ANATOMY AND MECHANISM OF THE PERINEUM.
An accurate knowledge of the anatomy and mechanism of the female
perineum is essential to an understanding of the nature and treatment
of injuries to this structure. The anatomical structures lying between
the anus behind and the symphysis pubis in front are those that most
directly interest the gynecologist. Proceeding from below upward, we
find the following structures lying in superimposed planes: the skin,
the superficial fascia, the deep layer of the superficial fascia, the
transversus perinæi and the sphincter vaginæ muscles, the anterior
layer of the triangular ligament, the posterior layer of the triangular
ligament, the levator ani muscle (Fig. 19).
[Illustration: FIG. 18, _A._--Superficial structures of the female
perineum (Weisse).]
[Illustration: FIG. 19.--Dissection of female perineum: on the left
side the perineal muscles are exposed by the reflection of the perineal
fascia; on the right side the muscles and the superficial layer of the
triangular ligament have been removed, thereby exposing the deep layer
of the ligament. _S. V._, Sphincter vaginæ muscle.]
The vagina passes through these structures. They surround and support
the ostium vaginæ as the fascia and muscles surround and support the
opening of the rectum or the anus. The muscles and fasciæ are attached
in the median line between the anus and the vagina, and therefore
this part of the body, which is called the perineum, is supported
or maintained in its proper position by these various structures.
The transversus perinæi arises from the ramus of the ischium and is
inserted in the perineum. The bulbo-cavernosus, or sphincter vaginæ,
arises in the perineum and is inserted in and about the clitoris. The
inner fibers of the levator ani arise from the symphysis pubis and are
inserted in the perineum and the lower part of the vagina (Fig. 20).
When these muscles contract, their action, therefore, is to draw the
perineum upward and forward. At the same time the anus is drawn upward
and forward, and so also is the posterior margin of the ostium vaginæ
and the lower portion of the posterior vaginal wall.
[Illustration: FIG. 20.--Dissection of female perineum, showing the
deeper structures after removal of the levator and sphincter ani
muscles.]
The vagina has no circular sphincter like the anus, but the vaginal
month is kept closed by the action of the transversus perinæi,
sphincter vaginæ, and levator ani muscles, which draw the perineum
forward, and thus keep the posterior vaginal wall in apposition with
the anterior wall.
[Illustration: FIG. 21.--Muscular floor of the pelvis seen from above.]
This sling of muscles and fascia, which surrounds and supports the
opening of the vagina, may readily be felt in the nulliparous woman by
introducing the finger in the vagina and pressing backward and outward
toward the ischio-rectal fossa. We then feel plainly, immediately
within the ostium vaginæ, a firm resisting band of tissue, apparently
about half an inch broad, embracing the posterior portion of the lower
vagina. This band is formed by the inner edges of the various muscles
and planes of fascia that have been described.
[Illustration: FIG. 22.--Sagittal section showing relations of the
several layers of fascia within the pelvic floor (Dickinson).]
The vagina extends, as a transverse slit in the pelvic floor, upward
and backward, approximately in the direction of a line drawn from
the ostium vaginæ to the fifth sacral vertebra. It is approximately
parallel with the conjugate of the brim, so that when the woman is
erect the long axis of the vagina is inclined at an angle of 60° to the
horizon. The vagina is not a vertical open tube: it is a slit in the
pelvic floor, in health always closed by the accurate apposition of the
anterior and posterior walls (Fig. 21). The anterior vaginal wall is
about 2½ inches long in a vertical mesial line. The posterior vaginal
wall is about 3½ inches long. The vaginal walls are triangular in
shape, being broader above than below. The shape of the normal vagina
at the pelvic outlet is shown by Fig. 23. The section here shows the
vaginal slit of the shape of the letter H. The portions of the
slit extending backward and somewhat outward are called the vaginal
sulci or furrows. They are directions of diminished resistance in which
tears are liable to occur.
[Illustration: FIG. 23.--Section illustrating the characteristic form
of the vaginal cleft (Henle): _Ua_, urethra; _Va_, vagina; _L_, levator
ani; _R_, rectum.]