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 membrane 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 hearing 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 muscular 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