Walls of the Thorax

What makes up the upper aperture ?

What makes up the lower aperture ?

What makes up the cavity of the Thorax ?

What is a costa ?

What are the True Ribs (vertebrosternal) ?
What are the False ribs ? (vertebrochondral)
What are the floating ribs ?

Why is the thoracic cage important ?

Where do the 2 Upper posterior intercostals arteries arise from , and where do the lower 9 come from ?