The physiological explanation of dreams must, it is plain, set out with
an account of the condition of the organism known as sleep. While there
is here much that is uncertain, there are some things which are fairly
well known. Recent physiological observation has gone to prove that
during sleep all the activities of the organism are appreciably lowered.
Thus, for example, according to Testa, the pulse falls by about
one-fifth. This lowering of the organic functions appears, under
ordinary circumstances, to increase towards midnight, after which there
is a gradual rising.

The nervous system shares in this general depression of the vital
activities. The circulation being slower, the process of reparation and
nutrition of the nerves is retarded, and so their degree of excitability
diminished. This is clearly seen in the condition of the peripheral
regions of the nervous system, including the sense-organs, which appear
to be but very slightly acted on by their customary stimuli.

The nervous centres must participate in this lethargy of the system. In
other words, the activity of the central substance is lowered, and the
result of this is plainly seen in what is usually thought of as the
characteristic feature of sleep, namely, a transition from vigorous
mental activity or intense and clear consciousness, to comparative
inactivity or faint and obscure consciousness. The cause of this
condition of the centres is supposed to be the same as that of the
torpidity of all the other organs in sleep, namely, the retardation of
the circulation. But, though there is no doubt as to this, the question
of the proximate physiological conditions of sleep is still far from
being settled. Whether during sleep the blood-vessels of the brain are
fuller or less full than during waking, is still a moot point. Also the
qualitative condition of the blood in the cerebral vessels is still a
matter of discussion.[72]

Since the effect of sleep is to lower central activity, the question
naturally occurs whether the nervous centres are ever rendered inactive
to such an extent as to interrupt the continuity of our conscious life.
This question has been discussed from the point of view of the
metaphysician, of the psychologist, and of the physiologist, and in no
case is perfect unanimity to be found. The metaphysical question,
whether the soul as a spiritual substance is capable of being wholly
inactive, or whether it is not in what seem the moment