The patient should be in the recumbent position, the head well towards the end of the table. The operation is performed without an anæsthetic or under local anæsthesia. A site is chosen at the outer angle of the fontanelle, about 1 inch away from the median antero-posterior line, thus avoiding all possibility of injuring the superior longitudinal venous sinus. The trocar and cannula, of small size, is passed directly inwards, towards the base of the skull, for a distance of not more than 2 inches. The trocar is withdrawn and the fluid allowed to escape slowly. If the cerebro-spinal fluid escapes at high pressure, the flow should be regulated by the finger placed over the mouth of the cannula, and, in any case, it is inadvisable to allow of the withdrawal of more than 50 c.c. (approximately 1¹⁄₂ ounces) at one sitting. The cannula is withdrawn and the site of tapping covered with collodion gauze. Even when adopting all precautions the operation is not without danger, and, added to this, is the fact that few surgeons care about introducing an instrument blindly into the cerebral cortex--the risk of puncturing one of the distended superficial cerebral veins is sufficiently obvious. _Through the frontal bone._ Tillmanns, in recommending this procedure, states that ‘the needle should be inserted about 2 centimetres from the central line and 3 centimetres from the precentral sulcus. You strike the ventricle at a depth of from 3 to 5 centimetres’. He claims that this method leads to satisfactory results. It is open, however, to all the objections of puncture through the fontanelle. _Over the descending cornu of the lateral ventricle._ This operation is strongly recommended by Keen on the ground that excellent drainage is supplied. A point is mapped out on the skull which lies 1¹⁄₄ inches behind the external auditory meatus and the same distance above Reid’s base-line. If the postero-lateral fontanelle be open a small trocar and cannula may be introduced at the upper angle of the space--thus avoiding the lateral sinus--and passed inwards in a direction towards the summit of the opposite ear. If the fontanelle be closed, a scalp-flap is framed and a bone-disk removed with a ¹⁄₄¹⁄₂ inch diameter trephine. The dura should not be opened. The evacuating instrument is then introduced through the membrane in the same direction as before. In either case it should not be passed for a greater distance than 1¹⁄₂ inches, and, in all cases, the exploration should be of a progressive nature, that is to say, the trocar should be withdrawn once for each ¹⁄₂ inch of brain substance perforated. The escape of cerebro-spinal fluid must be regulated in the manner previously described. If trephining has been necessitated, the bone-disk is not replaced, thus allowing of subsequent tappings through the trephine-hole, this gap now taking the place of a patent postero-lateral fontanelle.