174 THE NERVOUS SYSTEM 



forces its way between the two bony prominences. In its 

 course, the sciatic nerve crosses the posterior (or extensor) 

 aspect of the hip-joint, while its terminal branches are related 

 to the posterior (or flexor) aspect of the knee-joint. On 

 account of these relationships, the sciatic nerve is put on the 

 stretch when the hip-joint is flexed, provided that the knee- 

 joint is in the position of extension. In neuritis of the sciatic 

 nerve, this movement causes intense pain, which disappears 

 on flexion of the knee, on account of the resulting relaxation 

 of the terminal branches. 



The Sciatic Nerve terminates about the middle of the thigh 

 by dividing into the Tibial (Internal Popliteal) and the Common 

 Peroneal (External Popliteal) Nerves, but these two parts, 

 although wrapped up in the same fibrous sheath in the sciatic 

 nerve, are quite distinct from one another right up to their 

 origins from the sacral plexus. In the proximal part of the 

 thigh, the nerve supplies branches to the semimembranosus, 

 the semitendinosus and the long head of the biceps (through 

 its tibial part) and to the short head of the biceps (through 

 its peroneal part). The last-named branch may arise in the 

 distal part of the thigh from the common peroneal nerve itself. 



The Semimembranosus arises from the ischial tuberosity and passes 

 distally along the medial side of the back of the thigh. It forms one of 

 the proximo-medial boundaries of the popliteal fossa, and its tendon can be 

 readily palpated in that position when the knee is strongly flexed. It is 

 inserted into the posterior aspect of the medial condyle of the tibia. 



The Semitendinosus also arises from the ischial tuberosity and passes 

 distally on the surface of the preceding muscle. Its tendon can also be 

 palpated at the proximo-medial side of the popliteal fossa, and, finally, it 

 is inserted into the proximal part of the medial surface of the tibia. 



Both these muscles act as flexors of the knee and as medial rotators of 

 the leg on the femur. 



The Biceps has two heads of origin. The long head arises from the 

 ischial tuberosity and the short head from the dorsal aspect of the femur. 

 The tendon of the biceps forms the proximo-lateral boundary of the 

 popliteal fossa and it can he traced distally to its insertion into the head 

 of the t fibula, when the knee is strongly flexed. It acts as a flexor of the 

 knee and as a lateral rotator of the leg on the femur. 



