PHYSICS

Fig. 22.1. Lateral cross-section of the
human eye: 1 ­ cornea; 2 ­ iris;
3 ­ conjunctiva; 4 ­ ciliary muscle;
5 ­ sclera; 6 ­ lens; 7 ­ aqueous
humor; 8 ­ suspensory ligaments;
9 ­ vitreous humor; 10 ­ retina;
11 ­ optic disc; 12 ­ fovea;
13 ­ optic nerve
by the cornea-lens system onto the back surface of the eye, called
the retina. The central part of retina is called the fovea; it is here
that acuity of vision is sharpest. The spot at which the optic nerve
fibers exit the eye is the blind spot since it does not contain
receptor cells. The surface of the retina consists of millions of
sensitive structures called rods and cones. Rods are for dim light
and cones are for bright light and color. Both rods and cones
make direct synaptic connections with the interneurons called
bipolar cells, which connect the receptors with the ganglion cells
(fig. 22.2). The visual pigment found in rods of most vertebrates is
rhodopsin, a compound consisting of a protein called opsin and
a chromophore, retinal, which is responsible for the absorption of
1
1
Distant object
Near object
Fig.22.3. Horse's ramp-shaped retina
Fig.22.2. Five major cell types of the vertebrate retina:
1 ­ photoreceptor cells (either rods and cones); 2 ­ horizontal cells;
3 - bipolar cells; 4 ­ amacrine cells; 5 ­ ganglion cells
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light. Light striking the receptor cells, rods and cones, produces a
charge gradient across the membrane of the photoreceptor. This
process is called hyperpolarization of the receptor cell membrane
and is accompanied by a decrease in transmitter released at the
chemically mediated synapse with the bipolar cell. In turn, the
bi polar cell influences action potential frequencies in the
ganglion cell axons that carry the action potential to the brain
through the optic nerves. By this process, a distinct image of an
object is observed when the image falls on the retina.
In the horse, the eyeball is asymmetrical and the surface of the
retina does not form a true arc of the circle. The retina provides a
longer focal length for viewing downward (F) than for viewing
along the axis of the eye (F); it means that nearer objects are
automatically in focus without the use of accommodation
(fig. 22.3).
Birds. The eyeballs of birds are very large in relation to the size
of their bodies and occupy a considerable proportion of the entire
head. The size and resolution of the eye confers an adaptive
advantage in that it increases their performance and security.
Their visual precision facilitates finding food from several
hundred feet above the ground, allowing hawks to recognize
potential prey and precisely swoop down upon it. The human eye
has almost 200,000 cones per square millimeter. Hawks, in
contrast, which must spot small prey from considerable distances,
possess about five times the number of cones as humans! Owls
are active at night and as a consequence, have little need for
acute daytime vision. Birds vary, therefore, not only in the shape
of their eyeballs but also in their internal structure (fig. 22.4).
Most birds have retina with two adjuncts to visual acuity. One is
the presence of fovea, a small hypersensitive region in the retina
where the concentration of rods and cones is the highest and
therefore, vision the sharpest. The common form of fovea is a
narrow band. This is especially the case with insect eaters where
the fovea is elongated, minimizing the need for head movement
to keep a moving insect under observation. Fast flying birds, such
as swallows and swifts that catch minute insects in the air during
flight, need exceptional vision. Nearly all have a double fovea. The
other structure is known as the pecten. The pecten is a pleated fin
of pigmented, highly vascular tissue. It provides a supplementary
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