The human eye is a sensory organ, part of the sensory nervous system, that reacts to visible light and allows humans to use visual information for various purposes including seeing things, keeping balance, and maintaining circadian rhythm. The eye can be considered as a living optical device. It is approximately spherical in shape, with its outer layers, such as the outermost, white part of the eye (the sclera) and one of its inner layers (the pigmented choroid) keeping the eye essentially light tight except on the eye's optic axis. In order, along the optic axis, the optical components consist of a first lens (the cornea—the clear part of the eye) that accomplishes most of the focussing of light from the outside world; then an aperture (the pupil) in a diaphragm (the iris—the coloured part of the eye) that controls the amount of light entering the interior of the eye; then another lens (the crystalline lens) that accomplishes the remaining focussing of light into ; then a light-sensitive part of the eye (the retina) where the images fall and are processed. The retina makes a connection to the brain via the optic nerve. The remaining components of the eye keep it in its required shape, nourish and maintain it, and protect it. Three types of cells in the retina convert light energy into electrical energy used by the nervous system: rods respond to low intensity light and contribute to perception of low-resolution, black-and-white images; cones respond to high intensity light and contribute to perception of high-resolution, coloured images; and the recently discovered photosensitive ganglion cells respond to a full range of light intensities and contribute to adjusting the amount of light reaching the retina, to regulating and suppressing the hormone melatonin, and to entraining circadian rhythm. Humans have two eyes, situated on the left and the right of the face. The eyes sit in bony cavities called the orbits, in the skull. There are six extraocular muscles that control eye movements.