THE POSITION OF THE UTERUS AND THE MECHANISM OF ITS SUPPORT.
The uterus normally lies with its anterior surface in contact with
the posterior aspect of the bladder, no intestines intervening. The
absolute and relative positions of the uterus depend upon the degree
of distention of the bladder and the position of the woman. The uterus
is pushed backward and the fundus is turned upward by distention of
the bladder. When the woman is erect the uterus lies at a slightly
lower level than when the woman is on her back, and the intra-abdominal
pressure acting upon the posterior surface of the fundus turns the
uterus more forward, so that the fundus lies nearer the symphysis
pubis. Fig. 61 shows about the normal range of position.
[Illustration: FIG. 61.--Normal range of position of the uterus,
depending upon the distention of the bladder.]
It may be said that in the normal woman the long axis of the uterus is
approximately perpendicular to the long axis of the vagina (Fig. 62).
[Illustration: FIG. 62.--Median sagittal section of the normal female
pelvis.]
The uterus does not surmount the vagina with the axes of the two
structures in the same line, as is shown in some anatomical plates.
The cervix looks backward toward the coccyx, from the tip of which it
is situated 0.6 to 1.2 inches.
The uterus is maintained in position by a variety of factors. The
ligaments, which have been described, are eight in number--broad
ligaments, round ligaments, utero-sacral and utero-vesical ligaments.
With the exception of the round ligaments, which are muscular
structures, the uterine ligaments are formed by peritoneal folds,
including connective tissue, blood-vessels, lymphatics, and a small
amount of unstriped muscle.
When the woman is erect the insertions and origins of the various
uterine ligaments lie in the same horizontal plane. The insertion of
no ligament is higher than its origin in the uterus; therefore these
ligaments do not act as suspensory ligaments when the uterus is in its
normal position. The truth of this fact is repeatedly demonstrated at
operations. If the cervix be caught with a tenaculum when the woman
is on her back, the uterus may, with but very little force, be drawn
downward toward the ostium vaginæ to the extent of one or two inches;
and similarly, by a slight digital pressure on the cervix, the uterus
may be pushed upward from one to two inches above its normal position.
The ligaments of the uterus act as guys. They steady it, and prevent
too great lateral and fore-and-aft movement; they do not, when the
uterus is in its normal position or at its normal level, sustain it
against the force of gravity. When, however, the uterus, for any
reason, falls an inch or more below its normal level, the uterine
ligaments become suspensory in character.
In the normal woman the vagina is always closed. As has already
been said, it is a slit in the pelvic floor, valvular in character;
consequently the abdominal and pelvic viscera may be considered to be
contained in a closed vessel, in woman as well as in man. The uterus
floats in this closed vessel at a level which is consistent with its
own specific gravity. If, for any reason, the specific gravity of the
uterus were increased, it would sink below the level at which it is
normally situated.
Since, normally, there is no tendency in the uterus to change its
position, the pressure upon it must be equal in all directions. The
subject may perhaps be better understood by referring to a few simple
facts in hydrostatics. If a fluid contained in a closed vessel be
in a condition of equilibrium so that its various particles are at
rest, then the pressure upon any particle is equal and opposite in
all directions (Fig. 63); otherwise the particles would not be in
equilibrium, but would move. The bottom of such a vessel, however, is
not, like the particles of the fluid, surrounded on all sides by the
fluid, but above it is the fluid, and below it is the atmospheric air.
Any point upon the bottom of the vessel is subjected to a downward
pressure equal to the weight of the column of fluid above the point;
this downward pressure is resisted by the strength of the material
composing the vessel. If this material be yielding or elastic in
character, the pressure above will make the bottom protrude to a
certain extent. A particle within the fluid (like X immediately above
the bottom of the vessel) will be subjected to a downward pressure
equal to the weight of the column of fluid above it; but this pressure
will be counterbalanced not by any strength in the particle, but by a
counter-force acting from below equal and opposite to that acting from
above.
[Illustration: FIG. 63.--Vessel containing fluid in equilibrium. The
arrows indicate the direction of the pressure at various points.]
A similar state of things exists in the female pelvis. The uterus
floats at a certain level, and the intra-abdominal pressure acting from
above is counterbalanced by an equal force acting from below, while
the floor or bottom of this vessel (part of which is the perineum) is
subjected to a force from above equal to the intra-abdominal pressure,
and this force is opposed only by the strength of the perineum (see
Fig. 64).
[Illustration: FIG. 64.--Diagram representing the directions of the
intra-abdominal pressure upon the uterus in the uninjured woman.]
If the vagina were an open tube admitting air, so that the uterus
above was in contact with the contents of the pelvic vessel and below
with atmospheric air, then the condition of things would be altered.
In this case the uterus would in reality become part of the floor of
the vessel, and would be subjected to a pressure from above equal to
the intra-abdominal pressure, and to this pressure would be opposed
only the strength of the uterus and its attachments. Such a state
of things occurs when the perineum is torn and the vagina becomes a
patulous open canal, and not a closed slit. Therefore when the opening
of the vagina is torn and air constantly enters the vaginal canal, the
normal hydrostatic equilibrium of the pelvic contents is destroyed, the
resultant of the forces acting upon the uterus is downward, and the
organ has a tendency to fall or to prolapse (Fig. 65).
The normal perineum and vagina do not sustain the uterus by furnishing
a mechanical support from below, any more than the bottom of a vessel
sustains any single particle of fluid floating in it.
When the uterus tends to fall down or to prolapse, its progress is
opposed at a certain level by its various attachments. The ligaments
become suspensory in character as soon as their uterine attachments
are below their pelvic attachments. The cellular tissue, fat,
blood-vessels, etc. connected with the uterus restrain its downward
motion. And, finally, this motion is restrained by what has been called
the “retentive power of the abdomen,” which is merely the atmospheric
pressure acting from below on the contents of a vessel the top and
sides of which are closed.
[Illustration: FIG. 65.--Diagram representing the direction of the
intra-abdominal pressure in the woman with a laceration of the
perineum.]
Refer again to a simple physical example: If a glass tube be filled
with water, a finger placed over one end, and the tube inverted, the
water will not run out: it is sustained by atmospheric pressure acting
from below. If the finger be removed, atmospheric pressure also acts
from above, and the water will fall. If a hole be made in the side of
the tube, atmospheric pressure will act through it, and the water below
the hole will fall.
In order that the column of water be sustained, the sides of the tube
must be rigid or unyielding. If the sides of the tube yielded slightly
to atmospheric pressure, they would sink in and a certain amount of
water would escape.
The abdominal and pelvic cavities in the erect woman may be considered
as a tube filled with fluid contents. The top of the tube is closed by
the diaphragm; the sides are the more or less rigid abdominal walls
and the back; the floor is the perineum. When the floor is destroyed
a hole is made in the bottom of the tube: the contents tend to fall,
but the fall is resisted by atmospheric pressure acting from below. If
the diaphragm and the parietes were rigid as glass, there would be no
prolapse, any more than there is prolapse of the water in the glass
tube. If the parietes yield somewhat, the amount of fall or prolapse is
proportional. Thus the retentive power of the abdomen is dependent upon
the strength or rigidity of the abdominal walls.