001 Citation: Anile C, Ficola A, Santini P (2021) The intracranial system: A new interpretation of the Monro-Kellie doctrine. Arch Anat Physiol 6(1): 001-007. DOI: https://dx.doi.org/10.17352/aap.000016 http://dx.doi.org/10.17352/aap DOI: 2640-7957 ISSN: CLINICAL GROUP CLINICAL GROUP Introduction With the term “intracranial system” we define the complex system constituted by the brain parenchyma (including the vasculature and the cerebrospinal fluid [CSF]) and its envelopes, the dura mater, and the skull. The so-called Monro-Kellie [1] doctrine states the rule to which obeys this structure, that is the absolute constancy of the intracranial volume. The presence of CSF was ignored by Monro and Kellie; their doctrine was later enunciated by Cushing in 1925 [2] and formulated as we know today as the sum of the volume of the brain plus the CSF volume plus the intracranial blood volume is constant. This conclusion was different from this stated by original doctrine of Monro and Kellie in which they affirm: “For, whilst the heart is performing its systole, the arteries here, as elsewhere, may be dilating, and in the meantime, a quantity of blood, equal to that which is dilating them, is passing out of the head by the veins” [1]. This statement confirms not a static relationship between a closed box and its content, but a dynamic approach to explain the relationship between a pulsatile and continuous perfusion and a closed, totally filled, and non-expandable case of bone [3]. This doctrine implicitly affirms that the quantity of blood coming in and out from the brain is the same and that the blood coming out from the brain must be pulsatile and synchronous with arteries, in order to maintain constant the volume of blood. This conclusion is in obvious conflict with the assumptions made by a paper [4] in which the authors propose a continuous monitoring of the Monro-Kellie doctrine. They affirm: “1) The cross-sectional area of the insonated artery remains “constant” during the cardiac cycle (Toth et al 2000); and 2) The low-pulsatile venous outflow can be written as an averaged arterial inflow (Avezaat and van Eijndhoven, 1984)”. These assumptions appear to be very extreme “simplifications” of well-established notions as enunciated firstly by Starling in 1912 [5] and demonstrated later by Aaslid in normal humans [6] and by us in pigs [7] and in craniectomized decompressed humans [8]. Abstract In this paper we have reported our experience in several years in whom we have studied the phenomena correlated with the intracranial pressure. We have started to study different types of animals during an experimental condition of intracranial hypertension; we have observed many different configurations in spontaneous pathologic humans, and, eventually, in last years we have passed to confirm our ideas modelling an intracranial system in a physical phantom made by a centrifugal pump, stainless steel for base, glass for the wall, elastic tube for the cerebrovascular tree and collapsible tubes for venous drainage. We have noted that there is some confusion about the so-called Monro-Kellie doctrine; the major part of the scientist believe in a sort of static interpretation given by the sum of the compartment which constitute the intracranial system (parenchyma, blood and cerebrospinal fluid). But we believe in a dynamic interpretation in which there is a constant balance between the arterial inflow and venous outflow during each cardiac cycle. In this paper we have focused on the results, obtained in the different preparations, animals, humans and in vitro, all confirming the assumption that there is no difference between arterial inflow and venous outflow. Research Article The intracranial system: A new interpretation of the Monro- Kellie doctrine Anile Carmelo 1 *, Ficola Antonio 2 and Santini Pietro 3 1 Department of Neuroscience, Catholic University of Sacred Heart, Italy 2 Department of Engineering, University of Perugia (retired), Perugia, Italy 3 Policlinico Gemelli Foundation IRCCS, Rome, Italy Received: 07 April, 2021 Accepted: 16 April, 2021 Published: 17 April, 2021 *Corresponding author: Anile Carmelo, Full Professor (retired), Department of Neuroscience, Catholic Univer- sity of Sacred Heart, Italy, Tel: +39336248377, E-mail: Keywords: Intracranial pressure; Arterial inflow; Venous outflow; Starling’s resistor https://www.peertechzpublications.com