CHAP. XVIII.] THE SEMI-CIRCULAR CANALS. 



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entrance of a portion of the auditory nerve from the internal audi- 

 tory meatus. At the hinder part of the inner wall is the orifice of 

 the aqueductus vestibuli, a fine canal penetrating the vestibule from 

 the posterior surface of the petrous bone, and containing, as some 

 describe, a tubular prolongation of the lining membrane of the ves- 

 tibule, ending in a minute pouch between two layers of the dura 



Fig. 135. 



Interior of the osseous labyrinth. V. Vestibule, a v. Aqueduct of the vestibule, o. Fovea 

 semi-elliptica. r. Fovea hemispherica. S. Semi-circular canals, s. Superior, p. Posterior. 



1. Inferior, a a a. The ampullar extremity of each. C. Cochlea, ac. Aqueduct of the cochlea. 

 so. Osseous zone of the lamina spiralis, above which is the scala vestibuli, communicating with 

 the vestibule, st. Scala tympani below the spiral lamina. From Soemmerring. 



mater, within the cranial cavity. Breschet considers this to be an 

 evidence of a continuity once existing between the lining membrane 

 of the cranium, and that of the vestibule, and it is certain that in 

 most fishes the vestibule is a process of the cranial cavity, or sepa- 

 rated from it only by a membraniform septum. Whatever other use 

 the aqueduct of the vestibule may serve, it seems, certainly, to 

 convey small vessels to the internal ear. The lower part of the 

 inner wall presents a hemispherical depression (fovea hemispherica), 

 and immediately above it, and on the upper wall, another, trans- 

 versely oval and larger (fovea semi-elliptica). These are separated 

 by a small pyramided eminence. 



2. The semi-circular canals are three in number, all opening at 

 both ends into the vestibule, so that there would be six orifices, 

 were not one of the orifices common to two of the canals. The 

 canals are of unequal length, but all describe more than half a circle, 



