The middle layer of the eye consists of the iris, ciliary body, and choroid.

Figure 1. Section of the eye showing the external and middle portions of the eye.

Iris

Arising from the ciliary body and the sclera, the iris forms the contractile diaphragm of the eye and controls how much light enters the pupil. It is comparable to a camera's shutter.

The iris is highly vascularized, consisting of three structures:

  • Posterior pigmented layer
    • Highly pigmented epithelium
  • Dilator pupillae muscle
    • Specialized contractile epithelium -- not smooth muscle cells, but rather myoepithelial cells with contractile processes
    • Innervated by unmyelinated sympathetic fibers from the superior cervical sympathetic

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