THE SENSE OE HEARING. 



375 



The upper tube, when followed down to the base of the cochlea, is found 

 to open freely into the vestibule of the labyrinth ; it is therefore known 

 as the scala vestibuli. The lower tube ends blindly at the base of the 

 cochlea, but, where this part bulges into the tympanum as the "promontory" 

 of its inner wall, it is perforated by the aperture known as the fenestra 

 rotunda, whose proper membrane alone prevents the perilymph from escaping 





Pig. 198.— Diagram of a transverse section of a whorl of the cochlea (after Foster): Sc. V, scala vestib- 

 uli; Sc.T, scala tympani; C.Cld, canalis cochlearis; Lam.sp, Lamina spiralis; Og.sp, ganglion spirale; 

 it. aud, auditory nerve; m.R, membrane of Eteissner; Str.v, stria vascularis; Lg.sp, ligamentum Bpirale; 

 t.l, lymphatic epithelioid lining of basilar membrane on the tympanic side: m.b, basilar membrane; 

 Org.C, organ of Corti ; L.t, Labium tympanicum; lb, Limbus; L.v, Labium vestibulare; m.t, tectorial 

 membrane. 



into the middle ear. This tube is therefore known as the scala tympani. 

 From its central position the membranous cochlear canal is frequently known 

 as the scalamedia. The scala vestibuli and the scala tympani both decrease in 

 size as they wind from the base to the apex or cupola of the cochlea; the 



membranous cochlear canal, on the contrary, increases in seel ion from base to 

 apex until near the top; hence the width of the basilar membrane and the 



