quires a due supply of pure blood._ This organ
receives an unusually large supply of blood, in comparison with the
rest of the body. It is estimated that one tenth of all the blood sent
from the heart goes to this organ. If the arterial blood be altogether
withdrawn, or a person breathes air that is filled with carbonic gas,
the brain ceases its proper action, and sensibility with consciousness
becomes extinct.

_Illustrations._ 1st. If a person lose a considerable quantity of
blood, dizziness and loss of consciousness follow. This results from
the brain not receiving a sufficient amount of blood to sustain its
functions.

2d. When an individual descends into a well or pit that contains
carbonic acid, the blood is not changed or purified in the lungs, and
loss of consciousness and death soon follow.

799. The slighter variations in the state of the blood have equally
sure, though less palpable effects. If its vitality is impaired by
breathing an atmosphere so much vitiated as to be insufficient to
produce the proper degree of oxygenation, the blood then affords an
imperfect stimulus to the brain. As a necessary consequence, languor
and inactivity of the mental and nervous functions ensue, and a
tendency to headache, fainting, or hysteria, makes its appearance.

_Observations._ 1st. Let a person remain, for a time, in a crowded,
ill-ventilated, hall or church, and headache or faintness is generally
produced. This is caused by the action of impure blood upon the
brain.

2d. If a school-teacher wishes to have his pupils, on the day of
examination, appear creditably, he will be careful to have the room
well ventilated. Ventilating churches might prevent the inattention
and sleepiness that are observed during the afternoon service.

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798. Why does the brain require a due supply of pure blood? What is
the effect when a person loses a considerable quantity of blood? What
causes the loss of consciousness when carbonic acid is breathed? 799.
What effects are produced by slight variations in the quality of the
blood? From the following observations, give some of the effects of
impure blood on the brain.

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3d. In many instances, the transmission of imperfectly oxygenated
blood to the brain, is an influential cause in the production of
nervous disease and delicacy of constitution. The only efficient
remedy for these conditions is a supply of pure blood to the brain.