Page 61 - Human anatomy COMPLETELY DONE1

determined by the combination of excitation in these three types of receptors.
The retina of the human eye contains about 7 million cones and about 120 million
rods.
In addition to photoreceptors, the retina includes several layers of nerve cells.
The processes of retinal nerve cells form the optic nerve.
When light irritation occurs in the retina, the energy of light is converted into the
energy of nerve impulses. Under the influence of light quanta, visual pigments are
destroyed, generating electrical signals that are transmitted from rods and cones to the
optic nerve, and impulses are transmitted to the brain through its fibers.
Both cones and rods are missing at the exit point of the optic nerve from the
retina. This place is called a blind spot, its size is about 1.5 mm. If the light from the
object in question gets here, the object is not visible.
Most of the cones are located directly opposite the pupil - in the yellow spot. This
is the area of the clearest vision. There are almost no cones in the peripheral parts of
the retina, there are only rods there.
After leaving the eyeball, the optic nerve follows the upper tubercles of the
quadrilateral of the midbrain, where visual information is subjected to primary
processing. Along the axons of the neurons of the upper tubercles, visual information
enters the visual nuclei of the thalamus, and from there – into the occipital lobes of the
cerebral cortex. Here a visual image is formed, which we subjectively feel, that is, we
see.
The optical system of the eye forms on the retina not only a reduced, but also an
inverted image of the object. Signal processing in the central nervous system occurs in
such a way that objects are perceived in a natural position.

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