The uvea is the second membrane in the wall of the eye and is named for the fact that the eye resembles a purple grape when the outer membrane is removed. The uvea is located between the sclera and the retina, with a hole in front of it, the pupil, and the optic nerve crossing behind it. It is also called the uvea because it contains a lot of pigment, and it is also called the vascular membrane because it is rich in blood vessels. The uvea is divided into three contiguous parts: the iris, the ciliary body, and the choroid, from front to back. 1. Iris Anatomy: The iris is the most anteriorly located tissue of the uvea, after the cornea and before the lens, and is a disc-shaped membrane. Its root is connected to the anterior edge of the ciliary body and extends centrally to the front of the lens to form an important septum separating the anterior and posterior chambers of the eye. In its center or slightly to the nasal side is a round hole, the pupil, with a diameter of about 2.5 to 4 mm. The pupil adapts to changes in external light and dilates or narrows to regulate the amount of light entering the eye. The color of the iris depends mainly on the amount of pigment in two parts: the pigment cells in the iris stroma and the pigment epithelium on the posterior surface of the iris. In some people, the iris is blue at birth because there is not much pigment in the uvea at that time; after 3 to 6 months, the color of the iris changes as the pigment in the stroma increases. In Caucasians, the pigmented epithelial cells on the posterior surface of the iris and the iris stroma are rich in pigment, and the iris is dark brown; in Caucasians, the pigmented epithelial cells on the posterior surface of the iris are normally pigmented, while the pigmented cells in the iris stroma are lacking, and the iris is blue; in Albinos, however, not only is there a lack of pigment in the iris stroma, but there is also a lack of pigment in the pigmented epithelial cells on the posterior surface of the iris, so the iris is pink. Physiological function: Adjusts the pupil size and regulates the amount of light reaching the retina. 2. Ciliary body Anatomy: The ciliary body is the middle part of the uvea, anteriorly connected to the root of the iris and posteriorly migrating to the choroid. Physiological functions: atrial water production and circulation via: secretion of acidic mucopolysaccharide, formation of the lens suspensory ligament, this part of the function is completed through the ciliary epithelium; regulation of eye refraction, by the ciliary muscle to complete. When the ciliary muscle contracts, the anterior and posterior diameters of the lens increase (from 3.6 mm to 4 mm), the diameter decreases, and the surface projects forward, thus increasing the refractive power of the lens and enabling the eye to see objects at close distances clearly, thus playing a regulatory role; adjusting intraocular pressure. Anatomy: The choroid is the last part of the uvea, between the retina and the sclera, and is a soft, smooth, elastic and vascular brown film, mainly composed of blood vessels. Physiological functions: rich in blood, providing nutrition for the eye; rich in pigment, providing dark room for eye imaging.