should be used in pure air._ The purer the air we
breathe, the more stimulating the blood supplied to the muscles, and
the longer they can be used in labor, walking, or sitting, without
fatigue and injury; hence the benefit derived in thoroughly
ventilating all inhabited rooms. For the same reason, if the air of
the sick-room is pure, the patient will sit up longer than when the
air is impure.

_Observation._ It is a common remark that sick persons will sit up
longer when riding in a carriage, than in an easy chair in the room
where they have lain sick. In the one instance, they breathe pure air;
in the other, usually, a confined, impure air.

187. _The muscles should be exercised in the light._ Light,
particularly that of the sun, exercises more or less influence on man
and the inferior animals as well as on plants. Both require the
stimulus of this agent. Shops occupied by mechanics, kitchens, and
sitting-rooms, should be well lighted, and situated on the sunny side
of the house. Cellar kitchens and underground shops should be avoided.
For similar reasons, students should take their exercise during the
day, rather than in the evening, and, as much as possible, laborers
should avoid night toil.

_Illustrations._ Plants that grow in the shade, as under trees, or in
a dark cellar, are of lighter color and feebler than those that are
exposed to the light of the sun. Persons that dwell in dark rooms are
paler and less vigorous than those who inhabit apartments well
lighted, and exposed to the rays of the sun.

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186. Why should the muscles be used in pure air? Give a common
observation. 187. What effect has light on the muscular system? What
should the laborer avoid? Why should not students take their daily
exercise in the evening? How is the influence of solar light
illustrated?

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188. _Exercise should be regular and frequent._ The system needs this
means of invigoration as regularly as it does new supplies of food. It
is no more correct that we devote several days to a _proper_ action of
the muscles, and then spend one day inactively, than it is to take a
_proper_ amount of food for several days, and then withdraw this
supply for a day. The industrious mechanic and the studious minister
suffer as surely from undue confinement as the improvident and
indolent. The evil consequences of neglect of exercise are gradual,
and steal slowly upon an individual. But soo