73 
Organs of Hearing and Vision. 
ditorius externus is situated on the superior and posterior, or co- 
ronal, surface of the head, is small and much contracted, and sel- 
dom admits of any substance larger than a strong bristle. 
The tube is formed of tough, elastic, membranous substance, 
and runs outwards or laterad, forwards or antinead, and down- 
wards or basilad, a few lines more or less, according to the size 
of the animal, until it reaches within a short space of a mem- 
brane, stretched across the tube, where it enlarges to nearly twice 
the size of the other parts of the tube. 
This membrane may with propriety be termed the Merribrana 
Vestibuli. 
The tube then describes an angle by running downwards, 
backwards, and a little outwards, until it reaches the foramen 
oblongatum, to the edge of which it is attached in some subjects 
more closely than in others. 
In young fish of the $. canus , the meatus and auditory tube 
are more easily found than in the adult, or in some of the other 
species. In old fish, the meatus is generally almost obliterated ; the 
tube and membrana vestibuli are seen, but indurated and en- 
larged so as to be scarcely recognisable by the above description. 
This alteration of the parts may perhaps be the consequence 
of disease, or the effect of old age. In the ear of the adult of 
the Balcena Mysticetus, I have frequently found the stapes so 
firmly attached to the foramen ovale, that the union resembled 
ossification, and required considerable efforts to separate the bone 
from the foramen. The probable cause of this adhesion will be 
pointed out when describing the ear of that animal, so that one 
fact may, if possible, throw light on another. 
The following are the dimensions of the parts in a preparation 
of the S. canus , three feet in length. 
Distance between the orifices of the Meatus auditorius externus, 
Diameter of the tube at the external orifice, 
Do. do. inside of the cuticle, 
Do. do. Membrana Vestibuli, 
Length of the tube from the orifice to the Membrana Vestibuli, 
From that membrane to the Vestibule, - 
Depression in the Cranium, in length about 
21 lines 
25 of a line. 
i do. 
1 line. 
1| do, 
lido. 
6 do. 
In the species S. borealis , or Greenland shark, the magnitude 
of the semicircular canals is such as to surpass any idea which 
