The axilla, or armpit, is a four sided pyramid shaped walled by a number of muscles, and is a route for nerves, blood vessels and lymphatic vessels to get to and from the upper limb. There is a narrowing at the apex of this pyramid that can compress these structures, causing a number of problems.
Music:
Wake Up Call by Gregory David
https://www.epidemicsound.com/track/cxqg6GM6Kf/