of Rhodes. Spurious works Andronicus.

The American journal of obstetrics and diseases of women and children online

. (page 93 of 109)
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observations, if possible, in the light of the physiological con-
ditions that exist within the uterus during labor. The whole
is offered as a practical contribution toward arriving at a normal
mechanism of the third stage of labor.

The clinical facts given are based upon an observation of 150
consecutive deliveries. Unfortunately, accurate record was not
kept; the actual frequency therefore with which the various
conditions obtained cannot be given. The observations, how-
ever, are correct, and are as follows:

1. The placenta was usually delivered as an inverted cone,
the fetal surface presenting; the membranes trailing behind,
reversed.

2. Infrequently (in less than 10 per cent.) the placenta was
delivered edgewise; the membranes trailing behind, not reversed
(i.e., their anatomical relations preserved).

3. The placenta never appeared as an. inverted cone, the
maternal surface presenting.

4. Modifications of the "inverted cone" and the ''edgewise"
presentations were noted.

5. The rupture in the membranes in the "inverted cone"
presentation was central or nearly so; in the "edgewise'' presen-
tation usually lateral or marginal.

6. Stripping of portions of chorion from the underlying
amnion with subsequent retention, and detachment of the
membranes from the anterior margin of the placenta with or
without retention of portions of the membranes so separated
were noted frequently in the "edgewise" presentation. Strip-
ping of portions of the chorion or of portions of the amnion and
chorion was infrequently noted and detachment of the mem-
branes from their marginal insertion never observed in the
"inverted cone" presentation.

7. The passage of several ounces of blood immediately after
delivery of the placenta occurred frequently in, but was not
a characteristic feature of, the "inverted cone" presentation.
In^many cases the delivery of the placenta was practically
bloodless.



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harper: mechanism of the third stage of labor. 897

8. The membranes invariably followed the placenta in
expulsion.

In order to interpret the foregoing, it is necessary first to
review- certain anatomical and physiological facts concerning
the uterus in the third stage of labor.

The common implantation of the placenta is upon the anterior
or posterior wall near the fundus. Lower situations are less
frequent; those at the fundus rare.

The uterus in labor is divided physiologically into an active
upper and a passive lower segment. The dividing line between
these segments and the structure representing, functionally at
least, the internal os during labor is commonly considered to be
the margin of the contraction ring.

These features become most marked during the height of a
uterine contraction, and by some the contraction ring is claimed
to be existent only at that time. Between pains the divisions
may not appear, and the general shape of the uterus is ovoid.
During a uterine contraction the upper segment becomes thick-
ened and generally spherical in shape. The contraction ring
stands out prominently; the lower segment contracts less firmly,
or remains passive, and with the vagina becomes converted into
what might be termed a hollow cylinder.

For purposes of discussion, the term ** internal os" is applied
to the functionating contraction ring. The strength of the
contraction of the latter, measured by the firm grasp the structure
often maintains upon the after-coming membranes or upon a
douche tip introduced for purposes of intrauterine irrigation,
is a matter of common experience.

The essential cause of the separation of the placenta, com-
monly situated in the upper segment toward the fundus, is the
contraction of the uterine muscle. Such contraction decreases
the size of the placental site and compresses the placenta to
about one-half its normal size, when the organ becomes relatively
a firm body. Such contraction will partially separate the
placenta; the continued contraction will complete its detach-
ment, when the placenta becomes a foreign body to be acted
upon as such by the intrauterine forces.

When the roughly spherical upper segment contracts the
cavity becomes encroached upon, in general, equally from all
directions except from below where the internal os is located.
The margin of a circular, flattened body within the cavity,
especially if the diameter of the body approach that of the
3



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harper: mechanism of the third stage of labor.

interior of the cavity, will be in general equally pressed upon by
the contracting walls. As the result of this pressure, the body
will yield in the direction of least resistance, which is toward
the center of the cavity, and will become converted into a cone,
its apex and exterior being represented by the surface originally
directed toward the interior.

The contraction of the walls continuing, such a body will
be directed first toward the center of the uterine cavity. Such
direction will obtain for a like body in any position within the
cavity except in the region of the internal os. The second
direction imparted to the body will be downward toward the
internal os, since the pressure at this point upward toward the
center of the upper segment is nil.

Pressure toward the center of the "ring** from all points on
the margin of the contracting internal os will be approximately
equal. Contraction of the ring upon a body lying within it will
cause the body to yield in the directions of least resistance,
i.e., above and below. A circular, flattened body lying exactly
within the ring, when contracted upon, would yield at its center
which would be directed toward the vagina, since the resistance
in that direction is less than from above. A body entering
the contraction ring at an angle would have only the portions
of the margin in contact with the os acted upon. At the con-
traction of the ring, the margins in relation with it would become
approximated, bending the body upon itself but away from the
center of the ring; the extremities of the body (not being acted
upon by the contraction ring) would be directed upward toward
the upper segment and downward toward the vagina. The general
shape of the body (the placenta) then is that of a hollow cylinder,
in which form the contracting upper segment forces the placenta
through the ring and into the lower uterine segment and vagina.

The lower uterine segment and vagina are characteristically
less active. A body passing through these generally cylinder-
like structures will tend to accommodate itself to the general
shape of the canal formed by them. A circular, flattened body
would then become folded upon itself and lie with its long axis
in that of the canal. Such a body would appear at the outlet of
the vagina by its margin.

The clinical facts noted will be considered under separate
headings, as earlier given.

I. The placenta was usually delivered as an inverted cone,



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harper: mechanism of the third stage of labor. 899

the fetal surface presenting; the membranes trailing behind,
reserved.

It has been shown how a body like the placenta, freed from
its usual attachment in the upper segment, would be converted
into a shape resembling a cone and be directed first toward the
center of the upper segment then toward the internal os.
Since the apex of the cone is mobile and the base much less so,
being pressed upon by the uterine walls, the former will be
directed toward the internal os. The placenta presenting at
the internal os as an inverted cone favors dilatation of the latter
and the passage of the placenta through it.

When in the lower 'uterine segment and the vagina, the
placenta lies in a canal tubular in shape and characteristically
passive. Motion through this tube tends to change the shape of
the placenta as has been described. But the lower segment
and vagina are relatively short; the ordinarily weak forces
being active for not sufficient distance to alter markedly the
shape of the body passing through them. They do account,
however, for the variations (to be mentioned) from the type
of delivery that is common.

Negative pressure is probably the essential factor in main-
taining the union between the fetal membranes and the uterine
walls. With the escape of the uterine contents (the liquor
amnii and later the child), the membranes are thrown into
folds, as the uterus contracts, and may become partially sepa-
rated. Any anatomical adhesion of the chorion to the decidua
that may obtain could persist in spite of uterine contraction.
As the placenta is directed toward the center of the uterine cavity
and then through the internal os, it passes through the center
of the sac of membranes still in contact with the uterine walls,
forcibly separating the membranes in its advance. It finally
leaves the sac of membranes at the point of earlier rupture in
them. The membranes follow the placenta, which is presenting
by its fetal (amniotic) surface, and are delivered reversed, i.e.,
the amnion on the outside.

2. Infrequently (in less than lo per cent.) the placenta was
delivered edgewise; the membranes trailing behind, not reversed
{i.e., their anatomical relations preserved).

Various degrees of lower implantation of the placenta have
been met with. Separation of the placenta occurs along the
lines described for its common, higher implantation. As more
of the characteristically passive lower segment is encroached



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900 harper: mechanism of the third stage of labor.

upon, the separation from mere retraction of the placental site
becomes less complete and the pressure from above outward
is a more prominent factor in its complete detachment.

The placenta in the lower positions encroaches upon, or may
occupy, the lower uterine segment and comes in actual contact
with the functionating tontraction ring, which marks the upper
limit of this segment. When so situated, the powerfully con-
tracting ring becomes the principal factor in determining the
delivery in the way already described.

Passage through the tubular lower segment and vagina tends
to preserve the cylinder-shape assumed by the placenta. As
the relaxed outlet is approached, the placenta will unfold; the
margin (edge) presenting thus becoming straighter as the vaginal
outlet is larger and has preserved less tone.

For several reasons the membranes trail behind with their
anatomical relations preserved. The placenta being a solid
body and readily acted upon leaves the uterine cavity first,
forcibly separating the membranes from the walls of the uterine
cavity in its advance. Being at all times directed away from
the center of the contraction ring toward the uterine walls, the
placenta is not delivered through the center of the sac of mem-
branes as when under the full influence of the upper segment; the
membranes follow the placenta then not reversed, but with their
anatomical relations preserved, i.e., the chorion on the outside.

3. The placenta never appeared as an inverted cone, the
maternal surface presenting.

In the common and higher implantations, the pressure of the
contracting walls, applied most effectually about the margin of
the placenta, causes the latter to yield theoretically at its
center and be directed in the line of least resistance, i.e., toward
the center of the uterine cavity. The apex and outer surface
of the cone so formed (continued contraction of the uterine
walls maintains the shape) will be the surface originally directed
toward the center of the uterine cavity. The maternal surface
is not so directed. It could not become the apex and outer
surface of the cone and so delivered.

In the lower implantations the functionating contraction
ring and the cylindrical lower segment definitely determine the
shape the placenta is to assume in delivery.

4. Modifications of the "inverted cone*' and the "edgewise"
presentations were noted.

A frequent modification noted was the presentation of the



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harper: mechanism of the third stage of labor. 901

placenta by its fetal surface at a point not far from the margin;
occasionally a portion of the margin would appear, but in every
case the characteristic feature was the presentation of the
placenta by its fetal surface.

Such modifications are to be considered as transitions between
the "inverted cone" and the "edgewise" (cylindrical) presen-
tations; the transition from one to the other being more com-
plete as the lower uterine segment and vagina are of greater
length, of greater activity, or as the resistance offered to the
passage of the placenta (because of its size) is greater.

A less common occurrence was the bending back of the
placenta at its anterior margin, so exposing more or less of its
maternal surface. If the advance of the placenta in its cylin-
drical form is more rapid than that of the membranes through
the internal os, the tension of the membranes upon the placenta
is greatest at the anterior margin of the latter. The effect of this
pull is to bend the placenta back upon itself in the direction of
the internal os; the maternal surface then becomes exposed.
Therefore more of the maternal surface will appear as, with the
placenta advancing edgewise, the pull at the contraction ring
upon the trailing membranes is greater and as the latter resist
tearing at the anterior margin of the placenta.

This is to be looked upon as a variety of the true "edgewise"
presentation, occurring quite as frequently as the latter.

The edge of the placenta presenting will be less curved (i.e.,
the cylindrical shape of the placenta will be less preserved), as
the placenta can more readily unfold; this the placenta can do,
as it is smaller and the vaginal outlet larger.

5. The rupture in the membranes in the "inverted cone"
presentation was central or nearly so; in the "edgewise" pre-
sentation usually lateral or marginal.

When the membranes have ruptured spontaneously the rent
will appear at or near the point of least resistance, which point
the internal os would represent. When ruptured artificially, the
rent must appear at the exact location of the os. The rupture
in the membranes then indicates their lower extremity when in
their anatomical position within the uterus. With the site of
rupture as a fixed point and knowing the distance between the
point of rupture and the margin of the placenta, the height of
the implantation can be quite accurately determined.

Membranes expelled with the point of rupture central or
nearly so would denote a placental implantation (at or) near the



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902 harper: mechanism of the third stage of labor.

fundus. A placenta so situated is subject to the forces of con-
traction in the upper uterine segment. These forces tend to
invert the placenta in the manner in which it is found to be
delivered.

On the other hand, membranes expelled with the rupture
lateral or marginal indicates a low insertion of the placenta. A
placenta so situated is influenced in the main by the activity
of the contraction ring, the tendancy of which is to fold the
placenta upon itself as a cylinder in which form it is found to
present.

6. Detachment and retention of portions of the membranes.

The general characteristics of the fetal membranes should
be considered. The outer, or chorion, is friable, opaque, and
readily torn from the amnion to which it is only fairly adherent
and from the margin of the placenta to which it is attached.
Scattered over it there often appear small, irregular, adherent
patches of decidua. The inner, or amnion, is tough, fairly
transparent and thinner than the chorion though tearing much
less readily. Though the amnion is not attached to the margin
of the placenta, because of the adhesions between it and the
chorion, it may tear from the margin of the placenta to which
the chorion is attached.

When the placenta is delivered edgewise the membranes follow
with their anatomical relations preserved, i.e., the chorion is on
the outside. Following the passage of the placenta through
it, the internal os contracts down upon the friable chorion perhaps
irregularly roughened by adherent particle of decidua. (The
strength of the grip of the internal os upon the membranes has
been experienced by all who have endeavored to hasten the
delivery of the latter by pulling upon them.) Such contraction
may result in stripping portions of chorion from the underlying
amnion as the membranes advance.

The membranes may remain intact and be held tightly by the
internal os. As the placenta advances "edgewise'' (either
spontaneously or in response to Cred6 expression), the strain
upon the membranes becomes greatest at the anterior margin of
the placenta. The result of this pull may manifest itself in one
of two ways. If the membranes are tough and refuse to yield,
the placenta will bend at its anterior edge and be directed back
toward the cervix. As a result, more or less of the maternal
surface will be exposed (as has been mentioned). If the mem-
branes are less strong, they yield at the point of greatest stress.



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harper: mechanism of the third stage of labor. 903

which is at the anterior margin of the placenta, and are detached
at that point. The placenta continuing to advance will cause
the tear to extend around its margin, or the part of the mem-
branes already detached may be torn from the remainder still
adherent to the margin and be retained by the contracting os.

Any or all of these conditions may obtain depending upon
the toughness of the membranes, the strength of the contraction
of the internal os, and of the forces directing the placenta onward.

When the placenta is delivered as an inverted cone the
membranes follow reversed; that is, the shiny, tougher amnion
is on the outside. The same contraction of the internal os and
advance of the placenta occur. However, the pull upon the
membranes is less effectual because of the lessened resistance to
passage through the os the smooth surface of the amnion offers.
Too, the membranes do not tend to become detached for the
pull is distributed in general equally about the entire margin of
the ** inverted cone" and (also important) in a line in which the
membranes and placenta anatomically meet.

In either presentation portions of chorion may be retained
within the uterus, being stripped off in separation of the chorion
from the decidua.

That the conditions mentioned were noted rarely in the first
mechanism and with relative frequency in the second would
indicate that the essential factor in determining the second
mechanism (namely the contracting internal os) was an equally
important factor in bringing about separation and retention of
portions of the membranes.

7. The passage of blood immediately after delivery of the
placenta was not a characteristic feature of the ** inverted cone"
presentation.

By some, the essential cause of separation of the placenta
and its presentation as an inverted cone is claimed to be a
retroplacental hemorrhage. Such bleeding would separate the
placenta and ordinarily would not appear until the placenta,
acting as a tampon, had been delivered. But any blood in the
uterine cavity, with the placenta in the position described, would
be effectually held back until the placenta had been delivered.
If a true retroplacental hemorrhage were the important factor
in the separation of the placenta and its delivery as described,
we should expect to have found a relatively large amount of
blood immediately follow the placenta in its delivery in the
majority of instances; but such was not the case.



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904 harper: mechanism of the third stage of labor.

As routine in each case, the fundus was followed down as the
child was bom, and persistent, gentle massage maintained
throughout the third stage. The efforts were, as a rule, success-
ful, the fundus remaining firm; and, as has been stated, in many
cases the delivery of the placenta was practically bloodless.

Since the type of bleeding described did not occur in the
majority of "inverted cone" presentations (and, too, any
intrauterine bleeding at the time would manifest itself in like
manner), and since many placental deliveries were accompanied
by no bleeding, it may be argued that retroplacental hemor-
rhage is neither the cause of nor an important factor in the
separation of the placenta and its delivery as an inverted cone
with the fetal surface out.

8. The membranes invariably followed the placenta in
expulsion.

Were the membranes completely separated from the uterine
walls with the escape of the liquor amnii and the child, they
would readily be delivered before the still-adherent placenta;
especially would this be true if the insertion of the latter were
low. That they invariably follow the placenta in delivery
shows that the advance of this organ is the essential factor in
determining their separation.

summary.

A. The usual implantation of the placenta is near the fundus;
this we determine from the fact that the rupture in the mem-
branes is commonly near the center. The essential cause of
separation of the placenta is contraction of the uterine walls
with retraction of the placental site, and not retroplacental
hemorrhage.

The continued contraction of the active, upper uterine seg-
ment converts the placenta into a cone the apex of which is
directed first toward the center of the uterine cavity then toward
the internal os. The placenta thus travels through the center
of the sac of membranes, forcibly separating the latter and
reversing them as they follow the placenta advancing through
the internal os.

The grasp of the contracting internal os upon the tough,
shiny amnion (being on the outside of the reversed membranes)
exerts tension upon the entire margin of the advancing (cone-like)
placenta. With the mechanical advantages thus offered.



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harper: IfECED^NISM OF THE THIRD STAGE OF LABOR. 905

the membranes are commonly expelled intact and adherent to
the placenta presenting by its fetal (amniotic) surface.

Th^ presentation described is less typical as the lower uterine
segment and vagina are more active in their efforts to make
the long diameter of the placenta coincide with that of the canal.

Blood may or may not follow the delivery of the placenta as
described. When bleeding into the cavity does occur, the shape
of the placenta has assumed favors the accumulation of blood
behind it.

B. From the less common lateral or marginal rupture in the
membranes, we judge the unusual placental implantation in the
less active, lower uterine segment. The separation of the
placenta so situated is due less to retraction of the placental
site and more to forcible detachment in response to pressure
from above as more of the lower uterine segment is occupied by
the placenta.

The placenta encroaching at all upon the lower uterine seg-
ment is influenced directly by the active feature of that segment,
namely, the powerful internal os or contraction ring. Here the
entire margin is not pressed upon by the contracting walls, as in
the location higher up, but the greatest pressure is applied in a
line across the maternal surface corresponding to the location of
the **ring." As the contraction is increased, the placenta yields
in the directions of least resistance, that is above and below,
and becomes folded upon itself cylinder-like. In this form, and
hugging the uterine walls, the placenta leaves the os, traverses
the lower canal which tends to preserve its form, and presents
at the vulva by its edge.

Trailing behind and in a line about parallel with the long
diameter of the placenta are the membranes, their anatomical
relations preserved. Upon the roughened, friable chorion the
internal os contracts as the placenta advances. Not uncom-
monly, as a result of the grasp of the os upon the membranes
on the one hand and of the advance of the placenta on the other,
there occur stripping of the chorion from the underlying amnion,
detachment of the membranes from their anterior insertion, or
tearing of pieces of the membranes from the portions already
separated from their attachment. Any or all of the foregoing,
with or without subsequent retention of them within the internal
OS and the uterine cavity, may be noted in a single case.

Such possible retention emphasizes the pathological aspect
of this mechanism.



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906 NUTT: ECTOPIC GESTATION AT FULL TERIf.

More or less of the maternal surface of the placenta appears
as the advancing placenta bends under the pull of the membranes



Online Libraryof Rhodes. Spurious works AndronicusThe American journal of obstetrics and diseases of women and children → online text (page 93 of 109)