THE SENSE ORGANS 423 



endolymph. The base of each hair-cell is supplied with the dendritic 

 terminations of a nerve fiber from the cochlear nerve. 



Suspended in the endolymph above, and possibly in contact with the 

 hair, is the fibrous membrana tectoria, the function of which is prob- 

 lematic. It has been suggested that its vibrations are communicated to 

 the hairs. Others believe that, like a soft pedal, it acts as a dampener to 

 reduce vibration. 



Between the liquid-filled cavity of the internal ear and the air-filled 

 tympanic cavity, are the two membrane-covered openings, the fenestra 

 vestibuli or fenestra ovalis to which the stapes bone is attached, and the 

 fenestra cochleae or rotunda. By means of the stapes, vibrations are 

 transmitted through the fenestra vestibuli to the perilymph and to the 

 top of the scala vestibuli. These vibrations, passing down the scala 

 tympani, cause similar vibrations of the fenestra cochleae. It is still 

 a disputed question whether the vibrations are communicated to the 

 hair-cells through the endolymph and the tectorial membrane, or through 

 the vibrations of the basilar membrane of the spiral organ. 



The auditory centers of the brain are located in the temporal lobes 

 of the cerebral hemispheres. 



Development of the Human Ear 



The membranous sac of the ear begins in the two-somite embryo as a 

 thickened patch of ectoderm lateral to the hind-brain. By the time the 

 embryo has acquired eleven somites, this placode has begun to bend in 

 towards the brain wall to form a pit. In a five-weeks embryo, the otic 

 pit is converted into a spherical liquid-filled otic vesicle, which has lost 

 its original connexion with the ectoderm. This vesicle elongates dorso- 

 ventrally, and an endolymphatic duct grows from the dorsal side, not as a 

 remnant of the primitive invagination canal, but as a new formation. 

 By the elongation and spiral twisting of the ventral part of the vesicle, the 

 cochlear duct is formed. 



The dorsal portion of the vesicle, following the outgrowth of the 

 endolymphatic duct, becomes the utriculus. The two vertical canals 

 develop from a single dorsal hollow outpocketing of the utriculus; but 

 the lateral hollow outgrowth from which the horizontal semicircular duct 

 develops arises later, as might be expected from its phylogenetic history. 

 Each semicircular duct is formed by a partial fusion of the lateral walls 

 of its hollow anlage, by which the cavity is obliterated except at its 

 periphery, where it persists as the cavity of the definitive duct connected 

 with the utriculus at each end. At one of these ends the duct swells 

 out into an ampulla. 



That portion of the membranous sac which is intermediate between 

 the utriculus and the cochlea is converted into the sacculus. Its separa- 



