Physiology: The Science of the Body

by Ernest G. Martin

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.

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.