POEMS OF JOINTS. 97 



^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 phalanges 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 straightened 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 which allows us to open and close our mouths. The 

 latter is, however, not a perfect hinge-joint, since it per- 

 mits 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 forward 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 an- 

 other which remains stationary. We have a good example 

 of it between the first and second cervical vertebrae. The 

 iirst cervical vertebra or atlas* (Fig. 19*) has a very small 

 body and a very large arch, and its neural canal is subdi- 

 vided by a transverse ligament (L, Fig. 19) into a dorsal 

 and a ventral portion; in the former the spinal cord lies. 

 The second vertebra or axis (Fig. 20) has arising from its 

 body the stout bony peg, D, called the odontoid process. 

 This projects into the ventral portion of the space sur- 

 rounded by the atlas, and, kept in place there by the trans- 

 verse ligament, forms a pivot around which the atlas, car- 

 rying 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 permit 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 



*P'69. 



