REGION OF THE KNEE. 257 



Connected with the tendons of these muscles are bursal 

 sacs, which are of surgical importance, as if enlarged or 

 inflamed they may oifer some difficulties in diagnosis. 

 Thus, one exists between the inner head of the gastroc- 

 nemius and the condyle of the femur, and often commu- 

 nicates with the joint, another exists between the tendon 

 of the semi-membranosus and the tibia ; the bursa be- 

 neath the outer head of the gastrocnemius is generally a 

 prolongation of the synovial membrane of the joint, and 

 between the popliteal tendon and the posterior and exter- 

 nal lateral ligaments there often are found separate bursse. 

 The muscles above mentioned inclose a lozenge-shaped 

 space, containing a large quantity of fat and cellular 

 tissue, in which lie the popliteal vessels, nerves, and 

 some lymphatics ; as this fat and cellular tissue is con- 

 tinuous with that surrounding the muscles of the back 

 of the thigh and calf, any collections of pus in the space 

 are liable to extend up the limb or downwards amongst 

 the muscles of the back of the leg. Most superficial in 

 the ^space, on the outer side, is the external popliteal 

 nerve, which lies on the inner margin of the biceps ten- 

 don, and must be carefully avoided in tenotomy ; the 

 external saphena vein lies to its inner side, after having 

 perforated the popliteal aponeurosis. More internal 

 still is the internal politeal nerve, which is the continu- 

 ation directly downwards of the sciatic nerve, in the in- 

 ferior portion of the space ; this nerve gives off a leash 

 of branches which supply the muscles of the calf, and a 

 filament, the communicans poplitei, which joins a corre- 

 sponding one from the external popliteal, the communi- 

 cans peronei, forming a loop which generally lies in the 

 sulcus between the two heads of the gastrocnemius (ex- 

 ternal saphena). The sheath of the popliteal vessels is 



