JOINTS OR AKTIC0IAT10NS. 



47 



consisting exteriorly of rings of oblique fibres, and more 

 deeply of pulpy tissue, rich in corpuscles like cartilage- 

 corpuscles. These discs are so many elastic pads which are 

 useful in preventing shocks, and while little movement is 

 allowed by them between each pair of bones, the amount 

 permitted in the whole column is very considerable. 



Fig. 26. LUMBAR INTERVEIITEBRAL Disc. A, front view. B, hori- 

 zontal section. C, section from before backwards, a, fibrous 

 rings; 6, pulpy tissue; c, articular surfaces in contact; d, anterior 

 common ligament; e, posterior common ligament. 



Complete, joints are those in which the surfaces of bones 

 coated with articular cartilage glide one against the other, 

 while the ligaments which bind them together are placed 

 round about. Internal to the ligaments there extends from 

 the circumference of one articular surface to that of the 

 other, a delicate synovial membrane, so called because from 

 it there exudes sufficient fluid, termed synovia, to moisten 

 the surfaces of the shut cavity which it encloses. 



In the complete joints, the movements allowed vary greatly 

 both in character and degree. In. many, as, for example, 

 between, the individual bones constituting the second range of 

 the carpus, and between some of the tarsal bones, the move- 

 ment is extremely slight (arthrodia), and the principal advan- 

 tage gained appears to be elastic resistance to pressure, by the 

 weight being thrown upon tense ligaments. Others, in which 

 the movements are extensive, may be rudely compared with 

 joints made by mechanicians. Thus, the shoulder and the 

 hip maybe termed ball and socket joints, there being in each 

 a rounded surface which moves in all directions against a 



