(From Choroides 2129 chorion, and like ness). It is an epithet of several membranes, which, on account of the multitude of their blood vessels, resem-ble the chorion. It is the tunica retiformis oculi, a name of one of the coats of the eye. (See Retiformis.) It lines the sclerotis; is a thin vascular coat of a brownish colour, and generally said to derive its origin from the pia mater covering of the optic nerve. From the colour of part of this membrane it hath been called uvea; the external surface of which is called the iris; but at present the entire fore part only of this coat is called iris, and the rest choroides. It consists of two laminae; the exterior is slightly connected with the sclerotica, and is also covered with a black matter, called nigrum pigmen-tum. Both laminae are extremely vascular: the extremities of the vessels of the inner surface project from it, and are termed villi and papillae. As this internal lamina was first noted by Ruysch, it is called Ruyschi-ana tunica. The black substance which lies between the sclerotica and choroides, is also found betwixt it and the retina. Near where the sclerotica becomes transparent, the choroides is firmly united to it; and, at this circle of adhesion, the choroides seems to change its colour and texture, appearing as a whitish ring, of a compact substance, and is termed ciliare ligamentum. Here the internal lamina of the choroides dips inwards, to make what are termed the processes. The ciliary processes are on the inside, between the iris and choroides, as the ligamentum ciliare is on the outside. The choroides is continued on the inside of the transparent part of the sclerotis, and there forms the iris: the perjuration in the middle is called pupilla. The artery is a branch of the carotid. The veins empty themselves into the optic sinuses, which are again discharged into the internal jugulars; but some of these veins communicate with the external veins of the eye, so that part of the blood is emptied into the external jugulars. The nerves are from the ophthalmic branch of the fifth pair, and a branch of the third pair.

Opposite to the insertion of the optic nerve, the cho-roides is wanting: and thus is formed that white speck, on which, if the picture of an object falls, we are incapable of perceiving it.

Choroides plexus. A plexus of blood vessels; a congeries of blood vessels on the lateral ventricles of the brain. See Cerebrum.