The ciliary body is an internal structure of the eye. It is part of the uvea, the middle layer of the eye, and is located between the iris and the choroid, connecting these two parts. While less known than the cornea or retina, the ciliary body has a role in vision and maintaining the overall health of the eye.
Anatomy of the Ciliary Body
The ciliary body is a ring-shaped tissue extending from the base of the iris to the ora serrata, the anterior border of the retina. Located behind the iris and the sclera, it is not visible from the outside. The ciliary body consists of two main parts: the pars plicata and the pars plana. The pars plicata is the anterior portion, characterized by 70 to 80 folds known as ciliary processes, which are the primary site of aqueous humor production.
The pars plana is the flatter, smoother posterior section of the ciliary body that extends to the ora serrata, where the retina begins. This area is sometimes used as an entry point for surgical procedures to access the back of the eye. The ciliary body also contains the ciliary muscle, a smooth muscle that changes the shape of the lens to allow the eye to focus.
The ciliary muscle is composed of three different types of fibers: longitudinal, radial, and circular. The ciliary body is covered by a double layer of cells called the ciliary epithelium. This epithelium is involved in the secretion of aqueous humor and helps form the blood-aqueous barrier.
Functions of the Ciliary Body
A primary function of the ciliary body is producing and regulating aqueous humor. This clear fluid is secreted into the posterior chamber of the eye, the space between the iris and the lens. Aqueous humor is important for maintaining intraocular pressure, which helps the eye keep its shape. It also provides nutrients to the lens and cornea, which lack their own blood supply, and removes waste products.
Another function is accommodation, the process of changing the eye’s focus for different distances. When the ciliary muscle contracts, it reduces tension on the zonular fibers holding the lens in place. This allows the lens to become more convex, which is necessary for focusing on near objects. When the muscle relaxes, tension on the zonular fibers increases, causing the lens to flatten for focusing on distant objects.
The ciliary body also helps maintain the blood-aqueous barrier. This barrier prevents substances from the bloodstream from entering the eye, which protects the delicate intraocular environment from inflammation and other harmful conditions.
Disorders Associated with the Ciliary Body
Several disorders can affect the ciliary body. While glaucoma is a disease of the optic nerve, treatments often target the ciliary body’s role in producing aqueous humor. Medications for glaucoma, such as beta-blockers, work by reducing this fluid’s production, which in turn lowers intraocular pressure.
Inflammation of the ciliary body is a condition known as cyclitis, a form of uveitis. Symptoms can include a deep, aching pain in the eye, redness, blurred vision, light sensitivity, and floaters. Cyclitis can be caused by autoimmune diseases, infections, or trauma to the eye.
Tumors, such as melanomas, can also develop in the ciliary body. These tumors are rare but can be malignant and may affect vision by causing secondary glaucoma or retinal detachment.
Another common condition is presbyopia, or age-related farsightedness. Presbyopia occurs as the ciliary muscle loses its ability to contract effectively and the lens becomes less flexible with age, making it difficult to focus on near objects.
Direct trauma to the eye can also damage the ciliary body. An injury can lead to inflammation, bleeding, or a detachment of the ciliary body from the scleral spur, a condition called cyclodialysis. This can affect aqueous humor production and lead to changes in intraocular pressure.