The surface relief of the body is determined mainly by the muscles which are attached to the skeleton and move it and partly by the subcutaneous fat tissues. The muscles constitute the flesh of the body. There are also the vascular and intestinal muscles which are not externally visible and do not belong to surface anatomy. Every muscle is composed of small fibers which are bound together with tissue. Nervous stimulation causes the fibers to contract and this makes the muscle change its shape. If the nervous stimulation ceases so does the contraction of the fibers, and the muscle ceases to work. According to the demands on it the muscle grows in size or diminishes.
Muscles are attached very firmly to the bones by tendons. These are composed of strong, inelastic fibers, which can be seen and felt on the surface of the body when the muscles are tensed, for instance at the sides of the hollow of the throat, between the muscles of the forearm, wrist, and fingers, between the shin and toes, and at the sides of the kneecap when the knee is bent. The tendons running between the forearm and fingers and between
the shin and toes are appreciably longer than the corresponding muscles. Tendons, especially the largest, the Achilles tendon, which builds the contour of the heel, can easily be mistaken for bone, for they are tightly stretched and feel hard.
Except in sleep, the muscle fibers are always held slightly tensed, even when they are not working. They are like engines ticking over. This rest tension is called "tone." It influences the whole appearance of a person, and if the tone is too slack he looks, and probably is, slack, tired, or ill.
Every muscle can contract or relax either in jerks or very gradually. As long as there is any contraction the muscle is working, for there is no mechanism for maintaining it at a constant tension. If the muscle stops working, the limb which was held or moved by its power falls by force of gravity back to a position of rest.
Muscles only pull, they cannot push, and every muscle has its counterpart working in the opposite direction.
If the extensor muscles of an elbow were cut, the arm would still be able to lift something towards the body by tension of the flexor, but, except by force of gravity, it would be unable to put it down again. Usually only the most important functions of muscles are given in anatomy books, but it is interesting to note that some muscles, particularly in the chest, back, and shoulders, have subsidiary functions; when a joint is bent to its fullest extent the direction of a muscle's "pull" may be changed and its function as flexor or extensor altered as well.