The arterial pressure of man is not lowered by the ordinary fatigue of
daily life. It is only in extreme states of exhaustion that the
pressure may be found decreased when the subject is in the standing
position. The fall of pressure which does occur during rest or sleep
is mainly occasioned by the diminished rate of the heart. The increase
in the volume of the limbs is to be ascribed to the cessation of
muscular movement and to the diminution in the amplitude of
respiration. The duty of the heart is to deliver the blood to the
capillaries. From the veins the blood is, for the most part, returned
to the heart by the compressive action of the muscles, the constant
change of posture and by the respiration acting both as a force and
suction pump. All of these factors are at their maximum during bodily
activity and at their minimum during rest. On exciting a sleeper by
calling his name, or in any way disturbing him, the limbs, it has been
recorded, decrease in volume while the brain expands. This is so
because the respiration changes in depth, the heart quickens, the
muscles alter in tone, as the subject stirs in his sleep in reflex
response to external stimuli. Considering all these facts, we must
regard the fall of arterial pressure, the depression of the
fontanelle, and the turgescence of the vessels of the limbs as
phenomena concomitant with bodily rest and warmth, and we have no more
right to assign the causation of sleep to cerebral anæmia than to any
other alteration in the functions of the body, such as occur during
sleep.

We may well here summarize these other changes in function:

(1) The respiratory movement becomes shallow and thoracic in type.

(2) The volume of the air inspired per minute is lessened by one-half