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SLEEP AND THE THEORIES OF ITS CAUSE.


The theory of the origin of sleep which has gained the widest credence
is the one that attributes it to anæmia of the brain. It has been
shown by Mosso, and many others, that in men with defects of the
cranial wall the volume of the brain decreases during sleep. At the
same time, the volume of any limb increases as the peripheral parts of
the body become turgid with blood. In dogs, the brain has been
exposed, and the cortex of that organ has been observed to become
anæmic during sleep. It is a matter of ordinary observation that in
infants, during sleep, the volume of the brain becomes less, since the
fontanelle is found to sink in. It has been supposed, but without
sufficient evidence to justify the supposition, that this anæmia of
the brain is the cause and not the sequence of sleep. The idea behind
this supposition has been that, as the day draws to an end, the
circulatory mechanism becomes fatigued, the vasomotor center
exhausted, the tone of the blood vessels deficient, and the energy of
the heart diminished, and the circulation to the cerebral arteries
lessened. By means of a simple and accurate instrument (the
Hill-Barnard sphygmometer), with which the pressure in the arteries of
man can be easily reckoned, it has been recently determined that the
arterial pressure falls just as greatly during bodily rest as during
sleep. The ordinary pressure of the blood in the arteries of young and