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.