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 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