THE SENSE OF TOUCH.
846. SENSATION is the perception of external objects by means of the
senses. There are five senses, namely, _Touch_, _Taste_, _Smell_,
_Hearing_, and _Vision_.
847. TOUCH is the sense by which the mind becomes acquainted with some
of the properties of bodies, and enables us determine whether their
surfaces are smooth or rough, their relative temperature, and, to a
certain degree, their form and weight.
848. Some physiologists make a distinction between the sense of touch
and tact. Tact, or feeling, is more general, extending over the whole
surface of the skin and mucous membranes, while touch exists chiefly
in the fingers of man and in the noses of certain quadrupeds.
849. "In the exercise of these functions, tact is considered passive;
as, when any part of the system comes into contact with another body,
a sensation of its presence is given, without the exercise of
volition. On the contrary, touch is active, and is exercised
voluntarily, for the purpose of conveying to the mind a knowledge of
the qualities or properties of the surfaces of bodies; as when we feel
of a piece of cloth to ascertain its qualities, or a polished surface,
to prove its smoothness."
850. In man, the hand is admirably adapted to the exercise of touch.
"The fineness of the skin, its great sensibility, the species of
cushion formed by the sub-cutaneous fat at the extremities of the
fingers, the length and flexibility of these organs, and the
capability of opposing the thumb to the fingers, like a pair of
forceps, are so many conditions essentially favorable to the delicacy
of this sense, and enable us to appreciate with exactitude the
qualities of the bodies we may feel."
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846. Define sensation. How many senses have we? 847-851. _What is said
of the sense of touch?_ 847. Define touch. 848. What is the difference
between touch and tact? 849. In the exercise of these functions, which
is active, and which passive? 850. Why is the hand so admirably
adapted to the exercise of the sense of touch?
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851. The nerves that supply the sense of touch, proceed from the
anterior half of the spinal cord. Where this sense is most acute and
delicate, we find the greatest number of sensitive nervous filaments,
and those of the largest size.
_Observation._ In amputating limbs, and other surgical operations, the
division of the skin causes more pain than all the subsequent steps of
the operation, however protracted. The muscles, cellular membrane, and
fat have but little sensibility; while the bones, tendons, and
ligaments are insensible when not diseased, and may be cut without
causing pain.
HYGIENE OF THE SENSE OF TOUCH.
852. The sense of touch varies in different persons, and also in
individuals of different ages. Thus the sensibilities of the child are
more acute than those of the aged. Although there is an original
difference of sensibility from organization, still, the function of
the nerves of sensation is modified by certain influences.
853. _The healthy or unhealthy, active or inactive state of the brain,
influences the action of the sensitive nerves._ In sound and perfect
sleep, the brain is inactive. In this state, ordinary impressions made
upon the skin are not observed by the sleeping person. Thus the arm
may be blistered while sleeping, when exposed to the warm rays of the
sun, and the individual will not be aware of it at the time.
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851. From what do the nerves proceed that supply this sense? 852-864.
_Give the hygiene of the sense of touch._ 852. Does this sense vary in
different persons? 853. Mention a condition of the brain that
influences the nerves of sensation.
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854. If there is compression of the brain, as when the skull-bones are
depressed, or disease of this organ exists, as in severe typhus fever,
impressions made upon the nerves of the skin will not be noticed. The
same is true when the mind is engaged in intense thought or study;
heat or cold may be so intense as to disorganize the skin, and not to
be noticed.
855. The varying health or condition of the brain usually depresses or
increases the sensitiveness of the skin. This is seen in grief and
fear, which diminish, while hope and joy increase the impressibility
of this tissue. It is not uncommon to see the unfortunate insane
endure exposure to heat and cold with seeming impunity; whereas it
would induce almost insupportable suffering to the sane man. Diseases
of the heart, stomach, and lungs, alter the condition of the brain,
and modify, to a greater or less degree, the sensitiveness of the
skin.
856. _The state of the conducting nervous trunks influences the nerves
of sensation._ If a nervous trunk is compressed or divided, the parts
supplied by nervous filaments from this branch, will be insensible to
the impressions made upon them, and consequently such impressions are
not transmitted to the brain.
_Observation._ When the inside of the arm or lower extremities rests
upon a hard surface, the nerves may be compressed so as to deprive the
parts of sensibility. This condition is called "numbness."
857. _The quantity of blood supplied to the skin modifies its
sensitiveness._ If the quantity of blood is diminished, the
sensibility of the skin will be impaired. This is demonstrated by
noting the effects of cold upon the cutaneous tissue, the application
of which contracts the blood-vessels, and drives the circulating fluid
from this membrane, which is shown by the paleness, as well as by the
shrivelled appearance of the skin. And, if this tissue is wounded
while under the influence of cold, but little pain will be felt, and
this chilling influence may be carried so far as not only to deprive
the part of sensation, but of vitality.
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854. Mention other conditions that affect these nerves. 855. What is
the effect of the varying health or condition of the brain upon the
sensitiveness of the skin? Give instances of this effect. 856. What is
the result if a nervous trunk is divided or compressed? How may
"numbness" in the limbs be produced? 857. Does the quantity of blood
supplied to the skin affect its sensibility?
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858. The influence of the blood upon the sensibility of the skin, is
further demonstrated by the pain experienced when chilled extremities
are suddenly exposed to heat. The nerves, by the sudden dilatation of
the contracted blood-vessels, are put in vivid and rapid motion, which
causes the painful and tingling sensation that we experience. In every
part of the system, sudden changes produce unpleasant sensations, and
frequently a diseased condition of the organs.
_Observation._ When the hands, or other portions of the body, are
frozen, or severely chilled, safety and comfort demand that
circulation be restored to the parts by moderate exercise in a cool
room. Not unfrequently, the vitality of the limb is destroyed by
immersing it in hot water or holding it near the fire.
859. _The quality of the blood also influences sensation._ If the
brain and other parts of the nervous system receive impure blood,
their energy is depressed, and the sensibility of the skin rendered
more or less obtuse.
860. _The condition of the cuticle modifies the impression made upon
the cutaneous nerves._ 1st. When the cuticle has become thick and
hard, like horn, as on the inside of the mason's hand, it enables him
to ply his tools without much suffering, because the thickened cuticle
diminishes the impressions made upon the nerves.
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How is it demonstrated? 858. How is the influence of the blood upon
the skin further demonstrated? How should circulation be restored to
limbs frozen or severely chilled? What should be avoided? 859. Show
how the quality of the blood influences sensation. 860. Give the 1st
condition of the cuticle that influences the impressions made on the
cutaneous nerves.
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861. 2d. When the cuticle is very thin and delicate, as on the hand of
the lady who is unaccustomed to manual labor. Let her pursue some
manual employment for several hours, and the extreme tenuity, or
thinness of the cuticle, will not protect the nerves and parts below
from becoming irritated and inflamed.
862. 3d. When the cuticle is removed by blistering or abrasion, the
pain indicates that the naked nerves are too powerfully stimulated by
the contact of external bodies. 4th. When the cuticle is coated with
impurities, blended with the secretion from the oil-glands, the
sensibility of the skin is lessened.
863. _The sensibility of the cutaneous nerves is modified by being
habituated to impressions._ If, for example, an individual should
immerse his feet in moderately warm water, at first it might induce a
smarting sensation; in a short time, the nerves would not only become
habituated to the warm water, but its warmth night be considerably
increased. The same results follow, if an individual is exposed to a
cold element. The impressions at first are highly disagreeable; but as
soon as the nerves become accustomed to the surrounding atmosphere, it
may impart the most agreeable sensations.
_Illustration._ 1st. Let a person from the tropical regions go to a
colder climate, and the cool mornings of the latter will at first
affect him unpleasantly; but, after a few days' exposure to the cooler
air, the sensation will be far from disagreeable.
2d. Let a person enter a room moderately heated; gradually increase
the temperature, until it attains extreme summer heat; not only the
cutaneous nerves, but the whole system, become habituated to the high
temperature. From these facts we learn that the sensations, are not
always a correct index of the real temperature. A well-adjusted
thermometer will indicate it with unerring certainty.
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861. The 2d condition. 862. The 3d and 4th condition. 863. Show how
habit influences the sensibility of the cutaneous nerves. Give
illustration 1st. Illustration 2d.
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864. _Touch is modified, in a high degree, by education._ Thus the
blind, whose "windows of the soul" are closed to the beauties of the
external world, cultivate this sense to such a degree that they can
distinguish objects with great accuracy. And the rapidity with which
they read books prepared for their use, is a convincing proof of the
niceness and extent to which the cultivation of this sense can be
carried.
_Illustrations._ 1st. The cloth-dresser, by the aid of this sense,
distinguishes the quality, as well as the slightest difference of
texture, in the different pieces of cloth.
2d. The miller, from a similar education, quickly detects the quality
of flour or meal, by permitting it to pass between his fingers. The
difference in the texture of cloths, or the quality of the flour,
would not be distinguished by an individual whose tactile sense had
not been trained to make nice comparisons.
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864. Is this sense susceptible of improvement? What persons cultivate
it to a high degree? Give illustration 1st. Illustration 2d.
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