Remember it's the bottom of the pec that connects highest on the arm, and vis versa.

Thanks for your responses @squeen and @Gabriel Palma. I thought the pectoral muscle was as shown below (sorry, I don't have digital tools). But Proko mzde this stroke because the pectoral muscle goes in the armpit ? Thanks again

I agree with @squeen . That's the pectoral muscle. You can't actually see its attachment to the arm in that photo because it inserts in the interior side of the arm.