BASILAR MEMBRANE 285 



Organ of Corti 



The end organ of the auditory nerve is represented by the organ of 

 Corti. It consists of highly differentiated neuroepithelium whose 

 specialized cells are arranged in an invariable pattern extending from its 

 base to the apex of the cochlea. 



The superficial area of the organ has projecting above its surface an 

 inner row and three to five outer rows of auditory cells terminating in a 

 brush of cilia (hair cells) presumably projecting through a cuticular 

 membrane. There are about 3600 inner and about 18,000 outer hair 

 cells. The free margin of the tectorial membrane overhangs and rests 

 upon these hair cells. Presumably, any movement of the tectorial mem- 

 brane can be transmitted to the hair cells. An adequate stimulus for 

 their tactile nerve endings is not so much a change in pressure as a defor- 

 mation of the surface in which they are embedded. 



In evaluating the sensibility of any tactile area it is important to 

 remember that the hairs themselves form very effective tactile organs, 

 for there is a dense group of nerve fibers around the root of each hair. A 

 force applied to the free end of a hair acts on the long end of a lever with 

 the fulcrum at the surface of the membrane, so that a much greater force 

 can be developed at the root of the hair to distort the tissues in which the 

 ends of the acoustic nerve fibers are embedded. 



Basilar Membrane 



The basilar membrane is the membranous structure upon which rests 

 the epithelium of Corti's organ. Hardesty [1908] describes it as a flat 

 tendon strengthening the floor of the scala media. Its lower surface is 

 covered by a continuation of the lining membrane of the scala tympani. 

 Its intermediate layer is made up of tendinous bands which span the gap 

 between the inner margin of the bony spiral lamina and the margin of 

 the external spiral ligament. 



The internal bony shelf (modiolus) has its greatest width at the begin- 

 ning of the basal turn, from which point its width rapidly diminishes 

 towards the apex of the cochlea. The gap to be spanned by the basilar 

 membrane (Fig. VII-18) is relatively small at the beginning of the basal 

 turn of the scala media but progressively widens as the apex of the coch- 

 lea is approached. The shortest and thickest fibers bridging the gap to 

 form the basilar membrane are consequently located at the base of the 

 cochlea near the fenestra ovalis. The average length of the fibers in the 

 basal turn is 0.21 mm, increasing to 0.34 mm in the middle and to 0.36 

 in the apical turn. The total spiral length of the organ of Corti and its 



