The last inch or so of the rectum directed downwards and backwards is sometimes spoken of as the anal canal. It is surrounded by the internal and external sphincters, which, assisted by the levatores ani, compress it laterally, thus presenting an antero-posterior slit. It is related in front to the mass^of tissue known as the perineal body, where several perineal muscles meet ; behind to the tissue between the rectum and the coccyx, called the ano-coccygeal body ; while laterally it is in relation to the fat of the ischio-rectal fossa.
The mucous membrane of this part, becoming continuous with the anal skin at ' Hilton's white line,' presents a series of slight vertical ridges-columns of Morgagni-which become continuous above with the mucous membrane of the rectum proper at the ano-rectal line. At the anal extremity of each of the intervening depressions is a small anal valve, opening into a small rectal sinus in the submucous tissue. These small valves are sometimes torn by scybalous masses, and are said to give rise to fissure of the anus. The examining finger notes the smooth character of this portion of the mucous membrane and the underlying firm smooth surfaces of the resisting sphincters. Where a stricture exists, this is frequently detected only a short distance in, presenting a hard, possibly sharp, resisting ring.
The anus is capable of very considerable distension if done gradually, and a small hand has even been introduced for examination of various organs. As, however, the hand rapidly cramps from pressure, it is of little service. The rectum is supplied by the middle sacral and superior, middle, and inferior haemorrhoidal arteries, of which the first two are single, and the others dual vessels. The middle sacral arises from the aorta, and its supply is insignificant. The superior hemorrhoidal, the principal blood-supply, is the continuation of the inferior mesenteric. It supplies numerous branches to the rectum, which may be traced almost to the anus, piercing the muscular coat 3 inches above the anus, beyond which point it supplies chiefly mucous membrane. The middle hemorrhoidal arises from the internal iliac, reaches the rectum about 3 inches above the anus, and anastomoses with both the superior and inferior haemorrhoidal vessels. The inferior hemorrhoidal rises from the internal pudic of the internal iliac, and supplies chiefly the anal portion. The veins of the rectum are arranged in two main plexuses-the internal hemorrhoidal, situated in the submucous coat ; and the external hemorrhoidal, situated in the external coat. The veins constituting the internal haemorrhoidal plexus commence in a number of small anal veins, radially placed round the anus, and situated in the submucous tissue. They join to form larger and tortuous vessels, which, ascending in the columns of Morgagni, anastomose to form the plexus, and frequently present small dilatations, which may represent starting-points for haemorrhoids. Ultimately they pierce the muscular coat about the middle of the rectum, and join the superior haemorrhoidal vein.
Hemorrhoids consist of a varicosity of the veins of the internal haemorrhoidal plexus, and are classified as internal when they affect those in the region of the columns of Morgagni and are covered by mucous membrane, and as external when situated beneath the skin at the anal orifice. Internal haemorrhoids are not infrequently associated with prolapse of the mucous membrane, so that they may even appear externally, and are sometimes constricted by the sphincter, cure taking place by sloughing. Several anatomical causes may be found for haemorrhoids : (a) upright posture, with veins running vertically in a lax mucous membrane (scybalous masses may press on these vertical veins) ; (b) the superior haemorrhoidal, a long vein without valves, communicates with the portal system, and is liable to congestion ; it is also liable to be pressed on by loaded rectum. The various veins which pass out through the rectal wall unite to form the external haemorrhoidal plexus, and from this plexus the superior haemorrhoidals go to join the inferior mesenteric, which joins the portal vein ; the middle haemorrhoidal joins the internal iliac, and the inferior haemorrhoidal joins the internal pudic of the internal iliac. Thus portal and systemic systems communicate on the rectum.
The lymphatics pass chiefly to some rectal glands lying in the outer coat of the bowel in the course of the superior haemorrhoidal vein, and thence to the sacral glands in front of the sacrum. Some of these from the anal region join the cutaneous lymphatics, and so reach the superficial inguinal glands, and a few are said to pass to the internal iliac glands lying on the lateral pelvic wall. The rectum is supplied by the second, third, and fourth sacral nerves, together with sympathetic fibres from the mesenteric and hypogastric plexuses. The anal canal and external sphincter are supplied by the inferior haemorrhoidal branch of the internal pudic.
The sacral nerves convey motor impulses to the longitudinal muscle fibres and inhibiting impulses to the circular, while the sympathetic does the reverse. While the anal portion of the bowel is very sensitive, the portion above is very insensitive, so that considerable damage may be done without a patient being aware of the fact. In rectal affections pain is sometimes referred to the perineum, penis, and even down the thigh, owing to the pudic and small sciatic nerves arising from the same section of the cord. The fourth sacral nerve supplies both rectum and neck of bladder, and thus retention of urine frequently follows operations on the rectum, particularly, it is said, when the sphincter is stretched antero-posteriorly instead of laterally. The reflex defcecation centre is in the lumbar region, and may act even when separated from the brain. Destruction of the centre causes incontinence of faeces. Where the pelvic colon, or upper rectum, . is greatly distended with faecal matter, or is affected with carcinoma, the obturator nerve is sometimes pressed on, causing the patient to complain of pain in the knee.