The muscular system includes those tissues of the body, which by virtue of their composition of contractile tissue, are able to cause some type of movement. The term "contractile" refers to the fact that muscles are made of complex muscle fibers, composed of myofibrils. These myofibrils are composed of filaments that overlap each other in a way that allows them to slide past each other, changing the degree of overlap. This allows the muscle to grow shorter or longer, resulting in movement.

Some muscles are under voluntary control, meaning that an individual can decide to move those muscles. Voluntary muscles are also known as skeletal muscles, because most of these muscles are attached to the bones of the body, and are responsible for moving the skeletal system. Voluntary muscles not directly attached to bones include those muscles around the mouth and the anus. While...