d. The levator ani muscles, varying in thickness and strength, may be divided
into three parts: the iliococcygeal, the pubo-coccygeal, and the puborectal muscles.
The fibers of the levator ani blend with muscle fibers of the rectum and vagina. The
fibers (pubovaginal) of the pubococcygeal part of the levator ani muscles, lying directly
below the urinary bladder, are involved in the control of micturition. The pubococcygeal
fibers of the levator ani control and pull the coccyx forward and assist in the closure of
the pelvic outlet. The fibers pull the rectum, vagina, and bladder neck upward toward
the symphysis in an effort to close the pelvic outlet and are responsible for the flexure at
the anorectal junction. Relaxation of the fibers during defection permits a straightening
at this junction. During parturition, the action of the levator ani directs the fetal head into
the lower part of the passageway.
e. The uterus gains much of its support by its direct attachment to the vagina
and by indirect attachments to nearby structures such as the rectum and pelvic
diaphragm. The ligaments and muscles on each side of the uterus are the broad,
round, cardinal (Mackenrodt), and uterosacral ligaments and the levator ani muscles.
The uterus, which occupies a central place in the pelvis, is a pear-shaped organ
directed downward and backward. At the upper lateral points, the uterus receives the
uterine tubes (Fallopian tubes). The fundus of the uterus is the upper rounded portion
situated above the level of the tubal openings. The main portion of the uterus is called
the body. Below, the body of the uterus joins the cervix, from which it is separated by a
slightly constricted canal, called the isthmus. The cervix lies at the level of the ischial
spines. The lumen of the body of the uterus communicates with the cervical canal at
the internal orifice, called the internal os. The cervical canal ends at the vaginal
opening of the cervix called the external os. This is a small oval aperture situated
between two lips.
THE STRUCTURE OF THE UTERUS (FIGURE 2-3)
a. The Greek word for uterus is hystera. The uterus lies behind the bladder and
in front of the rectum (see figure 2-3). The uterine body has three layers: (1) the outer
peritoneal, or serous layer, which is a reflection of the pelvic peritoneum; (2) the
myometrium, or muscular layer, which houses involuntary muscles, nerves, blood
vessels, and lymphaticus; and (3) the endometrium, or mucosal layer, which lines the
cavity of the uterus.
b. The cervix consists of a supravaginal and a vaginal portion. The supravaginal
portion is closely associated with the bladder and the ureters. The vaginal portion of the
cervix projects downward and backward into the top of the vaginal vault.