ABSTRACT
The organs of equilibrium (labyrinth, vestibule), situated in mammals in the petrousbone, are extremely important for non-visual spatial perception.The labyrinth is a double organ, in the first place consisting of the utriculus and the sacculus. These contain lumps of calcium carbonate, the so-called otoliths, which by their weight can exert pressure on receptor cells. The organs register the direction of gravity and other linear accelerations. In the second place, the three semicircular canals, which are at right angles to each other (Fig. 3.1) and are filled with fluid, the endolymph.When the head is turned a floworiginates in the endolymph, giving rise to stimulation of the receptors in the dilated end of each canal, the so-called ampulla. In this way the canals register the turning of the head in the three
dimensions ofobjective space. In the central nervous system the nerve fibres arising from the labyrinth enter the vestibular nuclei, which are closely connectedwith the centres fromwhich movements are controlled: themotor centres in the spinal cord, the cerebrum and the cerebellum.There is a particularly close relationship between the organ ofequilibrium and themotor centres for eyemovements.Thiswill be considered in Chapter 5.
