The Region of the Knee Joint. 559 



the "nipping" of the movable body between the bones. 

 The semilunar cartilages may, very rarely, be "nipped," 

 and of the two, the internal is the one usually affected, 

 since the inner tuberosity of the tibia can be separated 

 from the femur by the action of the biceps, as already 

 mentioned. 



The Patella. The patella protects the front of the 

 joint and increases the leverage of the quadriceps exten- 

 sor by making it act at a greater angle. Its posterior sur- 

 face has seven facets : One, the inner, being in contact 

 with the internal condyle in all the movements of the 

 joint; while the others are arranged, in pairs, so 

 that, in extension of the leg, the lower pair rests on the 

 upper portion of the trochlear surface of the femur; in 

 mid-flexion, the middle pair rests on the middle of the 

 trochlear surface while, in flexion, the upper pair rests on 

 the lower part of this surface. Thus the points of contact 

 of the patella with the femur change, just as those of the 

 tibia and femur do, in flexion and extension. - * 



Fracture of the Patella. Hamilton states that mus- 

 cular violence, i.e., sudden forcible contraction of the 

 quadriceps extensor is responsible for the break in a trans- 

 versely fractured patella, and that, when so occurring, the 

 fracture is generally in the lower third of the bone. The 

 explanation of this is, that, in midflexion the position in 

 which the limb generally is, when fracture occurs 

 the middle of the patella rests on the middle of the 

 trochlear surf ace,, leaving, therefore, the upper and lower 

 portions of the bone unsupported, and hence, the lower 

 portion, being the weaker, may give way from any sudden, 

 severe strain. Stellate fractures of the patella generally 

 result from the application of direct violence. Bony 

 union in these fractures is very rare, partly from the pre- 



