The sense of balance and position.Two sets of receptors in the inner ear, or “labyrinth,” are responsible for our senses of balance and position (see Fig. 28, p. 543). The first of these consists of three semicircular canals which respond mainly when the head is rotated. They are in the form of hair- lined tubes oriented at right angles to each other: up-down, right-left, and backward-forward. The hairs contain sensitive nerve endings which are bent and activated by a watery liquid which flows one way or another when there is an acceleration or deceleration in the rate of rotation. The second set consists of the vestibular sac receptors (utricle and saccule) which contain the otolith (“ear stone”) organs that respond to the static force of gravity as well as to the straight-line motion. Pressure of the stones, or granules, on hair cells in the sacs produces nerve impulses that inform the brain of the upright position or tilt of the head.The equilibrium mechanism is largely automatic and works closely with our sense of movement (kinesthesis). When we are thrown off balance, sensory impulses from the inner ear set off reflex motor adjustments which enable us to regain our equilibrium. A good illustration of these adjustments is the cat’s ability to land on all fours after being turned upside down and dropped from a height. The head is twisted into a normal position first, followed by a reflex twisting of the trunk and body. Parachutists make similar movements during the free fall before the parachute is opened.The vestibular sense is involved in “motion sickness,” an ailment which is totally absent in people whose vestibular mechanisms are not functioning. At present there is no adequate explanation for the fact that some people become motion-sick when traveling in planes, autos and ships while others do not. However, we do know that this unpleasant feeling of nausea is caused by both physiological and psychological factors. The physiological effects appear to be due to reflex actions in the alimentary tract. Among the psychological factors are suggestion, emotional conditioning and past experience. Many people become ill because they expect to, and therefore pay excessive attention to slight discomfort. Nausea can also result from an inconsistent set of sensations, particularly in an airplane. If the plane is flying smoothly, and if we close our eyes or keep them focused on its interior, we do not usually experience vestibular sensations. But if we look out of the window, the combination of the unfamiliar sight of the earth or clouds moving rapidly by, plus the absence of a sensation of movement, may make us dizzy or sick to our stomach.Vestibular sensations may be counteracted by drugs such as Dramamine and Bonamine, or by lying down with the eyes closed. It is also possible to reduce or prevent motion sickness by becoming habituated to the stimuli and by anticipating the regularly recurring motions. The latter method of adjustment is utilized by acrobats, figure skaters, and ballet dancers. These performers learn to ignore vestibular sensations and rely on other senses during their complicated routines. One trick is to keep the head as motionless as possible while spinning around. This can be done by fixating on a stationary object. After a time the rapid eye movements (nystagmus) that accompany head rotation, one of the primary reasons for dizziness, become very slight.Candidates for commercial aviation, airborne military service and space flight must go through rigorous tests of equilibrium as well as long hours of practice in flight trainers. The object of their training is not merely to overcome any tendency to motion sickness, but to teach them to disregard misleading vestibular sensations and rely instead on their instruments when engaged in climbing, diving, or other maneuvers.