652 



Journal of Agriculture. 



[lo Nov., 1908. 



of swallowing or yawning, but sufficiently often to equalise the pressure 

 within and without the tympanic cavity and so to prevent the drum from 

 being sucked inwards or bellied outwards. The actual organ of hearing is 

 the COCHLEA, which is a bony structure bearing a striking resemblance to a 

 snail's shell (Fig. 56). The spiral cavity however is not single as in the 

 shell but divided, except at the extreme tip, by two partitions into three 

 spiral canals — the scala tymfani, the scala vestibuli, and the canalis 

 cochlccs (Fig. 58). All these canals are filled with a watery fluid. The 

 upper partition — Reissner's membrane — is very flimsy, but the lower one 

 is made up of a strong shelf of bone from which a m.embrane — the basilar 

 MEMBRANE — stretches to the opposite wall. This basilar membrane is- 

 composed of straight fibres which radiate out from the tip of the bony 

 shelf. Perched on the basilar membxane, and continuing with it up the 

 whorl, is the organ of Corti of which no detailed description need be 

 given here as so far its method of action has not been fully elucidated. It 

 will be enough to state that the auditory nerve has its endings in the organ; 

 of Corti, which latter may be looked on as an array of sound-receptors. 



jmR.. 



-9^p 



~ n 



Fig. 58. .Section through the cochh-a of the ear. ac, duct of coclilea ; sr 1 

 restibull ; nc. t. scala lympani ; v\ Ijony wall of cochlea ; c, organ of Corti on me 

 hanilarin ; iii.r., membrane of Reissner ; 11., nerve fibres of cochlear nerve. 



Sobotta 



scala 



tnembratia 



After 



The vibrations of the eaj drum are communicated to the bridge of 

 ossicles described above. The other end of the bridge is fastened to an 

 elastic window (Fig 56, 2) that shuts off the scala vestibuli from the tym- 

 panic cavity and so the vibrations are transmitted through this elastic 

 window to the fluid in the scala vestibuli. The vibrations pass up this 

 canal to the tip of the cochlea and then down the scala tympaiii and come to 

 a halt at another elastic window which shuts off the lower end of the scala 

 tymfani (Fig. 56, 9). The vibrations therefore course above and below the 



