THE SPECIAL SENSES.
SIGHT.
The eye is the organ through which we perceive, by the agency of light,
all the varied dimensions relations, positions, and visible qualities of
external objects.
The number, position, and perfection of the eyes, vary remarkably in
different orders, in many instances corresponding to the mode of life,
habitation, and food of the animal. A skillful anatomist may ascertain
by the peculiar formation of the eye, without reference to the general
physical structure, in what element the animal lives. Sight is one of
the most perfect of the senses, and reveals to man the beauties of
creation. The aesthetic sentiment is acknowledged to be the most
refining element of civilized life. Painting, sculpture, architecture,
and all the scenes of nature, from a tiny way-side flower to a Niagara,
are subjects in which the poet's eye sees rare beauties to mirror forth
in the rhythm of immortal verse.
In the vertebrates, the organs of vision are supplied with filaments
from the second pair of cranial nerves. In mammalia, the eyes are
limited to two in number, which in man are placed in circular cavities
of the skull, beneath the anterior lobes of the cerebrum. Three
membranes form the lining of this inner sphere of the eye, called
respectively the Sclerotic, Choroid, and Retina.
The _Sclerotic_, or outer covering, is the white, firm membrane, which
forms the larger visible portion of the eyeball. It is covered in front
by a colorless, transparent segment, termed the _cornea_, which gives
the eye its lustrous appearance. Within the sclerotic, and lining it
throughout, is a thin, dark membrane termed the _Choroid_. Behind the
cornea it forms a curtain, called the _iris_, which gives to the eye its
color. The muscles of the iris contract or relax according to the amount
of light received, thus enlarging or diminishing the size of the
circular opening called the _pupil_. The _Retina_ is formed by the optic
nerve, which penetrates the sclerotic and choroid and spreads out into a
delicate, grayish, semi-transparent membrane. The retina is one of the
most _essential_ organs of vision, and consists of two layers. A
spheroidal, transparent body, termed the _crystalline lens_, is situated
directly behind the pupil. It varies in density, increasing from without
inward, and forms a perfect refractor of the light received. The space
in front of the crystalline lens is separated by the iris into two
compartments called respectively the _anterior_ and _posterior
chambers_. The fluid contained within them, termed the _aqueous humor_,
is secreted by the cornea, iris, and ciliary processes. The space behind
the crystalline lens is occupied by a fluid, called the _vitreous
humor_. This humor is denser than the other fluids and has the
consistency of jelly, being perfectly transparent. "The function of the
crystalline lens is to produce distinct perception of form and
outline."[3] The transparent humors of the eye also contribute to the
same effect, but only act as auxiliaries to the lens.
[Illustration: Fig. 62.]
The figure on the next page represents the course of the rays of light
proceeding from an object _a b_, refracted by the lens, and forming the
inverted image _x y_ on the screen. All rays of light proceeding from
_b_ are concentrated at _y_, and those proceeding from _a_ converge at
_x_. Rays of light emanating from the center of the object _a b_ pursue
a parallel course, and form the center of the image. Rays of light
passing through a double convex lens converge at a point called the
_focus_. In the organ of vision, if perfect, the focus is on the retina,
which serves as a screen to receive the image or impression. We have a
distinct perception of the outline of a distant hill, and also of a book
lying before us. The rays of light we receive from these objects cannot
have the same focus. How, then, can we account for the evident
accommodation of the eye to the varying distances? Various theories have
been advanced to explain this adjustment; such as changes in the
curvature of the cornea and lens; a movement of the lens, or a general
change in the form of the eyeball, by which the axis may be lengthened
or shortened.
[Illustration: Fig. 63.]
Two facts comprise all the positive knowledge which we possess on this
subject. Every person is conscious of a muscular effort in directing the
eye to a near object" as a book, and of fatigue, if the attention is
prolonged. If, now, the eyes be directed to a distant object, there will
result a sense of rest, or passiveness. By various experiments it has
been proved that the accommodation or adjustment of the eye for near
objects requires a muscular effort, but for distant objects the muscles
are in an essentially passive condition. An increase in the convexity of
the crystalline lens is now admitted to be necessary for a distinct
perception of near objects. We may give two simple illustrations, cited
by Dr. Dalton in his recent edition of Human Physiology. If a candle be
held near the front of an eye which is directed to a distant object,
three reflected images of the flame will be seen in the eye, one on each
of the anterior surfaces of the cornea and lens, and a third on the
posterior surface of the latter. If the eye is directed to a near
object, the reflection on the cornea remains unchanged, while that on
the anterior surface of the lens gradually diminishes and approximates
in size the reflection on the cornea, thus giving conclusive evidence
that, in viewing a near object, the anterior surface of the crystalline
lens become _more convex_, and at the same time approaches the cornea.
Five or six inches is the minimum limit of the muscular adjustment of
the eye. From that point to all the boundless regions of space, to every
star and nebulae which send their rays to our planet, human vision can
reach. It is the sense by which we receive knowledge of the myriads of
worlds and suns which circle with unfailing precision through infinite
space.
HEARING.
[Illustration: Fig. 64.
Internal and external ear. 1. External ear. 2. Internal
auditory meatus. 3. Tympanum. 4. Labyrinth.
5. Eustachian tube.]
Hearing depends upon the sonorous vibrations of the atmosphere. The
waves of sound strike the sensitive portions of the ear, and their
impressions upon the auditory nerves are termed the sensations of
hearing. The ear is divided into three parts, called respectively the
External, Middle, and Internal ear.
The external organs of hearing are two in number, and placed on opposite
sides of the head. In most of the higher order of vertebrates, they are
so situated as to give expression and proportion to the facial organs,
and, at the same time, to suit the requirements of actual life.
The _External ear_ is connected with the interior part by a prolongation
of its orifice, termed the _external auditory meatus_. In man, this
gristly portion of the auditory apparatus is about one inch in length,
lined by a continuation of the integument of the ear, and has numerous
hairs on its surface, to prevent the intrusion of foreign substances.
Between the external MEATUS and the cavity of the middle ear is the
_membrana tympani_, which is stretched across the opening like the head
of a drum. The _tympanum_, or ear-drum, communicates with the pharynx by
the _eustachian tube_, which is a narrow passage lined with delicate,
ciliated epithelium. On the posterior portion it is connected with the
_mastoid cells_. Three small bones are stretched across the cavity of
the tympanum, and called, from their form, the _malleus, incus_ and
_stapes_, or the hammer, anvil, and stirrup. Agassiz mentions a fourth,
which he terms the _os orbiculare_. Each wave of sound falling upon the
membrana tympani, throws its molecules into vibrations which are
communicated to the chain of bones, which, in turn, transmits them to
the membrane of the _foramen ovale_. The three muscles which regulate
the tension of these membranes are termed the _tensor tympani, laxator
tympani_, and _stapedium tympani._
The _Labyrinth_, or _Internal_ ear, is a complicated cavity, consisting
of three portions termed the _vestibule, cochlea_, and _semi-circular
canals_. The vestibule is the central portion and communicates with the
other divisions. The labyrinth is filled with a transparent fluid,
termed _perilymph_, in which are suspended, in the vestibules and
canals, small membranous sacs, containing a fluid substance, termed
_endolymph_ (sometimes called _vitrine auditive_ from its resemblance to
the vitreous humor of the eye). The filaments of the auditory nerve
penetrate the membranous tissues of these sacs, and also of those
suspended at the commencement of the semi-circular canals. These little
sacs are supposed to be the seat of hearing, and to determine, in some
mysterious way, the quality, intensity and pitch of sounds.
The determination of the _direction_ of sound is a problem of acoustics.
Some have contended that the arrangement of the semi-circular canals is
in some way connected with this sensation. But this supposition,
together with the theory of the transmission of sound through the
various portions of the cranial bones, has been exploded.
From the foregoing description, it will be seen that the labyrinth and
tympanum are the most essential parts of the organs of hearing. In
delicacy and refinement this sense ranks next to sight. The emotions of
beauty and sublimity, excited by the warbling of birds and the roll of
thunder, are scarcely distinguishable from the intense emotions arising
from sight. It is a remarkable fact, that the refinement or cultivation
of these senses is always found associated. Those nations which furnish
the best artists, or have the highest appreciation of painting and
sculpture, produce the most skillful musicians, those who reduce music
to a science.
SMELL.
[Illustration: Fig. 65.
1. Frontal sinus. 2. Nasal bone. 3. Olfactory
ganglion and nerves. 4. Nasal branch
of the fifth pair. 5. Spheno-palatine ganglion.
6. Soft palate. 7. Hard palate, _a_.
Cerebrum, _b_. Anterior lobes, _c_. Corpus
callosum. _d_. Septum lucidum. _f_. Fornix.
_g_. Thalami optici. _h_. Corpora striata.]
Next in order of delicacy, and more closely allied with the physical
functions, is the sense of smell. Delicate perfumes, or the fragrance of
a flower, impart an exhilarating sensation of delight, while numerous
odors excite a feeling of disgust. The organ of smell is far less
complicated in its structure than the eye or the ear. It consists of two
cavities having cartilaginous walls, and lined with a thick mucous coat,
termed the _pituitary membrane_, over which are reflected the olfactory
nerves. Particles of matter, too minute to be visible even through the
microscope, are detached from the odorous body and come in contact with
the nerves of smell, which transmit the impressions or impulses thus
received to the brain. Fig. 65 shows the distribution of the olfactory
nerves in the nasal passages. The nose is supplied with two kinds of
filaments which are termed respectively nerves of _special_ and nerves
of _general sensation_. Compared with the lower animals, especially with
those belonging to the carnivorous species, the sense of smell in man is
feeble. The sensation of smell is especially connected with the
pleasures and necessities of animal life.
TASTE.
The sense of taste is directly connected with the preservation and
nutrition of the body. A delicious flavor produces a desire to eat a
savory substance. Some writers on hygiene have given this sense an
instinctive character, by assuming that all articles having an agreeable
taste are suitable for diet. The nerves of taste are distributed over
the surface of the tongue and palate, and their minute extremities
terminate in well developed _papillae_. These _papillae_ are divided
into three classes, termed, from their microscopic appearance,
_filiform_, _fungiform_ and _circumvallate_. The organ of taste is the
mucous membrane which covers the back part of the tongue and the palate.
The papillae of the tongue are large and distinct, and covered with
separate coats of epithelium. The filiform papillae are generally long
and pointed and are found over the entire surface of the tongue. The
fungiform are longer, small at the base and broad at the end. The
circumvallate are shaped like an inverted V and are found only near the
root of the tongue; the largest of this class of papillae have other
very small papillae upon their surfaces. It is now pretty satisfactorily
established that the circumvallate, or fungiform papillae are the only
ones concerned in the special sense of taste.
The conditions necessary to taste are, that the substance be in solution
either by artificial means, or by the action of the saliva; and that it
be brought in contact with the sensitive filaments imbedded in the
mucous membrane. The nerves of taste are both _general_ and _special_ in
their functions. If the general sensibility of the nerves of taste is
unduly excited, the function of sensibility is lost for some time. If a
peppermint lozenge is taken into the mouth, it strongly excites the
general sensibilities of taste, and the power of distinguishing between
special flavors is lost for a few moments. A nauseous drug may then be
swallowed without experiencing any disagreeable taste.
Paralysis of the facial nerve often produces a marked effect in the
sensibility of the tongue. Where this influence lies has not been fully
explained; probably it is indirect, being produced by some alteration in
the vascularity of the parts or a diminution of the salivary secretions.
TOUCH.
By the sense of touch, we mean the _general sensibility of the skin_.
Sensations of heat and cold are familiar illustrations of this faculty.
By the sense of touch, we obtain a knowledge of certain qualities of a
body, such as form consistency, roughness, or smoothness of surface,
etc. The tip of the tongue possesses the most acute sensibility of any
portion of the body, and next in order are the tips of the fingers. The
hands are the principal organs of tactile sensation. The nerves of
general sensibility are distributed to every part of the cutaneous
tissue. The contact of a foreign body with the back, will produce a
similar _tactile_ sensation, as with the tips of the fingers. The
sensation, however, will differ in _degree_ because the back is supplied
with a much smaller number of sensitive filaments; in _quality_ it is
the same.
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