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 anaesthetic save for one eye and one ear, and who fell
asleep when these were closed. Moreover, many men possess the power,
by an effort of will, of withdrawing from objective or subjective
stimuli, and of thus inducing sleep.

The histological theories of sleep are founded on recent extraordinary
advances in the knowledge of the minute anatomy of the central nervous
system, a knowledge founded on the Golgi and methylene blue methods of
staining. It is held possible that the dendrites or branching
processes of nerve cells are contractile, and that they, by pulling
themselves apart, break the association pathways which are formed by
the interlacing or synapses of the dendrites in the brain. Ramon y
Cajal, on the other hand, believes that the neuroglia cells are
contractile, and may expand so as to interpose their branches as
insulating material between the synapses formed by the dendrites of
the nerve cells. The difficulty of accepting these theories is that
nobody can locate consciousness to any particular group of nerve
cells. Moreover, the anatomical evidence of such changes taking place
is at present of the flimsiest character.

If these theories be true, what, it may be asked, is the agency that
causes the dendrites to contract or the neuroglia cells to expand? Is
there really a soul sitting aloof in the pineal gland, as Descartes
held? When a man like Lord Brougham can at any moment shu