In this form the bony cavities and projections are not spherical, but are grooved and ridged so that one bone can glide over the other in one plane only, to and fro, like a door on its hinges.

The knee is a hinge-joint; it can only be bent and straightened, in technical language, flexed and extended.

What kind of joints allow of the freest movement? What is meant by flexion of the thigh? By extension? By abduction? By adduction? By circumduction? By rotation? What movements do all ball and socket joints permit?

What sort of a joint is that at the shoulder? Why is more movement possible at it than at the hip-joint?

What is a hinge-joint ? Give an illustration.

Name other hinge-joints.

Between the phalanges of the fingers we find also hinge-joints ; another is found between the lower jaw and the cranium, allowing us to open and close the mouth. The latter is not, however, a perfect hinge-joint ; it permits also of slight lateral movements, and a gliding motion by which the lower jaw can be thrust forward so as to bring the lower range of teeth outside the upper.*