ENLAKGEMBNT OF BUES^. 147 



of the gastrocnemius, or in some cases more directly between 

 that head and the posterior ligament of the knee-joint. In by 

 far the greater number of cases the bursa communicates with 

 the knee-joint, and there is a great tendency for it to be dis- 

 tended with fluid when there is inflammation of the synovial 

 membrane of this joint. In the flexed position of the knee the 

 bursa becomes less tense, and it is possible to squeeze some of 

 its contents into the cavity of the articulation. In this way the 

 enlarged bursa is said to disappear in this position of the limb. 

 In extension fluid is forced from the interior of the joint into 

 the bursa, which now becomes palpable as a tense cyst lying 

 on the inner side of the posterior aspect of the joint, just 

 external to the tendon of insertion of the semi-membranosus 

 muscle. 



The bursa is probably the starting point in many instances 

 of a synovial cyst which later on may be entirely disconnected 

 with the joint, and may enlarge peripherally for some consider- 

 able distance. Such synovial cysts are frequently tuberculous in 

 nature. 



Sartorius bursa. The bursa placed between the expanded 

 tendinous insertion of the sartorius muscle and the insertions 

 of the gracilis and semi-tendinosus is sometimes the seat of 

 enlargement. The bursa does not communicate with the knee 

 joint. 



Sub-crureal bursa. This bursa lies between the quadriceps 

 tendon and the anterior surface of the lower part of the shaft of 

 the femur, and is in reality a prolongation upwards of the 

 synovial membrane of the joint. Hence it follows that a dis- 

 tension of the synovial cavity is usually accompanied by a 

 filling- up of this sac with fluid, which may readily be pressed 

 out of the sac into the joint, so as to make the sign of riding of 

 the patella easily obtainable. 



It follows from the position of this bursa and the two others 

 which communicate with the knee joint, namely the semi- 

 membranosus and the popliteal, that a punctured wound 

 involving any one of them, although appearing to be at some 



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