MUS'CLE, noun [Latin musculus, a muscle and a little mouse.]

1. In anatomy, the muscles are the organs of motion, consisting of fibers or bundles of fibers inclosed in a thin cellular membrane. The muscles are susceptible of contraction and relaxation, and in a healthy state the proper muscles are subject to the will, and are called voluntary muscles. But other parts of the body, as the heart, the urinary bladder, the stomach, etc. are of a muscular texture, and susceptible of contraction and dilatation, but are not subject to the will, and are therefore called involuntary muscles. The red color of the muscles is owing to the blood vessels which they contain. The ends of the muscles are fastened to the bones which they move, and when they act in opposition to each other, they are called antagonists.

Muscles are divided into the head, belly and tail. The head is the part fixed on the immovable joint called its origin, and is usually tendinous; the belly is the middle fleshy part, which consists of the true muscular fibers; the tail is the tendinous portion inserted into the part to be moved, called the insertion; but in the tendon, the fibers are more compact than in the belly of the muscle and do not admit the red globules.

2. A bivalvular shell fish of the genus Mytilus; sometimes written mussel.