_Technique._--The puncture may be made with the patient either lying on his left side, the spine being fully flexed by approximating the knees and shoulders; or sitting on the table with the knees drawn up and the body bent forward. The upper edge of the fourth lumbar spine is identified by drawing a horizontal line across the back at the level of the highest part of the iliac crests (Fig. 183). The space between the fourth and fifth lumbar vertebræ being the widest, is that usually selected. The skin having been purified, an exploring needle, about three inches long, is introduced about half an inch below the fourth lumbar spine in the middle line, and passed for about two inches in a direction forwards and slightly upwards. The needle usually encounters some resistance as it pierces the interspinous ligament, and then enters the sub-arachnoid space. If bone is struck, the needle should be withdrawn and introduced at a different level. If the cerebro-spinal fluid does not escape at once, a stylet should be passed through the needle to clear it of blood-clot or shreds of tissue. When the intra-thecal tension is normal, the fluid trickles away drop by drop, but if it is increased, as, for example, in meningitis, intra-cranial tumour, hydrocephalus, or uræmia, it may escape in a jet. [Illustration: FIG. 183.--Localisation of site for introduction of needle in Lumbar Puncture.] The _normal cerebro-spinal fluid_ is clear and colourless, has a specific gravity of 1004-1008, and contains a trace of serum globulin and albumose, some chlorides, and a substance which reduces Fehling's solution. Microscopically, it may contain some large endothelial cells and a few lymphocytes, 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 _Diagnostic Puncture._--Examination of the fluid withdrawn has proved useful in diagnosis in cases of intra-cranial and intra-spinal hæmorrhage, in various forms of meningitis, in cerebral abscess, and in some cases of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 hæmorrhage, 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 hæmorrhage there is no blood in the cerebro-spinal fluid.