138 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. 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 position, each vertebra must support the weight of all of the body parts positioned above it. Think about what this means. While a cervical vertebra supports only the weight of the head and neck, a lumbar ver- tebra supports the weight of the head, neck, arms, and the entire trunk positioned above that vertebra. The size and angulation of the vertebral processes also vary throughout the spinal column. This changes the orientation of the facet joints, which interconnect the vertebrae and serve to limit range of motion in the different spinal regions. The Spinal Curves The characteristic shapes of the vertebrae in the different spinal regions also form the normal spinal curves. As Figure 5.13 shows, the cervical and lumbar curves are posteriorly concave, while the thoracic, sacral, and coccyx curvatures are anteriorly concave. These alternating curves make the spine stronger and better able to resist potentially injurious forces than if it were straight. The thoracic and sacral curves are known as primary spinal curves because they are present at birth. The lumbar and cervical curves are referred to as secondary spinal curves. They develop after the baby begins to raise the head, sit, and stand, as increased muscular strength enables the young child to shift body weight to the spine. 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 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. 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 Scientific International Figure 5.15 The sacrum and coccyx. A—Anterior view. B—Posterior view.