&6 Mr. Home on the Organ of Hearing 
other animals, and also shewn that Hunter, unwilling to 
believe that there could be so great a deviation from the 
ordinary construction of this organ, was led into an error, 
which I can only attribute to his having formed to himself too 
strong a chain of analogies, I shall proceed in my description 
of the organ. 
Immediately behind the membrana tympani is a large cavity 
formed principally by the concave surface of a large hard bone 
peculiar to the whale, in the substance of which there is more 
earthy matter than in almost any other bone met with in ani- 
mal bodies. In its form it is not very unlike the shell called 
the concha Veneris, to which it has been compared. 
The cavity of the tympanum is of an oval shape, one end 
of which is bounded by the membrana tympani, the other 
forms the entrance of the Eustachian tube, and there the 
cavity is surrounded wholly by membrane inclosed in the 
substance of the skull. The large concave bone is only 
slightly connected with the petrous portion of the temporal 
bone, and is imbedded in a fatty substance of nearly an inch 
in thickness, with a smooth external surface. 
The Eustachian tube is two inches and a half long, it opens 
externally into the canal leading to the blow-hole ; its internal 
surface is honey-combed, which gives it a glandular appear- 
ance, and there are chords and septa crossing from side to side 
in different places : where it opens into the cavity, it has a val- 
vular structure. The cavity, as it corresponds in its principal 
uses with the tympanum of other animals, although it does 
not, as in them, contain the ossicula auditus, deserves to be 
called by the same name ; it is equal in size to a pint measure, 
and can only be filled from the Eustachian tube, there being 
