Thisis the presentation and position most frequently observed, perhaps fifteen out of sixteen times. In most Obstetrical works it is called thefirstposition.
By observing the following Plates, and referring to the previous explanations of the diameters of the Pelvis, and fœtal head, in Plates IX and XXV, it will be seen that through all its changes of position, while making its exit, the head always presents by one of its shortest diameters to one of the largest diameters of the Pelvis, so that the relation between them is invariably the best that could be established; and many peculiar turnings and revolutions occur, apparently for the express purpose of bringing this about.
Before the rupture of the membranes the head presents its occipito frontal diameter, which measures four inches, to the right oblique diameter of the Pelvis, which measures four inches and a half; while its bi-parietal diameter, which measures only about three inches and a half, is presented to the other oblique diameter, also measuring four and a half—(See PlateXIX.) Even here it will be seen that the passage is larger than the head which has to pass through it, but a more favorable position still can be obtained, by a slight movement of the Fœtus, whichnature accordingly accomplishes, and also follows by others, to preserve the advantage, which will next be described.
Movements of the Fœtal Head.—There are three of these peculiar movements, each of which takes place at a particular period of the labor, and must be described separately.
First movement, flexion, and descent.—Immediately after the Membranes are broken, the contractions of the Uterus force the head into the upper strait, by the occipito frontal diameter of four inches, as already shown; but then commences the first movement, which consists in a bending of the child's chin down upon its breast, so that the forehead is carried up into the Womb; and the most prominent point of the back of the head presents to the middle of the passage, by theoccipito bregmaticdiameter, which is only aboutthree inchesinstead of four. This of course makes the passage so much easier, and generally, immediately after this change, the head descends into the basin of the Pelvis.
It is not absolutely necessary however, for this movement to occur, for in a well formed Pelvis the head can descend without it, though not so easily; and sometimes, in fact, it does not take place, but this is unusual.
It must be recollected that this shifting of the head alters the position of thefontanelles—the posterior one being brought more to the centre of the strait, and the anterior one carried up out of reach, while previous to the movement they were both on a level. If an examination is not made therefore, till after this change, it may be difficult to determine the position, unless this is borne in mind. The following Plates show this very well.
PLATE XXVII.It will be seen here that the neck is straightened out, and that the two fontanelles are on a line with each other.Note.—The front of the bones are represented in this and the two following Plates, as if transparent, so that the head may be seen through them.
PLATE XXVII.
It will be seen here that the neck is straightened out, and that the two fontanelles are on a line with each other.
Note.—The front of the bones are represented in this and the two following Plates, as if transparent, so that the head may be seen through them.
head entering upper StraitPlate XXVII.This Plate represents the head just entering the upper Strait.
head entering upper Strait
Plate XXVII.This Plate represents the head just entering the upper Strait.
Plate XXVII.
This Plate represents the head just entering the upper Strait.
PLATE XXVIII.At this time the anterior fontanelle can scarcely be reached, but the posterior one is easily to be reached, being in the open passage, on the left side. The head is now fairly within the Pelvic cavity, but still lies across from right to left.Second Movement, Rotation of the Head.—When the Fœtus is thus brought to the bottom of the Pelvis, its head turns completely round, the back of it being brought to the front, or under the pubes of the mother, and its forehead turned against her Sacrum, as represented in Plate XXIX.
PLATE XXVIII.
At this time the anterior fontanelle can scarcely be reached, but the posterior one is easily to be reached, being in the open passage, on the left side. The head is now fairly within the Pelvic cavity, but still lies across from right to left.
Second Movement, Rotation of the Head.—When the Fœtus is thus brought to the bottom of the Pelvis, its head turns completely round, the back of it being brought to the front, or under the pubes of the mother, and its forehead turned against her Sacrum, as represented in Plate XXIX.
head descended lowerPlate XXVIII.This Plate represents the head descended still lower in the Pelvis.
head descended lower
Plate XXVIII.This Plate represents the head descended still lower in the Pelvis.
Plate XXVIII.
This Plate represents the head descended still lower in the Pelvis.
PLATE XXIX.Here the head is seen just beginning to turn—the right side, and part of the back of it, just passing under the pubes. As the rotation becomes complete the neck straightens, so that the two fontanelles are again found on the same level. Finally the back of the head fully emerges from under the pubic arch, and the chin slides gradually out after it beneath, so that the neck of the child is encircled by the ring of the Vulva.
PLATE XXIX.
Here the head is seen just beginning to turn—the right side, and part of the back of it, just passing under the pubes. As the rotation becomes complete the neck straightens, so that the two fontanelles are again found on the same level. Finally the back of the head fully emerges from under the pubic arch, and the chin slides gradually out after it beneath, so that the neck of the child is encircled by the ring of the Vulva.
head beginning to RotatePlate XXIX.This Plate represents the head still further down, and beginning to Rotate.
head beginning to Rotate
Plate XXIX.This Plate represents the head still further down, and beginning to Rotate.
Plate XXIX.
This Plate represents the head still further down, and beginning to Rotate.
PLATE XXX.Fig. 1.Fig. 2.rotation of the headInFig. 1, the back of the head is nearly under the pubic bone, while the forehead is just passing the external opening below.InFig. 2, the back of the head is completely extruded, and also the chin, so that the whole head is now born.—(p.is the pubic bone, in front.)
PLATE XXX.
PLATE XXX.
Fig. 1.Fig. 2.
Fig. 1.
Fig. 2.
rotation of the head
InFig. 1, the back of the head is nearly under the pubic bone, while the forehead is just passing the external opening below.InFig. 2, the back of the head is completely extruded, and also the chin, so that the whole head is now born.—(p.is the pubic bone, in front.)
InFig. 1, the back of the head is nearly under the pubic bone, while the forehead is just passing the external opening below.
InFig. 2, the back of the head is completely extruded, and also the chin, so that the whole head is now born.—(p.is the pubic bone, in front.)
In Fig. 1 it will be seen how the Os Coccygis, or lowest part of the back bone, (c,) is straitened out, backward, while the head is passing, as I explained before; and in Fig. 2 it has returned again to its natural position.
The reason for this rotation of the head will be obvious on calling to mind the form of the Pelvis and the external opening. On examining Plates IX and X, it will be seen that the longest diameter of the Vulva, or external opening, theantero posterior, is nearly at right angles to the longest diameters of the upper strait, theoblique. Now the longest diameter of the head is adapted to this oblique diameter, onenteringthe upper strait, as already explained, and it is necessary for it also to be adapted to the longest diameter of the external opening, the anterior posterior when making itsexit; but as the two are not parallel the head is compelled to turn, or rotate, in order to pass from one to the other. To accomplishthis however, the neck has to be twisted considerably. But when the head is fully delivered, it is immediately straitened again, by the back of the head turning towards the mother's left side, so that its proper relations with the trunk is re-established.
Third Movement, Rotation of the Shoulders.—When the head is delivered the shoulders come next, which also require to turn round a little, to adjust themselves to the long diameter of the lower strait; and as they turn within the Pelvis the head also turns, to correspond, and thus the back of it is brought opposite the middle of the mother's left thigh. Immediately this movement is effected, the shoulders rapidly escape through the external opening, the right one being in front, a little to the left of the symphysis pubes, and the left one behind, a little to the right of the os coccygis. The body then curves upwards, to accommodate itself to the curved axis of the Pelvis, and speedily follows the shoulders.
These curious movements cause the child to pass in a spiral direction, so that each part may pass through the Pelvis in the most favorable position. Sometimes all these movements are not effected, and yet the delivery may occur, though not so speedily, or safely, as when they are. The shoulders do not always fully rotate, but may nevertheless pass the opening, if the parts be large, and well relaxed. It is questionable however, if the head can ever pass the lower strait without rotating, when it enters the Pelvis diagonally, the occipit on one side and the forehead on the other; as it is necessary for either one or the other of these parts to pass under the pubes.
The right posterior occipital position.—This position, called thesecondby some authors, and thefourthby others, is exactly the reverse of the one just described, the head lying in the same direction, but the back of it being behind, to the right of the Sacrum, and the forehead to the left of the pubes.
Precisely the same movements are gone through in this position as in the other, excepting that the head has to rotate considerably further. In the former position the back of the head is only alittleto the left of the pubis, and therefore has not far to turn to pass under it; but in this position the back of the head isbehind, and therefore has to turn very far round to reach the same position. The rotation is therefore more difficult, not so soon effected, and sometimes likely to be dangerous to the child.
In the other movements there are no difference worthy of notice, but it must be borne in mind that they all occur the reverse way, to what they do in the first position, because the occiput is on therightside instead of the left.
The right anterior occipital position.—This position is precisely the same as the first, but on the other side. The back of the head is in front, but to therightof the pubis instead of the left, while the forehead is behind, to the left of the Sacrum. This will be apparent enough by observing Plate XXXI.
PLATE XXXI.This is the second most frequent position; the left anterior occipito iliac being themostfrequent.a.The Acetabulum, or socket of the hip joint.c.The cut edge of the Womb.c. The Foramen Ovale.d.The top of the ilium bone, called thecrest.h.The Symphysis Pubes.i.The ischium.k. k.The pubic bones.
PLATE XXXI.
This is the second most frequent position; the left anterior occipito iliac being themostfrequent.
a.The Acetabulum, or socket of the hip joint.c.The cut edge of the Womb.c. The Foramen Ovale.d.The top of the ilium bone, called thecrest.h.The Symphysis Pubes.i.The ischium.k. k.The pubic bones.
anterior occiput iliac positionPlate XXXI.Head in therightanterior occiput iliac position.
anterior occiput iliac position
Plate XXXI.Head in therightanterior occiput iliac position.
Plate XXXI.
Head in therightanterior occiput iliac position.
The mechanism of delivery is precisely the same, and all the movements occur in the same order and manner, as in the first position, but the reverse way. The rotation, for instance, being from right to left, instead of from left to right, and so of all the others.
The left posterior occipital position.—Delivery is effected the same in this as in the left anterior, or first position, excepting that the rotation is more extended, owing to the occiput being behind, as explained in the right posterior occipital position.
In all the other positions, and their varieties, there is nothing that calls for special notice, or that is material in practice, the delivery being nearly the same in them all. No matter what position the head is placed in, the back of it nearlyalwayscomes to the front, under the pubes, even though it have to turn half round to do so. The cause of this is supposed to be the peculiar form of the parts, which give it a screw like motion, in its descent, and the shape of the external opening, which, being longest from before to behind, can only allow the long diameter of the head to pass through in the same direction.
Sometimes, it is true, the occiput passes behind, instead of coming to the front, and then thechincomes under the pubes, while the occiput presses on the coccygis. This is very seldom observed, and when it occurs the labor is more difficult and tedious, though it may still terminate spontaneously.
The resistance of the soft parts, externally, appears to be the chief cause of the head turning; for when they are much relaxed, and the child's head small,it will sometimes pass without, or in the anterior posterior position. And sometimes, when the head is large, so that it distends the parts very much, the shoulders will pass cross wise, there being room enough for them without turning. In some females, formed large, whose organs are excessively relaxed, and whose children are small, the delivery takes place withoutanyof the movements being effected, the child passing straight through in whatever position it may happen to be: this is rarely seen however.
When there are twins they do not usually both present by the head, but one by the feet; and frequently the parts are so relaxed by the passage of the first, that the second is delivered without rotating at all, though in general it follows precisely the same movements. It sometimes happens however, that the second birth does not take place till some hours, or even days, after the first.
It may be said, in general, that all positions of the head are favorable to both mother and child, and may terminate spontaneously. It is seldom that anything more than ordinary assistance is required in any of them, and they could in general terminate without any at all, though sometimes with difficulty. The worst cases are those in which the head does not turn round, but remains across, or where the back of it turns behind, instead of coming to the front. In these cases there is great danger of the perineum, or external lips, being much lacerated, or even of an artificial passage being torn through the perineum, leading to the most serious after results. About one child also, out of every fifty, is lost in these unfavorable positions.