I have already mentioned several of the conditions under which the substance of the brain becomes infiltrated with serum. This occurrence is very frequent, and varies much in degree.

A slight amount of it may be recognized by the unusual moisture, and the brilliancy of a cut surface of the white substance, while its consistence remains unaltered.

A higher degree is marked by a soft pasty state of the texture of the brain, which becomes loose like pap, manifestly in consequence of infiltration of the tissue with a large quantity of serum. The ©edematous white substance has a dull white appearance; in the atrophied brain of an old person it is of a dirty white, or fawn color.

When oedema reaches its highest degree, it breaks down the texture of the brain into a diffluent watery pap, from which serum escapes in large quantity. This so-called "white softening" (p. 270) is seen mostly in the neighborhood of the ventricles, and occasionally around tumors and inflamed portions of the organ. In such cases the rest of the brain also partakes of the oedema, though in a less degree.

(Edema of the brain may be developed rapidly, and quickly reach an advanced degree, or it may come on gradually and increase slowly. Instances of the former course are best marked in cases of acute hydrocephalus, in the cerebral substance surrounding the ventricles; and, as I have before remarked, it is seen occasionally in the cerebrum, near tumors, spots of inflammation, etc.

Chronic oedema of the brain is a frequent consequence of the most dissimilar chronic diseases, especially of phthisis; whether there be oedema in other parts, or not. It is very common in persons advanced in life, in whose atrophied brain it probably arises from the congestions resulting from vacuum within the skull. And it is equally common in insane persons, accompanying other marks of marasmus of the brain.

(Edema generally occupies the brain entirely, but not equally: it diffuses itself from some one spot, diminishing as it advances through the rest of the organ. The substance around the ventricles, when in the state of so-called "white softening," usually forms such a central spot (Concentrationsherd).

The enlargement of the brain to which oedema gives rise reaches its greatest degree when the amount of oedema is considerable, and the brain has lost but little of its volume by previous atrophy. In combination with the inconsiderable effusion which goes on into the ventricles, oedema is very often the principal cause of the swelling of the brain in acute hydrocephalus.

It destroys life, both by the pressure which this swelling exerts, and by the softening and breaking down of the substance of the brain in the spots where it is much advanced.

It is very probable, also, that in advanced age, after having slowly and gradually reached a certain relative degree of intensity, it occasions those common and unexpected deaths which simulate apoplexy.

As a general rule, hemorrhage does not take place in an oedematous brain.