Physiology and Hygiene for Secondary Schools :: CHAPTER XV

CHAPTER XV - THE MUSCULAR SYSTEM

As already stated, the skeleton, the nervous system, and the muscular system are concerned in the production of motion. The skeleton and the nervous system, however, serve other purposes in the body, while the muscular system is devoted exclusively to the production of motion. For this reason it is looked upon as the special motor system. The muscular tissue is the most abundant of all the tissues, forming about 41 per cent of the weight of the body.

Properties of Muscles.—The ability of muscular tissue to produce motion depends primarily upon two properties—the property of irritability and the property of contractility. Irritability is that property of a substance which enables it to respond to a stimulus, or to act when acted upon. Contractility is the property which enables the muscle when stimulated to draw up, thereby becoming shorter and thicker (a condition called contraction), and when the stimulation ceases, to return to its former condition (of relaxation). The property of contractility enables the muscles to produce motion. Irritability is a condition necessary to their control in the body.

Kinds of Muscular Tissue.—Three kinds of muscular tissue are found in the body. These are known as the striated, or striped, muscular tissue; the non-striated, or plain, muscular tissue; and the muscular tissue of the heart. These are made up of different kinds of muscle cells and act in different ways to cause motion. The [pg 244]striated muscular tissue far exceeds the others in amount and forms all those muscles that can be felt from the surface of the body. The non-striated muscle is found in the walls of the food canal, blood vessels, air passages, and other tubes of the body; while the muscular tissue of the heart is confined entirely to that organ.

Striated Muscle Cells.—The cells of the striated muscles are slender, thread-like structures, having an average length of 1-1/2 inches (35 millimeters) and a diameter of about 1/400 of an inch (60 μ). Because of their great length they are called fibers, or fiber cells. They are marked by a number of dark, transverse bands, or stripes, called striations,83 which seem to divide them into a number of sections, or disks (Fig. 108). A thin sac-like covering, called the sarcolemma, surrounds the entire cell and just beneath this are a number of nuclei.84

Fig. 108

Fig. 108—A striated muscle cell highly magnified, showing striations and nuclei. Attached to the cell is the termination of a nerve fiber.

Within the sarcolemma are minute fibrils and a semiliquid substance, called the sarcoplasm. At each end the cell tapers to a point from which the sarcolemma appears to continue as a fine thread, and this, by attaching itself to the inclosing sheath, holds the cell in place. Most of the muscle cells receive, at some portion of their length, the termination of a nerve fiber. This penetrates the sarcolemma and spreads out upon a kind of disk, having several nuclei, known as the end plate.

[pg 245]The "Muscle-organ."—We must distinguish between the term "muscle" as applied to the muscular tissue and the term as applied to a working group of muscular tissue, which is an organ. In the muscle, or muscle-organ, is found a definite grouping of muscle fibers such as will enable a large number of them to act together in the production of the same movement. An examination of one of the striated muscles shows the individual fibers to lie parallel in small bundles, each bundle being surrounded by a thin layer of connective tissue. (See Practical Work.) These small bundles are bound into larger ones by thicker sheaths and these in turn may be bound into bundles of still larger size (Fig. 109). The sheaths surrounding the fiber bundles are connected with one another and also with the outer covering of the muscle, known as

Fig. 109

Fig. 109—Diagram of a section of a muscle, showing the perimysium and the bundles of fiber cells.

Fig. 110

Fig. 110—A muscle-organ in position. The tendons connect at one end with the bones and at the other end with the fiber cells and perimysium. (See text.)

The Perimysium.—The plan of the muscle-organ is revealed through a study of the perimysium. This is not limited to the surface of the muscle, as the name suggests, but properly includes the sheaths that surround the bundles of fibers. Furthermore,[pg 246] the surface perimysium and that within the muscle are both continuous with the strong, white cords, called tendons, that connect the muscles with the bones. By uniting with the bone at one end and blending with the perimysium and fiber bundles at the other, the tendon forms a very secure attachment for the muscle. The perimysium and the tendon are thus the means through which the fiber cells in any muscle-organ are made to pull together upon the same part of the body (Fig. 110).

Purpose of Striated Muscles.—The striated muscles, by their attachments to the bones, supply motion to all the mechanical devices, or machines, located in the skeleton. Through them the body is moved from place to place and all the external organs are supplied with such motion as they require. Because of the attachment of the striated muscles to the skeleton, and their action upon it, they are called skeletal muscles. As most of them are under the control of the will, they are also called voluntary muscles. They are of special value in adapting the body to its surroundings.

Structure of the Non-striated Muscles.—The cells of the non-striated muscles differ from those of the striated muscles in being decidedly spindle-shaped and in having but a single well-defined nucleus (Fig. 111). Furthermore, they have no striations, and their connection with the nerve fibers is less marked. They are also much smaller than the striated cells, being less than one one-hundredth of an inch in length and one three-thousandth of an inch in diameter.

In the formation of the non-striated muscles, the cells are attached to one another by a kind of muscle cement to form thin sheets or slender bundles. These differ from the striated muscles in several particulars. They are of a pale, whitish color, and they have no tendons. Instead of[pg 247] being attached to the bones, they usually form a distinct layer in the walls of small cavities or of tubes (Fig. 111). Since they are controlled by the part of the nervous system which acts independently of the will, they are said to be involuntary. They contract and relax slowly.

Fig. 111

Fig. 111—Non-striated muscle cells. A. Cross section of small artery magnified, showing (1) the layer of non-striated cells. B. Three non-striated cells highly magnified.

Work of the Non-striated Muscles.—The work of the non-striated muscles, both in purpose and in method, is radically different from that of the striated. They do not change the position of parts of the body, as do the striated muscles, but they alter the size and shape of the parts which they surround. Their purpose, as a rule, is to move, or control the movement of, materials within cavities and tubes, and they do this by means of the pressure which they exert. Examples of their action have already been studied in the propulsion of the food through the alimentary canal and in the regulation of the flow of blood through the arteries (pages 159 and 49). While they do not contract so quickly, nor with such great force as the striated muscles, their work is more closely related to the vital processes.

Structure of the Heart Muscle.—The cells of the heart combine the structure and properties of the striated and the non-striated muscle cells, and form an intermediate type between the two. They are cross-striped like the striated cells, and are nearly as wide, but are rather short (Fig. 112). Each cell has a well-defined nucleus, but the sarcolemma is absent. They are placed end to end to form fibers, and[pg 248] many of the cells have branches by which they are united to the cells in neighboring fibers. In this way they interlace more or less with each other, but are also cemented together. They contract quickly and with great force, but are not under control of the will. Muscular tissue of this variety seems excellently adapted to the work of the heart.

Fig. 112

Fig. 112—Muscle cells from the heart, highly magnified (after SchÄfer).

The Muscular Stimulus.—The inactive, or resting, condition of a muscle is that of relaxation. It does work through contracting. It becomes active, or contracts, only when it is being acted upon by some force outside of itself, and it relaxes again when this force is withdrawn. Any kind of force which, by acting on muscles, causes them to contract, is called a muscular stimulus. Electricity, chemicals of different kinds, and mechanical force may be so applied to the muscles as to cause them to contract. These are artificial stimuli. So far as known, muscles are stimulated naturally in but one way. This is through the nervous system. The nervous system supplies a stimulus called the nervous impulse, which reaches the muscles by the nerves, causing them to contract. By means of nervous impulses, all of the muscles (both voluntary and involuntary) are made to contract as the needs of the body for motion require.

Energy Transformation in the Muscle.—The muscle serves as a kind of engine, doing work by the transformation of potential into kinetic energy. Evidences of this are found in the changes that accompany contraction. Careful study shows that during any period of contraction oxygen and food materials are consumed, waste products, such as carbon dioxide, are produced, and heat is[pg 249] liberated. Furthermore, the blood supply to the muscle is such that the materials for providing energy may be carried rapidly to it and the products of oxidation as rapidly removed. Blood vessels penetrate the muscles in all directions and the capillaries lie very near the individual cells (Fig. 113). Provision is made also, through the nervous system, for increasing the blood supply when the muscle is at work. From these facts, as well as from the great force with which the muscle contracts, one must conclude that the muscle is a transformer of energy—that within its protoplasm, chemical changes take place whereby the potential energy of oxygen and food is converted into the kinetic energy of motion.

Fig. 113

Fig. 113—Capillaries of muscles.

Plan of Using Muscular Force.—Two difficulties have to be overcome in the using of muscular force in the body. The first of these is due to the fact that the muscles exert their force only when they contract. They can pull but not push. Hence, in order to bring about the opposing movements85 of the body, each muscle must work against some force that produces a result directly opposite to that which the muscle produces. Some of the muscles (those of breathing) work against the elasticity of certain parts of the body; others (those that hold the body in an upright position), to some extent against gravity; and others[pg 250] (the non-striated muscle in arteries), against pressure. But in most cases, muscles work against muscles.

Fig. 114

Fig. 114—The muscle pair that operates the forearm. For names of these muscles, see Fig. 119.

The striated, or skeletal, muscles are nearly all arranged after the last-named plan. As a rule a pair of muscles is so placed, with reference to a joint, that one moves the part in one direction, and the other moves it in the opposite direction. From the kinds of motion which the various muscle pairs produce, they are classified as follows:

1. Flexors and Extensors.—The flexor muscles bend and the extensors straighten joints (Fig. 114).

2. Adductors and Abductors.—The adductors draw the limbs into positions parallel with the axis of the body and the abductors draw them away.

3. Rotators (two kinds).—The rotators are attached about pivot joints and bring about twisting movements.

4. Radiating and Sphincter Muscles. —The radiating muscles open and the sphincter muscles close the natural openings of the body, such as the mouth.

The pupil should locate examples of the different kinds of muscle pairs in his own body.

Exchange of Muscular Force for Motion.—The second difficulty to be overcome in the use of muscular force in the body is due to the fact that the muscles contract through short distances, while it is necessary for most of them to move portions of the body through long distances. It may be easily shown that the longest muscles of the body do not shorten more than three or four inches during[pg 251] contraction. To bring about the required movements of the body, which in some instances amount to four or five feet, requires that a large proportion of the muscular force be exchanged for motion. The machines of the skeleton, while providing for motion in definite directions, also provide the means whereby strong forces, acting through short distances, are made to produce movements of less force, through long distances. The mechanical device employed for this purpose is known as

The Lever.—The lever may be described as a stiff bar which turns about a fixed point of support, called the fulcrum. The force applied to the bar to make it turn is called the power, and that which is lifted or moved is termed the weight. The weight, the power, and the fulcrum may occupy different positions along the bar and this gives rise to the three kinds of levers, known as levers of the first class, the second class, and the third class (Fig. 115). In levers of the first class the fulcrum occupies a position somewhere between the power and the weight. In the second class the weight is between the fulcrum and the power. In the third class the power is between the fulcrum and the weight.

Fig. 115

Fig. 115—Classes of levers. I. Two levers of first class showing fulcrums in different positions. II. Lever of second class. III. Lever of third class. F. Fulcrum. P. Power. W. Weight. a. Power-arm. b. Weight-arm.

Application to the Body.—In the body the bones serve as levers; the turning points, or fulcrums, are found at the joints; the muscles supply the power; and parts of the[pg 252] body, or things to be lifted, serve as weights. For these levers to increase the motion of the muscles, it is necessary that the muscles be attached to the bones near the joints, and that the parts to be moved be located at some distance from the joints. In other words the (muscle) power-arm must be shorter than the (body) weight-arm.86

Examining Fig. 116, it is seen that the distances moved by the power and weight vary as their respective distances from the fulcrum. That is to say, if the weight is twice as far from the fulcrum as the power, it will move through twice the distance, and if three times as far, through three times the distance. Thus the muscles, by acting through short distances (on the short arms of levers), are able to move portions of the body (located on the long arms) through long distances. Can all three classes of levers be used in this way in the body?

Fig. 116

Fig. 116—Motion producing levers. Diagrams show relative distances moved by the power and weight in levers having the power nearer the fulcrum than is the weight. F. Fulcrum. P, P'. Power. W, W'. Weight.

Classes of Levers found in the Body.—Practically all of the levers of the body belong either to the first class or the third class. In both of these the muscle power can be applied to the short arm of the lever, thereby moving the body weight through a longer distance than the muscle contracts (Fig. 116). In the levers of the second class, however, the weight occupies this position, being situated between the power and fulcrum (Fig. 117). The weight,[pg 253] therefore, cannot move farther than the power in this lever. It must always move a shorter distance. While such a lever is of great advantage in lifting heavy weights outside of the body, it cannot be used for increasing the motion of the muscles. For this reason no well-defined levers of the second class are present in the body.87

Fig. 117

Fig. 117—Weight lifting levers. Diagrams show relative distances moved by the power and weight in levers having the weight nearer the fulcrum than is the power. F. Fulcrum. P, P'. Power. W, W'. Weight.

Fig. 118

Fig. 118—Diagram of the foot lever. F. Fulcrum at ankle joint. W. Body weight expressed as pressure against the earth. While the muscle power acts through the distance ab, the fulcrum support (body) is forced through the distance FE.

Loss of Muscular Force.—Using a small spring balance for measuring the power, a light stick for a lever, and a small piece of metal for a weight, and arranging these to represent some lever of the body (as the[pg 254] forearm), it is easily shown that the gain in motion causes a corresponding loss in muscular power. (See Practical Work.) If, for example, the balance is attached two inches from the fulcrum and the weight twelve inches, the pull on the balance is found to be six times greater than the weight that is being lifted. If other positions are tried, it is found that the power exerted in each case is as many times greater than the weight as the weight-arm is times longer than the power-arm.

Applying this principle to the levers of the body, it is seen that the gain in motion is at the expense of muscular force, or, as we say, muscular force is exchanged for motion. This exchange is greatly to the advantage of the body; for while the ability to lift heavy weights is important, the ability to move portions of the body rapidly and through long distances is much more to be desired.

Important Muscles.—There are about five hundred separate muscles in the body. These vary in size, shape, and plan of attachment, to suit their special work. Some of those that are prominent enough to be felt at the surface are as follows:

Of the head: The temporal, in the temple, and the masseter, in the cheek. These muscles are attached to the lower jaw and are the chief muscles of mastication.

Of the neck: The sterno-mastoids, which pass between the mastoid processes, back of the ears, and the upper end of the sternum. They assist in turning the head and may be felt at the sides of the neck (Fig. 119).

Of the upper arm: The biceps on the front side, the triceps behind, and the deltoid at the upper part of the arm beyond the projection of the shoulder.

Fig. 119

Fig. 119—Back and front views of important muscles.

Of the forearm: The flexors of the fingers, on the front[pg 256] side, and the extensors of the fingers, on the back of the forearm (Fig. 119).

Of the hand: The adductor pollicis between the thumb and the palm.

Of the trunk: The pectoralis major, between the upper front part of the thorax and the shoulder; the trapezius, between the back of the shoulders and the spine; the rectus abdominis, passing over the abdomen from above downward; and the erector spinÆ, found in the small of the back.

Of the hips: The glutens maximus, fastened between the lower back part of the hips and the upper part of the femur.

Of the upper part of the leg: The rectus femoris, the large muscle on the front of the leg which connects at the lower end with the kneepan.

Of the lower leg: The tibialis anticus on the front side, exterior to the tibia, and the gastrocnemius, the large muscle in the calf of the leg. This is the largest muscle of the body, and is connected with the heel bone by the tendon of Achilles (Fig. 119).

The use of these muscles is, in most instances, easily determined by observing the results of their contraction.

HYGIENE OF THE MUSCLES

The hygiene of the muscles is almost expressed by the one word exercise. It is a matter of everyday knowledge that the muscles are developed and strengthened by use, and that they become weak, soft, and flabby by disuse. The effects of exercise are, however, not limited to the large muscles attached to the skeleton, but are apparent also upon the involuntary muscles, whose work is so closely related to the vital processes. While it is true that exercise cannot be applied directly to the involuntary muscles, it is also true that exercise of the voluntary muscles causes[pg 257] a greater activity on the part of those that are involuntary and is indirectly a means of exercising them.

Exercise and Health.—In addition to its effects upon the muscles themselves, exercise is recognized as one of the most fundamental factors in the preservation of the health. Practically every process of the body is stimulated and the body as a whole invigorated by exercise properly taken. On the other hand, a lack of exercise has an effect upon the entire body somewhat similar to that observed upon a single muscle. It becomes weak, lacks energy, and in many instances actually loses weight when exercise is omitted. This shows exercise to supply an actual need and to be in harmony with the nature and plan of the body.

How Exercise benefits the Body.—In accounting for the healthful effects of exercise, it must be borne in mind that the body is essentially a motion-producing structure. Furthermore, its plan is such that the movements of its different parts aid indirectly the vital processes. The student will recall instances of such aid, as, for example, the assistance rendered by muscular contractions in the circulation of the blood and lymph, due to the valves in veins and lymph vessels, and the assistance rendered by abdominal movements in the propulsion of materials through the food canal. A fact not as yet brought out, however, is that exercise stimulates nutritive changes in the cells, thereby imparting to them new vigor and vitality. While this effect of exercise cannot be fully accounted for, two conditions that undoubtedly influence it are the following:

1. Exercise causes the blood to circulate more rapidly.

2. Exercise increases the movement of the lymph through the lymph vessels.

The increase in the flow of the blood and the lymph[pg 258] causes changes to take place more rapidly in the liquids around the cells, thereby increasing the supply of food and oxygen, and hastening the removal of waste.

One should plan for Exercise.—Since exercise is demanded by the nature and plan of the body, to neglect it is a serious matter. People do not purposely omit exercise, but from lack of time or from its interference with the daily routine of duties, the needed amount is frequently not taken. Especially is this true of students and others who follow sedentary occupations. People of this class should plan for exercise as they plan for the other great needs of the body—food, sleep, clothing, etc. It is only by making a sufficient amount of muscular work or play a regular part of the daily program that the needs of the body for exercise are adequately supplied.

Amount and Kind of Exercise.—The amount of exercise required varies greatly with different individuals, and definite recommendations cannot be made. For each individual also the amount should vary with the physical condition and the other demands made upon the energy. One in health should exercise sufficiently to keep the muscles firm to the touch and the body in a vigorous condition.

Of the many forms of exercise from which one may choose, the question is again one of individual adaptability and convenience. While the different forms of exercise vary in their effects and may be made to serve different purposes, the consideration of these is beyond the scope of an elementary text. As a rule one will not go far wrong by following his inclinations, observing of course the conditions under which exercise is taken to the best advantage.

General Rules for Healthful Exercise.—That exercise may secure the best results from the standpoint of health, a number of conditions should be observed: 1. It should[pg 259] not be excessive or carried to the point of exhaustion. Severe physical exercise is destructive to both muscular and nervous tissues. 2. It should, if possible, be of an interesting nature and taken in the open air. 3. It should be counter-active, that is, calling into play those parts of the body that have not been used during the regular work.88 4. It should be directed toward the weak rather than toward the strong parts of the body. 5. When one is already tired from study, or other work, it should be taken with moderation or omitted for the time being. (For exercise of the heart muscle and the muscular coat of the blood vessels see pages 55 and 57.)

Massage.—In lieu of exercise taken in the usual way, similar effects are sometimes obtained by a systematic rubbing, pressing, stroking, or kneading of the skin and the muscles by one trained in the art. This process, known as massage, may be gentle or vigorous and is subject to a variety of modifications. Massage is applied when one is unable to take exercise, on account of disease or accident, and also in the treatment of certain bodily disorders. A weak ankle, wrist, or other part of the body, or even a bruise, may be greatly benefited by massage. The flow of blood and lymph is stimulated, causing new materials to be passed to the affected parts and waste materials to be removed. Massage, however, should never be applied to a boil, or other infected sore. The effect in this case would be to spread the infection and increase the trouble.

Summary.—Motion is provided for in the body mainly through the muscle cells. These are grouped into working parts, called muscles, which in turn are attached to the movable parts of the body. The striated muscles, as a[pg 260] rule, are attached to the mechanical devices found in the skeleton, and bring about the voluntary, movements. The non-striated muscles surround the parts on which they act, and produce involuntary movements. Both, however, are under the control of the nervous system. To bring about the opposing movements of the body, the striated muscles are arranged in pairs; and to increase their motion, the bones are used as levers. Physical exercise is necessary both for the development of the muscles and for the health and vigor of the entire body.

Exercises.—1. Compare the striated and non-striated muscles with reference to structure, location, and method of work.

2. In what respects is the muscular tissue of the heart like the striated, and in what respects like the non-striated, muscular tissue?

3. If muscles could push as well as pull, would so many be needed in the body? Why?

4. Locate muscles that work to some extent against elasticity and gravity.

5. Locate five muscles that act as flexors; five that act as extensors; two that act as adductors; and two as abductors. Locate sphincter and radiating muscles.

6. By what means does the nervous system control the muscles?

7. Give proofs of the change of potential into kinetic energy during muscular contraction.

8. Define the essential properties of muscular tissue and state the purpose served by each.

9. Describe a lever. For what general purpose are levers used in the body? What other purpose do they serve outside of the body?

10. Why are levers of the second class not adapted to the work of the body?

11. Name the class of lever used in bending the elbow; in straightening the elbow; in raising the knee; in elevating the toes; and in biting. Why is one able to bite harder with the back teeth than with the front ones when the same muscles are used in both cases?

12. Measure the distance from the middle of the palm of the hand to the center of the elbow joint. Find the attachment of the tendon of the biceps muscle to the radius and measure its distance to the[pg 261] center of the elbow joint. From these distances calculate the force with which the biceps contracts in order to support a weight of ten pounds on the palm of the hand.

13. How does exercise benefit the health? How does a short walk "clear the brain" and enable one to study to better advantage?

14. When exercisers taken for its effects upon the health, what conditions should be observed?

PRACTICAL WORK

The reddish muscle found in a piece of beef is a good example of striated muscle. The clear ring surrounding the intestine of a cat (shown by cross section) and the outer portion of the preparation from the cow's stomach, sold at the butcher shop under the name of tripe, are good examples of non-striated muscular tissue. The heart of any animal, of course, shows the heart muscle.

To show the Structure of Striated Muscle.—Boil a tough piece of beef, as a cut from the neck, until the connective tissue has thoroughly softened. Then with some pointed instrument, separate the main piece into its fiber bundles and these in turn into their smallest divisions. The smallest divisions obtainable are the muscle cells or fibers.

To show Striated Fibers.—Place a small muscle from the leg of a frog in a fifty-per-cent solution of alcohol and leave it there for half a day or longer. Then cover with water on a glass slide, and with a couple of fine needles tease out the small muscle threads. Protect with a cover glass and examine with a microscope, first with a low and then with a high power. The striations, sarcolemma, and sometimes the nuclei and nerve plates, may be distinguished in such a preparation.

To show Non-striated Cells.—Place a clean section of the small intestine of a cat in a mixture of one part of nitric acid and four parts of water and leave for four or five hours. Thoroughly wash out the acid with water and separate the muscular layer from the mucous membrane. Cover a small portion of the muscle with water on a glass slide and tease out, with needles, until it is as finely divided as possible. Examine with a microscope, first with a low and then with a high power. The cells appear as very fine, spindle-shaped bodies.

To illustrate Muscular Stimulus and Contraction.—Separate the muscles at the back of the thigh of a frog which has just been killed and draw the large sciatic nerve to the surface. Cut this as high up as possible and, with a sharp knife and a small pair of scissors, dissect it[pg 262] out to the knee. Now cut out entirely the large muscle of the calf of the leg (the gastrocnemius), but leave attached to it the nerve, the lower tendon, and the bones of the knee. Mount on an upright support, as shown in Fig. 120, and fasten the tendon to a lever below by a thread or small wire hook:

Fig. 120

Fig. 120—Apparatus for demonstrating properties of muscles.

1. Lay the nerve over the ends of the wires from a small battery which are attached to the support at A, and arrange a second break in the circuit at B. At this place the battery circuit is made and broken either by a telegraph key or by simply touching and separating the wires. Note that the muscle gives a single contraction, or twitch, both when the current is made and when it is broken.

2. Remove the current and pinch the end of the nerve, noting the result. With very fine wires, connect the battery directly to the ends of the muscle. Stimulate by making and breaking the current as before. In this experiment the muscle cells are stimulated by the direct action of the current and not by the current acting on the nerve.

3. With the wires attached to either the muscle or the nerve, make and break the current in rapid succession. This causes the muscle to enter into a second contraction before it has relaxed from the first, and if the shocks follow in rapid succession, to continue in the contracted state. This condition, which represents the method of contraction of the muscles in the body, is called tetanus.

NOTE.—In these experiments a twitching of the muscle is frequently observed when no stimulus is being applied. This is due to the drying out of the nerve and is prevented by keeping it wet with a physiological salt solution. (See footnote, page 38.)

To show the Action of Levers.—With a light but stiff wooden bar, a spring balance, and a wedge-shaped fulcrum, show:

1. The position of the weight, the fulcrum, and the power in the different classes of levers, and also the weight-arm and the power-arm in each case.

2. The direction moved by the power and the weight respectively in the use of the different classes of levers.

3. That when the power-arm and weight-arm are equal, the power equals the weight and moves through the same distance.

[pg 263]4. That when the power-arm is longer than the weight-arm, the weight is greater, but moves through a shorter distance than the power.

5. That when the weight-arm is longer than the power-arm, the power is greater and moves through a shorter distance than the weight.

To show the Loss of Power in the Use of the Body Levers.—Construct a frame similar to, but larger than, that shown in Fig. 120, (about 12 inches high), and hang a small spring balance (250 grams capacity) at the place where the muscle is attached. Fasten the end of a lever to the upright piece, at a point on a level with the end of the balance hook. (The nail or screw used for this purpose must pass loosely through the lever, and serve as a pivot upon which it can turn.) The lever should consist of a light piece of wood, and should have a length at least three times as great as the distance from the hook to the turning point. Connect the balance hook with the lever by a thread or string, and then hang upon it a small body of known weight. Note the amount of force exerted at the balance in order to support the weight at different places on the lever. At what point is the force just equal to the weight? Where is it twice as great? Where three times? Show that the force required to support the weight increases proportionally as the weight-arm and as the distance through which the weight may be moved by the lever. Apply to the action of the biceps muscle in lifting weights on the forearm.

A Study of the Action of the Biceps Muscle.—Place the fingers upon the tendon of the biceps where it connects with the radius of the forearm. With the forearm resting upon the table, note that the tendon is somewhat loose and flaccid, but that with the slightest effort to raise the forearm it quickly tightens. Now transfer the fingers to the body of the muscle, and sweep the forearm through two or three complete movements, noting the changes in the length and thickness of the muscle. Lay the forearm again on the table, back of hand down, and place a heavy weight (a flatiron or a hammer) upon the hand. Note the effort required to raise the weight, and then shift it along the arm. Observe that the nearer it approaches the elbow the lighter it seems. Account for the difference in the effort required to raise the weight at different places. Does the effort vary as the distance from the tendon?

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