Structural component of the bacteriophage tail which consists of a contractile sheath, a tube and a baseplate. The central cylindrical segment of the tail consists of a rigid tube, composed of multiple copies of gp19, surrounded by the outer contractile sheath assembled from gp18 subunits. A total of 138 copies of gp18 arranged into 23 hexameric rings constitutes the sheath. During infection, contraction of the sheath drives the central tube through the host outer membrane, creating a channel for DNA ejection from the capsid into the host cell. Hexamer. Belongs to the myoviridae tail sheath protein family.