(1) Cones. The cones of the retina are for acute vision and also receive
color information. The cones tend to be concentrated at the rear of the eyeball. The
greatest concentration is within the macula lutea at the inner end of the focal axis
(2) Rods. Light received by the rods is perceived in terms of black and
white. The rods are sensitive to less intensive light than the cones. The rods are
concentrated to the sides of the eyeball.
(3) Signal transmission. The stimulus from the photoreceptors (cones
and rods) is transferred to the bipolar cells. In turn, the stimulus is transferred to the
ganglion cells, the cells of the innermost layer of the retina. The axons of the ganglion
cells converge to the back side of the eyeball. The axons leave the eyeball to become
the optic nerve, surrounded by a dense FCT sheath. There are no photoreceptors in
the circular area where the axons of the ganglion cells exit the eyeball; thus, this area is
called the blind spot.
NERVOUS PATHWAYS FROM THE RETINAS
a. The two optic nerves enter the cranial cavity and join in a structure known
as the optic chiasma. Leading from the optic chiasma on either side of the brainstem is
the optic tract. In the optic chiasma, the axons from the nasal (medial) halves of the
retinas cross to the opposite sides. Thus, the left optic tract contains all of the
information from the left halves of the retinas (right visual field), and the right optic tract
contains all of the information from the right halves of the retinas (left visual field).
b. The optic tracts carry this information to the LGB (lateral geniculate body)
of the thalamus. From here, information is carried to the posterior medial portions
(occipital lobes) of the cerebral cortex, where the information is perceived as conscious
vision. Note that the right visual field is perceived within the left hemisphere, and the
left visual field is perceived within the right hemisphere.
c. The LGB also sends information into the midbrainstem. This information is
used to activate various visual reflexes.
FOCUSING OF THE LIGHT RAYS
a. The light rays, which enter the eyeball from the visual field, are focused to
ensure acute vision. The majority of this focusing is accomplished by the permanently
b. Fine adjustments of focusing, for acuteness of vision, are provided by the
crystalline lens (biconvex lens). See Figure 13-4. This is particularly important when
changing one's gaze between far and near objects.