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.]
By the introduction of a bundle of horsehair or catgut, passed through the cannula so as to project into the ventricular cavity, and, after the withdrawal of the cannula, tucked, with respect to the proximal ends, into the subdural, extra-dural, or subaponeurotic spaces, it is obvious that drainage may be established between the ventricles and the other regions. Experience showed, however, that drainage into the subaponeurotic space usually converted the condition of hydrocephalus into one of cephalocele (see Fig. 27), the fluid collecting as a localized fluid tumour over the region of exploration, whilst extra-dural drains did not permit of sufficiently rapid reabsorption of fluid. Subdural drainage gave the best results, the cerebro-spinal fluid being brought into relation with the pia-arachnoid meshwork of vessels. It would, of course, be infinitely preferable if the ventricular fluid could be brought into direct relation with the veins of the _subarachnoid_ space, for the cerebro-spinal tension and venous pressure are equal, and all excess of cerebro-spinal fluid would be absorbed as soon as it is formed. This course is, however, impossible to carry out. We have, therefore, to rest content with less direct contact, drainage into the _subdural_ space. This ventricular-subdural drainage, as obtained by horsehair, catgut, and silk, apparently leads to but temporary benefit, probably owing to falling together of the brain substance and obliteration of the adventitious passage.