Anatomy of iris and ciliary body

Detailed Anatomy of iris and ciliary body

Uveal tissue constitutes the middle vascular coat of the eyeball. From anterior to posterior, it can be divided into three parts, namely, iris, ciliary body and choroid.

iris

Iris is the anterior most part of the uveal tract. It is a thin circular disc corresponding to the diaphragm of a camera. In its centre is an aperture of about 4 mm diameter called pupil which regulates the amount of light reaching the retina.

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.

Illustration of iris and ciliary body

Ciliary body and muscles

The iris of eye consists of four layers which from anterior to posterior are

1. Anterior limiting layer. It is the anterior most condensed part of the stroma. It consists of
   melanocytes and fibroblasts. Previously, this layerwas called endothelial layer of iris which was a misnomer. This layer is deficient in the areas of crypts. The definitive colour of the iris depends on this layer. In blue iris this layer is thin and contains few pigment cells. While in brown iris it is thick and densely pigmented.
2. Iris stroma. It consists of loosely arranged collagenous network in which are embedded the
   sphincter pupillae muscle, dilator pupillae muscle, vessels, nerves, pigment cells and other cells which include lymphocytes, fibroblasts, macrophages and
   mast cells.
   • Sphincter pupillae muscle forms one millimetre broad circular band in the pupillary part of the iris. It is supplied by parasympathetic fibres through third nerve (see page 6). It constricts the pupil.
   • Dilator pupillae muscle lies in the posterior part of stroma of the ciliary zone of iris. Its myofilaments are located in the outer part of the cells of anterior pigment epithelial layer. It is supplied by cervical sympathetic nerves and dilates the pupil.
3. Anterior epithelial layer. It is anterior continuation of the pigment epithelium of retina and ciliary body. This layer gives rise to the dilator pupillae muscle.
4. Posterior pigmented epithelial layer. It is anterior continuation of the nonpigmented epithelium of ciliary body. At the pupillary margin, it forms the pigmented frill

Ciliary body and muscles

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Ciliary body is forward continuation of the choroid at ora serrata. In cut-section, it is triangular in shape. The anterior side of the triangle forms the part of the angle of anterior and posterior chambers. In its middle the iris is attached. The outer side of the triangle lies against the sclera with a suprachoroidal space in between. The inner side of the triangle is divided into two parts. The anterior part (about 2 mm) having finger-like ciliary processes is called
pars plicata and the posterior smooth part (about 4 mm) is called pars plana

From without inwards ciliary body consists of following five layers

1. Supraciliary lamina. It is the outermost condensed part of the stroma and consists of pigmented collagen fibres. Posteriorly, it is the continuation of suprachoroidal lamina and anteriorly it becomes continuous with the anterior limiting membrane of iris.
2. Stroma of the ciliary body. It consists of connective tissue of collagen and fibroblasts. Embedded in the stroma are ciliary muscle, vessels, nerves, pigment
   and other cells.
   ■■Ciliary muscle occupies most of the outer part of ciliary body. In cut section it is triangular in shape. It is a nonstriated muscle having three parts:
   • Longitudinal or meridional fibres which help in aqueous outflow;
   • Circular fibres which help in accommodation; and
   • Radial or oblique fibres act in the same way as the longitudinal fibres.

• Nerve supply. Ciliary muscle is supplied by parasympathetic fibres through the short ciliary
nerves.

3. Layer of pigmented epithelium. It is forward continuation of the retinal pigment epithelium. Anteriorly, it is continuous with the anterior pigmented epithelium of the iris.
4. Layer of nonpigmented epithelium. It consists mainly of low columnar or cuboidal cells, and is the forward continuation of the sensory retina. It continues anteriorly as the posterior (internal) pigmented epithelium of the iris.
5. Internal limiting membrane. It is the forward continuation of the internal limiting membrane of the retina. It lines the nonpigmented epithelial layer.

Functions of ciliary body

• Formation of aqueous humour.
• Ciliary muscles help in accommodation.

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