THE KNEE-JOINT. 261 



trochlea, but in extension it overlaps the outer portion 

 of the articular surface, and rises, if the extension be 

 extreme, half its diameter above it. 



The femur and the patella are united by the fibrous 

 capsule and by the tendon of the quadriceps extensor. 

 The great power exerted by the action of the quadriceps 

 on the patella above, and its strong attachment to the 

 tibia by the ligamentnm patellae below, explain how this 

 bone may be fractured transversely by muscular action. 



Dislocations of the Patella. This bone is most fre- 

 quently dislocated outwards. The bone lying above and 

 external to the external articular surface of the trochlea, 

 the outward traction of the extensor muscles favors this 

 form of displacement. Dislocation inwards is very rare. 

 Dislocation with the bone lying vertically either on the 

 outer or the inner condyle is also very rare. 1 



The Articulation between the Femur and the Tibia. 

 The inner articular surface of the condyle of the femur 

 is less prominent anteriorly than the external, but is set 

 lower and extends a little further backwards, and is more 

 oblique laterally. These surfaces are separated posteri- 

 orly by the intercondyloid notch. The head of the tibia 

 presents two concave articular surfaces, the external of 

 which is the rounder, separated by the spine, in front 

 and behind which is a rough depression to which are 

 attached the crucial ligaments. 



The femur and tibia are united by the capsular liga- 

 ment, posteriorly by the posterior ligament, which is a 

 prolongation upwards of the tendon of the semi-mem- 

 branosus, an internal and external lateral ligament, two 



1 Vide case in practice of author, British Medical Journal, De- 

 cember, 1872. 



