The spine (vertebral column) consists of thirty-three superimposed bones termed vertebrae. Of these the upper twenty-four remain separate throughout life and form three groups. The first seven are called cervical, the succeeding twelve thoracic (dorsal), and the last five lumbar. In adult life the last nine vertebras ankylose to form two composite bones named the sacrum and the coccyx. The sacrum is formed by the fusion of five vertebrae from the twenty-fifth to the twenty-ninth inclusive; the four terminal are vestigial, and form the coccyx. In order to gain a general notion of the characters of a vertebra, it is desirable to select a bone from the middle of the thoracic series.
A vertebra presents the following parts: The body or centrum is a solid disc of bone slightly concave on its superior and inferior aspects, and wider transversely than antero-posteriorly. The upper and lower surfaces are rough for intervertebral discs, and the margins are slightly lipped. The circumference of the body is, in front, concave vertically, but convex from side to side; posteriorly it is excavated, and presents foramina for the escape of veins from the cancellous tissue. On the sides of the body, at the upper and lower angles, there are four demi-facets; when two vertebra are superimposed, the adjacent demi-facets form a complete articular facet for the head of a rib.
The pedicles are two constricted short piers of bone projecting horizontally backwards from the upper angles of the posterior surface. The lower border of each pedicle is deeply notched; hence, when two vertebrae are in position the notches are converted into intervertebral foramina for the transmission of spinal nerves and vessels.
The laminae are broad plates of bone continuous with the pedicles; each lamina meets its fellow dorsally to complete the neural arch, and conjointly form the spinous process. The superior borders of the lamina are rough for the insertion of ligamenta subflava. The anterior surface, in its upper part, is smooth where it bounds the neural canal. The lower part is rough for the origin of the ligamenta subflava. This rough surface is continuous with the inferior border of the spinous process.
The spinous process projects backwards and downwards from the confluent laminaB. To its upper and lower borders the interspinous ligaments are attached; its tip is rounded for the supraspinous ligament. It is mainly a muscular process.
The articular processes are four in number: two are superior and have the articular facets directed backwards with a slight outward tendency; their anterior surfaces complete the intervertebral foramina; posteriorly their margins give attachment to capsular ligaments. The inferior articular processes are slightly concave oval facets on the lower and outer angles of the anterior surface of the laminae. They are directed forwards and slightly inwards.
The transverse processes are two in number, and jut outwards from the lamina between the superior and inferior articular processes. The tip presents an oval facet for articulation with the tubercle of the rib. When the rib is in situ its neck forms with the process a costo-transverse foramen. The transverse processes, in addition to supporting the ribs, afford powerful leverage to muscles.
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