THE EAR 295 



The epithelial cells covering the inner walls of the mem- 

 branous labyrinth form hair cells in the maculae and cristae 

 whose hairs extend into the cavity of the membranous laby- 

 rinth. These hair cells are neuro-epithelial cells in which 

 the fibres of the auditory nerve end. A branch of the 

 cochlear nerve goes to the macula sacculi, while the vesti- 

 bular nerve goes to the macula utriculi and the cristae. 



Upon both maculae lies a thin jelly-like membrane, the 

 membrane of the otoliths ; upon the surface of this membrane 

 lie the otoliths (consisting of calcium carbonate). 



The fibres of the auditory nerve to this part of the laby- 

 rinth are, according to the prevalent theory, not nerves of 

 hearing, but serve to perceive the position and movements of 

 the head. Their neuro-epithelial cells are supposed to be 

 mechanically stimulated either by the pull which the otoliths 

 exert because of their weight or by the hydrostatic pressure 

 of the endolymph, which varies with the different positions 

 of the head. They may also be stimulated by the move- 

 ments of the endolymph brought about by movements of the 

 head. 



Reflexly coordinated movements for the maintenance of 

 the normal position of the head and the equilibrium of the 

 body are in part called forth by impulses from the semicir- 

 cular canals and the otolith organs. The compensatory 

 movements of the eye (page 242) are also called forth by 

 the stimulations from the semicircular canals. Destruction 

 of the semicircular canals in animals is followed by disturb- 

 ances in the normal position and movements of the head and 

 of the whole body (forced positions and movements). It is 

 also followed by diminution of the energy and tonus of 

 skeletal muscles and disturbances in muscular sense. 



