The anus from Latin anus meaning "ring", "circle" is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces , unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may include: matter which the animal cannot digest, such as bones ;  food material after all the nutrients have been extracted, for example cellulose or lignin ; ingested matter which would be toxic if it remained in the digestive tract; and dead or excess gut bacteria and other endosymbionts. Amphibians, reptiles, and birds use the same orifice known as the cloaca for excreting liquid and solid wastes, for copulation and egg -laying. Monotreme mammals also have a cloaca, which is thought to be a feature inherited from the earliest amniotes via the therapsids. Marsupials have a single orifice for excreting both solids and liquids and, in females, a separate vagina for reproduction. Female placental mammals have completely separate orifices for defecation , urination , and reproduction; males have one opening for defecation and another for both urination and reproduction , although the channels flowing to that orifice are almost completely separate.
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Functional disorders of the anus and rectum
Rectum and Anus - Digestive Disorders - Merck Manuals Consumer Version
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.
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Rectum and Anus
In this report the functional anorectal disorders, the etiology of which is currently unknown or related to the abnormal functioning of normally innervated and structurally intact muscles, are discussed. These disorders include functional fecal incontinence, functional anorectal pain, including levator ani syndrome and proctalgia fugax, and pelvic floor dyssynergia. The epidemiology of each disorder is defined and discussed, their pathophysiology is summarized and diagnostic approaches and treatment are suggested.
The rectum is a chamber that begins at the end of the large intestine, immediately following the sigmoid colon, and ends at the anus see also Overview of the Anus and Rectum. Ordinarily, the rectum is empty because stool is stored higher in the descending colon. Eventually, the descending colon becomes full, and stool passes into the rectum, causing an urge to move the bowels defecate. Adults and older children can withstand this urge until they reach a bathroom. Infants and young children lack the muscle control necessary to delay bowel movement.