Chapter 14 - PELVIC GIRDLE: SACRUM, COCCYX, & OS COXAE

Abstract

The pelvic girdles of terrestrial vertebrates are connected to the vertebral column and are much larger than their homologs in fish. These adaptations are required for weight-bearing and muscle attachment in the terrestrial forms. The adult human bony pelvis is composed of four main elements: the right and the left os coxae, and the sacrum and coccyx. The sacrum and coccyx are parts of the axial skeleton and are actually variably fused vertebrae. The sacral vertebrae fuse during adolescence into one immobile, wedge-shaped bone, the sacrum. This bone is typically formed from five segments, but may have as few as four or as many as six. The sacrum is located at the base of the vertebral column. It articulates bilaterally with the two ossa coxa and inferiorly with the small coccyx. The coccyx, the vestigial tail, is highly variable in shape with three to five (most often four) variably fused segments. The rudimentary vertebrae of the coccyx have articular and transverse processes superiorly, but they lack pedicles, laminae, and spinous processes. The os coxae differ in males and females with its anatomy representing a compromise between the demands of locomotion and birthing. The os coxae are the parts of the bony pelvis and are formed ontogenetically from three different parts, the ilium, ischium, and pubis that fuse in early adolescence.