MUSCULAR SYSTEM, ANATOMY OF (see also MUS CLE, STRUCTURE OF). Here only the voluntary muscles, under the control of the will, are considered.
The voluntary muscles form the red flesh of an animal, and are the structures by which one part of the body is moved at will upon another. Each muscle is said to have an origin and an insertion, the former being that attachment which is usually more fixed, the latter that which is more movable. This distinction, though convenient, is arbitrary, as an example will show. The pectoralis major being attached to the front of the chest and to the upper part of the arm bone its contraction draws the arm towards the chest, whereas, when, in climbing a tree, the hand grasps a branch above, contraction will draw the chest towards the arm. Generally, a muscle is partly fleshy and partly tendinous; the fleshy con tractile part is attached at one or both ends to cords or sheets of white fibrous tissue, which in some cases pass round pullies and so change the direction of the muscle's action. The other end of these cords or tendons is usually attached to the periosteum of bones, with which it blends. In some cases, when a tendon passes round a bony pulley, a sesamoid bone is developed in it which diminishes the effects of friction. A good example of this is the patella in the tendon of the rectus femoris (fig. 1).
Every muscle is supplied with blood vessels and lymphatics (fig. 1), and with one or more nerves. The nerve supply is very important both from a medical and a morphological point of view. The attachments are also important, as determining the action of the muscle. This action cannot be understood by refer ence to the dead body alone, for every movement expresses the balanced contractions of numer ous muscles. (See C. E. Beevor, Croonian Lectures for 1903, Lon don, 1904.) Muscles may be fusiform, as in fig. I, conical, riband-like, or flattened into triangular or quad rilateral sheets. They may also be attached to skin, cartilage or fascia instead of to bone, while certain muscles surround open ings which they constrict and are called sphincters. The names of the muscles have gradually grown up, and no settled plan has been used in giving them. The German anatomists at the Basle confer ence lately proposed a uniform Latin and Greek nomenclature, which, though not altogether satisfactory, is gaining ground on the European continent. As there are some four hundred muscles
on each side of the body it will be impossible here to attempt more than a mere sketch ; for the details the anatomical textbooks must be consulted.