Sixth Period.—Evolution of the Head.—The occiput remaining behind the symphysis pubis, the sub-occipital region is kept immovable under the arch, and as the uterine contractions flex the head more and more, the chin is brought successively closer to the chest. One after the other, the follow ing diameters appear: sub-0.M., S.O.F., S.O.B., and finally the occi put, which emerges last of all. The head, therefore, is delivered by its sub-occipital diameters as in vertex presentations.
The Sacroiliac Left Posterior Position.—(Fig. 221.) The posterior plane of the foetus is directed backward and to the left, the anterior plane is in front and to the right, the intergluteal fossa is in the right oblique diameter of the pelvis. The coccyx points forward and to the right, the bis-iliac diameter is in the left oblique diameter of the pelvis. The left hip is directed forward and to the left; the right hip, backward and to the right.
The mechanism is the same, only the third step differs; the left hip executing a much more extensive movement from behind forward, and from the left to the right, so as to come under the symphysis, while the right hip occupies the sacral excavation. The torsion movement of the trunk is therefore much greater. The rotation of the head likewise con sumes a longer time, since the occiput has to traverse a greater distance before it comes back under the symphysis. Delivery takes place by the
sub-occipital diameters.
Position S.LR.P. (Fig. 222).
The back is turned backward and to the right; the anterior plane of the foetus, forward and to the left. The intergluteal fossa is in the left oblique diameter; the bis-iliac diameter in the right oblique diameter; the occiput points forward and to the left, the right hip forward and to the right, the left hip backward and to the left; the right buttock is lower, and during rotation places itself under the symphysis pubis. Rotation movement prolonged, owing to the great distance to be traversed.
Position S.I.R.A. (Fig. 223).
The fcetal back is directed to the right, forward, the anterior plane of the fcetus to the left, backward. The intergluteal fossa is in the right oblique diameter, the coccyx points to the left, backward, the right hip to the left, forward, the left hip to the right, backward; the bis-iliac diame ter is in the left oblique diameter; rotation, which brings the right hip first under the symphysis, takes place from left to right, while the occiput rotates from right to left.