Muscle is a
contractile tissue of animals and is derived from the
mesodermal layer of embryonic germ cells.
Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as
skeletal,
cardiac, or
smooth muscles. Their function is to produce
force and cause
motion. Muscles can cause either locomotion of the organism itself or movement of
internal organs. Cardiac and smooth
muscle contraction occurs without
conscious thought and is necessary for survival. Examples are the contraction of the
heart and
peristalsis which pushes food through the
digestive system. Voluntary contraction of the skeletal muscles is used to move the body and can be finely controlled. Examples are movements of the eye, or gross movements like the
quadriceps muscle of the
thigh.