Ciliary body

The ciliary body comprises the orbiculus ciliaris, the ciliary processes, and the Ciliaris muscle.

It is shaped like a triangle with apex towards the ora serrata posteriorly and base towards iris root anteriorly. The ciliary body faces the anterior chamber, posterior chamber, and vitreous cavity and is lined by two neuroepithelial layers, a non-pigmented layer internally and a pigmented layer externally. The outer layer of epithelium is pigmented, but the inner layer, which is in contact with the aqueous is non-pigmented. The nonpigmented epithelial cells secrete the aqueous humor.

The orbiculus ciliaris is a zone of about 4 mm. in width, directly continuous with the anterior part of the choroid; it presents numerous ridges arranged in a radial manner

The ciliary processes (processus ciliares) are formed by the inward folding of the various layers of the choroid, i.e., the choroid proper and the lamina basalis, and are received between corresponding foldings of the suspensory ligament of the lens. They are arranged in a circle, and form a sort of frill behind the iris, around the margin of the lens. They vary from sixty to eighty in number, lie side by side, and may be divided into large and small; the former are about 2.5 mm. in length, and the latter, consisting of about one-third of the entire number, are situated in spaces between them, but without regular arrangement. They are attached by their periphery to three or four of the ridges of the orbiculus ciliaris, and are continuous with the layers of the choroid: their opposite extremities are free and rounded, and are directed toward the posterior chamber of the eyeball and circumference of the lens. In front, they are continuous with the periphery of the iris. Their posterior surfaces are connected with the suspensory ligament of the lens.

Structure.—The ciliary processes are similar in structure to the choroid, but the vessels are larger, and have chiefly a longitudinal direction. Their posterior surfaces are covered by a bilaminar layer of black pigment cells, which is continued forward from the retina, and is named the pars ciliaris retinæ. In the stroma of the ciliary processes there are also stellate pigment cells, but these are not so numerous as in the choroid itself.

The aqueous humor is a secretion formed by the active intervention of the epithelial cells lining the apices of the ciliary processes.

The Ciliaris muscle (m. ciliaris; Bowman's muscle) consists of unstriped fibers: it forms a grayish, semitransparent, circular band, about 3 mm. broad, on the outer surface of the fore-part of the choroid. It is thickest in front, and consists of two sets of fibers, meridional andcircular. The meridional fibers, much the more numerous, arise from the posterior margin of the scleral spur; they run backward, and are attached to the ciliary processes and orbiculus ciliaris. One bundle, according to Waldeyer, is inserted into the sclera. The circular fibers are internal to the meridional ones, and in a meridional section appear as a triangular zone behind the filtration angle and close to the circumference of the iris. They are well-developed in hypermetropic, but are rudimentary or absent in myopic eyes. The Ciliaris muscle is the chief agent in accommodation, i. e., in adjusting the eye to the vision of near objects. When it contracts it draws forward the ciliary processes, relaxes the suspensory ligament of the lens, and thus allows the lens to become more convex.