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The Monro-Kellie doctrine in its own context. In 1783, Alexander Monro secundus maintained that because the brain is enclosed in a case of bone and its substance is nearly incompressible, the quantity of blood within the head must be the same, or very nearly the same, at all times. Years later, this hypothesis was experimentally supported by George Kellie, and became known as the Monro-Kellie doctrine. However, this doctrine encountered resistance. Its reviewers have agreed to point out the historically understandable error of not having considered the CSF as a normal intracranial volume. Yet, almost nothing has been published about some of the physiological, pathophysiological, and therapeutic ideas prevailing at that time that may have influenced Monro's hypothesis. Lastly, and perhaps most importantly, it is not clear why Monro, who knew the ventricular system in detail, did not include it as a potential compensatory compartment for changes in the intracranial blood volume.