直腸肛門の解剖と生理

Anatomy and physiology of the anorectum

要旨

　直腸肛門部の解剖と生理について解説する。S状結腸とRSの境界は，岬角と恥骨結節上縁を結んだ線である。RSとRaの境界は，第二仙骨下縁の高さである。RaとRbの境界は，腹膜反転部である。大腸内視鏡検査では，第二Houston弁が腹膜反転部に相当する。肛門管は恥骨直腸筋付着部上縁から肛門縁までの部分で，口側端にHerrmann 線があり，直腸粘膜と移行帯上皮の境界である。肛門管中央近くに歯状線が存在し，移行帯上皮と肛門上皮の境界である。肛門上皮と皮膚の境界が肛門縁である。肛門管の周りに内・外肛門括約筋，肛門挙筋が存在する。内肛門括約筋は不随意性の平滑筋で，外肛門括約筋は随意性の横紋筋である。肛門挙筋のなかの恥骨直腸筋は肛門の背側でU字を形成し，排便機能に大きく関与する。生理学上重要な排便機能や便通異常についても述べた。

　The anatomy and physiology of the anorectal region are described. The border between the sigmoid colon and the RS is a line connecting the promontory and the upper edge of the pubic tubercle. The border between the RS and the Ra is at the height of the lower edge of the second sacrum. The border between the Ra and the Rb is the peritoneal reflection. In colonoscopy, the second Houston valve corresponds to the peritoneal reflection. The anal canal is the area from the upper edge of the attachment of the puborectalis muscle to the anal verge, and contains the Herrmann line at the oral end, which is the boundary between the rectal mucosa and the transitional zone epithelium. The dentate line is located around the center of the anal canal, which is the border between the transitional zone epithelium and the anoderm. The border between the anoderm and the skin is the anal verge. The internal and external anal sphincter muscles and the levator ani are located around the anal canal. The internal anal sphincter muscle is an involuntary smooth muscle, and the external one is a voluntary striated muscle. The puborectalis muscle, which is part of the levator ani, forms a U-shape on the dorsal side of the anal canal and is deeply involved in defecation function. We also discuss physiologically important defecation functions and bowel movement abnormalities.