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Thoracic vertebra 9
Other Terms: Ninth dorsal vertebra, T9 vertebra
The thoracic vertebrae increase in size from cranial to caudal, the first thoracic vertebra being the smallest and the twelfth thoracic vertebra being the largest. The body is heart-shaped and has well-marked surfaces laterally for the reception of the heads of the ribs. The superior and inferior articular surfaces are flat. The short, thick pedicles are deeply notched from below and weakly notched on their superior surface. The laminae are flat and overlap the laminae of the subjacent vertebra. The long, slender spines form a tuberosity at their apex. The spines course in a dorsocaudal direction, overlapping their subjacent neighbor. The spines become shorter, wider, and more transverse caudally. The club-like transverse processes have costal facets on their anterior apical surface. Because of their association with the ribs, these vertebrae have the most limited range of movement.
Thorax arises from the Greek word thorakos meaning a breast-plate or cuirass. Since this vestment covered the upper portion of the trunk, this bodily region adapted its name from this covering. The word vertebra is an old Latin term that meant a joint or something to be turned. It arises from the Latin verto meaning to turn. In A.D. 30 Celsus used the word to designate any joint. It was only in later years that the bone arrived at its present meaning.
This vertebra articulates with four to six bones: the superjacent and subjacent vertebrae and one or two ribs on each side. Six articular surfaces unite each typical vertebra. The two superior articular facets articulate with the corresponding inferior articular facets of the adjacent vertebrae and the superior and inferior surfaces of the vertebral bodies form an articulation via the intervertebral disc of cartilage. The head of a rib articulates with the fovea on the body of the vertebra. The first eight thoracic vertebrae typically articulate with two ribs each, forming two fovea on the lateral sides of the body. These superior and inferior foveae are often called demifacets because they form half of a surface. Vertebrae nine through twelve usually articulate with one rib each, thus having only one fovea on each side of the body. The tubercle of each rib also articulates with the fovea on the anterior apex of the transverse process, except on vertebrae eleven and twelve.
Thoracic vertebrae ossify from three primary centers. During the third intrauterine month centers appear in the body and at the junction of each transverse process with the pedicle and lamina. These centers spread and are still separate at birth, uniting between the fourth and fifth year. At puberty, secondary epiphyseal centers appear at the apex of the transverse and spinous processes. These centers fuse by the eighteenth year. Secondary epiphyseal rings, for the body, arise during the seventeenth year and fuse in the mid-twenties.