The infiltration of the tissue of the pia mater with serum, which I have just brought forward as one result of congestion, constitutes oedema of the membrane. It is most commonly chronic: it may be combined with the other changes which have been enumerated as consequences of repeated and continued congestion; and it may advance till the pia mater measures several lines in thickness, and the serum pours forth in large quantity, when the membrane is cut or torn off. This is especially the case in atrophy of the brain. The infiltered pia mater may be easily separated in large pieces from the brain.

(Edema generally involves the entire pia mater, but it reaches by far its greatest amount over the convexity of the cerebral hemispheres. It may affect merely a very small section of the membrane; and it is thus strictly local when only a part of the brain is atrophied: when the wasting is limited to a few of the convolutions, the oedematous pia mater hangs over them like a loose bag.

In those situations where the cerebral arachnoid is stretched separately and like a bridge over certain parts of the brain, most of the serum is poured out in the free space between it and the pia mater.

The discrimination of an acute oedema, as ever proving the fatal result of an acute congestion of the membranes, is attended with difficulties of precisely the same nature as those which surround the question about the existence of such a disease as serous apoplexy. (Edema of the pia mater is associated with that disease.