986 



THE AUDITORY OSSICLES. 



[BOOK IIT.J 



branous labyrinth known as the utricle on the inside ; in the 

 dried bony labyrinth (Fig. 167 F.o.) it appears as a hole in 

 the vestibule. The other, round in shape, called the fenestra 

 rotunda (Figs. 166, 171 /./*.), lies between the tympanic cavity 

 and a part of the perilymph space which enters into the con- 

 struction of the cochlea ; as we shall see, the perilymph space 

 of the cochlea may be regarded as a peculiar tubular prolonga- 

 tion of that of the vestibule, and the membrane of the fenestra 

 rotunda closes as it were the end of this prolongation. 



Certain bones of the skull, converted by striking develop- 

 mental changes into a jointed chain of minute bones, the 

 auditory ossicles (Figs. 166 m. i. St., 172), are by processes of 

 growth thrust into the tympanic cavity in such a way that 

 they eventually seem to lie wholly in the cavity, and to form 



cp 



172. THE AUDITORY OSSICLES. (After Schwalbe and Helmholtz.) 

 Magnified four times. 



A. The malleus, cp. the head (caput) . * the articulating surface for the incus. 

 t. tooth locking with tooth of incus. Ig. is placed opposite the attachment of 

 the ligaments, p.f. processus gracilis or Folianus, represented as short, p.b. pro- 

 cessus brevis. m.br. handle (manubriuin) . 



B. The incus. * surface articulating with malleus, t. tooth locking with tooth 

 of malleus, p'.b'. processus brevis. p'.l'. processus longus. 



B'. The lower end of the processus longus seen sideway ; o. its expanded 

 termination or os orbiculare. 



C. The stapes, c. the head. /. the foot-plate. 



D. The three ossicles in connection. M. malleus, /. incus, st. stapes ; the 

 other letters as above. 



a bridge across the cavity between the tympanic membrane on 

 the outer side, and the fenestra ovalis on the mesial side (Fig. 

 173). The ossicles are three in number; to the tympanic mem- 

 brane is attached the malleus; this is joined to the incus, whicli 

 in turn is joined to the stapes, the end of which is attached to 

 the fenestra ovalis. 



612. The affections of consciousness, which we call sen- 

 sations of sound, are the result of auditory impulses reaching 



