UROGENITAL DIAPHRAGM
In the upper figure, the penis, bulbous spongiosum,
and both crura have been removed, exposing the inferior surface of the urogenital
diaphragm. This surface of the diaphragm is penetrated by the membranous
urethra and the ducts of Cowper (bulbourethral) glands that lie
within the confines of the inferior and superior fascial layers. Nerves,
arteries, and veins that supply both the corpus spongiosum and corpora
cavernosa also penetrate the inferior fascial layer of the urogenital
diaphragm. The deep dorsal vein of the penis, which drains the glans
penis and corpora cavernosa, passes through an aperture above the transverse
ligament of the pelvis. This ligament is formed by the fusion of the
superior and inferior fascial layers of the urogenital diaphragm. The urethra,
after passing through the urogenital diaphragm, pierces the dorsal surface of
the corpus spongiosum and is contained within it.
In the
middle figure, the inferior fascial layer of the urogenital diaphragm has been
removed. Note that the intramembranous Cowper (bulbourethral) glands and the deep
transverse perineal muscle are now exposed. This muscle lies between the
inferior and superior fascial layers of the urogenital diaphragm. Anteriorly,
the fibers of this muscle surround the membranous urethra and are termed
the membranous urethral sphincter. Injury to this muscle or its nerve
supply, for example at the time of radical prostatectomy for prostate cancer,
can result in urinary incontinence.
The
bulbospongiosus, ischiocavernosus, and trans- verse perineal muscles lie within
the superficial perineal compartment (see Plate 2-3). The bulbospongiosus
muscle envelops the posterior part (bulb) of the corpus spongiosum, and its
anterior fibers encircle both the corpus spongiosum and the paired corpora
cavernosa (see Plate 2-3). It takes origin from the perineal body in the
perineum as well as from a median raphe in the midline. It acts to expel
the last drops of urine from the urethra during micturition and to aid in the
penile erection.
The
paired, fusiform-shaped ischiocavernosus muscles arise from the inner surfaces
of the ischial tuber-osities and ischiopubic rami. They cover and
insert into the crura of the penis. They act to produce an erection by compressing the
crura.
The
superficial transverse perineal muscles are slender slips that arise from the
inner, anterior part of the ischial tuberosity and run transversely and insert
into the central perineal body. Here, they blend with the superficial
external anal sphincter (see Plate 2-4). The perineal branch of the pudendal
nerve supplies all of these perineal muscles.
The
lowest figure shows the relevant anatomic landmarks used to divide the perineum
into two topographic regions: the urogenital triangle anteriorly and the
anal triangle posteriorly. The bases of each triangle are shared and extend between the
bony ischial tuberosities, roughly paralleling the course of the
transverse perineal muscles. The apex of the urogenital triangle is the pubic
symphysis anteriorly and that of the anal triangle is the tip of the coccyx
posteriorly. Surgical procedures that traverse these anatomic regions,
especially the anal triangle, are commonly performed in urology to remove the
cancerous prostate (radical perineal prostatectomy).
