r,oi; PHYSIOLOGY 



INTERNAL EAR 



Within the petrous portion of the temporal bone are two mechanisms 

 anatomically in close relationship but physiologically entirely separate. 

 One of these mechanisms, which is called the cochlea, belongs to the auditory 

 organ ; the other, called the vestibule, consists of a series of organs which con- 

 cern equilibration and have no connection with hearing. 



THE LABYRINTHS lie one within the other ; the outer or osseous 

 labyrinth is hollowed out of the petrous portion of the temporal bone, and it 

 conforms roughly with the shape of the membranous labyrinth within it. 

 Between the two is liquid so that, as in the case of the brain, no constraint 

 is put by the external wall on the soft structures which it contains. This 

 liquid is called perilyrnph. The membranous labyrinth consists of a series 

 of hollow ducts and sacs which are filled with liquid called the endolymph. 

 The parts of the membranous labyrinth and the relative positions which 

 they occupy are shown in Fig. 306. From before backwards they will be 

 seen to consist of a spiral tube called the cochlea, the saccule, and the utricle 

 to which are connected the three semicircular canals. 



Of these structures the cochlea alone is concerned with hearing, as the 

 following evidence shows. 



(1) Destruction of the utricle and canals causes disturbed equilibration, 

 nystagmus and vomiting, but no deafness. 



(2) Destruction of the cochlea causes deafness but no disturbance of 

 equilibration. 



(3) Fishes, in which no evidence of hearing can be found, possess 

 utricle, saccule and canals, but no cochlea. 



THE COCHLEA is a tube 20 to 30 mm. long which is spirally wound 

 round a cone of bone called the modiolus, through the centre of which enters 

 the auditory nerve. From the modiolus a spiral lamina of bone extends 

 about two-thirds the way across the spiral cochleal canal so as partially to 

 divide it into two equal portions. From the outer edge of this lamina two 

 membranes extend to the walls of the canal, so that the latter is divided through- 

 out its length into three separate ducts. The upper duct is called the scala 

 vestibuli, the middle duct between the two membranes the scala media, 

 and the lower the scala tympani. The two membranes dividing 

 of! these ducts are quite different in structure ; whereas the upper, called 

 Reissner's membrane, is a thin layer of cells only, the lower is of complicated 

 arrangement and is called the basilar (base) membrane. To the latter is 

 attached a series of sensitive hair cells, called the organs of Corti, 

 connected to the fibres of the auditory nerve, which run through the osseous 

 spiral lamina to the body of the modiolus. To the upper edge of the spiral 

 lamina is attached a projecting ledge called the lamina tectoria ; this is so 

 mounted that it projects over but probably does not quite touch the tips of 

 the hair cells. If however the basilar membrane is displaced upwards, 

 the hairs touch the membrana tectoria, and the resulting stimuli are com- 

 municated to the auditory nerve. To prevent damage to the hairs, owing to 

 excessive motion of the basilar membrane, rods of Corti are placed between 



