e beneficial; while, by travelling two miles, the exhaustion
may be highly injurious. Exercise and labor should be adapted to the
strength of particular individuals. How little soever the strength,
that must be the measure of exertion. Any other rule would be fatal to
the hopes of invigorating the system, either by exercise or labor.

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Is there a limit to the muscles becoming powerful by action? How may
the strength of muscles be kept until advanced age? 175. What is the
effect when exercise is continued until there is a feeling of
exhaustion? Give a practical illustration. What rule is mentioned in
regard to exercise? 176. Can all persons take the same amount of
exercise? What rule is given as to the amount of exercise?

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177. _Relaxation must follow contraction, or, in other words, rest
must follow exercise._ The necessity of relaxation, when a muscle has
been called into action, is seen in the example of a boy extending his
arm with a book in his hand, as a penalty. The boy can keep the arm
extended but a short time, make what effort he may. It is also seen in
the restlessness and feverish excitement that are evinced by persons
gazing on troops during days of review. The same is noted in shopping.
Such employments call into action the muscles that support the spinal
column in an erect position, and the languor or uneasiness is muscular
pain. The long-continued tension of a muscle enfeebles its action, and
eventually destroys its contractility.

178. _In school, the small children, after sitting a short time,
become restless._ If their position be changed, their imperfectly
developed muscles will acquire tone, and will again support the spinal
column erect without pain. The necessity for frequent recesses in
school, is founded on the organic law of muscular action alternating
with rest. The younger and feebler pupils are, the greater the
necessity for frequent recesses. We would not have the teacher think
that one half of the time should be spent in recesses; or the mother,
that her daughter is going to school to play. But we do maintain that
recesses should be given, and that they should be short and frequent,
especially for small and feeble scholars.

179. _Exhaustion is the inevitable result of continued muscular
contraction._ For example, let a lady ply the needle quickly for some
hours, and the muscles of the back and right arm will become
exhausted, wh