983. _Why do we hear?_
Because the _tympanum_ of the ear _receives impressions from sounds_,
and transmits those impressions to the brain in a similar manner to
that in which the retina of the eye transmits the impressions made upon
it by light.
984. _Why is one part of the ear spread out externally?_
The external ear is a _natural ear-trumpet_, and serves to collect the
vibrations of sound, and to conduct them towards the internal ear.
985. _Why is the ear allowed to project, whilst the eye is carefully
enclosed?_
Because the external ear, being formed of tough cartilaginous
substance, and being very simple in its organisation, is but little
liable to injury.
986. _Why do hairs grow across the entrance of the ears?_
Because they prevent the intrusion of insects, and of particles of
dust, by which otherwise the faculty of hearing would be impaired.
The insect called the _earwig_ is popularly supposed to be so
named from its tendency to get into the human ear, and cause pain
and madness by penetrating to the brain. An earwig, however, is no
more likely to get into the ear than any other insect whose habit
it is to penetrate the corollas of flowers; and should an insect
enter the ear, it could get no further than the _membrane of the
tympanum_, which spreads all over the auditory passage, just as
the parchment of a drum spreads over the entire circumference of
that instrument. The fact is, that the wing of the insect, when
spread, _resembles the external ear_ in shape. It is similar to
the wing of the stag beetle (_see_ illustration), and this fancied
resemblance of the wing of the insect to the ear of man may have
given rise to the name of _ear-wing_, which became corrupted to
_earwig_.
[Verse: "Doth not the ear try words? and the mouth taste his
meat."--JOB XII.]
987. _Why is wax secreted at the entrance of the ear?_
Because, by the peculiar resinous property which it possesses, _it
improves the sound-conducting power_ of the auditory canal through
which it prevails.
[Illustration: Fig. 65.--THE STRUCTURE OF THE EAR.]
A A. Glands which secrete wax in the walls of the tube of the ear.
B. The membrane of the _tympanum_, or drum of the ear, formed in
the shape of a funnel.
C C. Bones which act as a sort of sounding-board to the ear,
giving strength to the vibrations.
D. The Eustachian tube, which opens into the root of the mouth,
and which serves to preserve an equilibrium in the density of the
air occupying the tubes of the ear.
E and F. The _labrynth_ of the ear, consisting of folds of
membraneous tubes, filled with fluid, which serves to undulate
with the vibrations of the _tympanum_, and thus gives clearness
and precision to the sounds.
The _auditory nerves_ are distributed in the tubes above described
(the _vestibule_ and the _cochlea_ E F), and the nerves receive
their impressions from the undulations of the fluid.
988. _Why do we sometimes hear singing noises in the ear?_
Because the ear is liable to inflammation from various causes, and
when the blood flows unduly through the vessels of the ear it _produces
a slight sound_.
[Verse: "Apply thine heart unto instruction, and thine ears to the
words of knowledge."--PROVERBS XXIII.]
989. _Why do people become deaf?_
Because the ear may be injured in various ways: the tympanum may be
impaired, the fluid of the ear dried up, or the nerves be pressed upon
by swellings in the surrounding parts. When, therefore, the _mechanism
of hearing_ is impaired, the sense of hearing becomes weakened, or
altogether lost.
990. _Why do persons accustomed to loud noises feel no inconvenience
from them?_
Because the _sensitiveness_ of the nerves of the ear becomes deadened.
They do not convey to the brain such intense impulses when they are
frequently acted upon by loud sounds.
991. _Why do persons engaged in battle often lose their hearing?_
Because the vibrations caused by the sounds of artillery are so violent
that they overpower the mechanism of the ear, and frequently _rupture
the connection of the fine nervous filaments_ with the textures through
which they spread.
The violent concussions of the air produced by volleys of cannon,
or by loud peals of thunder, have an overpowering effect upon
persons nervously constituted, and upon the organ of hearing,
which is more especially affected. As persons have been struck
blind by intense light, so others have been deafened by intense
sounds. In 1697 a butcher's dog was killed by the noise of the
firing to celebrate the proclamation of peace. Two troops of
horse were dismounted, and drawn up in a line to fire volleys. At
the moment of the first volley a large and courageous mastiff,
belonging to a butcher, was lying asleep before the fire. At
the noise of the first volley the dog started up, and ran into
another room, where it hid itself behind a bed; on the firing of
the second volley, it ran several times bout the room, trembling
violently; and when the third volley was fired it ran around once
or twice with great violence, and then dropped down dead, with
blood flowing from its mouth and nose. Persons who are painfully
affected by loud noises should put a little wool in their ears
when such noises are occurring; they will thereby save themselves
from temporary inconvenience, and probably preserve the sense of
hearing from permanent injury.
992. _Why do we smell?_
Because minute particles of matter, diffused in the air, come in
contact with the filaments of the _olfactory nerve_, which are
spread out upon the walls of the nostrils, and those nerves transmit
impressions to the brain, constituting what we call the _odour of
substances_.
[Verse: "And the Lord God formed man of the dust of the ground, and
breathed into his nostrils the breath of life; and man became a living
soul."--GENESIS II.]
[Illustration: Fig. 66.--SHOWING THE DISTRIBUTION OF THE NERVOUS
FILAMENTS UPON SENSITIVE MEMBRANES.]
A. The _olfactory nerve_, distributed in minute branches upon the
membrane of the nostril.
B. The _bulb_ of the _olfactory_ nerve.
C. The _roots_ from which the olfactory nerve originates.
D E. _Nerves of the palate_, showing the manner in which they are
passed through the bones of the roof of the mouth.
993. _Why do hairs grow across the passages of the nostrils?_
Because they form a _defence_ against the admission of dust and
insects, which would otherwise frequently irritate the nervous
structure of the nose.
994. _Why are the nostrils directed downwards?_
Because, as odours and effluvia _ascend_, the nose is directed towards
them, and thereby receives the readiest intimation of those bodies
floating in the air which may be pleasurable to the sense, or offensive
to the smell, and injurious to life.
[Verse: "Can that which is unsavoury be eaten without salt? or is
there any taste in the white of an egg?"--JOB VI.]
995. _Why is the nose placed over and near the mouth?_
Because, as one of the chief duties of that organ is to _exercise a
watchfulness_ over the purity of the substances we eat and drink, it is
placed in that position which enables it to discharge that duty with
the greatest readiness.