24 



THE FRAMEWORK OF THE BODY 



11. The ligament, then, secures firmness to the joint; it 

 must 'also have flexibility and smoothness of motion. This 

 is accomplished by a beautiful mechanism, the perfection of 

 which is only feebly imitated by the most ingenious contriv- 

 ance of man. The ends of the bones are covered by a thin 

 layer of cartilage which, being smooth and elastic, renders all 

 the movements of the joints very easy, and acts as a cushion. 

 Over the cartilage- is stretched a thin membrane, making a 



FIG. 7. THE LIGAMENTS OF THE WKIST 



FIG. 8. CELLS OF CARTILAGE 



compressed closed sack. This is called, the synovia! mem- 

 brane, and is constantly moistened by a colorless liquid, not 

 unlike the white of egg in appearance and consistency, whence 

 its name, synovial fluid (synovia = egg-like). 



12. Joints are classified as follows : The ball-and-socket 

 joint, which has the greatest freedom of motion, and. is exem- 

 plified by the hip and shoulder joints ; the hinge joint as shown 

 by the elbow ; and the gliding joint, where the motion is pro- 

 duced by one surface of bone gliding over another, as shown 

 by' the wrist and ankle joints. The junction of the first and 

 second vertebrae forms a pivot joint, which enables the head to 

 be rotated from right to left. The bones of the skull are 

 dove-tailed together by what are termed sutures. 



11. Uses of ligaments and cartilage ? 12. How are joints classified ? 



