Chapter 6: Skeletal System: Axial Division

Skeletal System: Axial Division anatomical chapter thoroughly details the axial division of the human skeletal system, outlining its composition of 80 bones that form the central longitudinal axis, offering protection for vital organs, providing extensive surfaces for muscle attachment, and housing key sensory organs. The discussion begins with the skull, which is structured into the eight bones of the protective cranium and the fourteen bones of the face, which form the entrances to the respiratory and digestive tracts. Important features of the cranium include the foramen magnum of the occipital bone, the large mastoid process of the temporal bones, the sella turcica of the sphenoid bone which acts as a protective cradle for the pituitary gland, and the cribriform plate of the ethmoid, perforated for the passage of olfactory nerves. The flat bones of the skull are joined by immovable, fibrous connections called sutures, such as the lambdoid and coronal sutures. Developmentally, the chapter addresses the unique aspects of infant skulls, specifically the presence of fontanels—flexible areas of connective tissue that facilitate cranial distortion during childbirth and accommodate rapid brain growth, disappearing by approximately age five. Attention then shifts to the 26-bone vertebral column, which is divided into the cervical, thoracic, and lumbar regions, followed by the fused sacrum and coccyx. This support column features four natural curves: the primary (accommodation) thoracic and sacral curves, and the secondary (compensation) cervical and lumbar curves. The vertebrae increase in size inferiorly, with the large lumbar vertebrae bearing the most weight. Specific vertebral features covered include the lack of a body on the atlas (C1) and the dens of the axis (C2), which together permit head movement. Clinical concepts related to spinal curvature are also introduced, including kyphosis (exaggerated thoracic curvature), lordosis (exaggerated lumbar curvature), and scoliosis (lateral spinal curvature). The final section examines the thoracic cage, comprising the thoracic vertebrae, 12 pairs of ribs, and the sternum. The ribs are categorized as true ribs (pairs 1–7) that connect directly to the sternum via costal cartilages, and false ribs (pairs 8–12), which include the floating ribs (pairs 11–12) that lack a sternal connection. The sternum itself consists of three parts: the manubrium, the body, and the xiphoid process.