The Structure of the Eye
The eye is a delicate instrument, and an essential part of the body. Its function is to provide us with sight. Eyes are extremely delicate and complicated organs, with several layers of tissue making up their structure. In this article, we’ll take a look at the eye as a whole, and also the layer that is most likely to be a source of dry eye – the outer lipid layer of the tear film.
There are many different individual parts of the eye, as you can see. For now, let’s focus on the top layers. The whites of our eyes are known as the sclera, and are a relatively tough layer that encompasses most of the eye. Light enters the eye through the cornea, which is the clear, curved layer in front of the iris and pupil. The cornea itself serves as a protective covering for the front of the eye, and it helps focus light on the retina at the back of the eye.
After passing through the cornea, light travels through the pupil (the black dot in the middle of the eye). The iris—the circular, colored area of the eye that surrounds the pupil—controls the amount of light that enters the eye. The pupil enlarges and shrinks, like the aperture of a camera lens, as the amount of light in the immediate surroundings changes. The size of the pupil is controlled by the action of the pupillary sphincter muscle and dilator muscle.
Behind the iris sits the lens. By changing its shape, the lens focuses light onto the retina. Through the action of small muscles (known as the ciliary muscles), the lens becomes thicker to focus on nearby objects and thinner to focus on distant objects.
The Tear Film
Even on a microscopic level, the eye is still exceedingly complicated. On the surface of our eyes there is a layer known as the tear film, which contains a further three layers, all incredibly thin. The top layer is what is known as the lipid layer, as it is made up of lipids (or oils). Its purpose is to prevent evaporation of the aqueous layer of water that sits behind it. This aqueous layer is what helps to keep our eyes fresh and full of moisture. Below the aqueous or water layer we have the mucin layer. It enables tears to adhere to the epithelium, keeping them in place.