset with certain terms that will locate definitely corresponding regions in all animals.

Man walks on two appendages (the legs); the long axis of his trunk is vertical; and above his body is his head. But dogs and other four-footed animals have a horizontal trunk with the head attached in front. Hence the same adjective cannot be used to describe the position of the head of man and of quadrupeds. Biologists have, therefore, adopted the term an-te'ri-or (Latin an'te before) which can be applied