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or contracts, only when it is being acted upon by some force outside of itself, and it relaxes again when this force is withdrawn.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—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 movements[85] 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—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.