Uvea
The uvea, also called the uveal layer, uveal coat, uveal tract, vascular tunic or vascular layer, is the pigmented middle layer of the three concentric layers that make up an eye, precisely between the inner retina and the outer fibrous layer composed of the sclera and cornea.
History and etymology
The originally medieval Latin term comes from the Latin word uva and is a reference to its grape-like appearance. 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 a technical term for part of the eye is ancient, but it only referred to the choroid in Middle English and before.Structure
Regions
The uvea is the vascular middle layer of the eye. It is traditionally divided into three areas, from front to back:- Iris
- Ciliary body
- Choroid
Function
- Nutrition and gas exchange: uveal vessels directly perfuse the ciliary body and iris, to support their metabolic needs, and indirectly supply diffusible nutrients to the outer retina, sclera, and lens, which lack any intrinsic blood supply.
- Light absorption: the uvea improves the contrast of the retinal image by reducing reflected light within the eye, and also absorbs outside light transmitted through the sclera, which is not fully opaque.
Pharmacology
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 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 is important in treating both acute and chronic glaucoma.