The eye is set is hollow socket called the orbit. The bony edges of the orbit, the frontal bone, and the cheekbone protect the eye from mechanical injury by blows or collisions.
The eyeball in the socket is a spherical structure approximately 1 inch in diameter with a pronounced bulge in front.
The outer area has three layers of tissue. The outer layer is called the sclera, a protective coating and covers about five-sixths of the eye surface. At the front of the eyeball, it is continuous with the bulging, transparent cornea. The middle layer of the eye coating is the choroid, a vascular lining of the posterior three-fifths of the eyeball. The choroid is continuous with the ciliary body and with the iris at the front. The innermost layer is the retina which collects the light.
The cornea itself is a strong, five-layered membrane through which light is admitted to the interior of the eye.
Light passes into a chamber of a clear, watery fluid, the aqueous humour, which separates the cornea from the crystalline lens.
The lens is a flattened sphere of many transparent fibres arranged in layers, connected by ligaments to a ring shaped ciliary muscle, which surrounds it. The ciliary muscle and surrounding tissues form the ciliary body. This is the muscle that bulges the lens for focussing, known as accommodation. In the normal eye accommodation is not necessary for distant objects. The lens, when flattened by the suspensory ligament, brings such objects to focus on the retina. For nearer objects the lens is increasingly rounded by the ciliary muscle contraction, which relaxes the suspensory ligament. A young child can see clearly at a distance as close as two and a half inches, but with increasing age the lens gradually hardens, so that the limits of close seeing are approximately 6 inches at the age of 30 and 16 inches at the age of 50. In the later years of life most people lose the ability to accommodate their eyes to distances within reading or close working range. This condition, known as presbyopia, can be corrected by the use of special convex lenses for the near range.
The pigmented iris hangs behind the cornea but in front of the lens, and has a circular opening in its centre which is controlled by a muscle around its edge by contracting and expanding and changing the amount of light let into the eye.
Behind the lens the eye cavern is filled with a transparent, jellylike substance, the vitreous humour, enclosed in a thin sac, the hyaloid membrane. The pressure of the vitreous humour keeps the eyeball distended.
The retina is composed largely of nerve cells. The light-sensitive receptor cells lie on the outer surface of the retina in front of a pigmented tissue layer. These cells are either rods or cones packed closely together (not unlike matches in a box). The cone-shaped cells of the retina are individually connected to other nerve fibres, so that stimuli to each individual cell gives fine detail. A brownish pigment present in the outer layer of the retina protects the cone cells of the retina from overexposure to light. If bright light strikes the retina, granules of this brown pigment migrate to the spaces around the cone cells, sheathing and screening them from the light. This is light adaptation. The rodshaped cells are connected in groups so that they respond to stimuli in a general area of total light but without detail. Seeing at night involves sensitising the rod cells by pigment called visual purple or rhodopsin, that is formed within the cells and produces dark adaptation. Visual purple is bleached by the action of light and must be reformed by the rod cells under conditions of darkness. This is why a person who steps from sunlight into a darkened room cannot see until the pigment forms. Vitamin A is necessary for the production of visual purple and inadequate Vitamin A causes night blindness. Directly in line with the pupil is a small yellow-pigmented spot, the macula lutea, in the centre of which is the fovea centralis, which has the greatest visual acuity of the eye. At the centre of the fovea, the sensory layer entirely cone-shaped cells. Around the fovea both rod-shaped and cone-shaped cells are present, with the cone-shaped cells becoming fewer toward the periphery of the sensitive area. At the outer edges are only rod-shaped cells.
Where the optic nerve enters the eyeball, below and slightly to the inner side of the fovea there are no receptor cells and there is the blind spot.
A person is not normally conscious of a central zone of sharpness surrounded by an area of increasing fuzziness because the eyes are constantly moving and shifting attention between objects in view. Six muscles move the eyeball up, down, left, right and obliquely. The eye muscles are extremely precise; it has been estimated that the eyes can move and focus on no less than 100,000 distinct points in the visual field. The muscles of the two eyes work together to converge the eyes on any point being observed to make the two images coincide. Double vision is when this does not happen well enough, but when it does we are able to estimate distance and size of objects.
The eyelids, being two folds of skin and tissue, upper and lower, are closed by muscles to form a protective covering over the eyeball against excessive light and mechanical injury. Inside the eyelid is a thin protective membrane, the conjunctiva, which doubles over to cover the visible sclera. The eyelashes, short hairs on the edge of either eyelid, screen dust particles and insects out of the eyes when the eyelids are partly closed. The eyebrows, located above each eye, also have a protective function in soaking up or deflecting perspiration or rain and preventing the moisture from running into the eyes. Each eye also has a tear gland, or lacrimal organ, situated at the outside corner of the eye. The salty secretion of these glands lubricates the forward part of the eyeball when the eyelids are closed and flushes away any small dust particles or other foreign matter on the surface of the eye. Normally the eyelids of human eyes close by reflex action about every six seconds, but if dust reaches the surface of the eye and is not washed away, the eyelids blink more and and more tears are produced. On the edges of the eyelids are the Meibomian glands which produce a fatty secretion that lubricates the eyelids themselves and the eyelashes.