MECHANICAL PHENOMENA.
Like the vertex, the head may occupy the positions M. I. R. P., M. I. L. A., M. I. R. A., and M. I. L. P., the right and left transverse positions being a tittle more frequent than in vertex cases. We shall take as a representa tive type, for our description, the position M.I.R.P., the most frequent of all.
1. Position M.LR.P. (Fig. 206).—This is the most frequent, corre sponding, as it does, to the most frequent vertex position, O.L. A.
Diagnosis.-1. Before Labor.—The diagnosis of face positions, before labor, is just about impossible. One can say that the position is right or left, but no more. Palpation, auscultation and vaginal touch simply sho* the face presentation M.I.R. 2. During Labor.—Palpation and ausculta tion, as before.
Vaginal Touch.—The head is moderately extended at the beginning of labor, and so we reach the forehead, which is nearly in the centre of the inlet. After rupture of the membranes, extension being complete, we find the following conditions: The full face is presenting, the forehead on the left, the chin on the right. The face occupies the left oblique diameter of the pelvis, the bregma being in contact with the anterior ex tremity of this diameter, the chin being in relation with the posterior extremity, the right sacroiliac symphysis. The diameter M.B. is in re lation with the left oblique pelvic diameter, the bi-malar diameter with the right diameter, and the circumference M.B. in relation with the out lines of the inlet. The nostrils look to the right and backward, the pelvic axis traverses the head in the diameter 0. F., the anterior foetal surface looks backward and to the right, the posterior surface forward and to the left. The right side of the foetus is forward, the left side behind. We find, here, the six periods or stages, as in vertex cases, except that the first period consists in extension of the head, not in flexion, and that the fourth period consists in flexion, and not, as in vertex cases, in extension. We, therefore, have arranged the following table: First Period.—Period of Diminution and Completion of Extension.— Th e first pain, finding the head in a position of well-marked extension,tends to increase the extension, thus depressing the chin and raising the neck on the dorsal side of the foetus. . This movement is not completed until
the occiput comes into contact with the back. At the same time the fcetus, bending upon itself, removes its back from the uterine wall, while its anterior surface approaches it. The bregma thus withdraws from the examining finger, while the chin approaches it. The result is, that the mento-bregmatic diameter leaves the superior strait and is replaced by the circumference sub-M.F., the latter diameter, or one very near it, coming to replace the diameter M.B. Now, this diameter, sub-M.F., is smaller than the M.B., and there is, therefore, diminution of the foetus. • Second Period.—Engagement or Descent.—The head, being thus forcibly extended, engages but does not descend as far as in the vertex presentations, and this depends on the length of the neck. As Cazeaux justly remarks, it is necessary, because of the forced extension of the head, in order that descent may occur, either that the superior part of the chest engage and . descend with the face, or that the neck be sufficiently elongated to equal the postero-lateral pelvic wall in length (3.7 inches). If this does not occur, the face can not descend to the pelvic floor. Descent is thus limited and the foetus remains higher than in vertex cases.
Third Period.—Internal Rotation of the Head.—This movement is indis pensable, since it alone allows the head to descend. Bringing, as it does, the chin under the symphysis, it permits the neck to come in contact with the anterior wall of the pelvis, which is only represented by the height of the symphysis. Since the latter is not higher than from 1+ to 11 inches, it follows that the neck may adapt itself to this wall and thus permit the expulsion of the chin beneath the symphysis. This rotation occurs more slowly in proportion as the chin is farther back, and it demands, for its occurrence, very energetic pains and normal relations between the size of the foetus and of the pelvis.