THE SKELETON 75 



bone of the thumb ; and others 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 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 articula- 

 tion 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 permits of a small amount of lateral 

 movement such as occurs in chewing, and also of a gliding move- 

 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 vertebrae. The first cervical vertebra 

 or atlas (Fig. 20) has a very small body and a very large arch, and 

 its neural canal is subdivided by a transverse ligament (L, Fig. 20) 

 into a dorsal and a ventral portion ; in the former the spinal cord 

 lies. The second vertebra or axis (Fig. 21) 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 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 ro- 

 tated until its back is turned upwards, it will be found that the 

 radius has partly rolled round the ulna. When the palm is up- 



