12. Is a generally closed parlor a healthful room?
13. Why can an idle scholar read his lesson and at the same time count the
marbles in his pocket?
14. In amputating a limb, what part, when divided, will cause the keenest
pain?
15. What is the effect of bad air on nervous people?
16. Is there any truth in the proverb that "he who sleeps dines"?
17. What does a high, wide forehead indicate?
18. How does indigestion frequently cause a headache?
19. What is the cause of one's foot being "asleep"? [Footnote: Here the
nervous force is prevented from passing by compression. Just how this is
done, or what is kept from passing, we can not tell. If a current of
electricity were moving through a rubber tube full of mercury, a slight
squeeze would interrupt it. These cases may depend on the same general
principle, but we can not assert it.--HUXLEY. The tingling sensation
caused by the compression is transferred to the foot, whence the nerve
starts.]
20. When an injury to the nose has been remedied by transplanting skin
from the forehead, why is a touch to the former felt in the latter?
21. Are closely curtained windows healthful?
22. Why, in falling from a height, do the limbs instinctively take a
position to defend the important organs?
23. What causes the pylorus to open and close at the right time?
24. Why is pleasant exercise most beneficial?
25. Why does grief cause one to lose his appetite?
26. Why should we never study directly after dinner?
27. What produces the peristaltic movement of the stomach?
28. Why is a healthy child so restless and full of mischief?
29. Why is a slight blow on the back of a rabbit's neck fatal?
30. Why can one walk and carry on a conversation at the same time?
31. What are the dangers of overstudy?
32. What is the influence of idleness upon the brain?
33. State the close relation which exists between physical and mental
health and disease.
34. In what consists the value of the power of habit?
35. How many pairs of nerves supply the eye?
36. Describe the reflex actions in reading aloud.
37. Under what circumstances does paralysis occur?
38. If the eyelids of a profound sleeper were raised, and a candle brought
near, would the iris contract?
39. How does one cough in his sleep?
40. Give illustrations of the unconscious action of the brain.
41. Is chewing tobacco more injurious than smoking?
42. Ought a man to retire from business while his faculties are still
unimpaired?
43. Which is the more exhaustive to the brain, worry or severe mental
application?
44. Is it a blessing to be placed beyond the necessity for work?
45. Show how anger, hate, and the other degrading passions are destructive
to the brain. [Footnote: "One of the surest means for keeping the body and
mind in perfect health consists in learning to hold the passions in
subservience to the reasoning faculties. This rule applies to every
passion. Man, distinguished from all other animals by the peculiarity that
his reason is placed above his passions to be the director of his will,
can protect himself from every mere animal degradation resulting from
passionate excitement. The education of the man should be directed not to
suppress such passions as are ennobling, but to bring all under
governance, and specially to subdue those most destructive passions,
anger, hate, and fear."]
46. Are not amusements, to repair the waste of the nervous energy,
especially needed by persons whose life is one of care and toil?
47. Is not severe mental labor incompatible with a rapidly growing body?
48. How shall we induce the system to perform all its functions regularly
49. How does alcohol interfere with the action of the nerves?
50. What is the general effect of alcohol upon the character?
51. Does alcohol tend to produce clearness and vigor of thought?
52. What is the general effect of alcohol on the muscles?
53. Does alcohol have any effect on the bones? The skin?
54. What is the cause of the "alcoholic chill"?
55. Show how alcohol tends to develop man's lower, rather than his higher,
nature.
56. When we wish really to strengthen the brain, should we use alcohol?
57. Why is alcohol used to preserve anatomical specimens?
58. What is meant by an inherited taste for liquor?
59. Ought a person to be punished for a crime committed during
intoxication?
60. Should a boy ever smoke?
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."
_ _
| 1. THE TOUCH...| 1. Description of the Organ.
| |_2. Its Uses.
| _
| 2. THE TASTE...| 1. Description of the Organ.
| |_2. Its Uses.
| _
| 3. THE SMELL...| 1. Description of the Organ.
| |_2. Its Uses.
| _ _
| | 1. Description of the | a. _External Ear._
| | Organ...............| b. _Middle Ear._
| 4. THE HEARING.| |_c. _Internal Ear._
| | 2. How we Hear
| |_3. Hygiene of the Ear.
| _
| | 1. Description of the Organ.
| | 2. Eyelids, and Tears.
| | 3. Structure of the Retina.
|_5. THE SIGHT...| 4. How we see.
| 5. The Use of the Crystalline Lens.
| 6. Near and Far Sight.
| 7. Color Blindness.
|_8. Hygiene of the Eyes.
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.
[Illustration:]
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.
[Illustration: _The Tongue, showing the several kinds of Papillæ--the
conical_ (D) _the whip like_ (K, I), _the circumvallate or entrenched_
(H, L); E, F, G, _nerves;_ C, _glottis._--LANKESTER.]
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.
[Illustration: A, b, c, d, _interior of the nose, which is lined by a
mucous membrane;_ n, _the nose;_ e, _the wing of the nose;_
q, _the nose bones;_ o, _the upper lip;_ g, _section of the
upper jaw-bone;_ h, _the upper part of the mouth, or hard palate;_
m, _frontal bone of the skull;_ k, _the ganglion or bulb of the
olfactory nerve in the skull, from which are seen the branches of the
nerve passing in all directions._]
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.
[Illustration: _The Ear._]
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.
[Illustration: _The Eye._]
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.
[Illustration: _The Eyelashes and the Tear Glands._]
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.
[Illustration: _Structure of the Retina._]
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.
[Illustration:]
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.
[Illustration: _Diagram showing how an image of an object is formed upon
the Retina by the Crystalline Lens._]