The term “anus” refers to the lower part of the digestive tract -the large bowel, Through this opening by-products of digestion are excreted (faeces).
Anal canal is surrounded by two cylindrical sphincters (internal and external sphincters), which by continuous contraction retain stool and prevent their automatic expulsion (incontinence). The external sphincter is more important than the interior sphincter.
The anal area is also characterized by the fact that its lower part has nerve endings and sensation, in contrast to the large intestine which does not appear any significant sensation.
It is also important that the anal canal is lined by cells with skin origin which can be exclusively infected by certain viruses. However, these viruses cannot infect cells that origin higher from large bowel.