SKELETAL MOVEMENTS. 479 



practically only the lines of union of different bones, which grow 

 together to form a single bone. 



2. SYMPHYSES, in which two bony substances are strongly 

 cemented together by ligaments, and a more or less thick adherent 

 layer of fibro-cartilage, are joints allowing of some movement, 

 which is, however, very limited. 



3. ARTHROSES, or true movable joints, such as are commonly 

 met with in the extremities. They are characterized by a syno- 

 vial sack lining the surrounding ligaments, and two smooth sur- 

 faces of cartilage which cover over the bony extremities taking 

 part in the articulation, and form what are called the articular 

 surfaces. The synovial sack is strengthened by a loose membran- 

 ous covering the capsular ligament which is attached round 

 the edge of the cartilages next to the periosteum, which here 

 ceases. 



The articular surfaces are always in exact and close contact, 

 being pressed together by the following influences : (1) The 

 elastic tension and tonic contraction of the surrounding'muscles, 

 which exert considerable traction on them. (2) The traction of 

 the surrounding ligaments, which in some cases holds the bones 

 firmly together, no matter what their relative positions may be. 

 This can be well seen in the knee joint, in which a comparatively 

 small number of the ligaments suffice to keep the articular sur- 

 faces in contact. (3) The atmospheric pressure also tends to 

 hold the bones in close apposition, as may be seen in the hip 

 joint, which is not easily disarticulated, even when all the sur- 

 rounding structures and the ligaments have been severed. 



The synovial joints may be classified according to the form of 

 their surfaces, or their mode of motion, as follows : 



1. Flat articular surfaces held together by a short, rigid cap- 

 sule, allowing of but very slight gliding movement ; examples of 

 this form of joint are to be found in the tarsus and the articular 

 processes of the vertebrae. 



2. Hinge joints, in which the surfaces are so adapted that only 

 one kind of motion can take place. A groove-like cavity in one 

 bone fits closely and glides around the axis of a roller on the 

 other bone, while the sides of the joint are kept tightly together 



