l
processes, and physical effects are involved in such a way that each
reenforces the others. The physical effect of the sleep instinct,
comparable with the pepsin secretion in the food instinct, or with the
hyperaemia of the sexual organs in the sexual instinct, is a change in
the cortex by which the sensory and motor brain centers are put out of
action. What kind of a change that is, is quite indifferent. It may be a
chemical one but more probably it is a circulatory one. Let us say it is
a contraction of blood-vessels which by the resulting anaemia makes the
sensory centers unfit for perception and the motor centers unfit for
action. In this way the brain becomes protected by sleep against the
demands of the surroundings. The mental reactions are eliminated and the
central nervous substance has an opportunity to build itself up. This
protective physical activity is now evidently itself controlled by a
subcortical center, just as secretion and sexual hyperaemia are
controlled. This center probably lies in the medulla oblongata.

Some theorists, to be sure, are inclined to think that the fatigued
brain cells enter directly through their exhaustion into the protective
sleep state. But that simplifies the situation too much. It is quite
true, as these theorists claim, that monotonous stimulation of the
senses produces sleep. But it is evident that the sleep occurs even then
not only in the particular overtired brain cells. A monotonous
stimulation of the acoustical center raises the threshold of perception
for all the senses and brings sleep to the whole brain. This control of
the whole apparatus is thus surely regulated by one definite center. But
this lower center, which controls the anaemia of the cortex, is itself
directly dependent again upon a mental condition, the mental experience
of fatigue. The fatigue sensation, which is possibly the result of toxic
processes, works on that lower sleep center, just as the appetizing
impression or the sensual images work on the centers of the other two
instincts. On the other hand this protective blood-vessel contraction
creates again as in the other cases a characteristic organic sensation,
the sensation of rest which arises when the threshold of perception and
activity is raised. The world begins to appear dim and far away, no
impulse for action excites us. This organic feeling of rest associates
itself with the fatigue feeling. The fatigue sensation, the subcortical
sleep ce