THE ORGANS OF SPECIAL SENSE.
=The Nose.=--The nose, the organ of the sense of smell, is composed
of a framework of bones and cartilages, the bridge being formed by
the two nasal bones, and the septum by the vomer and the triangular
cartilage. It consists of two parts, the _external nose_ and the
internal or _nasal fossæ_, which open to the face by the anterior nares
or nostrils and into the pharynx by the posterior nares. Externally it
is covered with skin, internally with ciliated mucous membrane. The
_fossæ_ have the inferior turbinated bones along their outer walls and
are divided into three parts known as the _superior_, the _middle_, and
the _inferior meatus_, the middle one connecting with the antrum of
Highmore, while into the inferior meatus the lachrymal canal empties.
There are many small muscles of which little use is made, although in
forced respiration, as in pneumonia, where every aid to breathing is
called into play, even the alæ nasi or nostrils are made to exert what
muscular power they possess in order to supply more air.
[Illustration: FIG. 18.--The nasal cavity. (After Sobotta.)]
Not only is most of the air breathed in through the nose and warmed in
its passage through, but the nose is the organ of smell and by means
of the peculiar property of its nerves protects the lungs against
deleterious gases and helps the taste discriminate. The _olfactory_
or _first cranial nerves_, after emerging from the brain, lie on the
under surface of the frontal lobe and rest on the ethmoid bone in what
is known as the _olfactory tract_. Each nerve ends in a bulb-like
termination called an _olfactory bulb_, which rests on the cribriform
plate and sends little terminal fibers down through to be distributed
to the nasal cavities, especially to the upper half of the septum of
the nose, the roof of the nose, and the anterior and middle turbinated
bones. For in the mucous membrane of the upper nasal cavity are
specially modified epithelial cells called _olfactory cells_, which
play an important part in the conduction of smell. Hence when one
wishes to smell anything especially well he sniffs it up.
Probably the _sensation of smell_ is caused by odoriferous particles
in the atmosphere being breathed into the nose, where they affect the
olfactory cells, which transmit the impulses to the olfactory nerve
and so to the brain. Whereas a certain amount of moisture in the nasal
cavity seems to be essential for accuracy of smell, the presence of
too much or too little interferes with it. The mucous membrane has a
certain power also of distinguishing different smells at the same time,
though this power varies greatly in different people, one smell often
wholly overpowering all others.
The cartilage below the bridge of the nose is sometimes attacked
in _syphilis_ and _cancer_, and _lupus_ often begins on the nose.
_Deviation of the septum_ may occlude all air from one side of the
nose, an effect also produced by _polypi_, generally of the turbinated
bone. Either condition is easily remedied. _Nosebleed_, though
generally unimportant, may be serious in adults.
=The Mouth.=--The mouth is of great importance as an entrance for fresh
air to the lungs when the nasal passages are for any reason impeded
and as the resonant chamber from which proceeds the voice, man’s
chief means of communication with his fellows. Its chief value may be
said, however, to reside in the fact that it is the vestibule of the
alimentary canal. It is an ovoid cavity lined with mucous membrane and
is bounded in front by the _lips_, at the sides by the _cheeks_, below
by the floor and tongue, and above by the _hard palate_ anteriorly and
by the _soft palate_ posteriorly, the _uvula_ depending from the latter
like a curtain between the mouth and the pharynx. Shape is given to the
mouth by the bones of the upper and lower jaw and its size is altered
by the lowering and raising of the latter, which is quite freely
movable.
[Illustration: FIG. 19.--The hyoid bone. (Toldt.)]
At the back of the mouth, at the entrance to the pharynx, are the
_anterior_ and _posterior pillars_ of the fauces, which contain
muscular tissue, and between which on either side are thick masses
of lymphoid tissue, the _tonsils_. The floor of the mouth is formed
largely by the _tongue_, which completely fills the space within the
lower teeth. Its base or root is directed backward and downward and is
attached by muscles to the hyoid bone and the lower jaw, the _hyoid
bone_ being a horseshoe-shaped bone lying just below and as it were
within the inferior maxillary. The base of the tongue is attached also
to the epiglottis and at the sides to the soft palate by the anterior
pillars. Except at its base and the posterior part of its under surface
the tongue is free, but a fold of mucous membrane, the _frenum_, holds
it somewhat in front. Thus it possesses great versatility of motion and
serves as an auxiliary in articulation, mastication, and deglutition.
=The Teeth.=--Securely embedded in either jaw are the teeth, nature’s
instrument for the first preparation of the food for digestion through
tearing and grinding. The _incisors_, which are in front, have wide
sharp edges for cutting the food. Next come the _canine_ teeth with a
sharp point for tearing it, while at the back are the _molars_ with a
broad flat top for grinding.
There are two sets of teeth: 1. the _temporary_ or _milk teeth_,
twenty in number--four incisors, two canines, and four molars in
each jaw--which appear at from six months to two years, and 2. the
_permanent teeth_, thirty-two in number--four incisors, two canines,
known as _eye teeth_ in the upper jaw and as _stomach teeth_ in the
lower jaw, four bicuspids, so called because they have two cusps where
the molars have four or five, and six molars in each jaw--which come
from the sixth to the twenty-first years. The first to appear are the
two lower middle incisors, which come at the age of six months. The
last to appear are the _wisdom teeth_, the farthest back of the molars,
which come at the age of twenty-one years or thereabouts.
Each tooth consists of a crown or body above the gum, a neck, and a
fang or root within the gum. The body is of dentine or ivory with a
thin crust of enamel and contains the _pulp_, a vascular connective
tissue containing many nerves. Beginning at the neck and covering the
fang is a layer of cement or true bone.
=The Sense of Taste.=--The sense of taste lies chiefly in the _taste
buds_ as they are called which are filled with _gustatory cells_
and are found in the papillæ of the tongue, principally in the
_circumvallate papillæ_ at the back of the tongue, which are few in
number and arranged in a V-shape. There is also a certain power of
taste in the tip and sides of the tongue but little in the upper
surface or dorsum. Only five special tastes can be distinguished:
bitter, sweet, acid, sour, and salt, but sometimes more than one
can be distinguished at a time, as bitter and sweet. Every one can
distinguish between different tastes but the power varies in different
people and with different conditions. Certain tastes seem to be better
distinguished in certain places, as sweet at the tip and bitter at the
back of the tongue. Moreover, the sense of taste is very dependent
upon the sense of smell, especially in the case of aromatic and savory
substances, which one really does not taste but smell. If one held his
nose and closed his eyes he would not know from the taste whether he
was eating onion or apple. This leads to the habit of pinching the nose
when taking nauseous medicines.
To be tasted a substance must be in solution. Friction against the
tongue, lips or cheek increase the sense of taste. A temperature of
100° Fahrenheit favors taste, while both great heat and great cold
impair it.
There are probably at least two nerves of taste, the lingual branch
of the trifacial or fifth cranial and the gustatory branch of the
glosso-pharyngeal.
Along with the sense of taste there are other senses in the mouth which
play an important part, such as _pressure_ and the _sense of heat and
cold_, and it is often hard to distinguish them from the pure sensation
of taste, which indeed is always accompanied by them.
=Salivary Glands.=--On either side of the mouth are three racemose
glands for the secretion of the _saliva_, which serves to soften and
lubricate the food and partially to digest starches by means of its
ferment, _ptyalin_. The _parotid gland_ is the largest and is below and
in front of the ear, opening by _Stensen’s duct_. The _submaxillary
gland_ is below the jaw toward the back on either side and its duct
is _Wharton’s duct_. The _sublingual gland_ lies beneath the mucous
membrane of the floor of the mouth and opens by eight to twenty tiny
ducts beside the frenum, the _ducts of Rivinus_. The activity of the
glands depends upon the blood supply; the more blood the greater their
activity.
[Illustration: FIG. 20.--Dissection of the side of the face, showing
the salivary glands: _a_, Sublingual gland; _b_, submaxillary gland,
with its duct opening on the floor of the mouth beneath the tongue at
_d_; _c_, parotid gland and its duct, which opens on the inner side of
the cheek. (After Yeo.)]
=The Tonsils.=--The tonsils vary in size and in _tonsillitis_ swell and
may even meet in the median line. They are frequently removed. When
they are enlarged one often gets a _third tonsil_ or _adenoids_, a
lymphoid growth at the back of the pharynx which causes mouth-breathing
by day and snoring by night. A child with adenoids is starved for air
and what air is breathed in is not warmed. The growth should be removed.
A short frenum produces _tongue-tie_, which may be remedied by
snipping. Cancer of the tongue is fairly common and necessitates
a radical operation. In mumps the parotid glands are inflamed and
enlarged.
=The Ear.=--The special organ of hearing is the ear, to which there are
three parts, the external, the middle, and the internal ear.
The _external ear_ consists of the _pinna_ or expanded cartilaginous
portion, for the concentration and direction of sound waves, and the
_external auditory canal_, partly cartilage, partly bone, which is
directed forward, inward, and downward and conveys sound to the middle
ear.
[Illustration: FIG. 21.--The small bones of the ear; external view
(enlarged). (After Gray.)]
The _middle ear_ or _tympanum_ is an irregular cavity in the petrous
portion of the temporal bone. Its outer wall is formed by the _membrana
tympani_ or _drum_, an oval translucent membrane placed obliquely at
the bottom of the external auditory canal. The middle ear communicates
with the inner ear through the _fenestra ovalis_ or oval window and
contains the _ossicles_, the _malleus_ or hammer, the _incus_ or anvil,
and the _stapes_ or stirrup, which are arranged in a movable chain from
the drum to the oval window. The _malleus_, which is connected with
the membrana tympani, articulates by its head with the body of the
incus, while the stapes articulates with the _incus_ by its head and is
connected by its base with the margin of the oval window. Connection
is made between the middle ear and the pharynx and the pressure of the
air upon the drum made equal on either side by means of the _Eustachian
tubes_. These tubes are about an inch and a half long, have cilia, and
convey wax and other matter from the ear to the pharynx. Occasionally
in a cold or for some other reason they become stopped up and trouble
results in the middle ear. Some of the _mastoid cells_ also connect
with the middle ear and may become infected, causing mastoid disease.
[Illustration: FIG. 22.--Interior view of left bony labyrinth after
removal of the superior and external walls: 1, 2, 3, the superior,
posterior, and external or horizontal semicircular canals; 4, fovea
hemi-elliptica; 5, fovea hemispherica; 6, common opening of the
superior and posterior semicircular canals; 7, opening of the aqueduct
of the vestibule; 8, opening of the aqueduct of the cochlea; 9, the
scala vestibuli; 10, scala tympani; the lamina spiralis separating 9
and 10. (From Quain, after Sömmerring.)]
The _internal ear_ consists of various chambers hollowed out in
the petrous portion of the temporal bone. There is an _osseous
labyrinth_, consisting of a central cavity known as the _vestibule_,
three semicircular canals, and the _cochlea_, and within the osseous
labyrinth, surrounded by _perilymph_, is the _membranous labyrinth_,
of like form, filled with the _endolymph_. Communication exists
externally with the middle ear by the _round_ and _oval windows_ and
internally with the _internal auditory canal_, through which passes
the eighth cranial or auditory nerve, the special nerve of hearing,
which is distributed to the inner ear only. When the auditory nerve
enters the ear through this internal auditory meatus it divides into
two branches, of which one goes to the vestibule and the other to the
_organ of Corti_, a group of specially modified epithelial cells in
the cochlea of the membranous labyrinth, which is very important in
transmitting the impulses to the brain. The nerve also breaks up into
very small branches and is distributed practically throughout the wall
of the labyrinth.
The _sensation of hearing_ is the result of impulses transmitted to the
auditory nerve and so conveyed to the auditory center in the brain.
It is caused by sound waves which travel through the air from their
point of origin and enter the external ear. This collects and selects
the waves of sound and helps one to a certain extent to determine the
direction from which the sound comes. As they pass through the external
meatus the sound waves are collected into a comparatively small area
for transmission to the middle ear, where, by means of the drum, they
set in vibration the chain of ossicles. Through these the vibrations
are in turn transmitted to the oval window, being intensified in the
process. Here again they are taken up by the perilymph, from which they
pass through the wall of the membranous labyrinth to the endolymph,
affecting the epithelial lining of the labyrinth in such a way that the
impulses are transmitted to the auditory nerve, more particularly in
the vestibule, from which the vibrations enter the cochlea. They also
affect the cells of the organ of Corti in like manner as they pass from
the perilymph to the endolymph. The membrane that covers the fenestra
rotunda or round window relaxes and expands as the vibrations strike
it, thus serving to eliminate the shock of impact.
_Musical sounds_ are caused by rhythmical or regularly repeated
vibrations, while irregular vibrations give rise to noises. In
musical sounds loudness is determined by the height or amplitude of
the vibrations, pitch by the length of the wave, and quality by the
number of so called partial tones. A sensation of sound cannot be
produced by less than 30 vibrations a second and the ordinary person
cannot hear more than 16,000 vibrations a second. Different sounds
can be distinguished when they follow each other as closely as by one
one-hundredth of a second.
All sound does not come through the canal of the ear. The bones of the
head vibrate and carry sound. So there are instruments for the deaf
which are put in the ear and others which are placed between the teeth.
The _semicircular canals_ are not essential to hearing but have
something to do with a person’s power of maintaining his equilibrium.
Injury to them may cause dizziness and loss of equilibrium.
=The Eye.=--One more feature, perhaps the most expressive, remains to
be described, the eye. The senses are all modifications of the original
cutaneous sensibility and the nerve of sight is no more sensitive to
light than any other nerve. It therefore needs an end organ that is
sensitive to the motions of the ether in order to give impressions of
light. This organ is provided in the eye, which is not only itself
capable of being moved in every direction, but is placed in the most
movable part of the body, the head, which can be turned in almost a
complete circle. The _eyeball_ is spherical and lies in the cavity of
the orbit upon a cushion of fat, where it has a large range of sight
but is securely protected from injury by its bony surroundings. The
sunken eyes following protracted illness are due to the using by the
system of the fat on which the eyeball ordinarily rests.
Each _orbital cavity_ is formed by the juncture of some seven bones and
communicates with the cavity of the brain through the _optic foramen_
and through the sphenoidal fissure. Above the orbits are arched
eminences of skin, the _eye-brows_, from which several rows of short
hairs grow longitudinally and which serve to protect the eyes and to
limit the amount of light to a certain extent, as in frowning.
Still further protection is afforded by the _eyelids_, longitudinal
folds of skin, the one above, the other below, which close like
curtains over the eye. Beneath the external layer of skin in the lids
is fatty tissue and then the orbicularis palpebrarum muscle by means
of which they are closed. They are kept in shape by the tarsal plates
or cartilages, in whose ocular surface are embedded the _Meibomian
glands_, whose secretion prevents the free edges of the lids from
sticking together. Along these edges grows a double or triple row
of stiff hairs, the _eye-lashes_, which curve outward so as not to
interfere with each other and also to prevent the entrance into the eye
of foreign bodies. Lining the inner surface of the lids and reflected
thence over the anterior surface of the sclerotic coat of the eye is
a mucous membrane, the _conjunctiva_, which is thick, opaque, and
vascular on the lids but thin and transparent on the eyeball. The
angles between the lids are known as the _internal_ and the _external
canthus_.
[Illustration: FIG. 23.--The external ocular muscles. (Pyle.)]
_Muscles_ and _Nerves_.--The eyeball is held in position by the ocular
muscles, the conjunctiva, and the lids, while surrounding it, yet
allowing free movement, is a thin membranous sac, the _tunica vaginalis
oculi_. The superior and inferior recti muscles at the upper and lower
edges of the ball turn the eye up and down; the internal and external
recti at the inner and outer edges turn the eye inward and outward; and
the superior and inferior oblique rotate the eye. The nerves supplying
these muscles are the third or motor oculi, the fourth and the sixth.
The _lachrymal gland_, which is about the size and shape of an almond,
is situated at the upper and outer part of the orbit. It secretes a
fluid which keeps the anterior surface of the eye bathed in moisture
and is ordinarily drained away through the lachrymal sac in the inner
canthus, whence it passes by the lachrymal ducts into the nose. When
the amount secreted is excessive, it overflows the lower lid as _tears_.
[Illustration: FIG. 24.--Diagram of the lacrimal apparatus. (Pyle.)]
_Coats of Eye._--The membranes or coats of the eye are three in number:
an outer or sclerotic, a middle or vascular, and an inner or sensitive.
The _sclerotic coat_ is a rather thick, fibrous, protective membrane.
Where it passes in front of the iris, however, it is thinner and
transparent and is known as the _cornea_. The cornea projects somewhat
and, as it were, resembles a segment of a smaller sphere set into the
rest of the sclerotic.
The middle or vascular coat, known as the _choroid_, carries
blood-vessels for the retina or sensitive coat in its inner layer
and has an outer layer of pigment cells that excludes light and
darkens the inner chamber of the eye. The folds of the choroid at its
anterior margin contain the ciliary muscles and are known as the
_ciliary processes_, while the name _iris_ is given to the little
round pigmented, perforated, curtain-like muscle just in front of the
crystalline lens. The posterior surface of the iris is covered with a
thick layer of pigment cells to prevent the entrance of light except
through the central opening or _pupil_, and its anterior surface also
has pigment cells that give it its color, though the difference in the
color of people’s eyes is due rather to the amount of pigment present
than to its color, a small amount of pigment being present in blue eyes
and a large amount in brown and black eyes. Variations in the size
of the pupil are brought about by contractions of the circular and
radiating fibers of the iris, contraction of the circular fibers making
it smaller and those of the radiating larger. The pupil is constricted
for near objects and during sleep, and is dilated for distant objects.
In a dull light also it dilates to let in more light, and in a bright
light it contracts. The appearance of the pupil is often important as a
means of diagnosis and in etherization.
[Illustration: FIG. 25.--Vertical section through the eyeball and
eyelids. (Pyle.)]
Lastly there is the innermost _sensitive coat_ or _retina_, which has
eight layers, the outer one containing some pigment cells and the
next the rods and cones, in which the power of perception is supposed
to lie, branches of the optic nerve being distributed over it in all
directions. In fact, the retina is formed by a membranous expansion of
the optic or second cranial nerve, the special nerve of sight, which
passes into the orbit through the optic foramen at the back and enters
the eyeball close to the _macula lutea_ or _yellow spot_. The exact
spot where the optic nerve enters the retina is not sensitive and is
known as the _blind spot_. In the center of the macula lutea, however,
which is in the middle of the retina posteriorly, is a tiny pit, the
_fovea centralis_, in which all the layers of the retina except the
rods and cones are absent, and at this point vision is most perfect.
It is, therefore, always turned toward the object looked at, and when
one wishes to see an object distinctly, he must keep moving his eyes
over it that the rays from each part may fall in turn upon the fovea
centralis.
Directly behind the pupil is the _crystalline lens_, a rather firm
gelatinous body enclosed in a capsule, which is transparent in life
but opaque in death. The lens is doubly convex and is held in place
by the suspensory ligaments, which arise from the ciliary processes.
In front of it is the anterior chamber of the eye, filled with a thin
watery fluid called the _aqueous humor_, while the larger space back of
it, occupying about four-fifths of the entire globe, is filled with a
jelly-like substance known as the _vitreous humor_.
The chief artery of the eye is the _ophthalmic_.
_Light Rays._--The eye is practically a camera and its principal
function is to reflect images. Although there are several refracting
surfaces and media, for practical purposes the cornea alone need be
considered. Except for those rays which enter the eye perpendicularly
to the cornea, whose line of entrance is called the _optic axis_, all
rays are refracted when they enter the eye and the point at which they
meet and cross each other behind the cornea is called the _principal
focus of the eye_. To focus properly, all the rays from any one point
on an object must meet again in a common point upon the retina, their
_conjugate focus_. In the normal eye all the rays from an object are
focused on the retina and form upon it an image of the object which, as
in the camera, is inverted, because of the crossing of the rays behind
the cornea. Once focused on the retina the light traverses the various
layers to the layer of rods and cones, where chemical action takes
place and affects the little filaments of the optic nerve, by which the
message is carried to the brain.
[Illustration: FIG. 26.--Diagram showing the difference between (_A_)
emmetropic, (_B_) myopic and (_C_) hypermetropic eyes. (American
Text-book of Physiology.)]
When the eye is at rest the pupil and lens are in their normal
condition and at such times the eye sees only distant objects. The
ability of the eye to focus upon objects at different distances is
called _accommodation_ and to accomplish it three things are necessary:
1. change in the shape of the lens; 2. convergence of the axes of the
eyes, and 3. narrowing of the pupils.
When the eye is directed toward distant objects, the muscle fibers
in the ciliary processes relax, causing tightening of the suspensory
ligaments and consequent flattening of the surface of the lens.
Otherwise an image would be formed in front of the retina; for the
greater the convexity of the lens, the greater the angle of refraction.
Such accommodation is passive and so not fatiguing. To look at nearby
objects, on the contrary, the ciliary muscles contract, drawing the
choroid forward and allowing the suspensory ligaments to relax, so that
the lens bulges in front. This is an exertion.
In order to accommodate properly, moreover, both eyes must work
together and the axes of both eyes must be directed toward the object.
Therefore, in looking at nearby objects the axes of the eyes converge,
drawn by the internal recti muscles. In _strabismus_ or _cross eye_,
where the axes of both eyes cannot be directed toward the object at the
same time, the rays fall upon one part of one eye and upon a different
part of the other eye and two separate images are seen.
Finally there is concentric narrowing of the pupil by contraction of
the circular fibers of the iris, by which means various side rays that
would come to a focus outside the retina are excluded.
All the muscles of accommodation, the ciliary muscles, the internal
recti, and the sphincter pupillæ, are under the control of the third
nerve.
Connected with this power of accommodation and dependent on it are the
two conditions of _near-sightedness_ or _myopia_ and _far-sightedness_
or _hypermetropia_.
The normal eye is emmetropic and is almost perfectly spherical, but
in the near-sighted or myopic eye the ball, instead of being round,
is flattened from above down and so bulges in front. Consequently,
owing to the greater distance from the lens to the retina, images are
formed in front of the retina. Only nearby objects can be seen clearly,
because the farther the object from the eye the farther in front of
the retina the image is formed. Concave glasses are worn to enable
near-sighted people to see at a distance. Hypermetropic or far-sighted
eyes are flattened from before backward and can see only objects at a
distance clearly, as those nearby form images behind the retina. For
such eyes convex glasses are worn.
As the ordinary person approaches middle life, he becomes able to see
better at a distance than near to. This _presbyopia_, as it is called,
which is practically far-sightedness, is due to a partial loss of the
power of accommodation in the lens, the result of a general loss of
elasticity in the parts.
Another very common defect is _astigmatism_, a failure of the rays to
focus upon a point, owing generally to a flattening in the surface of
the cornea.
_Color perception_ is also an important function of the eye. The waves
of hyperluminous ether when of a certain rate of vibration give the
sensation of heat and when their vibrations are more rapid they give
the sensation of light. Each of the primary colors of the spectrum
gives off a pretty definite number of light rays which travel through
the air and enter the eye, the number of rays determining the color
thrown upon the retina and the velocity determining the intensity of
the color. Occasionally when light is passing through into the eye it
is broken up as in a prism and the person gets a sensation as of all
sorts of colors, _chromatic aberration_. Total or partial absence of
sensitiveness to color is called _color blindness_. It is commonest
in the form of inability to distinguish between red and green and is
probably due to a defect in the retina.
Sometimes a hair follicle on the lid becomes infected and a _sty_
is formed. _Pink eye_ is conjunctivitis or inflammation of the
conjunctiva. A Meibomian duct may become stopped and cause bulging, or
there may be a sagging down or _ptosis of the upper lid_ in certain
diseases, as meningitis, apoplexy, and more especially syphilis.
_Rodent ulcer_ often begins by the eye or on the cheek.