s, or may be entirely devoid of cells. It does not
contain the antitoxins and opsonins which are normally found in the
plasma and lymph, hence the liability to infective meningitis after
injuries and operations on the central nervous system. With a view to
diminishing these risks, hexamine, which is excreted into the
cerebro-spinal fluid, is administered for its antiseptic properties in
cases of head injury and before intra-cranial operations.

_Diagnostic Puncture._--Examination of the fluid withdrawn has proved
useful in diagnosis in cases of intra-cranial and intra-spinal
haemorrhage, in various forms of meningitis, in cerebral abscess, and
in some cases of cerebral tumour.

The first few drops should be discarded, as they may be stained with
blood from the puncture, and about 5 c.c. collected in each of two
sterile tubes. To determine whether blood in the fluid is due to the
puncture or to a pre-existing intra-cranial or intra-thecal
haemorrhage, the fluid should be centrifugalised; in the former case
the supernatant fluid is clear and limpid, in the latter it retains a
yellow tinge. In extra-dural haemorrhage there is no blood in the
cerebro-spinal fluid.

In acute meningitis the fluid is turbid, and contains an excess of
albumin. Organisms also are present, such as the diplococcus
intracellularis in acute cerebro-spinal meningitis; staphylococci,
streptococci, and pneumococci, particularly in the intra-cranial
complications of middle ear disease. In all cases of acute microbic
infection, and especially in the suppurative forms, polynuclear
leucocytes are found in the fluid; while in chronic affections, such
as tubercle and syphilis, there is an excess of lymphocytes (Purves
Stewart). The detection of the tubercle bacillus is confirmatory of a
diagnosis of tuberculous meningitis, but, as it is often difficult to
find, its absence does not negative this diagnosis. In tuberculous
meningitis the clot which forms floats in the centre of the fluid, and
is translucent, grey, and flaky; in the pyogenic forms it is yellow,
and sticks to the side of the vessel.

In a few cases of malignant tumour of the spinal cord and its
membranes, characteristic cells have been found in the fluid after
centrifugalising.

In uraemia there is a diminution of chlorides, and an increase of
phosphates and sulphates.

The Wasserman test is sometimes positive in the cerebro-spinal fluid,
when it is negative in the blood.

_Therapeutic