The ciliary body is an inner eye structure, located at the border between the choroid and the iris. It is composed of several unique structures that give the ciliary body its unique shape and function. These structures include the ciliary muscle, ciliary processes, ciliary vessels and ciliary epithelia. The ciliary muscle is in charge of changing the shape of the lens, while the ciliary processes participate in the production of the fluid in the eye also known as the aqueous humor.
The ciliary body is attached to the lens by the collection of tiny fibrous cords known as the zonular fibers. This attachment is crucial in changing the eye focus by changing the shape of the lens, a process known as accommodation. In order to provide these functions, the ciliary body needs to have rich vascularization and innervation; vascularisation is provided by the branches of the ophthalmic artery, while the nerve supply comes from the ciliary ganglion.
This article will discuss the anatomy and function of the ciliary body.
|Definition||Inner eye structure composed of the ciliary muscle, ciliary processes, ciliary vessels and ciliary epithelia|
|Innervation||Parasympathetic component of oculomotor nerve (CN III)|
|Blood supply||Long posterior ciliary arteries; vorticose veins|
|Function||Production of the aqueous humor, change of eye focus (via the accommodation function)|
The ciliary body is a ring-like thickening located between the anterior border of the choroid and the posterior aspect of the iris. On the cross-section, the ciliary body is triangular with its base near the iris and the apex near the choroid. Together with the iris and choroid, the ciliary body comprises the uveal tract. This tract is sandwiched between an outer layer (sclera) and an inner layer (retina).
There are three specialized structures that make up the largest portion ciliary body. These structures include the:
- Ciliary muscle
- Ciliary processes
- Ciliary epithelium
The ciliary muscle is an intrinsic smooth muscle of the eye, that occupies the largest part of the ciliary body. The contraction of the ciliary muscle loosens the zonular fibers increasing the convexity of the lens, which induces accommodation. This muscle works in synergy with the other intrinsic muscles of the eye: dilatator pupillae and sphincter pupillae muscles, the two muscles which control the size of the pupil.
The ciliary processes are small, finger-like protrusions of the ciliary body, located on ints anterior surface. This anterior portion is also referred to as corona ciliaris or pars plicata. According to their size, the ciliary processes can be divided into major, intermediate and minor processes. These processes attach to the lens via the fibrous zonular fibers and thus participate in accommodation reflex. More importantly, the ciliary processes contain a specialized highly vascularized epithelium that secretes aqueous humor into the globe of the eye.
The ciliary epithelium is composed of two epithelial layers the pigmented layer and the non-pigmented layer. The pigmented one is the outer layer of the ciliary body and its processes contain cuboidal cells rich in melanin granules. The non-pigmented layer is the inner layer of columnar cells that are involved in the production of the aqueous humor.
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The blood supply of the ciliary body is provided by the two sets of branches of the ophthalmic artery: the anterior ciliary arteries and the long posterior ciliary arteries.
These tiny arteries anastomose with each other and comprise a vascular circle near the root of the iris, which they also supply.
The blood from the ciliary body is drained by the vorticose veins. Vorticose veins drain their blood into superior orbital and inferior orbital veins.
These fibers travel via the oculomotor nerve (CN III) to the ciliary ganglion. The ciliary ganglion then gives off short ciliary nerves (i.e. postganglionic fibers) that innervate the ciliary body.
There is scarce evidence that the ciliary body receives some sympathetic innervation via the superior cervical ganglion that inhibits the accommodation reflex.
The ciliary body plays several important roles in the proper functioning of the human eye. Firstly, the ciliary body is one of the main components of the accommodation reflex. The accommodation reflex is crucial for focusing on objects located in close proximity to the eye. Next, the epithelium of the ciliary processes produces the eye fluid or aqueous humor. The aqueous humor is a transparent water-like fluid similar to blood plasma. Its main functions are to provide nutrition to the surrounding tissues and maintain intraocular pressure. It prevents eye dryness and contains proteins (immunoglobulins) that protect the eye from external factors.
Glaucoma is one of the leading causes of blindness in the general population. Its most frequent cause is an obstruction of aqueous humor outflow through the trabecular meshwork. It can also be caused by the increased production of aqueous humor. The abnormal collection of aqueous humor increases the intraocular pressure which leads to the atrophy of the optic nerve (CN II) and progressive loss of vision.
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