These conditions are somewhat variously associated with each other and with lesions of other kinds. They mostly imply local or general alterations in the volume of the blood and lymphatic fluid in the brain, and such alterations have somewhat complex relations, chiefly arising from the fact that the skull, in the adult at least, is a closed cavity with rigid walls, whose contents as a whole are scarcely capable of variation, although the fluid constituents are variously interchangeable.