THE INTERNAL EAR 



331 



the tube and thus prevent its normal function, which is the trans- 

 mission of air to and from the tympanum and the equalization of 

 air-pressure on the two surfaces of the tympanic membrane, 

 (the external surface at the end of the auditory canal and the 

 internal surface within the tympanum). 



Clinical. Certain muscle fibers of the pharynx are so arranged 

 that in the act of swallowing the auditory canal is opened. This 

 fact is taken advantage of in passing the Eustachian catheter into 

 the tube. 



Two openings lead from the tympanum to the internal ear 

 the oval or vestibular window and the round or cochlear window. 



The round window is closed by a membrane called the secondary membrane 

 of the tympanum. The oval window is closed by a fibrous layer and the base 

 of the stirrup bone (p. 333). 



FIG. 211. BONY COCHLEA. 



i. Ampulla of superior semi- 

 circular canal. 2. Horizontal 

 canal. 3. _ Junction of superior 

 and posterior semicircular canals. 

 4. The posterior semicircular 

 canal. 5. Foramen rotundum. 

 6. Foramen ovale. 7. Cochlea. 

 (Brubaker.} 



FIG 212. i. Utricle. 2. 

 Succule. 3. Vestibular end of 

 cochlea. 4. Canalis reuniens. 

 5. Membranous cochlea. 6. 

 Membranous semicircular 

 canals. (Brubaker.} 



The internal ear is a cavity more deeply situated in the 

 petrous bone. It is extremely complicated, consisting of semi- 

 circular canals, vestibule, and cochlea, and well named the 

 labyrinth. There are three semicircular canals placed at right 

 angles to each other; the cochlea resembles a snail- shell in form, 

 and the vestibule is between them. 



The cochlea and the vestibule both communicate with the 

 tympanum, the cochlea by the round or cochlear window; the vesti- 

 bule by the oval or vestibular window. 



The illustration (Fig. 211) shows the shape of the osseous 

 labyrinth cut from the petrous bone. Observe the three semi- 



