The outside of a bone is called the cortex made up of minerals and protein. The inside of the bone is marrow. The bones are held together by specialised connective tissues called ligaments. This is called an articulation or joint. Bone ends would grate causing wear and tear if not for the cartilage. Although cartilage is tough it can be damaged by joint stress and trauma. Joints of the hip and shoulder are called ball & socket. The rest of the limb joints are of the hinge type they flex and extend in a plane from front to back. Dogs and humans skeletal anatomy have much in common. One difference – humans carry weight on their hip and the dog carries 75% of weight on his shoulder joints and front end.