468 THE THREE SEMICIRCULAR CANALS. 



From what has been stated it follows that there are two 

 cavities of the cochlea, each of which continues throughout its 

 whole extent. One begins at the vestibule ; the other at the 

 tympanum. But a membrane extended over the foramen 

 rotundum separates this last from the tympanum. They com- 

 municate with each other at their terminations. From their 

 origins they are denominated Scala Vestibuli and Scala 

 Tympani. 



As both the cochlea and the vestibule are filled with a fluid, 

 it is evident that a vibration produced on the membrane of the 

 foramen rotundum may be communicated through the two 

 scala3 to the vestibule. 



The three Semicircular Canals 



Are placed obliquely behind the vestibule. Their position 

 is such that one is Superior, another Posterior, and the third 

 Exterior. The superior and posterior are so placed that one 

 extremity of each may be considered as internal and the other 

 external. They unite at their internal extremities, which 

 therefore form but one orifice in the vestibule. Their other 

 extremities, being separated to a considerable distance, form 

 each one orifice ; while the external canal, which is smaller 

 than the others, opens by two orifices. 



Each of these canals is nearly of the same diameter, viz. 

 rather more than two lines. 



At one of their extremities, each one of them has an enlarge- 

 ment, which is called Ampulla ; and there is no other variation 

 of their diameters. 



The cavity of the labyrinth, thus complicated, is perforated 

 by many small foramina, through which various nerves are 

 transmitted. 



These foramina communicate with the large canal on the 

 posterior side of the petrous bone, called Meatus Auditorius 

 Internus ; which continues very near to the basis of the cochlea, 

 and transmits the seventh pair of nerves. 



The bottom of this cavity is divided by a ridge into two 

 unequal fossa? ; the uppermost of which is the least. 



