In all sanguineous animals membranes are found. And membrane
resembles a thin close-textured skin, but its qualities are different,
as it admits neither of cleavage nor of extension. Membrane envelops
each one of the bones and each one of the viscera, both in the
larger and the smaller animals; though in the smaller animals the
membranes are indiscernible from their extreme tenuity and minuteness.
The largest of all the membranes are the two that surround the
brain, and of these two the one that lines the bony skull is
stronger and thicker than the one that envelops the brain; next in
order of magnitude comes the membrane that encloses the heart.