608 ANIMAL PHYSIOLOGY. 
the stapes has by no means the range of movement of the han- 
dle of the malleus; in other words, there is loss in amplitude, 
but gain in intensity. A glance at Fig. 448 will show that the 
end attained by this arrangement of membrane and bony levers, 
which may be virtually reduced to one (as it is in the frog, etc.), 
is the transmission of the vibrations to the membrane of the 
fenestra ovalis, through the stapes finally, and so to the fluids 
within the internal ear. But it might be supposed that, for the 
Fie. 448.—Diagrammatic representation illustrating auditory processes (after Beaunis). A, 
external ear ; B, middle ear; C, internal ear ; 1, auricle ; 2, external auditory meatus; 3, 
tympanum ; 4, membrana bh orig 5, Eustachian tube ; 6, mastoid cells; 10, foramen 
rotundum ; 11, foramen ovale; 12, vestibule; 18, cochlea; 14, scala tympani; 15, scala 
vestibuli ; 16, semicircular canals. . F 
N. B.—The ear is so complicated an organ that it is almost impossible to give a diagram- 
matic representation of it at once simple and complete in a single figure. A agers 
of the whole series of cuts is therefore desirable. It is essential to understand how the 
end-organ within the scala media is stimulated. 
avoidance of shocks and the better adaptation of the apparatus 
to its work, some regulative apparatus, in the form of a nerv- 
ous and muscular mechanism, would have been evolved in the 
higher groups of animals. Such is found in the tensor tym- 
pani, laxator tympani, and stapedius muscles, as well as the 
Eustachian tube. 
Muscles of the Middle Ear.—The tensor tympani regulates the 
degree of tension of the drum-head, and hence its amplitude of 
vibration, having a damping effect, and thus preventing the ill 
results of very loud sounds. 
Ordinarily this is, doubtless, a reflex act, in which the fifth 
