STRUCTURE AND COMPOSITION OF BONE. JOINTS. 95 



with the same variety, but a much less range, of movement 

 between each of the remaining metacarpal bones and the 

 proximal phalanx of the finger which articulates with it. 



Hinge-joints. Another form of synovial joint is known as 

 a hinge-joint. In it the articulating bony surfaces are of 

 such shape as to permit of movement, to and fro, in one plane 

 only, like a door on its hinges. The joints between the pha- 

 langes of the fingers are good examples of hinge-joints. If 

 no movement be allowed where the finger joins the palm of 

 the hand it will be found that each can be bent and straight- 

 ened at its own two joints, but not moved in any other way. 

 The knee is also a hinge-joint, as is the articulation between 

 the lower jaw and the base of the skull whicli allows. us to 

 open and close our mouths. The latter is, however, not a 

 perfect hinge- joint, since it permits of a small amount of 

 lateral movement such as occurs in chewing, and also of a 

 gliding movement by which the lower jaw can be thrust for- 

 ward so as to protrude the chin and bring the lower row of 

 teeth outside the upper. 



Pivot-joints. In this form one bone rotates around 

 another which remains stationary. We have a good example 

 of it between the first and second cervical vertebrae. The 

 first cervical vertebra or atlas (Fig. 22) has a very small 

 body and a very large arch, and its neural canal is subdivided 

 by a transverse ligament (L, Fig. 22) into a dorsal and a ven- 

 tral portion ; in the former the spinal cord lies. The second 

 vertebra or axis (Fig. 23) has arising from its body the stout 

 bony peg, D, called the odontoid process. This projects into 

 the ventral portion of the space surrounded by the atlas, and, 

 kept in place there by the transverse ligament, forms a pivot 

 around which the atlas, carrying the skull with it, rotates 

 when we turn the head from side to side. The joints on each 

 side between the atlas and the skull are hinge-joints and per- 

 mit only the movements of nodding and raising the head. 

 When the head is leaned over to one side, the cervical part of 

 the spinal column is bent. 



Another kind of pivot- joint is seen in the forearm. If 

 the limb be held straight out, with the palm up and the elbow 

 resting on the table, so that the shoulder-joint be kept steady 

 while the hand is rotated until its back is turned upwards, it 

 will be found that the radius has partly rolled round the ulna. 

 When the palm is upwards and the thumb outwards, the 



