Essentials of Anatomy and Physiology

the retina we use most: for reading, for driving, for
recognizing people, and for any kind of close work.
People of all ages should be aware of this condition
and that smoking and exposure to ultraviolet rays are
risk factors.
When light strikes the retina, the rods and cones
generate impulses. These impulses are carried by gan-
glion neurons, which all converge at the optic disc

(see Figs. 9–5 and 9–6) and pass through the wall of

the eyeball as the optic nerve. There are no rods or

cones in the optic disc, so this part of the retina is

sometimes called the “blind spot.” We are not aware

of a blind spot in our field of vision, however, in part

because the eyes are constantly moving, and in part

because the brain “fills in” the blank spot to create a

“complete” picture.

206 The Senses

Sclera

Pigment cells Choroid

Bipolar

neurons

Ganglion

neurons

Light waves

Cone

Rod

Optic nerve

Optic nerve

fibers

Figure 9–6. Microscopic structure of the retina in the area of the optic disc. See text for

description.

QUESTION:Which type of neuron forms the optic nerve? Which cells are the photo-

receptors?