64 THE HUMAN BODY. 



Between the phalanges of the fingers we find also hinge- 

 joints ; another is found between the lower jaw and the 

 cranium, allowing us to open and close the mouth. 

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

 mits also of slight lateral movements, and a gliding motion 

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

 bring the lower range of teeth outside the upper.* 



Pivot-joints. In this form one bone rotates about 

 another. A good example is found between the first and sec- 

 ond cervical vertebrae (Figs. 13, 14). The odontoid process 

 of the axis reaches up into the neural arch of the atlas, 

 and, kept in place there by the transverse ligament which 

 does not let it press against the spinal cord, forms a pivot 

 around which the atlas rotates, carrying the skull with it 

 when we turn the head to right or left. 



A more complicated kind of pivot-joint is found in the 

 forearm. Lay the forearm and hand flat on a table, palm 

 uppermost ; without moving the shoulder-joint at all the 

 hand can then be turned over so that its back is upward. 

 In this movement the radius, which carries the hand, 

 crosses over the ulna. When the palm is, turned up 

 (summation) the radius and ulna are parallel (Fig. 27, A), 

 and the radius on the outside ; place a finger of the other 



Is the joint of the lower jaw with the skull a perfect hinge joint? 

 What movements can take place at it? 



What is a pivot- joint? Name an instance from the spinal column. 

 Describe the joint between atlas and axis. What happens to the head 

 when the atlas rotates on the odontoid process of the axis? 



Where do we find another kind of pivot-joint? Illustrate its 

 action. What happens when we turn the hand so that the palm 

 instead of being up shall be down? How can we observe the relative 

 change in position of radius and ulna while making this movement? 



* The object of these minor movements is to allow us to chew our foo<1 ; in car- 

 nivora, as cats, which bite, but do not chew, the lower jaw forms a perfect hinge' 

 joint with, the cranium. 



