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).