THE INTERNAL EAR. 



431 



media. The lamina spiralis is the spiral bony plate 

 which projects nearly at right angles from the modiolus, 

 and which incompletely separates the scala. It consists 

 of two lamellae, between which pass the branches of the 

 auditory nerve. Attached to the free edge of the osseous 

 spiral plate is the membranous spiral lamina, which is at- 

 tached to the inner surface of the cochlea ; it completes 

 the separation of scala vestibuli from the scala tympani. 



FlG. 189. SCHEMK OF THE DUCTUS COCHLEARIS AND THE ORGAN 



OF CORTI. 



N, eochlear nerve : K, inner, and P. outer hair-cells ; n. nerve-fibrils terminating in P ; 

 , a, supporting cells; d. cells in the sulcus spiralis : z, inner rod of Corti : Mb. Corti, mem- 

 brane of Corti, or the membrana tectoria ; o, the membrana reticularis ; H, G. cells filling 

 up the space near the outer wall. 



The membranous spiral lamina is formed of the following 

 structures: The membrana basilaris; it is a delicate 

 basement membrane stretched between the free edge of 

 the bony lamina and the inner surface of the cochlea, 

 Avhere it becomes thickened and ligamentous, forming the 

 ligamentum spirale. The investing endostium on the 

 upper surface of the osseous lamina spiralis becomes thick- 

 ened and forms the membrane of Todd. This divides into 



