SPECIAL SENSES 377 



transmitted to the inner ear at the fenestra ovahs through 

 these small bones. The air received into the middle ear 

 through the Eustachian tube serves to equalize the pressure 

 upon the tympanic membranes. When this tube is clogged 

 up with mucous, as when one has a cold, deafness re- 

 suits. 



The inner ear consists of an irregular cavity hollowed out 

 of the temporal bone, known as the bony labyrinth. It 

 consists of three parts, the vestibule (V), the semicircular 

 canals (B) and the cochlea (S) . The vestibule communicates 

 with the tympanic cavity by the fenestra ovalis (closed by 

 the stapes). It opens into the semicircular canal and into 

 the cochlea. These parts are all lined by a membrane, called 

 the membranous labyrinth. This membrane secretes a fluid, 

 the endolymph, which fills semicircular canals, vestibule and 

 cochlea. 



One of the canals lies in a horizontal plane (when the 

 body is upright) while the other two lie in a vertical plane 

 but at right angles to one another. The membrane lining 

 the three canals contains a series of nerve endings. Nerve 

 fibers from the auditory nerve (fig. 187, A) are distributed to 

 these endings. Our sensations of position in space, or 

 equilibrium, are gained through the nerve endings of the 

 semicircular canals. 



The cochlea is a complicated organ with a very delicate 

 set of sensory cells, known collectively as the organ of Corti 

 (fig. 187, c). These cells are the very delicate nerve 

 endings of fibers from another part of the auditory nerve, 

 as shown in the figure. The nerve endings in the organ 

 of Corti are stimidated by the vibrations in the endolymph 

 which bathes them. The endolymph receives vibrations 

 through the small chain of bones. 



Care of the ears.. — The inner ear is well protected and 

 can scarcely be injured by external causes. Colds or- sore 

 throat frequently cause stoppages of the Eustachian tube 



