Anterior and Posterior Segments
The mammalian eye can also be divided into two main segments: the anterior segment and the posterior segment.
The human eye is not a plain sphere but is like two spheres combined, a smaller, more sharply curved one and a larger lesser curved sphere. The former, the anterior segment is the front sixth of the eye that includes the structures in front of the vitreous humour: the cornea, iris, ciliary body, and lens.
Within the anterior segment are two fluid-filled spaces:
- the anterior chamber between the posterior surface of the cornea (i.e. the corneal endothelium) and the iris.
- the posterior chamber between the iris and the front face of the vitreous.
Aqueous humor fills these spaces within the anterior segment and provides nutrients to the surrounding structures.
Some ophthalmologists specialize in the treatment and management of anterior segment disorders and diseases.
The posterior segment is the back five-sixths of the eye that includes the anterior hyaloid membrane and all of the optical structures behind it: the vitreous humor, retina, choroid, and optic nerve.
The radii of the anterior and posterior sections are 8 mm and 12 mm, respectively. The point of junction is called the limbus.
On the other side of the lens is the second humour, the aqueous humour, which is bounded on all sides by the lens, the ciliary body, suspensory ligaments and by the retina. It lets light through without refraction, helps maintain the shape of the eye and suspends the delicate lens. In some animals, the retina contains a reflective layer (the tapetum lucidum) which increases the amount of light each photosensitive cell perceives, allowing the animal to see better under low light conditions.
The tapetum lucidum, in animals that have it, can produce eyeshine, for example as seen in cat eyes at night. Red-eye effect, a reflection of red blood vessels, appears in the eyes of humans and other animals that have no tapetum lucidum, hence no eyeshine, and rarely in animals that have a tapetum lucidum. The red-eye effect is a photographic effect, not seen in nature.
Some ophthalmologists specialise in this segment.
Read more about this topic: Mammalian Eye
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