Hygienic Physiology : with Special Reference to the Use of Alcoholic Drinks and Narcotics

Part 18

Chapter 183,829 wordsPublic domain

61. To what extent are we responsible for the health of our body?

62. Why does alcohol tend to collect in the brain?

63. Does the use of alcohol tend to increase crime and poverty?

VIII.

THE SPECIAL SENSES.

"See how yon beam of seeming white Is braided, out of seven-hued light; Yet in those lucid globes no ray By any chance shall break astray. Hark, how the rolling surge of sound, Arches and spirals circling round, Wakes the hush'd spirit through thine ear With music it is heaven to hear."

HOLMES.

"Let us remember that if we get a glimpse of the details of natural phenomena, and of those movements which constitute life, it is not in considering them as a whole, but in analyzing them as far as our limited means will permit. In the vibrations of the globe of air which surrounds our planet, as in the undulations of the ether which fills the immensity of space, it is always by molecules which are intangible for us, put in motion by nature, always by the infinitely little, that she acts in exciting the organs of sense, and she has modeled these organs in a proportion which enables them to partake in the movement which she impresses upon the universe. She can paint with equal facility on a fraction of a line of space on the retina, the grandest landscape or the nervelets of a rose leaf; the celestial vault on which Sirius is but a luminous point, or the sparkling dust of a butterfly's wing; the roar of the tempest, the roll of thunder, the echo of an avalanche, find equal place in the labyrinth whose almost imperceptible cavities seem destined to receive only the most delicate sounds."

THE SPECIAL SENSES

1. TOUCH.

DESCRIPTION.--Touch is sometimes called the "common sense," since its nerves are spread over the whole body. It is most delicate, however, in the point of the tongue and the tips of the fingers. The surface of the cutis is covered with minute, conical projections called _papillæ_ (Fig. 24). [Footnote: In the palm of the hand, where there are at least twelve thousand in a square inch, we can see the fine ridges along which they are arranged.] Each one of these papillæ contains its tiny nerve twigs, which receive the impression and transmit it to the brain, where the perception is produced.

USES.--Touch is the first of the senses used by a child. By it we obtain our idea of solidity, and throughout life rectify all other sensations. Thus, when we see anything curious, our first desire is to handle it.

The sensation of touch is generally relied upon, yet, if we hold a marble in the manner shown in Fig. 57, it will seem like two marbles; and if we touch the fingers thus crossed to our tongue, we shall seem to feel two tongues. Again, if we close our eyes and let another person move one of our fingers over a plane surface, first lightly, then with greater pressure, and then lightly again, we shall think the surface concave.

FIG. 57.

This organ is capable of wonderful cultivation. The physician acquires by practice the _tactus eruditus_, or learned touch, which is often of great service, while the delicacy of touch possessed by the blind almost compensates the loss of the absent sense. [Footnote: The sympathy between the different organs shows how they all combine to make a home for the mind. When one sense fails, the others endeavor to remedy the defect. It is touching to see how the blind man gets along without eyes, and the deaf without ears. Cuthbert, though blind, was the most efficient polisher of telescopic mirrors in London. Saunderson, the successor of Newton as professor of mathematics at Cambridge, could distinguish between real and spurious medals. There is an instance recorded of a blind man who could recognize colors. The author knew one who could tell when he was approaching a tree, by what he described as the "different feeling of the air."] (See p. 346.)

2. TASTE.

DESCRIPTION.--This sense is located in the papillæ of the tongue and palate. These papillaæ start up when tasting, as you can see by placing a drop of vinegar on another person's tongue, or your own before a mirror. The velvety look of this organ is given by hair-like projections of the cuticle upon some of the papillæ. They absorb the liquid to be tasted, and convey it to the nerves. [Footnote: An insoluble substance is therefore tasteless.] The back of the tongue is most sensitive to salt and bitter substances, and, as this part is supplied by the ninth pair of nerves (Fig. 56), in sympathy with the stomach, such flavors, by sympathy, often produce vomiting. The edges of the tongue are most sensitive to sweet and sour substances, and as this part is supplied by the fifth pair of nerves, which also goes to the face, an acid, by sympathy, distorts the countenance.

FIG. 58.

THE USE OF THE TASTE was originally to guide in the selection of food; but this sense has become so depraved by condiments and the force of habit that it would be a difficult task to tell what are one's natural tastes.

3. SMELL. [Footnote: The sense of smell is so intimately connected with that of taste that we often fail to distinguish between them. Garlic, vanilla, coffee and various spices, which seem to have such distinct taste, have really a powerful odor, but a feeble flavor.]

DESCRIPTION.--The nose, the seat of the sense of smell, is composed of cartilage covered with muscles and skin, and joined to the skull by small bones. The nostrils open at the back into the pharynx, and are lined by a continuation of the mucous membrane of the throat. The olfactory nerves (first pair, Fig. 55) enter through a sieve-like, bony plate at the roof of the nose, and are distributed over the inner surface of the two olfactory chambers. (See p. 346.) The object to be smelled need not touch the nose, but tiny particles borne on the air enter the nasal passages. [Footnote: Three quarters of a grain of musk placed in a room will cause a powerful smell for a considerable length of time without any sensible diminution in weight, and the box in which musk has been placed retains the perfume for almost an indefinite period. Haller relates that some papers which had been perfumed by a grain of ambergris, were still very odoriferous after a lapse of forty years. Odors are transported by the air to a considerable distance. A dog recognizes his master's approach by smell even when he is far away; and we are assured by navigators that the winds bring the delicious odors of the balmy forests of Ceylon to a distance of ten leagues from the coast. Even after making due allowance for the effects of the imagination, it is certain that odors act as an excitant on the brain, which may be dangerous when long continued. They are especially dreaded by the Roman women. It is well known that in ancient times the women of Rome indulged in a most immoderate use of baths and perfumes; but those of our times have nothing in common with them in this respect; and the words of a lady are quoted, who said on admiring an artificial rose, "It is all the more beautiful that it has no smell." We are warned by the proverb not to discuss colors or tastes, and we may add odors also. Men and nations differ singularly in this respect. The Laplander and the Esquimaux find the smell of fish oil delicious. Wrangel says his compatriots, the Russians, are very fond of the odor of pickled cabbage, which forms an important part of their food; and asaftida, it is said, is used as a condiment in Persia, and, in spite of its name, there are persons who do not find its odor disagreeable any more than that of valerian.--_Wonders of the Human body_.]

FIG. 59.

THE USES of the sense of smell are to guide us in the choice of our food, and to warn us against bad air, and unhealthy localities. (See p. 348.)

4. HEARING.

DESCRIPTION.--The ear is divided into the _external_, _middle_, and _internal_ ear.

1. _The External Ear_ is a sheet of cartilage curiously folded for catching sound. The auditory canal, _B_, or tube of this ear trumpet, is about an inch long. Across the lower end is stretched _the membrane of the tympanum_ or drum, which is kept soft by a fluid wax.

FIG. 60.

2. _The Middle Ear_ is a cavity, at the bottom of which is the Eustachian tube, _G_, leading to the mouth. Across this chamber hangs a chain of three singular little bones, _C_, named from their shape the _hammer_, the _anvil_, and the _stirrup_. All together these tiny bones weigh only a few grains, yet they are covered by a periosteum, are supplied with blood vessels, and they articulate with perfect joints (one a ball-and-socket, the other a hinge), having synovial membranes, cartilages, ligaments, and muscles.

3. _The Internal Ear_, or labyrinth, as it is sometimes called from its complex character, is hollowed out of the solid bone. In front, is the vestibule or antechamber, _A_, about as large as a grain of wheat; from it open three _semicircular canals_, _D_, and the winding stair of the _cochlea_, or snail shell, _E_. Here expand the delicate fibrils of the auditory nerve. Floating in the liquid which fills the labyrinth is a little bag containing hair-like bristles, fine sand, and two ear stones (_otoliths_). All these knocking against the ends of the nerves, serve to increase any impulse given to the liquid in which they lie. Finally, to complete this delicate apparatus, in the cochlea are minute tendrils, named the fibers of Corti, from their discoverer. These are regularly arranged,--the longest at the bottom, and the shortest at the top. Could this spiral plate, which coils two and a half times around, be unrolled and made to stand upright, it would form a beautiful microscopic harp of three thousand strings. If it were possible to strike these cords as one can the keyboard of a piano, he could produce in the mind of the person experimented upon every variety of tone which the ear can distinguish.

HOW WE HEAR.--Whenever one body strikes another in the air, waves are produced, just as when we throw a stone into the water a series of concentric circles surrounds the spot where it sinks. These waves of air strike upon the membrane. This vibrates, and sends the motion along the chain of bones in the middle ear to the fluids of the labyrinth. Here bristles, sand, and stones pound away, and the wondrous harp of the cochlea, catching up the pulsations, [Footnote: The original motion is constantly modified by the medium through which it passes. The bristles, otoliths, and Cortian fibers of the ear, and the rods and cones of the eye (p. 239) serve to convert the vibrations into pulsations which act as stimuli of the appropriate nerve. The molecular change thus produced in the nerve fibers is propagated to the brain.--See _Popluar Physics_, p. 182.] carries them to the fibers of the auditory nerve, which conveys them to the brain, and gives to the mind the idea of sound.

CARE OF THE EAR.--The delicacy of the ear is such that it needs the greatest care. Cold water should not be allowed to enter the auditory canal. If the wax accumulate, never remove it with a hard instrument, lest the delicate membrane be injured, but with a little warm water, after which turn the head to let the water run out, and wipe the ear dry. The hair around the ears should never be left wet, as it may chill this sensitive organ. If an insect get in the external ear, pour in a little oil to kill it, and then remove with tepid water. The object of the Eustachian tube is to admit air into the ear, and thus equalize the pressure on the membrane. If it become closed by a cold, or if, from any cause, the pressure be made unequal, so as to produce an unpleasant feeling in the ear, relief may often be obtained by grasping the nose and forcibly swallowing. (See p. 350.)

5. SIGHT.

FIG. 61.

DESCRIPTION.--The eye is lodged in a bony cavity, protected by the overhanging brow. It is a globe, about an inch in diameter. The ball is covered by three coats--(l) the _sclerotic_, _d_, a tough, horny casing, which gives shape to the eye, the convex, transparent part in front forming a window, the _cornea_, _d_; (2) the _choroid_, _e_, a black lining, to absorb the superfluous light [Footnote: Neither white rabbits nor albinos have this black lining, and hence their sight is confused.] and (3) the _retina_, _b_, a membrane in which expand fibers of the _optic nerve_, _o_. The _crystalline lens_, _a_, brings the rays of light to a focus on the retina. The lens is kept in place by the ciliary processes, _g_, arranged like the rays in the disk of a passion flower. Between the cornea and the crystalline lens is a limpid fluid termed the _aqueous humor_; while the _vitreous humor_--a transparent, jelly-like liquid fills the space (_h_) back of the crystalline lens. The pupil, _k_, is a hole in the colored, muscular curtain, _i_, the _iris_ (rainbow). (See p. 352.)

FIG. 62.

EYELIDS AND TEARS.--The eyelids are close-fitting shutters to screen the eye. The inner side is lined with a mucous membrane that is exceedingly sensitive, and thus aids in protecting the eye from any irritating substance. The looseness of the skin favors swelling from inflammation or the effusion of blood, as in a "black eye." The eyelashes serve as a kind of sieve to exclude the dust, and, with the lids, to shield against a blinding light. Just within the lashes are oil glands, which lubricate the edges of the lids, and prevent them from adhering to each other. The tear or _lachrymal_ gland, _G_, is an oblong body lodged in the bony wall of the orbit. It empties by several ducts upon the inner surface, at the outer edge of the upper eyelid. Thence the tears, washing the eye, run into the _lachrymal lake_, _D_, a little basin with a rounded border fitted for their reception. On each side of this lake two canals, _C_, _C_, drain off the overplus through the duct, _B_, into the nose. In old age and in disease, these canals fail to conduct the tears away, and hence the lachrymal lake overflows upon the face.

FIG. 63.

STRUCTURE OF THE RETINA.--In Fig. 63 is shown a section of the retina, greatly magnified, since this membrane never exceeds 1/80 an inch in thickness. On the inner surface next to the vitreous humor, is a lining membrane not shown in the cut. Next to the choroid and comprising about 1/4 the entire thickness of the retina, is a multitude of transparent, colorless, microscopic rods, _a_, evenly arranged and packed side by side, like the seeds on the disk of a sunflower. Among them, at regular intervals, are interspersed the cones, _b_. Delicate nerve fibers pass from the ends of the rods and cones, each expanding into a granular body, _c_, thence weaving a mesh, _d_, and again expanding into the granules, _f_. Last is a layer of fine nerve fibers, _g_, and gray, ganglionic cells, _h_, like the gray matter of the brain, whence filaments extend into _i_, the fibers of the optic nerve. (See p. 354.)

The layer of rods and cones is to the eye what the bristles, otoliths, and Cortian fibers are to the ear. Indeed, the nerve itself is insensible to light. At the point where it enters the eye, there are no rods and cones, and this is called the _blind spot_. A simple experiment will illustrate the fact. Hold this book directly before the face, and, closing the left eye, look steadily with the right at the left-hand circle in Fig. 64. Move the book back and forth, and a point will be found where the right-hand circle vanishes from sight. At that moment its light falls upon the spot where the rods and cones are lacking.

FIG. 64.

HOW WE SEE.--There is believed to be a kind of universal atmosphere, termed _ether_, filling all space. This substance is infinitely more subtle than the air, and occupies its pores, as well as those of all other substances. As sound is caused by waves in the atmosphere, so light is produced by waves in the ether. A lamplight, for example, sets in motion waves of ether, which pass in through the pupil of the eye, to the retina, where the rods and cones transmit the vibration through the optic nerve to the brain, and then the mind perceives the light. (Note, p. 236.)

THE USE OF THE CRYSTALLINE LENS. [Footnote: The uses of the eye and ear are dependent upon the principles of Optics and Acoustics. They are therefore best treated in Physics.]--A convex lens, as a common burning glass, bends the rays of light which pass through it, so that they meet at a point called the _focus_. The crystalline lens converges the rays of light which enter the eye, and brings them to a focus on the retina. [Footnote: The cornea and the humors of the eye act in the same manner as the crystalline lens, but not so powerfully.] The healthy lens has a power of changing its convexity so as to adapt [Footnote: The simplest way of experimenting on the "adjustment of the eye" is to stick two stout needles upright into a straight piece of wood,--not exactly, but nearly in the same straight line, so that, on applying the eye to one end of the piece of wood, one needle (A) shall be seen about six inches off, and the other (B) just on one side of it, at twelve inches distance. If the observer looks at the needle B he will find that he sees it very distinctly, and without the least sense of effort; but the image of A is blurred, and more or less double. Now, let him try to make this blurred image of the needle A distinct. He will find he can do so readily enough, but that the act is accompanied by a sense of fatigue. And in proportion as A becomes distinct, B will become blurred. Nor will any effort enable him to see A and B distinctly at the same time.--HUXLEY.] itself to near and to distant objects. (See Fig. 66.)

FIG. 65.

NEAR AND FAR SIGHT.--If the lens be too convex, it will bring the rays to a focus before they reach the retina; if too flat, they will reach the retina before coming to a focus. In either case, the sight will be indistinct. A more common defect, however, is in the shape of the globe of the eye, which is either flattened or elongated. In the former case (see _G_, Fig. 67), objects at a distance can be seen most distinctly-- hence that is called farsightedness. [Footnote: This should not be confounded with the long sight of old people, which is caused by the stiffness of the ciliary muscles, whereby the lens can not adapt itself to the varying distances of objects.] In the latter, objects near by are clearer, and hence this is termed nearsightedness. Farsightedness is remedied by convex glasses; nearsightedness, by concave. When glasses will improve the sight they should be worn; [Footnote: Dr. Henry W. Williams, the celebrated ophthalmologist, says that, in some cases, glasses are more necessary at six or eight years of age than to the majority of healthy eyes at sixty. Sometimes children find accidentally that they can see better through grandmother's spectacles. They should then be supplied with their own.] any delay will be liable to injure the eyes, by straining their already impaired power. Cataract is a disease in which there is an opacity of the crystalline lens or its capsules, which obscures the vision. The lens may be caused to be absorbed, or may be removed by a skillful surgeon and the defect remedied by wearing convex glasses.

FIG. 66.

FIG. 67.

COLOR-BLIND PERSONS receive only two of the three elementary color sensations (green, red, violet). The spectrum appears to them to consist of two decidedly different colors, with a band of neutral tint between. The extreme red end is invisible, and a bright scarlet and a deep green appear alike. They are unable to distinguish between the leaves of a cherry tree and its fruit by the color of the two, and see no difference between blue and yellow cloth. Whittier, the poet, it is said, could not tell red from green unless in direct sunlight. Once he patched some damaged wall paper in his library by matching a green vine in the pattern with one of a bright autumnal crimson. This defect in the eye is often unnoticed, and many railway accidents have doubtless happened through an inability to detect the color of signal lights.

CARE OF THE EYES.--The shape of the eye can not be changed by rubbing and pressing it, as many suppose, but the sight may thus be fatally injured. Children troubled by nearsightedness should not lean forward at their work, as thereby the vessels of the eye become overcharged with blood. They should avoid fine print, and try, in every possible way, to spare their eyes. If middle age be reached without especial difficulty of sight, the person is comparatively safe. Most cases of squinting are caused by longsightedness, the muscles being strained in the effort to obtain distinct vision. In childhood, it may be cured by a competent surgeon, who will generally cut the muscle that draws the eye out of place.

After any severe illness, especially after measles, scarlatina, or typhoid fever, the eyes should be used with extreme caution, since they share in the general debility of the body, and recover their strength slowly. Healthy eyes even should never be used to read fine print or by a dim light. Serious injury may be caused by an imprudence of this kind. Reading upon the cars is also a fruitful source of harm. The lens, striving to adapt itself to the incessantly varying distance of the page, soon becomes wearied. Whenever the eyes begin to ache, it is a warning that they are being overtaxed and need rest.