6o2 COMPARATIVE ANATOMY 



basilar membrane. In the region of the spiral organ, the epithelial lining 

 of the cochlear duct becomes a much thickened columnar epithelium, in 

 which are sensory hair-cells and supporting cells. Two kinds of support- 

 ing cells occur, pillar cells arranged like the rafters of a house, and Deiter's 

 cells, which support the sensory hair cells. Between them, is a liquid- 

 filled cavity, the tunnel. (Fig. 497) 



The hair cells do not extend through the entire thickness of the epithe- 

 lium, but are suspended with their rounded bases hanging between the 

 supporting cells. Four to six rows of hair cells extend through the entire 

 length of the cochlea. An inner row is separated by the pillar cells from 

 the outer rows. 



These hair-cells are estimated to be sixteen thousand in number. 

 Each has on its free surface about forty stiff hairs which project into the 

 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 rotimda. 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 hindbrain. 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. 



