[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.

*Cla.s.ses of Levers found in the Body.*-Practically all of the levers of the body belong either to the first cla.s.s or the third cla.s.s. 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 cla.s.s, however, the weight occupies this position, being situated _between_ the power and fulcrum (Fig. 117). The weight, 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 cla.s.s 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 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 _ma.s.seter_, 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 pa.s.s between the mastoid processes, back of the ears, and the upper end of the sternum. They a.s.sist 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 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_, pa.s.sing over the abdomen from above downward; and the _erector spinae_, 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 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 a.s.sistance rendered by muscular contractions in the circulation of the blood and lymph, due to the valves in veins and lymph vessels, and the a.s.sistance rendered by abdominal movements in the propulsion of materials through the food ca.n.a.l. 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 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 cla.s.s 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 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.)

*Ma.s.sage.*-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 ma.s.sage, may be gentle or vigorous and is subject to a variety of modifications. Ma.s.sage 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 ma.s.sage. The flow of blood and lymph is stimulated, causing new materials to be pa.s.sed to the affected parts and waste materials to be removed. Ma.s.sage, 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 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?