CHAP. XIX.] ORGANS OF SPECIAL SENSE. 225 



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-mem- 

 brane, and the stirrup is fastened into the oval window (also 

 covered by membrane) 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 membrane 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 membranous labyrinth 

 is filled with a watery fluid, called the endo-lymph, and is lined 

 for the most part by specially modified epithelium cells, which 

 are connected with the terminations of the auditory nerve. 



We may conceive of the internal ear as a bony cavity, elabo- 

 rately tunnelled from the expanded entrance or vestibule into 

 semicircular canals above, and into a canal resembling a snail's 

 shell below. In this bony labyrinth is a certain amount of 

 fluid, and a membranous bag, drawn out and expanded at inter- 

 vals to fit more or less closely within the bony labyrinth. This 

 membranous bag contains fluid, and is lined by a most highly 



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

 which one readily distinguishes behind the auricle. 



