Vertebrate sensory systems are similar in structure and function in different species, but they may be located in different parts of the body. Some species have senses humans do not have. Humans perceive smell when odorant molecules bind receptors in the olfactory epithelium of the nasal passages. The brain perceives a smell by evaluating the pattern of olfactory receptor cells that bind odorant molecules. Humans perceive taste when chemicals stimulate receptors within taste buds.
Humans perceive vision with a complex lens system in the eye.
The human eye contains three layers. The outer layer, the sclera, protects. It forms the transparent cornea in the front of the eyeball. The next layer, the choroid coat, is pigmented, located toward the rear of the eye, and absorbs light. In the front of the eye, the choroid coat forms the ciliary body, which controls the shape of the lens that focuses light on the photoreceptors, and the opaque iris.