"Movement through space is fundamental to life: movement of the entire organism, such as walking; movement of a part of the organism, such as an arm; movement of materials within the organism, such as food in the stomach; movement of offspring into the external world, as in birth. Each is a mechanism indispensable to virtually all members of the animal kingdom. In the end, it is muscle that accomplishes movement. As a consequence, muscle is the most abundant tissue in most animals and accounts for much of the energy-consuming cellular work in an active animal."