The colorful images of objects in the external world enter the eye through the cornea on the front surface of the eye and reach the fundus in turn through the pupil, lens, vitreous and other optical conduction systems of the eye, forming an optical image of the object. The light energy of the image is then converted by the photoreceptor cells in the retina into nerve impulses, which are transmitted out of the eye via bipolar cells and retinal ganglion cells in turn, and then through the optic nerve, optic cross, optic bundle, lateral geniculate body and optic radiation and other nerve conduction systems, and finally into the visual center of the posterior occipital region to produce vision.