THE REGION OF THE KNEE 441 



The Posterior Compartment of the Thigh contains the powerful 

 hamstring muscles, the sciatic nerve and its two terminal branches, the tibial 

 (int. popliteal) and the common peroneal (ext. popliteal) nerves. 



The hamstring muscles all arise from the ischial tuberosity. The 

 Biceps, which passes distally and laterally, possesses.' a second, or short 

 head of origin, which arises from the lateral lip of the linea aspera. These 

 two heads unite opposite the lateral epicondyle, and their common tendon, 

 which is easily felt through the skin, extends to the head of the fibula for its 

 insertion. The biceps flexes the knee-joint, and is capable of producing a 

 certain amount of lateral rotation ; in addition, it acts as an extensor of the 

 hip-joint. 



The Semimembranosus passes distally along the medial side of the posterior 

 compartment of the thigh, and is inserted into a groove on the postero-medial 

 aspect of the medial condyle of the tibia. The Semitendinosus is smaller in 

 bulk and lies on the surface of the semimembranosus. Its tendon (p. 443) 

 extends to the proximal part of the medial surface of the tibia, where it is 

 inserted behind the tendons of the gracilis and sartorius, from which it is 

 separated by a synovial bursa (p. 450). Like the biceps, both these muscles 

 extend the hip and flex the knee, but, since they are related to the medial 

 aspect of the latter joint, they cause a slight amount of medial rotation. 



All three muscles are supplied by the sciatic nerve (L. 4, 5, S. i, 2, and 3), 

 but the branch to the short head of the biceps frequently arises from the 

 common peroneal nerve (ext. popliteal). 



The Sciatic Nerve passes distally through the posterior 

 compartment of the thigh on the posterior surface of the 

 adductor magnus. Proximally it is crossed by the long head 

 of the biceps, and in the rest of its course it is overlapped by 

 the semimembranosus. It gives off branches of supply, which 

 enter the flexor muscles near their origin, and, together with 

 the obturator nerve, it supplies the adductor magnus. In the 

 middle third of the thigh, it ends by dividing into the common 

 peroneal (L. 4, 5, S. i and 2) and the tibial (L. 4, 5, S. i, 2, and 

 3) nerves. 



The perforating arteries from the profunda femoris (p. 410) 

 enter the posterior compartment of the thigh by piercing the 

 adductor magnus close to its insertion into the linea aspera. 

 They form a longitudinal series of anastomoses with one another. 

 Proximally the first perforating artery takes part in the crucial 

 anastomosis (p. 418) ; distally, the third and fourth perforating 

 arteries anastomose with the proximal muscular branches of 

 the popliteal (p. 445). 



THE REGION OF THE KNEE. 



Surface Landmarks. As there is little elasticity in the 

 ligamentum patellae, the patella remains at a constant distance 



