DIAGRAM SHOWING THE RATE OF GROWTH IN MANDIAGRAM SHOWING THE RATE OF GROWTH IN MAN
cells. If we accept the rough estimate given above, about 60 per cent is then used for the actual manufacture of new protoplasm; the remainder is worked over by cells specially devoted for the purpose and put into place to serve as supporting structure, or to be held in reserve as fat. Living protoplasm is chemically a very complex mixture. In consistency it resembles a rather thin, transparent jelly; the thickness of the jelly depends on how much water it contains and this varies greatly in different kinds of protoplasm. The watery part of the protoplasm has dissolved in it several substances; among them may be mentioned ordinary table salt; also salts of potash and lime. Only tiny amounts of these are present, but it is a curious fact that without these tiny amounts of salts protoplasm cannot live. The chief solid substance in protoplasm is protein; this
A FACTORY’S COOPERATION IN AIDING PHYSICAL FITNESS—THE UNDERWEIGHT EMPLOYEES ARE GIVEN A MILK LUNCHA FACTORY’S COOPERATION IN AIDING PHYSICAL FITNESS—THE UNDERWEIGHT EMPLOYEES ARE GIVEN A MILK LUNCH
Photo, Paul Thompson THE WEIGHT TEST IN A CHILD’S PHYSICAL EXAMINATION TO DETERMINE HOW NEARLY IT APPROACHES CORRECT STANDARDS OF PHYSICAL DEVELOPMENTPhoto, Paul ThompsonTHE WEIGHT TEST IN A CHILD’S PHYSICAL EXAMINATION TO DETERMINE HOW NEARLY IT APPROACHES CORRECT STANDARDS OF PHYSICAL DEVELOPMENT
material, which is one of the most complex substances known to chemistry, has certain peculiarities which seem to fit it specially to serve as the chemical basis of life. Evidently of all the foodstuffs protein is the most important for the manufacture of new protoplasm, in other words for growth. In the case of a tiny one-celled animal, whose body is made up of protoplasm, not much else would be needed, but any animal that has a bony skeleton has to build this up to keep pace with the growth of the soft parts of the body. For this purpose mineral substances are needed, chiefly lime salts.
In addition to these foods which are actually used for making new body substance it has recently been discovered that proper growth in the higher animals, including man, depends on the presence in the diet of certain dietary accessories, whose use is not at all understood, although there is no doubt of their importance. These materials, to which has been given the rather cumbersome name of “growth-promoting vitamines,” are found dissolved in certain food fats. Apparently they are insoluble in water and soluble in oil. Most animal fats appear to contain them in small amounts, while most vegetable fats do not. Milk and eggs, which are growth foods in an especial sense, are richer in these accessories than any other articles of the diet. The discovery of these facts has emphasized the importance of including animal fats in the diet of growing children, milk and eggs particularly. Since milk is also rich in the lime salts which are necessary for bone formation it forms the best single foodstuff for children that there is. When very young children have to be fed on cow’s milk, whichdiffers somewhat in proportion from mother’s milk, it is often found necessary to feed the top milk diluted with water, instead of the whole milk. When this is done, lime water is usually used in part for diluting the milk, instead of all ordinary water. In this way the proportion of lime is brought up enough to insure that the child will get plenty of it.
In addition to the use of protein as a growth food it has another use which no other kind of foodstuff can share. This is also because protein is the foundation material of living protoplasm. We do not know a great deal about what goes on in living protoplasm to make up what we call the life processes, but we do know that these processes are of a chemical nature, and that in connection with them there is a steady wastage of protein. The protein that thus goes to waste is broken down into simpler chemical compounds which are expelled from the cells. Why this occurs we do not know, but since it does it is evident that unless the wastage is made good the time will presently come when so much protein will have been lost from the protoplasm that it can no longer exist as such and must die. As a matter of fact, one might go on a diet excessively rich in starchy foods and fats and still starve to death if there were no protein present. This use of protein is called cell maintenance to distinguish it from the other special use of protein in cell growth. Evidently, whatever may be missing from the diet, protein must not be left out. Fortunately most of our common foods contain protein. It is especially abundant in lean meat, in dried beans and peas, and in grain. Potatoes and most garden vegetables are deficient in protein, as are almost all common fruits. Breadand meat are our chief stand-bys as furnishers of protein.
Just as there are vitamines that are important for growth, so are there vitamines that are necessary for cell maintenance. Many years ago Dr. Sylvester Graham made himself prominent by arguing that the outer coats of wheat grains contain something that is needed in the diet, which is removed in the process of manufacturing white flour. He accordingly invented a form of flour, familiar to us all under his name, which includes some of the bran from the outer layers of the wheat. This idea, which originated with Dr. Graham, has since been substantiated, although not precisely as Graham had it. We know that there are necessary accessories to the diet, but we know, also, that they are much more widespread than Graham thought. They occur in so many kinds of foodstuffs that anyone who eats a mixed diet usually gets enough of them for his needs. The ill effects of their lack are most evident when the diet is restricted to a few kinds of food which happen not to contain them. A striking example of bodily injury directly due to the absence of these vitamines from the food is seen among Orientals whose diet is apt to be made up of rice plus small amounts of other substances. Of recent years the natives of Japan and China and the Philippines have suffered much from a disease of the nerves known as beriberi. Investigation has shown that this disease is due to the absence from the diet of needed vitamines, and dates from the time when rice-milling machinery was introduced. The old hand methods of milling rice were so imperfect that much of the hull was left clinging to the grains, but machinerypolishes the rice clean of every trace of hull. The hulls of rice contain the accessory that is wanting from the polished grains. Wherever it has been possible to bring about the use of unpolished (brown) rice instead of the usual polished kind, beriberi has disappeared. Or the same result can be secured by adding small amounts of beans to the diet. It is probable, also, that the hulls of most grains, including wheat, contain some of the same, or a similar accessory, so to that extent Dr. Graham was right in emphasizing the importance of adding hulls to the flour. Quite recently it has been shown that raw foods are richer in these accessories than cooked, and that ordinary compressed yeast contains more of them than any other easily obtainable material. Many people are being benefited by taking part or all of a yeast cake daily in a glass of milk.
For growth, or the making of new protoplasm, and for maintenance, or the repair of protoplasmic wastage, then, we must eat protein-containing foods, also foods containing various kinds of salts, and foods containing the necessary vitamines. All these are to provide required materials; the actual substances built into the protoplasm. There remains the requirement of power, for both growth and maintenance represent chemical activity on the part of the cell, and this activity depends on power just as does any other activity. In saying this we are merely saying over again in different words what was set down at the very beginning of the book as the chief sign of life, namely, the necessity on the part of living cells of continuous power development. The use of food as a source of energy or power has been talked about already, but it is necessary to say something about the different sorts ofpower development that may go on in cells, and since we shall have to talk about this a good deal, right here is a good place to bring in for the first time a word that has come to be used whenever the matter of the chemical activities of living cells is being mentioned. The word ismetabolism; when we speak of cell metabolism we mean the chemical processes that are going on in the cells. Hereafter, instead of saying power development, the word metabolism will be used as meaning practically the same thing.
First of all, in describing the various kinds of metabolism that cells may show, we have the metabolism of rest. By this we mean the power development that is going on when the cell is doing nothing more than keeping alive; neither growing nor showing any special activity. This is evidently the minimum amount that any cell can show, so it is often referred to as thebasic metabolism. We know of at least two things that may change the amount of basic metabolism; the first of these is a change in temperature; when a cell is cold, its basic metabolism is less than when it is warm. There is a very simple chemical reason for this, namely, that chemical processes as a rule go on more slowly the lower the temperature. Since all metabolism consists of chemical processes, this rule applies not only to basic metabolism, but to all other kinds as well, and, as we shall see, explains why the lower animals show such marked differences in behavior in cold and warm weather. The second thing that influences the amount of basic metabolism is the percentage of water in the protoplasm of the cell. Highly organized animals, like ourselves, are destroyed if the cells lose more than a small fractionof their water, but there are many of the lower animals that can be dried until their bodies contain only a very little water and still live. This applies to microscopic forms that live in puddles and similar places; when the puddle dries up the animal dries up too, until all that is left of it is a tiny particle of highly concentrated protoplasm. But this tiny particle preserves all the original cells, or at least enough of them to make a fresh start, and a very sluggish metabolism goes on in each cell. Of course, the advantage of this is that the stored food materials will not be used up as rapidly as they would if metabolism went on at the usual rate, and so there is a better chance that the animal may survive until more water falls or drains into the puddle, or until the particle of dust which the animal has become may be blown by the wind where it will fall into another one. Whenever either of these things happens the protoplasm takes up water again and the former rate of metabolism is resumed. It is only by means of this reduction in rate of metabolism that many kinds of animals are able to persist, for in large parts of the globe there is a period of each year when conditions become so unfavorable that the usual rate of metabolism could not possibly be maintained.
Next in order to basic metabolism comes the metabolism of growth, by which we mean the energy necessary for the making of new protoplasm. Not a great deal is known about growth metabolism; in fact, about the only reason for believing that it requires any energy at all is that the metabolism of young animals, whenever it has been studied, has been found to be greater in proportion than that of animals that are fully grown. It is hardto account for this, unless the growth process itself, namely, the making of new protoplasm, requires energy. When we think of the extreme complexity of living protoplasm, we can easily believe that its formation involves the expenditure of energy, perhaps in considerable amounts.
The last kind of power development to be considered is the metabolism of special activity. Most kinds of cells, particularly in highly organized animals, have some special kind of work to do. For example, the muscle cells have the task of making the motions; the gland cells of manufacturing the secretions, and so on. These we speak of as the particular functions of the cells, and the metabolism by which they are performed asfunctionalmetabolism. In some of the lower animals one can scarcely tell where basic metabolism leaves off and functional begins. There is a small shrimp, about a half inch long, that is found quite commonly in small ponds. This little animal has several pairs of legs by which he swims about, and the strokes of these legs go on continuously, day and night, with almost no interruptions, at the rate of a hundred or more a minute, for days or even weeks. It looks as though this, and other animals, that are continuously on the move, were organized without any sharp line between basic and functional metabolism; their protoplasm liberates energy by the oxidation of food, and various things happen as the result; among them are the maintenance of the protoplasm and the making of motions. In the higher animals the distinction between basic and functional metabolism is sharp, and, necessarily so, for the well-being of any of the higher animals requires that he shall have pretty complete controlover the activities of his protoplasm, and this he could not have if the functional metabolism were blended in with the basic. In other words, it is as important for bodily well-being that the cells be able to become inactive as that they be capable of activity.
AGOOD deal has been said thus far about living cells without anything at all having been said to tell what they look like, or how they are made up, beyond the statement that they consist of living protoplasm, which is of a jellylike consistency. To look at living cells through a microscope would almost surely be a disappointment at first, for protoplasm is so transparent that not much of its form can be seen on direct inspection. Fortunately for our knowledge of how cells are made up, protoplasm that has been properly killed and preserved takes stain very well, and different chemical substances in the protoplasm stain differently. Thus features that could not be made out at all in the living cells become clearly visible after killing and staining. The first thing that attracts the attention when cells thus prepared are studied is that every cell has somewhere within it, and usually near its middle, a spot which is more deeply stained than any other part of the cell. This indicates the presence of a substance or substances that take stain more readily than the mass of the protoplasm. This peculiarity led to the naming of the deeply staining portion of the protoplasmchromatin, referring to the ease of staining. The part of the cell which contains chromatin is called thenucleus. In many kinds of cells the nucleus can be made out by an expert observerwithout resorting to stains, although the details of structure cannot be seen in that way.
NUCLEUS OF A CELL, SHOWING CHROMATIN NETWORK (After Martin’s “Human Body”)NUCLEUS OF A CELL, SHOWINGCHROMATIN NETWORK(After Martin’s “Human Body”)
We now know that the nucleus, or rather the chromatin that it contains, plays a remarkable and interesting rôle in the life of the cell. To this we shall return presently. The remainder of the protoplasm, outside of the nucleus, shows the greatest possible variety of form, according to the kind of cell at which we happen to be looking. In some of the simpler types this part of the protoplasm seems to be merely a nearly uniform mass, perhaps with tiny particles scattered through it. In other types the protoplasm is drawn out into long slender threads, and these threads may have many branches; or the protoplasm may be distorted into a thin shell inclosing a mass of fat; or it may be subdivided into dense and thin portions with sharp lines of division between them. These various forms are related to the special functions which the cells have, and we shall learn more about them as we take up the different functions in order. On the whole, study of cell structure shows clearly that the protoplasm outside the nucleus carries on the greater part of the metabolism or power development, and is correspondingly important as the seat of the special functions shown by the cell. If it is a muscle cell,this is the part that does the moving; if a gland cell, this is the part that secretes. Nevertheless, the nucleus is a vital part of the cell. It has been definitely proven that a cell from which the nucleus is lost cannot survive more than a brief time. To gain some idea of the actual part played by the nucleus, we shall have to return to it in some detail.
DIAGRAM SHOWING CELL SUBDIVISION A, a cell; B to F, successive stages in its subdivision; a, cell-sac; b, cell contents; c, nucleus. (From Martin’s “Human Body”)DIAGRAM SHOWING CELL SUBDIVISIONA, a cell; B to F, successive stages in its subdivision; a, cell-sac; b, cell contents; c, nucleus. (From Martin’s “Human Body”)
Before undertaking a further description of the nucleus itself, we shall be helped to an understanding of its function if we trace briefly the history of the cells which make up our body. At the beginning, as we probably all know, we start life as a single cell. This cell, after a series of events which will be described in a later chapter, begins the process known as development. Development consists of a series of subdivisions of cell material. At first the single cell divides into two; each of these then divides, giving four. At the next stage eight areformed, then sixteen and so on, until finally the millions of cells that make up the body are produced, all derived from the original single cell. We know that in the adult body there are very many different kinds of cells. Since they are all derived from a single cell, these differences must have put in their appearance during the course of the various cell divisions. In fact, this happens all along; at definite points in the process the two cells that come from the subdivision of some particular one will not be alike. The special kinds of cells that are thus produced become the starting points for whole masses of similar cells in the fully developed body. In human beings, and probably in most other kinds of animals, the very first subdivision does not result in any difference between the cells. The proof of this is that sometimes, in fact fairly often, the two cells become separated. When this happens twinning results, and the twins are exactly alike, being known as “identical twins.” Not only are they alike in all other respects, but they are always of the same sex, a fact that has escaped the attention of some writers of fiction, who have made twins, identical in all other features, brother and sister, instead of both boys or both girls. Twins that are not identical come from different original cells that happened to start developing together. Such twins need have no more resemblance than any members of the same family, and may or may not be of the same sex.
In every cell division the first step consists in a division of the chromatin of the nucleus, which is followed by a division of the rest of the protoplasm. The process by which the chromatin is subdivided is so curious as to be worth a brief description. The
DIAGRAM ILLUSTRATING DIVISION CHANGES IN A CELL NUCLEUS WITH FOUR CHROMOSOMES (From Martin’s “Human Body”)DIAGRAM ILLUSTRATING DIVISION CHANGES IN A CELL NUCLEUSWITH FOUR CHROMOSOMES(From Martin’s “Human Body”)
chromatin material is not a simple lump in the nucleus. It looks rather like a tiny string of beads thrown down carelessly, so as to become all mixed together. Each bead is a single bit of chromatin, and these bits are strung on a tiny thread. In an ordinary cell the beads are so mixed together that no order can be distinguished among them, but if a cell that is about to begin dividing is looked at it is found that the string has straightened itself out, and also that it has broken into pieces. The individual pieces are calledchromosomesand their number is always the same for any one kind of animal or plant. There is a parasitic worm whose cells have only four chromosomes, and the number ranges from this up to as many as forty-eight in human beings. It may be that other species have even more, but they become so hard to count when there are as many as forty-eight that the number cannot be stated with certainty. So far as can be judged, the number of chromosomes has little to do with the complexity of the animal or plant, for some complex forms have few chromosomes, and some simple forms many.
At the same time as the chromatin is breaking up into chromosomes two tiny spots put in their appearance in the protoplasm of the cell on opposite sides of the nucleus, and tiny threads extend from one spot to the other through the nucleus. There are as many threads as there are chromosomes, the whole group making up a spindle-shaped figure. The chromosomes now become arranged at the middle of the spindle, and apparently each chromosome becomes fastened to a thread. Next each chromosome splits lengthwise through the middle and by what looks like a shortening of the threads the split halvesare pulled apart and drawn to opposite tips of the spindle. The purpose of this elaborate scheme seems to be to insure an exactly equal division of the chromosomes between the cells, and the necessity of such an equal division will become clear when we learn something of what the chromatin is for. Meanwhile the description of cell division can be finished by saying that after the halves of the chromosomes are pulled apart the whole mass of protoplasm divides through the middle. As we stated above, sometimes the cells thus produced are alike and sometimes they are different, according to whether they are destined to become parts of similar or of different structures. In either case the chromatin material that goes into the two cells is exactly alike, so that if the cells themselves become different there must have developed a difference in the protoplasm at the two ends of the cell from which they came. Our bodies are made up of millions of cells, of a great many different kinds, but however different they may be the chromatin of each exactly duplicates that of every other one, or did when the cells were first formed; there is reason to believe that the chromatin may become changed during the lifetime of the cells, at least in some cases.
We may be interested in inquiring how long this process of cell division keeps up. Many children do not get through growing until they are twenty years old or more. Does cell division keep on during all this time? More than that; are there any cases of cell division that continue after full growth is reached? The answer to both these questions can be given in a brief paragraph. There are some tissues, particularly the outer layer of the skin, theconnective tissues, the blood-corpuscle-forming tissues, and the reproductive tissues, in which cell division continues during all or most of life. The others finish at birth or shortly thereafter. We are born with the precise number of muscle cells with which we shall die, unless accident deprives us of some meanwhile; and if this happens no new ones will be formed to replace those that are lost. The same is true of gland cells. The last cell divisions among nerve cells are believed to occur within a few months after birth. As most of us have observed in our own cases, bodily injuries, if at all severe, are followed by the formation of scars. This means that connective tissue has grown in to fill the place of the cells destroyed by the injury, which cannot be replaced by cells of their own sort, since they have lost the power of cell division.
We have tried, in the above paragraphs, to get some idea of what living cells are like, and how they are derived, but have not attempted any detailed picture of particular kinds of cells. That will have to wait till we reach the story of the different kinds of bodily activity, when the cells that carry on each kind will have to be described more exactly. Something has also been told of the chromosomes, but the full account of them and their meaning is to be taken up in a later chapter, devoted to the matter of heredity and reproduction. In what remains of the present chapter we wish to talk about the conditions in which cells live so that we shall easily picture how they carry on their metabolism.
As an introduction to this topic a word may be said about the wide differences of complexity that are found in animals. They range from the simplestimaginable, a single cell with its nucleus and with protoplasm that appears almost uniform throughout, to a highly organized body like that of man, composed of millions of cells of many different kinds. Between these extremes almost every possible form is seen. The one-celled animals themselves show a wide range of complexity, and as soon as animals begin to be formed of numbers of cells grouped together the possibilities of complexity increase in proportion. One important difference between one-celled and many-celled animals needs to be emphasized; that is the matter of size. There are definite limits to the size that a single cell may attain; these limits are just over the boundary of naked eye vision. If animals are to attain larger sizes, they must necessarily be composed of many cells. The life of a single-celled animal presents no special problem, since it has only to take in through its outer layer from the surrounding water the various food materials and the oxygen which its metabolism requires, and to discharge into the same water any chemical products that may result from that same metabolism, and the question of whether it will live or die depends only on whether the water in which it happens to be contains sufficient materials and is otherwise suitable as a place to live. A many-celled animal, whose cells are arranged in not more than two layers, is in practically the same situation, for every cell has a frontage on the water and so can carry on interchanges of material directly; but the moment complexity reaches a stage where any cells are buried beneath other cells some special arrangement must be provided so that the buried cells can obtain the needed substances for their metabolism. The arrangement consists, in general, of furnishingwhat may be called an internal water frontage for the buried cells. In other words, complex animals have spaces all through their bodies, and these spaces are filled with fluid. There are no living tissues so dense that the cells of which they are composed are completely cut off from contact with body fluid. In thinking of our own bodies we should realize that this same arrangement applies; every one of our millions of living cells has contact with the fluid with which all the spaces of our bodies are filled, and it is from this fluid that the cells obtain the materials for their metabolism, and into this same fluid they discharge whatever substances their metabolism may produce.
The total amount of body fluid is not large, for the spaces among the cells are in most cases extremely tiny; it follows that with all the millions of cells absorbing food materials and oxygen from this fluid and discharging waste materials into it the time will soon come when no more food or oxygen will be left to be absorbed and there will be no more capacity for holding waste substances. If this state of affairs were actually to happen, metabolism would come to an end and death would be the result; evidently there must be some means of keeping the body fluids constantly renewed in respect to the things which the cells need for their metabolism, and constantly drained of the waste substances which the cells pour out. The way in which this renewal is accomplished is simple; part of the body fluid is separated off from the rest in a system of pipes, known to us as the blood vessels, and this part is kept in motion; at intervals along the system are stations at which the moving fluid can exchange substances with the fluid which actually comes in
DIAGRAM SHOWING HOW THE MOVING BODY FLUID RENEWS THE STATIONARY, AND IS ITSELF RENEWED IN LUNGS, DIGESTIVE TRACT, AND KIDNEYSDIAGRAM SHOWING HOW THE MOVING BODY FLUID RENEWS THE STATIONARY, AND IS ITSELF RENEWED IN LUNGS, DIGESTIVE TRACT, AND KIDNEYS
contact with the cells; thus the stationary fluid can obtain from the moving fluid the materials which the cells, in turn, are constantly withdrawing from it, and can pass on to the moving fluid the products with which the cells are continuously charging it. All that is necessary to complete the successful operation of the system is to have additional stations at which the moving fluid can obtain supplies of food materials and of oxygen, and stations where it can get rid of the wastes which it accumulates from the stationary fluid, and there must be a pump by which the moving fluid is kept in motion. We are familiar with the moving fluid under the name of blood; the system of pipes in which it moves are the blood vessels; the pump which keeps it in motion is the heart;the various supply stations include the digestive organs, the lungs, and the kidneys. In later chapters the operation of all these stations will be described in detail. The present outline has been given to show in a general way how the problem of metabolism is handled in highly organized bodies in which the individual cells have no direct access to food or oxygen supplies.
SINCE protoplasm is so very soft and fragile it must be supported in all animals and plants except the very tiniest. The nature of the supporting framework has a great deal to do with both the form and the working of the body, so it is desirable that we become familiar with it before trying to go further in the examination of the living protoplasm itself.
A large heavy body like that of man requires an arrangement for support that shall meet several conditions. In the first place there must be strength and stiffness, combined with flexibility, so that the body as a whole shall be firm, yet not rigid. The weight, also, must be kept as small as possible. Then every single cell, and every grouping of cells that we call an organ, must be supported in its place securely but without hindering the free performance of its function. Not only must the protoplasm be held in place, but on account of its fragility it has also to be protected against injury; the vital parts require more careful protection than those that are less immediately essential to life. Finally, bodily motions of all sorts depend on the framework to give purchase to the muscles, which are the actual organs of motion, and so to make their movements effective. For support, for protection, and for motion, then, the framework is important.
The material that does the real supporting is not, of course, alive, for living protoplasm lacks the necessary qualities needed here. It is manufactured and put in place, however, by living cells. They do this by withdrawing the special materials needed from the body fluid which surrounds them; in large part what they get from the fluid is not the finished substance but material from which the living cells make the finished substance. It is then passed outside their bodies and deposited in the surrounding space. Of course this is a gradual process. Bit by bit the structure, bone, cartilage, or connective tissue, as the case may be, is built up by the combined activities of many cells.
Of the three kinds of supporting material mentioned above, bone is the most familiar. No description of its appearance is necessary, for everyone has seen it as it appears in meat animals and in poultry, and it looks precisely the same in man. There are several things about bone, however, that are worth describing. One is the arrangement by which the very hard, compact material is deposited in large masses without cutting off the cells which are doing the depositing from their contact with the body fluid, and so destroying them and bringing their work to an end. The way this is managed can be made out by examination of the figure, showing the structure of bone. At the beginning the bone cells are lying near one of the tiny blood vessels known ascapillaries, which are the exchange stations for material between blood and the stationary part of the body fluid. Thus these cells are favorably located for obtaining materials from which bone can be constructed. As they proceed with the formation of bone they always leave tiny passages openbetween themselves and the blood capillary. Finally the capillary may become completely surrounded by bone, but all along it will be left the passages through which fluid can make its way from the blood to where the cells are imprisoned within the bony walls of their own construction. The metabolism of bone cells is not on a very active scale; the amount of bone substance that a single bone cell has to produce in a day is only a fraction of the amount of saliva, for instance, that a single cell of the salivary gland turns out in the same time; so the bone cell can manage even though its supply of material has to come to it through a few very tiny passages in the bone.
CROSS SECTION OF COMPACT BONE FROM THE SHAFT OF THE HUMERUS A, bone cells; B, blood capillaries. (From Martin’s “Human Body”)CROSS SECTION OF COMPACT BONE FROM THE SHAFT OF THE HUMERUSA, bone cells; B, blood capillaries. (From Martin’s “Human Body”)
Another interesting feature of bone is the ease with which it can be remodeled. We are apt to think of bone as permanent, after it has once been formed, but as a matter of fact bone is about as subjectto change as any of the softer tissues. This is because there are in and around the bones, in addition to the bone-forming cells, a great many cells of different appearance which may be named bone-destroying cells. These latter have the ability to dissolve out the hard material which the bone-forming cells have deposited. Good examples of their work are seen in the hollows of the long bones. We know, of course, that the bones in a child’s leg are so much smaller than those in the leg of an adult that they could almost be fitted into the hollows of the latter. Evidently the bone substance has been moved bodily outward during the course of growth. As the bone-forming cells add material to the outer surface of the bone, the bone-destroying cells dissolve it away from the inner surface. The same thing happens all over the body. A child’s face grows by an increase in size of the bones. Again the inner surfaces are dissolved away. Apparently one condition which makes the bone-destroying cells active is constant pressure. A good example of this is seen in what is known as a gumboil. If a tooth becomes ulcerated, gas and pus are formed at its root, and cannot escape since this is completely surrounded by bone. They accordingly press upon the surrounding bone, and also upon the sensitive tissues, resulting in extreme pain. The pressure upon the bone starts the bone-destroying cells into great activity and in the course of a few days they will dissolve a hole right through the bone, allowing the gas and pus to escape to the outside, and relieving the pain.
Of recent years school authorities have had much to say about the importance of adjusting school seats and desks so that they shall be at the proper heightfor the particular children that are to occupy them. This is because if the feet hang clear of the ground for hours at a time, as they will if the seat is too high, or if the body must be screwed around to enable the child to work at his desk, as happens when the desk is too low, there is real danger that some of the bones may become misshapen. Most of the stoop shoulders and many of the crooked backs that we see are the result of the habitual taking of wrong postures. Children, and adults as well, should form habits of standing and sitting so straight that none of the bones are put under a pressure that may tend to distort them.
After the teeth are lost the bony sockets in which they lie are dissolved away, making the jaws much shallower than formerly, a fact that accounts for the shortening of the distance between chin and nose in aged people. An important result of this dissolving away of bone by the bone-destroying cells is that the bones are kept as light as possible, without undue sacrifice of strength.
A second kind of supporting material is cartilage. This is both softer and more flexible than bone. It is found in places where flexibility is more important than great strength, as in the ears, the parts of the nose just below the bridge, the Adam’s apple and wind pipe. The chief difference in make-up between bone and cartilage is that while in bone about three-fourths of the nonliving substance consists of lime salts, in cartilage there is almost none of this material, organic substances making up the entire mass. There are no living cells in the body that are more poorly located with respect to obtaining supplies from the body fluids than the cartilage cells, for as these deposit the cartilage around themselves theyleave no definite passages through which fluid may pass; the material incloses the cells completely. Although cartilage looks as though it were altogether nonporous, there must be some degree of sponginess present, since the cells do succeed in getting the materials on which their life depends. Cartilage seems to be a more primitive kind of supporting substance than bone. This is shown by the fact that it makes up the entire skeleton in the lowest fishes, and also by the fact that in the higher animals, including man, the bony skeleton starts, in large part, as cartilage. In the parts in which this happens a mass of cartilage is deposited in the place which is later to be occupied by bone. Then at certain points the cartilage begins to be dissolved away by cartilage-destroying cells, which are precisely like bone-destroying cells, and the bone-forming cells come in and build up the real bone as fast as the cartilage is removed. This process of replacing cartilage by bone is practically completed at birth, except in the long bones of legs and arms. These bones, which will about double in length during the growth of the body to adult size, as well as the other bones, which grow to some extent, retain plates of cartilage near each end during all the growth period, and the increase in length is obtained by a continuous formation of new cartilage, which is continuously replaced by bone.
The third kind of supporting material is connective tissue. This is composed of tiny threads or fibers, some of which are inelastic, others are very elastic. The inelastic fibers are found in places where a flexible, but unyielding support is required; the elastic fibers are located where elasticity is particularly important. Either kind of fiber may begrouped into sheets, or into loose networks, or into stout cords. A good example of inelastic connective tissue in sheet form is in themesenterywhich holds the organs of the abdominal cavity in place. Just under the skin, anchoring it loosely to the underlying muscles, is connective tissue in network form. The tendons by which most of the muscles are attached to the bones upon which they pull are made up of inelastic connective tissue in the form of cords. The best example of elastic connective tissue is in the large arteries, which are just as elastic as best quality rubber tubing. Another good example is the large and strong elastic cord which passes along the back of the neck in cattle and sheep, and helps to support the weight of the head. Connective tissue fibers are deposited by living connective tissue cells. Since connective tissue is of open and relatively loose construction, there is no problem presented in supplying the cells with material. The meshes among the fibers are filled with fluid, and this fluid has ready connection, in turn, with the blood. Use is made of the abundance of body fluid in the connective tissue spaces whenever a subcutaneous injection is given, for what is done is to inject the desired material into the fluid which fills the spaces in the connective tissue just beneath the skin, trusting that it will pass from there to the blood, which it does rather gradually, and so is distributed about the whole body.
While we are on the topic of the supporting framework, something must be said about the grouping of the bones into what we know as the skeleton. Of course it is evident that the effectiveness of the bony part of the framework depends almost altogether on the way in which the individual bones are grouped together. If the whole skeleton were
THE BONY AND CARTILAGINOUS SKELETON (From Martin’s “Human Body”)THE BONY AND CARTILAGINOUS SKELETON(From Martin’s “Human Body”)
composed of one great bone, or of different bones anchored solidly together, the body would be perfectly rigid; since motion is necessary to life, flexible connections between some of the bones are absolutely essential. Our movements are actually madeby means of muscles, but nearly all of them become effective through the motions of bones to which the muscles are fastened. The bones are often very irregular in shape; careful study shows that the irregularities are due either to provision for the contact of one bone with another in the joints, a contact that must allow in most cases for motion of one on the other, or to provision of places to which muscles can be fastened in such a way as to make their pull effective. It is, of course, out of the question for us to examine the skeleton bone by bone. Figures are given of a number of typical bones: all that we can do in addition is to mention some of the interesting features of the skeleton.
The skeleton of the head is called the skull; its chief features are the brain case, the eye sockets, and the parts about the nose and mouth. The brain case is made up of eight bones firmly joined together by saw-tooth margins to make up a roughly spherical box which holds the brain, and protects this delicate and vitally important organ from all injury except the most severe. There are a number of small openings out from the brain case through which nerves pass, and one large opening below and at the back through which the spinal cord merges into the brain. The bones which make up the sides of the brain case are much thickened just behind the ears. A hollow extends from each ear into the bone, and within this hollow, securely protected from harm, is the actual organ of hearing. There are extensions of the hollow backward which are not occupied by any organs, and which communicate with the cavity of the ear. These sometimes become infected from the ear, causing the condition known as mastoiditis. Not only is this condition excruciatingly painful, buton account of the thin layer of bone which separates it from the brain itself it is highly dangerous. For this reason any ear trouble should be carefully watched lest it develop into mastoid trouble.
A SIDE VIEW OF THE SKULLA SIDE VIEW OF THE SKULL
Of the bones that make up the eye sockets not much need be said, except that they have a great deal to do with determining the shape of the upper part of the face and so the appearance. There are bones within the nostrils that are very irregular in outline. Their effect is to increase greatly the surface over which the air that is breathed must pass, enabling it to become both warm and moist before entering the lungs. The jaw bones serve as receptacles for the teeth; the lower jaw, which is theonly movable bone of the head, except for the tiny bones within the ears, has also the duty of operating as a mill in reducing the food to suitable form for swallowing. To aid in this function the lower jaw is hinged to the rest of the skull in such a way that it not only opens and closes but can slide forward and back or from side to side. All these motions are used in chewing. There are twenty-two bones altogether in the skull, not counting the three tiny ones in each ear which will be described later.
FRONT VIEW OF TRUNK AND LIMB ARCHES c, collar bone; s, shoulder blade; Oc, innominate bone (From Martin’s “Human Body”)FRONT VIEW OF TRUNK AND LIMB ARCHESc, collar bone; s, shoulder blade; Oc, innominate bone (From Martin’s “Human Body”)
SIDE VIEW OF THE SPINAL COLUMN C 1-7, cervical; D 1-12, dorsal; L 1-5, lumbar; S 1, sacrum; Co 1-4, coccygeal. (From Martin’s “Human Body”)SIDE VIEW OF THE SPINAL COLUMNC 1-7, cervical; D 1-12, dorsal; L 1-5, lumbar; S 1, sacrum; Co 1-4, coccygeal. (From Martin’s “Human Body”)
The body consists of trunk and limbs, and each part has its skeleton. The skeleton of the trunk consists of the spinal column, the rib cage, the shoulder girdle, and the hip girdle. The skeletons