The eyes are expansions of the optic nerve, while latter is direct prolongation of the brain. The nerve spreads out into a thin layer called retina in which are situated microscopic bodies called rods and cones. It is the excitation of those rods and cones in retina that sends up sensory impulse to the brain, which the brain translates as vision. The eye-globe is situated in the orbital cavity amidst a cushion of fat and is moved in various directions by muscles. The globe consists of an anterior and a posterior chamber and the external coating of the eye in its front portion is called cornea, behind the sclerotic coat. The cornea is glassy and transparent; the partition between the two chambers is formed by the crystalline lens being suspended by a circular screen (iris) which is hanging like a curtain from a band of muscles (ciliary muscles). It is this iris that forms the black center of the eye and the aperture through it is called the pupil. The size of the pupil is controlled by the ciliary muscles. The posterior globe is full of a viscid fluid called vitreous humor. The glassy cornea, the aqueous humor (or watery fluid in the anterior chamber), the lens and the vitreous humor all serve to focus rays of light on the retina so as to form a true image thereon for the brain to perceive.