400 GENERAL ANATOMY. 



The muscles which surround the articulations, although they 

 do not enter essentially into their composition, contribute 

 powerfully to their solidity. 



630. Firmness and mobility are variously combined in the 

 diarthrodial articulations. 



These articulations possess very diversified motions, as 

 sliding, rotation, angular opposition and circumduction. The 

 sliding motion exists in all the diarthrodial articulations. The 

 other motions, on the contrary, occur only in a cerain number 

 of them. Rotation is peculiar to certain articulations. Some- 

 times it is exercised upon a single pivot, as around the odon- 

 toid process of the second vertebra. Sometimes there are 4;wo, 

 as in the double articulation of the bones of the fore-arm with 

 each other. Sometimes it is round an ideal axis that a bone 

 turns, as is exemplified in the femur. The motion of opposi- 

 tion, or angular motion, is that in which the bones form more 

 or less open angles with each other, according to the degree 

 of motion. It is distinguished into opposition limited to two 

 motions of flexion and extension, as at the elbow, the knee, 

 &c.; and into vague opposition, which may take place in four 

 principal directions, and in all the intermediate directions, of 

 which examples are offered by the arm, the thigh, the thumb, 

 &c. Circumduction, which exists in all the articulations pos- 

 sessing vague opposition, is a motion by which the bone which 

 moves describes a cone whose summit corresponds to the 

 central extremity of the bone, and the base to its opposite 

 extremity. 



The firmness of these articulations, like that of the others, 

 is in the inverse ratio of their mobility. 



631. Several kinds of diarthrosis are distinguished, de- 

 pending upon the configuration of the surfaces, the means of 

 union; and the motions of these articulations. 



The close and planiform diarthrosis, articulus adstrictus, 

 the amphiarthrosis of some, the motus obscurusof Columbus, 

 is that in which the surfaces are superficial, the ligaments 

 strong and tight, the motions obscure and confined to sliding, 

 but capable of being performed in several directions. Of this 

 kind are the articulations of the articular processes of the ver- 



