(1) Any enveloping structure, such as the membranous covering of a muscle, nerve, or blood vessel. Any sheathlike structure. SYN: vagina (1). (2) The prepuce of male animals, especially of the horse. (3) A specially designed tubular instrument through which special obturators or cutting instruments can be passed, or through which blood clots, tissue fragments, and calculi can be evacuated. (4) A tube used as an orthodontic appliance, usually on molars. [A.S. scaeth]