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The ciliary body (Latin: corpus ciliare) is the thick part in front of the choroid. The ciliary body is made of blood vessels, connective tissue, and smooth muscle cells. The ciliary body works as an anchor for the lens through zonular fibers, also known as suspensory ligaments. The ciliary body is involved in the suspension of the lens and the process of accommodation.
Structure of the ciliary body
The frontal thick part of the ciliary body is called corona ciliaris (pars plicata), but the dorsal thinner part - orbiculus ciliaris (pars plana). There are approximately 70 radial extensions starting from corona ciliaris, known as the ciliary processes to which fibers of the lens capsule attach. Orbiculus ciliaris is found next to the ora serrata.
The ciliary body is approximately 6 mm wide. The ciliary body is triangular-shaped when looking from a cross-section. Its base points to the anterior chamber, while the anterior outer angle is pointing at the scleral spur. The ciliary body's apex projects laterally and posteriorly where it connects with the choroid. The posterior border of the pars plana has a scalloped edge. A double-layered epithelium covers the inner surface of the ciliary body.
The anterior part of the ciliary body is where the anterior ciliary arteries and the long posterior arteries meet. The ciliary body is a passage for the nerves supplying the anterior tissues of the eyeball. On the dorsal part, the ciliary body is continuing with the choroid but anteriorly with the peripheral border of the iris. As the name of the structure points, the ciliary body is a complete ring running around the inner surface of the anterior part of the sclera.
Within the ciliary body is the ciliary muscle made of differently orientated fiber groups - circular fibers, radial fibers, longitudinal fibers. The muscle's action causes the lens to stretch differently, which results in the lens changing its curvature. This change allows the eye to accommodate viewing objects at a different distance. The ciliary processes produce aqueous humor that is gathered in the eye's chamber.
Layers of the ciliary body
The ciliary body has four layers: the ciliary epithelium, the ciliary stroma, the ciliary muscle, and the supraciliary layer.
The ciliary epithelium is the innermost layer. The ciliary epithelium has two layers of cubital cells covering the inner part of the ciliary body. The inner layer of the epithelium is unpigmented, but the outer layer has melanin in it. The inner layer has columnar cells covering the pars plana and cuboidal cells over the ciliary process. The outer layer contains cuboidal cells.
The pigmented epithelium is connected to the retinal pigment epithelium at the ora serrata, while the unpigmented epithelium is continuous with the neural retina. Anteriorly, the ciliary epithelium connects with the double epithelium on the posterior surface of the iris. The basal part of the unpigmented cells faces the eye's interior part, whereas the base of the pigmented cells faces the stroma.
This characteristic causes the apical regions of both cell types to meet each other, with ciliary channels separating them. The ciliary epithelium secretes aqueous humor.
The ciliary stroma is made of loose connective bundles with a rich network of blood vessels and melanocytes. The present blood vessels are the ciliary arteries, veins, and capillaries. Branches of the long posterior ciliary arteries form a major arterial circle located at the base or periphery of the iris.
The ciliary muscle is made of smooth muscle fibers and is responsible for the substance of the ciliary body. The muscle fibers have three groups: longitudinal (external, closest to the sclera, go into the stroma of the choroid), radial (through the first to the third layer), circular (internal, around the eyeball, close to the periphery of the lens). When the muscle contracts, the longitudinal and circular fibers pull the ciliary body forward in accommodation. This relieves the tension in the suspensory ligaments creating the elastic lens more convex and increasing the refractive power of the lens. The postganglionic parasympathetic fibers from the oculomotor nerve are innervating the ciliary muscle.
The supraciliary layer is a thin outermost layer separating the sclera from the ciliary muscle. The supraciliary layer is made of collagen fibers. The layer works as an alternative uveoscleral passage for the drainage of aqueous humor.
Function of the ciliary body
The ciliary body is involved in the suspension of the lens and the process of accommodation. The anterior surface of the ciliary processes produces the aqueous humor into the posterior chamber, while the posterior surface - glycosaminoglycans for the vitreous body.
Vasculature and innervation of the ciliary body
Blood supply and venous drainage
The ciliary body is supplied by the anterior ciliary arteries and the long posterior ciliary arteries forming the major arterial circle.
Venous drainage happens through the choroid into the vortex veins. The vortex veins can drain into the superior ophthalmic vein and then into the cavernous sinus, while some - into the inferior ophthalmic.
The short ciliary nerves innervate the ciliary body. The short ciliary nerves carry the parasympathetic fibers from the oculomotor nerve (CN III).