A useful modification has been invented by Professor Naegelé, which supplies a considerable defect in the two above-mentioned instruments, viz. the necessity of using both hands to open the blades, thereby requiring that the hand which guides the instrument in the vagina should be removed at this moment: for this purpose the blades do not cross at the lock as the others do, by which means the grasp of one hand is sufficient to squeeze the handles together, and thus make the blades diverge in order to dilate the opening. A similar one has been invented by the surgical instrument maker, Mr. Weiss, but it does not appear to be quite so safe. The object of these instruments is not merely to bore through the skull, but to break down the parietal bone to a certain extent, in order to enlarge the opening: a slight curve of the blades is advantageous, because their points thus impinge more directly upon the skull, and enter it at once without running the risk of slipping along the surface. _Indications._ "The perforation is indicated, first, in all cases where the labour is dangerous for the mother, and where the antero-posterior diameter, although more than two inches and a half, is so small that the head which presents, cannot be delivered by the forceps. Secondly, it is indicated where the head is much larger than natural, as in hydrocephalus." (Naegelé, _MS. Lectures_.) For a more detailed and special account of the precise circumstances under which it will be required, we must refer to those different forms of DYSTOCIA, where it is occasionally required, particularly our fourth species, viz. DYSTOCIA PELVICA. Much discrepancy of opinion has existed as to how far the operation itself was justifiable, and has, therefore, given rise to very different results in the practice of different schools. The most obstinately prejudiced against perforation was the late celebrated Benjamin Osiander, of Göttingen, who asserted, that it was never necessary, for, where others were obliged to open the head, he would deliver the patient by means of his forceps, an instrument which, from its great length and the various hooks &c. for applying additional hands, was capable of exerting a degree of force which nothing could justify. In France, the predilection for using exceedingly powerful forceps to a degree, which in this country and the greater part of Germany would be looked upon as very injurious, if not dangerous, has tended to render the perforation a comparatively rare operation: thus out of somewhat more than twenty thousand labours at the Maternité, of Paris, only sixteen were delivered by this means. Of the ninety-six cases in whom the forceps was applied, no mention is made as to the result with respect to the mothers; but, from the description of a forceps case at the Hôtel Dieu which we have received from an eye-witness, the force used must have been carried to a most unwarrantable extent.