The so-called Monro-Kellie doctrine states that with an intact skull, the sum of the volume of the brain plus the cerebrospinal fluid volume plus the intracranial blood volume is constant. Therefore an increase in one should cause a reduction in one or both of the remaining two. The researcher who expressed the doctrine in such a way was indeed Harvey Cushing in 1925, during a lecture in Edinburgh. The original Monro-Kellie doctrine is the result of observations on autopsies and several animal experiments. What the original doctrine states is a dynamic explanation of the intracranial system, aimed at explaining how a pulsatile and continuous perfusion may occur in a closed-non-expandable and fully filled system. During each cardiac cycle the quantity of blood within the head must be the same: during the systole, the brain arteries dilate, and, in the mean time, a quantity of blood, equal to that which is dilating them, is passing out of the head through the veins. During the succeeding diastole, the quantity which dilated the brain arteries passes into the corresponding veins and, at the same time, as much passes from the sinuses out of the head, as enters into the head from the arteries situated between the heart and the head. Monro implicitly states that the blood coming out from the cerebral veins into the sinuses must be pulsatile and (almost) synchronous with the arteries. That is deeply different from the concept of a constant content of a rigid case, as expressed by Cushing.
Monro-Kellie doctrine, intracranial system, cerebral blood flow, cerebral autoregulation, intracranial veins.