The<unk> is part of the anterior segment of the human eye. In front of the<unk> is the iris, which regulates the amount of light entering into the eye. The<unk> is suspended in place by the suspensory ligament of the<unk>  a ring of fibrous tissue that attaches to the<unk> at its equator and connects it to the ciliary body. Posterior to the<unk> is the vitreous body, which, along with the aqueous humor on the anterior surface, bathes the<unk>  The<unk> has an ellipsoid, biconvex shape. The anterior surface is<unk> curved than the posterior. In the adult, the<unk> is typically circa 10 mm in diameter and has an axial length of about 4 mm, though it is important to note that the size and shape can change due to accommodation and because the<unk> continues to grow throughout a person's lifetime. Microanatomy The<unk> has three main parts: the<unk> capsule, the<unk> epithelium, and the<unk> fibers. The<unk> capsule forms the outermost layer of the<unk> and the<unk> fibers form the bulk of the interior of the<unk>  The cells of the<unk> epithelium, located between the<unk> capsule and the outermost layer of<unk> fibers, are found only on the anterior side of the<unk>  The<unk> itself lacks nerves, blood vessels, or connective tissue.<unk> capsule The<unk> capsule is a smooth, transparent basement membrane that completely surrounds the<unk>  The capsule is elastic and is composed of collagen. It is synthesized by the<unk> epithelium and its main components are type IV collagen and sulfated glycosaminoglycans (GAGs). The capsule is very elastic and so allows the<unk> to assume a more spherical shape when not under the tension of the zonular fibers (also called suspensory ligaments), which connect the<unk> capsule to the ciliary body. The capsule varies from 2 to 28 micrometres in thickness, being thickest near the equator and thinnest near the posterior pole.<unk> epithelium The<unk> epithelium, located in the anterior portion of the<unk> between the<unk> capsule and the<unk> fibers, is a simple cuboidal epithelium. The cells of the<unk> epithelium regulate most of the homeostatic functions of the<unk>  As ions, nutrients, and liquid enter the<unk> from the aqueous humor, Na+/K+-ATPase pumps in the<unk> epithelial cells pump ions out of the<unk> to maintain appropriate<unk> osmotic concentration and volume, with equatorially positioned<unk> epithelium cells contributing most to this current. The activity of the Na+/K+-ATPases keeps water and current flowing through the<unk> from the poles and exiting through the equatorial regions. The cells of the<unk> epithelium also serve as the progenitors for new<unk> fibers. It constantly lays down fibers in the embryo, fetus, infant, and adult, and continues to lay down fibers for lifelong growth.<unk> fibers Pattern of<unk> fibers (anterior and lateral aspect) The<unk> fibers form the bulk of the<unk>  They are long, thin, transparent cells, firmly packed, with diameters typically 4–7 micrometres and lengths of up to 12 mm long. The<unk> fibers stretch lengthwise from the posterior to the anterior poles and, when cut horizontally, are arranged in concentric layers rather like the layers of an onion. If cut along the equator, it appears as a honeycomb. The middle of each fiber lies on the equator. These tightly packed layers of<unk> fibers are referred to as laminae. The<unk> fibers are linked together via gap junctions and interdigitations of the cells that resemble "ball and socket" forms. The<unk> is split into regions depending on the age of the<unk> fibers of a particular layer. Moving outwards from the central, oldest layer, the<unk> is split into an embryonic nucleus, the fetal nucleus, the adult nucleus, and the outer cortex. New<unk> fibers, generated from the<unk> epithelium, are added to the outer cortex. Mature<unk> fibers have no organelles or nuclei. Development Development of the human<unk> begins at the 4 mm embryonic stage. Unlike the rest of the eye, which is derived mostly from the neural ectoderm, the<unk> is derived from the surface ectoderm. The first stage of<unk> differentiation takes place when the optic vesicle, which is formed from outpocketings in the neural ectoderm, comes in proximity to the surface ectoderm. The optic vesicle induces nearby surface ectoderm to form the<unk> placode. At the 4 mm stage, the<unk> placode is a single monolayer of columnar cells. As development progresses, the<unk> placode begins to deepen and invaginate. As the placode continues to deepen, the opening to the surface ectoderm constricts and the<unk> cells forms a structure known as the<unk> vesicle. By the 10 mm stage, the<unk> vesicle has completely separated from the surface ectoderm. After the 10 mm stage, signals from the developing neural retina induces the cells closest to the posterior end of the<unk> vesicle begin to elongate toward the anterior end of the vesicle. These signals also induce the synthesis of crystallins. These elongating cells eventually fill in the lumen of the vesicle to form the primary fibers, which become the embryonic nucleus in the mature<unk>  The cells of the anterior portion of the<unk> vesicle give rise to the<unk> epithelium. Additional secondary fibers are derived from<unk> epithelial cells located toward the equatorial region of the<unk>  These cells lengthen anteriorly and posteriorly to encircle the primary fibers. The new fibers grow longer than those of the primary layer, but as the<unk> gets larger, the ends of the newer fibers cannot reach the posterior or anterior poles of the<unk>  The<unk> fibers that do not reach the poles form tight, interdigitating seams with neighboring fibers. These seams are readily visible and are termed sutures. The suture patterns become more complex as more layers of<unk> fibers are added to the outer portion of the<unk>  The<unk> continues to grow after birth, with the new secondary fibers being added as outer layers. New<unk> fibers are generated from the equatorial cells of the<unk> epithelium, in a region referred to as the germinative zone. The<unk> epithelial cells elongate, lose contact with the capsule and epithelium, synthesize crystallin, and then finally lose their nuclei (enucleate) as they become mature<unk> fibers. From development through early adulthood, the addition of secondary<unk> fibers results in the<unk> growing more ellipsoid in shape; after about age 20, however, the<unk> grows rounder with time and the iris is very important for this development. Several proteins control the embryonic development of the<unk>  among these, primarily, PAX6, considered the master regulator gene of this organ. Other effectors of proper<unk> development include the Wnt signaling components BCL9 and Pygo2. Variation In many aquatic vertebrates, the<unk> is considerably thicker, almost spherical, to increase the refraction. This difference compensates for the smaller angle of refraction between the eye's cornea and the watery medium, as they have similar refractive indices. Even among terrestrial animals, however, the<unk> of primates such as humans is unusually flat. In reptiles and birds, the ciliary body touches the<unk> with a number of pads on its inner surface, in addition to the zonular fibres. These pads compress and release the<unk> to modify its shape while focusing on objects at different distances; the zonular fibres perform this function in mammals. In fish and amphibians, the<unk> is fixed in shape, and focusing is instead achieved by moving the<unk> forwards or backwards within the eye. In cartilaginous fish, the zonular fibres are replaced by a membrane, including a small muscle at the underside of the<unk>  This muscle pulls the<unk> forward from its relaxed position when focusing on nearby objects. In teleosts, by contrast, a muscle projects from a vascular structure in the floor of the eye, called the falciform process, and serves to pull the<unk> backwards from the relaxed position to focus on distant objects. While amphibians move the<unk> forward, as do cartilaginous fish, the muscles involved are not homologous with those of either type of fish. In frogs, there are two muscles, one above and one below the<unk>  while other amphibians have only the lower muscle. In the most primitive vertebrates, the lampreys and hagfish, the<unk> is not attached to the outer surface of the eyeball at all. There is no aqueous humor in these fish, and the vitreous body simply presses the<unk> against the surface of the cornea. To focus its eyes, a lamprey flattens the cornea using muscles outside of the eye and pushes the<unk> backwards.
Lens 
Eye
Globe 
Optician
DermAtlas
Pituitary gland
Heart
Optic stalk