ear the front of an eye which is directed to a distant object,
three reflected images of the flame will be seen in the eye, one on each
of the anterior surfaces of the cornea and lens, and a third on the
posterior surface of the latter. If the eye is directed to a near
object, the reflection on the cornea remains unchanged, while that on
the anterior surface of the lens gradually diminishes and approximates
in size the reflection on the cornea, thus giving conclusive evidence
that, in viewing a near object, the anterior surface of the crystalline
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
gristly portion of the auditory apparatus is about one inch in length,
lined by a continuation of the integument of the ear, and has numerous
hairs on its surface, to prevent the intrusion of foreign substances.
Between the external MEATUS and the cavity of the middle ear is the
_membrana tympani_, which is stretched across the opening like the head
of a drum. The _tympanum_, or ear-drum, communicates with the pharynx by
the _eustachian tube_, which is a narrow passage lined with delicate,
ciliated epithelium. On the posterior portion it is connected with the
_mastoid cel