4. Answer the following questions: (a) What is a ball and socket joint? (b) Which of the skull bones are movable? (c) Why can our elbow not move backwards?

Ans: (a) The rounded end of one bone fits into the hollow space of other bone. This is called ball and socket joint. Ball and socket joints allow movements in all the directions, e.g. shoulder and hip can be moved in all directions. (b) In skull, only lower jaw is movable. (c) Our elbow cannot move backwards because the elbow has a hinge joint that allows movement in only one direction.