Chap. XX] INTERNAL AND EXTERNAL SENSES 419 



The cochlea opens from the front end of the vestibule and saccule. 

 It resembles a snail shell and consists of a spiral tube of two and 

 one-half turns around a central pillar called the modiolus. 



Projecting from the modiolus is a thin lamina or plate of bone. 

 At its outer margin this lamina connects with a membrane which 

 extends to the outer wall of the cochlea. This lamina and mem- 

 brane divide the spiral canal into two passages or scahne. The lower 

 portion of this membrane is called the basilar membrane, and con- 

 sists of a network of fibres which forms the foundation for thou- 

 sands of cells which serve as the end organs of the auditory nerve. 

 These end organs constitute a structure that is known as the organ 

 of Corti. They receive nerve-fibres which arise in the ganglia con- 

 tained in the cavity of the modiolus. Both the modiolus and lam- 

 ina are pierced by numerous openings for the passage of these 

 nerves. 



The semicircular canals are three bony canals lying above and 

 behind the vestibule, and communicating with it by five openings, 

 in one of which two tubes join. They are known as the posterior, 

 vertical, and horizontal canals, and their position is such that each 

 one is at right angles to the other two. One end of each tube is 

 enlarged and forms what is known as the ampulla. The membrane 

 of the ampulla is covered with cells that are similar to those found 

 in the utricle and saccule. These hair-cells serve as end organs for 

 the vestibular branch of the auditory nerve. 



The auditory nerve. — The eighth or auditory nerve is a sensory 

 nerve and contains two distinct sets of fibres, which differ in their 

 function, origin, and destination. One set of fibres is known as 

 the cochlear division and the other as the vestibular. 



The fibres of the cochlear nerve arise from bipolar cells that are 

 situated in the modiolus of the cochlea. One axis cylinder from 

 each cell passes through the foramina of the modiolus or lamina, 

 and terminates in and around the cells that constitute the organ 

 of Gorti. The other axis cylinder passes through the intepial 

 auditory meatus to a portion of the brain, called the cochlear 

 root of the auditory nerve. This root is located at the lower edge 

 of the pons Varolii. The nerve-fibres which pass from the ear to 

 the pons or from the pons to the ear are not continuous strands, 

 as there are several relays of ganglia in which the axones of one 

 cell interlock with the dendrites of another cell. 



