What is the X in X-mas?
muscular system n.
All the muscles of the body collectively, especially the voluntary skeletal muscles.
The system in the body composed of muscle cells and tissues that brings about movement of an organ or body part. There are three kinds of muscle: skeletal muscle, which is attached to bones and allows the voluntary movement of limbs; smooth muscle, which is found in internal organs and aids in the involuntary movements that occur in the circulatory, digestive, excretory, reproductive, and respiratory systems; and cardiac muscle, which forms the powerful walls of the heart. (See illustrations, previous two pages.)