The Lumbar Vertebrae are the largest segments of the vertebral column, and can at once be distinguished by the absence of the foramen in the transverse process, the characteristic point of the cervical vertebrae, and by the absence of any articulating facet on the side of the body, the distinguishing mark of the thoracic vertebrae.

The body is large, and has a greater diameter from side to side than from before backward, slightly thicker in front than behind, flattened or slightly concave above and below, concave behind, and deeply constricted in front and at the sides, presenting prominent margins, which afford a broad basis for the support of the superincumbent weight. The pedicles are very strong, directed backward from the upper part of the bodies; consequently, the inferior intervertebral notches are of considerable depth. The laminas are broad, short, and strong, and the spinal foramen triangular, larger than in the dorsal, smaller than in the cervical, region. The spinous processes are thick and broad, somewhat quadrilateral, horizontal in direction, thicker below than above, and terminating by a rough, uneven border.

The superior articular processes are concave, and look backward and inward; the inferior, convex, look forward and outward; the former are separated by a much wider interval than the latter, embracing the lower articulating processes of the vertebra above. The transverse processes are long, slender, directed transversely outward in the upper three lumbar vertebrae, slanting a little upward in the lower two. They are situated in front of the articular processes, instead of behind them as in the thoracic vertebra, and are homologous with the ribs. Of the three tubercles noticed in connection with the transverse processes of the twelfth thoracic vertebra, the superior ones become connected in this region with the back part of the superior articular processes, and have received the name of mammillary processes; the inferior are represented by a small process pointing downward, situated at the back part of the base of the transverse process, and called the accessory processes: these are the true transverse processes, which are rudimental in this region of the spine; the external ones are the so-called transverse processes, the homologue of the rib, and hence sometimes called costal processes. Although in man these are comparatively small, in some animals they attain considerable size, and serve to lock the vertebrae more closely together.

The Fifth Lumbar vertebra is characterized by having the body much thicker in front than behind, which accords with the prominence of the sacro-vertebral articulation; by the smaller size of its spinous process; by the wide interval between the inferior articulating processes; and by the greater size and thickness of its transverse processes, which spring from the body as well as from the pedicles.