262 



THE ARTICULATIONS 



of the internal condyle of the femur; in flexion the upper pair of facets rest 

 on the lower part of the trochlear surface of the femur; in mid-flexion the 

 middle pair rest on the middle of the trochlear surface; while in extension 

 the lower jmir of facets on the patella rest on the upper portion of the trochlear 

 surface of the femur. 



This diflferenee may be descril)ed as the shifting of the points of contact of the 

 articular surface. 



2. It differs from a true hinge in that, in passing from a state of extension to 

 one of flexion, the tibia does not revolve round a single transverse axis drawn 

 through the lower end of the femur, as the ulna does round the lower end of the 

 humerus. The articular surface of the tibia slides forwards in extension and back- 

 Avards in flexion; thus the axis round which the tibia revolves upon the femur is a 

 shifting one, as is seen by reference to fig. 250, B, C, D. 



3. Another point of difference is that extension is accompanied by rotation out- 

 wards, and flexion by rotation inwards. This rotation occurs round a vertical axis 



Fig. 251. — Section of Knee, showing Crlcials in Extension. 



Anterior crucial ligament 



SPINE OF TIBIA 

 Transverse ligament 



Slip from external flbro-cartilage to 



I femur (ligament of Wrisbergi 



Posterior crucial ligament 



External semilunar cartilage 



Coronary ligament 



Antero-superior tibio-flbular ligament 



drawn through the middle of the outer condyle of the femur and the outer tuber- 

 osity of the ti))ia, and is most marked at the termination of extension and at the 

 commencement of flexion. This rotation of the leg at the knee is a true rotation 

 about a vertical axis, and tlius differs from the oblicjuity of the flexion and extension 

 movements at the ell)o\v wliich is due to the oblique direction of the articular sur- 

 faces of tlie bom-s. 



4. The antero-posterior spiral curve of the femoral condyles is sucli, that the 

 anterior part is an arc of a greater circle than the posterior; hence certain ligaments 

 which are tightened during extension are relaxed during flexion, and thereby a con- 

 sideralile amount of rotatory movement is permitted in the flexed position. Tlie 

 axis of tliis rotation is vertical, and passes througli the inner tuliercle of the spiiK^ 

 of thetiltia, so that the outer tuberosity moves in the arc of a larger circle than 

 does the inner, and is therefore re(iuired "to move more freely and easilv; hence the 

 shape of the external articular facet and the loose connection of "the external 

 semilunar cartilage whicli is a(hi])ted to it. 



In extension, all the ligaments arc on the stretch with the exception of the 



