The eye is the receptor organ of the visual system. Photons of light entering the eye are focussed by the cornea and the accomodative lens onto the retina. Photoreceptor cells (rods and cones) act as transducers to convert the light signals into electrical signals, further processed by other neurones in the retina: bipolar cells, horizontal cells, amacrine cells, retinal ganglion cells, and interplexiform cells. The action potentials generated are transmitted via the optic nerve (cranial nerve II) to - chiefly - the lateral geniculate nucleus (visual information), superior colliculus (somatic reflexes), and pretectal areas (autonomic reflexes). The lateral geniculate nucleus projects to the the primary visual cortex of the occipital lobe (calcarine cortex, striate cortex, or Brodmann's area 17); the final area of neural processing of visual information.
The eye is situated in the anterior part of the orbital cavity. It consists of the segments of two spheres: a small transparent anterior segment, the aqueous humour, and a large posterior opaque one, the vitreous body. These are separated by the transparent, biconvex lens which is held in place by the suspensory ligaments. The aqueous humour is further divided into anterior and posterior chambers by the iris. Aqueous flows from the posterior chamber, around the iris, and into the anterior chamber, where it drains into the canal of Schlemm. The eyeball consists of three basic layers: outermost, the sclera, continuous with the transparent cornea anteriorly; intermediate, the pigmented vascular choroid, continuous with the ciliary body and iris anteriorly; and innermost, the retina. Shown on the diagram above is the visual axis, the line of best vision, passing through the fovea centralis, which is a pit at the point on the retina with the greatest concentration of cones (and no rods) and the greatest visual acuity. This is the point that our eyes move to focus fine details on. The optical axis is a line perpindicular to the anterior and posterior surfaces of the eye and passing through the centre of it.
