236 MECHANISM OF THE KNEE-JOINT. 



devoted much attention to the configuration, movements, and 

 relations of the opposite cartilaginous surfaces of the diarthro- 

 dial joints. The details with reference to special articulations 

 I shall reserve for future communications ; at present I will 

 merely refer to certain general results, which bear more parti- 

 cularly on the mechanism of the knee-joint. 



In all diarthrodial articular surfaces there are facets, more 

 or less pronounced, situated at the opposite extremities of the 

 lines of movement. These may be conveniently designated 

 terminal facets. They are not primarily engaged in guiding 

 or conditioning the movements, although they frequently 

 modify their initial and terminal portions. These terminal 

 facets are essentially surfaces against which, and on which, 

 the opposite bones come to rest. 



The proper acting area in each of the opposite articular 

 surfaces is, after eliminating the terminal facets, comparatively 

 limited, and may generally be observed to present two facets. 

 These opposite facets act in pairs (a pair consisting of one on 

 each surface), acting together in the movement in one direc- 

 tion ; the other pair in the opposite alternate movement. The 

 pair of facets not in action break contact, or gape, during the 

 action of the facets engaged ; and the chink between them is 

 occupied by synovia or by a fatty pad. 



But even the acting facets, during their movements, are 

 only partially in contact. They are only coincident when the 

 facets approach or have reached the limit of their movement 

 in their proper direction ; that is, when the flexion-facets 

 as they may be conveniently termed have completed flexion, 

 or when the extension-facets have completed extension. In 

 these latter stages of the movements of the acting facets, when 

 they come into contact, the corresponding terminal facets close 

 also. The acting facets may, even at the conclusion of their 

 proper action, break contact, and the entire joint rest on the 

 corresponding terminal facets. 



