74 Mr Buchanan on the Comparative Anatomy of the 
may be formed of the parts, from the dissection of the organ in 
the human subject. The superior size of the parts may be esti- 
mated from the dimensions of a cast of the left ear of that ani- 
mal now before me, which I took a few hours after it was killed *. 
The entrance of the meatus internus is about three lines in 
diameter, and situated in the inferior part of the organ. 
It runs downwards, and a little outwards, about two lines 
and a half ; then contracts suddenly, and runs horizontally out- 
wards nearly a line, when it enters the vestibule, parallel with 
the floor of that cavity. 
The vestibule is large, of an irregular triangular figure, the 
perpendicular of which may be said to present to the brain, 
while the base runs horizontally outwards and backwards. 
The circumference of the greatest diameter of the vestibule 
is twenty-five lines, and the height of the cavity, from the high- 
est to the lowest points, twenty-two lines. 
On the inside of the superior part of the external angle of the 
vestibule is a longitudinal ridge, which, in the sketch of the 
cast, is seen as a depression ; and on the floor, there is a corres- 
pondent ridge on the opposite side, that runs upwards on the 
parietes, next to the brain, until it arrives at the top of the ca- 
vity, where it unites and forms a septum, which separates the 
foramen oblongatum from the foramen rotundum. 
The floor of the vestibule is more tough and hardened than 
any other part of the labyrinth, particularly that which is di- 
rectly under the sabulous body, when it has a white, scaly, 
opaque appearance, approaching towards ossification. 
The whole of the vestibule is lined with a reflection of the 
dura mater, which is closely attached to the parietes of that ca- 
vity, where it is considerably less dense than in the inside of the 
cranium, and still less in the cartilaginous, semicircular canals, 
where it is almost pellucid in the adult fish, and beautifully 
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. 
See Plate V, Figs. 1. and 2. where the parts are shewn the natural size. 
