154 ON THE OEGAN OF HEARING 
affect the canals, the vibrations are instantly communicated 
to the sac. 
The spherical sac, in these animals, is connected with the 
great sac by a short membranous tube, similar to that 
which we find in the shark. In the inside of the spherical 
sac of the shark, as well as in that of the other cartilagi- 
nous fishes, the nerves terminate in a remarkable manner, 
which will be described in the account of the distribution 
of the auditory nerve. The structure of this sac is similar 
to that of the great sac, but more strong and elastic ; and 
it may be remarked, that nearly the whole of the cavity in 
which it is lodged is occupied, particularly the inferior 
parts, to which, as formerly mentioned, it is firmly at- 
tached. 
There are three membranous semicircular canals which 
terminate in the great sac, and occupy the centre of the 
cartilaginous canals. These may be termed, from their 
situation^ the posterior, anterior, and Iwrizontal or ea:ter^ 
nat 
The posterior membranous canal forms a circle, and has 
but one passage into the great sac, which is situate on ra- 
ther the posterior side of the canal and upper part of the 
sac, and nearly in a line, sometimes rather above the upper 
edge of the sabulous body. The anterior side of the tube 
is in contact with, and firmly adhering to the posterior part 
of the end of the anterior canal. In the Thornback there 
is a small funnel-shaped tube, of about four or five lines in 
length, which leads from the posterior canal as it begins to 
descend, opens into the great sac, and forms the common 
aperture. 
The common opening of the posterior canal of the Shark 
is formed so as to resemble a tube of communication, but 
the length seldom exceeds half a line. The superior part 
of the canal is firmly attached to the roof of the vestibule, 
3 
