XIII 



BONES AND JOINTS 



179 



CCLfS 



the bones are covered with a thin layer of cartilage which 

 is very smooth and well oiled. A closed sac of very 

 thin connective tissue occurs between the adjacent sur- 

 faces of some joints. It secretes the synovial fluid, 

 which keeps the surfaces 

 moist and enables the bones 

 to move with little friction. 



The bones are held to- 

 gether by means of strong 

 bands of connective tissue 

 known as ligaments. In some 

 cases a bone has a large 

 head that fits into a deep 

 socket of another, as at the 

 hip and shoulder, so that 

 the air pressure alone may 

 be sufficient to keep them in ^ 

 place. But even those bones 

 have a large number of very 

 strong ligaments as well as 

 muscles that extend from 

 one bone to the other and 

 hold them together very 

 firmly. 



Joints may be divided into several classes as the 

 ball-and-socket, pivot, hinge, saddle, gliding, and suture. 

 Where the round head of one bone fits into a deep 

 cavity or socket of another, as at the hip and shoulder, 

 in such a way as to permit rolling motion in any direc- 

 tion, we have a ball-and-socket joint. The pivot joint is 



tib 



FIG. 95. The right-knee joint. The 

 outer half of the femur and patella 

 cut away. (Huxley.) fern, fe- 

 mur ; pat, patella ; tib, tibia ; 

 fib, fibula; caps, capsule of joint; 

 /, /, ligaments ; c, cartilage ; e, 

 tendon. 



