1. a muscle that contracts without conscious control and found in walls of internal organs such as stomach and intestine and bladder and blood vessels (excluding the heart) • Syn: involuntary muscle • Hypernyms: muscle, musculus • Hyponyms: myometrium, musculus sphincter ani internus 2. muscle tissue that does not appear striated under the microscope; has the form of thin layers or sheets • Hypernyms: muscle, muscular tissue • Part Meronyms: smooth muscle cell