Fifth Period.—External Rotation of the Head.—This movement con sists in external rotation of the head, which turns the occiput towards the thigh of the mother, which corresponds with the side of the pelvis which it originally occupied, i.e. the left thigh in the position O. L. A., while the face is turned toward the opposite thigh. This movement has long been known as the movement of restitution (Fig. 201). It was supposed that, in the movement of internal rotation, only the head rotated, the neck being twisted, and hence external rotation, corresponding to the un twisting of the neck, gave the name of movement of restitution to the stage in question. There is, in reality, a slight twisting and untwisting of the neck, but that is not the true cause of the external rotation of the head. Gerdy first showed that this movement, which always occurs at the time of a pain, is only the external expression of a movement of the shoulders, within the pelvis, by which the bi-acromial diameter passes from the transverse to the antero-posterior diameter of the outlet, the head merely following this internal rotation of the shoulders. A fact which proves that the neck has been slightly twisted is that often, just when the head has been expelled, it executes a slight rotatory movement which causes it to assume a slightly oblique position relatively to the vulva. This movement is really due to an untwisting of the neck, and is the genuine movement of restitution of the books.
Sixth Period. of the external rotation has occurred, the trunk is expelled. The anterior shoulder, the right one in the position 0. L. A., takes its position below the symphysis and appears at the vulva. It becomes fixed beneath the symphysis, as the occiput did, and thus partly liberates itself, but, in reality, it is the posterior, or left shoulder which, pushed by the uterine contraction, sweeps over the whole posterior pelvic wall and is completely expelled before the other. The rest of the body escapes with a spiral movement, and, generally, very rapidly. But, in some cases, we see the expulsion of the nates effected in the same way as that of the shoulders, the anterior appearing first under the symphysis, the posterior side being completely expelled before the former. This description most exactly suits those cases in which the foetuses are very large.
2. Position 0. L. P.
The details given in our description of the position 0. L. A. enable us to deal more concisely with the other positions. We have already seen by what means the position 0. L. P. is recognized, and have now only to describe the mechanism which embraces the same six periods or stages.
First head is in the right oblique diameter, the occiput behind and to the left, the forehead in front and to the right, the posterior fontanelle behind and to the left, on a level with the left sacro-iliac synchrondrosis, the anterior fontanelle at the right ilio-pectineal eminence. The posterior surface of the foetus is behind and on the left, the anterior surface looks forward and to the right, the diameter 0. F. occupies the right oblique diameter, the circumference 0. F. is in rela tion with the margin of the pelvic inlet, at first, and the diameter B. P. lies in the left oblique diameter.
First head is flexed, as in the left anterior position, and the foetus is diminished in size.
Second or engagement occurs more slowly than in the anterior position it is always less deep, and the occiput, descending less, is still in relation with the sacro-iliac syn chondrosis when rotation commences.
Third Period.—Internal Rotation.—This is the longest stage. The oc ciput starts from the posterior part of the pelvis and the path which it has to pursue is much longer, and the conditions for accommodation less favorable. The occiput again turns from left to right, and finishes by passing beneath the symphysis. The body accompanies this movement, and, when rotation has once occurred, the fourth, fifth and sixth periods follow, as in the position O. L. A.
3. Position O. R. P.
In this position, the head is placed in the left oblique diameter, the posterior fontanelle at the level of the right sacro-iliac symphysis, the anterior fontanelle on a level with the left ilio-pectineal eminence, the diameter O. F. in the left oblique diameter, and the diameter B. P. in the right oblique diameter. The back looks backward and to the right, the anterior festal plane forward and to the left, and the circumference O. F. is in relation with the superior strait.
First Period.—.111exion.
Second Period. — Engagement or Descent takes place slowly, and is less complete than in anterior positions. The occiput descends along the right sacro-iliac synchondrosis, and is still in contact with this articula tion when rotation begins.
Third Period.—Internal Rotation.—This is very long, on account of the distance through which the occiput must pass, turning as it does from right to left in order to get beneath the symphysis. The trunk follow% and rotation becomes complete.
Fourth Period.—Expulsion.—As in the position O. L. A.
Fifth Period.—External Rotation.—The occiput turns toward the woman's right thigh, the face toward the left thigh. This movement corresponds to the internal movement of the shoulders. The slight twisting of the neck is rather more pronounced than in the anterior positions.
Sixth Period.—Expulsion of Trunk.—As in position O. L. A.
4. Position O. R. A.
This is the rarest original position of all. It is generally secondarily produced, during rotation in the position O. R. P. We subjoin the rela tions of the fcetus: The posterior fontanelle is forward and on the right, on a level with the right ilio-pectineal eminence, the anterior fontanelle behind and on the left, on a level with the left sacro-iliac symphysis, the diameter O. F. in the right oblique diameter, and the circumference O. F. in relation with the superior strait. The back of the foetus looks forward and to the right, and its anterior surface backward and to the left. The first to the fourth periods, inclusive, are similar in mechanism to the descrip tion already given. The occiput turns toward the woman's right thigh, and the face toward the left thigh.
Such is the normal mechanism of labor. But irregularities or anomalies may arise in either of the periods, which, although they do not deprive delivery of its physiological character, and do not modify its general physiognomy, yet deserve to attract our whole attention. These irregu larities are often the cause of the obstetrician's intervention (vide Dystocia). However, they more frequently still permit labor to be regularly and spon taneously completed.