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INTRACRANIAL COMPLICATIONS                     355

further intracranial complications, e.g. sinus thrombosis, brain abscess or


In order to understand the changes which occur in meningitis it is necessary
to discuss briefly the secretion, circulation and absorption of the cerebro-
spinal fluid. The fluid is secreted mainly by the choroid plexuses of the
cerebral ventricles. Starting in the lateral ventricles, the fluid flows through
the foramen of Monro to reach the third ventricle. Here its volume is aug-
mented by the choroid plexus of that cavity. It then passes through the
aqueduct of Sylvius into the fourth ventricle. The fluid now passes out
into the cisterna magna of the subarachnoid space through the minute
foramina of Magendie and of Luschka, which perforate the lower part of
the ventricular roof, and permeates upwards and forwards round the sides
of the medulla and over the cerebellar hemispheres. It also passes through
the opening in the tentorium and over the cerebral hemispheres. The fluid
also passes caudally into the spinal theca—the most convenient anatomical
site at which to obtain fluid for investigation, i.e. by lumbar puncture. The
cisterna pontis and cisterna lateralis—which contain the seventh and eighth
nerves—communicate freely with the cisterna magna. Absorption of cerebro-
spinal fluid takes place almost entirely from the subarachnoid space overlying
the cerebral hemispheres. It will thus be seen that a meningitis commencing
on the posterior surface of the petrous bone tends to be confined, for a
time at any rate, below the tentorium and only spreads slowly upwards
along the Sylvian fissure.

The cells of the choroid plexus form a selective barrier between the blood
and the cerebrospinal fluid, allowing certain substances, e.g. chlorides, to
pass freely and keeping back others, e.g. protein. When, however, there is
widespread meningeal inflammation, the barrier between the blood and
fluid becomes less complete and the chemical composition of the cerebro-
spinal fluid approximates to that of the blood. Glucose in the cerebrospinal
fluid tends to disappear, due to the action of bacteria. So long as the menin-
gitis is limited to a small area, the percentages of the inorganic salts in the
fluid remain unaltered, even though the fluid be turbid with pus cells and
contains an excess of protein, whereas when the meningitis becomes more
widespread, the chlorides are diminished.

Meningitis may be serous or purulent, localized or diffuse, and may run
an acute or a chronic course. It may be the result of either acute or chronic
suppuration in the ear. The infection may pass through an erosion of the
bone situated in the groove of the transverse sinus, or the posterior surface
of the petrous bone—more rarely in the roof of the middle ear. It may also
arise, in the absence of defects in the bone, through the inner ear and internal
auditory meatus and perilymphatic aqueduct or along the sheaths of the
vessels which run between the middle ear and in the meninges, e.g. those
which pass from the inner wall of the tympanic antrum to the fossa sub-
arcuata and posterior cranial fossa. Lastly, infection may come through
the blood stream in septicaemic cases.

There are two degrees of meningitis: (1) in which the organisms have not
reached the internal surface of the arachnoid, e.g. extradural abscess; and