ANATOMY OF THE EAR 259 called the periotic capsule and this in turn changes to dense bone in which some cartilage cells persist throughout life. This dense enchondral bone forms the bony labyrinth embedded, fossil-like, in the petrous temporal bone. The interior of the bony labyrinth is lined by endosteum. The central part of the bony labyrinth is the vestibule in the lateral wall of which lies the fenestra vestibuli (oval window) closed by the footplate of the stapes and the annular ligament. The medial or inner wall of the vestibule is directed towards the internal acoustic meatus and is perforated by nerve endings. It contains two depressions—the spherical recess for the saccule and the elliptical recess for the utricle. Below the elliptical recess there is the small opening of the aqueduct of the vestibule which transmits the endolymphatic duct. The posterior, superior and inferior walls of the vestibule contain the five openings of the semicircular canals. The lateral or horizontal canal has an opening at each end but the superior and posterior vertical canals have a common opening at their non-ampullated ends, the crus commune (Figs. 139, 140). The bony cochlea, anterior to the vestibule, is a spiral tube wound two and a half times round its central axis or modiolus. Its base is directed towards the fig. 142. Vertical coronal section through right ear, axial section through cochlea. 1, Tensor tympani; 2, Helicotrema; 3, Canal for greater superficial petrosal nerve; 4, Modiolus; 5, Facial nerve; 6, Cochlear aqueduct; 7, Edge of ; 8, Carotid , -bone capsule of cochlea canal; 9, Tubal cells; 10, Tuba! part of tym- panic cavity. internal acoustic meatus and its apex towards the internal carotid artery (Fig. 142). From the modiolus, which contains the cochlear nerve, a hollow bony ledge (spiral lamina) projects into the lumen of the tube transmitting the branches of the nerve. This ledge commences in the vestibule just above the fenestra cochleae (round window) and ends near the apex of the cochlea at the hamulus or hook. The basilar membrane of the membranous cochlea is attached to the edge of the bony spiral lamina and the outer surface of the membranous cochlea is attached to the inner wall of the bony cochlea thus dividing each coil of the bony cochlea into three parts: (1) the upper part, the scala vestibuli, commences from an opening in the vestibule below the spherical recess, (2) the lower part, the scala tympani, begins at the fenestra cochleae (round window), (3) the membranous cochlea lying between the upper and lower parts is in effect part of the scala vestibuli separated from it by a membrane (Reissner's membrane) and is known also as the scala media. At the apex ofthe cochlea the membranous cochlea ends blindly and in this region, known as the helicotrema (Figs. 140, 142 and 143), the scala vestibuli and scala tympani are continuous with one another.