132 THE JOINTS IX GENERAL. 



Amphiarthrosis means the mixed articulation, or that in which 

 there is mediate union by some intervening- substance, with partial 

 mobility. The articulations between the bodies of the vertebrae, that 

 between the two ossa pubis at the symphysis, and that between the two 

 first pieces of the sternum, may be taken as examples of this mode of 

 connection. Some of the joints of this kind pass on the one hand into 

 synarthrosis, and on the other into diarthrosis. 



" Diarthrosis includes the complete joints, with synovial cavities sepa- 

 rating the surfaces of the articular bones, and is attended with consider- 

 able yet varying degrees of mobility. In tliis form of joint, })lates of carti- 

 lage cover the articular parts of the bones and present within the joint 

 free surfaces of remarkable smoothness, and these surfaces are further 

 lubricated by the synovial fluid secreted from the delicate membrane 

 which lines the fibrous coverings and all other parts of the articulating 

 cavity except the cartilage. This membrane is continuous with the 

 margin of the articular cartilages, and along with them completely 

 encloses the joint cavity. The bones are further held together by 

 fibrous tissue in the various forms of ligaments, such as membranous 

 capsules, flat bands, or rounded cords. These ligaments, it is true, are 

 not so tight as to maintain the bones in close contact in all positions of 

 the joint, but are rather tightened in some positions and relaxed in 

 others, so that they may be looked upon chiefly as controllers of move- 

 ments. The bones are likewise held together in diarthrt>dial joints, by 

 atmospheiic pressure, and by the surrounding muscles. Certain forms 

 of diarthrodial joint have received special names. The term fjimihjmiis_ 

 is used to distinguish a hinge joint, or one which admits only of flexion 

 and extension, like the elbow, knee and ankle. Eiiaj'th'ojds (Cruveil- 

 hier) is the_ ball-and-socket form of joint, like the shoulder and hip, 

 allowing motion in every direction. Arfhrodia is a joint with nearly 

 fiat surfaces, Avhich admits of gliding movement merely, as in the arti- 

 culations of the carpus, tarsus, and articular processes of the vertebrce. 



Vaeious Kinds of Movement. — The various movements of tho 

 bones in diarthrodial joints are distinguished by different terras ac- 

 cording to their directions, viz., angular movement, circumduction, 

 rotation, and gliding ; but it is proper to remark that although different 

 kinds of movement, answering to these several terms, may readily be 

 recognised, yet they are rarely of only one kind in any joint, but rather 

 several kinds of movement are frequently combined, and they also run 

 into one another in great variety. 



Angular movement is movement in such a manner as to increase 

 or diminisli the angle between two bones, so that they shall lie more or 

 less nearly in a straight line. The different kinds of angular movement 

 arc designated by ditferent terms according to the directions in which 

 they take place with reference to the limb ox body : thus flexion and 

 cxlension indicate angular movements, which have the effect of bending 

 or straightening parts upon one another or upon the trunk of the body : 

 adduction and ahdKctioji indicate angular movement to and from the 

 mesial plane of the body, or, when fingers and toes are referred to, these 

 terms may be used to denote movement to and from the middle line of 

 the hand or foot. 



Coapintion. is a forni of angular movement, in which, as in tlie movement of 

 the patella on the femm- the articular siu'face of one bone travels over that of 

 another, so as to bring different pai-ts of the surfaces successively into contact in 



