Anus, the term applied by anatomists to the lower, or (in the case of animals) the posterior aperture of the intestinal canal; the rectum terminating externally in the anus. With regard to its anatomy, it is sufficient to state that it is kept firmly closed on ordinary occasions by the external and internal sphincter muscles, the former of which contracts the integument around the opening, and, by its attachment to the coccyx behind, and to a tendinous centre in front, helps the levator ani muscle in supporting the aperture during the expulsive efforts that are made in the passage of the fæces or intestinal evacuations; while the latter, or internal sphincter, is an aggregation of the circular muscular fibres of the lowest part of the rectum, and acts in contracting the extremity of the tube. The main function of the levator ani muscle is expressed in its name, it being the antagonist of the diaphragm and other muscles which act in the expulsion of the fæces. The integument around the anus lies in radiating plaits, which allow of its stretching without pain during the passage of the fæces; and the margin is provided with a number of sebaceous glands, which, in some of the lower animals, secrete strongly odorous matters (see ANAL GLANDS).
Infants are occasionally born with an imperforate anus, or congenital closure of the rectum. In the simplest and most common form of this affection, the anus is merely closed by thin skin, which soon becomes distended with the Meconium (q.v.), and can easily be divided. Cases where the obstruction is at a higher point are more serious; but even these can generally be relieved by a surgical operation. When this is neglected or is unsuccessful, death speedily takes place, with symptoms resembling those of intestinal obstruction from any other cause. Spasm of the sphincter ani is characterised by violent pain of the anus, with difficulty in passing the fæces. It is almost always caused by the presence of an ulcer or fissure, or by some morbid condition in neighbouring parts. Ulceration occurring as a breach of surface at one or more points around the anus, but not extending within the orifice, is sometimes met with; but more common and important is fissure of the anus, a term applied to a crack, or superficial ulceration, situated between the folds of the skin and mucous membrane at the verge of the anus, and extending within the rectum. It gives rise to intense pain during the passage of the evacuations, and for some hours afterwards to great discomfort, smarting, and itching. The treatment to be adopted is to endeavour to procure regular and somewhat soft evacuations, and to sponge with warm water immediately afterwards, the parts being dried with a soft cloth. In slight and recent cases, one or two applications of solid nitrate of silver, with the use of a mild astringent lotion, will sometimes cure the disease. Where the pain is severe, division of the base of the fissure by the knife, or by stretching, is usually necessary. Pruritus ani, which simply means intense itching and irritation of this part, is usually a symptom of morbid changes rather than a special disorder; but sometimes occurs alone, and is often a very distressing and obstinate affection. It is often associated with an unhealthy state of the intestinal secretions, or with simple constipation; with a congested state of the mucous membrane; with a disordered condition of the womb; with the presence of thread-worms in the rectum, &c.; and it is peculiarly common in persons whose occupations are sedentary. The affection is often much aggravated by the patient's being unable to refrain from scratching the parts, which leads to excoriations, ulcerations, thickening of the skin, &c. Treatment must of course first be directed to these associated conditions, when present; and with their removal the irritation often disappears. A lotion containing carbolic acid, hydrocyanic acid or cocaine, or an ointment composed of a drachm of calomel to an ounce of lard, often gives relief. To prevent the reappearance of ulcerations, &c., a strong alum lotion should be used to bathe the parts night and morning. The other principal affections of the anus are Fistula, Piles, and Prolapsus, which are discussed in special articles.