Pelvis - kotsuban (English spelling) pelvis

Japanese: 骨盤 - こつばん（英語表記）pelvis

Commonly known as the hipbone, it is a ring-shaped skeleton resembling a mortar or a water basin. Its Latin scientific name, Pelvis, means "basin." The pelvis is a skeleton that forms an extremely thick and solid wall, unlike the bones of the skull and thorax, and supports the movable parts of the spine from the lumbar vertebrae onwards, as well as the weight of the trunk. It is particularly useful for holding the intestines and pelvic organs, and is also the base of the lower limbs. The pelvis is made up of four bones: the left and right hip bones that make up the outer and front walls, and the sacrum and coccyx that make up the back wall. The left and right hip bones are formed by the fusion of the ilium, pubis, and ischium, and are joined at the pubic symphysis in the front, and at the back, they are joined at the ear-like surfaces on the left and right sides of the central sacrum to form the sacroiliac joint. The coccyx is attached to the lower end of the sacrum.

When viewed from the inner wall of the pelvis, the promontory at the front edge of the upper end of the sacrum, the arcuate line running as the bony ridge on the inner surface of the ilium, the upper edge of the pubic symphysis, and the circular line passing through these are called the line of division, and the surface of this line of division is used to distinguish the upper part into the greater pelvis and the lower part into the lesser pelvis. The greater pelvic cavity is completely open at the front, while the lesser pelvic cavity (also called the pelvic cavity) is completely cylindrical. The greater pelvic cavity is located at the lower part of the abdominal cavity, and the broad surfaces of the wings of the ilium are on both outer walls, and it expands overall forward and upward. This space surrounded by the line of division is called the superior pelvic inlet. The lower edge of the lesser pelvis is surrounded by an irregular line and is not flat, but the space within this line is called the inferior pelvic inlet. The anterior midline of the inferior pelvic inlet is at the junction of the left and right pubic bones, and below this junction is the inferior pubic angle. The arched line extending from the inferior angle of the pubic bone to the lower edges of the left and right pubic bones and ischium is called the pubic arch.

There are three main diameters that indicate the width of the upper pelvic opening: the anteroposterior diameter, the horizontal diameter, and the oblique diameter. The anteroposterior diameter (vertical diameter) is the line that connects the upper edge of the pubic symphysis and the promontory, and is also called the true symphysis in anatomical terms. In men, it is about 10.5 cm, and in women, it is about 11.5 cm. In addition to this, the distance from a point 1 cm below the upper edge of the pubic symphysis to the promontory is called the obstetric symphysis diameter, which is closely related to the progress of childbirth. The horizontal diameter is the maximum distance between the lines of equinus, and is about 18 cm in men and about 12.5 cm in women. The oblique diameter is the distance that runs diagonally from the sacroiliac joint on one side to the iliopectineal eminence on the other side, and is about 11 cm in men and about 12 cm in women. There are two diameters to indicate the width of the lower pelvic opening: the anteroposterior diameter and the horizontal diameter. The anteroposterior diameter is from the tip of the coccyx to the lower edge of the pubic symphysis, and is about 10.5 cm in men and 11 cm in women, but the distance varies slightly because the coccyx moves backwards. The transverse diameter is the distance between the ischial tuberosities on both sides, and is about 9.5 cm in men and 11 cm in women.

Many measurements of the pelvis have been selected and are used for anatomical and anthropological purposes. However, in the case of women, when considering it as the birth canal through which the fetus passes, the measurements of the pelvis are particularly important in childbirth. However, since it is generally difficult to measure inside the pelvis, in clinical practice, external pelvic measurements are used to indirectly calculate the size of the pelvis. For example, the external symphysis (anteroposterior diameter) is the distance between the upper edge of the pubic symphysis and the tip of the spinous process of the fifth lumbar vertebra, and the inter-crest diameter is the distance between the left and right iliac crests. In addition, the inter-crest diameter is the distance between the anterior superior iliac spines of the left and right iliac bones, which is the indirect horizontal diameter of the upper pelvic opening. In the normal anatomical standing position, the position of the pelvis in the body is oblique to the body axis, and the surface of the upper pelvic opening is inclined forward by about 50 to 60 degrees from the horizontal plane (this is called the pelvic inclination). The surface of the lower pelvic opening is inclined by 10 to 15 degrees from the horizontal plane.

The pelvis is the most sex-specific of the skeletons of men and women. Sex-specific differences in the pelvis are said to appear in the fourth month of gestation, but there are no significant differences until infancy, and the upper pelvic opening of both sexes is oval in shape with a long anterior-posterior diameter. However, from around the age of 10, sex-specific differences become evident, and by puberty, the difference between men and women is complete. However, the size of the pelvis is not only sex-specific, but also varies greatly among individuals of the same sex. In general, the female pelvis is the birth canal for the fetus at the time of delivery, so it takes on a shape suited to this purpose. For this reason, compared to men, the greater pelvis is wider and spreads outward, and the lesser pelvis is lower and wider. In addition, the sacral promontory does not protrude as much and is flat from the front, so the upper pelvic opening is large and close to an oval shape. The lower pelvic opening is also wide, and the sacrum is wide and short, making it easier for the fetus to escape.

Furthermore, because the height of the pubic symphysis is lower in women than in men, the subpubic angle is an obtuse angle (70-80 degrees). In men, it is an acute (50-60 degree) arch. Therefore, when distinguishing between men and women using old bones, examining this subpubic angle can reliably determine the gender. Looking at the female pelvis as a whole, it is delicate and weak, with loose ligaments and shallow muscle attachments.

When people generally refer to the pelvic cavity, they are referring to the lesser pelvic cavity, whose lower pelvic opening is closed by soft tissues such as muscles and tendons, and which is penetrated by the urethra and rectum in men, and the urethra, rectum, and vagina in women.

[Kazuyo Shimai]

Animal pelvis

The pelvic girdles are a structure consisting of the three bones (ilium, pubis, and ischium) that make up the left and right pelvic girdles of vertebrates, and the part of the spine that connects them (sacrum). In mammals, these bones are united to form a single structure. The shape of the pelvis differs between males and females, and the upper cavity of the pelvis (pelvic cavity) is larger in females than in males to support the uterus during pregnancy.

The three bones of the mammalian pelvic girdle are initially connected by cartilage, but later ossification makes the boundaries unclear and they are called the hip bone. The left and right hip bones are fused to the sacrum on the dorsal side and are connected by the pelvic symphysis on the ventral midline. The pelvic symphysis consists of two parts, the pubic symphysis and the ischial symphysis. The place where the femur fits is called the pelvic acetabulum, which is surrounded by the ilium above, the pubic bone in the lower front, and the ischium in the lower back. The pubic symphysis relaxes at the end of pregnancy due to the action of a hormone called relaxin, and quickly returns to its original shape once delivery is complete.

[Seiichiro Kawashima]

Names of the various parts of the pelvis and differences by gender
©Shogakukan ">

Names of the various parts of the pelvis and differences by gender

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

俗に腰骨(こしぼね)ともいわれるもので、全体としてはすり鉢、あるいは水盤の形に似た輪状の骨格である。ラテン語学名Pelvisは「たらい」の意。骨盤は頭蓋(とうがい)や胸郭の骨とは違って、きわめて厚い堅固な壁を形成している骨格で、腰椎(ようつい)以上の脊柱(せきちゅう)可動部分を支えるほか、体幹の重量をも支えている。とくに腸管や骨盤臓器の保持に役だち、下肢骨の基部ともなっている。骨盤は4個の骨が組み合わさって構成されている。すなわち、外側と前方の壁をつくる左右の寛骨(かんこつ)、および後壁をつくる仙骨と尾骨である。左右の寛骨は腸骨、恥骨、坐骨(ざこつ)が化骨癒合したもので、前方では恥骨結合の部分で結合し、後方では中央部の仙骨の左右側面にある耳状面で結合して仙腸関節をつくっている。尾骨は仙骨の下端に付着している。

　骨盤内壁面からみて、仙骨上端前縁にあたる岬角(こうかく)、腸骨内面の骨稜(こつりょう)として走る弓状線、恥骨結合上縁、これらを通る輪状の線を分界線とよび、この分界線の面を境にして上方を大骨盤、下方を小骨盤に区別する。大骨盤腔(くう)は前方がまったく開放されているが、小骨盤腔（骨盤腔ともいう）は完全な筒形を形成している。大骨盤腔は腹腔下部にあたり、両外側壁に腸骨翼の広い面があり、全体として前上方に拡張している。分界線に囲まれたこの空間部分を骨盤上口(じょうこう)とよぶ。小骨盤の下縁は不規則な線で囲まれ、平面ではないが、この線内の空間部分を骨盤下口(かこう)とよぶ。骨盤下口の前正中部は左右恥骨の結合部にあたり、結合部の下側に恥骨下角(かかく)ができる。この恥骨下角の部分から左右の恥骨と坐骨の下縁に至る弓状線を恥骨弓とよぶ。

　骨盤上口の広さを示すものとして、前後径、横径、斜径の3主要径がある。前後径（縦径）は恥骨結合上縁と岬角とを結ぶ線で、解剖学上では真結合線ともいう。男性は約10.5センチメートル、女性は約11.5センチメートルである。前後径では、このほか、恥骨結合上縁から下方1センチメートルの点から岬角までの距離を産科結合径といい、分娩(ぶんべん)の経過に密接な関係がある。横径は分界線間の最大距離で、男性は約18センチメートル、女性は約12.5センチメートルである。斜径は一側の仙腸関節から対側の腸恥隆起まで斜めに走る距離で、男性は約11センチメートル、女性は約12センチメートルである。骨盤下口の広さを示す場合は二つの径、すなわち前後径と横径がある。前後径は尾骨尖(せん)から恥骨結合下縁までで、男性は約10.5センチメートル、女性は約11センチメートルであるが、尾骨が後方に動くため、距離に多少の幅がある。横径は左右両側の坐骨結節間の距離で、男性は約9.5センチメートル、女性は約11センチメートルである。

　骨盤には多くの計測径が選定されており、解剖学上、また人類学上の目的で利用される。しかし、女性の場合には胎児が通過する産道として考えるとき、骨盤の計測値は、分娩上とくに重要なものとなる。ところが、骨盤内の計測は一般にむずかしいものであるため、臨床上は骨盤外計測を用い、間接的に骨盤の大きさを算出する。たとえば、外結合線（前後径）として恥骨結合上縁と第5腰椎棘突起(きょくとっき)先端との距離、稜間径として左右の腸骨稜間の距離がある。また、棘間径として左右腸骨の上前腸骨棘間の距離を測るが、これは骨盤上口の間接的な横径となる。体内における骨盤の位置は、解剖学的な正常立位では体軸に対して斜めとなり、骨盤上口の面が水平面に対して約50～60度前方に傾斜している（これを骨盤傾度という）。骨盤下口の面は水平面に対して10～15度の傾きである。

　骨盤の形態は男女の骨格のなかでは、もっとも性差が著しい。骨盤の性差は胎生4か月には現れるといわれるが、幼年期まではあまり著しい相違がなく、骨盤上口をみても、男女とも前後径の長い卵円形を示している。ところが10歳前後から性差が著明に現れ始め、思春期では完全に男女差ができる。しかし、骨盤の大きさは性差ばかりでなく同性間における個人差も大きい。一般に女性骨盤は分娩時の胎児の産道となるため、これに適した形態をとる。このため、男性に比べて、大骨盤は広く外方に広がり、小骨盤は低くて広い。また、仙骨岬角の突出が弱く、前面から平らなので、骨盤上口は大きく楕円(だえん)形に近くなる。骨盤下口も広く、仙骨が幅広く短いので、それだけ胎児の脱出が容易となる。

　なお、恥骨結合の高さは女性の場合は男性より低いため、恥骨下角(かかく)が鈍角（70～80度）となる。男性の場合は鋭角（50～60度）の弓状を示している。したがって、古い骨で男女の鑑別をする場合、この恥骨下角を調べると確実に性別が判定できる。女性骨盤を全体としてみると、繊細で弱く、靭帯(じんたい)は緩く、筋の付着も浅い。

　一般に骨盤腔という場合は小骨盤腔をさし、骨盤下口は筋や腱(けん)の軟部組織で閉じられており、男性では尿道、直腸、女性では尿道、直腸、腟(ちつ)が貫いている。

［嶋井和世］

動物の骨盤

脊椎動物の左右の腰帯をつくる3骨（腸骨、恥骨、坐骨）とそれを結合する脊椎骨の部分（仙骨）からなる構造体をいう。哺乳(ほにゅう)類ではこれらの骨が合体して単一の構造体をなしている。骨盤の形態は雌雄で異なり、雌では妊娠時に子宮を支えるために、骨盤の上部腔所（骨盤腔）が雄に比べて大きい。

　哺乳類の腰帯の3骨は初め軟骨で結合しているが、のちに化骨して境界が不明瞭(ふめいりょう)となると寛骨とよばれる。左右の寛骨は、背側で仙骨と融合し、腹正中線上で骨盤結合により結ばれている。骨盤結合は恥骨結合と坐骨結合の2部分よりなる。大腿骨(だいたいこつ)が収まるところを骨盤臼(きゅう)といい、骨盤臼は上を腸骨、下前方を恥骨、下後方を坐骨が囲んでいる。恥骨結合は、妊娠期の終わりになると、リラキシンというホルモンの作用によって緩み、分娩が終わると急速にもとに戻る。

［川島誠一郎］

[参照項目] | 寛骨 | 骨盤計測 | 産道 | 恥骨 | 腸骨 | 尾骨

骨盤の各部名称と性別による差異

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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