EQUILIBRIUM, a state of balance as to the forces acting upon any body. The condi tion• is generally considered in respect to the action of gravity, and especially as to the centre of gravity of the body under observation. In accordance with that law of physics that the centre of gravity tends always to occupy the lowest possible position — that is, the position in which it is nearest to the centre of the earth — equilibrium appears in three forms: (1) stable equilibrium, when the centre of gravity is below the point of support; (2) unstable equilibrium, when the centre of gravity is above the point of support; (3) neutral equilibrium, when the centre of gravity and the point of support coincide. An illustration may be found in a wheel supported free of the ground on its axle. If the wheel is of uniform build in all of its parts it will be in neutral equilibrium, re maining at rest in any position to which it may be revolved on the axle. If, however, a weight be attached to the rim of the wheel at its low est point, when the wheel is moved so as to raise the weight ever so little the wheel will swing back until the weight is again at the low estpoint, in which position it is in stable equilibrium. If the wheel is then revolved so that the weight is directly above the axle, and carefully balanced in that position, the wheel will be in unstable equilibrium, for when dis turbed it will immediately revolve so as to bring the weight to the lowest point again, and come to rest in stable equilibrium. See APPLIED ME CHANICS; MECHANICS.
In physiology, the ability to maintain the body by proper muscular force under nervous control, so that it can perform co-ordinated movements or resist the force of gravity. By equilibrium is here meant the control of the body in the upright position, apart from the localized processes of co-ordination. Loss of
equilibrium shows itself particularly in walk ing and running. Here the centre of gravity of the body is constantly changing and the ability of the individual to hold himself erect depends upon a number of features. The eye, the tactile sense of the feet, the joint-senses, the muscular sense that weighs the various muscular move ments and the higher cerebral centres are all involved, and loss of equilibrium or inco-ordina tion may result from disease or injury to any of these functions. Loss of eyesight does not necessarily involve any loss of equilibrium, but loss of tactile sense of the feet, such as is seen in locomotor ataxia or in people who have had their feet frozen, almost invariably produces a loss of equilibrium. In the same disease (loco motor ataxia) and in forms of neuritis the loss of muscular sense and joint-sense prodnces similar phenomena. For the higher cerebral centres of control there is good reason to believe that the semi-circular canals in the in ternal ear constitute a special sense-organ for the determination of the direction of the move ments of the head which are so essential in the preservation of general equilibrium. Diseases of the semi-circular canals are frequently ac companied with dizziness and vertigo. It has been suggested that the sacculus and utriculus of the internal ear act to maintain equilibrium while at rest. It also seems probable that cer tain areas in the cerebellum are closely associated with the equilibrium. See CEREBELLUM ; INCO ORDINATION ; LOCOMOTOR ATAXIA.