A given receptor is described based on each classification criterion: receptor distribution, stimulus origin, and modality of stimulus. The eyes, for example, are because they are located in the head (receptor distribution); because they detect stimuli outside the body (stimulus origin); and because they detect light (modality of stimulus). In comparison, receptors that detect stretch of blood vessels are classified as because they are distributed throughout the body, because they detect stimuli within the body, and because they detect changes in distension of the organ wall.