Middle Layer

The middle layer is highly vascular. If one were to peel the sclera away from this layer (not an easy task), the remaining structure would resemble a grape, as this middle layer, which is called the uvea, is heavily pigmented as well as being vascular. The anterior part of the uvea forms the bulk of the iris body and hence inflammation of the iris is called either anterior uveitis or iritis. The posterior part of the uvea is called the choroid.

The iris is the most anterior part of the uvea. It is a thin circular disc perforated centrally by the pupil. Contraction of the iris sphincter muscle constricts the pupil, while contraction of the dilator pupillae muscle dilates the pupil.

The ciliary body is part of the uveal tissue and is attached anteriorly to the iris and the scleral spur; posteriorly it is continuous with the choroid and retina. The ciliary body is also referred to as the intermediate uvea.

The ciliary body is triangular in cross-section. The anterior side of the ciliary body is the shortest and borders the anterior chamber angle; it gives origin to the iris. The outer side of the triangle (mainly ciliary muscles) lies against the sclera. The inner side is divided into two zones: (1) the pars plicata forms the anterior 2 mm and is covered by ciliary processes and (2) the pars plana constitutes the posterior 4.5-mm flattened portion of the ciliary body. The pars plana is continuous with the choroid and retina.

The choroid consists of the following:

• Bruch's membrane - membrane on the external surface of the retinal pigment epithelium (RPE). It consists of the basement membrane of RPE cells and chorio-capillaris. Between the two layers of basement membrane are the elastic and collagenous layers. Small localised thickenings of Bruch's membrane (which increase with age) are called drusen.

• The choriocapillaris - a network of capillaries supplying the RPE and outer retina.

• Layer of larger choroidal blood vessels external to the choriocapillaris.

• Pigmented cells scattered in the choroid external to the choriocapillaris.

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