Skeletal system and its functions

Skeletal system and its functions

The skeletal system — the framework of bones and cartilages — is, like muscle, derived from the embryonic mesoderm. Muscles attach to bones via tendons and use the bones as rigid levers to convert contraction into movement. Across the animal kingdom, three broad skeleton designs exist:

• Hydrostatic skeleton — a fluid-filled body cavity encircled by muscle, found in soft-bodied invertebrates (e.g. earthworm); movement comes from muscles squeezing the incompressible fluid.
• Exoskeleton — a rigid, hard case on the outside of the body, found in many invertebrates (e.g. cockroach); it both protects and anchors the muscles from without.
• Endoskeleton — bones and cartilages located inside the body and surrounded by muscle, found in vertebrates, including humans; it grows with the body and can support far greater body size than an exoskeleton.

The adult human skeleton (Figure 9.6) is built from 206 bones and cartilages, grouped into two principal divisions: the axial skeleton (80 bones — skull, hyoid, vertebral column, thoracic cage) and the appendicular skeleton (126 bones — limbs and their girdles).

Beyond providing the levers for locomotion, the skeletal system performs several other essential functions:

• Support — forms a rigid frame that bears the body's weight against gravity.
• Shape — gives and maintains the overall body form.
• Protection — shields delicate internal organs (e.g. the skull protects the brain, the ribcage protects the heart and lungs).
• Mineral reservoir — stores calcium and phosphate, and stores fat (as triglyceride) in yellow bone marrow as an energy reserve. …