72 PHYSIOLOGY OF THE HUMAN BODY. 
the head, which ordinarily receives the name of ear for itself, and of the 
passage seen entering the head. This passage is completely closed on the 
inside by a membrane called the membrane of the drum of the ear, and 
behind this is the middle ear. The latter is a cavity filled PE air 
received through a tube communicating with the nose, and, across wie 
there stretches a chain of curiously shaped: little bones. ” ‘These bones 
form the communication between the membrane of the drum and one of 
the membranes covering the two openings into the internal ear, and their 
use seems to be to communicate the sound-vibrations with greater intensity 
than if they had been communicated direct to the inner membrane. 
The internal ear consists of three parts—the vestibule, the semicircular 
canals, and the cochlea [Latin, ‘the shell of a snail’]. The vestibule is 
the middle part V, and com- 
municates with the middle ear 
by an opening covered with a 
of the bony chain mentioned 
above is attached ; the other 
two parts are extensions of 
this cavity (and open into it, 
as into a hall; hence its 
name) chiefly for the purpose 
of giving a greater extent of 
surface for the membrane in 
which the fibres of the audi- 
tory nerve are distributed. 
The semicircular eanals are 
three passages, S, in the bone, 
all lined with the same mem- 
brane as the vestibule. The cochlea, C, is a spiral canal, very like the 
inside of a snail-shell, and also lined with the membrane of the vestibule. 
The passage is divided into two by a partition running through it length- 
wise. The two passages thus formed only communicate with each other 
at the inner end; at the entrance of the spiral, the one passage opens into 
the vestibule, the other into the middle ear, the latter opening being 
closed with a membrane. All these cavities are completely filled with 
a fluid, which is made to vibrate when the vibrations of the air which strike 
on the membrane of the drum are communicated, through the chain of 
bones in the middle ear, to the membrane covering the entrance to the 
vestibule. These vibrations act on the fibres of the auditory nerve dis- 
tributed over the membrane lining the cavities; and the impressidus 
being communicated to the brain, produce the sensation of sound. 

membrane, to which the end — 

ee 
