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 anaemia of the brain instantaneously abolishes consciousness,
partial anaemia 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 anaesthesia is not in
all cases associated with cerebral anaemia. Thus, while during
chloroform anaesthesia the arterial pressure markedly falls, such is
not the case during anaesthesia produced by ether or a mixture of
nitrous oxide and oxygen.

The arterial pressure of man is not lowered by the ordinary fatigue of
daily life. It is only in extreme states of exhaustion that the
pressure may be found decreased when the subject is in the standing
position. The fall of pressure which does occur during rest or sleep
is mainly occasioned by the diminished rate of the heart. The increase
in the volume of the limbs is to be ascribed to the cessation of
muscular movement and to the diminution in the amplitude of
respiration. The duty of the heart is to deliver the blood to the
capillaries. From the veins the blood is, for the most part, returned
to the heart by the compressive action of the muscles, the constant
change of posture and by the respiration acting both as a force and
suction pump. All of these factors are at their maximum during bodily
activity and at their minimum during rest. On exciting a sleeper by
calling his name, or in any way disturbing him, the limbs, it has been
recorded, decrease in volume while the brain expands. This is so
because the respiration changes in depth, the heart quickens, the
muscles alter in tone, as the subject stirs in his sleep in reflex
response to external stimuli. Considering all these facts, we must
regard the fall of arterial pressure, the depression of the
fontanelle, and the turgescence of the vessels of the limbs as
phenomena conc