e the intra-cranial tension and cause symptoms of compression, or
even to press upon cortical centres and produce localised paralyses.
As discharge can escape from the wound in the scalp, the puffy tumour
does not necessarily form.

_Treatment._--When the abscess is secondary to middle ear disease, the
mastoid must be opened, the eroded bone exposed, and sufficient of it
removed with rongeur forceps to admit of free drainage. When the
infection has spread from the frontal sinus, the skull is trephined in
the frontal region, the precise site being indicated by the oedematous
area in the scalp, and the diseased bone is removed. In cases of
compound fracture, drainage is established by enlarging the scalp
wound, and removing loose, depressed, or inflamed portions of bone; if
the bone is comparatively intact, it must be trephined, and further
bone is removed with rongeur forceps over the entire area in which the
dura has been separated.

#Lepto-meningitis.#--If the infection spreads to the adjacent
arachno-pia (_localised lepto-meningitis_), adhesions usually form,
and shut off the infected area from the general arachno-pial space.

Pus may form among these adhesions, constituting a _sub-dural
abscess_, and may infiltrate the superficial layers of the cortex
(_purulent encephalitis_, or _meningo-encephalitis_) (Fig. 194). The
symptoms are similar to those of extra-dural abscess, but may be more
severe; and it is seldom possible to distinguish between them before
exposing the parts by operation. The treatment is carried out on the
same lines.

[Illustration: FIG. 194.--Diagram of Sub-Dural Abscess.]

_Acute General Lepto-Meningitis._--In bone lesions, particularly
compound fractures, infection of the arachno-pia may take place
before protective adhesions form, and a diffuse lepto-meningitis
results. The open structure of the arachno-pial membrane favours the
rapid spread of the infection, which may extend over the surface of
the hemispheres, or downwards towards the base (_basal meningitis_),
or in both directions. The process is at first attended with a copious
effusion of cerebro-spinal fluid into the arachno-pial space and into
the ventricles (_serous lepto-meningitis_), but this fluid tends to
become purulent, the pus forming in a thin layer over the surface of
the brain, and in the sulci between the convolutions (_purulent
lepto-meningitis_). The membranes are congested and thickened, the
veins of the arachno-pia