For the eye to see accurately, the cornea and lens of the eye must focus the light on the retina, or the part of the eye that sends a visual picture down the optic nerve to the brain. The section of the retina that the focused light hits is called the macula. In the very center of the macula lies the fovea centralis, the point on the retina that is the absolute center of a person's direct vision. The rim of the fovea, called the foveal rim, is the retina's thickest area.
On the fovea centralis, there are only cones (which sense the color of light) and not rods (which sense the dim light and light movement). Because of this, dim objects such as faraway stars can be difficult to see if they are looked at straight on. Instead, they can be viewed by looking slightly to one side of the star, so that the light is focused on a different part of the macula that contains rods as well. The density of the rods on the fovea is what determines a person's visual acuity. This is because when light falls on a dense cluster of rods on the fovea, those rods take in more information about the light than they would if there a smaller number on that particular area.
The foveola is the very center of the fovea, and it has the most tightly packed cones of all. This area, as well as much of the rest of the fovea, is not nourished directly by blood vessels like most other parts of the body. Instead, it receives oxygen from the choroid's blood vessels. In conditions of bright light, this oxygen is insufficient for long periods of time, which is one reason why extremely bright lights can be harmful to the eye.