evaporate flow toward the inner angle of the eye, where they drain down ducts into the nose. MIDDLE LAYER.—The middle layer of the eye is called the choroid. This layer is a highly vascular, pigmented tissue that provides nourishment to the inner structures. Continuous with the choroid is the ciliary body. The ciliary body is formed by a thickening of the choroid and fits like a collar into the area between the retina and iris. Attached to the ciliary the body are the suspensory ligaments, which blend with the elastic capsule of the lens and holds it in place. Iris.—The iris is continuous with the ciliary body. The iris is a circular, pigmented muscular structure that gives color to the eye. The iris separates the anterior cavity into anterior and posterior chambers. The opening in the iris is called the pupil (fig. 1-49). The amount of light entering the pupil is regulated through the constriction of radial and circular muscles in the iris. When strong light is flashed into the eye, the circular muscle fibers of the iris contract, reducing the size of the pupil. If the light is dim, the pupil dilates to allow as much of the light in as possible. The size and reaction of the pupils of the eyes are an important diagnostic tool. Lens.—The lens is a transparent, biconvex (having two convex surfaces) structure suspended directly behind the iris. The optic globe posterior to the lens is filled with a jellylike substance called vitreous humor, which helps to maintain the shape of the eyeball by maintaining intraocular pressure. The lens separates the eye into anterior and posterior cavities. INNER LAYER.—The inner layer of the eye is called the retina (fig. 1-48). It contains layers of the nerve cells, rods, and cones that are the receptors of the sense of vision. The retina is continuous with the optic nerve, which enters the back of the globe and carries visual impulses received by the rods and cones to the brain. The area where the optic nerve enters the eyeball contains no rods and cones and is called the optic disc (blind spot) (fig. 1-50). Rods.—Rods respond to low intensities of light and are responsible for night vision. They are located in all areas of the retina, except in the small depression called the fovea centralis, where light entering the eye is focused, and which has the clearest vision. Cones.—Cones require higher light intensities for stimulation and are most densely concentrated in the fovea centralis. The cones are responsible for daytime vision. Vision Process The vision process begins with rays of light from an object passing through the cornea. The image is then received by the lens, by way of the iris. Leaving the lens, the image falls on the rods and cones in the retina. The image then is carried to the brain for interpretation by the optic nerve (fig. 1-51). Note the image received by the retina is upside down, but the brain turns it right-side up. REFRACTION.—Deflection or bending of light rays results when light passes through substances of varying densities in the eye (cornea, aqueous humor, lens, and vitreous humor). The deflection of light in the eye is referred to as refraction. ACCOMMODATION.—Accommodation is the process by which the lens increases or decreases its curvature to refract light rays into focus on the fovea centralis. CONVERGENCE.—The movement of the globes toward the midline, causes a viewed object to come into focus on corresponding points of the two 1-46 SYMPATHETIC MOTOR NERVE RADIALLY ARRANGED SMOOTH MUSCLE FIBERS OF THE IRIS CIRCULARLY ARRANGED SMOOTH MUSCLE FIBERS OF THE IRIS PUPIL PARASYMPATHETIC MOTOR NERVE FIBER IN DIM LIGHT IN BRIGHT LIGHT HM3F0149 Figure 1-49.—Anterior view of the eye.