Keen’s method of ventricular puncture presents many advantages over other methods, though still open to the objection that the actual central puncture is done blindly. =Ventricular-subdural drainage.= A point is mapped out on the scalp which corresponds to the surface-marking of the descending horn of the lateral ventricle (see p. 3), and, with this point as a guide, a scalp-flap is framed, the base of which lies immediately below the indicated spot whilst its convexity is situated between 1¹⁄₂ and 2 inches above. This flap should not include the pericranium. The flap is turned down to its base, the pericranium stripped aside and a disk of bone removed, at the upper part of the exposed bone, with a ¹⁄₄ or ¹⁄₂ inch diameter trephine. The bone is usually very thin. [Illustration: A B FIG. 26. DIAGRAMMATIC REPRESENTATION OF THE AUTHOR’S OPERATION FOR HYDROCEPHALUS INTERNUS. A. The osteoplastic exposure of the brain (A, The bone; B, Upper two-thirds of trephine hole; C, Dura mater; D, The four dural flaps; E, Site of brain perforation; F, The brain; G, Line of fracture of bone-flap; H, The bone-flap; I, The Scalp; J, Lower third of trephine hole). B. Ventriculo-subdural drainage (_s._, The scalp; _b._, The bone; _d._, The dura mater; _v._, The lateral ventricle; _t._, The drainage medium between the ventricular cavity and the subdural space; _s.l.s._, Superior longitudinal sinus; _f.c._, Falx cerebri).] When this disk is removed, the dura is separated from the bone, and, with the aid of a strong pair of scissors, the bone is cut in such a manner as to form a bone-flap, the margins of which lie well within those of the scalp-flap (see Fig. 26). This flap is broken across at its base, turned down, and covered with gauze. At the lower portion of the exposed dura mater, a crucial incision is made through the dura mater and a blunt-pointed trocar and cannula introduced at the centre of the exposed brain, all visible vessels being avoided. The diagnosis is now confirmed--by the withdrawal of the trocar and the escape of cerebro-spinal fluid. [Illustration: FIG. 27. THE CONVERSION OF HYDROCEPHALUS INTERNUS INTO CEPHALOCELE.]