Page:The Harveian Oration,1902.djvu/42

34 in bulk to the effused matter will be pressed out of the cranial cavity. It has been argued in opposition to this doctrine that the free ebb and flow of the cerebro-spinal fluid between the cerebral and spinal cavities must allow of considerable variation in the volume of blood in the cranium, but under normal conditions the amount of this fluid is so small as to be practically a negligible quantity. The variations in blood-supply extend for the most part only to the relative proportion between the arterial and venous. The brain fills the cranial cavity as a hand a glove, and is closely appressed to the interior of the skull-cap. The pressure exerted from within outwards varies greatly; directly with the venous, and only proportionately with the arterial pressure. It is increased with each pulsation of the hearty and with each expiration if at all forcible. The average is about lOO millimetres H^O, but the pressure is of importance purely in its bearing on the circulation. The functions of the brain cease if the arterial pressure is too low to cause an effective flow throughout the capillaries, or if the venous pressure is too high to permit of the arteries emptying them-