Ribs

Japanese: 肋骨 - ろっこつ

In vertebrates, this refers to a cartilaginous or bony long bone that connects to the vertebrae and curves in an arch toward the ventral side, surrounding the thoracic or abdominal cavity. Cyclostomes do not have ribs. In reptiles and above, the curved osteoclastic long bones are divided into true ribs, which connect to the sternum via the costal cartilage at their anterior end (abdominal end), false ribs, which connect indirectly via the costal cartilage of the upper rib, and floating ribs, which do not connect to the sternum. The mammalian albino rat has 13 pairs of ribs, of which the first through seventh ribs are true ribs, the eighth through tenth ribs are false ribs, and the last three ribs, eleventh through thirteenth ribs, are floating ribs.

In terms of development, ribs are divided into two types. The first type arises from the connective tissue in the partition that separates the muscular system into the dorsal and abdominal parts and is connected to the transverse processes of the vertebrae. It is found in cartilaginous fish and amphibians and above and is called the dorsal rib or upper rib. The second type extends just below the wall of the body cavity with the blood vessels open to the left and right. It is found in bony fish and is called the gastric rib or lower rib. However, some species (such as carp) have both dorsal and gastric ribs.

[Masayuki Uchibori]

Ribs in humans

Human ribs are long and slender, curved in an arch, and form most of the side wall of the thorax. There are 12 pairs of ribs, of which the upper seven pairs form joints with the thoracic vertebrae at their rear ends, and their front ends are connected to the outer edge of the sternum via seven pairs of costal cartilages. The first through seventh ribs are called true ribs (sternal ribs). The lower five pairs (8th through 12th ribs) are called false ribs (arch ribs), and form joints with the thoracic vertebrae at their rear ends. Of the lower five pairs, the costal cartilages of the upper three pairs of ribs are connected to the costal cartilage immediately above them and connect to the sternum, but the front ends of the lower two pairs (11th and 12th pairs) are free and do not reach the sternum. These are called floating ribs, and even the 10th rib can become free.

The central ribs (around the sixth rib) show the most common shape. At the rear end is the rib head, which articulates with the thoracic vertebrae, followed by a slender rib neck about 2 centimeters long. The rib neck transitions to the rib body, but at the border is the costal tubercle, and from around the outside of this tubercle the degree of curvature of the rib body suddenly changes, bending forward and inward toward the outer edge of the sternum, where it connects to the sternum by costal cartilage. The point where the strong bend begins is called the costal angle.

The first through seventh ribs gradually get longer, and the ribs below them get shorter again. The first, second, eleventh, and twelfth ribs are different from the general shape described above. The first rib is the shortest and most curved, and is also wide. The second rib is also strongly curved, and is about twice as long as the first rib. The eleventh and twelfth ribs are weakly curved, and have no rib neck or rib tubercle. The twelfth rib is particularly short, and may be absent. The direction of the inclination of the ribs also varies depending on the part of the rib. From the upper ribs, it gradually slopes downward and forward, becoming most steep around the ninth rib, and then gradually easing downward again.

There is a groove called the costal groove on the inside of the rib, and the intercostal artery, vein, and nerve run through this groove. The 11th and 12th ribs do not have this groove. The ribs are connected to the sternum by costal cartilage, which gives the rib cage elasticity, reduces the impact on the sternum and ribs, and increases the efficiency of the rib cage movement during breathing.

[Kazuyo Shimai]

Names of the ribs
©Shogakukan ">

Names of the ribs

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

脊椎(せきつい)動物において、椎骨と結合し、胸腔(きょうこう)や腹腔を囲んで弓状に彎曲(わんきょく)して腹方に向かう軟骨性または硬骨性の長骨をいう。円口類には肋骨はない。爬虫(はちゅう)類以上では湾曲した破骨性長骨がその前端（腹端）にある肋軟骨を介して胸骨と結合する真肋骨、上位助骨の肋軟骨を介して間接的に結合する仮肋骨(かろっこつ)、胸骨と結合しない浮肋骨(ふろっこつ)がある。哺乳(ほにゅう)類のシロネズミでは13対の肋骨があり、第1～第7肋骨の7個が真肋骨であり、第8～第10肋骨の3個は仮肋骨で、終わりの第11～第13肋骨の3個は浮肋骨である。

　発生学上は肋骨を2種に分ける。第一は筋系を背部と腹部に分ける隔壁にある結合組織より発生し椎骨の横突起に連結したもので、軟骨魚類および両生類以上のものにみられ、背肋骨(はいろっこつ)または上肋骨(じょうろっこつ)とよばれる。第二は血道突起が左右に開いたまま体腔壁直下に伸長したもので、硬骨魚類にみられ、腹肋骨(ふくろっこつ)または下肋骨(かろっこつ)とよばれる。しかし、一部のもの（たとえばコイ）には背肋骨と腹肋骨の両方がある。

［内堀雅行］

ヒトにおける肋骨

ヒトの肋骨は、細長く、弓状に曲がっており、胸郭の側壁の大部分を形成している。数は12対あるが、上位7対はそれぞれ後端で胸椎(きょうつい)と関節をつくり、前端はそれぞれ7対の肋軟骨を介して胸骨外側縁と連結している。この第1肋骨から第7肋骨までを真肋(しんろく)（肋硬骨、胸骨肋）とよぶ。下位5対（第8～第12肋骨）は仮肋(かろく)（弓肋）とよび、後端で胸椎と関節をつくる。下位5対のうち、上位の3対の肋骨の肋軟骨は、すぐ上位の肋軟骨に結合して胸骨と連絡しているが、下位2対（第11対と第12対）は前端が胸骨に達しないで遊離している。これを浮肋(ふろく)（浮遊肋骨）とよび、第10肋骨でも遊離することがある。

　肋骨は中央部の肋骨（第6肋骨あたり）がもっとも一般的な形態を示している。すなわち、後端には胸椎と関節をつくる肋骨頭があり、ここから長さ2センチメートルくらいの細長い肋骨頸(けい)が続く。肋骨頸から肋骨体に移るが、その境に肋骨結節があり、この肋骨結節の外側あたりから肋骨体の部分は急に彎曲(わんきょく)の度合いが変わり、前内方へと胸骨外側縁に向かって屈曲し、肋軟骨によって胸骨と結合する。強い屈曲を始める部分を肋骨角とよぶ。

　第1肋骨から第7肋骨までは漸次長くなり、それ以下の肋骨は、ふたたびしだいに短くなる。肋骨のうち、第1、第2、第11、第12肋骨は前述した一般的な形態とは異なっている。第1肋骨はもっとも彎曲して短く、また幅も広い。第2肋骨も彎曲が強度で、長さは第1肋骨の倍くらいである。第11、第12肋骨は彎曲が弱く、肋骨頸、肋骨結節もない。第12肋骨はとくに短く、欠如している場合もある。また、肋骨は部位によって傾斜の方向が異なっている。上位肋骨から徐々に前下方に傾き、第9肋骨あたりでもっとも傾斜が強くなり、それから下方は、またしだいに緩くなる。

　肋骨の内側には肋骨溝(こう)とよぶ溝が走るが、この溝の中を肋間動・静脈、肋間神経が走っている。第11、第12肋骨にはこの溝がない。肋骨は胸骨と肋軟骨によって結合することによって、胸郭に弾性を与え、胸骨や肋骨への衝撃を緩め、呼吸運動における胸郭の運動の効率を高めている。

［嶋井和世］

肋骨の各部名称

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