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CHAPTER III. 
Plate V.— the human ear, and tympanum of the elephant. 
Fig. 1. The organ of hearing; a, the external ear ; b , the meatus 
cmditorius extemus , or outward passage of the ear; leading to c, the 
membrana tympani, or drum; d, the ossicida auditus, or little bones 
of the ear; e, the semicircular canals ; f the cochlea ; g, a section of 
the eustachian tube , which extends from the cavity of the tympa¬ 
num, to the back of the mouth or fauces. 
Fig. 2. The bones of the ear magnified, a , the malleus, or mal¬ 
let, connected by a process to the tympanum; the round head is 
lodged in the body of, b , the incus , or anvil, and the incus is united 
to, c, the os orbiculare, or round bone, and this to, d, the stapes , or the 
stirrup. These bones are named from their shape, and the names 
assist in conveying an idea of their form. They are united by lig¬ 
aments, and form an uninterrupted chain to transmit the vibrations 
of the atmosphere. 
Fig. 3. The labyrinth , so named from the intricacy of its cavi 
ties; it is situated in the petrous part of the temporal bone, and con¬ 
sists of the vestibule , or central cavity , three semicircular canals, and 
cochlea , so named from its resemblance to the windings of a snail 
shell, and is best explained by the plate, Fig. 1, and 3. 
The vibrations of sounds, striking against the membrana tympa¬ 
ni, are propagated by the intervention of these four little bones, to 
the water contained within the cavities of the labyrinth; and by 
means of this water the impression is conveyed to the extremities 
of the auditory nerve , and finally to the brain. 
Fish require no tympanum, nor external opening to the ear; the 
fluid in which they live is the medium for conducting sounds 
through the bones of the head. 
Fig. 4. The tympanum of the elephant, of its natural size, show¬ 
ing its radiated fibres, supposed to be muscular. 
