The part labelled 1 is the ligament.

Ligaments, tough fibrous bands of connective tissue, connect bones to bones, primarily at joints, providing stability and limiting excessive movement, thus protecting joints from injury. They are like strong, flexible straps or ropes that hold bones together, especially at joints.They help stabilize joints by preventing excessive movement or twisting, ensuring that bones stay in the correct position. 

Two bones are held together by ligaments, which are tough, fibrous bands of connective tissue that connect bones to other bones and provide stability to joints.