148 
ON THE ORGAN OF HEARING 
and still less so in the cartilaginous semicircular canals-, 
where it is almost pellucid in the adult fish-, and beauti- 
fully transparent in the young. 
There are three semicircular canals which arise from 
and communicate with the vestibule, similar to those of 
the human subject ; and from their relative situations to 
that cavity, may be termed the Posterior, Anterior, and 
External or Horizontal. The extremities of these canals, 
adjoining to the vestibule, are considerably enlarged, so 
as to have a similar appearance to that in the human 
subject termed the Ampullae, whereas the proper am- 
pullae of these cartilaginous canals are situate at a consider- 
able distance from the vestibule, and are of an oblong 
figure. The posterior canal is the largest, and measures 
Jjfty-three lines, or nearly five inches and a half in length, 
and two lines by two and three quarters in diameter. It 
runs in a longitudinal direction from and to the vestibule, 
and, when viewed internally, has the appearance of a circle 
attached to the ampulla of the external canal. The circum- 
ference of the ampulla is sixteen lines, and the average cir- 
cumference of the other parts of the tube eight lines and 
three quarters. The external canal is the next in size, and 
measures thirty-six lines in length, and seven lines in cir- 
cumference. The ampulla of this canal, at its greatest 
circumference, measures only ten lines and a half. The 
canal runs in a diagonal direction, the one extremity arising 
from the superior and posterior parts of the vestibule, and 
the other from a globular space communicating with the 
inferior and anterior parts of that cavity. The anterior 
canal, although the smallest of the three, is yet of consider- 
able size, measuring thirty-one lines in length, and from 
five to seven lines in circumference. 
These canals are partially flattened, so that if any of 
them were divided at a little distance from the ampulla. 
