Anus - Komon

Japanese: 肛門 - こうもん

It is located at the bottom of the rectum, which is the digestive tract, and is the opening to the outside. When the rectum penetrates the pelvic diaphragm that blocks the lower opening of the pelvis inside the pelvis, it becomes the anal canal, and opens to the outside world at the anus at its lower end. The length of the anal canal is about 3 to 4 centimeters. The upper half of the inner wall of the anal canal is covered with mucosal epithelium (mucosal part), but the lower half is covered with stratified squamous epithelium that has the same properties as skin (transitional part), and at the anus it has completely the same properties as skin. Therefore, there are many black melanin pigment cells in the anus. There are large sweat glands (also called apocrine glands, and the perianal glands are an example of this), as well as hair and sebaceous glands. The part where the mucosal epithelium transitions to stratified squamous epithelium is just surrounded by the internal anal sphincter, and is called the hemorrhoidal ring (anal ring) because it is raised in a ring shape. Five to ten longitudinal folds are seen running upward from the hemorrhoids, called the anal pillars, but these folds disappear as they move upward. Between the anal pillars there is a depression called the anal sinus. At the lower end of the anal sinus there is a semi-lunar mucosal fold attached to the side, called the anal valve. The line connecting the base of the anal pillars to the height of the anal valve is called the pectinate line, and forms the boundary between the mucosal and cutaneous parts of the anal canal. Around this boundary, a white ring-shaped line is called Hilton's line, and it may be seen. This is a connective tissue fiber continuing from the external longitudinal muscle of the rectum, and it has few blood vessels, which is why it looks whitish. Large glands also open here.

The anal canal has a well-developed anal sphincter, which is involved in opening and closing the anal canal. In the middle of the anal canal, the internal circular muscle (smooth muscle) of the rectum continues to develop well and thickens to form the internal anal sphincter. This is the lowest end of the internal circular muscle that surrounds the digestive tract. The internal anal sphincter is controlled by the autonomic nervous system, contracting when stimulated by the sympathetic nervous system and relaxing when stimulated by the parasympathetic nervous system. The external anal sphincter surrounds the outside of the internal anal sphincter in a ring shape, and this muscle extends down to the subcutaneous tissue below the anal canal. Since the external anal sphincter is a voluntary muscle, its contraction can be regulated by will and it has a strong contractile force. When the external longitudinal muscle of the rectum reaches the bottom, it penetrates between the internal and external anal sphincters, reaches the subcutaneous tissue of the anus, and extends radially from the anus. This muscle is called the corrugator ani, and creates radial wrinkles around the anus. Outside the internal and external anal sphincter is the levator ani, which holds the anus up. The levator ani is part of the pelvic diaphragm, which is made up of several muscles and aponeurosis. Most of the muscle fibers originate from the inner surface of the superior pubic ramus, run backwards on both sides of the bladder and prostate (vagina in women), attach to the anus, and some reach the tailbone. When abdominal pressure is increased, the contraction of this muscle causes tension in the perineum.

The most common anal disease is hemorrhoids. A venous plexus is developed beneath the mucous membrane of the anal canal, spreading out between the muscular layer of the anal canal. Therefore, contraction of the muscular layer can easily cause blood in the venous plexus to stagnate, easily forming varicose veins. Hemorrhoids are the protuberances that form beneath the mucous membrane due to varicose veins. Hemorrhoids that form in the upper part of the anal canal (above the pectinate line) are called internal hemorrhoids, while those that form below are called external hemorrhoids; however, because the lower part of the anal canal is particularly sensitive, external hemorrhoids are generally more painful.

[Kazuyo Shimai]

Animal anus

This refers to the part at the end of the digestive tract that opens onto the body surface and expels feces and other substances from the body. In many cases, there are muscles around the anus, such as a sphincter, that control the opening and closing of the anus. In vertebrate animals, such as amphibians, reptiles, and birds, the end of the digestive tract is the cloaca, where the reproductive ducts and urethra also open, and so the exit to the outside of the body is called the cloaca. In fish and mammals, the reproductive ducts and urethra open separately from the anus.

[Sadao Yasugi]

[Reference] | Hemorrhoids

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

消化管である直腸の最下部にあり、外部への開口部となる。直腸は骨盤内で骨盤下口をふさいでいる骨盤隔膜を貫くと肛門管となり、その下端部の肛門で外界に開いている。肛門管の長さは約3～4センチメートルである。肛門管内壁の上半部は粘膜上皮で覆われているが（粘膜部）、下半部は皮膚と同じ性状の重層扁平(へんぺい)上皮で覆われるようになり（移行部）、肛門では完全に皮膚と同じ性状となる。したがって、肛門には黒色のメラニン色素細胞も多い。汗腺(かんせん)としては大型汗腺（アポクリン腺ともいい、肛門周囲腺はその一例）があるほか、毛、脂腺も存在している。粘膜上皮から重層扁平上皮に移行する部分は、ちょうど内肛門括約筋に囲まれた部分で、輪状に隆起しているため痔帯(じたい)（痔輪・肛門輪）とよばれている。痔帯から上方に向かって5～10条の縦走ひだがみられ、これを肛門柱(ちゅう)（直腸柱）とよぶが、このひだは上方に向かうにしたがって消失してしまう。肛門柱の間には肛門洞とよぶくぼみができる。肛門洞の下端は半月状の粘膜ひだが横につき、これを肛門弁とよぶ。肛門柱の基部と肛門弁の高さの部分をつないだ線を櫛状線(しつじょうせん)といい、肛門管の粘膜部と皮膚部の境となっている。この境あたりはヒルトンの線とよばれ、白色の輪状線がみられることがある。これは直腸の外縦走筋から続く結合組織性線維がついたもので、血管に乏しく、白っぽくみえるわけである。ここにはまた大型の腺が開口している。

　肛門管には肛門括約筋が発達しており、肛門管の開閉に関係している。肛門管の中部あたりでは直腸の内輪走筋（平滑筋）の続きがよく発達し、肥厚して内肛門括約筋をつくる。これは消化管を取り囲む内輪走筋の最下端部になるわけである。内肛門括約筋は自律神経の支配を受け、交感神経の刺激で収縮し、副交感神経の刺激で弛緩(しかん)する。外肛門括約筋は内肛門括約筋の外側を輪状に取り巻くが、この筋は肛門管の下方の皮下まで広がっている。外肛門括約筋は随意筋であるため、意志によって収縮の調節ができ、収縮力も強い。直腸の外縦走筋は下方に達すると内・外肛門括約筋の間に入り込み、肛門部皮下まで到達して肛門を中心に放射状に広がっている。この筋は肛門皺皮筋(こうもんしゅうひきん)とよび、肛門周囲の放射状のしわをつくっている。内・外肛門括約筋の外側には肛門挙筋があり、肛門を上につり上げている。肛門挙筋はいくつかの筋と腱膜(けんまく)で構成されている骨盤隔膜の筋の一部で、筋線維の大部分は恥骨上枝の内面からおこり、膀胱(ぼうこう)、前立腺（女性では腟(ちつ)）の両側を後方に走り、肛門につき、一部は尾骨に達している。腹圧を高めた場合、この筋の収縮で会陰(えいん)部が緊張する。

　肛門疾患でもっとも一般的なのは痔（痔核）である。肛門管の粘膜下には静脈叢(そう)が発達していて、肛門管の筋層の間に広がっている。したがって、筋層の収縮によって静脈叢の血液がうっ滞しやすく、静脈瘤(りゅう)をつくりやすい。静脈瘤によって粘膜下にできる隆起物が痔核である。肛門管の上部（櫛状線より上方）にできる痔核を内痔核、下方にできるのを外痔核とよぶが、肛門管の下方はとくに敏感なため、外痔核のほうが痛みは一般に強い。

［嶋井和世］

動物の肛門

消化管末端が体表に開口して糞(ふん)などを体外に排出する部分をいう。多くの場合は、肛門周囲に括約筋などの筋肉があって肛門の開閉を支配する。脊椎(せきつい)動物のうち両生類、爬虫(はちゅう)類、鳥類では、消化管末端が総排出腔(こう)となって生殖輸管や尿道もここに開口するために、その体外への出口は総排出口とよばれる。魚類と哺乳(ほにゅう)類では、生殖輸管および尿道が肛門とは別に開口している。

［八杉貞雄］

[参照項目] | 痔

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例

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