The eye is a complex and delicate organ. It has many functional parts that all work together to make sight possible. Though many of the parts are the same in different species, animals have developed certain adaptations that best suit their needs.
Eye anatomy
The eye has three main layers: the outer fibrous tunic, middle vascular tunic, and inner nervous tunic. The names are clues as to their basic structures and functions, but a closer look at the components of each layer will make understanding the mechanism of sight much easier.
Fibrous tunic: The fibrous tunic is the outermost layer of the eye. An opaque (not transparent) network of collagen (fibrous protein) and elastic fibers, called the "sclera," covers the posterior (back) three fourths of the eye. The sclera is tough and somewhat stretchy, like a thick balloon filled with the gelatinous contents of the eye. The rest of the fibrous tunic, the anterior (front) quarter of the eye, is a clear structure called the "cornea." It is made up of extremely thin layers of cells arranged in a unique fashion so the cornea is transparent. A normal cornea allows light to enter the eye.
Vascular tunic: The vascular tunic, as the name implies, is a network of blood vessels that supply oxygen and nutrients to the tissues of the eye. The actual area where this network is located is beneath the portion covered by the sclera and is called the "choroid." Anterior to the choroid is a circular structure called the "ciliary body." The ciliary body has muscles that act on suspensory ligaments called "zonules," which suspend the lens in the correct position. The ligaments are either taut or relaxed based on the action of the ciliary muscles. The tension on the ligaments changes the shape of the lens, depending on the distance of the object being viewed. This process is called "accommodation" and will be discussed in more detail in the following section. The iris is the colored portion of the eye. At its position in front of the ciliary body, it is the most anterior portion of the vascular tunic, and it divides the front portion of the eye into two chambers — the anterior and posterior chambers. The opening in the middle of the iris is called the "pupil," which appears as the dark center of the eye. The iris either dilates or constricts the pupil to regulate the amount of light entering the eye. In bright light the pupil will be small, but in dim light the pupil will be very large to let in as much light as possible.
Nervous tunic: The nervous tunic is a layer of photoreceptor cells called the "retina." These cells are able to change light into electrochemical signals, which are transmitted to the nervous system. There is a roughly circular opening where the optic nerve and blood vessels exit, called the "optic disc." Often the optic disc is called the "blind spot," because there are no photoreceptor cells there, so no images can actually be perceived at that position. There are two types of photoreceptor cells which perform different functions and are named for the shape of the cell. These are the rods and cones. The rods are very light sensitive, so they are most abundant in nocturnal species. The cones need bright light, and they are for sharp image formation and perception of color. Domestic mammals have mostly rods, and are unable to distinguish colors well. Some reptiles and most birds can see color, though, since they have many cones. There is a centrally located indentation at the back of the retina. It is called the "fovea centralis," and it is much more pronounced in larger animals. Surrounding the fovea is a slightly raised ring of cells called the "macula lutea." Because most of the light is focused on this region, the concentration of photoreceptor cells is increased greatly. The anterior edge of the retina is non-visual and does not contain any photoreceptors, as light does not come into contact with that surface. A line called the "ora serrata" demarcates the division between the visual and non-visual retina. This name was given because the line appears jagged, or serrated, in humans. In domestic animals, however, the division is not serrated, and it is sometimes referred to as the "ora ciliaris retinae." Most often the term "ora serrata" is still used to describe the structure in animals as well as humans.