The aqueous humor is similar in ionic composition to plasma but contains less than 0.1% protein (compared with 7% protein in plasma). The aqueous humor passes from the ciliary body toward the lens, and then between the iris and lens, before it reaches the anterior chamber of the eye (see Fig. 24.6). In the anterior chamber of the eye, the aqueous humor passes laterally to the angle formed between the cornea and iris. Here it penetrates the tissues of the limbus as it enters the labyrinthine spaces of trabecular meshwork and finally reaches the canal of Schlemm, which communicates with the veins of the sclera (see Folder 24.1).
The choroid is the portion of the vascular coat that lies deep to the retina.
The choroid is a dark brown vascular sheet only 0.25 mm thick posteriorly and 0.1 mm thick anteriorly. It lies between the sclera and retina (see Fig. 24.1).
Two layers can be identified in the choroid:  The choriocapillary layer, an inner vascular layer  Bruch’s membrane, a thin, amorphous hyaline membrane
The choroid is attached firmly to the sclera at the margin of the optic nerve. A potential space, the perichoroidal space (between the sclera and retina), is traversed by thin, ribbon-like, branching lamellae or strands that pass from the sclera to the choroid. These lamellae originate from the suprachoroid lamina (lamina fusca) and consist of large, flat melanocytes scattered between connective tissue elements including collagen and elastic fibers, fibroblasts, macrophages, lymphocytes, plasma cells, and mast cells. The lamellae pass inward to surround the vessels in the remainder of the choroid layer. Free smooth muscle cells, not associated with blood vessels, are present in this tissue. Lymphatic channels called epichoroid lymph spaces, long and short posterior ciliary vessels, and nerves on their way to the front of the eye are also present in the suprachoroid lamina.
Most of the blood vessels decrease in size as they approach the retina. The largest vessels continue forward beyond the ora serrata into the ciliary body. These vessels can be seen with an ophthalmoscope. The large vessels are mostly veins that course in whorls before passing obliquely through the sclera as vortex veins. The inner layer of vessels, arranged in a single plane, is called the choriocapillary layer. The vessels of this layer provide nutrients to the cells of the retina. The fenestrated capillaries have lumina that are large and irregular in shape. In the region of the fovea, the choriocapillary layer is thicker, and the capillary network is denser. This layer ends at the ora serrata.
Bruch’s membrane measures 1 to 4 m in thickness and lies between the choriocapillary layer and the pigment epithelium of the retina. It runs from the optic nerve to the ora serrata, where it undergoes modifications before continuing into the ciliary body. Bruch’s membrane is a thin, amorphous refractile layer, also called the lamina vitrea. The transmission electron microscope (TEM) reveals its structure as consisting of a multilaminar sheet containing a center layer of elastic and collagen fibers. Five different layers are identified in Bruch’s membrane:  The basal lamina of the endothelial cells of the choriocap illary layer  A layer of collagen fibers approximately 0.5 m thick  A layer of elastic fibers approximately 2 m thick  A second layer of collagen fibers (thus forming a “sandwich” around the intervening elastic tissue layer)  The basal lamina of the retinal epithelial cells
At the ora serrata, the collagenous and elastic layers disappear into the ciliary stroma, and Bruch’s membrane becomes continuous with the basal lamina of the RPE of the ciliary body.