This is, very exactly, the mechanism of the spontaneous evolution in those cases in which the posterior plane of the child was primarily in front; that is in the first position of the right shoulder, and in the second position of the left. For in this latter, there is this difference, only, that the movement of the rotation must be in the opposite direction, that is, the head must turn from the right to the left, and from behind forwards, and the breech must turn from left to right and from before backwards.
But where the sternal plane of the foetus is originally turned forwards, as in the first position of the left shoulder, and in the second position of the right, the movement is somewhat different. M.P. Dubois, who had an opportunity to see two such cases, states that at the moment of the disengagement of the breech at the anterior perineal commissure, the entire trunk of the child experienced a movement of torsion, that brought the dorsal plane of the foetus still farther forwards and upwards, which, without this movement, would have been directed towards the anus. Thus it happens, and we cannot but declare it a very remarkable circumstance, that even here we find the same general law, that we have already seen to regulate every natural labor, that whatever may be the primitive situation of the posterior plane of the foetus, it ultimately places itself in relation with the anterior parts of the pelvis.
As was observed at first, it is easy to submit the mechanism of spontaneous evolution to the same divisions as the delivery by the face. We have, in fact, a first period of flexion of the trunk of the foetus towards the side opposite to that which presents; a second one of descent, interrupted by the third movement or stage of rotation; a fourth period of deflexion or disengagement; and according to the observations of M. Dubois, for the dorso-posterior positions, we may add a fifth movement, or period of exterior rotation.
Such is the account given by M. Cazeaux; illustrated by cuts from the same work. Of course no one at the present day would think of allowing any of these unnatural presentations to continue for the sake of observing a possible spontaneous evolution. Speedy interposition should be instituted, for the humane purpose of saving both mother and child.
