The joint which allows movement in one direction only is a:

Solve the following crossword puzzle with the help of clues given alongside.
Across:
3. Places where two or more bones are joined together (6)
4. A soft, elastic structure of the skeleton (9)
8. A type of muscle taking part in movement (6)
9. Structures because of which body of a bird becomes light (6, 5)
Down: 
1. Form the muscular system (7)
2. Body in which the two ends are tapering and the middle portion is broader (11)
5. Structures provided with feathers in birds (5)
6. Hard structures in the human skeleton (5)
7. Made up of 33 small bones and protects the spinal cord (8)