SEMICIRCULAR CANALS COCHLEA. 463 



(which is continuous internally with the lining membrane of the vestibule, 

 and externally with the dura mater), and to a small vein. 



The Openings for arteries and nerves are situated in the internal wall of 

 the vestibule, and correspond with the termination of the meatus audito- 

 rius internus. 



The SEMICIRCULAR CANALS are three bony passages communicating 

 with the vestibule, into which they open by both extremities. Near one 

 extremity of each of the canals is a remarkable dilatation of its cavity, 

 which is called the ampulla (sinus ampullaceus). The superior , or per- 

 pendicular canal (canalis semicircularis verticalis superior), is directed 

 transversely across the petrous bone, forming a projection on the anterior 

 face of the latter. It commences, by means of an ampulla, in the supe- 

 rior ventricle of the vestibule, and terminates posteriorly by joining with 

 the oblique, and forming a common canal, which opens into the upper 

 part of the posterior ventricle. The middle or oblique canal (canalis semi- 

 circularis verticalis posterior) corresponds with the posterior part of the 

 petrous portion of the temporal bone : it commences by an ampullary di- 

 latation in the posterior ventricle, and curves nearly perpendicularly up- 

 wards to terminate in the common canal. In the ampulla of this canal 

 are numerous minute openings for nervous filaments. The inferior or 

 horizontal canal (canalis semicircularis horizontalis) is directed outwards 

 towards the base of the petrous bone, and is shorter than the two preced- 

 ing. It commences by. an ampullary dilatation in the superior ventricle, 

 and terminates in the posterior ventricle. 



The COCHLEA (snail- shell) forms the anterior portion of the labyrinth, 

 corresponding by its apex with the anterior wall of the petrous bone, and 

 by its base with the anterior depression at the bottom of the cul de sac of 

 the meatus auditorius internus. It consists of an osseous and gradually 

 tapering canal, about one inch and a half in length, which makes two 

 turns and a half spirally around a central axis, called the modiolus. 



The central axis, or modiolus, is large near its base, where it corre- 

 sponds with the first turn of the cochlea, and diminishes in diameter to- 

 wards its extremity. At its base, it is pierced by numerous minute open- 

 ings, which transmit the filaments of the cochlear nerve. These openings 

 are disposed in a spiral manner : hence they have received, from Cotun- 

 nius,* the name of tractus spiralis foraminulentus. The modiolus is every- 

 where traversed, in the direction of its length, by minute canals, w^hich 

 proceed from the tractus spiralis foraminulentus, and terminate upon the 

 sides of the modiolus, by opening into the canal of the cochlea or upon 

 the surface of its lamina spiralis. The central canal of the tractus spiralis 

 foraminulentus is larger than the rest, and is named the tubulus centralis 

 modioli ; it is continued onwards to the extremity of the modiolus, and 

 transmits a nerve and small artery (arteria centralis modioli). 



The interior of the canal of the cochlea is partially divided into two 

 passages (scalae) by means of a thin and porous lamina of bone (zonula 

 ossea lamina; spiralis), which is wound spirally around the modiolus in 

 the direction of the canal. This bony septum extends for about two 

 thirds across the diameter of the canal, and in the fresh subject, is pro 



* Dominico Cotunnius, an Italian physician; his dissertation " De Aquaednctibtu 

 Auris Humance Internse" was published at Naples in 1761. 



