The human body has a number of organs that are specialized receptors adapted to detect stimuli.
The eye detects light energy (photons) and transmits information about intensity, color, and shape to the brain. The eyeball is covered by a thick, opaque layer known as the Sclerasclera, which is also known as the white of the eye. Beneath the sclera is the choroid layer, which helps to supply the retina with blood. The choroid is a dark, pigmented area that reduces reflection in the eye. The innermost layer of the eye is the retina, which contains the photoreceptors that sense light.
The transparent cornea at the front of the eye bends and focuses light rays. The rays then travel through an opening called the pupil, whose diameter is controlled by the pigmented, muscular iris. The iris responds to the intensity of light in the surroundings (light makes the pupil constrict). The light continues through the lens, which is suspended behind the pupil. The lens, the shape and focal length of which is controlled by the ciliary muscles, focuses the image onto the retina.
In the retina are photoreceptors that transduce light into action potentials. The two main types of photoreceptors are cones and rods. Cones respond to high-intensity illumination and are sensitive to color, whereas rods detect low-intensity illumination and are important in night vision. Cones and rods contain various pigments that absorb specific wavelengths of light. Cones contain three different pigments that absorb red, green, and blue wavelengths; the rod pigment, rhodopsin, only absorbs a single wavelength. The photoreceptor cells synapse onto bipolar cells, which in turn synapse onto ganglion cells. Axons of the ganglion cells bundle to form the optic nerve, which conducts visual information to the brain. The point at which the optic nerve exits the eye is called the blind spot because photoreceptors are not present there. The fovea, a small area of the retina above the blind spot, is densely packed with cones and is important for high-acuity vision.
The eye also contains a jelly-like material called vitreous humor that helps maintain its shape and optical properties. Aqueous humor is a more watery substance that fills the space between the lens and the cornea.