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. It is a highly vascular, pigmented tissue that provides nourishment to the inner structures. Continuous with the choroid is the  ciliary  body,  whose  muscular  structure  at- taches   to   the   lens   by   means   of   suspensory ligaments and produces changes in the thickness of the lens. This permits the eye to focus to long- range  or  close-up  vision. The  iris  is  continuous  with  the  ciliary  body. It is a circular, pigmented muscular structure that gives color to the eye. The opening in the iris is called  the  pupil  (fig  3-46).  The  amount  of  light entering the pupil is regulated through the con- striction  of  radial/circular  muscles  in  the  iris. When strong light is flashed into the eye, the cir- cular 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 possi- ble. The size and reaction of the pupils of the eyes are  an  important  diagnostic  tool. The lens is a transparent, biconvex structure suspended directly behind the iris. It separates the interior eye into anterior and posterior cavities. The  anterior  cavity  contains  a  watery  solution called aqueous humor, which helps to give the cor- nea  its  curved  shape.  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 and prevents misshaping by main- taining intraocular pressure. INNER LAYER.— The inner layer of the eye is called the retina (fig. 3-47). It contains different layers of nerve cells, rods, and cones that are the receptors of the sense of vision. The retina is con- tinuous 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  blind  spot. The 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. The 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 Figure 3-46.—Eye, anterior view. Deflection  or  bending  of  light  rays  results when light passes through substances of varying densities  in  the  eye  (cornea,  aqueous  humor, crystalline lens, and vitreous humor) (fig. 3-48). The  deflection  is  referred  to  as  refraction.  Accom- modation is the process performed by the lens that increases or decreases its curvature to refract light rays  into  focus  on  the  fovea. The  constriction  of  the  pupil  by  the  iris regulates the amount of light entering the eye. This process protects the retina from excessive stimula- tion and prevents a scattering of light rays that would  produce  blurred  vision. A  movement  of  the  globes  toward  the  midline, which causes a viewed object to come into focus on  corresponding  points  of  the  two  retinas,  is called   convergence.   This   gives   clear,   three- dimensional  vision. The end receptors or nerve endings in the rods and  cones  that  have  been  stimulated  by  light  con- duct   impulses   to   the   occipital   lobes   of   the cerebrum, where they are interpreted into vision (fig.  3-48). 3-36