The skin of the back is carefully sterilised; painting with tincture of iodine serves admirably. The ampoule containing the tropacocaine is opened, and the drug sucked into the syringe through a spare cannula. The loaded syringe is then placed on a sterile towel at the back of the patient. With a little practice there is no difficulty in making the lumbar puncture. The fourth lumbar spine is located by noting the level of the highest point on the iliac crest--this may be indicated by an assistant. A line joining the highest points on the two iliac crests will pass through the tip of the fourth lumbar spine. When this process has been carefully identified, the needle is introduced half-an-inch to one side of the median plane and midway between the third and fourth spine. Some surgeons prefer to go in exactly in the middle line to make sure of entering the middle of the subarachnoid space, but in this position the tough supra-spinous and interspinous ligaments are met with, and to avoid the resistance of these it is best to keep a short distance out from the median plane. By carefully noting the direction of the needle, the cysterna terminalis can always be entered. The needle is passed forwards, very slightly upwards, and slightly medially so as to hit off the centre of the subarachnoid space. As the needle passes through the ligamentum flavum, there is a sudden diminution of resistance and immediately afterwards the point of the needle lies in the subarachnoid space. The passage of the needle through the membranes is sometimes accompanied by a slight pricking pain. The stylet is withdrawn at this stage and the cerebro-spinal fluid usually trickles out drop by drop. The syringe is picked up, carefully emptied of air bubbles, and fitted on to the needle. The piston is withdrawn until the syringe is filled with cerebro-spinal fluid, which mixes freely with the anæsthetic solution, and the contents then slowly injected. The 10 c.cm. syringe is to be preferred for this purpose as it is essential to mix the tropacocaine thoroughly with the cerebro-spinal fluid. If the smaller syringe is used, it should be refilled with cerebro-spinal fluid and emptied a second time so as to ensure thorough diffusion of the drug. The needle is then withdrawn and the puncture sealed with collodion. The injection should never be made until a free flow of cerebro-spinal fluid is obtained, since this is the only certain indication that the needle has entered the subarachnoid space. If failure is met with in the space between the third and fourth spines, the interspinous space above or below should be tried.