The anatomy structure of the anus starts from the dentate line down to the anal margin. It is about 3-4cm long and is an anatomical anal canal.
1. Anal canal
The anal canal is the end of the digestive tract, from the dentate line up to the anal margin, about 3-4 cm long, and is an anatomical anal canal. Some people extend the upper boundary of the anal canal to 1.5cm above the dentate line, which is the plane of the anorectal ring, called surgical anal canal, which is generally rarely used. The surface layer of the anal canal has columnar epithelium and transitional epithelium in the upper segment, and transitional epithelium and squamous epithelium in the lower segment. The front of the male anal canal is adjacent to the urethra and prostate, while the female is the uterus and vagina; the back is the tailbone, surrounded by internal and external sphincter muscles.
The tooth line is the boundary line between the rectum and the anal canal. It consists of the anal flap and the lower end of the anal column. The line is serrated, so it is called the tooth line (or comb line) and is an important anatomical mark. In the embryonic period, the tooth line is the junction of the endoderm and ectoderm, so the blood vessels, nerves and lymph sources on and below the tooth line are different, and the symptoms and signs of the dentition are also different. The clinical importance of the dental line is as follows: ①Above the dental line is mainly supplied by the superior and inferior rectal arteries, and below the dental line is the anal artery supply. The venous plexus above the dental line belongs to the internal hemorrhoidal venous plexus, which flows back to the portal vein. If varicose, internal hemorrhoids are formed. The venous plexus below the tooth line belongs to the external hemorrhoidal venous plexus, which stays in the inferior vena cava, and varicose forms external hemorrhoids. Infections above the dental line can cause liver abscess through the portal vein; infections below the dental line spread to the whole body from the inferior vena cava. ②The mucosa above the dentinal line is innervated by autonomic nerves, and there is no pain; the anal canal below the dentate line is innervated by spinal nerves, and the pain response is acute. Therefore, the injection and surgical treatment of internal hemorrhoids must be performed above the dentinal line. Do not involve the area below the dentinal line to prevent pain and edema. ②The lymph above the dentinal line mainly flows back to the lymph nodes around the abdominal aorta, and the lymph below the dentinal line mainly flows back to the inguinal lymph nodes. Therefore, rectal cancer metastasizes to the abdominal cavity, while anal cancer metastasizes to the bilateral inguinal lymph nodes.
The mucous membrane above the tooth line, due to contraction of the sphincter. There are 6-10 longitudinal strip folds, about 1-2cm in length, called rectal column (anal column), which can disappear when the rectum is dilated. The rectal column contains the terminal branch of the superior rectal artery and the vein of the same name formed by the superior rectal venous plexus. The internal hemorrhoids are formed by the varicose venous plexus and enlarged.
The lower ends of the rectal columns are connected by a half-moon-shaped mucosal fold, which is called anal flap. The rectal mucosa between the anal valve and the rectal column forms many pockets called anal sinuses (anal recesses). The sinus mouth is upward, about 3-5mm deep, with anal gland opening at the bottom. There are 2-8 triangular papillary protrusions under the anal flap, called anal papilla. Torn anal flap can cause anal fissure, anal sinusitis and anal papillitis. There are 4-8 anal glands in the normal anal canal, most of which are concentrated on the back wall of the anal canal, and each anal gland opens at the anal sinus. The anal gland has a tubular part under the mucosa called anal gland duct. The anal duct is divided into botryoid branches in the submucosa. 2/3 of the anal gland extends downwards and outwards to the inner sphincter layer. A few can pass through this muscle to the joint longitudinal muscle Layer, very few can enter the external sphincter, and even to the ischiorectal space. Most of the anal glands are the entrance of infection, and a few are also the sites of adenocarcinoma.
The white line is located between the dentate line and the anal margin. A groove can be felt during digital rectal examination. It is the junction of the lower edge of the internal sphincter muscle and the lower part of the external sphincter skin. It is generally invisible and can only be touched, so the term white line It's not very precise, it should be called the internal and external sphincter groove, or intersphincter groove for short.
The above picture is anal canal
The upper end of the rectum is on the plane of the third sacral vertebra, connected to the sigmoid colon, and connected to the anal canal at the dentate line. It is about 12-15cm long. The upper end of the rectum is similar in size to the colon, and its lower end expands into the ampulla of the rectum, which is the temporary storage site for stool before it is discharged, and the lower end becomes thinner to connect to the anal canal. The position of the rectum in the pelvic cavity is closely related to the ventral surface of the sacral spine, and has the same curvature as the sacral spine. The rectum has protruding curves to the left and right on the frontal plane. When performing sigmoidoscopy, you must pay attention to these curves to avoid damaging the intestinal wall. The upper 1/3 of the rectum is covered with peritoneum on the front and both sides; the middle 1/3 has peritoneum in front of it, and folds forward to form rectal bladder depression or rectal uterine depression; the lower 1/3 is all outside the peritoneum, so the rectum is abdominal cavity Half of the intestines inside and outside. There is no real mesangium in the rectum, but on the upper and back, the peritoneum often surrounds the blood vessels and cellulite in the upper rectum. Therefore, some people call it the mesangium. There are lateral ligaments on both sides to fix the rectum to the side wall of the pelvis. The mucosa of the rectal ampulla has three folds on the upper, middle and lower sides, which contain circular muscle fibers, called rectal flaps. The middle flap is often opposite to the peritoneal reflex plane. However, the number of rectal valves can vary, up to five. The rectal flap disappears when the rectum is inflated, and the rectal flap has the effect of preventing the discharge of feces.
The above picture shows the entire structure of the anus
3. The anal canal and rectal muscles have two different functions. One is the voluntary muscle, located outside the anal canal, namely the external anal sphincter and the levator ani muscle; the other is the involuntary muscle, located in the anal canal wall, namely The internal sphincter of the anal canal; the middle muscle layer is the joint longitudinal muscle, which has both voluntary and involuntary muscle fibers, but the latter are more. The above muscles can keep the anal canal closed and open.
(1) Internal anal sphincter: The rectal muscle is also divided into outer longitudinal muscle and inner circular muscle. The circular muscle thickens at the lower end of the rectum to form the internal anal sphincter. Its functions: ①When there is no defecation, the internal sphincter is in a continuous and involuntary contraction state, closing the anal canal. ②When defecation, it has a "force" effect, squeezing out the fecal mass and emptying the anal canal. ③When actively closing the anal canal, the internal sphincter can supplement voluntary muscles (such as external sphincter, puborectalis). ④ It can be fully relaxed to ensure sufficient expansion of the anal canal.