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ON THE ORGAN OF HEARING 
into a round cavity, which forms the ampulla. It then con- 
tracts to the size of the diameter of the posterior tube, and 
runs backwards with a gentle curve, so as to describe near- 
ly a semicircle, and terminates in the inner, anterior, and 
superior part of the great sac. 
The external canal arises from the external and rather 
superior part of the spherical sac, and passing horizontally 
outwards, and then inwards, with a circular sweep, com- 
pletes the semicircle, and opens into the great sac near to, 
but rather above, the entrance of the posterior canal. 
The ampullae of the horizontal and anterior canals are 
considerably above the level of the posterior ampulla (in 
its natural position), owing to the meatus internus, and 
also the spherical sac, being placed below them. The 
walls of the membranous canals are strong, elastic, and 
very thick, when compared with their caliber ; but do not 
occupy one-third of the caliber of the cartilaginous canals. 
There is little perceptible difference between the membra- 
nous canals of the young and those of the adult, except in 
size— and that is chiefly observable in length, the diameter 
of the canals increasing very little in proportion to the age 
of the animal. The membranous tubes of the Skate do 
not fill above one-fifth of the caliber of the canals. The 
intermediate space in both is partially filled with transpa- 
rent reticulated membranous substances, resembling some- 
what thecordae tendineae in miniature, but extremely slender, 
which connect and retain the membranous tubes at an 
equal distance from the sides of the cartilaginous canals, 
support the vessels of nutrition, and convey them to and 
from the tubes, while, at the same time, the elasticity of the 
fibres admits of considerable vibratory action in the canals. 
The membranous canals are very vascular, but in the 
healthy state do not admit any of the red globules ; and 
in the aged become nearly cartilaginous, so that a great 
