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 shut himself
away from the outer world and fall asleep, does his soul break the
dendritic contacts between cell and cell; and when he awakes, does it
make contacts and switch the impulses evoked by sense stimuli on to
one or other tract of the axons, or axis cylinder processes, which
form the association pathways? Such a hypothesis is no explanation; it
simply puts back the whole question a step further, and leaves it
wrapped in mystery. It cannot be fatigue that produces the
hypothetical interruptions of the dendritic synapses and then induces