Eye and Rods and Cones

The sensory organs for vision - the eyes - are at the front of the head, but the actual visual sense is provided by areas of the brain at the back and sides. Nerve impulses generated by "rods" and "cones" in the retinas of the eyes travel along the optic nerves to the optic chiasma, where they partially cross over. "Mixed" impulses from both eyes pass through the optic tracts to the striate cortex at the back of the brain and end in the temporal lobe area so that right and left halves of the visual field merge. When light rays reach the retina (the film of the eye's camera), light energy is converted into electrical nerve signals. Crisscrossed with blood vessels, the retina has three layers of microscopically thin nerve cells. Nearest to the lens is a layer of ganglion cells, then a layer of bipolar cells and finally the photoreceptors. It is the photoreceptors that actually process the packets of light energy or photons that impact on the retina, so light must pass through the ganglions and bipolar cells to get to others. There are two types of photoreceptor cells which, because of their shapes, are called "rods" and "cones." Rods are sensitive enough to respond to a single photon, the basic unit of light, but together they create only one coarse, gray image, which is just adequate for seeing in poor light. Fine detail and color come from the cones, but they need a lot more light and work best in broad daylight. Inside the human eye, there are eighteen times more rods than cones. These are arranged in such a way as to produce the best possible combination of night and day vision. During the 16th and 17th centuries, when witches were said to destroy their victims by the power of the evil eye, hundreds of women were executed just on the evidence that someone had died after receiving an angry look from them. The judges who presided at the trials were so afraid of receiving a curse from these women when passing sentence that some of the defendants were led into the courtroom backwards.