EARS 469 



and communicates at its other end by a narrow passage, the 

 heHcotrema, with the lower chamber, the scala tympani, which is 

 shut off from the middle ear by the membrane of the fenestra 

 rotunda. These two chambers contain perilymph, and the varia- 

 tion in pressure set up by the movements of the stapes are damped 

 by the resistance of helicotrema and fenestra rotunda. The middle 

 chamber, or scala media, contains endolymph, and its lower wall, 

 the basilar membrane, bears the organ of Corti. This consists of 

 a number of hair-like processes connected with branches of the 

 auditory nerve, and it is these which respond to the vibrations 

 of sound. Quality, pitch and loudness can all to some extent be 

 distinguished. The ability to determine the direction from which a 

 sound comes depends largely on the phase-difference between the 

 waves arriving at the two ears. The utriculus and the three 

 semicircular canals (two vertical, one horizontal) are sensitive to 

 the position of the animal's head and to both linear and radial 

 accelerations. By means of them the animal is able to maintain 

 its normal posture. The ductus endolymphaticus runs from the 

 sacculus to a small expansion under the brain. 



Vestibule Fenestra Scala ScaLa 



/ova lis vcstibuli media 



Stapes 



^////////^/////^^^ 



/^ .^1 111111 1 111111 1 / 1 1 1 1 1 11111 1 1 1 11111 1 1 1 III! II ii i Jiim iiinniuniimrTTTT^ 



/ v/////Ay///\/////\//A<(////////y/ y- . 



FenesVa / Basilar \ \ Helicotrema 



rotunda Reissner''s membrane A, \ 



membrane ^gmpani O'-g^" o^ C<^rt. 



Fig. 368. — A very diagrammatic representation of the mammalian cochlea, 

 wound. Diagonal shading indicates bone. The stapes is in the middle 

 and the fenestra rotunda also looks into this space.— From \ app. An 

 Physiology, 1939. Clarendon Press. 



un- 



ear, 



i»uil 



OTHER SENSES 



The olfactory sense cells are situated in the Schneiderian 

 membrane which lines the olfactory chambers (p. 428). They 

 respond to air-borne particles of chemical substances. Minute 



