Thus it is then that, particularly at the superior strait, the forceps is lacking as a tractor, and this is why Hubert, Morals, Hermann and Hart mann endeavored to modify the instrument, so as to enable traction to be made in the axis of the pelvic strait. Tarnier's forceps, with its indicator needle and its traction rods, constitutes, to a degree, real progress in this direction. Unfortunately it is, in particular, in the cavity and at the inferior strait that this forceps can be used to the greatest advantage, and here the classic forceps can exert traction better in the axis of the pelvis.
At the superior strait, above all higher, the classic forceps and that of Tarnier present the same disadvantages. Whatever the instrument, it cannot make traction backwards. But with Levret's forceps we are aware of resistance offered, we can vary the direction of traction, we are con scious of all the force we employ, and of the results. Nothing of the kind with Tarnier's instrument. It is the needle alone which guides the ac eoucheur, it is it alone which registers the amount of traction. Do we make traction in the pelvic axis? Not at all at the superior strait; only a trifle in the cavity and at the inferior strait, since it is impossible to know the mathematical axis of the pelvis. Nevertheless, the younger French practitioners are daily making greater use of the Tarnier instru ment. In England, Alexander Russell Simpson reported on this instru before the Edinburgh Obstetrical Society in July, 1884. He has adapted the principle of axis traction to Simpson's forceps.
[In this country the opinion of those who have used Tarnier's forceps is not in agreement with that expressed by Charpentier. For ourselves, it is at the superior strait or above it that we should use Tarnier's instru ment, or one similar to it, for the reason that here, in particular, back ward traction is indispensable to a greater extent than is possible by means of the Simpson or Eliot forceps, which are the favorites with us. In the cavity, or at the inferior strait and outlet, we personally do not desire a better instrument than the Vienna Simpson. At the superior strait we do not contend that any axis•tractor as yet devised will allow us to pull in a mathematical axis. All we believe we accomplish is to make our traction more in the axis of the pelvic inlet, for the very reason that we are enabled to pull in a direction further backwards. We have never
noted in such cases that the result of our traction seemed to be rather to tilt the head forward.
In regard to Tarnier's instrument, Lusk says: " In all high operations I cannot too strongly recommend the ingenious forceps of M. Tarnier. To one accustomed only to the familiar forceps, the facility with which delivery can be accomplished by Tarnier's instrument is incredible. It is a question whether axis•traction forceps should be employed at all at the inferior strait."—Ed.] The Forceps as a forceps compresses the foetal head, and thus diminishes its volume. But this compression has its limits, and is purely accessory to traction. Hersent, Baudelocque, Petrequin, Delore, Chassagny, Budin, have made interesting researches on this point, which agree in the essentials.
Baudelocque drew the following conclusions: The diminution in the foetal head varies according to the state of ossification of the sutures and the fontanelles, but it is never as great as has been claimed, rarely exceeding four to five lines when the instrument is applied to the sides. The divergence of the blades is not an accurate measure of the com pression force. The diameter in which the head is compressed does not diminish in the same proportion in which the other is increased. The increase is scarcely one quarter of a line.
P6trequm, on the other hand, claims that the diameter opposed to the ne grasped by the forceps increases almost constantly. In agreement ith Baudelocque, he points out that the bi-parietal diameter diminishes ut little, and that the occipito-frontal is always more reducible.
Delore (1865), in studying this subject, drew up the following tables: Pressure between the Blades of a Forceps.
Diameter 0.F., pressure of 231 pounds, no fracture; Diameter B.F., ressure of 220 pounds, no fracture.
Resuming, he believes that the force should never exceed 176 pounds, and from his researches he deduces the following conclusions: - 1. The head resists the more, the greater the pressure surface.
2. It is difficult to foretell the amount of pressure which will produce a fracture.
3. A persistent depression is not always accompanied by fracture.