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the main part of the axial skeleton of vertebrate animals and of man. In phylogeny the spine replaces the chorda of lower chordates. In ontogeny, the development of the cartilaginous or, more commonly, the bony vertebrae making up the spine results in the contraction of the chorda found in embryos and subsequent elimination of the chorda. However, cyclostomes, dip-noans, holocephalans, and acipenserids do not have vertebral bodies even when adult. Instead, paired arches—superior in cyclostomes and superior and inferior in the others—rest freely on a well-developed chorda.
Historically, the appearance of the spine was related to increased mobility and speed in water. The loss of the flexibility that the chorda had provided was compensated for by the greater elasticity and strength of the spine, properties important for the functioning of the truncal musculature. The supportive role of the spine increased markedly when vertebrates began living on land. The need to strengthen the spine gave rise to articular processes of the vertebrae, which formerly had been jointed only by the vertebral bodies and connected by ligaments.
The secondary function of the spine is to protect the spinal cord, which is contained in a special canal formed by the vertebral bodies from below and by the widened arches from above. Functionally, the spine is divided into segments whose number increases from the lower vertebrates to the higher. Fishes have two segments, a truncal segment with strong ribs and a caudal segment whose vertebrae bear the hemal arches that protect the caudal artery and vein.
Terrestrial vertebrates have a spine consisting of four or five segments: cervical, thoracic, lumbar (absent in some groups), sacral, and caudal. The need to move the head gave rise to the cervical segment. The ribs of this segment are poorly developed or absent, while the vertebrae generally have well-developed condyles that permit great flexibility of the neck. Amphibians have only one cervical vertabra (the head can only bend down), reptiles have an average of eight (four to nine or more), birds have 11 to 25, and mammals have seven; only the manatees and didactyl sloths have six, whereas tridactyl sloths have nine or ten. The first two cervical vertebrae of amniotes, the atlas and epistropheus, have a shape permitting both tilting and rotating of the head.
The thoracic segment has well-developed ribs, most of which articulate with the sternum to form the thorax; amphibians do not have a thorax. In birds, some of the thoracic vertebrae fused to form a single structure permitting flight.
The lumbar segment has rudimentary ribs and is generally more mobile than the thoracic segment. Among amphibians and reptiles there are no clear-cut distinctions between the segments; all the vertebrae situated between the cervical and sacral segments are usually called spinal.
The sacral segment is well developed in all terrestrial vertebrates because of the special role of the rear extremities in locomotion. The vertebrae of this segment are fixed to the pelvic bones by modified ribs joined with the transverse processes to form the sacrum. Amphibians have one sacral vertebra and reptiles usually have two. The two sacral vertebrae of birds fused with the lumbar, posterior thoracic, and anterior caudal segments to form a single bone, the complex sacrum that supports the pelvis. In mammals, the sacrum also consists of one to ten (generally two to four) fused vertebrae, of which only the first and second are true sacral vertebrae.
The caudal segment, usually very mobile, performs a variety of functions. Among reptiles it is well developed, consists of many vertebrae, and is important in locomotion. The loss of the locomotor role of the tail in mammals led to its contraction. In tailless amphibians, the caudal vertebrae are fused into a single bone, the rodlike urostyle. Several of the caudal vertebrae in birds are also fused, forming the pygostyle which supports the rectrices.
V. B. SUKHANOV
In man, the spine consists of five segments: the cervical (seven vertebrae), thoracic (12), lumbar (five), sacral (five vertebrae fused into a single bone, the sacrum), and coccygeal (generally a single bone, the coccyx, consisting of three or four vertebrae).
The spine provides solid bony protection for the spinal cord enclosed within.
The shape of man’s spine has been conditioned by his upright mode of locomotion. Thickening evenly downward, the human spine has alternating flexures: convex in the cervical and lumbar segments (lordosis) and concave in the thoracic and sacral segments (kyphosis). With this structure, part of the pressure of a standing person’s weight is removed from the vertebrae and transmitted to the paravertebral ligaments. The flexures in the spine. modify the stresses caused by walking and running. Because of the flexibility of the vertebrae, the spine bends easily when the trunk moves: the spine is capable of bending, straightening up, tilting to one side, and rotating. Mobility is greatest in the cervical and lumbar segments and least in the thoracic segment. Man’s posture depends on the condition of the spine and its ligaments and surrounding muscles. Posture is also influenced by such static disturbances during growth as poor posture while studying. Among postural defects are leveling of the spine’s flexures (flat back) and enlargement of these flexures (sway-back).
Diseases of the spine include rare congenital structural anomalies, acquired diseases, and traumas. Anomalies include the fusion of two or three vertebrae, the presence of additional vertebrae, flattening of the vertebrae (platyspondylisis), absence of bony fusion of an arch with a vertebral body (spondylolysis), and the splitting of vertebral arches. Most congenital anomalies are painless. Acquired diseases of the spine are far more common and may develop at any age. They include anteroposterior and lateral deviations, such inflammatory diseases as tuberculosis of the spine, osteomyelitis of the spine, and chronic infectious spondylitis, and spondylosis and intervertebral osteochondrosis.
Among the traumas are injury to the spine’s ligamentous system: ruptures and sprains of the ligaments, injury to intervertebral disks, injury to joints and ligaments (subluxations and dislocations of the vertebrae), and injury to bones (fractures). Dislocations and subluxations usually occur in the cervical segment because of the segment’s greater mobility; these traumas are dangerous because of proximity to the medulla oblongata. Fractures of the vertebrae, the commonest form of trauma, may cause partial flattening of the spine as a result of compression. When a spinal fracture is combined with injury to the spinal cord, the posttraumatic period is very painful. Orthopedics and traumatology study diseases of the spine and deal with their treatment and prevention.
REFERENCESBazilevskaia, Z. V. Zakrytye povrezhdeniia pozvonochnika. Moscow, 1962.
Chaklin, V. D. Osnovy operativnoi ortopedii i travmatologii. Moscow, 1964.
Tsiv’ian, Ia. L. Khirurgiia pozvonochnika. Moscow, 1966.
Kaplan, A. V. Zakrytye povrezhdeniia kostei i sustavov, 2nd ed. Moscow, 1967.
Watson-Jones, R. Perelomy kostei i povrezhdeniia sustavov. Moscow, 1972. (Translated from English.)
V. F. POZHARISKII