The choroid is a thin, highly vascular membrane, of a dark brown or chocolate color, investing the posterior five-sixths of the globe; it is pierced behind by the optic nerve, and in this situation is firmly adherent to the sclera. It is thicker behind than in front. Its outer surface is loosely connected by the lamina suprachorioidea with the sclera; its inner surface is attached to the pigmented layer of the retina.
Structure.—The choroid consists mainly of a dense capillary plexus, and of small arteries and veins carrying blood to and returning it from this plexus. On its external surface is a thin membrane, the lamina suprachorioidea, composed of delicate non-vascular lamellæ—each lamella consisting of a net-work of fine elastic fibers among which are branched pigment cells. The spaces between the lamellæ are lined by endothelium, and open freely into the perichoroidal lymph space, which, in its turn, communicates with the periscleral space by the perforations in the sclera through which the vessels and nerves are transmitted.
Internal to this lamina is the choroid proper, consisting of two layers: an outer, composed of small arteries and veins, with pigment cells interspersed between them; and an inner, consisting of a capillary plexus. The outer layer (lamina vasculosa) consists, in part, of the larger branches of the short ciliary arteries which run forward between the veins, before they bend inward to end in the capillaries, but is formed principally of veins, named, from their arrangement, the venæ vorticosæ. They converge to four or five equidistant trunks, which pierce the sclera about midway between the sclero-corneal junction and the entrance of the optic nerve. Interspersed between the vessels are dark star-shaped pigment cells, the processes of which, communicating with those of neighboring cells, form a delicate net-work or stroma, which toward the inner surface of the choroid loses its pigmentary character. The inner layer (lamina choriocapillaris) consists of an exceedingly fine capillary plexus, formed by the short ciliary vessels; the net-work is closer and finer in the posterior than in the anterior part of the choroid. About 1.25 cm. behind the cornea its meshes become larger, and are continuous with those of the ciliary processes. These two laminæ are connected by a stratum intermedium consisting of fine elastic fibers. On the inner surface of the lamina choriocapillaris is a very thin, structureless, or faintly fibrous membrane, called the lamina basalis; it is closely connected with the stroma of the choroid, and separates it from the pigmentary layer of the retina.
One of the functions of the choroid is to provide nutrition for the retina, and to convey vessels and nerves to the ciliary body and iris.
Tapetum.—This name is applied to the outer and posterior part of the choroid, which in many animals presents an iridescent appearance.