232 THE HUMAN BODY 



casts of its interior are represented from different aspects in Fig. 71. 

 The vestibule is the central part and has on its exterior the oval 

 foramen (Fv) into which the base of the stirrup-bone fits. Behind 

 the vestibule are three bony semicircular canals, communicating 

 with the back of the vestibule at each end, and dilated near one 

 end to form an ampulla (Vpa, Vaa, and ha). The horizontal 

 canal lies in the plane which its name implies, and has its am- 

 pulla at the front end. The two other canals lie vertically, the 

 anterior at right angles, and the posterior parallel, to the median 

 anteroposterior vertical plane of the head. Their ampullary ends 

 are turned forwards and open close together into the vestibule; 

 their posterior ends unite (Vc) and have a common vestibular 

 opening. 



The bony cochlea is a tube coiled on itself somewhat like a snail's 

 shell, and lying in front of the vestibule. 



The Membranous Labyrinth. The membranous vestibule, 

 lying in the bony one, consists of two sacs communicating by a 

 narrow aperture. The posterior is called the utriculus, and into it 

 the membranous semicircular canals open. The anterior, called the 

 sacculus, communicates by a 

 tube with the membranous 

 cochlea. The membranous sem- 

 icircular canals much resemble 

 the bony, and each has an am- 

 pulla; in most of their extent 

 they are only united by a few ir- 

 regular connective-tissue bands 

 with the periosteum lining the 

 bony canals; but in the ampulla 



One side of the membranous tube FIG. 72. A section through the cochlea 

 , , . , , in the line of its axis. 



js closely adherent to its bony 



protector; at this point nerves enter the former. The relations 

 of the membranous to the bony cochlea are more complicated. 

 A section through this part of the auditory apparatus (Fig. 72) 

 shows that its osseous portion consists of a tube wound two and 

 a half times (from left to right in the right ear and vice versa) around 

 a central bony axis, the modiolus. From the axis a shelf, the 

 lamina spiralis, projects and partially subdivides the tube, 

 tending farthest across in its lower coils. Attached to the ou1 



