3-56. THE EYE
a. The eye is specialized for the reception of light. The optic nerve conveys the
impulses to the visual area of the cerebral cortex where the sensation is made
b. The eyeball is divided into an anterior chamber and a posterior chamber
separated by the crystalline lens. The anterior chamber contains the aqueous humor
while the posterior chamber is filled with the vitreous humor.
c. The retina is a delicate nerve membrane that receives the images and
transfers these images to the optic nerve and ultimately to the cerebral cortex. It
contains two kinds of light-sensitive cells: cones and rods.
(1) Cones are concentrated near the center of the retina and dispersed to a
lesser degree away from the center. They are sensitive to light in the daylight range
(from 0.1 to 1,000 millilamberts).
(2) Rod cells are concentrated away from the optical center. They see in
terms of shades of gray and do not differentiate colors. Sensitivity of the rods extends
as low as 0.0001 millilamberts.
3-57. THE EAR
a. The ear (figure 3-27) consists of the outer ear, the middle ear, and the
inner ear. The pinna, or external part of the outer ear, is composed of cartilage covered
with skin. It projects from the side of the head and collects sound vibrations that are
conducted by the external auditory canal to the middle ear.
b. The middle ear is the irregular space in the temporal bone filled with air and
containing the auditory ossicles: the incus (anvil), malleus (hammer), and stapes
(stirrup). They conduct vibrations from the tympanic membrane to the internal ear. The
middle ear is also connected with the nasopharynx by the auditory (eustachian) tube,
which equalizes the air pressure in the middle ear with the exterior. Another
communication exists between the middle ear and the mastoid air cells.
c. The inner ear contains the receptors for hearing and equilibrium in
relationship with the petrous portion of the temporal bone. Sound waves, which
traverse the external auditory canal to the tympanic membrane (ear drum), produce
vibrations in the auditory bones. The vibrations are transmitted to a fluid and thence to
the fibers of the hair cells of the organ of Corti, which lies at the base of the cochlear
duct. These fibers, in turn, initiate impulses in the auditory nerve, which travel to the
auditory center of the cerebral cortex.