HEARING. 609 
is usually the afferent nerve concerned. It is well-known that, 
when we are aware that an explosion is about to take place, we 
are not as much affected by it, which would seem to argue a 
voluntary power of accommodation ; but of this we must speak 
with caution. ; 
According to some authorities the laxator tympani is not a 
muscle, but a supporting ligament for the malleus. The stape- 
dius, however, has the important function of regulating the 
movements of the stapes, so that it shall not be too violently 
driven against the membrane covering the fenestra ovalis. 
The two muscles, stapedius and tensor, suggest the accom- 
modative mechanism of the iris. The motor nerve of the sta- 
pedius is derived from the facial; of the tensor, from the tri- 
geminus through the otic ganglion. 
The Eustachian Tube.—Manifestly, if the middle ear were 
closed permanently, its air would gradually be absorbed. The 
drum-head would be thrust in by outward pressure, and become 
useless for its vibrating function. The Eustachian tube, by 
communicating with the throat, keeps the external and internal 
pressure of the middle ear balanced. Whether this canal. is 
permanently open, or only during swallowing, is as yet unde- 
termined. 
One may satisfy himself that the middle ear and pharynx 
communicate, by closing the nostrils and then distending. the 
upper air-passages by a forced expiratory. effort, when a sense 
of distention within the ears is experienced, owing to the rise 
of atmospheric pressure in the tympanum. 
Pathological—Inflammation of the tympanum may result in 
adhesions of the small bones to other parts or to each other, or 
to occlusion of the Eustachian tube from excess of secretion, 
cicatrices, etc., in consequence of which the relations of atmos- 
pheric pressure become altered, the membrani tympani being 
indrawn, and the whole series of conditions on which the nor- 
mal transmission of vibrations, depends disturbed, with the 
natural result, partial deafness. The hardness of hearing ex- 
perienced during a severe cold in the head (catarrh, etc.) is 
owing in great part to the occlusion of the Eustachian tube, 
which may be either partial or complete. 
By filling one or both of the ears external to the mem- 
brana tympani with cotton-wool, one may satisfy himself how 
essential for hearing is the vibratory mechanism, which is, of 
course, under such circunistances inactive or nearly so; hence 
the deafness. 
39 
