Chap, xxi.] THE KNEE JOINT. 437 



communicated with the synovial cavity in seven cases 

 out of ten in young children, and in eight cases out of 

 ten in adults. 



It will thus be seen that when this communica- 

 tion exists, a stab over the femur, about two inches 

 above the trochlear surface of the bone, or about 

 the same distance above the top of the patella, 

 when the limb is extended, will practically open the 

 knee-joint. 



Cases are reported of extravasation of blood into 

 this bursa, that, although at first limited to the sac, 

 have, on rough handling, extended into the knee- 

 joint, a circumstance leading to the supposition that 

 in some cases the orifice of communication may be 

 very small. 



The crucial ligaments, although more or less com- 

 pletely invested by the synovial membrane, are yet 

 entirely outside the synovial cavity. 



The upper third of the patellar ligament is in 

 relation to the synovial membrane, from which, how- 

 ever, it is separated by a pad of fat. The lower two- 

 thirds of the ligament are in relation to the bursa and 

 fatty tissue that intervene between the band and the 

 tibia. A knife passed horizontally backwards at the 

 apex of the patella, would, when the limb is extended, 

 just miss the joint line between the femur and tibia, 

 and would hit the latter bone (Fig. 45). If, how- 

 ever, there be any effusion in the joint, or the limb be 

 a little flexed, a knife so introduced would pass 

 between the two bones (Fig. 46). 



Joint disease. Owing to its superficial position, 

 the knee-joint is the articulation that is most frequently 

 the seat of inflammations due to injury and exposure 

 to cold. When distended with fluid, the effusion soon 

 shows itself above and at the sides of the patella, by 

 bulging forward the synovial sac, which is here more 

 nearly in relation to the surface than it is elsewhere. 



