152 ON THE ORGAN OF HEARING 
and reaches the membrana vestibule, which stretches across 
and divides the tube, and thence to the orifice of the mea- 
tus, situate on the coronal surface of the head, as formerly 
mentioned. In the young subject of the Squalus caninus 
this process is easily found, and will admit of a large bristle; 
but in the adult it is extremely small, sometimes scarcely 
discernible *. It is beautifully seen in the Thornback, and 
is of a trumpet-like form ; but the caliber is so small when 
it reaches the upper part of the vestibule, that it will not 
admit of a substance thicker than a human hair. The sac 
consists of two fine transparent membranous layers, between 
which are situate an almost innumerable multitude of ves- 
sels, glands, and nervous filaments. There are numerous 
short lines of a serpentine figure, which appear more bright 
than the other parts, and these are formed in a range of 
semitransparent points resembling minute glands ; so that 
I am persuaded these glands secrete the gelatinous humour 
in the sac. It likewise appears to be plentifully supplied 
with nervous filaments from the sabulous plexus. We must 
therefore suppose that the sac is possessed of great sensibi- 
lity, and consequently of muscularity similar to the iris, 
but in a less degree ; and that, when the liquor amplexus 
is agitated, the sac itself assists in communicating the vibra- 
tions to the sabulous humour and semicircular canals. 
In order to keep this great sac extended, there is a 
strong ligament that arises from the upper and anterior 
part of the body of the sac, directly above the process; 
which runs straight outwards, and is attached to the oppo- 
site and anterior extremity of the vestibule. By means of 
this ligament, and the reticulated cellular body, the supe- 
* Toward the latter end of this essay, I shall endeavour to shew the 
utility of this gradual contraction of the meatus externus. 
