control over the lower reflex centers in the spinal cord.

Thus, in sleeping dogs, after the spinal cord has been divided in the
dorsal region, reflexes can be more easily evoked from the lumbar than
from the cervical cord, because the former is freed from the
inhibitory control of the brain.

The strength of stimulus necessary to pass the threshold of
consciousness and to produce an awakening has been measured in various
ways. It has been determined that it takes a louder and louder sound
or a stronger and stronger electric shock to arouse a sleeper during
the first two or three hours of slumber; after that period, the sleep
becomes lighter and the required stimulus need be much less.

The alternative theories which have been suggested to account for the
onset of sleep may be classed as chemical and histological.

In relation to the first, it has been suggested that if consciousness
be regarded as dependent upon a certain rate of atomic vibration, it
is possible that this rate depends on a store of intramolecular
oxygen, which, owing to fatigue, may become exhausted; or it may be
supposed that alkaloidal substances may collect as fatigue products
within the brain, and choke the activity of that organ. Against this
theory may be submitted the facts that monotony of stimulus will
produce sleep in an unfatigued person, that over-fatigue, either
mental or bodily, will hinder the onset of sleep, that the cessation
of external stimuli by itself produces sleep. As an example of this
last, may be quoted the case recorded by Strumpel of a patient who was
completely anæsthetic save for one eye and one ear, and who fell
asleep when these were closed. Moreover, many men possess the power,