XVIII. THE HUMAN MACHINE AND ITS NEEDS
_Problem.--To obtain a general understanding of the parts and uses of the bodily machine._
LABORATORY SUGGESTIONS
_Demonstration._--Review to show that the human body is a complex of cells.
_Laboratory demonstration_ by means of (_a_) human skeleton and (_b_) manikin to show the position and gross structure of the chief organs of man.
Man and his Environment.--In the last chapter we saw that one factor in the improvement of man lies in giving him better surroundings. It will be the purpose of the following chapters to show how man is fitted to live in the environment in which he is placed. He comes in contact with air, light, water, soil, food, and shelter which make his somewhat artificial environment; he must adapt himself to get the best he can out of this environment.
The Needs of Living Things.--We have already found that the primary needs of plants and animals are the same. They both need food, they both need to digest their food and to have it circulate in a fluid form to the cells where it will be used. They both need oxygen so as to release the energy locked up in their food. And they both need to reproduce so that their kind may be continued on the earth. What is true of plants and other animals is true of man.
The Needs of Simple and Complex Animals the Same.--The simplest animal, a single cell, has the same needs as the most complex. The _cell_ paramoecium feeds, digests, oxidizes its food, and releases energy. The _cells_ of the human body built up into tissues have the same needs and perform the same functions as the paramoecium. It is the _cells_ of the body working together in groups as tissues and organs that make the complicated actions of man possible. Division of labor has arisen because of the complex needs and work of the organism.
[Illustration: The human body seen from the side in longitudinal section.]
The Human Body a Machine.--In all animals, and the human animal is no exception, the body has been likened to a machine in that it turns over the _latent_ or potential energy stored up in food into _kinetic_ energy (mechanical work and heat), which is manifested when we perform work. One great difference exists between an engine and the human body. The engine uses fuel unlike the substance out of which it is made. The human body, on the other hand, uses for fuel the same substances out of which it is formed; it may, indeed, use part of its own substance for food. It must as well do more than purely mechanical work. The human organism must be so delicately adjusted to its surroundings that it will react in a ready manner to stimuli from without; it must be able to utilize its fuel (food) in the most economical manner; it must be fitted with machinery for transforming the energy received from food into various kinds of work; it must properly provide the machine with oxygen so that the fuel will be oxidized, and the products of oxidation must be carried away, as well as other waste materials which might harm the effectiveness of the machine.
Most important of all, the human machine must be able to repair itself.
In order to understand better this complicated machine, the human body, let us briefly examine the structure of its parts and thus get a better idea of the interrelation of these parts and of their functions.
The Skin.--Covering the body is a protective structure called the skin.
Covered on the outside with dead cells, yet it is provided with delicate sense organs, which give us perception of touch, taste, smell, pressure, and temperature. It also aids in getting wastes out of the body by means of its sweat glands and plays an important part in equalizing the temperature of the body.
[Illustration: Skeleton of a man. _CR._, cranium; _CL._, clavicle; _ST._, sternum; _H._, humerus; _V.C._, vertebral column; _R._, radius; _U._, ulna; _P._, pelvic girdle; _C._, carpals; _M._, metacarpals; _Ph._, phalanges; _F._, femur; _Fi._, fibula; _T._, tibia; _Tar._, tarsals; _MT._, metatarsals.]
Bones and Muscles.--The body is built around a framework of bones. These bones, which are bound together by tough _ligaments_, fall naturally into two great groups, the bones of the body proper, vertebral column, ribs, breast bone, and skull, which form the _axial_ skeleton, and the appendages, two sets of bones which form the framework of the arms and legs, which with the bones which attach them to the axial skeleton form the _appendicular_ skeleton.
To the bones are attached the muscles of the body. Movement is accomplished by contraction of muscles, which are attached so as to cause the bones to act as levers. Bones also protect the nervous system and other delicate organs. They also help to give form and rigidity to the body.
[Illustration: Diagram showing action of biceps muscle. _a_, contracted; _b_, extended; _h_, humerus; _s_, scapula.]
Hygiene of Muscles and Bones.--Young people especially need to know how to prevent certain defects which are largely the result of bad habits of posture. Standing erect is an example of a good habit, round shoulders a bad habit of this sort. The habit of a wrong position of bones and muscles once formed is very hard to correct. This can best be done by certain corrective exercises at home or in the gymnasium.
Round shoulders is most common among people whose occupation causes them to stoop. Drawing, writing, and a wrong position when at one's desk are among the causes. Exercises which strengthen the back muscles and cause the head to be kept erect are helpful in forming the habit of erect carriage.
Slight curvature of the spine either backward or forward is helped most by exercises which tend to straighten the body, such as stretching up with the hands above the head. Lateral curvature of the spine, too often caused by a "hunched-up" position at the school desk, may also be corrected by exercises which tend to lengthen the spinal column.
[Illustration: Three classes of levers in the human body; bones and muscles act together. _A_, a lever of the first class; _B_, a lever of the second class; _C_, a lever of the third class.]
It is the duty of every girl and boy to have good posture and erect carriage, not only because of the better state of health which comes with it, but also because one's self-respect demands that each one of us makes the best of the gifts that nature has given us. An erect head, straight shoulders, and elastic carriage go far toward making their owner both liked and respected.
[Illustration: Bad posture in the schoolroom may cause permanent injury to the spine.]
Other Body Structures.--In spaces between the muscles are found various other structures,--blood vessels, which carry blood to and from the great pumping station, the heart, and thence to all parts of the body; connective tissue, which holds groups of muscle or other cells together; fat cells, scattered in various parts of the body; various gland cells, which manufacture enzymes; and the cells of the nervous system, which aid in directing the body parts.
Body Cavity.--Within the body is a cavity, which in life is almost completely filled with various organs. A thin wall of muscle called the _diaphragm_ divides the body cavity into two unequal spaces. In the upper space are found the _heart_ and _lungs_, in the lower, the digestive tract with its glands, the _liver_, _kidneys_, and other structures (see page 267).
Digestion, Absorption, and Excretion.--Running through the body is a food tube in which undigested food is placed and from which digested or liquid food is absorbed into the blood so that the cells of the various organs which do the work may receive food. Emptying into this food tube are various groups of gland cells, which pour digestive fluids over the solid foods, thus aiding in changing them to liquids. Solid wastes are passed out through the posterior end of the food tube, while liquid wastes are excreted by means of glands called _kidneys_.
Work done by Cells.--Food, prepared in the digestive tract, and oxygen from the lungs are taken by the blood to the cells. Bathed in liquid food, the cells do their work; they promote the oxidization of food and the exchange of carbon dioxide for oxygen in the blood, while other wastes of the cells are given off, to pass eventually through the kidneys and out of the body.
The Nervous System.--The smooth working of the bodily machine is due to another set of structures which direct the working of the parts so that they will act in unison. This director is the nervous system. We have seen that, in the simplest of animals, one cell performs the functions necessary to its existence. In the more complex animals, where groups of cells form tissues, each having a different function, a nervous system is developed.
_The functions of the human nervous system are:_ (1) _the providing of man with sensation, by means of which he gets in touch with the world about him;_ (2) _the connecting of organs in different parts of the body so that they act as a united and harmonious whole;_ (3) _the giving to the human being a will, a provision for thought._ Cooperation in word and deed is the end attained. We are all familiar with examples of the cooperation of organs. You see food; the thought comes that it is good to eat; you reach out, take it, raise it to the mouth; the jaws move in response to your will; the food is chewed and swallowed. While digestion and absorption of the food are taking place, the nervous system is still in control. The nervous system also regulates pumping of blood over the body, respiration, secretion of glands, and, indeed, every bodily function. Man is the highest of all animals because of the extreme development of the nervous system.
Man is the thinking animal, and as such is master of the earth.
REFERENCE READING FOR THIS AND SUCCEEDING CHAPTERS ON HUMAN BIOLOGY
ELEMENTARY
Hunter, _Laboratory Problems in Civic Biology_. American Book Company.
Davison, _The Human Body and Health_. American Book Company.
Gulick, _The Gulick Hygiene Series_. Ginn and Company.
Overton, _General Hygiene_. American Book Company.
Ritchie, _Human Physiology_. World Book Company.
Sharpe, _Laboratory Manual in Botany_, pages 218-225.
American Book Company.
ADVANCED
Halliburton, _Kirk's Handbook of Physiology_. P. Blakiston's Son and Company.
Hough and Sedgwick, _The Human Mechanism_. Ginn and Company.
Howell, _Physiology_, 3d edition. W. B. Saunders Company.
Schafer, _Textbook of Physiology_. The Macmillan Company.
Stiles, _Nutritional Physiology_. W. B. Saunders Company.
Verworn, _General Physiology_. The Macmillan Company.
XIX. FOODS AND DIETARIES
_Problems.--A study of foods to determine:--_ _(a) Their nutritive value._ _(b) The relation of work, environment, age, sex, and digestibility of foods to diet._ _(c) Their relative cheapness._ _(d) The daily Calorie requirement._ _(e) Food adulteration._ _(f) The relation of alcohol to the human system._
LABORATORY SUGGESTIONS
_Laboratory exercise._--Composition of common foods. The series of food charts supplied by the United States Department of Agriculture makes an excellent basis for a laboratory exercise to determine common foods rich in (_a_) water, (_b_) starch, (_c_) sugar, (_d_) fats or oils, (_e_) protein, (_f_) salts, (_g_) refuse.
_Demonstration._--Method of using bomb calorimeter.
_Laboratory and home exercise._--To determine the best individual balanced dietary (using standard of Atwater, Chittenden, or Voit) as determined by the use of the 100-Calorie portion.
_Demonstration._--Tests for some common adulterants.
_Demonstration._--Effect of alcohol on protein, _e.g._ white of egg.
_Demonstration._--Alcohol in some patent medicines.
_Demonstration._--Patent medicines containing acetanilid.