**2.3 Anal canal**

The rectum enters the pelvic floor and becomes the anus. The anal canal is defined as from the dentate line to the anal verge by anatomists, but most surgeons consider the anal canal from the anorectal ring to the anal verge [20]. The anorectal ring is where the rectum enters the pelvic floor and is angled by the puborectalis muscle. This ring can be palpated by a meticulous digital rectal exam. The dentate line, which divides the upper two-thirds and lower third of the anal canal, is an anatomically important landmark of the anal canal, and there are 6–14 longitudinal folds on the dentate line known as columns of Morgagni (**Figure 7**). The upper and lower part of the anal canal differs in venous and lymphatic drainage, innervation, and the epithelial surface based on the dentate line. Above the dentate line, the blood drains into the portal venous system, and lymphatics drains to the superior

**Figure 6.** *Rectovaginal septum. In female, the rectovaginal septum was consistent with the denonvilliers' fascia in male.*

#### **Figure 7.**

*Anal canal and anal sphincter complex. (a) The dentate line divides the upper two thirds and lower third of the anal canal, and there are longitudinal folds known as columns of Morgagni. The external sphincter consists of three separate parts: Subcutaneous, superficial, and deep part [21]. (b) External anal sphincter. Lt. Hemipelvis.*

rectal and iliac lymphatic chains. Below the dentate line, the blood drains into the caval system, and lymphatics drain into the inguinal lymph nodes.

There are two sphincter muscles surrounding the anus, the internal sphincter and the external sphincter. The internal sphincter is connected from the inner circular smooth muscle of the rectum and descends to 1–1.5 cm below the dentate line. Its length is about 2.5–4 cm and the mean thickness is about 0.5 cm. It is an involuntary smooth muscle and plays an important role in the maintenance of fecal incontinence because it contributes a majority of the resting pressure of the anal canal. The outer longitudinal muscle of the rectum conjoins the fibers from the puborectalis muscle and is located between the external and internal sphincter. The external sphincter muscle is a striated muscle surrounding the internal sphincter in the shape of a cylinder, and it extends slightly below the internal sphincter. The external sphincter consists of three separate parts: subcutaneous, superficial, and deep part. The subcutaneous external sphincter attaches to the perianal skin encircling the anus. The external anal sphincter is innervated by the rectal branch

of the pudendal nerve and is under voluntary control [20, 22, 23]. The intersphincteric groove between the internal and external sphincter is an important landmark in surgery for patients with distal rectal cancer such as intersphincteric resection (ISR) [24].
