320 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



auricle or 

 pinna 



semicircular canal 



nerve 



_ ductus 



en.d.oltjiiiphaticu3 



LOl sac 



e diate 

 ot 

 hlecL 



i lymph. 



External 

 ear 



Fig. 17.9. The human mechanism of hearing and adjacent organ of balance, the 

 semicircular canals. A diagram of the general structure shown by a cut through the 

 temporal bone. The inner ear is structurally but not functionally associated with 

 the semicircular canals. The pharyngo-tympanic duct (Eustachian tube) opens into 

 the pharynx. Pressure of air against the outer side of the eardrum is evident in 

 travel through a deep tunnel. It is balanced when the mouth is opened and air goes 

 through the Eustachian tube and presses against the inner side of the eardrum. 

 (Courtesy, Ham: Histology, ed. 2. Philadelphia, J. B. Lippincott & Co., 1953.) 



and the vestibular cavity above and the tympanic below it. The latter two 

 are continuous, one into the other at the tip of the cochlea. At the base of the 

 cochlea the vestibular cavity comes to an end against the membrane filling 

 the oval window and the stapes. At the base of the cochlea, the tympanic 

 cavity is also ended by a membrane that closes the round window. When 

 the membrane of the oval window is pushed in toward the vestibular cavity 

 by vibrations in the middle ear, the fluid in the cavities is moved, finally 

 pushing against the resilient membrane in the round window (Fig. 17.9). As 

 the minute vibrations surge along through the fluid from the oval to the 

 round window, they vibrate the basilar membrane on the floor of the 

 cochlear duct which contains the actual organ of hearing, the organ of 

 Corti. Fibers of the auditory nerve extend to the receptor cells in this organ. 

 These cells are similar to those of taste and smell in that their hairlike 

 processes protrude into the fluid which floods over them. A delicate mem- 

 brane (tectorial) projects like a miniature porch roof over and so close to 

 the processes that the slightest jar of the basilar membrane brings them in 

 touch with it. Thus the receptor cells are stimulated, and they in turn excite 

 impulses that are transmitted to the brain by fibers of the auditory nerves. 



Summary of Action. Sound vibrations move along the chain of bones 

 in the middle ear and against the membrane of the oval window, thereby 



