r--a condition which is usually due to the spread of
infection from a localised osseous lesion, such as erosion of the
tegmen tympani in chronic suppuration of the middle ear, of the wall
of the sigmoid groove in mastoid disease, or of the posterior wall of
the frontal sinus in suppuration of that cavity. It also occurs in
relation to septic lesions of the cranial bones such as a broken-down
gumma, after operations on the cranial bones, and in cases of compound
fracture attended with a mild degree of infection and with imperfect
drainage. In contusion of the skull without an external wound, the
infection may take place through the blood stream.

The layer of the dura in contact with the affected portion of
bone is inflamed, thickened, and covered with a layer of
granulations--_external pachymeningitis_--and between it and the bone
there is an effusion of fluid. Up to this point the process is largely
protective in its effects, and gives rise to no symptoms, beyond
perhaps some pain in the head.

In the majority of cases, however, suppuration occurs between the dura
and the bone--_suppurative pachymeningitis_--and leads to the
formation of an _extra-dural abscess_ (Fig. 192). When this happens
in association with disease in the middle ear or frontal sinus, it is
attended with severe headache referred to the seat of the abscess, a
sudden rise of temperature preceded by shivering, and other evidence
of the absorption of toxins. Over the situation of the abscess, the
scalp becomes swollen and oedematous--a condition which Percival Pott,
in 1760, first observed to be characteristic of extra-dural
suppuration, hence the name, _Pott's puffy tumour_, applied to it
(Fig. 193). Under these circumstances the abscess is seldom of
sufficient size to cause a marked increase in the intra-cranial
tension, or to give rise to localised cerebral symptoms by pressing on
the brain.

[Illustration: FIG. 192.--Diagram of Extra-Dural Abscess.]

[Illustration: FIG. 193.--Pott's Puffy Tumour in case of extra-dural
abscess following compound fracture of orbital margin; infected with
road-dust; operation; recovery. At the time of the photograph the man
was unconscious.]

When associated with a punctured wound implicating the skull, an
extra-dural abscess may develop within a few days of the injury, or
not till after the lapse of several weeks, and it may spread over a
wide area and come to encroach on the cranial cavity sufficiently to
rais