subclavius

noun

sub·​cla·​vi·​us ˌsəb-ˈklā-vē-əs 

plural subclavii -vē-ˌī 

: a small muscle on each side of the body that arises from the junction of the first rib and its cartilage, inserts into the inferior surface of the clavicle, and acts to stabilize the clavicle by depressing and drawing forward its lateral end during movements of the shoulder joint