332 ANATOMY AND PHYSIOLOGY 



circular canals each with bulb-like extremity, or ampulla, opening 

 into the vestibule. (Two are joined together at one extremity, 

 leaving five openings for the three canals.) 



Observe the two windows, round and oval in the vestibule wall, 

 open in the dried bone, as in the illustration, but closed in life by 

 the lining membrane of the internal ear. 



The cochlea is a spiral canal winding two and one- half 

 turns about a central stem of petrous bone (the modiolus). It is 

 divided into three canals or scala, reaching from base to apex of the 

 spiral. In one of these the membranous cochlea lies; of the two 

 others, one opens into the vestibule and the other ends at the 

 round window, separated from the tympanum by the secondary 

 membrane. 



The internal ear has a fibro-serous lining which contains a 

 clear watery fluid called perilymph. 



Lying in the perilymph and bathed by it is the membranous 

 labyrinth having all parts and shapes of the osseous labyrinth. 



Fig. 212 shows the membranous labyrinth which lies within the 

 bony labyrinth, surrounded by perilymph. It contains a fluid 

 called endolymph which bathes the fine nerve-fibers of the auditory 

 nerves. 



Note the membranous semicircular canals, with their ampullae , 

 membraneous cochlea and the two portions of the membranous vesti- 

 bule. Within these (called the saccule and utricle), the ampullae, 

 and the cochlea, the terminal fibers of the auditory nerve are 

 distributed. 



Ossicles. A chain of three ossicles (or little bones) is sus- 

 pended across the tympanum the malleus, incus, and stapes. 

 The malleus (ox hammer) is attached by the handle to the membrane 

 of the drum, the incus (or anvil) comes next, and then the stapes 

 (or stirrup) with its base fitting the oval window of the middle ear. 

 Any vibration of the membrane of the tympanum is at once trans- 

 mitted by this chain of bones across the tympanum, to the oval 

 window. 



The base of the stapes occupies the oval window (fenestra ovalis); 

 its movements are transmitted to the perilymph and through this 

 to the membranous labyrinth, thence to the endolymph within it 

 and the auditory nerves. 



