When we use the eye too
long, or in too bright a light, it becomes bloodshot. The increased
action of its vessels and nerves gives rise to a sensation of fatigue
and pain, requiring us to desist. If we relieve the eye, the
irritation gradually subsides and the healthy state returns. But, if
we continue to look intently, or resume our employment before the eye
has regained its natural state by repose, the irritation at last
becomes permanent, and disease, followed by weakness of vision, or
even blindness, may ensue.

809. Phenomena precisely analogous occur, when, from intense mental
excitement, the brain is kept long in a state of excessive activity.
The only difference is, that we can always see what happens in the
eye, but rarely what takes place in the brain; occasionally, however,
cases of fracture of the skull occur, in which, part of the bone being
removed, we can see the quickened circulation in the vessels of the
brain, as easily as those of the eye.

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807. What is one great evil attending the absence of some imperative
employment to exercise the mind and brain? What is the true remedy for
these evils? 808. From what other cause do evils arise to the brain?
Explain the evil of it by the excessive use of the eye. 809. What is
the only difference in the analogy of the phenomena of the eye and
brain? Has the analogy been verified?

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810. Sir Astley Cooper had a young man brought to him, who had lost a
portion of his skull, just above the eyebrow. "On examining the head,"
says Sir Astley, "I distinctly saw that the pulsation of the brain was
regular and slow; but at this time he was agitated by some opposition
to his wishes, and directly the blood was sent with increased force to
the brain, and the pulsation became frequent and violent."

811. Indeed, in many instances, the increased circulation in the
brain, attendant on mental excitement, reveals itself when least
expected, and leaves traces after death, which are very perceptible.
When tasked beyond its strength, the eye becomes insensible to light,
and no longer conveys any impressions to the mind. In like manner, the
brain, when much exhausted, becomes incapable of thought, and
consciousness is almost lost in a feeling of utter confusion.

812. _At any time of life, excessive and continued mental exertion is
hurtful_; but in infancy and early youth, when the structure of the
brain is still immature