PIVOT JOINTS 323 



the inner sides of the two lateral masses of the atlas, and 

 keeps the hinder side of the neck of the odontoid peg 

 in its place (Fig. 100, A). By this arrangement, the atlas 

 is enabled to rotate through a considerable angle either 

 way upon the axis, without any danger of falling forwards 

 or backwards — accidents which would immediately destroy 

 life by crushing the spinal cord. 



The lateral masses of the atlas have, on their upper 

 faces, concavities (Fig. 100, A, a) into which the two con- 

 vex, occipital condyles of the skull fit, and in which they 



Fig. 100. 



A. The atlas viewed from above ; a a, upper articular surfaces of its 

 lateral masses for the condyles of the skull ; 6, the peg of the axis 

 verti ra. 



B. Side viev7 of the axis vertebra ; a, articular surface for the lateral 

 mass of the atlas ; 6, peg or odontoid process. 



play upward and dowmward. Thus the nodding of the 

 head is effected by the movement of the skull upon the 

 atlas ; while, in turning the head from side to side, the 

 skull does not move upon the atlas, but the atlas slides 

 round the odontoid peg of the axis vertebra. 



The second kind of pivot-joint is seen in the forearm. 



If the elbow and forearm, as far as the wrist, are made 

 to rest upon a table, and the elbow is kept firmly fixed, 

 the hand can nevertheless be freely rotated so that either 

 the palm, or the back, is turned directly upwards. When 

 the palm is turned upwards, the attitude is called 



t2 



