CHAPTER I THE HUMAN BODY A COLONY OF CELLS (Part 2)

the most important things in all the world to you. If they become diseased, you will fall sick; and if they fail in their work, your life must cease. When the bricks crumble, the house falls; and when the cells are dead, the body is dead, for the life is in the cell. There is a third reason why you should understand the THE HUMAN BODY A COLONY OF CELLS 13 cell, and that is because in physiology we constantly study the work of the different kinds of cells. If you did not under- stand the figures 1, 2, 3, and 4, you would get very little pleasure or profit from trying to solve problems in arith- metic; and if you do not understand what a cell is and how the body is made up of cells, you will think that physi- ology is a dull subject indeed, and it will never mean much to you. But understanding the cell, you can study the differ- ent parts of the body intelligently, and in all the world you will find nothing more wonderful or interesting than your own body. Anatomy, Physiology, and Hygiene. Anatomy is the study of the structure of the body,—the study of the way all its organs are composed of tissues and its tissues made of cells, and of how all the organs and tissues are joined together to make one body. Physiology is the study of the function of the different cells, tissues, and organs, —the study of the work which all the different parts of the body do. Hygiene is the study of how to keep the body in health, These three subjects we must study in this book. Summary. The human body, like all other living things, is composed of cells. Each cell isa little piece of protoplasm that takes in food and grows and is alive. Cells are formed only by the division of other cells, and living things can come only from other living things of the same kind. They can- not come from dead matter, although this has often been believed. When a one-celled animal divides, the new cells separate. In many-celled animals the cells remain together after divi- sion, and the bodies of the larger animals are great colonies of cells. Each cell of the human body corresponds to an entire one-celled animal. Each cell in the body, like a one- 14 HUMAN PHYSIOLOGY celled animal, must have food and oxygen and must get rid of its wastes. It would not be possible for a cell in the body to supply all of its own wants, so each cell spends all its time in one kind of work, and depends on other cells for many things that are necessary to its life. It follows, therefore, that when part of the cells in the body fail in their work the whole body must die. The division of labor among the cells causes the body in a striking way to resemble a community where the people have different occupations and each one has many of his wants supplied by others. As the workmen who do the same kind of work in a fac- tory are often found together, so the cells that do the same kind of work in the body are found in groups. Such a group of cells—cells that do the same kind of work —is called a tissue. An organ is a part of the body that, like the hand or the eye, is fitted for some special work. It may be composed chiefly of one or of many kinds of tissues, A clear idea of the cell is a very great help in understand- ing all living things, in caring for our own bodies, and in the * study of physiology. Anatomy is the study of the structure of the body, Physiology is the study of the function of the different organs of the body, and Hygiene is the study of how to keep the body in health. QUESTIONS Of what is the human body composed? Describe or draw a cell. What kind of objects are composed of cells? How are new cells formed? Where do living things come from? Give some of the beliefs that have been held by different people in regard to the ori- gin of animals and plants. Describe the process of cell division. THE HUMAN BODY A COLONY OF CELLS 15 In a one-celled animal what do the cells do after division? What do the cells do after division in a many-celled animal? What in the human body corresponds to an entire one-celled animal? What are some of the needs of a cell? Explain why a cell in the body could not provide for all its own wants. Name some of the different kinds of cells in the body and explain the function (work) of each. What happens to the body if part of the cells fail in their work? Give examples of cells that must do their work to keep the body alive. Mention some ways in which the body corresponds to a community of people. What is a tissue? Name some of the body tissues. What is an organ? Name some organs of the body and give their functions. Give three reasons why it is important to understand the cell. © What is Anatomy? Physiology? Hygiene? If a cell and a peach were compared, what part of the cell would correspond to the seed of the peach? to the flesh of the peach? to the skin of the peach? In an egg there is one living cell lying on the side of the yolk. What is necessary to make this cell grow and divide? Of what use are the yolk and white of the egg to the cell within the egg? Into what have the yolk and the white been changed by the time the egg hatches? . Does the cell in a duck’s egg grow into a duck and the cell in a hen’s egg into a chick because the food supply in the eggs is different, or because the living cells in the eggs are different? Are the cells of your body always composed of the same materials, no matter what kind of food you eat? Can a branch be transferred from one tree to another and still live? Can a piece of tissue be transferred from one person to an- other? Ask a physician if cells from the body of an animal can be transplanted to the human body. What happens in a wound when it heals? Ask a physician how the cells in a scar differ from the cells in other parts of the skin.