出典(authority):フリー百科事典『ウィキペディア(Wikipedia)』「2014/06/12 14:58:04」(JST)
The vertebral column, also known as backbone or spine, is a bony structure found in vertebrates. It is formed from individual bones called vertebrae (singular: vertebra).
In human anatomy, the vertebral column usually consists of 24 articulating vertebrae,[1] and nine fused vertebrae in the sacrum and the coccyx.[citation needed] It is situated in the dorsal aspect of the torso, separated by intervertebral discs. It houses and protects the spinal cord in its spinal canal, and hence is commonly called the spine, or simply backbone.
There are normally 33 vertebrae in humans, including the five that are fused to form the sacrum (the others are separated by intervertebral discs) and the four coccygeal bones that form the tailbone.
The upper three regions comprise the remaining 24, and are grouped under the following names:
This number is sometimes increased by an additional vertebra in one region, or it may be diminished in one region, the deficiency often being supplied by an additional vertebra in another. The number of cervical vertebrae is, however, very rarely increased or diminished.[citation needed]
In animals, vertebrae are defined by the regions of the vertebral column they occur in. Cervical vertebrae are those in the neck area. With exception of two sloth genera (Choloepus and Bradypus) and the manatee (Trichechus),[2] all mammals have seven cervical vertebrae.[3] In other vertebrates the number of cervical vertebrae can range from a single vertebra in amphibians, to as many as 25 in swans or 76 in the extinct plesiosaur Elasmosaurus. The dorsal vertebrae range from the bottom of the neck to the top of the pelvis. Dorsal vertebrae attached to ribs are called thoracic vertebrae, while those without ribs are called lumbar vertebrae. The sacral vertebrae are those in the pelvic region, and range from one in amphibians, to two in most birds and modern reptiles, or up to 3 to 5 in mammals. When multiple sacral vertebrae are fused into a single structure, it is called the sacrum. The synsacrum is a similar fused structure found in birds that is composed of the sacral, lumbar, and some of the thoracic and caudal vertebra, as well as the pelvic girdle. Caudal vertebrae compose the tail, and the final few can be fused into the pygostyle in birds, or into the coccygeal or tail bone in chimpanzees (and humans).
Individual vertebrae are composed of a centrum (body), arches protruding from the top and bottom of the centrum, and various processes projecting from the centrum and/or arches. An arch extending from the top of the centrum is called a neural arch, while the hemal arch or chevron is found underneath the centrum in the caudal (tail) vertebrae of fish, most reptiles, some birds, some dinosaurs and some mammals with long tails. The vertebral processes can either give the structure rigidity, help them articulate with ribs, or serve as muscle attachment points. Common types are transverse process, diapophyses, parapophyses, and zygapophyses (both the cranial zygapophyses and the caudal zygapophyses).
The centra of the vertebra can be classified based upon the fusion of its elements. In aspidospondyly, bones such as the neural spine, the pleurocentrum and the intercentrum are separate ossifications. Fused elements, however, classify a vertebra as having holospondyly.
A vertebra can also be described in terms of the shape of the ends of the centra. Centra with flat ends are acoelous, like those in mammals. These flat ends of the centra are especially good at supporting and distributing compressive forces. Amphicoelous vertebra have centra with both ends concave. This shape is common in fish, where most motion is limited. Amphicoelous centra often are integrated with a full notochord. Procoelous vertebrae are anteriorly concave and posteriorly convex. They are found in frogs and modern reptiles. Opisthocoelous vertebrae are the opposite, possessing anterior convexity and posterior concavity. They are found in salamanders, and in some non-avian dinosaurs. Heterocoelous vertebrae have saddle-shaped articular surfaces. This type of configuration is seen in turtles that retract their necks, and birds, because it permits extensive lateral and vertical flexion motion without stretching the nerve cord too extensively or wringing it about its long axis.
The vertebrae of lobe-finned fishes consist of three discrete bony elements. The vertebral arch surrounds the spinal cord, and is of broadly similar form to that found in most other vertebrates. Just beneath the arch lies a small plate-like pleurocentrum, which protects the upper surface of the notochord, and below that, a larger arch-shaped intercentrum to protect the lower border. Both of these structures are embedded within a single cylindrical mass of cartilage. A similar arrangement was found in the primitive Labyrinthodonts, but in the evolutionary line that led to reptiles (and hence, also to mammals and birds), the intercentrum became partially or wholly replaced by an enlarged pleurocentrum, which in turn became the bony vertebral body.[4] In most ray-finned fishes, including all teleosts, these two structures are fused with, and embedded within, a solid piece of bone superficially resembling the vertebral body of mammals. In living amphibians, there is simply a cylindrical piece of bone below the vertebral arch, with no trace of the separate elements present in the early tetrapods.[4]
In cartilagenous fish, such as sharks, the vertebrae consist of two cartilagenous tubes. The upper tube is formed from the vertebral arches, but also includes additional cartilagenous structures filling in the gaps between the vertebrae, and so enclosing the spinal cord in an essentially continuous sheath. The lower tube surrounds the notochord, and has a complex structure, often including multiple layers of calcification.[4]
Lampreys have vertebral arches, but nothing resembling the vertebral bodies found in all higher vertebrates. Even the arches are discontinuous, consisting of separate pieces of arch-shaped cartilage around the spinal cord in most parts of the body, changing to long strips of cartilage above and below in the tail region. Hagfishes lack a true vertebral column, and are therefore not properly considered vertebrates, but a few tiny neural arches are present in the tail.[4]
The general structure of human vertebrae is fairly typical of that found in mammals, reptiles, and birds. The shape of the vertebral body does, however, vary somewhat between different groups. In mammals, such as humans, it typically has flat upper and lower surfaces, while in reptiles the anterior surface commonly has a concave socket into which the expanded convex face of the next vertebral body fits. Even these patterns are only generalisations, however, and there may be variation in form of the vertebrae along the length of the spine even within a single species. Some unusual variations include the saddle-shaped sockets between the cervical vertebrae of birds and the presence of a narrow hollow canal running down the centre of the vertebral bodies of geckos and tuataras, containing a remnant of the notochord.[4]
Reptiles often retain the primitive intercentra, which are present as small crescent-shaped bony elements lying between the bodies of adjacent vertebrae; similar structures are often found in the caudal vertebrae of mammals. In the tail, these are attached to chevron-shaped bones called haemal arches, which attach below the base of the spine, and help to support the musculature. These latter bones are probably homologous with the ventral ribs of fish. The number of vertebrae in the spines of reptiles is highly variable, and may be several hundred in some species of snake.[4]
In birds, there is a variable number of cervical vertebrae, which often form the only truly flexible part of the spine. The thoracic vertebrae are partially fused, providing a solid brace for the wings during flight. The sacral vertebrae are fused with the lumbar vertebrae, and some thoracic and caudal vertebrae, to form a single structure, the synsacrum, which is thus of greater relative length than the sacrum of mammals. In living birds, the remaining caudal vertebrae are fused into a further bone, the pygostyle, for attachment of the tail feathers.[4]
Aside from the tail, the number of vertebrae in mammals is generally fairly constant. There are almost always seven cervical vertebrae (sloths and manatees are among the few exceptions), followed by around twenty or so further vertebrae, divided between the thoracic and lumbar forms, depending on the number of ribs. There are generally three to five vertebrae with the sacrum, and anything up to fifty caudal vertebrae.[4]
The vertebral column in dinosaurs consists of the cervical (neck), dorsal (back), sacral (hips), and caudal (tail) vertebrae. Dinosaur vertebrae possess features known as pleurocoels, which are hollow depressions on the lateral portions of the vertebrae, which served to decrease the weight of these bones without sacrificing strength. Pleurocoels are filled with air sacs, according to some researchers. The sauropod dinosaurs are known for their unusually long tails, which were anchored in some cases by over 50 caudal vertebrae. In many hadrosaur and theropod dinosaurs, the caudal vertebrae were reinforced by tendons. The presence of three or more sacral vertebrae, in association with the hip bones, is one of the defining characteristics of dinosaurs. The occipital condyle is a structure on the posterior part of a dinosaur's skull which articulates with the first cervical vertebra.[5]
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リンク元 | 「頚椎」「椎骨」「spinal column」「vertebral」「椎」 |
概念 | 疫学 | 自覚症状 | 他覚症状 | ||||
頚椎症状 | 神経根症状 | 脊髄症 | 神経根症 | 脊髄症 | |||
頚椎椎間板ヘルニア | 椎間板の退行変性に基づく線維輪断裂部からの椎間板組織の脱出。後方正中ヘルニア→脊髄症。後側方のヘルニア→神経根圧迫 | 30-50歳代。男性。中下位頚椎 | 喉頭・頚部から肩甲背部の疼痛、しびれと頚椎運動制限を呈する。通常頚椎の運動時に増悪し、安静にて軽快する。 | 一側の肩甲背部の疼痛、上肢へ放散する疼痛、しびれと感覚障害、脱力、筋萎縮、筋の線維性攣縮などを呈する | 感覚以上は手指、手掌全体に及ぶしびれ感が主体で、体幹、下肢に広がる。運動系では、手指巧緻運動障害を訴える。下肢痙攣麻痺(ぎこちない歩行、階段下降時に手すりが必要、走れない)。進行すると膀胱直腸障害を自覚 | 神経障害部位に一致した上肢の筋力低下、および筋萎縮、感覚障害、腱反射減弱。Spurlingテスト陽性が多い。 | 上肢に障害髄説に一致した腱反射低下、筋力低下。それ以下は錐体路障害による腱反射亢進(Hoffmann反射、Rossolimo徴候、Mendel-Bekhterev反射、膝・足クローヌス陽性)。手指巧緻運動障害。感覚障害は初期に上肢、故知に体幹・下肢に拡大。腹壁反射、睾丸挙筋反射消失。Babinski反射陽性。排尿障害は軽微 |
変形性頚椎症 | 椎間板の退行変性により、椎間板腔の狭小化、椎体近縁の骨硬化・骨棘形成、椎間関節の狭小化などを生じる。これにより、可動域制限、疼痛、こり感などの局所症状を呈した状態 | 中下位頚椎。高齢者ではC3-4椎間。 | 椎間板および椎間関節の変形などによる頚肩部の疼痛、運動制限 | ||||
頚椎症性神経根症 | 変形性頚椎症に加え、神経根症を呈した状態 | 圧迫に伴う神経根刺激症状として、上肢のしびれ、放散痛、感覚異常(後根)がある。 | Jacksonテスト、Spurlingテスト陽性。神経脱落症状としては、感覚鈍麻、脱失および上肢の脱力、筋萎縮筋の線維束攣縮が見られる。 | ||||
頚椎症性脊髄症 | 頚椎症性神経根症に加え、脊髄症を合併した状態。 | 上肢における巧緻運動障害、myelopathy hand、下肢腱反射亢進、病的反射の亢進、痙性歩行障害などの痙性麻痺および神経因性膀胱などが見られる。 | |||||
頚椎後縦靭帯骨化症 | 椎体および椎間板の後面にあり脊柱管の前壁をなす後縦靭帯が肥硬・骨化し、脊髄を緩徐に圧迫して脊髄症状を引き起こす疾患。 | 後縦靭帯骨化:男性4%、女性2%。 | 頚椎可動性の減少、肩こり、頚部痛が見られる。重要な障害は圧迫による脊髄症の麻痺症状である。一般に脊髄症は緩徐に進行する。外傷を契機に急激に悪化する場合もある。受診時に、多くの患者は種子のしびれや巧緻運動障害、下肢の痙性麻痺による歩行障害を呈する。 |
頚椎 | cervical vertebrae | 7 |
胸椎 | thoracic vertebrae | 12 |
腰椎 | limbar vertebrae | 5 |
仙椎 | sacral vertebrae | 5 |
尾椎 | coccygeal vertebrae | 1(5-6) |
環椎 | 軸椎 | 頚椎 | 胸椎 | 腰椎 | |
椎体の特徴 | なし | 歯突起 | 小型で、横径が前後径より長い。上面は凹み、下面は膨らむ | 心臓型である。肋骨頭と関節する窩(facet)を持つ | 巨大。上方ないし下方から見ると腎臓型 |
椎孔の特徴 | 歯突起が通る | 大型で三角 | 円形で、頚椎と腰椎よりも小型である。 | 三角形。胸椎よりも大きく、頚椎よりも小さい。 | |
横突起の特徴 | 横突孔がある。 | 横突孔がある。 | 横突孔がある。第7頚椎では小型あるいは欠如。椎骨動脈と伴行静脈と交感神経叢が横突孔を通るが、ただし第7頚椎では小さな頚骨静脈を通すのみ。前結節と後結節がある。 | 長く丈夫で、後外側に伸びる。第1から第12胸椎に向かって短くなる(第1~第10胸椎には肋骨の結節と関節するための関節面がある) | 長く細い。この突起の基部の後面に副突起がある。 |
関節突起の特徴 | 後頭顆を受ける | 下関節面は下前方に向く。 | 上関節面は上後方に向く。下関節面は下前方に向く。 | 上関節面は後方やや外側に向く。下関節面は、前方やや内側に向く。 | 上関節面は後内側(ないし内側)を向く。下関節面は、前外側(ないし外側)を向く。上関節突起の後面には乳様突起がある。 |
棘突起の特徴 | なし | 先が二分する | 第3~第5頚椎では短く先が二分する。第6頚椎では長いが、第7頚椎ではさらに長い。 | 長く後下方に傾斜する。先端は下位の椎体の高さに達する。 | 短く丈夫である。 |
その他 | 前弓、後弓、前結節、後結節、歯突起窩 | 歯突起 |
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