es, Eustachian, ear drum)]

The iris corresponds to the adjustable diaphragm of the camera. Just
behind the pupil is the lens of the eye, which also is adjustable by
the action of a little muscle, called the "ciliary muscle". This
muscle corresponds to the focussing mechanism of the camera; by it the
eye is focussed on near or far objects. The eye really {195} has two
lenses, for the cornea acts as a lens, but is not adjustable. The
"aqueous and vitreous humors" fill the eyeball and keep it in shape,
while still, being transparent, they allow the light to pass through
them on the way to the retina. The retina is a thin coat, lying inside
the choroid at the back of the eyeball, and having the form of a
hollow hemisphere. The light, coming through the pupil and traversing
the vitreous humor, strikes the retina from the inside of the eyeball.
Other accessory apparatus of the eye includes the lids, the tear
glands, and the muscles that turn the eyeball in any direction.

[Illustration: Fig. 30.--Two views of the internal ear. These views
show the shape of the internal ear cavity. The sense organs lie inside
this cavity. Notice how the three semi-circular canals lie in three
perpendicular planes. (Figure text: cochlea, vestibule, 3 Canals)]

The ear is about as complex a piece of mechanism as the eye. We speak
of the "outer", "middle" and "inner" ear. The outer, in such an animal
as the horse, serves as a movable ear trumpet, catching the sound
waves and concentrating them upon the ear drum, or middle ear. The
human external ear seems to accomplish little; it can be cut off
without noticeably affecting hearing. The most essential part of the
external ear is the "meatus" or hole that allows the sound waves to
pass through the skin to the tympanic membrane or drum head. The sound
waves throw this membrane into vibration, and the vibration is
transmitted, by an assembly of three little bones, across the
air-filled cavity {196} of the middle ear to an opening leading to the
water-filled cavity of the inner ear. This opening from the middle to
the inner ear is closed by a membrane in which one end of the assembly
of little bones is imbedded, as the other end is imbedded in the
tympanic membrane; and thus the vibrations are transmitted from the
tympanic membrane to the liquid of the inner ear. Once started in this
liquid, the vibrations are propagated through it to the sense cells of
the cochlea and stimulate them in the w