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26-511: The originally medieval Latin term comes from the Latin word uva ("grape") and is a reference to its grape-like appearance (reddish-blue or almost black colour, wrinkled appearance and grape-like size and shape when stripped intact from a cadaveric eye). In fact, it is a partial loan translation of the Ancient Greek term for the choroid , which literally means “covering resembling a grape”. Its use as

52-448: A body of capillary adjacent to the optic nerve called the choroidal gland. Though its function is not known, it is believed to be a supplemental oxygen carrier. Melanin , a dark colored pigment, helps the choroid limit uncontrolled reflection within the eye that would potentially result in the perception of confusing images. In humans and most other primates , melanin occurs throughout the choroid. In albino humans, frequently melanin

78-417: A result, some forms of visual field loss that occur without involving the macula are termed macular sparing . (For example, visual field testing might demonstrate homonymous hemianopsia with macular sparing .) In the case of occipitoparietal ischemia owing to occlusion of elements of either posterior cerebral artery , patients may display cortical blindness (which, rarely, can involve blindness that

104-405: A size of 5.5 mm (0.22 in) is much larger than the clinical macula which, at a size of 1.5 mm (0.059 in), corresponds to the anatomical fovea. The macula is responsible for the central, high-resolution, color vision that is possible in good light. This kind of vision is impaired if the macula is damaged, as in macular degeneration . The clinical macula is seen when viewed from

130-468: A technical term for part of the eye is ancient, but it only referred to the choroid in Middle English and before. The uvea is the vascular middle layer of the eye. It is traditionally divided into three areas, from front to back: The prime functions of the uveal tract as a unit are: In addition, some uveal regions have special functions of great importance, including secretion of the aqueous humour by

156-407: Is absent and vision is low . In many animals, however, the partial absence of melanin contributes to superior night vision . In these animals, melanin is absent from a section of the choroid and within that section a layer of highly reflective tissue, the tapetum lucidum , helps to collect light by reflecting it in a controlled manner. The uncontrolled reflection of light from dark choroid produces

182-404: Is clinically significant for diseases affecting choroidal blood supply. The macula responsible for central vision and the anterior part of the optic nerve are dependent on choroidal blood supply. The structure of choroidal vessels can be revealed by optical coherence tomography , and blood flow can be revealed by Indocyanine green angiography , and laser Doppler imaging . Teleosts bear

208-622: Is important in treating both acute and chronic glaucoma . The normal uvea consists of immune competent cells, particularly lymphocytes, and is prone to respond to inflammation by developing lymphocytic infiltrates. A rare disease called sympathetic ophthalmia may represent 'cross-reaction' between the uveal and retinal antigens (i.e., the body's inability to distinguish between them, with resulting misdirected inflammatory reactions). See uveitis , choroiditis , iritis , iridocyclitis , anterior uveitis , sympathetic ophthalmia , and uveal melanoma . Choroid The choroid , also known as

234-489: Is seen when viewed from the pupil, as in ophthalmoscopy or retinal photography . The anatomical macula is defined histologically in terms of having two or more layers of ganglion cells . The umbo is the center of the foveola which in turn is located at the center of the fovea. The fovea is located near the center of the macula. It is a small pit that contains the largest concentration of cone cells . The retina's receptor layer contains two types of photosensitive cells,

260-429: Is subdivided into the umbo , foveola , foveal avascular zone , fovea , parafovea , and perifovea areas. An even smaller central region of highest receptor density (40–80 μm) is sometimes referred to as the foveal bouquet . The anatomical macula at 5.5 mm (0.22 in) is much larger than the clinical macula which, at 1.5 mm (0.059 in), corresponds to the anatomical fovea. The clinical macula

286-439: Is thus responsible for the central, high-resolution, color vision that is possible in good light; and this kind of vision is impaired if the macula is damaged, for example in macular degeneration . The clinical macula is seen when viewed from the pupil, as in ophthalmoscopy or retinal photography. Whereas loss of peripheral vision may go unnoticed for some time, damage to the macula will result in loss of central vision, which

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312-421: Is usually immediately obvious. The progressive destruction of the macula is a disease known as macular degeneration and can sometimes lead to the creation of a macular hole. Macular holes are rarely caused by trauma, but if a severe blow is delivered it can burst the blood vessels going to the macula, destroying it. Visual input from the macula occupies a substantial portion of the brain's visual capacity. As

338-411: The choroidea or choroid coat , is a part of the uvea , the vascular layer of the eye . It contains connective tissues , and lies between the retina and the sclera . The human choroid is thickest at the far extreme rear of the eye (at 0.2 mm), while in the outlying areas it narrows to 0.1 mm. The choroid provides oxygen and nourishment to the outer layers of the retina . Along with

364-442: The ciliary body and iris , the choroid forms the uveal tract . The structure of the choroid is generally divided into four layers (classified in order of furthest away from the retina to closest): There are two circulations of the eye: the retinal (in the retina) and uveal , supplied in humans by posterior ciliary arteries , originating from the ophthalmic artery (a branch of the internal carotid artery ). The arteries of

390-431: The ciliary processes , control of accommodation (focus) by the ciliary body, and optimisation of retinal illumination by the iris's control over the pupil . Many of these functions are under the control of the autonomic nervous system . The pupil provides a visible example of the neural feedback control in the body. This is subserved by a balance between the antagonistic sympathetic and parasympathetic divisions of

416-443: The rod cells and the cone cells. Because the macula is yellow in color, it absorbs excess blue and ultraviolet light that enter the eye and acts as a natural sunblock (analogous to sunglasses) for this area of the retina. The yellow color comes from its content of lutein and zeaxanthin , which are yellow xanthophyll carotenoids , derived from the diet. Zeaxanthin predominates at the macula, while lutein predominates elsewhere in

442-465: The uveal circulation, supplying the uvea and outer and middle layers of the retina, are branches of the ophthalmic artery and enter the eyeball without passing with the optic nerve. The retinal circulation, on the other hand, derives its circulation from the central retinal artery, also a branch of the ophthalmic artery, but passing in conjunction with the optic nerve. They branch in a segmental distribution to end arterioles and not anastomoses . This

468-445: The autonomic nervous system. Informal pharmacological experiments have been performed on the pupil for centuries, since the pupil is readily visible, and its size can be readily altered by applying drugs—even crude plant extracts—to the cornea. Pharmacological control over pupil size remains an important part of the treatment of some ocular diseases. Drugs can also reduce the metabolically active process of secreting aqueous humour, which

494-464: The choroid from his dissections on eyes of cadavers . Choroid is the most common site for metastasis in the eye due to its extensive vascular supply. The origin of the metastases are usually from breast cancer, lung cancer, gastrointestinal cancer, and kidney cancer. Bilateral choroidal metastases are usually due to breast cancer, while unilateral metastasis is due to lung cancer. Choroidal metastases should be differentiated from uveal melanoma , where

520-512: The eye), the macula appears yellow, a color that is not visible in the living eye except when viewed with light from which red has been filtered. Structures in the macula are specialized for high- acuity vision. Within the macula are the fovea and foveola that both contain a high density of cones , which are nerve cells that are photoreceptors with high acuity. In detail, the normal human eye contains three different types of cones, with different ranges of spectral sensitivity. The brain combines

546-451: The latter is a primary tumour arising from the choroid itself. Macula The macula (/ˈmakjʊlə/) or macula lutea is an oval-shaped pigmented area in the center of the retina of the human eye and in other animals. The macula in humans has a diameter of around 5.5 mm (0.22 in) and is subdivided into the umbo , foveola , foveal avascular zone , fovea , parafovea , and perifovea areas. The anatomical macula at

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572-447: The patient denies having, as seen in Anton's Syndrome ), yet display sparing of the macula. This selective sparing is due to the collateral circulation offered to macular tracts by the middle cerebral artery . Neurological examination that confirms macular sparing can go far in representing the type of damage mediated by an infarct, in this case, indicating that the caudal visual cortex (which

598-470: The photographic red-eye effect on photos, whereas the controlled reflection of light from the tapetum lucidum produces eyeshine (see Tapetum lucidum ). The choroid was first described by Democritus (c. 460 – c. 370 BCE ) around 400 BCE, calling it the "chitoon malista somphos" (more spongy tunic [than the sclera ]). Democritus likely saw the choroid from dissections of animal eyes. About 100 years later, Herophilos (c. 335 – 280 BCE) also described

624-429: The pupil, as in ophthalmoscopy or retinal photography. The term macula lutea comes from Latin macula , "spot", and lutea , "yellow". The macula is an oval-shaped pigmented area in the center of the retina of the human eye and other animal eyes . Its center is shifted slightly away from the optical axis (laterally, by 5°=1.5 mm). The macula in humans has a diameter of around 5.5 mm (0.22 in) and

650-408: The retina. There is some evidence that these carotenoids protect the pigmented region from some types of macular degeneration . A formulation of 10  mg lutein and 2 mg zeaxanthin has been shown to reduce the risk of age-related macular degeneration progressing to advanced stages, although these carotenoids have not been shown to prevent the disease. After death or enucleation (removal of

676-423: The signals from neighboring cones to distinguish different colors. There is only one type of rod, but the rods are more sensitive than the cones, so in dim light, they are the dominant photoreceptors active, and without information provided by the separate spectral sensitivity of the cones it is impossible to discriminate colors. In the fovea centralis , cones predominate and are present at high density. The macula

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