The Anatomy of the Anus

Dentate line
The part which borders the anus from the rectum, about 1.5 cm inside from the anal verge. The part inside, or going inside the dentate line is called the rectum, and the part outside of it is the anus.

Because the sensory nerve which feels pain reaches here, illlness in the anus comes with acute pain. The surface is strong and does not bleed easily because it is covered by anal epithelium which is like skin.

The sensory nerve does not reach here, so illness inside here does not produce pain unless it pressures it surroundings. The surface is frail and bleeds easily because it is covered by a mucous membrane.

Anal crypt
About 10 indented areas at the dentate line. The analgland opens up behind it. When bacteria enters and infects this area, anal fistula is formed from the periproctal abscess.

Internal sphincter
It is at the lower edge of the muscle which makes up the wall of the rectum. Although one cannot move it at his own will because it is controlled by the autonomous nerve system, it is the role of this muscle to keep the anus closed.

External sphincter
Strong muscle that one can tighten at will since it is controlled by the spinal nerve. There are a lot of venous plexus, which is the part where veins gather together like meshes of a net.

Venous plexus
Blood vessels which exist like meshes of a net around the anus. Hemorrhoids is made up of venous plexus which became varices.