Eye Anatomy - The Retina

The eye’s retina is the inside back surface and is often compared to camera film – with good reason. It is densely filled with light-sensitive cells that receive images brought in by light rays. The cells are of two types:

Six to seven million cones – which provide color perception and sharp vision in bright light;
75 to 150 million rods – which provide vision in dim light;

In the 1990s, a third type of retinal cell was discovered -- photosensitive ganglion cells. They detect the rising and setting of the sun and affect the body’s sleep-wake pattern. The first published writing about them was in 2000.

The retina (about 0.5 mm thick) is the innermost of three layers that make up the eyeball’s wall. The layer outside the retina is the choroid. It is filled with blood vessels that bring nutrients and oxygen to the retina and carry away its waste products. The outermost layer of the eyeball wall is the sclera, which we see in front as the “white” of the eye and the cornea. It is a tough, protective layer.

The Macula Lutea

While most of the rods are located around the retinal periphery, the cones are clustered in the central retina, especially in the macula lutea – the “yellow spot”. The macula is an oval area near where the optic nerve leaves the retina, and in its center is a tiny depression called the fovea.

The fovea has no blood vessels but instead has the most dense concentration of cones and provides our sharpest vision. When we look directly at an object in bright light, there is a straight line from the object to the center of the eye’s lens to the fovea.

The yellow pigment in the macula absorbs excess light inside the eye like a pair of natural sunglasses. For more about pigment in the eye, please see Eye Anatomy: Iris and Pupil.

The macula can deteriorate as we age and you can read more at our page on Age-Related Macular Degeneration.

Converting Light to Neural Energy

Cones and rods all connect to the 1.2 million optic nerve fibers that cover the retina. The light-sensitive cells convert the image information brought by light rays to nerve energy, and the nearby nerve fibers pick it up. They converge at a point near the center of the retina, forming what we call the Optic Nerve.

This point of convergence has no light-sensitive cells, since the nerve fibers are occupying it. There is therefore no vision here – it is the eye’s blind spot.

The fibers travel in a bundle to the brain’s vision center at the back of the head. The brain interprets the neural data and supplies names for the images.

The Vitreous and the Retina

While the choroid is the layer on the outside of the retina, on its interior surface the retina is next to a fluid called the Vitreous Humor, Vitreous Gel, or just the Vitreous. This is a transparent gel that allows light to pass through unimpeded. It makes up most of the eyeball’s mass, filling the relatively large area between the lens and the retina.

Normally we have no awareness of the eyeball’s interior, but floaters and flashes are an exception. As we age, the vitreous slowly shrinks and may form little clumps of tissue that float and cast shadows on the retina. If you look at a field of new snow or a blue sky you may notice these irregularly shaped strings and clumps. They move away when you try to focus on them.

Retinal Detachment

Normally floaters are not harmful, but if you notice a sudden increase in their number and size, and if you also notice sudden flashes of light, you could be experiencing a detached retina and should call your eye doctor immediately.

If the retina becomes detached from the blood supply in the choroid, retinal cells could die and vision would thus be impaired. A detached retina that is not treated quickly enough can cause blindness. Retinal detachment is often part of Diabetic Retinopathy.

To speak to an experienced ophthalmologist in your area, please visit our eye doctor directory today.