126 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. foramina on either side. The sacral canal ends with a large opening called the sacral hiatus, which is the point of administration for certain anesthetics. The coccyx, or tailbone, is located at the bottom of the spine. It includes three to five irregularly shaped, fused vertebrae. Structures of the Vertebrae Although no two vertebrae are exactly alike, most of the vertebrae have several structural features in common (Figure 5.16): • The vertebral body is the thick, disc-shaped portion that bears weight and forms the anterior portion of the vertebra. • The vertebral arch is the round projection of bone on the posterior aspect of the vertebra. It surrounds a hole known as the vertebral foramen (foh-RAY-mehn), through which the spinal cord passes. • The transverse processes are bony projections on the lateral sides of the vertebral arch. • The spinous process is a bony projection that extends posteriorly. • The superior and inferior articular processes are indentations or facets where a vertebra articulates, or joins, with the vertebrae immediately above and below it. These articulations are called facet joints. There is a progressive increase in vertebral size from the cervical region down through the lumbar region (Figure 5.17). This gradual size increase serves a functional purpose. When the body is in an upright below (Figure 5.15). Laterally, the alae of the sacrum articulate with the hip bones, forming the sacroiliac joints. The fused spinous processes form a prominent ridge, known as the median sacral crest, down the midline of the posterior surface, with posterior sacral Posterior arch Superior articular facet Superior articular facet Odontoid process Body Spinous process Pedicle Posterior tubercle Atlas Axis © Body Scientifi c International Figure 5.14 The first and second cervical vertebrae, the atlas and the axis. B A Posterior view Anterior view Coccyx Coccyx Ala S1 S2 S3 S4 S5 Co1 Co2 Co3 Co4 Transverse process Superior articular process Sacral canal Median sacral crest Auricular surface Posterior sacral foramina Sacral hiatus Anterior sacral foramina © Body Scientifi c International Figure 5.15 The sacrum and coccyx. A—Anterior view. B—Posterior view.