THE MECHANISM OF DELIVERY, IN A PRESENTATION OF THE HEAD.
THE LEFT ANTERIOR OCCIPITO ILIAC POSITION.
This is the presentation and position most frequently observed, perhaps
fifteen out of sixteen times. In most Obstetrical works it is called
the _first_ position.
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 Plate XIX.) 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, which nature 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 the
_occipito bregmatic_ diameter, which is only about _three inches_
instead 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 the _fontanelles_--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.
[Illustration: 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.
[Illustration: 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.
[Illustration: Plate XXIX.
This Plate represents the head still further down, and beginning to
Rotate.]
[Illustration: PLATE XXX.
In _Fig. 1_, the back of the head is nearly under the pubic bone, while
the forehead is just passing the external opening below.
In _Fig. 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, the _antero posterior_, is nearly at right
angles to the longest diameters of the upper strait, the _oblique_.
Now the longest diameter of the head is adapted to this oblique
diameter, on _entering_ the 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 its _exit_; 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 accomplish this 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.
MECHANISM OF DELIVERY IN ALL OTHER POSITIONS OF THE HEAD.
_The right posterior occipital position._--This position, called the
_second_ by some authors, and the _fourth_ by 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 a _little_ to 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 is _behind_, 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 the _right_ side
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 the _right_ of 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 the _most_ frequent.
_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 the _crest_.
_h._ The Symphysis Pubes.
_i._ The ischium.
K. K. The pubic bones.
[Illustration: Plate XXXI.
Head in the _right_ anterior 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.
GENERAL REMARKS ON THE DIFFERENT POSITIONS OF THE HEAD.
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 nearly _always_ comes 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 the _chin_ comes 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 without _any_ of 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.