8T 



ponding depressions in the astragalus, at the moment the Hmb ar-' 

 rives at the greatest degree either of flexion or of extension. 

 V iThe shape of the surfaces of the astragalus concerned in the arti- 

 culation, is not that of a given circle throughout, for towards either 

 extremity, the descent is more rapid, or, in other words, answers to an 

 arc of a smaller circle. Hence, when one of the projections of the 

 tibia has arrived at its corresponding cavity in the astragalus, which 

 happens when the limb is either completely flexed or completely ex- 

 tended, the rapid cur^'e of the articulating surface presents a conside- 

 rable obstruction to change of position. Thus, the form of the articu- 

 lating surfaces, in itself, to a certain degree explains the phenomenon, 

 but its chief cause is to be found in the disposition and arrangement 

 of the ligaments. 



The external malleolus of the tibia is divided by a deep groove, for 

 the passage of a tendon, into an anterior and posterior tubercle; from 

 the latter of which and close to the edge of the articulating surface, 

 arises a strong and broad ligament, that is inserted into the os calcis. 

 Under this lies another ligament, which arising from the anterior 

 tubercle, is also inserted into the os calcis. It is to be observed, that 

 the origin of the latter is anterior to that of the former, but its inser- 

 tion posterior, so that these lateral ligaments cross each other in the 

 form of an X. The external articulating protuberance of the astra- 

 galus on which the tibia revolves, has, as has been already stated, a 

 nearly circular outline, and the attachments of the ligaments just de- 

 scribed, are at points on the outside of the os calcis, which would 

 lie nearly in the circumference of that circle, were it continued from 

 the articulating surface ; so that each of these ligaments has one of its 

 extremities fixed in a certain point of the circumferance, while its 

 opposite extremity revolves during the motion of the joint, nearly in 

 the circumference of the same circle. This observation applies like- 



o 2 



