HAMSTRING GROUP 475 



and semimembranosus rotate the thigh and the leg medial ward; the biceps, 

 lateralward. The semitendinosus and the long head of the biceps constitute a 

 superficial layer; the semimembranosus and the short head of the biceps a deep 

 layer. The semitendinosus and the long head of the biceps arise by a common 

 tendon from the tuber of the ischium. The somewhat fusiform semitendinosus 

 gives rise to a tendon in the lower half of the thigh. The tendon curves forward 

 behind the knee to be inserted under that of the sartorius into the medial side of 

 the tibia. The penniform short head of the biceps arises from the linea aspera 

 in the lower part of the thigh, and is inserted, together with the fusiform long 

 head, into a tendon that passes over the lateral side of the knee and is attached 

 to the head of the fibula. The semimembranosus arises from the tuber ischii 

 through a long, flat, triangular tendon. The belly of the muscle increases in 

 thickness toward the knee. It is inserted by a strong tendon on the back of the 

 medial condyle of the tibia. From the tendons of all the hamstring muscles 

 expansions are sent into the crural fascia. 



The muscles of this group are all supplied by the tibial portion of the sciatic, 

 except the short head of the biceps, which is supplied from the peroneal portion. 



The femoral head of the biceps is characteristic of the anthropoid apes and man. In many 

 mammals its place is taken by a slender muscle, the tenuissimus, which extends from the caudal 

 vertebrae, the sacro-tuberous (great sacro-sciatic) ligament, or the gluteal fascia to the fascia 

 of the back of the leg. In some forms this muscle is broad instead of slender. According to 

 Testut, the long head of the biceps may be looked upon as arising by two fascicuU, one primi- 

 tively attached to the posterior part of the ilium, the other to the caudal vertebrae or coccyx. 

 The sacro-tuberous (great sacro-sciatic) ligament represents the reduced upper portion of this 

 muscle. In the fcstus the origin of the muscle extends higher on the sacro-tuberous Ugament 

 than in the adult. In many of the lower mammals the origins of the semimembranosus and 

 semitendinosus take place in part from the sacro-caudal vertebrae. 



In the mammals below man the insertion of the biceps, gracilis, and semitendinosus takes 

 place chiefly into the fascia of the back of the leg, and extends more distally than in man. This 

 insertion of these flexor muscles is associated with a permanent position of flexion of the leg at the 

 knee. In the human embryo likewise these muscles are inserted more distally than in the adult. 

 In the lower primates the semimembranosus is chiefly a medial rotator of the leg. 



Biceps femoris (Figs. 408, 413). — Long head. — Origin. — From a tendon common to it and 

 the semitendinosus. This tendon arises from the more medial of the two facets on the back of 

 the tuber of the ischium and from the sacro-tuberous (great sacro-sciatic) ligament. It is 

 continued for a third of the distance to the knee as a septum between the biceps and the semi- 

 tendinosus, and for a short distance as an aponeurotic sheath on the deep surface of the biceps. 



Structure and insertion. — The fibre-bundles begin to arise from the tendon some distance 

 from the ischium. They form a thick fusiform belly which is inserted into the deep surface of 

 a tendon that begins laterally on the back of the muscle about the middle of the thigh. The 

 insertion of the fibre-bundles of the long head continues on the medial margin of the deep surface 

 of the tendon nearly as far as the lateral condyle of the femur. 



Short head. — Origin. — By short tendinous fibres from the lateral lip of the linea aspera of 

 the femur from the middle of the shaft to the bifurcation of this line, the proximal two-thirds 

 of the supracondylar ridge, and the lateral intermuscular septum. 



Structure and insertion. — The fibre-bundles take a nearly parallel course, to be inserted on 

 the deep surface of the common tendon of insertion. The most distal fibres are inserted nearly 

 to the skeletal attachment of the tendon. The tendon is inserted into the superior extremity 

 of the head of the fibula, into the lateral condyle of the tibia, and into the fascia of the leg. 



Nerve-supply. — Commonly two branches are given to the long head of the biceps. One 

 of these branches is given off proximal to the ischium, and enters the proximal third of the deep 

 surface of the muscle. The other is given off more distally and usually enters the rniddle third. 

 Either or both branches may be doubled or the two may be combined for some distance in a 

 common trunk. The nerve-fibres arise usually from the fu-st, second, and thu-d sacral nerves. 

 The branch to the short head arises from the peroneal (external pophteal) portion of the sciatic 

 nerve about the middle of the thigh. It enters the posterior surface near the lateral margin of 

 the muscle, and passes distally across the muscle bundles about midway between the tendons of 

 origin and insertion. The nerve-fibres come chiefly from the fifth lumbar, first and second 

 sacral nerves. 



Action. — To extend and adduct the thigh and flex the leg. The short head acts only on the 

 leg. The long head acts as a lateral rotator of the thigh, and of the leg when flexed. 



Relations. — The upper extremity of the muscle is covered by the gluteus maximus. ^ Below 

 this the long head and tendon of insertion lie beneath the fascia lata and overlie the short head. 

 Ventral to the muscle he the tendon of origin of the semimembranosus, the adductor magnus 

 and vastus lateralis muscles, and the lateral head of the gastrocnemius. The medial border ia 

 in contact with the semitendinosus and semimembranosus. Distally it forms the upper lateral 

 border of the pophteal space. The sciatic nerve runs between it and the adductor magnus. 



Variations. — The short head is rarely absent. It may be more isolated from the long 

 head than usual, and at times has a separate tendon of insertion. It may itself be divided into 

 two distinct laminae. Its origin may take place higher up on the femur than usual or from the 

 fascia lata. Variations of this sort suggest the tenuissimus muscle of some of the lower mammals 



