THE SKELETON 



71 



IT 



ment by which the lower jaw can be thrust forward so as to pro- 

 trude 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 vertebra. The first cervical vertebra 

 or atlas (Fig. 23) has a very small body and a very large arch, and 

 its neural canal is subdivided by a transverse ligament (L, Fig. 23) 

 into a dorsal and a ventral portion; in 

 the former the spinal cord lies. The 

 second vertebra or axis (Fig. 24) 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 trans- 

 verse 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 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 humems;/?, radius; t/, ulna, 

 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 up- 

 wards and the thumb outwards, the lower end of the radius can 

 be felt on the outer side of the forearm just above the wrist, and if 

 this be done while the hand is turning over, it will be easily dis- 

 cerned that during the movement this end of the radius, carrying 

 the hand with it, travels around the lower end of the ulna so as to 

 get to its inner side. The relative position of the bones when the 

 palm is upwards is shown at A in Fig. 40, and when the palm is 

 down at B. The former position is known as supination; the latter 



