that the increased rapidity of the return of blood
through the veins, is, of itself, a sufficient cause for the
accelerated movements of the heart during active exercise.

_Observation._ The quantity of blood in different individuals varies.
From twenty-five to thirty-five pounds may be considered an average
estimate in a healthy adult of medium size. The time in which the
blood courses through the body and returns to the heart, is different
in different individuals. Many writers on physiology unconditionally
limit the period to three minutes. It is undeniable that the size and
health of a person, the condition of the heart, lungs, and brain, the
quantity of the circulating fluid, the amount and character of the
inspired air, and the amount of muscular action, exert a modifying
influence. The time probably varies from three to eight minutes.

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366. What causes the accelerated movements of the heart during active
exercise?

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_Note._ Let the pupil review the anatomy and physiology of the
circulatory organs from fig. 75, or from anatomical outline plates,
No. 6 and 7.

CHAPTER XIX.

HYGIENE OF THE CIRCULATORY ORGANS

367. If any part of the system is deprived of blood, its vitality will
cease; but, if the blood is lessened in quantity to a limited extent,
only the vigor and health of the part will be impaired. The following
conditions, if observed, will favor the free and regular supply of
blood to all portions of the system.

368. _The clothing should be loosely worn._ Compression of any kind
impedes the passage of blood through the vessels of the compressed
portion. Hence, no article of apparel should be worn so as to prevent
a free flow of blood through every organ of the body.

369. The blood which passes to and from the brain, flows through the
vessels of the neck. If the dressing of this part of the body is
close, the circulation will be impeded, and the functions of the brain
will be impaired. This remark is particularly important to scholars,
public speakers, and individuals predisposed to apoplexy, and other
diseases of the brain.

370. As many of the large veins lie immediately beneath the skin,
through which the blood is returned from the lower extremities, if the
ligatures used to retain the hose, or any other article of apparel, in
proper position, be tight and inelastic, the passage of blood through
these vessels will be obstructed,