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 demon