Chap, xxi.] THE KNEE JOINT. 439 



flexion. The pain of acute synovitis is due mainly 

 to the increasing distension of the joint with fluid, 

 and it is natural that the patient should in- 

 stinctively place the limb in the position in which 

 the joint will hold the greatest amount of fluid, 

 and in which the interarticular tension is reduced 

 to a minimum. It cannot be said, however, that the 

 greater the degree of flexion the greater the capa- 

 city of the joint cavity. By experimental injections 

 into the knee_, Braune found (a) "that the capacity of 

 the synovial cavity reaches its maximum in a definite 

 degree of flexion, and that the angle at which this 

 happens is twenty-five degrees ; " (b) that "the mini- 

 mum of the capacity of the synovial cavity coincides 

 with the maximum of flexion." Thus it happens that 

 the tension within an inflamed knee will be greater in 

 extreme flexion than it is in full extension. 



(2) By flexing the limb, the more powerful liga- 

 ments (such as the ligamentum posticum, the posterior 

 crucial and lateral ligaments) are relaxed, while the 

 ligaments rendered tense by the position are the 

 patellar and the anterior part of the capsular, the 

 latter of which is but a yielding membrane. 



(3) The sensory nerves of the joint being disturbed, 

 contraction of muscles may be anticipated from reflex 

 action, and of the muscles so excited the flexors may 

 be expected to have the advantage, as being the more 

 powerful and the more favourably placed for acting 

 upon the articulation. 



Dislocation of the semi-lunar cartilages. 

 One or other of these cartilages may be displaced from 

 its attachments to the tibia, and become nipped or 

 locked between that bone and the femur. The result 

 is a sudden pain in the limb associated with a fixing of 

 the knee in the ' flexed position. The accident is 

 usually brought about by a twist given to the leg when 

 the knee -joint is more or less bent. It may be noted 



