The General Plan of the Body. Before the principal systems of organs in the body are described, we should know something of its general plan.
The body includes head and trunk, arms and legs. The arms and the legs are practically solid masses of skin, muscle, bone, and other tissue, but the trunk contains two cavities which are not quite filled by the organs in them. The digestive system, a long tube, passes from the mouth down through both these cavities. The upper or thoracic cavity – (the chest) contains the heart and the lungs, as well as the upper part of the digestive system. The lower or abdominal cavity is nearly filled by the stomach, intestines, and other organs connected with the lower part of the digestive system. The cavities are separated by a dome-shaped muscle, the diaphragm.
The Bony System.The first thing we need in building any kind of machine is a framework, to support and hold together all the different parts. This framework in the human body is made up of the bones. The body is not stiff and rigid, however; it contains many separate bones, which are joined together in such a way that they can move easily at the joints. (See Fig. 6, page 20.)
There is, for example, a row of bones running up the center of the back, which make up the backbone. A series of bones runs down each arm and leg, and branches out into a chain of bones in each finger and toe. A series of hooplike bones, the ribs, protects the organs in the trunk; and a case of bones, the skull, incloses the delicate brain.
The Muscular System.About half the total weight of the body is made up of the muscles, which are the organs of movement. (See Fig. 7.) They hold the parts of the body in position and give it form and contour.
When you bend your forearm up, the motion is brought about by the shortening or contraction of a stout muscle, attached at one end near the shoulder and at the other end just below the elbow. Muscles move the tongue about and grind the teeth together when we chew our food. Another set of muscles keeps the eye fixed on what we are looking at. Other muscles move the tongue and lips and throat when we speak.
The muscles, however, do more than cause motions that can be seen. There are many movements going on inside the body that we cannot see at all. The heart is a sort of muscle which, by its movements, forces the blood to all parts of the body. The stomach and the other parts of the digestive system contain muscles which move the food along while it is being digested.
The Digestive System.In the digestive system the food is made liquid and changed into a form in which it can be absorbed. It is first broken up in the mouth by the chewing action of the teeth. Then it passes through the throat and a narrow tube (the esophagus) and goes into the stomach. From the stomach it gradually finds its way into the small intestine, a long coiled tube in which most of the digestion takes place; and finally whatever remains goes to the large intestine.
Digestive juices of various kinds are poured at many points into the mouth, stomach, and intestine, from organs, called glands, in which they have been made. After the food mass has been changed by the action of these juices, the part which the body can use is absorbed through the walls of the small intestine and passes into the blood. The waste material, which is of no use to the body, passes out through the large intestine.
The Respiratory System.The oxygen which the body needs is taken in by a pair of organs called the lungs, which occupy a large part of the chest and open into the throat by the windpipe. The lungs are bags or sacs with elastic walls, and every time we breathe, the lungs are enlarged so as to draw in air through the throat.
The lungs are liberally supplied with blood vessels. The walls between the blood vessels and the air passages in the lungs are very thin, and through these walls oxygen from the air passes into the blood. At the same time, carbon dioxide, one of the waste products of the body, passes from the blood into the air spaces and is expelled with the out-going breath.
The parts of the body that work together in respiration, or breathing, form the respiratory system.
The Circulatory System.We have seen how the food material passes into the blood after it has been changed in the digestive system, and how the oxygen reaches the blood in the lungs. The blood carries these necessary things to all parts of the body where they are needed.
The blood flows unceasingly through a system of fine tubes, the blood vessels, and the force that drives it is the beat of the heart, which by its regular contractions squeezes the blood into the vessels. Through the walls of the blood vessels, oxygen and food pass into the various tissues of the body. The organs concerned in the work of sending the blood through the body are called the organs of circulation.
The Excretory System.The blood not only carries food and oxygen, but it also takes away from the tissues the waste matter that is constantly forming and carries it to the special organs which discharge it from the body. The principal organs which serve to get rid of these waste materials are the lungs, the intestines, the liver, the kidneys, and the skin. Each of these organs takes out certain kinds of waste material. They are called the organs of excretion.
The Skin.The skin forms a cover for the soft internal parts to keep them from being injured or from drying up, and it is the organ through which we perceive heat and cold and exercise the sense of touch. It plays an important part in keeping the temperature of the body constant, and assists in excretion by throwing off some of the wastes in the form of perspiration.
The Nervous System.Perhaps the most important of all the systems of organs is the nervous system, which carries messages from one part of the body to another and keeps all the parts working together.
The nervous system includes first of all the brain, which lies inside the skull, and the spinal cord, which runs down from the base of the brain through the backbone. From both brain and spinal cord, bundles of fine white threads, called nerves, run out to all parts of the body. Some of these nerve threads bring in messages from the sense organs, and some carry messages out to the muscles and glands and other parts of the body, to set them in action. The brain and the spinal cord are the chief managers of the whole complex living machine.
The Sense Organs.In order that the body may per-form successfully the various things it has to do, it must have some means of learning what is going on around it. This is accomplished by means of the sense organs. If you try to walk or to draw or do almost anything with your eyes shut, you will realize how much you depend on the messages which come from outside. The chief sense organs of the body are the eyes and the ears. In the skin are special organs for receiving impressions of pressure, of heat, and of cold. Similar organs in the tongue receive impressions of taste, and others in the nose receive impressions of smell.
Each of the systems of organs which have been briefly considered here will be discussed in detail in succeeding chapters of the book.
HEALTHY LIVING QUESTIONS FOR DISCUSSION AND REVIEW
1. Name the systems of organs in the body. What is the special work of each?
2. Show by examples how the systems of the body are de-pendent on one another.
3. Describe some of the movements of the body brought about by the muscles.
4. What are some of the changes produced in the food in the process of digestion?
5. What two different processes go on in the lungs?
6. What are some of the uses of the skin?
7. Of what use are the sense organs?