ion of brain
substance through an acquired opening in the skull and dura mater,
such as may result from a compound fracture or a gun-shot wound. The
protrusion is due to increased intra-cranial tension, and is almost
invariably associated with infection of the brain and its membranes,
and with the presence of a foreign body or fragments of bone. Other
things being equal, a hernia is more likely to occur through a small
than through a large opening in the skull.

So long as the extruded portion of brain matter is small, it pulsates,
but as it increases in size and is pressed upon by the edges of the
opening through which it escapes, the pulsation ceases, and the
herniated portion may become strangulated and undergo necrosis.

In cases of compound fracture, and in other conditions associated with
necrosis of bone, masses of redundant granulation tissue growing from
the soft parts outside the skull may simulate a hernia cerebri.

The _treatment_ consists in counteracting the septic infection by
purifying the protruding mass, and if necessary by enlarging the
opening in the skull with rongeur forceps to admit of the removal of
foreign bodies or bone fragments and to relieve the inter-cranial
tension. Steps must also be taken to prevent meningitis, which, if it
occurs, is usually fatal. Pressure over the hernia, with the object
of returning it to the skull, is to be avoided, and the herniated
portion should not be cut away unless it is sloughing, or has become
pedunculated. It may be got rid of by painting it with 40 per cent.
formalin, which causes a dry, horny crust to form on the surface; this
is picked off, and the formalin re-applied.

After the hernia has disappeared and the wound is aseptic, steps
should be taken to close the gap in the skull. This may be done by an
osteo-plastic operation in which a flap, comprising a segment of the
outer table, is raised from an adjacent part of the skull and placed
in the gap; or by transplanting a portion of periosteum-covered bone
from the scapula, tibia, or other suitable source. An alternative
method is to implant a plate of celluloid, silver or other metal, or a
portion of the fascia lata, in the gap. When a permanent hole is left
in the bone, the patient should wear over it a leather or metal shield
to protect the brain.

The protrusion of brain resulting after a decompression operation
deliberately performed for the relief of intra-cranial tension, unless
it becomes in