HEARING. II/ 



brana tympani is tense, the tooth of the incus is kept locked 

 against the notch of the malleus. When in this con- 

 dition, the two bones rotate on an axis of which one 

 extremity is at the tip of the short process of the incus, 

 the other corresponding approximately to the attachment 

 of the ligamentum anterius of the malleus : hence the 

 axis of rotation nearly coincides with the upper border 

 of the tympanic ring. The two bones may be compared 

 when in action to a bell-crank lever of which one limb, 

 representing the handle of the malleus, is approximately 

 half as long again as the other (the long process of the 

 incus), and forms with it an acute angle. Consequently 

 every motion of the tympanic membrane which is trans- 

 mitted to the stapes is diminished by about a third. 

 When the membrana tympani is relaxed, the distance 

 between the tip of the long process of the incus and the 

 handle of the malleus slightly increases. Nothing can be 

 certainly stated as to the uses of the muscles of the 

 middle ear. 



The internal ear. The essential part of the organ of 

 hearing is the cochlea and particularly the organ of Corti. 

 The organ of Corti consists of a series of arches, about 

 3,000 in number, of extreme minuteness, the span of which 

 increases gradually from the base of the cochlea to the 

 helicotrema, and of epithelial elements (the hair-cells) in 

 relation with these arches, which are intimately connected 

 with the terminations of the cochlear nerve. These struc- 

 tures rest upon a ribbon-shaped fibrous membrane (the 

 lamina spiralis membranacea or membrana basilaris) 

 which is wide at the helicotrema, narrow at the base of 

 the cochlea. It is attached by its edges to bone, and con- 

 sists chiefly of fibres which run transversely to its length, 

 and are believed to be tense. Corti's organ is contained 

 in a spiral tube, triangular in section, the duct of the 

 cochlea, one side of which is formed by the membrana 

 basilaris, the other by the membrane of Reissner. 



