The anal canal refers to the part from the dentate line to the anal margin, with an average length of 2.5 cm for adults.The surgical anal canal refers to the part from the anal margin to the plane of the anorectal ring. The average length of adults is 4 cm. The surgical anal canal has 4 boundaries;(1) The anal margin, also known as the anal opening, is the lowest boundary of the digestive tract;(2) The intersphincter groove, that is, the white line, is between the anal margin and the dental line, about 1cm away from the anal margin, facing the junction of the internal and external sphincters.If you put your index finger into the anal canal, you can feel a concave groove between the internal anal sphincter and the lower part of the external anal sphincter skin, that is, the intersphincter groove;(3) Tooth line, at the junction of skin and mucosa above the white line, there is a ring of zigzag lines about 2.5 cm from the anal marginTooth line；(4) The surface between the tooth line and the white line is smooth and shiny, which is called anal comb.
Internally, there are 6~10 longitudinal mucosal wrinkled walls in the anal canal, called anal column.The circular line at the upper end of the flat anal column is the anal straight gut line.A half moon shaped mucosal fold between the lower ends of adjacent anal columns is called anal flap.The small recess formed by the anal flap and the lower end of the adjacent anal column is called the anal sinus.The opening of the anal sinus is upward, and there is often fecal debris in the sinus. After infection, it is easy to cause anal sinusitis. In serious cases, it can form anal fistula or ischiorectal abscess.A zigzag circular line is formed by connecting the lower end of the anal column and the edge of the anal flap, which is called a dentate line or anal skin line.The epithelium, blood supply, lymphatic drainage and nerve distribution covered by this line and below are completely different, which is of clinical significance.Slightly below the dentate line, there is a smooth area with circular protuberance, called anal comb. Because the deep surface of its epithelium contains venous plexus, it is light blue on the living body.The lower edge of the anal comb is a slightly wavy line, called white line, which is about 1.5cm away from the anus.The shallow groove that can be touched during clinical examination is the white line, also called the intermuscular groove, which is the junction of the internal and external anal sphincters.The anal mucosa and subcutaneous veins anastomose into a cluster, which may be silted up due to poor blood flow, resulting in varicose hemorrhoids.Those above the dentate line are internal hemorrhoids, those below the dentate line are external hemorrhoids, and those crossing the dentate line or below are mixed hemorrhoids.
The anus is the opening at the end of the anal canal, which is equivalent to 4 cm below the tip of the tailbone, and usually presents sagittal longitudinal fissure.Due to the constriction of the anal sphincter, the skin around the anus forms radial wrinkles, containing sweat glands and sebaceous glands.
The anal canal is the end of the digestive tract, from the tooth line to the anal margin, about 3~4cm long.Some people extend the upper boundary of the tube to the plane of the anorectal ring, which is called the argonaut tube of surgery.The front of the anal canal is connected with the perineal body, and in the male, it is adjacent to the urethra membrane, the urethra bulb and the posterior edge of the urogenital septum by means of the perineal body;In women, the perineum is adjacent to the vaginal vestibule and the lower 1/3 of the vagina.The posterior part is connected to the tailbone through the anal caudal ligament;On both sides are the ischiorectal fossa, surrounded by internal and external anal sphincters and joint longitudinal muscles.The inner layer of anal canal is transitional epithelium on the head side and squamous epithelium on the tail side. The surface is smooth and white, without sweat glands and sebaceous glands.If too much is removed during operation, it may lead to anal skin defect, membrane eversion and mucus overflow.The anal canal is a muscular channel connecting the rectum and the external anal sphincter. It is divided into five layers from the inside out: mucosa, submucosa, internal anal sphincter, joint longitudinal muscle and external anal sphincter.
Current situation of anatomy
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Anatomically or embryologically, the anal canal is about 3-4cm from the anal margin to the dentate line.In 1985, the World Anatomical Nomenclature Committee defined the upper edge of the anorectal ring as the upper boundary of the anal canal, and accordingly renamed the rectal column as the anal column, and the rectal sinus as the anal sinus, and described the anal canal with four lines and three bands.
Four line
one
Anal skin line: the boundary between anal canal and skin, namely anus;
two
White line: Hilton line, 1cm above the anal skin line, at the junction of the lower part of the skin of the internal sphincter and the external sphincter, also known as the intersphincter groove, is difficult to identify with the naked eye;
three
Dentate line: 2cm away from the anal margin, which is the only circular line visible to the naked eye among the four lines describing the anal canal;
four
Anal straight line: also known as Hermann line, about 1.5cm on the dentate line.
Triple band
one
Columnar zone: located between the anorectal line and the dentate line, it is the location of the rectal column, and is about 0.5-1.5cm in the annular area;
two
Middle zone: the area between the dentate line and the white line, also known as hemorrhoid zone;
three
Skin band: the area between the white line and anal skin line, which is transformed squamous epithelium.
Anal column
Also called rectal column or Morgagnis column, they are about 6-14 vertical mucosal folds on the inner wall of the rectal cavity. The length of the column is 1-2cm and the width is 0.3-0.6cm. They are the result of the contraction of the anal sphincter. When the rectum expands, this column disappears.The lower edge of each rectal column is connected by half moon shaped mucosal folds, which are called anal flaps or rectal flaps. There are 6-12 of these half moon shaped mucosal folds, which are the remains of the original anal membrane and have no function of the flaps.The lower ends of the two adjacent anal columns and the anal flap together form a small recess, called anal recess, 6-8, in the shape of a funnel, with the sinus opening facing the upper part of the intestinal cavity, the sinus bottom extending outward and downward, the sinus depth is generally about 0.3-0.5cm, and there is an opening of the anal gland at the bottom of the anal sinus or on the anal flap.There are about 2-6 triangular epithelial processes under the anal flap, 0.1-0.3cm high, called anal papilla.
