smooth muscle

:  muscle tissue that lacks cross striations, that is made up of elongated spindle-shaped cells having a central nucleus, and that is found especially in vertebrate hollow organs and structures (as the small intestine and bladder) as thin sheets performing functions not subject to direct voluntary control and in all or most of the musculature of invertebrates other than arthropods—called also nonstriated muscle, unstriated muscle; compare cardiac muscle, striated muscle