healthy men averages 110-120 mm. of mercury. In sleep, the pressure
may sink to 95-100 mm.; but if the pressure be taken of the same
subject lying in bed, and quietly engaged on mental work, it will be
found to be no higher. By mental strain or muscular effort, the
pressure is, however, immediately raised, and may then reach 130-140
mm. of mercury. It can be seen from considering these facts that the
fall of pressure is concomitant with rest, rather than with sleep. As,
moreover, it has been determined on strong evidence that the cerebral
vessels are not supplied with vasomotor nerves, and that the cerebral
circulation passively follows every change in the arterial pressure,
it becomes evident that sleep cannot be occasioned by any active
change in the cerebral vessels. This conclusion is borne out by the
fact that to produce in the dog a condition of coma like to sleep, it
is necessary to reduce, by a very great amount, the cerebral
circulation. Thus, both carotids and both vertebral arteries, can be
frequently tied at one and the same time without either producing coma
or any very marked symptoms. The circulation is, in such a case,
maintained through other channels, such as branches from the superior
intercostal arteries which enter the anterior spinal artery. While
total anæmia of the brain instantaneously abolishes consciousness,
partial anæmia is found to raise the excitability of the cortex
cerebri. By estimation of the exchange of gases in the blood which
enters and leaves the brain, it has been shown that the consumption of
oxygen and the production of carbonic acid in that organ is not large.
Further, it may be noted that the condition of anæsthesia is not in
all cases associated with cerebral anæmia. Thus, while during
chloroform anæsthesia the arterial pressure markedly falls, such is
not the case during anæsthesia produced by ether or a mixture of
nitrous oxide and oxygen.