This is an eye ball once again. The visual axis is the direction where you are looking at and the image of that point comes to the fovea, the center of the retina. There is optic disc nearby where nerves and blood vessels come into the retina and go out from the retina. No other organs can exist there, particularly the receptors that respond to the incoming light. We can not see the light falling on the optic disc and we call the place the blind spot. You may experience by this figure.
Now going back to the eye ball you notice there is a hollow at the fovea. Why this hollow? Please look at the next next slide.