474 



Comparative Animal Physiology 



to vibrate as a unit. This motion causes a rocking of the stapes, which is 

 fastened eccentrically to the inner ear drum covering an oval window in the 

 bony inner surface of the middle ear. There are two small muscles in the 

 middle ear, one attached to the malleus and one to the stapes. Contraction 

 of these muscles reduces the amplitude of the vibrations and thereby de- 

 creases the sensitivity of the ear. Contraction is usually under reflex control, 

 but in exceptional cases it may be voluntary. Below the oval window is 

 another opening, the round window, which is also covered by a membrane, 

 the auxiliary ear drum. The bones of the middle ear do not make contact 

 with the membrane of the round window, the function of this membrane 

 being to relieve the pressure exerted on the inner ear by the stapes. When 

 the covering of the oval window is pushed inward by the stapes, the cover- 

 ing of the round window bulges outward. The cavity of the middle ear is 

 connected to the oral cavity by the eustachian tube, the function of which 

 is to equilibrate air pressure of the middle ear with that of the environment 

 and to serve as a drainage canal. 



TYMPANIC ME 



Fig. 154. Diagrammatic representation of the ear. A, Schematic diagram of middle 

 and internal ear. The oval window (unlabeled) lies above the round window and con- 

 tacts the stapes. B, Schematic diagram of the ossicles of the middle ear, showing position 

 of various members at rest and after inward displacement of tympanic membrane. Dotted 

 lines and arrows represent path of sound waves. After Stevens and Davis'" from Fulton.^" 



The principal function of the middle ear is to transmit sound waves from 

 air to liquid. Because of the ratio of the areas of the tympanic membrane 

 and the stapes (90 to 3.2 square millimeters) and the reduction in total am- 

 plitude of vibration (by a factor of 2) there exists a very considerable in- 

 crease (roughly, 50 times) in the force which is available for driving the 

 fluid of the scala vestibuli. This increase in force is of the proper order of 

 magnitude to overcome the increase in the greater cross sectional impedance 

 of the liquid as compared with air. In general, the middle ear seems to be 

 a very efficient system for transmitting vibrations from air to liquid. Lower 

 animals which have no middle ear are at a distinct disadvantage. 



Inner Ear. The inner ear is a Huid-filled structure housed in a complex 

 system of cavities and tunnels known as the osseous labyrinth. The inner 

 ear has two functions: detection of sound and detection of movement. De- 

 tection of sound is accomplished by the cochlea, a spiral bony labyrinth 



