sense, faculty by which external or internal stimuli are conveyed to the brain centers, where they are registered as sensations. Sensory reception occurs in higher animals through a process known as transduction, in which stimuli are converted into nerve impulses and relayed to the brain. The four commonly known special senses (sight, hearing, smell, and taste) are concerned with the outer world, and external stimuli are received and conducted by sensory receptors concentrated in the eye, ear, olfactory organ, and the taste buds. The so-called somatic senses respond to both external and internal stimuli. Although most of the somatic receptors are located in the skin (conveying the external sensations of touch, heat, cold, pressure, and pain), others are located in internal organs (e.g., the heart and the stomach). Somatic sensations such as hunger, thirst, and fatigue are thought to originate in specific areas of the nervous system. The sense of balance, or equilibrium, is related to the flow of endolymph, a fluid found in the inner ear.