lens become _more convex_, and at the same time approaches the cornea.
Five or six inches is the minimum limit of the muscular adjustment of
the eye. From that point to all the boundless regions of space, to every
star and nebulae which send their rays to our planet, human vision can
reach. It is the sense by which we receive knowledge of the myriads of
worlds and suns which circle with unfailing precision through infinite
space.


HEARING.


[Illustration: Fig. 64.
Internal and external ear. 1. External ear. 2. Internal
auditory meatus. 3. Tympanum. 4. Labyrinth.
5. Eustachian tube.]

Hearing depends upon the sonorous vibrations of the atmosphere. The
waves of sound strike the sensitive portions of the ear, and their
impressions upon the auditory nerves are termed the sensations of
hearing. The ear is divided into three parts, called respectively the
External, Middle, and Internal ear.

The external organs of hearing are two in number, and placed on opposite
sides of the head. In most of the higher order of vertebrates, they are
so situated as to give expression and proportion to the facial organs,
and, at the same time, to suit the requirements of actual life.

The _External ear_ is connected with the interior part by a prolongation
of its orifice, termed the _external auditory meatus_. In man, this