The Perineum Is a lozenge-shaped area, nearly 4 inches long and 3½ inches broad at the widest part, which corresponds to the outlet of the pelvis, being bounded in front by the symphysis ; laterally by the rami of the pubes and ischium, tuber ischii, and great sacro-sciatic ligament ; posteriorly by the coccyx. Above it is separated from the pelvis by the pelvic diaphragm, consisting of the levator ani and coccygei muscles, which are covered on their pelvic aspect by the rectovesical fascia, and on their perineal aspect by the anal fascia. These muscles, originating from a line extending from near the lower border of the symphysis to the ischial spine, slope down and inwards to meet one another in the middle line, leaving, however, passages for urethra and rectum, and, in addition (in the female), for the vagina. As the muscles thus diverge from the pelvic wall, they leave a lateral space on either side, which they, covered by anal fascia, bound on the inner side, and which is bounded externally by the pelvic parietes, covered by obturator fascia, called the ischio-rectal fossa (see Figs. 29, 30, and 31). The perineum is subdivided into an anterior urogenital and a posterior rectal triangle by a transverse line crossing the mid-point of the perineum about 1 inch in front of the anus, and a little in front of the tuber ischii.
The urogenital space forms practically an equilateral triangle, whose sides are about 3¼ inches in length, and it is bounded on its deep surface by the triangular ligament. The skin and superficial fascia do not call for special comment, but the deep layer of superficial fascia-Colles's fascia-has an important surgical bearing. Attached laterally to the rami of the pubes and ischium, Colles's fascia, which is of a membranous structure, dips down posteriorly, and becomes continuous with the base of the triangular ligament, while anteriorly it is continuous with the dartos coat of the scrotum and fascia of the penis, and, continuing up in front of the spermatic cord, becomes continuous with the deep layer of the superficial fascia of the abdomen (Scarpa's fascia). Thus the structures contained in the urogenital triangle are contained in a space which is closed, except in front, by Colles's fascia superficially, and the triangular ligament deeply. When, therefore, on rupture of the urethral bulb which lies in this space, extravasation of urine occurs, the urine is forced to come forward, distending the scrotum and penis, and finding its way up in front of the cord to the front of the abdomen.
As a result of gonorrhoeal infection of the urethra, periurethral abscess is apt to occur in this region, and may burrow ultimately to the surface and discharge. If it also communicates with the urethra, a perineal urinary fistula would result. Extravasation may also cause such a fistula. Situated in the middle line of the triangle is the bulb of the corpus spongiosum, enclosing the urethra, and surrounded by the accelerator urinae muscle, while lying along the rami of the pubes and ischium on either side are the crura of the corpora cavernosa, which converge to meet the corpus spongiosum under the pubic arch, and form the body of the penis. Thus, the urogenital triangle is divided into two smaller triangles, the corpus spongiosum forming the inner side common to both, the crura forming the outer sides of the two triangles, while the transversus perinaei muscles converging to the central point of the perineum from the rami of the ischium form the third sides. The space is traversed from behind forwards by the superficial perineal vessels and nerves (from the internal pudic arterial and venous trunks), which enter it by piercing the base of the triangular ligament. The transverse perineal arteries (internal pudic) also pierce the triangular ligament, and accompany the muscles of that name, while the long pudendal nerve (small sciatic) enters the space through Colles's fascia.
The central point of the perineum lies about a finger-breadth in front of the anus, and at it the antero-posterior accelerator urinae and sphincter ani, and the lateral transversus perinaei muscles meet. Further, it marks the centre of the base of the triangular ligament, and it is used as a landmark in certain operations, the incisions not being carried beyond it in order to avoid injury to the bulb, which lies just in front of it.
The floor of the triangle is formed by the triangular ligament, which is composed of two layers, enclosing certain structures, the anterior layer being equivalent to the deep perineal fascia, while the deep or posterior layer is derived from the obturator layer of pelvic fascia. Attached laterally to the pubic arch, the layers of the triangular ligament join one another at the base of the triangle, which is about 1½ inches deep. The anterior layer presents a small opening at the apex, under the pubic arch, through which the dorsal vessels of the penis pass. Between the layers of the triangular ligament is the membranous urethra, surrounded by the compressor urethrae. This lies about 1 inch below the symphysis, and ¾ inch above the central point. When ruptured, as by a blow in the perineum, extravasation takes place between the layers of the triangular ligament, by which it is limited at first. The anterior layer of the ligament is pierced by the artery to the bulb, close to the urethra, and by the artery to the corpus cavernosum near the subpubic angle. In addition to the membranous urethra and compressor muscle, there are, between the layers of the triangular ligament, the termination of the internal pudic artery, dividing into dorsal artery of penis and artery of the corpus cavernosum, artery to the bulb, Cowper's gland, and the dorsal vein and nerve of the penis. Lying on the deep surface of the deep layer of the triangular ligament is the prostate gland, surrounded by its capsule, derived from rectovesical layer of pelvic fascia, prostatic plexus, etc. In cutting down on the prostate through the perineum, seven layers of alternate fascia and muscle are met with : (i) Superficial fascia ; (2) superficial perineal muscles ; (3) anterior layer of triangular ligament ; (4) compressor urethrae muscle ; (5) posterior layer of triangular ligament ; (6) levator ani muscle ; (7) prostatic capsule.