What is the relationship between the anal canal and the adjacent tissues? The anal canal is connected to the lower end of the rectum and is the final passage for defecation. The superior plane of the anal canal: in men, it is at the same level as the tip of the prostate; in women, it is at the same level as the body of the perineum. The anal canal is surrounded by the internal and external sphincters, the joint longitudinal muscles and the anal levator muscle. The anterior part of the anal canal is in contact with the perineum; in males, it is adjacent to the urethral membrane, the urethral bulb and the posterior border of the urogenital diaphragm; in females, it is adjacent to the vestibule and the lower third of the vagina. Posteriorly, it is connected to the coccyx by the anal caudal ligament; on both sides is the sciatic rectal fossa. In addition to the above-mentioned tissues, there are two special structures adjacent to the anal canal: the front is a tough tendon structure formed by the muscles and fascia of the anus, rectum and perineum, called the perineal center point, also known as the perineal body; the back is the anal caudal body, also known as the anal caudal gland, located under the tip of the tailbone and below the anal levator muscle. The anal caudal body is embedded in a pseudocapsule of adipose tissue and fibrous connective tissue, which originates from the middle sacral artery, a group of convoluted vascular tissues with input arteries and output veins, the role of the anal caudal body is unknown so far. What are the 4 boundaries within the anal canal from top to bottom? What is the clinical significance of each? The internal part of the anal canal can be divided into four boundaries from top to bottom. (1) The rectal line of the anal canal, which is about 1.5 cm above the dentate line, i.e. the plane of the rectal ring of the anal canal. Because the anal canal rectal ring consists of the puborectalis muscle, the deep and latent layer of the external sphincter, and part of the internal sphincter, which is the main tissue for maintaining normal defecation, this line is often used clinically as a marker to check the functional status of the sphincter around the anal canal and to provide a better anatomical location for surgery. (2) The dentate line, also called the comb line. It is the boundary between the skin of the anal canal and the mucosa of the rectum. Since the tissues, blood supply, lymphatic return, innervation, and embryonic formation are different above and below the dentate line, it has an important role in clinical diagnosis and treatment of diseases, and most anorectal diseases also occur near the dentate line. (3) Anal line, this line is located in the middle part of the anal canal, about 1 cm below the dentate line, and a groove can be felt by inserting a finger into the anal canal and pressing on the four walls, which is in the shape of a ring. The upper part of this line is the internal sphincter and the lower part of the external sphincter is the lower part of the skin. Therefore, it is also called the intersphincter line. It is also called the white anal line because it lacks blood vessels and is lighter in color. It is used clinically as an important marker to distinguish the internal and external sphincter. (4) Anal canal skin line, which is the demarcation line between the anal canal and the skin of the body, and is also called the anal margin in some cases. What are the muscles around the anorectum? What are the physiological functions of each? The muscles around the anorectum can be divided into two categories, one is the random muscles, including the external sphincter and the anal levator; the other is the involuntary muscles, including the internal sphincter and the joint longitudinal muscles. (1) External sphincter: The external anal sphincter is divided into three layers by the joint longitudinal muscle fibers, in order of subcutaneous layer, latent layer and deep layer. Its physiological function is to close the anus normally, stretch it during defecation to help defecate, and close it immediately after defecation. (2) Anal levator muscle: Generally, it is divided into three parts: puborectalis, pubococcygeus, and iliococcygeus. It plays an important role in supporting the pelvic viscera, helping defecation and sphinctering the anus. (3) Internal sphincter: It is made up of the rectal circular muscle bundle that extends downward and thickens. The physiological functions of the internal sphincter are to close the anus normally, to squeeze the fecal mass through the anal canal during defecation, and to close the anal canal after defecation. (4) Joint longitudinal muscle: This longitudinal muscle is formed by the convergence of the anal raphe and rectal longitudinal muscle at the upper end of the anal canal, and it travels downward between the internal and external sphincters. These two fibers gradually decrease downward, and almost completely disappear in the plane of the lower edge of the internal sphincter, replaced by elastic fibers, but there are a small number of terminal fibers containing smooth muscle. Its physiological function is to fix the anal canal and assist the anal sphincter function.4. What are the perianorectal gaps? There are seven perianorectal interstices, which are filled with fatty connective tissue, and through which lymph, blood vessels and nerves pass, and in which branches of the anal ducts are located. Due to their anatomical location and special structure, they are often prone to abscesses and secondary anal fistulas. The perianorectal space is clinically divided into two categories: supra-anal raphe (deep space) and infra-anal raphe (superficial space). (1) Supra-anal raphe: A pelvic rectal gap: on both sides of the rectum, one on each side. The upper is the peritoneum, the lower is the anal raphe, the front is the bladder or vagina, and the back is the lateral rectal ligament, and its position is deep, so it is not easy to find when an abscess is formed. b Posterior rectal gap (presacral gap): located between the rectum and the sacrum. The upper is the reflexed peritoneum, the lower is the anal raphe, the anterior is the rectum, and the posterior is the sacrum and the presacral fascia, which can be connected with the pelvic rectal gap on both sides. (2) Subanal raphe interstitial space: A Scirorectal interstitial space: located between the anal canal and the sciatic tuberosity, one on each side, superiorly by the anal raphe and inferiorly by the skin, and can be connected through the anterior and posterior sides of the anal canal. B Posterior deep anal interstitial space: located at the posterior side of the anal canal and the anal canal, superiorly by the anal raphe, inferiorly by the superficial part of the external sphincter, and posteriorly by the coccyx. Infection of the posterior anal canal space can spread to both sides of the scirorectal space and form posterior horseshoe-shaped abscesses and sinus tracts.C Superficial posterior anal canal space: superficial external sphincter superiorly, subcutaneous external sphincter anteriorly, and skin inferiorly; infection in this space is limited to subcutaneous tissue and does not affect the anal canal, scirorectal space, or deep posterior anal canal space.D Deep anterior anal canal space: superficial external sphincter inferiorly, attached to the central tendon of the perineal body, and superiorly bounded may extend to the rectovaginal diaphragm, posteriorly to the deep external sphincter, and anteriorly to the urogenital diaphragm. This gap is connected posteriorly to the scirorectal space.E Anterior superficial anal canal gap: the same as the posterior superficial anal canal gap, infection is confined to the subcutaneous tissue.F Perianal subcutaneous gap: located around the lower third of the anal canal, the upper part of which is the outward extension of the longitudinal rectal muscle fibers, the lower part is the skin, and the inner part is the lower part of the external sphincter skin.G Sphincter gap: at the joint longitudinal layer between the internal and external sphincters, infection often comes from the anal glands and spreads outward, upward, and downward. The infection often originates from the anal glands and spreads outward, upward and downward to form different kinds of perianal abscesses and anal fistulas. 5. What are the characteristics of anorectal blood circulation? The arterial supply of the anorectum comes from the superior and inferior rectal arteries, the anal artery and the middle sacral artery. The superior rectal artery is the terminal branch of the inferior mesenteric artery. The inferior rectal artery is divided from the internal iliac artery, one on each side, and enters the rectum through the lateral rectal ligament. The anal artery divides from the 2 lateral internal pubic arteries and distributes the anal canal region through the sciatic rectal fossa. The middle sacral artery divides from the posterior wall of the bifurcation of the abdominal aorta, descends immediately anterior to the sacrum, and distributes in the rectum. The distribution status of the veins in the anorectal region is similar to that of the arteries, with the superior and inferior rectal veins, the internal pubic vein, and the anal vein. The superior rectal vein starts from the venous plexus under the rectal mucosa above the dentate line and merges into several veins, which pass through the rectal wall and enter the portal vein through the inferior mesenteric vein. The venous plexus below the dentate line converges into several branches, partly into the anal canal vein and the internal pubic vein, partly into the inferior rectal vein, and directly into the internal iliac vein and into the inferior vena cava. The anorectal blood circulation is partly in the portal vein system and partly in the inferior vena cava system, and there are traffic branches between them, so varicose veins are easily formed here, especially the recto-anal veins have no venous valves, so the venous plexus is easily dilated into hemorrhoids.