276 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



to those described for the muscles concerned. They 

 elongate at points where the articular surfaces are 

 spread apart, and correspondingly shorten where the 

 flexure produces a folding. These changes proceed 

 more slowly than those of the muscles and tendons. 

 Very remarkable are the changes undergone by the 

 articular cartilages. When a joint is permanently 

 flexed, a part of the extremity of each bone is separated 

 from contact with the other, and the articulation is 

 finally destroyed at this point, because the cartilage 

 begins to vanish. One must conclude that the exist- 

 ence of the articular cartilages is dependent on their 

 mutual contact ; for dislocated articular surfaces which 

 remain in contact with soft tissues only, lose their car- 

 tilaginous covering. . . . Finally it is possible by a 

 consideration of the etiology of the effects of joint con- 

 tractions to reach some hitherto unnoticed conclusions 

 regarding the changes of articular surfaces, and bone 

 forms. The results of joint-contraction are most con- 

 spicuous when the latter occurs in childhood. During 

 maturity, a dislocation which causes an articular bor- 

 der or prominence to rest abnormally on the opposed 

 articular face in the act of walking, will be followed by 

 the penetration of the former into the latter, and a de- 

 formation of the articulation ; but the corresponding 

 changes under like conditions in the growing skeleton 

 are much more conspicuous." 



Hiitter thus describes the formation of new artic- 

 ular surfaces as a consequence of dislocation of joints. 

 " If the head of the femur or humerus leaves its socket, 

 and rests on the side of the ilium or the scapula, the 

 periosteum of the bone which receives the new impact 

 is excited to active bone-production, and the result is 

 the deposit of new osseous tissue. The thin bones 



