Читать книгу A Practical Physiology: A Text-Book for Higher Schools - Albert F. Blaisdell - Страница 16

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64. Motion in Animals. All motion of our bodies is produced by means of muscles. Not only the limbs are moved by them, but even the movements of the stomach and of the heart are controlled by muscles. Every part of the body which is capable of motion has its own special set of muscles.

Even when the higher animals are at rest it is possible to observe some kind of motion in them. Trees and stones never move unless acted upon by external force, while the infant and the tiniest insect can execute a great variety of movements. Even in the deepest sleep the beating of the heart and the motion of the chest never cease. In fact, the power to execute spontaneous movement is the most characteristic property of living animals.

65. Kinds of Muscles. Most of the bodily movements, such as affect the limbs and the body as a whole, are performed by muscles under our control. These muscles make up the red flesh or lean parts, which, together with the fat, clothe the bony framework, and give to it general form and proportion. We call these muscular tissues voluntary muscles, because they usually act under the control of the will.

The internal organs, as those of digestion, secretion, circulation, and respiration, perform their functions by means of muscular activity of another kind, that is, by that of muscles not under our control. This work goes on quite independently of the will, and during sleep. We call the instruments of this activity involuntary muscles. The voluntary muscles, from peculiarities revealed by the microscope, are also known as striped or striated muscles. The involuntary from their smooth, regular appearance under the microscope are called the unstriped or non-striated muscles.

The two kinds of muscles, then, are the red, voluntary, striated muscles, and the smooth, involuntary, non-striated muscles.

66. Structure of Voluntary Muscles. The main substance which clothes the bony framework of the body, and which forms about two-fifths of its weight, is the voluntary muscular tissue. These muscles do not cover and surround the bones in continuous sheets, but consist of separate bundles of flesh, varying in size and length, many of which are capable of independent movement.

Each muscle has its own set of blood-vessels, lymphatics, and nerves. It is the blood that gives the red color to the flesh. Blood-vessels and nerves on their way to other parts of the body, do not pass through the muscles, but between them. Each muscle is enveloped in its own sheath of connective tissue, known as the fascia. Muscles are not usually connected directly with bones, but by means of white, glistening cords called tendons.

Fig. 30.--Striated (voluntary) Muscular Fibers.

 A, fiber serparating into disks;

 B, fibrillæ (highly magnified);

 C, cross section of a disk

If a small piece of muscle be examined under a microscope it is found to be made up of bundles of fibers. Each fiber is enclosed within a delicate, transparent sheath, known as the sarcolemma. If one of these fibers be further examined under a microscope, it will be seen to consist of a great number of still more minute fibers called fibrillæ. These fibers are also seen marked cross-wise with dark stripes, and can be separated at each stripe into disks. These cross markings account for the name striped or striated muscle.

The fibrillæ, then, are bound together in a bundle to form a fiber, which is enveloped in its own sheath, the sarcolemma. These fibers, in turn, are further bound together to form larger bundles called fasciculi, and these, too, are enclosed in a sheath of connective tissue. The muscle itself is made up of a number of these fasciculi bound together by a denser layer of connective tissue.

Experiment 17. To show the gross structure of muscle. Take a small portion of a large muscle, as a strip of lean corned beef. Have it boiled until its fibers can be easily separated. Pick the bundles and fasciculi apart until the fibers are so fine as to be almost invisible to the naked eye. Continue the experiment with the help of a hand magnifying glass or a microscope.

67. The Involuntary Muscles. These muscles consist of ribbon-shaped bands which surround hollow fleshy tubes or cavities. We might compare them to India rubber rings on rolls of paper. As they are never attached to bony levers, they have no need of tendons.

Fig. 31.--A, Muscular Fiber, showing Stripes, and Nuclei, b and c. (Highly magnified.)

The microscope shows these muscles to consist not of fibers, but of long spindle-shaped cells, united to form sheets or bands. They have no sarcolemma, stripes, or cross markings like those of the voluntary muscles. Hence their name of non-striated, or unstriped, and smooth muscles.

The involuntary muscles respond to irritation much less rapidly than do the voluntary. The wave of contraction passes over them more slowly and more irregularly, one part contracting while another is relaxing. This may readily be seen in the muscular action of the intestines, called vermicular motion. It is the irregular and excessive contraction of the muscular walls of the bowels that produces the cramp-like pains of colic.

The smooth muscles are found in the tissues of the heart, lungs, blood-vessels, stomach, and intestines. In the stomach their contraction produces the motion by which the food is churned about; in the arteries and veins they help supply the force by which the blood is driven along, and in the intestines that by which the partly digested food is mainly kept in motion.

Thus all the great vital functions are carried on, regardless of the will of the individual, or of any outward circumstances. If it required an effort of the will to control the action of the internal organs we could not think of anything else. It would take all our time to attend to living. Hence the care of such delicate and important machinery has wisely been put beyond our control.

Thus, too, these muscles act instinctively without training; but the voluntary need long and careful education. A babe can use the muscles of swallowing on the first day of its life as well as it ever can. But as it grows up, long and patient education of its voluntary muscles is needed to achieve walking, writing, use of musical instruments, and many other acts of daily life.

Fig. 32.--A Spindle Cell of Involuntary Muscle. (Highly magnified.)

Experiment 18. To show the general appearance of the muscles. Obtain the lower part of a sheep's or calf's leg, with the most of the lean meat and the hoof left on. One or more of the muscles with their bundles of fibers, fascia, and tendons; are readily made out with a little careful dissection. The dissection should be made a few days before it is wanted and the parts allowed to harden somewhat in dilute alcohol.

68. Properties of Muscular Tissue. The peculiar property of living muscular tissue is irritability, or the capacity of responding to a stimulus. When a muscle is irritated it responds by contracting. By this act the muscle does not diminish its bulk to any extent; it simply changes its form. The ends of the muscle are drawn nearer each other and the middle is thicker.

Muscles do not shorten themselves all at once, but the contraction passes quickly over them in the form of a wave. They are usually stimulated by nervous action. The delicate nerve fibrils which end in the fibers communicate with the brain, the center of the will power. Hence, when the brain commands, a nervous impulse, sent along the nerve fibers, becomes the exciting stimulus which acts upon the muscles and makes them shorter, harder, and more rigid.[10]

Muscles, however, will respond to other than this usual stimulus. Thus an electrical current may have a similar effect. Heat, also, may produce muscular contraction. Mechanical means, such as a sharp blow or pinching, may irritate a muscle and cause it to contract.

We must remember that this property of contraction is inherent and belongs to the muscle itself. This power of contraction is often independent of the brain. Thus, on pricking the heart of a fish an hour after removal from its body, obvious contraction will occur. In this case it is not the nerve force from the brain that supplies the energy for contraction. The power of contraction is inherent in the muscle substance, and the stimulus by irritating the nerve ganglia of the heart simply affords the opportunity for its exercise.

Contraction is not, however, the natural state of a muscle. In time it is tired, and begins to relax. Even the heart, the hardest-working muscle, has short periods of rest between its beats. Muscles are highly elastic as well as contractile. By this property muscle yields to a stretching force, and returns to its original length if the stretching has not been excessive.

Fig. 33.--Principal Muscles of the Body. (Anterior view.)

69. The Object of Contraction. The object of contraction is obvious. Like rubber bands, if one end of a muscle be fixed and the other attached to some object which is free to move, the contraction of the muscle will bring the movable body nearer to the fixed point. A weight fastened to the free end of a muscle may be lifted when the muscle contracts. Thus by their contraction muscles are able to do their work. They even contract more vigorously when resistance is opposed to them than when it is not. With increased weight there is an increased amount of work to be done. The greater resistance calls forth a greater action of the muscle. This is true up to a certain point, but when the limit has been passed, the muscle quickly fails to respond. Again, muscles work best with a certain degree of rapidity provided the irritations do not follow each other too rapidly. If, however, the contractions are too rapid, the muscles become exhausted and fatigue results. When the feeling of fatigue passes away with rest, the muscle recovers its power. While we are resting, the blood is pouring in fresh supplies of building material.

Experiment 19. To show how muscles relax and contract. Lay your left forearm on a table; grasp with the right hand the mass of flesh on the front of the upper arm. Now gradually raise the forearm, keeping the elbow on the table. Note that the muscle thickens as the hand rises. This illustrates the contraction of the biceps, and is popularly called "trying your muscle" Reverse the act. Keep the elbow in position, bring the forearm slowly to the table, and the biceps appears to become softer and smaller,--it relaxes.

Experiment 20. Repeat the same experiment with other muscles. With the right hand grasp firmly the extended left forearm. Extend and flex the fingers vigorously. Note the effect on the muscles and tendons of the forearm. Grasp with the right hand the calf of the extended right leg, and vigorously flex the leg, bringing it near to the body. Note the contractions and relaxations of the muscles.

70. Arrangement of Muscles. Muscles are not connected directly with bones. The mass of flesh tapers off towards the ends, where the fibers pass into white, glistening cords known as tendons. The place at which a muscle is attached to a bone, generally by means of a tendon, is called its origin; the end connected with the movable bone is its insertion.

There are about 400 muscles in the human body, all necessary for its various movements. They vary greatly in shape and size, according to their position and use. Some are from one to two feet long, others only a fraction of an inch. Some are long and spindle-shaped, others thin and broad, while still others form rings. Thus some of the muscles of the arm and thigh are long and tapering, while the abdominal muscles are thin and broad because they help form walls for cavities. Again, the muscular fibers which surround and by their contraction close certain orifices, as those of the eyelids and lips, often radiate like the spokes of a wheel.

Muscles are named according to their shape, position, division of origin or insertion, and their function. Thus we have the recti (straight), and the deltoid (Δ, delta), the brachial (arm), pectoral (breast), and the intercostals (between the ribs), so named from their position. Again, we have the biceps (two-headed), triceps (three-headed), and many others with similar names, so called from the points of origin and insertion. We find other groups named after their special use. The muscles which bend the limbs are called flexors while those which straighten them are known as extensors.

After a bone has been moved by the contraction of a muscle, it is brought back to its position by the contraction of another muscle on the opposite side, the former muscle meanwhile being relaxed. Muscles thus acting in opposition to each other are called antagonistic. Thus the biceps serves as one of the antagonists to the triceps, and the various flexors and extensors of the limbs are antagonistic to one another.

71. The Tendons. The muscles which move the bones by their contraction taper for the most part, as before mentioned, into tendons. These are commonly very strong cords, like belts or straps, made up of white, fibrous tissue.

Tendons are most numerous about the larger joints, where they permit free action and yet occupy but little space. Large and prominent muscles in these places would be clumsy and inconvenient. If we bend the arm or leg forcibly, and grasp the inside of the elbow or knee joint, we can feel the tendons beneath the skin. The numerous tendons in the palm or on the back of the hand contribute to its marvelous dexterity and flexibility. The thickest and strongest tendon in the body is the tendon of Achilles, which connects the great muscles in the calf of the leg with the heel bone (sec. 49).

When muscles contract forcibly, they pull upon the tendons which transmit the movement to the bones to which they are attached. Tendons may be compared to ropes or cords which, when pulled, are made to act upon distant objects to which one end is fastened. Sometimes the tendon runs down the middle of a muscle, and the fibers run obliquely into it, the tendon resembling the quill in a feather. Again, tendons are spread out in a flat layer on the surface of muscles, in which case they are called aponeuroses. Sometimes a tendon is found in the middle of a muscle as well as at each end of it.

Fig. 34.--The Biceps Muscle dissected to show its Tendons.

72. Synovial Sheaths and Sacs. The rapid movement of the tendons over bony surfaces and prominences would soon produce an undue amount of heat and friction unless some means existed to make the motion as easy as possible. This is supplied by sheaths which form a double lining around the tendons. The opposed surfaces are lined with synovial membrane,[11] the secretion from which oils the sheaths in which the tendons move.

Little closed sacs, called synovial sacs or bursæ, similarly lined and containing fluid, are also found in special places between two surfaces where much motion is required. There are two of these bursæ near the patella, one superficial, just under the skin; the other deep beneath the bone (Fig. 29). Without these, the constant motion of the knee-pan and its tendons in walking would produce undue friction and heat and consequent inflammation. Similar, though smaller, sacs are found over the point of the elbow, over the knuckles, the ankle bones, and various other prominent points. These sacs answer a very important purpose, and are liable to various forms of inflammation.

Experiment 21. Examine carefully the tendons in the parts dissected in Experiment 18. Pull on the muscles and the tendons, and note how they act to move the parts. This may be also admirably shown on the leg of a fowl or turkey from a kitchen or obtained at the market.

Obtain the hoof of a calf or sheep with one end of the tendon of Achilles still attached. Dissect it and test its strength.

73. Mechanism of Movement. The active agents of bodily movements, as we have seen, are the muscles, which by their contraction cause the bones to move one on the other. All these movements, both of motion and of locomotion, occur according to certain fixed laws of mechanics. The bones, to which a great proportion of the muscles in the body are attached, act as distinct levers. The muscles supply the power for moving the bones, and the joints act as fulcrums or points of support. The weight of the limb, the weight to be lifted, or the force to overcome, is the resistance.

74. Levers in the Body. In mechanics three classes of levers are described, according to the relative position of the power, the fulcrum, and the resistance. All the movements of the bones can be referred to one or another of these three classes.

Levers of the first class are those in which the fulcrum is between the power and the weight. The crowbar, when used to lift a weight at one end by the application of power at the other, with a block as a fulcrum, is a familiar example of this class. There are several examples of this in the human body. The head supported on the atlas is one. The joint between the atlas and the skull is the fulcrum, the weight of the head is the resistance. The power is behind, where the muscles from the neck are attached to the back of the skull. The object of this arrangement is to keep the head steady and balanced on the spinal column, and to move it backward and forward.

Fig. 35.--Showing how the Bones of the Arm serve as Levers.

 P, power;

 W, weight;

 F, fulcrum.

Levers of the second class are those in which the weight is between the fulcrum and the power. A familiar example is the crowbar when used for lifting a weight while one end rests on the ground. This class of levers is not common in the body. Standing on tiptoe is, however, an example. Here the toes in contact with the ground are the fulcrum, the power is the action of the muscles of the calf, and between these is the weight of the body transmitted down the bones of the leg to the foot.

Levers of the third class are those in which the power is applied at a point between the fulcrum and weight. A familiar example is where a workman raises a ladder against a wall. This class of levers is common in the body. In bending the forearm on the arm, familiarly known as "trying your muscle," the power is supplied by the biceps muscle attached to the radius, the fulcrum is the elbow joint at one end of the lever, and the resistance is the weight of the forearm at the other end.

Experiment 22. To illustrate how the muscles use the bones as levers. First, practice with a ruler, blackboard pointer, or any other convenient object, illustrating the different kinds of levers until the principles are familiar. Next, illustrate these principles on the person, by making use of convenient muscles. Thus, lift a book on the toes, by the fingers, on the back of the hand, by the mouth, and in other ways.

These experiments, showing how the bones serve as levers, may be multiplied and varied as circumstances may require.

75. The Erect Position. The erect position is peculiar to man. No other animal naturally assumes it or is able to keep it long. It is the result of a somewhat complex arrangement of muscles which balance each other, some pulling backwards and some forwards. Although the whole skeleton is formed with reference to the erect position, yet this attitude is slowly learned in infancy.

In the erect position the center of gravity lies in the joint between the sacrum and the last lumbar vertebra. A line dropped from this point would fall between the feet, just in front of the ankle joints. We rarely stand with the feet close together, because that basis of support is too small for a firm position. Hence, in all efforts requiring vigorous muscular movements the feet are kept more or less apart to enlarge the basis of support.

Now, on account of the large number and flexibility of the joints, the body could not be kept in an upright position without the cooperation of certain groups of muscles. The muscles of the calf of the leg, acting on the thigh bone, above the knee, keep the body from falling forward, while another set in front of the thigh helps hold the leg straight. These thigh muscles also tend to pull the trunk forward, but in turn are balanced by the powerful muscles of the lower back, which help keep the body straight and braced.

The head is kept balanced on the neck partly by the central position of the joint between the atlas and axis, and partly by means of strong muscles. Thus, the combined action of these and other muscles serves to balance the body and keep it erect. A blow on the head, or a sudden shock to the nervous system, causes the body to fall in a heap, because the brain has for the time lost its power over the muscles, and they cease to contract.

Fig. 36.--Diagram showing the Action of the Chief Muscles which keep the Body Erect. (The arrows indicate the direction in which these muscles act, the feet serving as a fixed basis.) [After Huxley.]

Muscles which tend to keep the body from falling forward.

 A, muscles of the calf;

 B, of the back of the thigh;

 C, of the spinal column.

Muscles which tend to keep the body from falling backward.

 D, muscles of the front of the leg;

 E, of the front of the thigh;

 F, of the front of the abdomen;

 G, of the front of the neck.

76. Important Muscles. There are scores of tiny muscles about the head, face, and eyes, which, by their alternate contractions and relaxations, impart to the countenance those expressions which reflect the feelings and passions of the individual. Two important muscles, the temporal, near the temples, and the masseter, or chewing muscle, are the chief agents in moving the lower jaw. They are very large in the lion, tiger, and other flesh-eating animals. On the inner side of each cheek is the buccinator, or trumpeter's muscle, which is largely developed in those who play on wind instruments. Easily seen and felt under the skin in thin persons, on turning the head to one side, is the sterno-cleido-mastoid muscle, which passes obliquely down on each side of the neck to the collar bone--prominent in sculpture and painting.

The chest is supplied with numerous muscles which move the ribs up and down in the act of breathing. A great, fan-shaped muscle, called the pectoralis major, lies on the chest. It extends from the chest to the arm and helps draw the arm inward and forward. The arm is raised from the side by a large triangular muscle on the shoulder, the deltoid, so called from its resemblance to the Greek letter delta, Δ. The biceps, or two-headed muscle, forms a large part of the fleshy mass in front of the arm. Its use is to bend the forearm on the arm, an act familiarly known as "trying your muscle." Its direct antagonist is the three-headed muscle called the triceps. It forms the fleshy mass on the back of the arm, its use being to draw the flexed forearm into a right line.

On the back and outside of the forearm are the extensors, which straighten the wrist, the hand, and the fingers. On the front and inside of the forearm are the flexors, which bend the hand, the wrist, and the fingers. If these muscles are worked vigorously, their tendons can be readily seen and felt under the skin. At the back of the shoulder a large, spread-out muscle passes upward from the back to the humerus. From its wide expanse on the back it is known as the latissimus dorsi (broadest of the back). When in action it draws the arm downward and backward, or, if one hangs by the hands, it helps to raise the body. It is familiarly known as the "climbing muscle."

Fig. 37.--A Few of the Important Muscles of the Back.

Passing to the lower extremity, the thigh muscles are the largest and the most powerful in the body. In front a great, four-headed muscle, quadriceps extensor, unites into a single tendon in which the knee-cap is set, and serves to straighten the knee, or when rising from a sitting posture helps elevate the body. On the back of the thigh are several large muscles which bend the knee, and whose tendons, known as the "hamstrings," are readily felt just behind the knee. On the back of the leg the most important muscles, forming what is known as the calf, are the gastrocnemius and the soleus. The first forms the largest part of the calf. The soleus, so named from resembling a sole-fish, is a muscle of broad, flattened shape, lying beneath the gastrocnemius. The tendons of these two muscles unite to form the tendon of Achilles, as that hero is said to have been invulnerable except at this point. The muscles of the calf have great power, and are constantly called into use in walking, cycling, dancing, and leaping.

77. The Effect of Alcoholic Drinks upon the Muscles. It is found that a man can do more work without alcohol than with it. After taking it there may be a momentary increase of activity, but this lasts only ten or fifteen minutes at the most. It is followed by a rapid reduction of power that more than outweighs the momentary gain, while the quality of the work is decidedly impaired from the time the alcohol is taken.

Even in the case of hard work that must be speedily done, alcohol does not help, but hinders its execution. The tired man who does not understand the effects of alcohol often supposes that it increases his strength, when in fact it only deadens his sense of fatigue by paralyzing his nerves. When put to the test he is surprised at his self-deception.

Full intoxication produces, by its peculiar depression of the brain and nervous system, an artificial and temporary paralysis of the muscles, as is obvious in the pitifully helpless condition of a man fully intoxicated. But even partial approach to intoxication involves its proportionate impairment of nervous integrity, and therefore just so much diminution of muscular force. All athletes recognize this fact, as while training for a contest, rigid abstinence is the rule, both from liquors and tobacco. This muscular weakness is shown also in the unsteady hand, the trembling limbs of the inebriate, his thick speech, wandering eye, and lolling head.

78. Destructive Effect of Alcoholic Liquors upon Muscular Tissue. Alcoholic liquors retard the natural chemical changes so essential to good health, by which is meant the oxidation of the nutritious elements of food. Careful demonstration has proved also that the amount of carbon dioxide escaping from the lungs of intoxicated persons is from thirty to fifty per cent less than normal. This shut-in carbon stifles the nervous energy, and cuts off the power that controls muscular force. This lost force is in close ratio to the retained carbon: so much perverted chemical change, so much loss of muscular power. Not only the strength but the fine delicacy of muscular action is lost, the power of nice control of the hand and fingers, as in neat penmanship, or the use of musical instruments.

To this perverted chemical action is also due the fatty degeneration so common in inebriates, affecting the muscles, the heart, and the liver. These organs are encroached upon by globules of fat (a hydrocarbon), which, while very good in their proper place and quantity, become a source of disorder and even of death when they abnormally invade vital structures. Other poisons, as phosphorus, produce this fatty decay more rapidly; but alcohol causes it in a much more general way.

This is proved by the microscope, which plainly shows the condition mentioned, and the difference between the healthy tissues and those thus diseased.

Fig. 38.--Principal Muscles on the Left Side of Neck.

 A, buccinator;

 B, masseter;

 C, depressor anguli oris;

 D, anterior portion of the digastric;

 E, mylo-hyoid;

 F, tendon of the digastric;

 G, sterno-hyoid;

 H, sterno-thyroid;

 K, omo-hyoid;

 L, sternal origin of sterno-cleido-mastoid muscle;

 M, superior fibers of deltoid;

 N, posterior scalenus;

 O, clavicular origin of sterno-cleido-mastoid;

 P, sterno-cleido-mastoid;

 R, trapezius;

 S, anterior constrictor;

 T, splenius capitis;

 V, stylo-hyoid;

 W, posterior portion of the digastric;

 X, fasciculi of ear muscles;

 Z, occipital.

[Note. It was proposed during the Civil War to give each soldier in a certain army one gill of whiskey a day, because of great hardship and exposure. The eminent surgeon, Dr. Frank H. Hamilton of New York, thus expressed his views of the question: "It is earnestly desired that no such experiment will ever be repeated in the armies of the United States. In our own mind, the conviction is established, by the experience and observation of a life, that the regular routine employment of alcoholic stimulants by man in health is never, under any circumstances, useful. We make no exceptions in favor of cold or heat or rain."

"It seems to me to follow from these Arctic experiences that the regular use of spirits, even in moderation, under conditions of great physical hardship, continued and exhausting labor, or exposure to severe cold cannot be too strongly deprecated."

A. W. Greely, retired Brigadier General, U.S.A., and formerly leader of the Greely Expedition.]

79. Effect of Tobacco on the Muscles. That other prominent narcotic, tobacco, impairs the energy of the muscles somewhat as alcohol does, by its paralyzing effect upon the nervous system. As all muscular action depends on the integrity of the nervous system, whatever lays its deadening hand upon that, saps the vigor and growth of the entire frame, dwarfs the body, and retards mental development. This applies especially to the young, in the growing age between twelve or fourteen and twenty, the very time when the healthy body is being well knit and compacted.

Hence many public schools, as well as our national naval and military academies, rigidly prohibit the use of tobacco by their pupils. So also young men in athletic training are strictly forbidden to use it.[12] This loss of muscular vigor is shown by the unsteady condition of the muscles, the trembling hand, and the inability to do with precision and accuracy any fine work, as in drawing or nice penmanship.