Larynx - larynx

The laryngeal cavity is an important organ that functions as the airway for the respiratory system and also as a vocal organ. It is a tubular organ about 5 cm long that begins at the bottom of the hyoid bone on the midline of the anterior neck and extends to the height of the 4th to 6th cervical vertebrae. The wall of the tube is made up of six types of laryngeal cartilage (one thyroid cartilage, cricoid cartilage, epipatic cartilage, one pair of arytenoid cartilage, small corniculate cartilage, and cuneate cartilage) and is connected by ligaments and laryngeal muscles, and inside the larynx is the laryngeal cavity. The inner wall of this cavity is covered with mucous membrane. The upper part of the laryngeal cavity begins at the laryngeal opening, and the lower part continues to the trachea, bordering the cricoid cartilage. The spoon-shaped epiglottis protrudes backward from the anterior wall of the laryngeal opening and serves as the lid of the laryngeal opening. The thyroid cartilage is the largest of the laryngeal cartilages, and constitutes most of the anterior and lateral walls of the larynx. The thyroid cartilage is made up of two plates (left and right) that extend to the left and right, and join at an acute angle at the midline, forming the laryngeal prominence (Adam's apple), which protrudes subcutaneously. The prominence at the top end is particularly pronounced, and becomes more pronounced in males during puberty. The upper edge of the laryngeal prominence has a deep notch that can be felt subcutaneously. The scientific name for the thyroid cartilage is thyreos, which means shield, and comes from the fact that the rectangular shields carried by ancient Greek soldiers had a notch at the top center through which one could see forward. Below the thyroid cartilage is the cricoid cartilage, which also touches the subcutaneous layer. The cricoid cartilage is the upper end of the trachea.

On both outer walls of the inner surface of the laryngeal cavity, there are upper and lower folds running back and forth at almost the middle height, dividing the laryngeal cavity into two parts, upper and lower. The lower folds are called vocal cords, and the glottis is formed between the opposing vocal cords. The average length of the vocal cords in Japanese men is 2 cm, and 1.5 cm in women. The glottis is the narrowest part of the laryngeal cavity, and the glottis is formed by the vocal cords and the glottis. Speech is produced by the subtle tension of the vocal cords due to the action of the laryngeal muscles and the opening and closing of the glottis. The folds above the vocal cords are called the vestibular folds (false vocal folds), and the area above this to the laryngeal opening is called the laryngeal vestibule. A deep spindle-shaped depression is formed toward the outside between the vocal cords and the vestibular folds, and this is called the laryngeal chamber. The mucous membrane of this area is particularly rich in secretory glands, which help to lubricate the upper surface of the vocal folds. The mucous membrane of the larynx is made of ciliated mucous epithelium (pseudociliated epithelium) like the mucous membrane of the trachea and bronchi, but only the surface of the vocal folds is covered with stratified squamous epithelium, the same as the skin.

The laryngeal muscles are divided into the external laryngeal muscles and the intrinsic laryngeal muscles. The intrinsic laryngeal muscles are located inside the larynx and change the relative positions of the cartilages, controlling the opening and closing of the glottis and the tension of the vocal cords. A part of the thyroarytenoid muscle, one of the intrinsic laryngeal muscles, runs through the vocal folds, so the muscle fibers of this part are called the vocal cord muscles.

The larynx is a part of the airway and serves as the voice box, but it also has a closing function, closing the cavity during swallowing to prevent food from entering the airway. In this case, the laryngeal opening and glottis are closed.

[Kazuyo Shimai]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

呼吸器官の気道となると同時に発声器官としての役割をもつ重要な器官。部位は前頸部(ぜんけいぶ)の正中線で舌骨の下部から始まり、第4～第6頸椎(けいつい)の高さにわたっている、長さ約5センチメートルの管状構造の器官である。管壁は6種類の喉頭軟骨（1個の甲状軟骨、輪状軟骨、喉蓋(こうがい)軟骨と、1対の披裂(ひれつ)軟骨、小角軟骨、楔状(けつじょう)軟骨）と、これらを結合する靭帯(じんたい)や喉頭筋が付着して構成され、喉頭内部には喉頭腔(くう)が存在する。この内壁は粘膜で覆われている。喉頭腔の上方は喉頭口で始まり、下方は輪状軟骨を境にして気管へと続いている。喉頭口には前壁からさじのような形をした喉頭蓋（会厭(ええん)）が後方に突出して喉頭口の蓋(ふた)となっている。喉頭軟骨のうちでは甲状軟骨が最大で、喉頭の前壁と側壁の大部分を構成している。甲状軟骨は左右に広がる2枚の板（左板と右板）が壁をつくり、両板は正中線で鋭角的に結合し、この結合部は喉頭隆起（アダムのリンゴ）となり、皮下で隆起している。とくに上端部の突出が著しく、また思春期には男性で著明となる。喉頭隆起の上端縁には深い切れ込みがあり、皮下に触れることができる。甲状軟骨の学名はthyreosといい、盾の意であるが、これは古代のギリシア兵士が持った長方形の盾の上端中央に前方をのぞける切れ込みがあったことに由来する。甲状軟骨の下方には輪状軟骨があり、これも皮下に触れる。輪状軟骨は気管の上端にあたる。

　喉頭腔内面の両外側壁にはほぼ中央の高さで前後に走る上・下のヒダ（襞）があり、喉頭腔を上・下の2部分に区分している。下方のヒダを声帯ヒダ（声帯）とよび、相対する声帯ヒダの間に声門裂ができる。声帯ヒダの長さは日本人の場合、男2センチメートル、女1.5センチメートルが平均とされる。声門裂は喉頭腔ではもっとも狭い部分で、声帯ヒダと声門裂によって声門が形成される。喉頭筋の働きによる声帯ヒダの微妙な緊張状態と声門裂の開閉とによって発声が生じる。声帯ヒダの上方のヒダは前庭ヒダ（仮声ヒダ）といい、これより上方の喉頭口までを喉頭前庭とよぶ。声帯ヒダと前庭ヒダとの間には外側方に向かって紡錘形の深いくぼみができ、これを喉頭室とよぶ。この部分の粘膜にはとくに豊富な分泌腺(せん)があり、声帯ヒダの上面を滑らかにするのに役だっている。喉頭粘膜は気管や気管支の粘膜と同様に線毛をもった粘膜上皮（多列線毛上皮）であるが、声帯ヒダの表面だけは皮膚と同じ重層扁平(へんぺい)上皮で覆われている。

　喉頭筋には外喉頭筋群と内喉頭筋群とが区別される。内喉頭筋群は喉頭内に局在し、軟骨の相互の位置を変化させ、声門の開閉や声帯の緊張度の調節をつかさどっている。内喉頭筋の一つである甲状披裂筋の一部は声帯ヒダの中を走るので、この部分の筋線維を声帯筋とよぶ。

　喉頭は気道の一部であるとともに発声器となるが、また閉鎖作用をもち、食物が食道へ送られる嚥下(えんげ)の際、内腔を閉じて、食物の気道への流入を防いでいる。この場合には喉頭口と声門裂の閉鎖が行われる。

［嶋井和世］

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