It has already been mentioned that the motions of the iris alter the size of the pupillary aperture through which the rays of light must pass, and while it regulates the amount of light admitted, it also acts like the diaphragm of an optical instrument, and always cuts off a large amount of the marginal rays. The great importance of not allowing the rays which would traverse the margin of the lens to enter the eyeball can be understood after what has been said of spherical and chromatic aberration. The iris also contracts when the eye is adjusted for near vision, independent of the amount of light by which the object is illuminated. This action is of advantage, because the more convex the lens becomes in viewing near objects, the greater is the aberration of the marginal rays. If the iris did not contract in near vision, the closer an object was brought to the eye the greater would be the tendency to indistinctness caused by spherical aberration.

In structure, the iris consists of a framework of delicate connective tissue, like that of the choroid coat, containing many blood vessels. On its posterior surface is a dense layer of pigment cells called the uvea, which gives the eye its color. The act of contracting the pupil is performed by a very definite set of instriated muscular fibres, forming the sphincter which surrounds the margin of the pupil. The sphincter muscle seems always to be more or less in action, because if it be paralyzed, the pupil dilates. The muscular character of the dilator mechanism has been doubted from the fact that radiating muscular fibres have not been satisfactorily demonstrated under the microscope. Certainly the sphincter is the most distinctly contractile, for strong electric stimulation always causes contraction of the pupil, and shortly after death the pupils dilate from the relaxation of the sphincter. If we admit the active dilatation of the pupil, we must assume that the power of the sphincter dies more quickly than that of the dilator, or that it at once relaxes when it has lost the stimulus reflected from the retina. There apears to be no difficulty in explaining the dilatation of the pupil as the effect of the elasticity of the tissue and the changes in vasomotor influences.

Section through the ciliary region, showing the relation of the iris.

Fig. 228. Section through the ciliary region, showing the relation of the iris (/) to the choroid (g) and the ciliary muscle {a), which arises from the margin of the cornea at (e), and passes toward the choroid to the right, where it separates the latter from the sclerotic. Some bundles of circular fibres are shown, one marked (d).