Читать книгу Physiology: The Science of the Body - Ernest G. Martin - Страница 10

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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 as capillaries, 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 open between 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”)

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 subject to 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 height for 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 they leave 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 be grouped into sheets, or into loose networks, or into stout cords. A good example of inelastic connective tissue in sheet form is in the mesentery which 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”)

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 made by 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, but on 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 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 the only 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.