Revelation, Rationality, Knowledge & Truth — Page 542
ORGANIC SYSTEMS AND EVOLUTION the human eye. ' Cones are conical in shape. The light which is focused on the retina stimulates the cones and rods. The cones perform the major function of splitting the light into various colours. If defective, the person would become colour-blind. During the full light of day the cones are sufficient to perform all the functions of sight. Rods are rendered useless yet they have their own importance in dim or night vision. In dim light, or total darkness, it is the rods which perform the function of vision but they can only differentiate between black and white. Cones cannot work at all under such conditions. During very dim light, colours become faded or totally disappear. When a person moves from a brightly lit place to a dark room the time he takes to begin to see things again is the time taken by the rods. to become fully reactivated. The cones and rods transfer their stimulation to the ganglia which are situated near the front of the retina. When stimulated, they start impulses which stimulate the ganglia in front. . From the ganglia more than half a million nerve fibres carry the impulses to a large cranial nerve called the optic nerve. The spot where the optic nerve joins the retina is called the blind spot because there are no cones and rods there. . F. ROM THE BACK of each eyeball, separate optic nerves take up the function of transmission of sight to the occipital lobe of the cerebrum which make the centre of vision. This centre is divided into two lobes, one for each eye. Some of the optic fibres cross from the right eyeball to the left, and from the left to the right. Thus what one sees with each eye is interpreted in both lobes. 10 The image formed by the retina is inverted but the centre of vision reerects it. The centre of vision performs other fantastic things as well. The image is in fact very tiny but is enlarged 509