The eyeball is suspended in the orbit and faces outward. Helping to fill the orbit
are a number of structures associated with the eyeball; these are the adnexa. Among
these other structures is the lacrimal apparatus.
a. The lacrimal gland is located in the upper outer corner in front. Via small
ducts, it secretes the lacrimal fluid into the space between the external surface of the
eyeball and the upper eyelid.
b. The inner surface of the eyelids and the outer surface of the eyeball are
covered by a continuous membrane known as the conjunctiva. The lacrimal fluid keeps
the conjunctiva transparent. Also, with the blink reflex, the lacrimal fluid washes away
any foreign particles that may be on the surface of the conjunctiva.
c. The free margins of the upper and lower eyelids have special oil glands.
The oily secretion of these glands helps prevent the lacrimal fluid from escaping.
d. With the movement of the eyeball and the eyelids, the lacrimal fluid is
gradually moved across the exterior surface of the eyeball to the medial inferior corner.
Here, the lacrimal fluid is collected into a lacrimal sac, which drains into the nasal
chamber by way of the nasolacrimal duct. Thus, the continuous production of lacrimal
fluid is conserved by being recycled within the body.
Section III. THE SPECIAL SENSE OF HEARING (AUDITORY SENSE)
If a medium is set into vibration within certain frequency limits (average
between 25 cycles per second and 18,000 cycles per second), we have what is called a
sound stimulus (Figure 13-5). The sensation of sound, of course, occurs only when
these vibrations are interpreted by the cerebral cortex of the brain at the conscious
a. The human ear is the special sensory receptor for the sound stimulus. As
the stimulus passes from the external medium (air, water, or a solid conductor of sound)
to the actual receptor cells in the head, the vibrations are in the form of (1) airborne
waves, (2) mechanical oscillations, and (3) fluid-borne pulses.