Shoulder girdle - Health

Part of the skeleton of the forelimbs of vertebrates, also called the forelimb girdle. It connects the free limbs, which are the parts of the forelimbs that protrude from the body, to the spine, and is located inside the trunk. The part that connects the free limbs of the hind limbs to the spine is called the pelvic girdle or hindlimb girdle. The shoulder and pelvic girdles together are called the limb girdle. In other words, paired appendages (paired limbs), including fish, can be divided into free limbs and limb girdles.

The shoulder and pelvic girdle have basically the same structure, but the names of the bones that make them up are different. Regarding the joints of the free limbs, in the shoulder girdle, the dorsal part is called the scapula, the anterior part on the ventral side is called the anterior coracoid cartilage, and the posterior part is called the coracoid bone. The anterior coracoid cartilage becomes the clavicle, which connects the scapula to the sternum. However, among bony fishes, holostei and teleostei lack the clavicle. In the pelvic girdle, the dorsal part is called the ilium, the anterior part on the ventral side is called the pubis, and the posterior part is called the ischium, and in amphibians and above, the whole forms a single pelvic girdle.

In fish, the limb girdle stabilizes fin movement, while in land animals it mediates the transfer of body weight to the free limbs. In fish, the dermal shoulder girdle is well developed, reinforcing the endoskeletal shoulder girdle, which is attached to the skull via the upper clavicle. However, in amphibians, the upper clavicle has become smaller, causing it to lose its connection to the skull, allowing the head to move freely relative to the torso. The upper clavicle does not exist in animals other than reptiles. Functionally, the endoskeletal shoulder girdle (known as the shoulder girdle) is important, and all the major limb muscles are attached to it. In land animals, the limb girdle becomes larger as the amount of limb muscles attached to it increases. The coronel was small in the fossil pelycosaurs, mammal-like reptiles. However, as we progressed from therapsids, the ancestors of mammals, to the monotreme, marsupial, and placental stages of mammals, not only did it become larger, but the shape of the coronel also changed significantly with changes in leg posture and attachment muscles.

[Seiichiro Kawashima]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

脊椎(せきつい)動物の有対肢のうち前肢の骨格の一部で、前肢帯ともいう。前肢の体外へ突出した部分である自由肢を脊柱に結合する役を果たし、体幹内にある。後肢の自由肢を脊柱に結ぶものを腰帯(ようたい)または後肢帯という。肩帯と腰帯をあわせて肢帯という。つまり、魚類を含め、対(つい)をなす付属肢（有対肢）は自由肢と肢帯に分けられる。

　肩帯と腰帯は基本的には同じ構造をもつが、構成する骨の名称が異なる。自由肢の関節部について、肩帯では、背側のものを肩甲骨、腹側の前方部を前烏喙(うかい)軟骨、後方部を烏喙骨（烏口(うこう)骨）という。前烏喙軟骨は鎖骨となり、肩甲骨と胸骨とをつなぐ。ただし硬骨魚類のうち全骨類と真骨類は鎖骨を欠く。腰帯では、背側を腸骨、腹側の前方部を恥骨(ちこつ)、後方部を坐骨(ざこつ)といい、両生類以上では全体としてひとかたまりの骨盤肢帯をつくる。

　肢帯は、魚類ではひれの運動を安定させ、陸生動物では体重が自由肢へかかるのを仲介する。魚類では内骨格性の肩帯を補強する皮骨性肩帯が発達していて、上鎖骨を介して頭蓋(とうがい)に結合している。しかし両生類では、上鎖骨の縮小により頭蓋との結合が消失し、頭は胴に対して自由に動けるようになった。上鎖骨は爬虫(はちゅう)類以上には存在しない。機能的には内骨格性の肩帯（いわゆる肩帯）が重要で、おもな体肢筋はすべてこれに付着する。陸生動物では、肢帯に付着する肢筋の量の増加に伴い肢帯が大きくなる。烏喙骨は、哺乳(ほにゅう)類様の爬虫類である盤竜類（化石）では小さかったが、哺乳類の祖先である獣弓類から哺乳類の単孔類、有袋類、有胎盤類段階へと進むにつれて、大きくなっただけでなく、脚(あし)の姿勢と付着筋の変化に伴い、烏喙骨の形も著しく変化した。

［川島誠一郎］

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