Eyes collects light from the surrounding environment. The light passes through the clear cornea and the clear fluid (aqueous humour) of the front chamber of the eye (anterior chamber). By the actions of the dynamic coloured diaphragm, the iris, the amount of light entering the eye is controlled. The light is then focused onto the retina (detecting layer), by the lens. The lens is held in place by zonules (these suspensory ligaments are string like structures that radiate out from it like the spokes on a bicycle wheel). These zonules are attached to the ciliary body which is a circular muscle that when contracted, causes the zonules to relax and the lens to become deeper from front to back (fatter). This makes its focal length short for focusing near objects.
This muscle shows changes with age so that focusing on near objects such as reading and looking at things up close more difficult. Hence the need for reading glasses is common over the age of 40. This is called presbyopia. When we wish to look into the distance the ciliary muscle relaxes increasing its effective diameter in the eye; the zonules tighten and the lens is pulled into a thin ellipse with a long focal length.
The pressure in the eye is also maintained by the continuous production of fluid by the ciliary body and the draining the fluid into the angle of the eye. The angle (trabecular meshwork) is situated in the slit like gap between the iris and the cornea. The balance between them means that the pressure can be regulated in the normal range.
The posterior segment or back chamber of the eye is far bigger and constitutes the bulk of the eye. The total distance between the tip of the cornea and the tip of the back of the eye is typically 22-24mm. This chamber contains a gel like substance akin to egg white.
The retina is a complex multi-layered structure containing tightly packed cells including photoreceptors which detect light using photopigments and convert it into electro-chemical impulses carried by nerves to the optic nerve. When we look into the eye we see the beginning of this nerve as the optic disc. The optic nerve is the large nerve that exits the back of the eye and takes the nerves on their long journey to the back of the brain, the visual cortex and other areas of the brain. The retina wraps around almost the entire internal surface of the sclera.
That is why we can see so far to the sides of us even when our eyes are pointing straight ahead giving us ‘wrap-around vision. Herbivores and birds have their eyes on the sides of their heads which is why they have a much bigger field of vision than primates and most carnivores. The advantage to them is that the two fields of vision of the two eyes overlap. The brain uses this overlap to generate a sense of depth and distance very handy for hunting prey or fine hand eye coordination of picking fruit or jumping from branch to branch.
The eye and particularly the retina is very metabolically active. It is akin to the brain in this way. That should be of no surprise since the eye is derived from the early brain forming in the foetus. The retina therefore needs a lot of oxygen and nutrients and a way for the waste products to be carried away. It therefore has a dual blood supply. The inner retina is supplied by the ophthalmic artery. This enters at the back of the eye via the optic nerve emerging from the optic disc. It branches into a network of ever finer arteries, then arterioles and finally into tiny capillaries no thicker than the width of a single red blood cell. The capillaries act like an exchange station for nutrients, oxygen that are taken up by the cells of the retina. The cells get rid of their carbon dioxide vein and the tissues then drain into tiny venules which join together to form veins, joining to create bigger veins and finally they all unite to form the central retinal vein. This exits via the optic disc through the scleral wall to the optic nerve and beyond.
The outer retina is supplied by the choroid. This is a tissue that is almost entirely blood vessel and a little connective tissue just to support them. There is a very thin membrane that sits between the choroid and the retina called Bruch’s membrane.
The sclera is the white outer tough coat of the eye. This means that it can be kept rigid at a pressure of 15-21mmHg (above atmospheric pressure) to keep it rigid and every structure in alignment. This is also where the muscles that move the eyes attach. They originate on the bones of the eye socket and are knitted into the fibres of the sclera. Hence you can look in any direction and the two eyes work in concert, such that they are both looking at the same object of interest.
Finally the front part of the sclera and the insides of the lids are covered by a thin mucus membrane that helps to lubricate the front of the eye and keep the cornea constantly wetted by the tears. The water component of the tear film is created by the lacrimal gland sitting between the eye and the bone of the orbit above and to the outer side of the eye. The mucus and oily part of the tear film are produced by the conjunctiva and lids.
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