Retina and Fovea
retina, a ten-layered sheet of nerve cells, lines the inside of
the eye. The retina receives images in the form of lightwaves. Cells
of the retina perform the initial processing of images and transmit
the resulting neural impulses to the brain via the optic nerve.
The retina of each eye has approximately 126 million receptor cells:
120 million rods and 6 million cones. Rods are responsible for our
night vision and cones for our day and color vision.
opposite the lens, the retina contains a small, circular, yellow-colored
area, the macula lutea. This is the area of the retina responsible
for high acuity vision. When the eye is focused on any object, the
object’s retinal image is always focused on the macula. The
macula contains the fovea and foveal pit where visual acuity is
highest. The foveal pit contains dense, highly packed red and green
cones. Significantly, it contains no blue cones, and no
rod photoreceptors. It is here that we experience our sharpest,
most acute vision and color discrimination.
fovea and foveal pit are where visual acuity and color vision
are the best because of the high concentration of cones. The
fovea is located directly opposite the eye's lens system for
optimum focusing. (Image courtesy of the University
of Utah, John A Moran Eye Center, WebVision at www.webvision.med.utah.edu)
yellow pigment of the macula lutea absorbs short-wave length (blue)
light. The lens absorbs short-wave length light as well. Even if
short-wave light reaches the fovea, recall the foveal pit contains
no blue cones. Therefore, we are effectively color blind for blue
at the point of sharpest focus on the retina. The eye does not use
blue to resolve details. Blue is better perceived in the periphery
of the retina.
ganglion nerve cells that collect and aggregate nerve impulses converge
at the optic disk and form the optic nerve. At the optic disk there
are no photoreceptor cells which creates a blind spot on the retina.