vI FORM AND FUNCTION 133 
also a great extension, called, because it is shaped like 
a spiral shell, the cochlea, and into this too the 
fluid extends. A membranous bag, also filled with 
fluid, extends throughout the ramifications of the 
cochlea and the labyrinth. On the inside of the 
membranous bag within the labyrinth, at certain 
points where it is attached to the bony wall, are hairs 
which are believed to communicate with the nerve of 
hearing. So far, 1 have been describing the ear 
proper. The rest of the machinery has for its object 
the communication of the vibrations of sound to the 
fluid in the bony labyrinth, from which they pass to 
that in the membranous bag which lies in it, from that 
to the hairs which connect with the nerve. The 
apparatus for conveying sound vibrations to the laby- 
rinth is rather complicated. There is, to begin with, 
a membrane which stretches across the external 
aperture of the ear. When a sound sets the air moving 
in waves, which we speak of as vibrations, they strike 
against the membrane. To this membrane and the 
chamber behind is given the name of the drum of the 
ear, and on the further side of the drum is the 
labyrinth described above. Three bones united 
together have their one end resting against the outer 
membrane just mentioned, the other against another 
membrane that at one point takes the place of bone in 
the bony labyrinth. Thus, the vibrations of the outer 
membrane are transmitted by the united three bones 
to the window of membrane in the bony wall of the 
labyrinth, from there to the fluid in the labyrinth, 
next to the fluid within the membranous bag, and lastly 
to the hairs within the bag that connect with the nerve, 
