470 COCHLEA. 



They have an ampulla at one extremity, as they arise from the 

 sac above mentioned, and are cylindrical during the remainder 

 of their course ; they are transparent, and have smaller diame" 

 ters than the bony canals, although they are rather longer. 



The Auditory Nerve. 



Upon this structure, viz. the Sacs in the Vestibule, the Mem- 

 branous Tubes in the Semicircular Canals, and the Lamina 

 Spiralis of the Cochlea, are expanded the fibres of the Audi- 

 tory Nerve. 



This nerve, with the Portio Dura, and its appendage, the 

 Portio Media, composes the seventh pair of nerves of the 

 brain. 



It is called Portio Mollis, and is very distinct from the Portio 

 Dura, although they pass together along the Meatus Audito- 

 rius Internus. 



Corresponding to the foramina and the cribriform structure 

 of the bottom of the meatus auditorius internus, the Auditory 

 Nerve passes into the labyrinth in branches, or fibrillae, of 

 various sizes. One portion of them enters the vestibule, and 

 has been traced upon the alveus or utriculus, and its internal 

 surface; and also upon the semicircular membranous tubes. 

 Another portion seems exclusively appropriated to a part of 

 these tubes. And a third is spent upon the sacculus sphericus. 



These nervous fibres seem to terminate in a pulpy expansion 

 on the internal surface of the aforesaid sacs and canals, in a 

 way which has some analogy with the termination of the 

 optic nerve. 



A large bundle of these fibrillae enters the cochlea at its 

 base ; and the largest of them passes through a foramen, 

 mentioned before, along the centre of the modiolus to the 

 infundibulum. 



These fibrillae divide most minutely, and passing between 

 the plates of the Lamina Spiralis, as well as the other parts 

 of the bony structure of the cochlea, at length form a plexus, 

 which has the appearance of a pulpy membrane, that is 



