CHAPTER I THE HUMAN BODY A COLONY OF CELLS (Part 1)

CHAPTER I THE HUMAN BODY A COLONY OF CELLS WueEn you view a brick house from a distance you do not see the bricks of which the house is built; but if you look at the house through a telescope or come close to it, you see clearly the bricks in the walls of the house. The house which from a distance appears to be one object is seen to be com- Fic. 1, Cells. A is a single cell as it appears under the microscope; B is a cell showing that it has length, breadth, and thickness; and Cis a group of cells. A cell found alone usually has a somewhat spherical shape, as shown in 4. When cells grow in groups they press against each other and usually have an irregular shape, as shown inc posed of a great number of smaller objects built together to form one whole. The human body is composed of many small parts called cells, When we look at the body we cannot see the cells; but when a small portion of flesh or skin or other part of the body is examined under the microscope, the little parts which 4 HUMAN PHYSIOLOGY make up the body can be distinctly seen. As the walls of the house are built of bricks, so the human body is built of cells. The Cell. A cell is a small portion of a transparent, jelly-like material called profoplasm.! Usually a cell has a thin wall about it, so that it is like a little sac filled with a clear, half-liquid substance. In each cell is a nucleus, which is a denser portion of the protoplasm. Both the nucleus and the less dense material? around it take in food and grow; both of them are alive. Taken together they are the protoplasm, the living substance of the cell. Living Things composed of Cells. As a heap of sand is composed of small grains, so are living things composed of very tiny cells# Every blade of grass, every weed, every flower, and every tree is made of cells. Every animal, whether it be large or small, whether it live in the water, on the land, or in the air, is composed of cells. Dead materials, like earth, stones, water, and air, are not made of cells, but there is nothing living that is not composed of cells. How Cells are formed. The ancient Egyptians thought that crocodiles and frogs came from the mud of the river Nile, and a great Grecian philosopher believed that insects sprang from the dew. A wise old German once told the people how mice could be created from wheat and stagnant water. Two hundred years ago, it was commonly believed that maggots came from meat and cheese, and that worms, insects, snails, and eels came out of decaying matter and 1In the back of the book the pupil will find a glossary that gives the pronun- ciations and meanings of many of the more difficult words. 2 The lighter portion of the protoplasm is called cyfoplasm. 5A few cells, for example, a frog's egg, are large enough to be seen with the naked eye, In general, however, cells are very small; so small that it would require twenty-five hundred cells from the human body to make a row an inch long. THE HUMAN BODY A COLONY OF CELLS 5 mud. Fifty years ago, many physicians and other scientific men believed that disease germs and other little microbes were formed from unclean and decaying matter, and many persons still think that this is true. We know now that all these ideas are incorrect. All living things, from the smallest germ to the greatest whale, are made of cells, and a cell can come, not from dead matter, but only from another living cell. Fic. 2. Cell division. The nucleus of the cell divides and part goes to each end of the cell, A wall is formed across the cell, dividing it into two parts, each of which {sacell. All new cells are formed in this way. The nucleus of a cell divides, and part of it goes to each end of the cell. Then a wall forms across the cell, and divides it into two parts. Each part is a new cell with its own nucleus, and each part grows as large as the parent cell, All new cells are formed in this way. “Every cell comes from a cell.” One-Celled and Many-Celled Animals. In a drop of stag- nant water many hundreds of little animals may sometimes be found,—animals so small that you can see them only with a microscope. One of these little animals has only one cell in its body. The animal is a single cell that swims about alone and lives by itself. When this cell divides, the two new cells separate, and each one forms a new animal. 6 HUMAN PHYSIOLOGY The bodies of all the larger animals (for example, the body of the chick in an egg) begin with a single cell, but when this cell divides, the new cells do not separate, like those of the one-celled animals. The cells remain together and keep on dividing and dividing until, in the body of a large animal, like a man, there are millions and millions of cells, more than you could count in many years. The Fic. 3. A one-celled animal dividing. This cell swims about in the water by means of small hair-like cifia which beat the water. When it divides, the new cells separate instead of remaining together as they do in the many-celled animals, difference between the little one-celled animals and the larger many-celled animals is therefore this: in one-celled animals, the cells separate after they divide and each cell lives alone. In the many-celled animals, the cells remain together after division, and live in a great colony. Your body, therefore, is a great colony of cells, and each cell in it corresponds to an entire one-celled animal. You might almost think of yourself as made up of a great com- munity of little animals, yet this idea would not be wholly correct, The cells of our bodies have learned to live to- gether. They would die if separated, and it takes them all to make one complete animal. THE HUMAN BODY A COLONY OF CELLS 7 Different Kinds of Cells do Different Kinds of Work. The single cell of a one-celled animal must do many different kinds of work to live. It has no hands to get food for it, no teeth to chew the food, and no stomach to digest it. It has no lungs to breathe in oxygen, and no kidneys to throw off its poisonous wastes. It lives all alone, with no other cells to help it, and it must do everything for itself. Each cell in the human body has the same needs as the little animal cell which lives alone. Each must have food, must get oxygen from the air, and must get rid of its poison- ous wastes. Many of our cells are shut up in the center of the body, where they can get neither food nor oxygen for themselves, and their waste matter would poison both them- selves and their neighbors if there were not some way of getting it entirely out of the body. Each cell cannot take care of itself, as does the little animal in the drop of water, You can easily see how much it would be to the advantage of all the cells in the body for each one to give up trying to do everything for itself, and for all of them to unite and work for the good of the whole community. This they have done. They have divided the work, and each cell has taken for its own some special task, The cells of the stomach digest the food; the bone cells build up a strong frame- work to support the body ; the muscle cells move the body; the kidney cells throw out 4 giandof the stom- wastes; the lung cells take in oxygen from ach. The function the air; and the cells of the blood (red blood c= OF these cells 10 digest the food, corpuscles) carry oxygen through all the body to the cells. Each cell is a skilled workman doing some particular work for the body as a whole, and not 8 Connective tissue. In its first stage connective tissue is a group of cells which build around themselves a mass of jelly-like material, as shown in A. ‘This material hardens into the fibers that are seen between the cells in B. Alll through the body a framework of connective tissue runs, holding the cells, organs, and tissues in place. Cells of the outer layer of the skin, ‘These cells form a protective covering One of the nerve cells from the brain, ‘These cells are asso- ciated with thought. for the body. ‘The outer cells die and dry up until they are mere scales. A muscle cell from the stomach, The muscle cells have the work of moving the body. Bone cells. deposit around themselves bone material (@), thus building bones to support the body. ‘The bone cells build a network of fibers like dense connective tissue and then fill the spaces between the fibers with hard mineral matter. a is a cavity from which the bone cell has been removed. Fat cells, Food for the body is stored in these cells, Large quantities of fat collect in the cell, and crowd the protoplasm (@ and 4) to one side, A fat cell is little more than a bag of oil, Fic, 5. Cells from the human body. Each kind of cell in the body has a Particular work to do for the body as a whole. THE HUMAN BODY A COLONY OF CELLS 9 an unskilled laborer trying to do all the many different kinds of work necessary to provide for its own wants. The Cells Dependent on Each Other. You will now under- stand that the cells in the body must depend on each other for many things. If the stomach fails to digest the food, there will be a lack of food in all the cells. If the kidneys do not throw off the wastes, all the cells will be poisoned. If the lungs stop taking in oxygen, all the cells must die for lack of oxygen. If part of the cells fail in their work, all the cells must suffer, and death usually comes to the body because part of the cells have ceased to work. The Body compared to a Community. The resemblance between the body and a community of people must now be very clear to you. In both the body and the com- munity we have individuals, each leading his own life and yet making a part of a greater whole. In the community we have individuals of dif- ferent occupations, — doctors, teachers, car- penters, blacksmiths, grocers, and milkmen. In the body we have, as we have seen, cells of different kinds, —muscle cells, bone cells, digestive cells, and many others. In both the body and the community the individual the tining of the tra- does not provide everything that he uses, chea. ais a cell that but depends on others for many things, ™anulactures sticky ‘ é mucus (é) inwhich dust The carpenter builds houses for the milk- and germsfrom the air man and the grocer, and these persons re aie alae bring the carpenter his food. The stom- Oe ae ach cells digest food for the cells of the the mucus, dust, and lungs, and the lung cells take in oxygen #erms up out of the air passages and lungs, for the stomach cells. Communities are prosperous and the body is healthy only 10 HUMAN PHYSIOLOGY when the individuals do their work faithfully and well; for no person in a community lives to himself, and no cell in the body lives to itself, but each has a share in the life of all. New individuals from time to time are born into the community and other individuals die. New cells Fic. 7. Cells from the blood, is a white corpuscle whose : function isto kill diseasegerms; are constantly being formed in the Bisanedgeviewand Cisaside body, and every day millions of cells view of the red corpuscles that carry oxygen through the body. i the body are destroyed. Com- munities increase in size when the number of births in them exceeds the number of deaths, and the body grows when the number of new cells formed in it is greater than is the number of the cells that die. Thus in many ways the body resembles a community of people where each individual is doing something for the good of all. Tissues. If you have now in mind what has been said about cells and their work, you will have no difficulty in understanding what is meant by ¢issues. In a factory we usually find the workmen who do the same kind of work all collected in one part of the factory. So in the body we usually find grouped together the cells which do the same kind of work. The great group of nerve cells is in the brain. The muscle cells are collected in the muscles, and the kidney cells in the kidneys. Where cells of one kind are grouped, a tissue is formed, so we may say that @ tissue is a group of cells which do the same kind of work. Muscle tissue is made of muscle cells, fatty tissue of fat cells, and nerve tissue of nerve cells. _ Each tissue is a group of cells which do some particular work for the body as a whole, and in return have many things done for them by the cells of other tissues. THE HUMAN BODY A COLONY OF CELLS Ir Organs. Ax organ is a part of the body that does a special work. The kidney is an organ for throwing out wastes, the lungs are organs for taking in oxygen, the eye is an organ for seeing, and the stomach is an organ for storing and digest- ing food. Some organs, like the liver, the kidneys, and the heart, are chiefly of one kind of tissue. Other organs have in them many kinds of tissues. The hand is an example of an organ of tlfis kind, bone, connective tissue, muscle, and skin all being united in it to form an organ for grasping. Why you should understand the Cell. It is very impor- tant for you to understand the cell, because a clear idea of the cell will give you a new way of thinking about all living things. Having an understanding of it, you will not only think of an animal as a living thing, but you will also think of the millions of cells in its body, each filled with living protoplasm. When you see a plant cut down, or an insect, or a frog, or a bird killed, you will not only think of how the plant or animal dies, but you will think of how all the little cells in its body also die; and from thinking of the living objects about you in this new way, you will become interested in many things which you do not now notice, and will understand much that now seems strange to you. When you see a rose living for several days after it has been cut from the bush and placed in water, or when you see a branch of a plant living and growing when placed in the soil, you will know that when a part of a plant is cut off from the body of the plant, its cells need not always die. If you read that when a starfish, or certain kinds of worms, are cut into pieces, each piece grows into a complete animal, you will see that in these animals, as in plants, the cells are more independent than in the human body, and that a group of them is able to live without the rest of 12 HUMAN PHYSIOLOGY the body. Also, when you learn to think of animals as groups of cells, you will not wonder that a frog’s legs should twitch and jerk in the skillet; or that after the head has been cut off, a snake’s tail can live and move, and a turtle can walk about. For you will understand that in these animals, as in the rose, part of the cells can live for a time without the others, and that the muscle cells are living and moving after the brain cells are dead. All these thingstand many others you can clear up for yourself, if you will think about the cell instead of about the body as a whole. A second very important reason why you should under- stand the cell is that you may intelligently care for your own bodies. Each of the great multitude of cells in your body is following out its own little life, and each is industriously work- ing for the good of the community in which it lives. We hope that you will understand that your cells must have proper food, oxygen, exercise, and rest, and that they must get rid of their waste matter. You should realize that any medicine you take can act on the body only by being carried by the blood through the body and entering the living proto- plasm of the cells. If it helps the cells, it is beneficial to the body, and if it injures the cells, it injures the body. You should understand how reckless it is to take in among all these delicate cells patent medicines, headache remedies, alcohol, or tobacco, unless you are perfectly certain that these things will not harm the cells. For the cells of your body are