12« 



ARTHROLOGY. 



or disc is softer towards the centre, ^vhere occasionally there may 

 be one, or even two, narrow cavities, and authorities differ as to 

 whether sach cavities are lined by synovial membrane 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 illustration of the 

 mixed class. 



The classification of the joints is here tabulated for reference. 



Synarthrosis 

 (Immovable). 



Sutui-a. 



( Dentata. 

 Vera. J Serrata. 

 ' Limbosa. 



Notha. 



Synchondrosis. 

 Schindylesis. 

 ' Gomphosis. 



SqiTamosa. 

 Harmonia. 



( Artlirodia. 

 Diarthrosis ' Enarthrosis. 

 (Movable). 1 Ginglymus. 



Diarthrosis rotatorius. 



Amphiarthrosis 

 (Mixed). 



MOTION IN JOINTS. 



The following terms express the 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 jDartial 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. 



