There are also muscular fibres in the uterus which have a longitudinal as well as some that have a circular course, and the action of the former tend after a time to retract the os, over the fœtal head.
The second obstacle is the bony brim of the pelvis into which the head of the fœtus cannot pass until its long diameter is adapted to certain diameters of the pelvis. The diameter of the bony pelvis is diminished over one-fourth of an inch by the soft parts upon it, but the oblique diameter of the pelvis will admit the long diameter of the head of the child, which does not often exceed 4½ inches. The head usually presents in this way, and passes in a somewhat spiral manner until it arrives at the outlet where the diameters are adjusted to each other. The head is, however, too large to pass, even in this way, were it not that it admits of a degree of compression to facilitate the entrance and progress through; this moulding is effected by the continued pains. The head of the child which presents at the brim with the occiput towards the left acetabulum rotates during the passage, so that the occiput at its exit is directly under the symphasis pubis; the cause of the rotation is found in the form and direction of the passage and in the shape and size of the fœtal head.
This presentation and position is the most common one, though either of the following is liable to occur. By naming the position we indicate just how a presenting part lies, or is turned. We adopt the following classification, which accords with several good authors:
PRESENTATIONS AND POSITIONS.
| Presentations. | No. | Position. | Name of Position. |
|---|---|---|---|
| A—Vertex or head | 1 | Occiput to left acetabulum. | Left occipito-iliac anterior. |
| 2 | Occiput to right acetabulum. | Right occipito-iliac anterior. | |
| 3 | Occiput to Symphasis pubis. | Occipito pubic. | |
| 4 | Occiput to r. sacro-iliac junc. | R. occipito-iliac posterior. | |
| 5 | Occiput to l. sacro-iliac junc. | L. occipito-iliac posterior. | |
| 6 | Occiput to promon’y of sacrum | Occipito sacral. | |
| B—Breach, including inferior extremities. | 1 | Sacrum to left acetabulum. | Left sacro-iliac anterior. |
| 2 | Sacrum to right acetabulum. | Right sacro-iliac anterior. | |
| 3 | Sacrum to symphasis pubis. | Sacro pubic. | |
| 4 | Sacrum to r. sacro-iliac junction. | Left sacro-iliac posterior. | |
| 5 | Sacrum to l. sacro-iliac junction. | Right sacro-iliac posterior. | |
| 6 | Sacrum to promont’y of sacrum. | Sacro sacral. | |
| C—Body, including shoulders, elbow and hand. | |||
| D—Face, including six varieties. | |||
The right occipito-iliac posterior (A 4) position is not a very uncommon one, but that variety which is described and named as the left occipito-iliac anterior (A 1), in which the occiput is directed in front and to the left, is most frequent. These and other vertex presentations may be recognized even in the commencement of labor through the vaginal walls, the head being known by its rounded spheroidal surface.
Supposing that we have a case of the kind that is most common (A 1), and that labor has begun, we may introduce the finger through the os uteri and we encounter a rounded, smooth and resistant surface, which is the anterior part of the head, and then by directing the finger upwards and backwards it will come in contact with the sagittal suture.
If the direction of the suture is oblique, and if it runs from before backwards and from the left towards the right, the position must be either the left anterior or the right posterior occipito-iliac one. (A 1 or A 4).
To complete the diagnosis we follow with the finger the sagittal suture until it reaches the fontanelle, and this determines the position. If the posterior fontanelle is found to the left and in front, and the anterior one is to the right and behind, the position is A 1, or the left antero-occipito-iliac one. The back of the fœtus is turned forwards and towards the left side, while its face and anterior plane is turned backwards and towards the right, and the occipito-frontal diameter of the child’s head corresponds to the oblique diameter of the pelvic brim.
As the labor progresses and the head is forced down in the pelvis, it is also more strongly flexed on the chest and the occiput is pressed down in the excavation. With the occiput thus presenting, it traverses all the space between the superior and inferior straits until it reaches the floor of the pelvis; there it makes what is sometimes called the pivot turn—it executes a movement of rotation, which carries the occiput behind the symphasis pubis and the forehead towards the hollow of the sacrum; then the head being pressed forwards and stretching the perineum, the forehead and face being disengaged from it, emerge; then after the perfect expulsion of the head it again rotates, the occiput turns somewhat to the left thigh and the face towards the right thigh.