ANATOMY IN A NUTSHELL. 577 



The eighth nerve (auditory) which is the portio mollis of the seventh nerve. 

 This nerve has no neurilemma. 



The organ of Corti is the terminal auditory apparatus in the membranous 

 cochlea. 



The internal auditory meatus is the opening into the internal oar from the 

 cranial cavity for the seventh ami eighth nerve and stylo-mastoid artery. 



The vestibule is an oval cavity at the entrance to the cochlea within the 

 internal car. Vertically it is about one-fifth of an inch, laterally about one- 

 tenth of an inch. It contains a fluid called perilymph and the utricle and 

 saccule, and membranous labyrinth. It is situated between the cochlea and 

 the semicircular canals internal to the tympanum. It has the following points 

 for consideration: 



1. The fenestra ovalis is on its outer or lateral wall and communicantes 

 with the tympanum. It is closed by the base of the stapes and its annular 

 ligament which is from the periosteal lining of the vestibule. 



2. The fovea hemispherica is a small circular depression at the inner 

 portion of the inner or median wall at the bottom of which are numerous mall 

 openings for the vestibular branch of the auditory nerve. 



- 3. The crista vestibuli is posterior to the fovea hemispherica. It is a 

 vertical crest. 



The fovea cochlearis is a small depression which is perforated for the 

 passage of the filaments of the auditory nerve. 



5. The aqueductus vestibuli is on the posterior portion of the inner wall. 

 It transmits a small vein and lodges the ductus endolymphaticus which con- 

 nects the membranous labyrinth with the general cerebral lymph spaces. 



6. The fovea he.mielliptica is an oval fossa on the roof. 



In the posterior portion of the vestibule are the five openings of the semi- 

 circular canals, while at the anterior portion of the vestibule is an opening 

 leading into the scala vestibuli of the cochlea. 



The semicircular canals are three C-shaped bony tubes aboul one-twentieth 

 of an inch in diameter which are situated above and behind the vestibule. 



1. The superior SEMICIRCULAR canal is nearly an inch long and lies in a 

 saggital plane of the body. 



2. The external semicircular canal is abut one-fifth of an inch long 

 and lies horizontally. 



3. The posterior semicircular canal is nearly an inch long and lie- in a 

 coronal plane. From this we can see that the three semicircular canal- are at 

 right angles to one another and each forms more than a semicircle. The ampulla 

 is about one-tenth of an inch in diameter. 



The cochlea is a bony tube about one and a half inch long situated anteriorly 

 to the vestibule. It resembles a snail shell and coils around a central axis two 

 and three-fourths times. It has the following points for consideration : 



1. The modiolus (central axis) around which is wound a spiral tube. 

 This axis has numerous canals in it for branches of the auditory nerve and 

 artery, the largest one of these canals is canalis centralis modioli. 



2. The base of the cochlea is two-fifths of an inch in diameter and i- 



