Anatomical consideration of the eye:

Light enters from the top through the cornea which is a closed structure that is curved and hence, refracts the light entering. This light then passes though the aqueous humor behind the cornea and hits the lens. The lens is also a curved structure and so the cornea and lens work together to refract light (to receive +- 180 degrees of space) onto retina.

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Behind the cornea is the anterior chamber which contains aqueous fluid. This chamber is separated from the posterior chamber by the lens and the ciliary muscles.

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The cornea does not change shape (i.e. it’s fixed) regardless of whether you are looking at an object which is near or far. The lens ,on the other hand, undergoes accommodation meaning that it either becomes rounder (closer objects) or flatter (further away) depending on the distance of the object you’re looking at. This changing of shape is due to the action of the ciliary muscles which are attached to the lens via zonule fibers (part of the suspensory ligament).

The fluid in the anterior chamber is constantly recycled, there are ducts that bring fluid in and others that remove fluid. This fluid is important as it provides hydration and nourishment to the lens and removes waste products from the muscle.

The posterior chamber is filled with vitreous humor which is responsible for retaining the shape of the eyeball. This fluid doesn’t get recycled and hence you have the same fluid for your whole life basically0. As we age, there can be build up of proteins and other waste materials in the vitreous humor which light can’t pass through which produce spots (”floaters”) in our vision.

So light has travelled through the cornea, aqueous humor, lens and vitreous humor. It then travels to the back of the eye where it contacts the retina.

The retina contains two important structures which are found at the back of the eye. These are:

1. the optic disk- where optic nerve exits eye/has access to the eye (its our blind spot as there are no photoreceptors there)
2. the fovea- it’s the central part of the macula lutea and has the highest concentration of cone photoreceptors (has most visual acuity)

There is a single layer of cells located between the photoreceptors and the choroid which is called the retinal pigment epithelium. It is very important for retinal health and ensuring function/survival of photoreceptors

This retinal pigment epithelium sits on top of the choroid which goes all around the eye and provides vasculature (vascular layer) and nourishment to the eye.

The outermost layer is the sclera which is a fibrous protective capsule (cornea is a specialized part of the sclera).

In the anterior chamber if there are problems with the rate of drainage vs production it can lead to a build up of fluid within this chamber resulting in increased intraocular pressure. This compresses the neural layer at the back of the eye which can lead to nerve damage/death in retina. This is known as Glaucoma.

Visual accommodation:

Zonule fibers make up the suspensory ligaments

Accommodated (<6m away):