Читать книгу Physiology: The Science of the Body - Ernest G. Martin - Страница 12

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OUR account of the body has now reached the point where we can take up in detail the special activities of the different kinds of cells. The first to be considered is motion, both because it is the familiar sign of life, as pointed out in the first chapter, and because it has so much to do with everything that enters into life. There are probably no animals that live out their entire lives without making any active motions, although some parasites, like the tapeworm, are stationary most of the time. There are a number of different ways in which movements are brought about. The very simplest animals, which consist of nothing but a bit of protoplasm, move by causing the protoplasm to flow bodily in one direction or another, a projection of part of the protoplasm being balanced by withdrawal of an equal part on the opposite side, and the whole mass progresses in the direction of the first projection. Next beyond this simplest means comes motion by tiny threads of protoplasm which project beyond the surface of the cell and whip back and forth. The stroke of these threads or cilia, as they are called, is stronger in one direction than in the other, so the effect of their beating is to propel the cell of which they are part in one direction through the water; or if they are on a surface which is stationary they set up a current in the water itself. This latter is the means by which oysters and similar animals which are anchored to the rocks get their food supplies. In some one-celled animals there are only one or two large cilia at one end; these beat back and forth, propelling the animal much as a fish swims.

The commonest, as well as the most effective, means of making motions is by cells specially developed for that purpose. These are called muscle cells, and every highly organized animal depends on them for most if not all of the motions which take place in its body. In muscle cells the functional metabolism takes the form of forcible changes in shape of the cells by which bodily motions are brought about. A muscle cell might be described as a mechanical device for transforming the chemical energy of burning fuel into the energy of motion. We have something comparable in the automobile cylinder, where the energy obtained from the explosive burning of the air-gas mixture drives the piston and so propels the car. Of course the two devices are not even remotely alike in the actual way in which they operate; their resemblance is purely the general one of converting one type of energy (chemical) into another type (motion).

There are three kinds of muscle cells in our bodies. The simplest are those that are found in the wall of the stomach and intestines and other internal organs that are capable of movements; the next kind is found only in the heart; the third, and most complex, makes up the bulk of our muscular tissue; it is the muscle that moves the bones. The first kind, because it shows no particular markings when examined through the microscope, is usually called smooth muscle; the second kind is known as heart muscle; the third kind, because it moves the skeleton, is named skeletal muscle. We shall devote most of our attention to this third kind of muscle, because it is a much more efficient machine than the others, and also because it has to do with our general bodily movements instead of with motions of internal organs.

A single skeletal muscle cell is an exceedingly slender fiber, much smaller than the finest thread; it may also be very short, not more than a twenty-fifth of an inch long, or it may be as much as an inch long. A muscle is made up of many of these fibers grouped side by side in bundles, and also, if the muscle is long, placed end to end. The fibers are held in place and fastened together by connective tissue. Lean meat consists of thousands of these muscle cells with their connective tissue fastenings. In coarse meat there is relatively more connective tissue and less actual muscle tissue than in the finer grades. In every muscle the connective tissue is loose enough to allow body fluid to penetrate among the muscle cells. Blood vessels are also distributed through the mass of the muscles between and among the cells; thus their nutrition is provided for.

Although not all muscle cells are exactly equal in power, on the whole the force that muscle can show is the force of one cell multiplied by the number of cells that can join in the pull. A strong muscle must have many cells side by side; in other words it must be thick. Also, the distance through which muscles can make movements depends on their length, so a muscle that has to pull for a considerable way must be long, and since single muscle cells are short there will have to be a good many cells end to end to make the whole muscle long enough for its task. The actual make-up and arrangement of muscles in the body depends in part, therefore, on the thickness and length needed for the particular work to be done, and in part on the architecture of the part of the body where the muscle is located. For example, the strongest muscle in the body is that by which one rises on the toes. This muscle operates by pulling upward at the back of the heel. If it were located right at the ankle, where it would have to be if attached directly to the place where its force is exerted, the resulting clumsiness can easily be imagined. By shifting it up to the middle of the lower leg room is found for the large mass of muscle needed for the work. The connection with the heel is made by means of a long and very strong tendon, known as the tendon of Achilles, because that was the part Achilles’ mother failed to immerse when she was dipping the infant in the river Styx to make him invulnerable. Other equally good examples are the muscles for operating the fingers. If placed in the hands, the latter would be too bulky and clumsy for any kind of efficient use. By placing them up in the forearms out of the way, and connecting them with the fingers by long tendons, delicacy is secured for the hands.

The muscles of the arms and legs are arranged in groups about the joints, and these groups always include opposing sets. Thus if the joint is a simple hinge, as at the elbow, where the only motion possible is back and forth, there will be one muscle to bend the joint and another opposing muscle to straighten it out again. The first is known as a flexor; the second as an extensor. In the arm the biceps, on the upper surface, is the flexor and the triceps, on the under side, the extensor. Joints that permit of motion in several directions have correspondingly more opposing sets of muscles acting upon them. The same scheme applies to the trunk, but since in the trunk instead of a few very movable joints we have the whole row of slightly movable vertebræ, the grouping of muscles is more complicated. Not all the skeletal muscles work about joints. The tongue, the muscles of the lips and about the eyes, those along the front of the abdomen, and some others are attached to bones only at one end or not at all, and do their work by pulling upon one another.

THE BICEPS MUSCLE AND THE ARM BONES (From Martin’s “Human Body”)

In earlier paragraphs we have seen that the movements made by muscles represent their functional metabolism, and also that the actions of whole muscles are merely the sum of the actions of the individual cells. Our present task is to see how muscles act; in other words to examine their functional metabolism. One feature that must be in mind from the very beginning is that the functional metabolism of muscle cells is under control; they do not go off at random, but only when started. This is more or less true of the functional metabolism of all the cells in highly organized animals. The agency that starts them off is named a stimulus. To picture how stimuli act we shall have to think for a moment of the state of affairs in cells at rest. As we have tried to make clear, cells at rest are not stagnating; a more or less active basic metabolism goes on within them all the time. This metabolism is of such a sort that it does not disturb the balance existing within the cell. The various chemical processes go on, using up material and producing wastes, but without arousing the additional chemical processes of functional metabolism. Meanwhile the substances that are required for this latter are present in the cell, so that when the disturbance that we call a stimulus comes along there is an increase in the total amount of metabolism, the extra chemical processes being those which perform the special function of the cell. In the case of muscle cells the stimulus ordinarily reaches them by way of the nervous system, although electric shocks, sharp blows, some irritating chemicals, and perhaps one or two other kinds of disturbance can act as stimuli. The effect of the stimulus is to start certain chemical processes; these in turn bring about the forcible shortening which is the thing that happens in active muscle. In skeletal muscle the shortening may be very rapid; the muscle can contract and relax again more quickly than the eye can follow. This is true at the temperature of our bodies. In cold-blooded animals, like fish or frogs, muscles become sluggish when they are cold. We see here one of the advantages we enjoy in having bodies that stay at the same temperature the year around; if our bodies cooled off in cold weather as do those of frogs, we should have to do as they do, become inactive whenever the weather becomes cold. As each muscle cell shortens it pulls upon the connective tissue that surrounds it; this communicates with the connective tissue of other cells, and all the connective tissue within the mass of the muscle fastens to the very stout sheets or cords of the same at the ends which are called tendons, by which the muscles are attached to the bones. Thus, although the pull of any single cell is so feeble as to be scarcely measurable, when hundreds or thousands of them pull all at once the effect may be very powerful.

We are familiar with the very wide range of effort that our muscles can show. They may contract with utmost delicacy, as when we hold a humming bird’s egg in our fingers, or they may pull with a force, in our largest muscles, of several hundred pounds. Of course this possibility of variation is of great advantage in our use of our muscles. It depends upon the very large number of individual fibers of which even our smallest muscles are made up. Whenever any single fiber contracts, it pulls to its full extent; if only a few become active, the pull of the whole muscle will be slight; as more come into action, more force will be exerted; the muscle will show its utmost power when all the fibers are contracting at once. We are conscious of greater mental effort when we make a powerful muscular contraction. This can be explained as due to the greater nervous discharge required to excite all the muscle fibers at once.

One feature of muscular action calls for an additional word. This is the temporary loss of power, resulting from too long-continued use, which is called fatigue. We know that a well-constructed machine can operate day in and day out without having to stop to rest; why cannot our muscles do the same? Evidently the necessity of resting cuts down the possibilities of life more than any other one thing; our real life is only two-thirds as long as it counts up in years because we have to spend one-third of the time in sleep. Of course muscular fatigue is not the only kind; there is nervous fatigue, as well, about which something will be said later. The activity of our muscles is based on functional metabolism; it follows, therefore, that fatigue is also due to metabolism. We can think of two ways in which metabolism might cause fatigue; the first of these is by using up the materials which furnish energy; clearly no cells can go on working after they have exhausted their supplies of fuel. The second results from the fact that metabolism produces waste products. It is a familiar fact of chemistry that when the substances formed in chemical processes are not removed they interfere with the processes themselves. In active muscles very rapid metabolism is going on and large quantities of waste substance are being formed; these have to be discharged from the cells into the surrounding fluid, and removed from there in turn by the blood. We can easily imagine that this might not take place as fast as necessary to keep the cells from becoming more or less clogged; in fact this clogging is exactly what happens, so that muscles begin to show fatigue some time before their supplies of fuel material are used up.

One familiar fact of muscular fatigue is that soreness, which indicates that fatigue has really

Photo, Metropolitan Museum MUSCULAR DEVELOPMENT OF AN ATHLETE—THE DISCUS THROWER OF MYRON

A MODERN “VICTORY”—MISS SABIE AT PRACTICE

been present in large amount, occurs much more often when we use our muscles in ways to which we are not accustomed than when they are exercised according to habit. It is the experience of every one who does manual labor that when he gets a new job, one that calls for different use of the muscles than he has been in the habit of, his muscles are very sore until he is “broken in.” After that, although he does as much or even more work than at first, he no longer becomes sore. This is explained as being due to two things. First, whenever we make an unaccustomed movement we overstimulate our muscles; that is, we call more fibers into action than are necessary to do the job; as the motion becomes familiar we cut down the action to that which just meets the demand. Thus there is a great deal more metabolism than necessary when unfamiliar motions are being made. Then, secondly, there is a spot in every muscle cell, just at the point where the nerve makes its connection with the muscle, that is more easily fatigued than any other part of the muscle cell. This spot, by becoming fatigued first, tends to cause metabolism to stop in time to prevent the rest of the cell from being seriously fatigued. Only when we are so much interested in what we are doing that we pay no attention to the fatigue of this safety spot, or when necessity keeps us at work after we would quit if we had our own way, do we push the metabolism so far that muscular soreness results. Other types of fatigue, including feelings of exhaustion, are due to effects on the nervous system, and will be described when we have that system before us.

Before we leave the subject of the skeletal muscles it will be interesting to say a word about the different kinds of motions that they bring about. We have already seen that they work by pulling at the joints, and we have no intention of enlarging on that topic. What we want to do here is to group the bodily motions into a few classes, regardless of what joints are actually moved. First, and most important, comes locomotion; by that we mean any motions that move the body from one place to another. Under that head we have walking, running, swimming, jumping; in birds, flying. Next in order comes grasping; this includes all motions by which we take hold of anything. We can realize the importance of this group of movements when we think that our fore limbs are specifically grasping organs, while in the great majority of animals they are organs of locomotion along with the hind limbs. Originally grasping had to do, undoubtedly, with the taking of food and not much else. In civilized man we have in addition the use of all kinds of tools from the coarsest to the finest. In most four-legged animals the chief organ of grasping is the mouth. We still use our mouths to some extent as grasping organs, and could probably learn to make even more use of them in that direction if forced to it. Chewing and swallowing make up a group of movements concerned primarily with the handling of food after it has been grasped. Not much need be said about them. Of small extent but great importance are the motions connected with sense perception; these include chiefly the motions of the body, neck, and eyes in vision; we are constantly turning to look at something; in such animals as horses movements of the ears help greatly in hearing; and both man and animals make sniffing motions to increase the keenness of smelling. There is a group of motions devoted to voice production (including breathing). In man the vocal cords, tongue, and muscles of the cheeks are the chief muscles that have to do with the voice, not including the muscles of breathing, which, of course, are essential. The interesting things about the vocal cords are the excessive fineness of their operation, enabling expert singers to produce tones that vary by only a few vibrations a second, and the amazing exactitude of the control that the nerves have over them, so that good singers can set them at the tension needed for producing a particular tone with absolute certainty. The tongue is not a single muscle, but a mass of several muscles working one upon the other. It plays a part both in voice production and in the chewing of food. As an organ of voice production it helps by changing the shape of the mouth cavity. Speech depends very largely on this, since not the tension of the vocal cords but the shape of the mouth and throat determines the making of letters and syllables.

In addition to these familiar uses of the muscles there is a use which is just as important but about which we are apt to think less. This is their use in connection with posture, the taking and holding of particular bodily positions. Posture is unlike other muscular activities in several things. In the first place there is a steady, but rather feeble, tension which can be held without marked fatigue for long periods; all other forms of muscular contraction become severely fatiguing rather quickly if held steadily. In the second place the nervous control of posture seems to be different in some respects from our ordinary control of our muscles. Finally there is some doubt as to whether the contractions of the muscles themselves are the same. Measurements of the functional metabolism of posture show that it is much less than would be expected if the muscular action were of the ordinary type. This, of course, explains why posture is less fatiguing than other forms of activity.

The other two kinds of muscle, heart muscle and smooth muscle, must have a word of description. Heart muscle contracts quickly and powerfully, as does skeletal. It differs from skeletal in not depending on nervous stimulation to make it contract; the heart can be cut clean out of the body and will go on beating for a short time; in cold-blooded animals, like frogs or turtles, for a long time. This could not be true if the heart muscle had to be aroused to activity by nerves. Besides being automatic, heart muscle shows the peculiarity that whenever it contracts all the fibers join. We do not have a varying strength of pull shown by heart muscle as we do in skeletal. As we shall see, it would be a serious disadvantage rather than an advantage if heart muscle were to be like skeletal in this respect.

Smooth muscle has the duty of operating the internal organs. For this function no great strength is required; the motions do not have to be powerful. Nor is rapid motion important. Smooth muscle does not have to be so highly developed, then, as is skeletal. It is sluggish and rather feeble in its actions. There are, however, two points of superiority about smooth muscle, which fit in well with its special task. The first of these is its freedom from fatigue. There are in the body numerous smooth muscle masses that are in contraction practically all the time. This would be impossible if fatigue were to develop. These masses make up what are called the sphincters, rings of muscle surrounding openings like that from the esophagus to the stomach or from the stomach to the small intestine. It is the duty of these sphincters to hold the openings closed all the time except occasionally when they open for just an instant to let material through. The second point about smooth muscle which fits it for its work is that it is capable of stretching out greatly or contracting sharply without much difference in the force with which it is pulling. For example, at the beginning of a meal the walls of the stomach are drawn up, so that the food that is swallowed enters a small space. With the progress of the meal the stomach enlarges, so that at the end it has a much greater bulk than at the beginning. But the actual pressure of the stomach upon its contents is about the same as at the beginning. If the stomach were an ordinary elastic bag this could not happen; the walls would have to stretch as the stomach filled, and the stretching would mean greater pressure. Since the stomach walls are of smooth muscle they adjust themselves to the progress of the meal. It is important to note that there is a limit to this possibility of adjustment. If one is so greedy as to keep on stuffing after the stomach has reached its full size, stretching does occur, and if this is repeated it may lead to a diseased condition known as “dilated stomach,” which will cause much digestive trouble.