128 ARTHROLOGY. 



there may be one, or even two, nan-ow cavities, and authorities 

 diflfer as to whether such cavities are lined by synovial mem- 

 brane or not. Since there are really no frictional surfaces in 

 such a joint, motion depends upon the flexibility of the disc. 

 The joints between the vertebral centra afford the best illustra- 

 tion of the mixed class. 



The classification of the joints is here tabulated for reference. 



Synarthrosis 

 (Immovable). 



Sutura. 



( Dentata. 

 Yera. } Serrata. 

 I Limbosa. 



Notha. / Squamosa. 

 I Harmonia. 



Synchondrosis. 

 Schindylesis. 

 Gom ptosis. 



Di arthrosis 

 (Movable). 



Arthrodia. 

 Enarthrosis. 

 Ginglymus. 

 Diarthrosis rotatorius. 



Amphiarthrosis 

 (Mixed). 



MOTION IN JOINTS. 



The following- terms express tne various movements; allowed 

 by joints : — extension tends to bring two bones as nearly into a 

 straight line as the structure of the joint will permit ; flexion is 

 the reverse of this, and diminishes the angle that extension 

 increases ; abduction expresses the outward movement of a limb 

 or bone from the central plane of the body ; adduction is the 

 reverse action ; rotation signifies the partial revolution of a bone 

 or number of bones, as it were, on their own axis ; circumduc- 

 tion implies the movement of the distal end of a bone or limb, 

 when it describes a curve, as the arc of a circle, ellipse, &c. The 

 term gliding explains itself, and is peculiar to diarthroses, other 

 joints having no frictional surfaces. 



