Structure of Human eye

The structure of the mammalian eye can be divided into three main layers or tunics whose names reflect their basic functions: the fibrous tunic (also called tunica fibrosa oculi), the vascular tunic (also know as Uvea), and the nervous tunic.

Fibrous tunic: The fibrous tunic, is the outer layer of the eyeball consisting of the cornea and sclera.

Sclera: Sclera is a opaque (usually white), fibrous, protective layer containing collagen and elastic fibers. In children it is thinner and shows some of the underlying pigment, appearing slightly blue. In the old, however, fatty deposits on the sclera can make it appear slightly yellow.

Cornea: The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber, providing most of an eye's optical power.Together with the lens, the cornea refracts light, and as a result helps the eye to focus. The cornea has unmyelinated nerve endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary reflex to close the eyelid. Because transparency is of prime importance the cornea does not have blood vessels; it receives nutrients via diffusion from the tear fluid.The human cornea has five layers. 1: Corneal epithelium: a thin epithelial multicellular layer of fast-growing and easily-regenerated cells, kept moist with tears. 2: Bowman's layer (also erroneously known as the anterior limiting membrane, when in fact it is not a membrane but a condensed layer of collagen): a tough layer that protects the corneal stroma, consisting of irregularly-arranged collagen fibers. 3: Corneal stroma (also substantia propria): a thick, transparent middle layer, consisting of regularly-arranged collagen fibers along with sparsely populated keratocytes. 4: Descemet's membrane (also posterior limiting membrane): a thin acellular layer that serves as the modified basement membrane of the corneal endothelium. 5: Corneal endothelium: a simple squamous or low cuboidal monolayer of mitochondria-rich cells responsible for regulating fluid and solute transport between the aqueous and corneal stromal compartments. (The term endothelium is a misnomer here. The corneal endothelium is bathed by aqueous humour, not by blood or lymph, and has a very different origin, function, and appearance from vascular endothelia.) Vascular tunic: The vascular tunic, also known as the tunica vasculosa oculi, is the middle vascularized layer which includes the iris, pupil, ciliary body, and choroid.

Pupil: Pupil is a hole that is located in the center of the iris. It controls the amount of light that enters the eye. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye.

Iris: The iris consists of pigmented fibrous tissues known as stroma. The stroma connects a sphincter muscle which contracts the pupil, and a set of dilator muscle which opens it.From anterior (front) to posterior (back), the layers of the iris are:Anterior border layer Stroma of iris Iris sphincter muscle Iris dilator muscle Anterior pigment myoepithelium Posterior pigment epithelium Anterior surface featuresThe Crypts of Fuchs are a series of openings located on either side of the collarette that allow the stroma and deeper iris tissues to be bathed in aqueous humor. Collagen trabeculae that surround the border of the crypts can be seen in blue irises. The pupillary ruff is a series of small ridges at the pupillary margin formed by the continuation of the pigmented epithelium from the posterior surface. The Circular contraction folds, also known as contraction furrows, are a series of circular bands or folds about midway between the collarette and the origin of the iris. These folds result from changes in the surface of the iris as it dilates. Crypts at the base of the iris are additional openings that can be observed close to the outermost part of the ciliary portion of the iris. Posterior surface featuresThe Radial contraction folds of Schwalbe are a series of very fine radial folds in the pupillary portion of the iris extending from the pupillary margin to the collarette. They are associated with the scalloped appearance of the pupillary ruff. The Structural folds of Schwalbe are radial folds extending the length of the iris that are much broader and more widely-spaced. The Circular contraction folds are a fine series of ridges that run in a circular pattern over the entire posterior surface. Choroid: The choroid, also known as the choroidea or choroid coat, is the vascular layer containing connective tissue, of the eye lying between the retina and the sclera. In humans its thickness is about 0.5 mm. The choroid provides oxygen and nourishment to the outer layers of the retina . Along with the ciliary body and iris, the choroid forms the uveal tract.The structure of the choroid is generally divided into four layers:Haller's layer - outermost layer of the choroid consisting of larger diameter blood vessels - layer of medium diameter blood vessels - layer of capillaries (synonyms: Lamina basalis, Complexus basalis, Lamina vitra) - innermost layer of the choroid

Nervous tunic: The nervous tunic, also known as the tunica nervosa oculi, is the inner sensory which includes the retina.Retina is consists of Fovea, Ganglion cells, Optic disc Macula and Photoreceptor layer (rods/cones).Fovea is located in the center of macula and is responsible for sharp central vision.Ganglion cells recieve the visual information from the photoreceptors and collectively transmit visual information from retina to several regions in the thalamus, hypothalamus and mesencephalon of brain.Optic nerve carries the ganglion cells to the brain. Macula is an oval yellow spot near the center of the retina and is responsible for high acuity vision.

Human retina contains three types of cone cells that are repsonsible for the photopic vision. Cones are less sensitive to light than the rod cells in the retina (which support vision at low light levels), but allow the perception of color. Cone cells are somewhat shorter than rods, but wider and tapered, and are much less numerous than rods in most parts of the retina, but greatly outnumber rods in the fovea. Structurally, cone cells have a cone-like shape at one end where a pigment filters incoming light, giving them their different response curves. They are typically 40-50 m long, and their diameter varies from .50 to 4.0 m, being smallest and most tightly packed at the center of the eye at the fovea. The S cones are a little larger than the others. Like rods, each cone cell has a synaptic terminal, an inner segment, and an outer segment as well as an interior nucleus and various mitochondria. The synaptic terminal forms a synapse with a neuron such as a bipolar cell. The inner and outer segments are connected by a cilium. The inner segment contains organelles and the cell's nucleus, while the outer segment, which is pointed toward the back of the eye, contains the light-absorbing materials.

Like rods, the outer segments of cones have invaginations of their cell membranes that create stacks of membranous disks. Photopigments exist as transmembrane proteins within these disks, which provide more surface area for light to affect the pigments. In cones, these disks are attached to the outer membrane, whereas they are pinched off and exist separately in rods. Neither rods nor cones divide, but their membranous disks wear out and are worn off at the end of the outer segment, to be consumed and recycled by phagocytic cells.Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision.Rods are a little narrower than cones but have the same structural basis. The pigment is on the outer side, lying on the pigment epithelium. This end contains many stacked disks. Rods have a high area for visual pigment and thus substantial efficiency of light absorption. Because they have only one type of light-sensitive pigment, rather than the three types that human cone cells have, rods have little, if any, role in color vision.Like cones, rod cells have a synaptic terminal, an inner segment, and an outer segment. The synaptic terminal forms a synapse with another neuron, for example a bipolar cell. The inner and outer segments are connected by a cilium, which lines the distal segment. The inner segment contains organelles and the cell's nucleus, while the rod outer segment (abbreviated to ROS), which is pointed toward the back of the eye, contains the light-absorbing materials.