ELEMENTARY PHYSIOLOGY 



knee ; and (4) pivot joints, in which a projection of one bone 

 serves as a pivot for the rotation of the other, as the joint formed 

 by the atlas and the axis. 



In each of these joints the articulating surfaces of the bones 

 are covered with a thin layer of articular cartilage, and thi? 



Fig. 42. Section through the 

 Pivot Joint formed by the 

 Atlas and the Axis. 



i, section through the odontoid peg, 

 showing synovial cavities before and 

 behind ; 2, cut portion of the atlas : 

 3, the transverse ligament which 

 holds the peg ; 4, surface of the atlas 

 which articulates with the skull. 



Fig. 43. A lumbar Vertebra, 



Viewed from above, showing the inter- 

 vertebral disc of cartilage. 



Fig. 44. Section through Lumbar Vertebrae, 



Showing an intervertebral disc of cartilage. 



cartilage is again covered with a membrane called the synovial 

 membrane (Gr. syn, together ; and oon, an egg), from which 

 exudes a fluid (synovia) resembling the white of egg, the use of 

 which is to lubricate or moisten the joints. The synovial mem- 

 brane forms a closed sac or cavity, called the synovial cavity, 

 which receives and contains the synovia. 



An imperfect movable joint is one in which the bones are 

 separated from each other by a layer of cartilage. The bones, 



