Our backdoor areas are full of nerve endings which feel amazing to vast majority of both men and women, when gently stroked. Not only that, according to a well known sexologist Jack Morin, PhD the folds of anal tissue give the anal canal a mind blowing capacity for expansion, which varies greatly according to personal preference, degree of relaxation, amount of practice, and other aspects. Imagine, for instance, that during rectal surgery, under anesthesia, a person's anal muscles can easily be dilated so that the surgeon's entire hand can pass through the anal canal. On the other hand, In the intimate arena a similar expansiveness is called upon in the activity known as "fisting" or "handballing," which involves inserting several fingers or an entire hand into the anus and rectum. Although hardly a mainstream activity, anal fisting is more popular than most people realise according to Dr. Morin.(more on fisting in Chapter 9). These extremes of anal expansion are not experienced by most people, though they do illustrate that anal tissue can be easily and safely accommodating a finger and such.

Source:

Anal Pleasure and Health: A Guide for Men, Women ands Couples, 2011, by Jack Morin, PhD