The anatomical structure of the anus is from the dentate line up to the anal verge, about 3-4 cm long, and is the anatomical anal canal. 1. Anal canal The anal canal is the end of the digestive tract and is anatomical from the dentate line up to the anal verge, about 3-4 cm long. Some people extend the upper boundary of the anal canal to 1.5cm above the dentate line, that is, the plane of the rectal ring of the anal canal, called the surgical anal canal, which is generally used sparingly. The superficial layer of the anal canal is composed of columnar and migratory epithelium in the upper segment and migratory and squamous epithelium in the lower segment. In men, the front of the anal canal is adjacent to the urethra and prostate, and in women, the uterus and vagina; the back is the tailbone, surrounded by the internal and external sphincter. Zhao Fengying, Department of Anorectal Medicine, Liaocheng Hospital The dentate line is the junction line between the rectum and the anal canal, consisting of the anal flap and the lower end of the anal column, which is serrated, so it is called the dentate line (or comb line) and is an important anatomical landmark. In the embryonic period, the dentate line is the junction of the endoderm and ectoderm, so the blood vessels, nerves and lymphatic sources above and below the dentate line are different, and the symptoms and signs are also different. The clinical importance of the dentate line is as follows: ① Above the dentate line is mainly supplied by the superior and inferior rectal arteries, and below the dentate line is supplied by the anal artery. The venous plexus above the dentate line is the internal hemorrhoid plexus, which returns to the portal vein and forms internal hemorrhoids if varicose. The venous plexus below the dentate line is the external hemorrhoid plexus, which returns to the inferior vena cava and forms external hemorrhoids if varicose. Infection above the dentate line can lead to liver abscess through the portal vein; infection below the dentate line spreads from the inferior vena cava to the whole body. The mucosa above the dentate line is innervated by the vegetative nerve and no pain is felt; the anal canal below the dentate line is innervated by the spinal nerve and the pain response is acute. Therefore, the injection and surgical treatment of internal hemorrhoids should be carried out above the dentate line and should not involve the area below the dentate line to prevent pain and edema reaction. The lymph above the dentate line mainly returns to the lymph nodes around the abdominal aorta, while the lymph below the dentate line mainly returns to the inguinal lymph nodes. Therefore, rectal cancer metastasizes to the abdominal cavity, while anal canal cancer metastasizes to bilateral inguinal lymph nodes. The mucosa above the dentate line, due to the contraction of the sphincter. There are 6-10 longitudinal striated folds, about 1-2 cm long, called rectal pillars (anal pillars), which can disappear when the rectum is dilated. The rectal column contains the terminal branch of the superior rectal artery and the eponymous vein formed by the superior rectal plexus, from which the internal hemorrhoid is formed by varicose and enlarged veins. The lower end of each rectal column is connected by a semilunar mucosal crease, which is called the anal flap. The rectal mucosa between the anal flap and the rectal column forms many pouch-like nooks called anal sinuses (anal saphenous fossa). The opening of the sinus is upward, about 3-5 mm deep, with an opening for the anal glands at the bottom. There are 2-8 triangular papillary protrusions below the anal flap, called anal papillae. Tearing of the anal flap can lead to anal fissure, sinusitis and anal papillitis. There are 4-8 anal glands in the normal anal canal, mostly concentrated in the posterior wall of the canal, each opening at the anal sinus. The anal glands have a tubular part under the mucosa called the anal duct, which is divided into grape-like branches in the submucosa. 2/3 of the anal glands extend downward to the internal sphincter layer, and a few may cross the muscle to the joint longitudinal layer, and a very few may enter the external sphincter and even the scirorectal space. The anal glands are mostly the entry point for infection and, in a few cases, for adenocarcinoma. The white line is located between the dentate line and the anal margin, and a groove can be felt during rectal palpation, which is the junction between the lower edge of the internal sphincter and the lower part of the external sphincter. The above picture is the anal canal. 2. Rectum The upper end of the rectum is in the plane of the third sacral vertebra, connected to the sigmoid colon, and connected to the anal canal at the tooth line. The upper end of the rectum is similar in size to the colon, and the lower end of the rectum is enlarged into a rectal jug, which is the temporary storage site before the feces is expelled, and the lower end becomes thinner and connects to the anal canal. The location of the rectum in the pelvic cavity is closely related to the ventral surface of the sacral vertebrae, which has the same curvature as the sacral vertebrae. When performing sigmoidoscopy, attention must be paid to these bends to avoid damage to the intestinal wall. The upper 1/3 of the rectum is covered with peritoneum in front and on both sides; the middle 1/3 is covered with peritoneum in front and is reflexed forward to form a rectal bladder sink or rectal uterine sink; the lower 1/3 is located outside the peritoneum, so the rectum is a half of the intestinal tract inside and outside the abdominal cavity. The rectum has no real mesentery, but its upper posterior, the peritoneum is often wrapped around the rectum on the blood vessels and cellular tissue, therefore, some people called rectal mesentery. On both sides there are lateral ligaments that anchor the rectum to the lateral wall of the pelvis. The mucosa of the rectocecal abdomen has three folds, upper, middle and lower, containing annular muscle fibers, called the rectal flap. The middle flap is often opposite to the plane of peritoneal reflexion. However, the number of rectal flaps can vary, up to 5. The rectal flap disappears when the rectum is distended, and the rectal flap has the function of preventing feces from being expelled. The above pictures show all the structures of the anus. 3. There are two kinds of muscles with different functions in the anal canal and rectum, one is the random muscle, which is located outside the anal canal, i.e. the external anal sphincter and the anal raphe; the other is the involuntary muscle, which is inside the wall of the anal canal, i.e. the internal anal sphincter; the middle muscle layer is the joint longitudinal muscle, which has both random and involuntary muscle fibers, but the latter is more frequent. The above muscles can keep the anal canal closed and open. (1) internal anal canal sphincter: the rectal muscle layer is also divided into the outer longitudinal muscle and the inner ring muscle. The circular muscle is thickened at the lower end of the rectum to form the internal anal canal sphincter. Its function: ① when there is no defecation, the internal sphincter is in a continuous involuntary contraction state, closing the anal canal. When defecation occurs, it has the function of “forcing”, pushing out the fecal mass and emptying the anal canal. ③When actively closing the anal canal, the internal sphincter has the function of supplementing the random muscles (such as external sphincter and puborectalis muscle). ④It can be sufficiently relaxed to ensure adequate dilatation of the anal canal.