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Mr. Home on the Organ of Hearing 
the animal is under water; but the degree of pressure to 
which it is liable, is incompatible with the nicer vibrations 
required to impress the internal organ so as to convey to it 
distinct sounds, and it is for this last purpose that the mem- 
brane is stretched across the cavity of the tympanum. 
This membrane, from being connected with the concave 
bone, will have its vibrations increased, and the bone being 
imbedded in a fatty covering, none of the vibratory motion 
impressed upon it can be carried off from the opposite side, 
but the whole will be communicated to the malleus, and so 
on to the cochlea and semi-circular canals. 
EXPLANATION OF THE PLATES. 
Plate I. 
, An external view of the parts in which the organ of hear- 
ing is contained. 
aa. The external surface of the large concave bone which 
forms the cavity of the tympanum. 
b. The petrous portion of bone, in which the cochlea and 
semi-circular canal are contained. 
c. A bony process connecting the petrous portion with the 
skull. 
dd. The meatus auditorius externus. 
e. The surface of the membrana tympani to show its mus- 
cular fibres, the external covering having been removed. 
f. A portion of the malleus, one of the small bones of the 
ear. 
g. The anterior portion of the cavity of the tympanum 
