Chapter IV: 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.

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.

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.

=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.

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.

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_.

_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_.

_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.

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.

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.