It has been a general rule with almost every modification of forceps, that the greatest distance between their blades should not be less than two inches and a half, for as this is the breadth of the basis cranii in the foetal head, it would be impossible to compress the head beyond this extent. The form of the head curvature will determine the situation of the point where the blades are most distant from each other: in some forceps it is about one-third the length of the blades from their extremities; in some it is nearly equidistant; whereas, in others it is nearer to the lock; the medium between these extremes is the best. The extremities of the blades ought to be at least half an inch apart: in this country they are usually somewhat more; on the Continent they are much less, being rarely more than one or two lines asunder. The fenestræ, or open spaces in the blades, should be wide and ample, for not only are the projecting parts of the head allowed to protrude between them, but the pressure of the blades is diffused over a larger extent of surface: this is remarkably seen in the forceps of the late Dr. Hopkins and that of Professor Davis, both of which are extensively used. It is also important that the edge at the extremities of the blades should be well rounded and not too thin; it is thus less liable to catch against corrugations either of the vagina or foetal scalp. The greatest breadth of the fenestræ is generally towards the extremities of the blades; in some, their edges are parallel; whereas, in those of Drs. Orme and Lowder the greatest breadth is near the lock: upon the whole, an oval shaped fenestra is the best, for it can be easily introduced, and has the advantages of a wide blade. In 1751 and the following year another curve was given to the blades of the forceps by the celebrated M. Levret of Paris, and by the equally distinguished Dr. Smellie of London, by which the instrument was adapted to the curve formed by the axes of the brim, cavity, and outlet of the pelvis, and by which the head could be seized much higher in the pelvis than by the straight forceps. Each have an equal claim to the merit of having invented this "pelvic curvature," as it has been called: the priority of the invention is perhaps due to Levret; but as he made a secret of it for some years, it is impossible to ascertain the precise fact. The pelvic curve, as it is called,[81] is especially adapted to the long forceps, which thus becomes an instrument of very considerable power. Numerous modifications of these curved forceps have since been made, but they are merely varieties of the original ones invented by Smellie and Levret, which have become the national instruments of their respective countries.