1 8 ANATOMY AND PHYSIOLOGY 



To hold the bones together, bands or cords of white fibrous 

 tissue are provided, strong and flexible, but not elastic. They 

 are called ligaments. The ligaments pass from one bone to 

 the other on every side of the joint, like a capsule, completely 

 enclosing it, and the capsule thus formed is lined by synomal 

 membrane, so named because it secretes a fluid called synovia 

 (the lubricating fluid or "joint-oil") which resembles in appear- 

 ance the white of egg and prevents friction. 



The synovial membrane not only lines the capsule but is 

 attached to the margins of the articular cartilages. 



Seven varieties of movement are allowed by these joints. 

 They are: 



Flexion, or bending. 



Extension, or straightening. 



Rotation, or rolling. 



Circumduction, a free sweeping movement of the distal end 

 of a limb in a circle. 



Abduction, or moving away from a middle line. 



Adduction, or moving toward a middle line. 



Gliding (which explains itself). 



Movable joints are classified according to the movements of 

 individual joints, or by peculiarities of structure. The most 

 important are the following: 



Class. Motions. Example. 



Hinge (ginglymus) Flexion and extension Elbow, Knee. 



Ball and socket (Enar- 



throsis) In all directions Shoulder, Hip. 



Pivot (Trochoides) Rotation within a ring Head of Radius. 



Rotation of ring around a 



pivot Atlas and axis. 



Arthrodia Gliding Wrist joints. 



There are other joints in which motion is so slight that they 

 are not classed as movable, nor do they possess a^cavity containing 

 synovia. They have been well described by the term yielding. 

 In these the bones are usually connected by fibro- cartilage discs. 

 Examples are found in the joints of the pelvis (page 50) and in 

 the spinal column (page 42). They are sometimes classified as 

 slightly movable. 



