Anatomy - Seton drainage for high transsphincteric anal fistula

(1) Longitudinal muscular tunic, (2) Circular muscular tunic, (3) Levator ani muscle, (4) Puborectalis muscle, (5) Deep anal sphincter, (6) Anal columns, (7) Superficial anal sphincter, (8) Subcutaneous anal sphincter, (9) Kohlrausch fold, (10) Internal anal sphincter, (11) Proctodeal gland, (12) Corrugator ani muscle

Three muscles form the closure apparatus in the wall of the lower rectum:

1. The internal anal sphincter is a thickening of the last circular fibers of the smooth colon musculature and is innervated by the sympathetic nervous system.
2. The levator ani muscle, on the other hand, is voluntarily innervated (sacral plexus) and includes the puborectalis muscle attached to the pubic bone. It acts as a large sling around the anal canal, bending it functionally forward.
3. The external anal sphincter is also striated and is suspended between the center of the perineal region (perineal body) and the coccyx. It is voluntarily innervated by the pudendal nerve. Its contraction closes the anal canal terminally.
   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 (anal columns) that have dense arterial plexuses with venous drainage. Upon contraction of the sphincter muscles, they quickly fill and the mucosa swells, adheres together, and thus provides a gas-tight seal. Hemorrhoids and venous thromboses are known vascular complications of this region.
Defecation occurs partly through the relaxation of the closure mechanisms (initiated by voluntary muscles, emptying of the erectile tissue) and partly through active abdominal pressure and intestinal peristalsis.

Functionally, the rectum serves as a fecal reservoir, preventing continuous defecation. It directly follows the sigmoid colon and, like it, has 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 extends 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 are 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.

Read more about the histology of the rectum here.