retinas. This process, called convergence, produces
clear, three-dimensional vision.
HEARING
The ear is the primary organ of hearing. Its major
parts are illustrated in figure 1-52. The ear is divided
into three parts: the external, middle, and inner ear.
External Ear
The external (outer) ear is composed of two parts,
the auricle and the external auditory canal
(fig. 1-52). The auricle, or pinna, is a cartilaginous
structure located on each side of the head. The auricle
collects sound waves from the environment, which are
then conducted by the external auditory canal to the
eardrum. The lining of the external auditory canal
contains glands that secrete a wax-like substance
called cerumen. Cerumen aids in protecting the
eardrum against foreign bodies and microorganisms.
The tympanic membrane, or eardrum, is an oval
sheet of fibrous epithelial tissue that stretches across
the inner end of the external auditory canal. The
eardrum separates the outer and middle ear. The sound
waves cause the eardrum to vibrate, and this vibration
transfers the sounds from the external environment to
the auditory ossicles.
Middle Ear
The middle ear is a cavity in the temporal bone,
lined with epithelium. It contains three auditory
ossicles—the malleus (hammer), the incus (anvil), and
the stapes (stirrup)—which transmit vibrations from
the tympanic membrane to the fluid in the inner ear
1-47
HM3f0151
OPTIC
NERVE
IMAGE ON
RETINA
OBJECT
IRIS
LENS
PUPIL
CORNEA
Figure 1-51.—The vision process.
SCLERA
CHOROID
RETINA
OPTIC DISK
(BLIND SPOT)
NASAL
SIDE
TEMPORAL
SIDE
FOVEA CENTRALIS
(AREA OF SHARPEST VISION)
HM3f0150
Figure 1-50.—Ophthalmoscope view of the eye.
