THE MBMBEANOTJS LABYRINTH 473 



secretes a fluid called the perilj^mph. It contains a closed 

 membranous sac, or capsule, called the membranous laby- 

 rinth. This sac fills almost completely the spaces of the 

 bony labyrinth. The bon}/^ labyrinth is usually divided 

 into three areas. The central part, about an eighth of an 

 inch in diameter, is called the vestibule. The part back of 

 this consists of three tubes called the semicircular canals. 

 The part in front of the vestibule is a single tube 

 coiled like a snail shell and called the cochlea. Into the 

 osseous labyrinth enter the branches of the eighth pair of 

 cranial nerves (auditory). 



The bony labyrinth.— (See Fig. 224.) The vestibule lies 

 just back of the tympanum, and communicates with it by 

 the membrane-covered fenestra ovalis. The three semi- 

 circular canals lie toward the back of the head and com- 

 mimicate with the vestibule by five openings, two of the 

 canals uniting before entering the vestibule. The cochlea 

 lies forward of the vestibule. It is a spiral tube consisting 

 of two and a half turns about a central pillar. The large 

 end opens into the vestibule. It communicates with the 

 tjTnpanum by the membrane-covered fenestra rotunda. 



The membranous labyrinth. — This closed membra- 

 nous sac lies in the bony labyrinth surrounded by 

 the ];)eril)miph of that cavity. It conforms very closely 

 to it in shape. Internally it secretes a fluid called the 

 endol5Tnph, which fills it and gives it shape. In 

 this liquid are tiny particles of lime cafled otoliths. The 

 membranous vestibule is divided into two sacs, the utricu- 

 lus and the sacculus, connected by a small opening. The 

 utriculus is the larger and from it arise three membranous 

 semicircular canals. The sacculus is smaller and commun- 

 icates with the membranous cochlea. The membranous 



