The term “anus” refers to the lower part of the digestive tract (the end of the colon). It is the point where the products of the food’s digestion are excreted (faeces).
The anus is surrounded by two sphincters (internal and external muscles), which manage to hold the stool and prevent their automatic expulsion(incontinence) by contracting. The external sphincter is much more important than the interior one.
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, so this area is an infection point of viruses which prefer them. However, these viruses cannot infect cells from the colon and the intestine.