MECHANISM.
The characteristic features which we have just assigned to the differ ent vertex presentations are those presented before labor and at the begin ning of labor. They will permit us to closely follow the different periods of labor, i.e., to follow the movements which the foetus passively executes, under the influence of the pains, during its expulsion from the genital canal.
1. Position O.L.A.
We take, as a type, the position 0.L.A., the most frequent vertex po sition. There are six periods.
1. Period of moulding (flexion.) 2. Period of engagement.
3. Period of internal rotation of the head.
4. Period of expulsion of the head.
5. Period of external rotation of head and internal rotation of the trunk.
6. Period of expulsion of the trunk.
First Period, that of Moulding.—This period is called that of flexion, by certain authors. (Fig. 194.) When labor is about to begin, the head is in the following position: The occiput is at the left ilio-pectineal emi nence, the forehead at the right sacroiliac symphysis, the sagittal suture in the left oblique diameter, the posterior fontanelle in front and on the left, the anterior fontanelle behind and on the right. The posterior foetal surface is directed forward and to the left, the anterior surface backward and to the right, the right lateral surface and the right shoulder lcroking forwards and to the right, the left lateral plane and the left shoulder backward and to the left. The head is semi-flexed. The diameter 0.F. lies in the left oblique diameter. The bi-parietal diameter, B.P., in the right oblique diameter, and the circumference which passes through the ends of the diameter O.F. is that which corresponds to the margin of the superior strait. According, then, to our view, the head presents perpen dicularly to the superior strait, and what has been called the obliquity of NaegelO and of P. Dubois does not exist.
These two authors stated that the head presented, at the superior strait, slightly inclined toward its anterior parietal bone, the parietal eminence on that side being lower, and so more accessible than that of the other parietal. Contested, even, by Velpeau and Cazeaux, the obliquity
of Naegele has been recently denied, in tote, by Matthews Duncan and by Knneke. Kuneke gives to the perpendicular descent of the lead into the pelvis the name of synclitic movement, and supposes this to remain until the exit of the head, while, according to Matthews Duncan and Playfair, Naegele's obliquity is produced when the head reaches the floor of the pelvis. In short, the synclitic movement gives place, at that time, to an asynclitic movement. In other words, Kuneke and Duncan say that the presenting point, " the point of the head which is traversed. by the axis of the genital canal," is always, or should always be, on the me dian line and on the sagittal suture, not, as Dubois and Naegele say, on the anterior parietal bone. But we agree, with Matthews Duncan, that if the head descends thus ce perpendicularly until it meets the resistance offered by the lower half of the sacrum and the pelvic floor, i.e., with the axis of the superior strait traversing the sagittal suture, it is no longer so when the head has passed beyond the middle of the pelvis. Then, so soon as it has passed the transverse plane cutting the middle of the third sacral vertebra, the head is inclined, and the pelvic axis tra verses a point on the anterior parietal bone, instead of passing through the sagittal suture. In short, the obliquity of Naegele is produced in the lower half of the pelvis. If it be true, as Cazeaux observes, that the palpating finger reaches the anterior parietal eminence, and not the sag ittal suture, this is because of the forward and downward inclination of the pelvis, and because the vaginal axis crosses the axis of the superior strait at nearly a right angle. The finger, thus, necessarily reaches that part of the head which is in front, i.e., the anterior parietal bone, the right one in left positions, and the reverse.