Fundamentals of Anatomy and Physiology

The Nervous System: The Brain, Cranial Nerves, Autonomic Nervous System, and the Special Senses 265

Ciliary body
and muscle^
Suspensory
ligament

Conjunctiva^
Iris^
Pupil

Posterior cavity

Anterior chamber (vitreous humor) of anterior cavity

(aqueous humor)

Cornea

Lens

Canal of Schlemm

External eye muscle

Figure 11- 10 The anatomy of the eye, transverse view.

Sclera

Choroid

Retina

Retinal arteries

and veins^

Fovea Macula

centralis lutea

Optic

nerve^

Optic disc

(blind spot) ® Blood vessels Learning

Cengage ©

Ganglion cell

Rods—

Retina

dim light (^)
Secondary Cell (^)
neuron bodies ®
(^)
Cones—^ Learning^
responsible (^)
(^) for colors and (^) Cengag (^) e (^)
bright lights^
Optic nerve (11)
©^
Figure 11- 11 The layers of the retina illustrating the
rods and cones and other cellular layers.
like when we look directly at an object. Medial to the fo-
vea centralis is the optic disk. It is here that nerve fibers
leave the eye as the optic nerve. Because the optic disk has
no receptor cells, it is called the blind spot.
Both rods and cones contain light-sensitive pig-ments.
Rod cells contain the pigment called rhodopsin (roh-
DOP-sin). Cone cells contain a slightly different pigment.
When exposed to light, the rhodopsin breaks down into a
protein called opsin and a pigment called retinal.
Manufacture of retinal requires vitamin A. Some-one with
a vitamin A deficiency may experience night blindness,
which is difficulty seeing in dim light.
Sight is one of our most important senses. Humans
depend on sight as their main sense to survive and interact
with our environment. We educate ourselves via visual in-
put through reading, color interpretations, and movement.
People who lose their sight tend to develop acuity with the
other senses like smell and sounds, senses that our dog and
cat companions have developed to a high degree.
The Sense of Hearing and Equilibrium
The external, inner, and middle ear contain the organs of
balance and hearing (Figure 11-12). The external ear is that
part of the ear that extends from the outside of the head to
the eardrum. Medial to the eardrum is the air-filled
chamber called the middle ear, which contains the auditory
ossicles: the malleus, incus, and stapes. The ex-ternal and
middle ear are involved in hearing. The inner ear is a group
of fluid-filled chambers that are involved in both balance
and hearing.
The external ear consists of the flexible, visible part of
our ear called the auricle (AW-rih-kl) composed mainly of
elastic cartilage. This connects with our ear canal known as
the external auditory meatus (AW-dih-tor-ee mee-
ATE-us). The auricle allows sound waves to enter the ear
canal, which then directs those waves to the delicate
eardrum or tympanic (tim-PAN-ik) membrane. The ear
canal is lined with hairs and modified sebaceous glands
called ceruminous (seh-ROO-men-us) glands that
produce earwax or cerumen. The hairs and earwax pro-
tect the eardrum from foreign objects. The thin tympanic
membrane, which is silvery gray in color, is very delicate
and sound waves cause it to vibrate.