Anatomy - Ventral mesh rectopexy according to D'Hoore, laparoscopic

Functionally, the rectum serves as a fecal reservoir, preventing continuous defecation. It directly follows the sigmoid colon and, like the latter, exhibits an S-shaped anterior-posterior and lateral curvature (sacral, anorectal, and lateral flexures). Typically, the rectum is divided into three sections:

1. Ampulla recti (main reservoir, ventral to the sacrum)
2. Anal canal (see below, closure region between the anorectal flexure and the skin opening)
3. Between 1 and 2, an unspecified section that emerges at a right angle from 1 and transitions at a right angle into 2. Here, typical folds (Kohlrausch folds) are found in the mucosa.

Adjacent structures of the rectum include dorsally the sacrum and coccyx, laterally the regional lymph nodes next to the internal iliac artery and vein, the sacral plexus, and parts of the autonomic nervous system, as well as the ureter and adnexa. Ventrally, in females, are the uterus and vagina, and in males, the bladder and prostate/seminal vesicle. Caudally is the pelvic floor.

Three muscles form the anal sphincter complex in the wall of the lower rectum:

1. The M. sphincter ani internus represents a thickening of the last circularly arranged fibers of the smooth colon musculature and is innervated by the sympathetic nervous system.
2. The M. levator ani, on the other hand, is voluntarily innervated (sacral plexus) and includes the M. puborectalis attached to the pubic bone. It acts as a large loop around the anal canal anteriorly, functionally bending it.
3. The M. sphincter ani externus is also striated and suspended between the center of the perineal region (Centrum perinei) and the coccyx. It is voluntarily innervated by the N. pudendus. Its contraction results in the terminal closure of the anal canal.

The different innervation of the three closing muscles provides additional security against failures and resulting incontinence.
In the mucosa of the anal canal, there are numerous longitudinal folds (Columnae anales) that contain dense arterial (!) plexuses with venous drainage. Upon contraction of the sphincter muscles, they quickly fill up, causing the mucosa to swell, adhere together, and thus create a gas-tight seal. Hemorrhoids and venous thromboses are known vascular-related complications of this region.

Defecation occurs partly through the relaxation of the closure mechanisms (initiated by the voluntary muscles, emptying of the erectile tissue) and partly through active abdominal pressure and intestinal peristalsis.

The rectum is supplied arterially by three main vessels:

• the superior rectal artery (from the inferior mesenteric artery) via the mesocolon sigmoideum (note: transection of this artery at the level of the sigmoid leads to ischemia of the upper rectum!)
• the middle rectal artery (from the internal iliac artery), running cranially to the levator ani muscle
• the inferior rectal artery (from the internal pudendal artery), below the levator ani muscle

Under the mucosa of the rectum, the veins run as the rectal venous plexus. They drain the blood of the organ in the upper section as the superior rectal vein via the inferior mesenteric vein to the liver, in the middle and lower sections via the middle and inferior rectal veins and via the internal iliac vein to the inferior vena cava (porto-caval anastomosis).

Corresponding to the vascular supply, the lymph of the rectum is also drained: The large group of inferior mesenteric lymph nodes forms its own group at the upper rectum (superior rectal lymph nodes), similarly, the internal iliac lymph nodes take lymph from the middle rectum (from the pararectal lymph nodes), as well as the superficial inguinal lymph nodes take lymph from the lower anal region, anus, and skin of the perineal region.

The perineal region encompasses the area caudal to the pelvic diaphragm (urogenital and anal region) and is confined

• cranially by the fascia diaphragmatis pelvis inferior
• ventrally by the symphysis
• laterally by the ischium
• dorsally by the lower edge of the gluteus maximus muscle

The posterior perineal region corresponds to the anal region and is referred to in nomenclature as the ischioanal fossa, while the anterior perineal region corresponds to the pubic region and can be further subdivided into three overlapping sections: the most superficial being a subcutaneous perineal space (between the subcutaneous fascia = Colles' fascia and the fascia perinei = Buck's fascia), a superficial perineal space between the fascia perinei and the perineal membrane, and a deep perineal space cranial to the perineal membrane.

The dorsally located ischioanal fossa consists mainly of adipose tissue and numerous vessels/nerves (branches of the inferior rectal artery and the internal pudendal artery as well as the pudendal nerve to the anal region) under the skin. At the transition to the sacral region, an epithelial-lined cavity can form as a tract between the coccyx tip and the anal margin in the gluteal fold. This can be encapsulated as a cyst (pilonidal cyst/dermoid cyst) or may have an external connection (pilonidal sinus).

The ventrally located superficial perineal space (spatium superficiale perinei) contains the superficial perineal muscles and the conduits to the external genitalia (perineal artery and artery of the bulb of the vestibule as well as branches of the pudendal nerve to the labia/clitoris or to the scrotum).

The ventral deep perineal space (spatium profundum perinei) is less clearly defined, extends dorsally into the ischioanal fossa, and contains the deep perineal muscles and further profound conduits.