INTERNAL EAR OR LABYRINTH. 373 



which open by foramina, corresponding to similar canals 

 on the lamina spiralis, for transmitting the auditory ves- 

 sels and nerves. One of these canals, larger than the rest, 

 the tubulus centralis modioli, passes on to the infundibulum, 

 and conducts the terminal branch of the cochlear nerve, 

 and the arteria centralis modioli. 



The aqueduct of the cochlea extends from the scala of the 

 tympanum, into which it opens, near the fenestra rotunda, 

 to the lower surface of the petrous bone near the jugular 

 fossa like the aqueduct of the vestibule, it simply carries 

 a vein, and is closed by the dura mater. 



Semicircular Canals, (Fig. 108.) These consist of three 

 bony tubes, representing so many cylinders of equal diam- 

 eters, and forming about three-fourths of a circle. They 

 are situated within the substance of the petrous bone be- 

 hind the vestibule, into which they open by five distinct 

 orifices. Two of these canals are perpendicular, and the 

 third horizontal. They are the anterior and superior ver- 

 tical, posterior and inferior vertical, and the horizontal. 



The superior vertical makes a projection on the upper 

 surface of the petrous bone crosses it transversely, its 

 outer extremity opening, by an ampulla, into the superior 

 cornu of the vestibule while its inner unites with the 

 upper crus of the posterior vertical, forming a common 

 tube, which enters,, by a single orifice, into the posterior 

 cornu of the vestibule. 



The posterior vertical canal is at right angles to the su- 

 perior, and parallel to the posterior surface of the petrous 

 bone. It opens, by one extremity, into the posterior cornu ; 

 and by the other it forms, with one end of the superior, a 

 common canal, opening also into the same cornu of the 

 vestibule. 



The horizontal is the shortest of the canals, and enters, 

 by an ampulla at one end, into the superior cornu ; and at 

 the other end, into the posterior cornu of the vestibule. 

 The semicircular canals are thus seen to have, each at one 

 )f their extremities, a dilatation or ampulla one at the 

 uter end of the superior vertical, one at the inferior 



