Chapter 71: Lesser Pelvis & Perineum Anatomy

The lesser pelvis is defined as a bowl-shaped structure formed by the sacrum, pubis, ilium, and ischium, surrounding the central pelvic canal, which, in females, constitutes the birth canal. The walls of the pelvis are partially formed by piriformis and obturator internus, but the inferior limit is defined by the pelvic diaphragm, a broad muscular sheet primarily composed of levator ani (subdivided into pubococcygeus, iliococcygeus, and pubo-analis) and ischiococcygeus, whose constant activity supports the viscera, reinforces the external anal sphincter, and helps maintain continence. The connective tissue framework is organized into parietal pelvic fascia, which includes the well-differentiated obturator fascia and the clinically significant presacral fascia (a landmark whose integrity is vital in rectal tumour surgery), and visceral pelvic fascia (endopelvic fascia), which creates supportive lateral attachments, often referred to as 'ligaments' (e.g., cardinal ligament). Vascular supply originates from the common iliac artery, which bifurcates into the external iliac artery (principal supply to the lower limb) and the internal iliac artery, which shows considerable branching variation and divides into anterior and posterior trunks supplying the pelvic organs and gluteal/hip regions, respectively, giving rise to vessels like the superior and inferior gluteal, uterine, and internal pudendal arteries. Venous drainage primarily converges into the internal and external iliac veins to form the common iliac vein, noting that pelvic veins provide a major avenue for collateral venous return if systems like the external iliac are blocked. Innervation is managed by the somatic lumbosacral and sacral plexuses, providing key branches such as the sciatic and pudendal nerves, alongside the autonomic pelvic splanchnic nerves (parasympathetic) and the sympathetic trunks which unite anterior to the coccyx at the ganglion impar. The perineum, located inferior to the pelvic diaphragm, is partitioned by the inter-ischial line into the posterior anal triangle, containing the ischio-anal fossa and external anal sphincter, and the anterior urogenital triangle. The urogenital triangle is further separated into the deep and superficial perineal spaces by the strong perineal membrane. Essential to urinary continence is the urethral sphincter mechanism, comprising smooth and striated muscles like the compressor urethrae, which works in combination with fascial tissues to allow the urethra to be compressed closed, particularly during increases in abdominal pressure.