Chapter 15
The Special Senses
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15
rod or cone to the
inner segment
. Te inner segment then con-
nects to the
cell body
, which is continuous with an
inner fiber
bearing
synaptic terminals
.
In rods, the outer segment is slender and rod shaped (hence
their name) and the inner segment connects to the cell body by
the
outer fiber
. By contrast, the cones have a short conical outer
segment and the inner segment directly joins the cell body.
Functional Anatomy of the Photoreceptors
Photoreceptors are modified neurons, but structurally they re-
semble tall epithelial cells turned upside down with their “tips”
immersed in the pigmented layer of the retina
(Figure 15.15a)
.
Tese “tips” are the receptive regions of rods and cones and are
called the
outer segments
. Moving from the pigmented layer into
the neural layer, a connecting cilium joins the outer segment of a
Concave lens moves focal
point further back.
Eyeball
too short
Eyeball
too long
Focal
plane
Focal point is on retina.
Uncorrected
Focal point is in front of retina.
Corrected
Uncorrected
Focal point is behind retina.
Corrected
Convex lens moves focal
point forward.
Myopic eye
(nearsighted)
Hyperopic eye
(farsighted)
Emmetropic eye
(normal)
Figure 15.14
Problems of refraction.
The refractive power of the cornea, which normally
supplies about two-thirds of the light-bending power of the eye, is ignored here.
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