The human skeletal system provides the fundamental framework for the body, offering structural support and protecting internal organs. This complex arrangement of bones is classified into two major divisions to simplify the study of its anatomy and function. The pelvis, a large basin-shaped structure located at the base of the trunk, connects these two divisions. Its central location and composite nature often create confusion regarding its anatomical classification.
Defining the Two Skeletal Divisions
The two major classifications of the human bony framework are the axial skeleton and the appendicular skeleton. The axial skeleton forms the central axis of the body, providing primary support and encasing the major organs of the head and trunk. This strong, relatively immobile structure includes the bones of the skull, the vertebral column, and the thoracic cage (ribs and sternum).
This central division maintains the body’s posture and stability, also serving as an attachment point for muscles that control movement of the head, neck, and trunk. The appendicular skeleton, in contrast, is dedicated to movement and interaction with the environment.
The appendicular skeleton consists of the bones of the upper and lower limbs, as well as the girdles that connect them to the center of the body. The pectoral girdle attaches the upper limbs, while the pelvic girdle attaches the lower limbs. These bones are more mobile and are the primary agents of locomotion and manipulation.
The Components of the Pelvic Girdle
The structure commonly referred to as the pelvis is a large, bony basin that anchors the lower limbs and supports the weight of the upper body. It is not a single bone but a ring-like assembly of four distinct bones. It is composed of the two large hip bones, which are joined anteriorly and articulate with the vertebral column posteriorly.
Each hip bone (coxal bone or os coxae) is formed by the fusion of three separate bones during development. These three fused components are the ilium, the ischium, and the pubis. The ilium is the broad, flaring upper part; the ischium is the lower, posterior portion; and the pubis is the anterior part.
Connecting these two hip bones to the trunk are the sacrum and the coccyx, which are extensions of the spinal column. The sacrum is a triangular bone formed by the fusion of five vertebrae. The coccyx, or tailbone, is a small bone inferior to the sacrum. The combination of the two hip bones, the sacrum, and the coccyx forms the complete bony pelvis.
Determining the Classification of the Pelvis
The classification challenge arises because the term “pelvis” refers to a composite structure containing bones from both divisions. The hip bones (ilium, ischium, and pubis) collectively form the pelvic girdle, which is classified as part of the appendicular skeleton. This classification is based on their primary function: connecting the lower limbs to the central trunk, enabling locomotion and weight transfer. The hip bones articulate with the femur at the hip socket, making them functionally part of the lower limb system.
Conversely, the sacrum and the coccyx are classified as belonging to the axial skeleton. They are the terminal segments of the vertebral column, which is a defining feature of the axial skeleton. Therefore, the answer to whether the pelvis is part of the axial skeleton is complex: the entire bony pelvis is a blend of both divisions.
The pelvic girdle component (the two hip bones) is appendicular, while the sacrum and coccyx component is axial. When anatomists refer specifically to the “pelvic girdle,” they mean the appendicular portion that attaches the limbs.