Dog Owner's Home Veterinary Handbook.pdf - Mr. Walnuts

lens. The large vitreous chamber, containing a clear jelly, fills the cavity
behind the lens and in front of the retina.
The lens is held in place by a series of strands called the suspensory ligaments.
These attach to the ciliary body, a structure composed of muscle, connective
tissue, and blood vessels. The ciliary body secretes the fluid that fills
the anterior and posterior chambers. Contraction of the ciliary muscles
changes the curvature of the lens, which enables images of objects at different
distances to be focused onto the retina.
Light enters the eye by passing through the cornea and anterior chamber
and then through the pupil and lens. It then travels through the vitreous and
is received by the retina. The retina is a layer of photoreceptor cells that convert
light into electrical impulses. These impulses are then carried to the
brain via the optic nerves.
How Dogs See
THE EYES • 171
Dogs have relatively poor vision in some areas when compared with people.
Dogs are nearsighted and accommodate poorly. Most dogs see at about 20/75—
remember that for people normal eyesight is 20/20. Accommodation is the
process during which the lens changes shape to focus light on the retina. Dogs
accommodate poorly because the ciliary muscles that change the shape of the
lens are relatively weak.
The dog’s retina contains a small number of cone cells that distinguish
between blue, yellow, and gray. However, the canine retina lacks photoreceptors
for red and green, and thus is similar to the retina of people who are redgreen
colorblind. While dogs do perceive some colors, it is believed that the
ability to perceive subtle shades of gray is the most important function of the
cone cells. Dogs can detect degrees of brightness.
On the positive side, dogs have large pupils and a wide field of vision, making
them adept at following moving objects. Dogs also have an abundance of
rods in the retina, which are the cells that detect light. Along with the cone
cells that distinguish shades of gray, the rods enable dogs to see very well in
relative darkness. Dogs also have a fair degree of binocular vision and depth
perception. Furthermore, any shortcoming dogs may have in eyesight is more
than made up for by their superior senses of hearing and smell.
Visual abilities vary somewhat with breed, head shape, and eye shape. A
dog with a long muzzle will have “visual streaks,” which are areas in the retina
with extra cells for vision over a wide area and for detecting up motion. Many
of the sighthounds fit that description. A dog with a short muzzle and prominent
eyes will have an “area centralis.” This is a central spot on the retina
with extra cells to pick up details. These are the dogs who tend to watch television
more closely.