Anal canal , the terminal portion of the digestive tract , distinguished from the rectum because of the transition of its internal surface from a mucous membrane layer endodermal to one of skinlike tissue ectodermal. The anal canal is 2. The canal is divided into three areas: the upper part, with longitudinal folds called rectal columns; the lower portion, with internal and external constrictive muscles sphincters to control evacuation of feces ; and the anal opening itself. The anal canal connects with the rectum at the point where it passes through a muscular pelvic diaphragm. The upper region has 5 to 10 rectal columns, each column containing a small artery and vein.
The rectum is the concluding part of the large intestine that terminates in the anus. The average length of the human rectum may range between 10 and 15 cm. Its diameter can be compared to that of the sigmoid colon the part of the large intestine nearest the rectum at its onset. However, it becomes larger near the anus, where it forms the rectal ampulla. The key role of the rectal ampulla is to act as a temporary storehouse for feces. The expansion of the rectal walls causes the stretch receptors within the walls to stimulate the urge to defecate.
In humans , the anus from Latin anus meaning "ring", "circle"   is the external opening of the rectum. Two sphincters control the exit of feces from the body during an act of defecation , which is the primary function of the anus. These are the internal anal sphincter and the external anal sphincter , which are circular muscles that normally maintain constriction of the orifice and which relaxes as required by normal physiological functioning.
Anogenital distance AGD is the distance from the midpoint of the anus to the genitalia, the underside of the scrotum or the vagina. It is considered medically significant for a number of reasons, in both humans and animals, including sex determination and as a marker of endocrine disruptor exposure. Such endocrine disruption may affect the development of the brain. Studies show that the human perineum is twice as long in males as in females,  but males have more variance. Measuring the anogenital distance in neonatal humans has been suggested as a noninvasive method to determine male feminisation and thereby predict neonatal and adult reproductive disorders.