4. Compression in the occipito-frontal diameter is 'better supported than in the bi-parietal.
5. The head compressed transversely augments in particular in the trachelo-bregmatic diameter.
6. The pressure exerted by the blades is about equal to one half the traction force.
Budin has made similar researches: " After having, by means of trac tions measured by the dynamometer, caused the head to partially pass through the superior strait of a contracted pelvis, I froze it in this posi tion. Then removing it, hard as a rock, I measured directly the diame ters, and proved, with Petrequin and Delore, that if the head is com pressed in one direction, the opposite diameters increase. Further, the sub-occipito bregmatic suffered the greatest modification, and also the trachelo-bregmatic." The following table gives Budin's results: The diminution in the head, therefore, to be inoffensive, should scarcely exceed .39 of an inch, and this, according to the advocates, is one of the advantages of Tarnier's forceps. The compression exerted by the screw is determined before traction is made, and this traction being made through rods independent of the handles, and on parallel rods, compression will remain constant and will not vary, as must happen with the classic for ceps, where the accoucheur instinctively compresses with the increase in the resistance to traction. We have elsewhere seen that this proposition is not absolutely exact, because we are often obliged to screw down further, a proof that the head is more compressed at the end than at the begin ning of the tractions.
The Forceps as a Dynamic, or Oxytocic Agent.—The forceps, further, awakens uterine contractions, often, indeed, to such an extent as to in terfere considerably with the introduction of the second blade. This action, however, is far from being constant, and it has been much ex aggerated by Baudelocque, Stein, Kilian, etc. At times they are only momentarily increased; and then, again, they are entirely wanting.
The forceps, we have seen, may be applied in presentations of the head, of the face, of the after-coming head, of the decapitated head. In each instance the head may be more or less elevated, and it may be situated: I. Below the superior strait, in the excavation, and more or less near the inferior strait. 2. At the level of the superior strait, in part engaged, but projecting more or less above the excavation. 3. Entirely above the superior strait, and more or less movable.
The general rules, of course, are the same in each instance, but the par ticular rules vary not only in each case, but also in the different positions of the head. We will study each in succession.
Application of the Forceps in Vertex Presentations.
A. The Head in the Cavity, or at the Level of the Inferior Strait.— The vertex may present in 0.P., 0.S., O.I. L. A., O. I. L. P., O. I. L. T., O. I. R. A., O. I. R. P., O. I. R. T.
1. Occipito-pubic Position.—The occiput, according to the descent of the head, is situated either behind the symphysis or beneath. The fore head is backward, the bi-parietal diameter is in the transverse of the cav ity, or of the inferior strait—in other words, the sides of the head corre spond to the sides of the pelvis. The forceps are applied directly, that is to say, the blades are parallel both to the head and to the pelvis. There fore left blade to left side of head, and right blade to right side of head. The forceps applied, the lesser curve lies directly under the lower border of the symphysis. The blades must be pushed in less deeply the lower the head. It is in every way advantageous to begin here with the left blade. Extraction is usually easy. We must simply make traction at first downward, and then in front, until the occiput is engaged under the symphysis (this is very important), and then only, slowly lift the handles to extend the head outwardly, even as in normal labor, by its sub-occipito breginatic, sub-occipito frontal, sub-occipito-mental, diameters. (See Fig. 91.) (We are speaking now of the classic forceps, that of Tarnier telling us by its needle in what direction we must make traction.) Never hurry, unless the interest of mother or of child calls for haste. Give the soft parts time to relax; watch carefully the perineum, ready to perform episiotomy in case of rapid delivery being needed, always bearing in mind the principle that except in an emergency, it is far better to work slowly than hastily in the delivery of tile head. Let the perineum relax, hold back the head if need be, and allow it, as far as is possible, to be born only in the intervals of the pains. (See Figs. 91 and 92.) The head delivered and, then alone, remove the forceps, contrarywise to the opinion of Mme. Lachapelle, [and contrary also to the opinion of the generality of American accoucheurs, for reasons we have already given—Ed.], who taught that the forceps should be removed as soon as the head was free from the bony walls.