AskNott got it.
It’s a cross section of a human shoulder.
Here’s a somewhat gruesome color pic of the same area (Medical text, probably not for the squeamish):
http://www.vh.org/adult/provider/anatomy/HumanAnatomy/2Section/03.html
Here’s a CT scan of the same area:
http://www.meddean.luc.edu/lumen/MedEd/GrossAnatomy/cross_section/thorax/1297fre.jpeg
Compare the left shoulder on this one with the pic.