The head, placed perpendicularly to the plane of the superior strait, is semi•exed. It represents a lever, the arms of which are not equal. The shorter arm corresponds to the distance between the angle of the occipital bone and the foramen magnum. the longer arm to that part of the head between the foramen and the chin. According to the depth of its en gagement, the head is acted upon by two forces, one of which is the uterine contraction, and the other the resistance offered by the borders of the pelvis, the pelvic walls or the perineal floor, as the case may be. The superior force, acting downward, is transmitted by the spine to the foramen magnum. Since this foramen divides the base of the cranium into two unequal arms, the force will act more strongly on the short arm, i.e., on the occiput, than on the other arm, i.e., the chin. The chin will be thus elevated and the occiput depressed. The chin, in being raised, is approxi mated to the chest and flexion is complete. On the other hand, the re sistance of the superior strait, of the pelvic walls or of the perineum, as the case may be, acting in the opposite direction, i.e., from below upward, will contribute to the same result, viz., the elevation of the forehead and the completion of flexion. This movement is complete when the chin is in contact with the upper piece of the sternum. The head is flexed, whether the chin is elevated or the occiput depressed. The higher in the pelvis this resistance is felt, the sooner flexion will occur. Placing the finger on the anterior fontanelle, one can follow the progressive descent of the occiput and the progress of flexion, which is complete in propor tion as the posterior fontanelle is near the centre of the pelvis. Now, in the beginning, flexion having hardly commenced, the anterior fontanelle is nearer to the pelvic centre, and it will be felt to withdraw from the cen tre as the posterior fontanelle tends to approach this point. The result of flexion is to substitute a smaller circumference and diameter for the frontal circumference and the occipito-frontal diameter, which were in relation with the superior strait.• We allude to the sub-occipito-bregmatic cicumference and the sub-occipito-bregmatic diameter. A diameter of 3.70 inches thus replaces one of 4.48 inches, and, as the foetus has become more and more flexed upon itself under the influence of the pains, so as to occupy the least possible space, a diminution in the size of the foetus has really taken place. This reduction in size is accomplished by the completion of flexion. In face presentations, on the contrary, it is effected by extension. Let us add Pajot's remark that the foetus " before flexion, could be considered as a broken and waving branch, the mobility of which was particularly resident in the articulation of the head and of the trunk. Now, a solid body, thus situated, is in an unfavorable position to transmit force applied to its extremities. But the cephalic extremity, so soon as it is fixed upon the thorax, is very fortunately so placed as to participate in the impulsion imparted to the whole festal mass, and, besides, the head, being compressed by the pelvic canal, can, in virtue of its com pressibility, and under the influence of the total uterine effort, adapt it self to the shape and size of the pelvis and thus add a further reduction in volume to that already obtained by its changed position." The term period of moulding is thus quite justifiable.
Second of Engagement of flexed, the head passes through the superior strait and the whole pelvis to the pelvic floor, where it is arrested. Since, however, the posterior pelvic wall is much longer than the anterior, it follows that the posterior part of the head has a longer journey to make than the anterior part, and that the head thus describes a sort of arc of a circle around the anterior end of the bi-parietal diameter, which is almost immobile, in front and on the right, while the posterior extremity rapidly descends along the posterior pelvic plane. During the descent the head has passed through the cer vix, and it is during the second part of the descent that, according to Matthews Duncan, the head descends by an asynclitic movement. When the descent is completed the two parietal eminences are at the same level, and rest upon the perineum. Although one may separate these two periods, for purposes of study, they are, in reality, rarely distinct. Most frequently flexion and descent occur simultaneously. It is even the rule that flexion does not become complete until the head encounters the resistance of the pelvic floor. This rule obtains especially in primiparte, in whom the engagement is quite pronounced, even at the beginning of labor, and in whom it is not rare to find the head, still retained by the incompletely dilated cervix, in the lower part of the pelvis. Nevertheless, everything depends on the pelvic resistance, and, as we shall see when we come to consider the mechanism of labor in contracted pelvis, where pel vic resistance is most marked, the engagement can only occur after exag gerated flexion of the head has taken place. In such cases flexion neces sarily occurs above the superior strait, thus furnishing the proof of the utility of the first period of fetal progression, i.e., of complete flexion.
Third Period.—Internal Rotation.—Upon its arrival at the pelvic floor, the head executes a movement of rotation which brings the occiput to the front. This movement occurs, no matter what the primitive position may be, whether left or right, anterior, transverse or posterior. In the position 0.L. A.. which we are now considering, the movement takes place from left to right, i.e., the occiput passes beneath the symphysis pubis. This rotation will take place more easily if the occiput is originally far forward in the pelvis.
The causes of this rotation have been the subject of much discussion and have been explained in a great many different ways. The theory of the pelvic inclined planes, adopted by Baudelocque and his pupils, down to Velpean and Dubois, was the first which was generally recognized.
The pelvis was held to present two inclined planes, one anterior and one posterior. When the occiput was originally placed in the anterior plane it turned forward, and when it occupied the posterior plane it turned backward. But, Naegel( having shown that the occiput comes forward even when it was, originally, behind, it was necessary to find other ex planations. Paul Dubois, after experimenting on the cadaver, arrived at the conclusion that the cause of the rotation lies, 1st, in the volume, form and mobility of the parts to be expelled; and 2d, in capacity, form and resistance of the genital canal. He attaches much importance to the resistance of the perineum. Hubert de Louvain and Cazeaux sought to explain the phenomenon on mathematical and mechanical principles, but their explanations, which might perhaps account for the rotation in anterior positions, are insufficient for posterior positions. De Soyre seems to return to Baudelocque's inclined planes, and gives two explanations according as the head is originally in front or behind. " If it is in front, he assumes flexion to be as complete as possible, and, as a result, the occiput to be depressed and to rotate only when it reaches the perineum.' The perineum represents a wall inclined from behind forward, from below upward and from without inward. Now the occiput, upon which the uterine forces are expended, is placed on the side and is pushed by the uterine contractions against the perineal floor, which will direct it forward and inward. In posterior positions, the flexed head descends to the pel vic floor, the resistance of which first augments the flexion. But, the direction of the pelvic floor being from below upward, and from behind forward, the head follows this inclined plane until, the occiput remaining constantly behind, the anterior part of the cranium comes to be applied to the osseous rim of the pelvis on the opposite side. The occiput is directed upward and forward, i.e., it turns toward the symphysis and the more in proportion as the perineum is more resistant." Truly, the re sistance of the perineum aids rotation, but if perineal resistance were the sole cause, rotation ought never to be absent in primiparie, the perinea of whom are so resisting. Now, it is especially with them that rotation - often fails, and intervention becomes necessary. Tyler Smith, Leishman and Playfair, refer rotation to the inward projection of the sciatic spines, which contract the transverse diameter of the inferior strait. " As the pains depress the occiput, its backward rotation is prevented by the left sciatic spinous process, while its forward rotation is favored by the smooth, angular surface of the ascending ramus of the ischium. In the same way, the spinous process of the opposite side prevents the forward rotation of the forehead, which is directed backward toward the concavity of the sac rum by the smooth surface of the sacro-sciatic ligaments. These ments give, thus, the shape of the thread of a screw to the interior of the pelvis, and, as the pains push forward the head, they impart to it the rota tion which is so important in adapting it to the largest dimensions of the inferior strait." (Playfair.) In Germany, this movement is attributed to the increasing resistance which the head experiences on the part of the posterior pelvic wall and of the perineum. Playfair's theory, which is good for anterior positions, does not explain rotation in the posterior positions. Tarnier holds that: 1st, rotation does not occur until flexion is complete; and 2d, that when the head is flexed and engaged it is di rected downward and forward, as a result of the inclination of the pelvic floor. Rotation, like flexion, is explained by the unequal length of the arm of the lever. His explanation somewhat resembles that of de Soyre. Depaul, recognizing' the inadequacy of these explanations, limits himself to the indisputable statement that the head turns, and that this rotation extends to the whole body of the child. Pajot says that the causes are multiple, but it is, above all, the shape of the head which decides the character of the movement. " The occiput, being only a mathematical point, will oscillate, slip and escape, under the influence of the pains, to be replaced by a more extended surface. The head in the position O.R. P., for example, must rest, because of the direction of the superior strait, upon the right posterior lateral region, which extends from the oc ciput toward the parietal eminence of the same side. The occiput will, therefore, be carried forward, less on account of the direction of the forces which impel it than because of the necessity for accommodation of the cephalic surfaces to the pelvic surfaces, and so, step by step, until the occiput, having passed beneath the ischio-pubic ramus, there encoun ters an inclined plane which forces it to advance beneath the pubes and to thus complete its anterior rotation." He finds, here, the application of the following mechanical law: When a solid body is contained in an other, which is the seat of alternating movement and repose, if the sur faces are smooth and rounded, the contained body will constantly tend to accommodate its form and dimensions to the capacity of the containing body. This explains why rotation fails when the fcetus is too large or too small, and when the pains are too feeble. Usually, rotation occurs slowly and progressively, but sometimes, particularly in multiparty, this movement occurs very rapidly, even under the influence of a single pain. It is not rare, in the case of women who have borne several children, to see the head, arrested by the last resistance of the cervix, pass through the cervix under the influence of a more energetic contraction, descend to the vulva and be expelled, either by this single pain or by the suc ceeding one.