MECHANISM OF THE KNEE-JOINT. 237 



The movements of opposite diarthrodial surfaces on one 

 another appear to be in every instance a combination of 

 gliding and rolling ; the amount of gliding being directly, and 

 the rolling inversely, as the coincidence of the opposite arti- 

 cular surfaces. 



An important addition has recently been made to our 

 conceptions of the mechanical constitution of joints, by the 

 nearly simultaneous publication of memoirs by Langer and 

 Henke, and of a report of these memoirs, with original obser- 

 vations on the same subject, by Meissner. As these new 

 observations refer principally to the ankle and elbow-joint, 

 and as I shall have afterwards to refer to them in detail, I 

 shall at present only allude to such points as bear on the 

 subject of this communication. 



The important fact announced by these three observers is 

 the screw configuration of the articular surfaces of the elbow, 

 ankle, and calcaneo-astragaloid joints. The method of inves- 

 tigation adopted was to trace lines on one of the articular sur- 

 faces by means of a steel point passed a little beyond the 

 opposite surface previous to putting the joint through its 

 movements. These lines are termed " ganglinii" " go-lines," 

 and in all the joints examined are arranged obliquely to their 

 axes of rotation. Langer, acting on the happy idea of pro- 

 longing the screw, by uniting in one direction a number of 

 plaster casts of the same articular surface, succeeded in form- 

 ing continued screws from the upper articular surface of the 

 astragalus in the horse, panther, and human subject. 



Langer concludes that the " go-line" of the ankle-joint in 

 all the Mammalia is a portion of a helix, and that therefore 

 the astragaloid surface is a segment of a cylindrical or conical 

 male screw, while the tibio-fibular surface is a segment of the 

 corresponding female screw. The right ankle-joint is a left- 

 handed screw combination ; the left ankle-joint a right- 

 handed. When, therefore, the foot is conceived to be fixed, 



