JOINTS 31 



arches and supporting trusses of bony tissue, which 

 branch out from the dense bone of the shaft. This ex- 

 pansion distributes over a broad surface the pressure 

 between the ends of the bones, and also gives a wide 

 foundation for the attachment of the ligaments and 

 muscles. 



Structure of a joint. The joint ends of the bone are 

 further adapted to slip easily upon one another and are 

 protected from injury by a thin cushion of smooth 

 elastic cartilage, the articular cartilage. The articular 

 cartilage is kept moist and slippery by a thin fluid, the 

 synovial fluid, which is secreted by a sleeve of mem- 

 brane called the synovial membrane. This membrane, 

 which makes a closed cavity of the joint, is so loose 

 that it is not stretched or torn by any movements of 

 the joint. Normally, the amount of fluid secreted is 

 small, but in diseased conditions or after injury the 

 secretion may be so increased as greatly to distend the 

 synovial membrane and the surrounding ligaments. 

 When this happens in the knee joint, it is popularly 

 called water on the knee. 



Range of movement. Some joints permit but a slight 

 slip, such as that which occurs between the sacrum and 

 the hip bone. Others have a wider but still compara- 

 tively restricted movement, as in the knuckles of the 

 fingers. Still others have a relatively free movement, 

 as the shoulders. The main exceptions to the presence 

 of joints at the points of contact between bones, are 

 found in the skull, where the bones are so firmly held 

 by the interlocking of their jagged edges that all move- 

 ment is made impossible soon after birth; in the pelvic 

 bones, which in youth become perfectly blended to form 

 the hip bones; and in the sacrum, which, as we have 

 seen, results from the very early fusion of five vertebra. 



