The Muscular System : Part 2

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 16 of 27)

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. Each cell has a well-defined nucleus, but the sarcolemma is absent. They are placed end to end to form fibers, and 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.

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or contracts, only when it is being acted upon by some force outside of itself, and it relaxes again when this force is withdrawn.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 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. 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.

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 movements 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 (the non-striated muscle in arteries), against pressure. But in most cases, muscles work against muscles.

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.

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 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. 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.

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 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.

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. Therefore 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?

D.C. Heath and Co. - Publishers
Original copyright 1909