Chap. VI] 



ARTICULATIONS 



85 



ilia, the articular surfaces being covered by cartilage and held 

 together by ligaments. 



The fibro-cartilage between these joints (symphysis pubis and 

 sacro-iliac) becomes thickened and softened during pregnancy 

 and allows of a certain limited motion which is essential to a nor- 

 mal parturition. 



Syndesmosis. — When the bony surfaces are united by an 

 interosseous ligament, as in the lower tibio-fibular articulation, it 

 is called syndesmosis. 



5VN0VIAL FOLD 



DIARTHROSES, OR FREELY MOVABLE JOINTS 



This division includes the complete joints, and are the only 

 joints in which the three following conditions are found : — 



(1) The bones are united by fibrous ligaments, forming more 

 or less perfect capsules. The ligaments are not always 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 in many cases they are to be looked on chiefly 

 as controllers of movements, and not as serving solely to hold 

 the bones together. The bones 

 are partly held together in these 

 joints by atmospheric pressure 

 and largely by the surrounding 

 muscles. 



(2) A secreting membrane 

 (synovial) lines the capsule and 

 is so arranged that it dips in 

 between the edges of the oppos- 

 ing articular cartilages. (See 

 Fig. 64.) 



(3) Each articular end of the 



bone is covered by hyaline cartilage, which provides surfaces of 

 remarkable smoothness, and these surfaces are lubricated by the 

 synovial fluid secreted from the delicate synovial membrane which 

 lines the cavity of the joint. 



The varieties of joints in this class have been determined by 

 the kind of motion permitted in each. They are as follows : — 



(1) Gliding joint. — The articular surfaces are nearly flat, and 



Fig. G4. — A (Vj.mplete Joint. The 

 synovial membrane is represented by 

 dotted lines. 



