Expansion of the cerebrospinal-fluid space is a new conception introduced in view of the fact that in the closed cranial cavity in case of an increase in cerebrospinal fluid the relation volume/pressure undergoes characteristic changes. The physical laws resulting from this have already been considered in the Monro-Kellie doctrine and concern the three main parts of the cranial cavity: brain, cerebrospinal fluid, and vessels. The capability of compensation, the compliance and the elasticity or elastantia of the cerebrospinal-fluid space can be measured on the basis of the dynamic changes proceeding in the space, the pressure/volume index (PVI) being a measure of the degree of filling of the intracranial space.