Final answer:
Bones contain a matrix of collagen fibers that support calcium salt crystals, which solidify to give bones their hardness and strength, allowing them to act as levers for muscles to facilitate movement.
Step-by-step explanation:
The phrase 'matrix hard owing to calcium salts; provides levers for muscles to act on' refers to bone, which plays a critical role in our bodies by providing structure, protection, and facilitating movement.
Bones contain a matrix of collagen fibers that serve as a base for calcium phosphate and calcium carbonate to combine and create hydroxyapatite.
Hydroxyapatite crystals, along with various inorganic salts like magnesium hydroxide, fluoride, and sulfate, calcify on the collagen fibers to form a hard tissue that allows bones to be strong yet flexible.
Bones act as levers for muscles. The muscle must span a joint and contract for a bone to move, leveraging bones as rigid objects to create motion. Calcium is also crucial, not just for the hardness of the bones, but in facilitating muscle contraction, which is essential for the movement of these levers.