PHYSIOLOGY OF MOVEMENT. 



729 



joint being determined by muscular action ; for when by the action of 

 muscular force the positions of the bones are changed, the original 

 position is not regained in the cessation of that force. 



The form of the joint, in which two bones are united end to end, is 

 subject to considerable variation, depending on 

 and governing the direction in which the move- 

 ment may take place. The articular extremities 

 of the bone are covered with articular cartilage, 

 surrounded by closed serous sacs containing 

 a serous fluid, the synovia, which tends to 

 diminish friction between the movable parts. 

 Since the space between articular surfaces con- 

 tains only the synovial fluid, it ma}- be regarded 

 as a vacuum, and atmospheric pressure is itself 

 sufficient to keep' the articular surfaces in con- 

 tact, even sustaining the entire weight of the 

 limbs and thus sparing muscular action. The 

 movement between the joint ends is not only 

 governed by the character of the joint, which 

 we will find may be resolved into several differ- 

 ent types, but is further restricted by the cap 

 snlar ligament which holds the joints in appo- 

 sition and by the tendinous bands which 

 surround them, in all cases only those move- 

 ments being possible in which the articular 

 surfaces remain in contact. 



The articulations are divided into three 

 classes : the immovable articulations, or the 

 synarthroses; the mixed, or amphiarthroses ; 

 and the movable, or diarthroses. 



To the first class belong the sutures and 

 other articulations where the surfaces of the 

 bones are in almost direct contact, not sepa- 

 rated by a synovial cavity, and immovably 

 connected with each other. In the second class 

 the osseous surfaces are connected together 

 by disks of fibro-cartilage, as between the bodies 

 of the vertebrae, or the articulating surfaces are 

 covered with fibro-cartilage, partly lined by 

 synovial membrane, and bound together by external ligaments, as in the 

 sacro-iliac and pubic symphyses. 



The third class includes the greatest number of joints in the animal 

 body, and as mobility is their distinguishing characteristic they are the 



Fig. 297.— Inferior Extrem- 

 ity of Man. {Colin.) 



AB C. line of action of biceps flexor. 

 (D, lever of third class.) E, gastroc- 

 nemius. 



