230 ANATOMY FOE NUESES. [CHAP. XIX. 



cates above with a number of bony cavities in the mastoid por- 

 tion of the temporal bone. 1 The cavities, called mastoid cells, 

 are lined with mucous membrane, which is continuous with that 

 covering the cavity of the tympanum. 



Stretching across the tympanic cavity is a chain of tiny mov- 

 able bones, three in number, and named from their shape the 

 malleus or hammer, the incus or anvil, and the stapes or stirrup. 

 The hammer is firmly attached to the drum-membrane, and the 

 stirrup is fastened into the oval window (also covered by mem- 

 brane) leading into the inner ear. The anvil is placed between 

 the hammer and stirrup, and attached to both by delicate 

 articulations. These little bones are set in motion with every 

 movement of the drum-membrane. Vibrations of the mem- 

 brane are communicated to the hammer, taken up by the anvil 

 and transmitted to the stirrup, which is driven slightly forward, 

 and sets in motion the membrane covering the oval opening 

 leading into the internal ear. 



The internal ear or labyrinth receives the ultimate termina- 

 tions of the auditory nerve, and is, therefore, the essential part 

 of the organ of hearing. It consists of (1) a bony labyrinth, 

 which is composed of a series of peculiarly shaped cavities, hol- 

 lowed out of the petrous portion of the temporal bone, and 

 named from their shape the vestibule, the semicircular canals, 

 and cochlea (snail-shell). This bony labyrinth is lined by a 

 serous membrane, which secretes a watery fluid called the peri- 

 lymph ; and lying within the bony labyrinth and peri-lymph is 

 (2) a membranous labyrinth, which is composed of a series of 

 sacs or tubes, fitting more or less closely within the vestibule, 

 semicircular canals and cochlea, the two former being concerned 

 with the sense of equilibrium, the last with the sense of hear- 

 ing. The membranous labyrinth is filled with a watery fluid 

 called endo-lymph. In its walls terminate the dendrones of the 

 auditory nerve. Before its termination, the auditory nerve 

 divides into two branches, the cochlear supplying the cochlea, 

 the vestibular supplying the vestibule and semicircular canals. 

 The cells of origin of these two branches constitute two ganglia 

 situated in the region of the labyrinth. Their dendrones are 

 distributed to the epithelial lining of the membranous sac, while 



1 The mastoid portion of the temporal bone is that rounded mass of bone 

 which one readily distinguishes behind the auricle. 



