The iris arises from the anterior border of the ciliary body (Fig. 24.7) and is attached to the sclera about 2 mm posterior to the corneoscleral junction. The pupil is the central aperture of this thin disc. The iris is pushed slightly forward as it changes in size in response to light intensity. It consists of highly vascularized connective tissue stroma that is covered on its posterior surface by highly pigmented cells, the posterior pigment epithelium (Fig. 24.8). The basal lamina of sphincter m. pigment layer iris dilator m. canal of Schlemm ciliary muscle ciliary process zonular fibers
FIGURE 24.6 • Schematic diagram of the structure of the eye. This drawing shows a horizontal section of the eyeball with color-coded layers of its wall. Upper inset. Enlargement of the anterior and posterior chambers is shown in more detail. Note the location of the iridocorneal angle and canal of Schlemm (scleral venous sinus), which drains the aqueous humor from the anterior chamber of the eye. Lower inset. Typical organization of the cells and nerve fibers of the fovea.
these cells faces the posterior chamber of the eye. The degree of pigmentation is so great that neither the nucleus nor character of the cytoplasm can be seen in the light microscope. Located beneath this layer is a layer of myoepithelial cells, the anterior pigment myoepithelium. The apical (posterior) portions of these myoepithelial cells are laden with melanin granules, which effectively obscure their boundaries with the adjacent posterior pigment epithelial cells. The basal (anterior) portions of myoepithelial cells possess processes containing contractile elements that extend radially and collectively make up the dilator pupillae muscle of the iris. The contractile processes are enclosed by a basal lamina that separates them from the adjacent stroma.
Constriction of the pupil is produced by smooth muscle cells located in the stroma of the iris near the pupillary margin of the iris. These circumferentially oriented cells collectively compose the sphincter pupillae muscle.
The anterior surface of the iris reveals numerous ridges and grooves that can be seen in clinical examination with the ophthalmoscope. When this surface is examined in the light microscope, it appears as a discontinuous layer of fibroblasts and melanocytes. The number of melanocytes in the stroma is responsible for variation in eye color. The function of these pigment-containing cells in the iris is to absorb light rays. If there are few melanocytes in the stroma, eye color is derived from light refected from the pigment present in the cells of the posterior surface of the iris, giving it a blue appearance. As the amount of pigment present in the stroma increases, the color changes from blue to shades of greenish blue, gray, and, fnally, brown.
The sphincter pupillae is innervated by parasympathetic nerves; the dilator pupillae muscle is under sympathetic nerve control.
The size of the pupil is controlled by contraction of the sphincter pupillae and dilator pupillae muscles. The process of adaptation (increasing or decreasing the size of the pupil) ensures that only the appropriate amount of light enters the eye. Two muscles are actively involved in adaptation:  The sphincter pupillae muscle, a circular band of smooth muscle cells (Plate 106, page 924), is innervated by parasympathetic nerves carried in the oculomotor nerve (cranial nerve III) and is responsible for reducing pupillary size in response to bright light. Failure of the