THE EAR AND HEARING. 
215 
which had occluded the external canal, may often count for 
more than the lavement of the membrane), or by removing 
the obstruction caused at the pharyngeal end of the eusta- 
chian tube by the congestion of the surrounding tissue, the 
accumulation of mucus, &c., and thereby facilitating the 
vibration of the membrane. 
Against this statement that the tympanum is not really a 
drum Dr. Edward Clarke (quoted by Brown Sequard, “ Journal 
de la Pliys.,” i., 644) adduces his observations of seventy-five 
cases of perforated membranes, with the following results :— 
Tick of watch not beard in. 5 cases') Normal limit 
Only heard when close to ear .. .. 11 ,, [ of hearing 
Only within distance of 6 to 12 inches, 16 ,, j about 14 ft. 
Only within 4 feet.43 ,, J (Clarke.) 
There is, however, a source of fallacy in these valuable 
observations—It seems to have been forgotten that only one 
tympanum was perforated in his subjects, and therefore, if 
there were no other aural defects, they ought to have been 
able to recognise the tick of a watch with the sound ear any¬ 
where within 14ft. If they could not, their defective hearing 
might reasonably be attributed to causes other than a per¬ 
forated tympanum. [The watch used in the experiment 
must have been one of the portly forms worn by our great 
grandfathers.] 
The modus overcindi of hearing may be summarised some¬ 
what as follows. The aerial waves proceeding from the 
vibrating body are collected by the pinna of the ear, focussed 
by the concha, and conducted into the external meatus, where 
they strike upon the tympanic membrane, producing 
vibrations in it. The vibration of this membrane causes the 
process of the malleus attached to it to swing at the same 
rate ; the head of the malleus consequently turns through 
a small arc on its pivot (the processus gracilis). But the 
turning of the head of the malleus involves that of the head 
of the incus upon its pivot (the short process). Hence the 
long process of the incus swings through an arc estimated as 
about two-thirds that described by the handle of the malleus, 
the decrease in the extent of the push being compensated for 
by a proportional increase in force, which is an advantageous 
change in so confined a space as the tympanum. The long 
process of the incus being articulated with the stapes, 
involves this bone in its movements, pulling it out of and 
pushing it into the fenestra ovalis with equal rate and force. 
But every pull and push of the stapes imparts a correspond¬ 
ing set of shakes to the perilymph, which fills the bony 
labyrinth and cochlea external to the membranous labyrinth 
