In addition to the anteroposterior flexures, the rectum also has three lateral flexures. They are formed by the submucosal transverse folds that protrude into the lumen of the rectum. These submucosal folds are also known as the valves of Houston, and they include the following:

Superior rectal valve - upper convexity on the left side of the rectum;
Middle rectal valve - intermediate convexity on the right side;
Inferior rectal valve - lower convexity on the left side of the intestine.

Unlike the sigmoid colon, the rectum lacks the haustra, taeniae coli, and epiploic appendices. The proximal diameter of the rectum is similar to that of the sigmoid colon, but it enlarges distally and forms a dilation called the rectal ampulla. The rectal ampulla is a terminal part of the rectum that further transitions into the anal canal, which ends with an opening known as the anus.

Anal canal

The anal canal is a continuation of the rectum. It is also the final portion of the large intestine and the entire gastrointestinal tract. It extends between the anorectal junction and the anus and is about 0.98 - 1.57 inches (2.5 - 4 cm) long. Its primary functions include the defecation and maintenance of fecal continence.

The anal canal ends with an opening called the anus that connects the lower gastrointestinal tract with the external environment. From the anorectal junction, the anal canal descends in the posterior direction. Within the perineum, it is found in the anal triangle between the perineal body and the anococcygeal ligament. Like the distal one-third of the rectum, the anal canal is also a completely extraperitoneal organ.

Overall, the anal canal can be subdivided into two zones or segments - superior (upper) and inferior (lower). Both are separated by a demarcation line called the dentate or pectinate line. It is an irregular zig-zagging line that demarcates the superior two-thirds of the anal canal from its inferior one-third. Both segments between the dentate line have a different embryological origin; therefore, the neurovascular supply for each part is different.

Upper segment - in this zone, the mucous membrane forms several longitudinal folds called the anal columns (columns of Morgagni). The most distal ends of the anal columns merge, forming a horizontal row of mucosal folds known as the anal valves. The location of the anal valves corresponds to the dentate line. Above the valves and between two anal columns, small recesses filled with mucus called the anal sinuses (sinuses of Morgagni) are present.
Lower segment - inferior zone located below the dentate line.

As mentioned before, the anal canal is responsible for defecation and fecal continence. Therefore, it is surrounded by a muscular sphincter system that is formed by two sphincters that prevent unwanted passage of feces.

The internal anal sphincter circularly surrounds the upper two-thirds of the anal canal. It is a thickening of the circular muscle fibers of the muscularis externa layer. The internal anal sphincter is involuntary - its contractions are activated by the sympathetic nervous system and inhibited by the parasympathetic nervous system.
The external anal sphincter is a voluntary sphincter containing skeletal muscle fibers that are controlled by the somatic nervous system. It is closely related to the puborectalis of the levator ani, and together with the internal anal sphincter, all mentioned muscles form the anorectal sling. This sphincter attaches anteriorly to the perineal body and posteriorly to the coccyx via the anococcygeal ligament. Superiorly, it merges with the puborectalis muscle.

Anterior, the anal canal relates to the perineal body, which separates it from the root of the penis in men and the vagina in women. Lateral to the anal canal are the ischioanal (ischiorectal) fossae containing adipose tissue. And finally, posterior to it is the tip of the coccyx.

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Rectum and anal canal (part 2)