Ciliary (ciliary) body

The ciliary body (corpus ciliare) is the middle thickened part of the vascular tract of the eye, which produces intraocular fluid. The ciliary body provides support for the lens and provides an accommodation mechanism, in addition, it is the heat collector of the eye

Under normal conditions, the ciliary body, located under the sclera in the middle between the iris and the choroid, is not accessible to inspection: it is hidden behind the iris. The area of the ciliary body is projected on the sclera in the form of a ring 6-7 mm wide around the cornea. On the outer side, this ring is slightly wider than on the nasal side.

The ciliary body has a rather complex structure. If you cut the eye along the equator and look from the inside at the anterior segment, you will clearly see the inner surface of the ciliary body in the form of two round dark-colored belts. In the center, surrounding the lens, rises a folded ciliary crown 2 mm wide (corona ciliaris). Around it is the ciliary ring, or flat part of the ciliary body, 4 mm wide. It goes to the equator and ends with a serrated line. The projection of this line on the sclera is in the area of attachment of the rectus muscles of the eye.

The ring of the ciliary crown consists of 70-80 large processes oriented radially towards the lens. Macroscopically, they resemble cilia, hence the name of this part of the vascular tract - "ciliary, or ciliary, body". The tops of the processes are lighter than the general background, the height is less than 1 mm. Between them there are tubercles of small processes. The space between the equator of the lens and the process part of the ciliary body is only 0.5-0.8 mm. It is occupied by a ligament that supports the lens, which is called the ciliary belt, or Zinn's ligament. It is a support for the lens and consists of the finest threads coming from the anterior and posterior capsules of the lens in the equator area and attached to the processes of the ciliary body. However, the main ciliary processes are only part of the attachment zone of the ciliary zonule, while the main network of fibers passes between the processes and is fixed along the entire length of the ciliary body, including its flat part.

The fine structure of the ciliary body is usually studied on a meridional section, which shows the transition of the iris into the ciliary body, which has the shape of a triangle. The wide base of this triangle is located in front and represents the branched part of the ciliary body, and the narrow apex is its flat part, which passes into the posterior section of the vascular tract. As in the iris, the ciliary body is divided into an outer vascular-muscular layer, which has a mesodermal origin, and an inner retinal, or neuroectodermal, layer.

The outer mesodermal layer consists of four parts:

• suprachoroid. This is a capillary space between the sclera and the choroid. It can expand due to the accumulation of blood or edematous fluid in eye pathology;
• accommodation, or ciliary, muscle. It occupies a significant volume and gives the ciliary body its characteristic triangular shape;
• vascular layer with ciliary processes;
• Bruch's elastic membrane.

The inner retinal layer is a continuation of the optically inactive retina, reduced to two layers of epithelium - the outer pigmented and the inner non-pigmented, covered by the border membrane.

For understanding the functions of the ciliary body, the structure of the muscular and vascular parts of the outer mesodermal layer is of particular importance.

The accommodation muscle is located in the anterior-outer part of the ciliary body. It includes three main portions of smooth muscle fibers: meridional, radial, and circular. The meridional fibers (Brücke's muscle) are adjacent to the sclera and are attached to it at the inner part of the limbus. When the muscle contracts, the ciliary body moves forward. The radial fibers (Ivanov's muscle) fan out from the scleral spur to the ciliary processes, reaching the flat part of the ciliary body. Thin bundles of circular muscle fibers (Müller's muscle) are located in the upper part of the muscular triangle, form a closed ring, and act as a sphincter when contracted.

The mechanism of contraction and relaxation of the muscular apparatus underlies the accommodative function of the ciliary body. When all portions of differently directed muscles contract, the effect of a general decrease in the length of the accommodative muscle along the meridian (pulls forward) and an increase in its width in the direction of the lens occurs. The ciliary belt narrows around the lens and approaches it. The Zinn ligament relaxes. The lens, due to its elasticity, tends to change its disc-shaped form to a spherical one, which leads to an increase in its refraction.

The vascular part of the ciliary body is located medially from the muscular layer and is formed from the large arterial circle of the iris, located at its root. It is represented by a dense interweaving of vessels. Blood carries not only nutrients, but also heat. In the anterior segment of the eyeball, which is open to external cooling, the ciliary body and iris are a heat collector.

The ciliary processes are filled with vessels. These are unusually wide capillaries: if erythrocytes pass through the retinal capillaries only having changed their shape, then up to 4-5 erythrocytes fit in the lumen of the capillaries of the ciliary processes. The vessels are located directly under the epithelial layer. This structure of the middle part of the vascular tract of the eye ensures the function of secreting intraocular fluid, which is an ultrafiltrate of blood plasma. Intraocular fluid creates the necessary conditions for the functioning of all intraocular tissues, provides nutrition to avascular formations (cornea, lens, vitreous body), maintains their thermal regime, and maintains eye tone. With a significant decrease in the secretory function of the ciliary body, intraocular pressure decreases and atrophy of the eyeball occurs.

The unique structure of the vascular network of the ciliary body described above also has negative properties. In wide, tortuous vessels, blood flow is slow, which creates conditions for the settling of pathogens. As a result, any infectious diseases in the body can lead to inflammation in the iris and ciliary body.

The ciliary body is innervated by branches of the oculomotor nerve (parasympathetic nerve fibers), branches of the trigeminal nerve and sympathetic fibers from the plexus of the internal carotid artery. Inflammatory phenomena in the ciliary body are accompanied by severe pain due to the rich innervation of the branches of the trigeminal nerve. On the outer surface of the ciliary body there is a plexus of nerve fibers - the ciliary ganglion, from which branches extend to the iris, cornea and ciliary muscle. An anatomical feature of the innervation of the ciliary muscle is the individual supply of each smooth muscle cell with a separate nerve ending. This is not found in any other muscle of the human body. The expediency of such a rich innervation is explained mainly by the need to ensure the performance of complex centrally regulated functions.

Functions of the ciliary body:

• lens support;
• participation in the act of accommodation;
• production of intraocular fluid;
• thermal collector of the anterior segment of the eye.

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