Pajot's explanation seems to us the most rational of all. Without ad vancing any explanation of our own, we desire to record the results ob tained by rotation in the position 0.L.A., now being considered. The occiput having engaged beneath the symphysis pubis, and the child's body having participated in this rotation, the shoulders and the upper part of the body have descended into the pelvis, the back of the foetus looks forward, the anterior surface backward, and the bi-acromial diameter is in relation with the transverse diameter of the pelvis. The diameter sub occipito-frontal lies in the coccygeo-pubic diameter, the bi-parietal diame ter in the bi-ischiatie diameter, the sub-occipito-frontal circumference corresponding with the inferior strait. It is, thus, only held back by the coccyx and laterally and in front by the perineum. Now, the coccyx is mobile, and its recession to the rear enlarges the antero-posterior diame ter of the inferior strait. It is, therefore, in the most favorable condi tion for expulsion.
Fourth Period.—Erpulsion. —This stage is accomplished by a move ment of extension. Rotation having taken place, the occiput, under the influence of the pains, is engaged beneath the symphysis and fixed there by the sub-occipital region. It is thus withdrawn from the influence of uterine action, which, however, continuing, will produce two effects: 1st, the descent of the body of the foetus; and 2d, the bending of the foetus on its posterior plane, i.e., upon the maternal abdominal wall. This bending will separate the chin from the chest a little, and will thus begin the movement of extension. Again, the pains will tend to engage the shoulders as deeply as possible, but the neck, being fixed below the sym physis, the uterine forces, having produced their full effect on the shoulders and body of the fothis, and not being able to act on the occiput, will be transmitted through the foramen magnum to the anterior arm of the cephalic lever, the chin. This is, indeed, the only remaining mobile part. The uterine action, being expended on the chin, will depress it, and will thus force the head to bo extended, and finally expelled. The occiput being disengaged and the neck fixed beneath the symphysis, the sub-occipital region becomes the pivot around which the head describes its movement of extension, and the chin being pressed more and more backward, the head will be expelled by the successive liberation of the sub-occipito-bregmatic, of the sub-occipito-frontal and of the sub•occipito mental diameters. All these diameters are shorter than the coccygeo-pu bic, so that only the perineum opposes itself to the exit of the head,which will be rapid or slow, according to the amount of perinea} resistance.
Tarnier rejects the above explanation of Cazeaux and Pajot. Here is his view of the case: " The trunk becomes engaged in the pelvis, while the head distends the perineum, and the chin remains applied to the chest, not only until the occiput takes its place below the pubic arch, but even until the bregma appears at the posterior commissure. Then the peri neum acts like an elastic band, which, on the one hand, pushes the head upward, beneath the pubic bone, and on the other, quickly glides over the face, which it leaves uncovered. The liberation of the occiput and of the vertex only commences when the head is pushed backward by the trunk, but, at this moment, the perineum, hitherto passively distended, resumes its activity and, retracting and gliding over the face, impresses upon the whole head a movement of extension, which makes the pubic arch its central point. Moreover, it is in this second period of expulsion of the vertex that the movement of extension is truly evident." We cannot entirely agree with this explanation, which, if applicable to the end of extension, does not apply to its beginning. After its rotation the occipnt is not suddenly but progressively liberated, and it is not brought into close contact with the perineum until it is completely freed. Until that time it remains at a certain distance from the perineum, and if we introduce the finger posteriorly, we feel that the head is retained behind by its frontal eminences, on the sides by the bi-parietal eminences, and that there is a certain space between the perineum and the bregmatic region. In order that the bregma may be brought into close contact with the perineum, the occiput must be liberated from under the symphysis, and the bregma must descend, which it cannot do at this moment, unless the head becomes extended. If we suppose flexion to persist, as Tarnier says, the perineum will not only have no tendency to produce extension, but will tend to prevent it. But if we suppose the first degree of exten sion to have been produced by the mechanism suggested by Cazeaux and Pajot, at the moment when the bregina appears at the vulva, then the perineum can act as Tarnier supposes it to do, and can thus complete extension, and lead to the final expulsion of the head. however this may be, the head is expelled by its sub-occipital diameters, in performing a movement of extension. When the head is once freed and the perineum retracted, the head falls over the anus, by its own weight, and remains immovable a short time, when the fifth period begins.