QUADRICEPS EXTENSOR 3G5 



this armngement is to allow of the free action of the long head of the l:)iceps 

 which crosses over this part of the senii-niemhranosiis. Otherwise the swelling 

 of the fibres of the biceps during contraction might have pressed upon and inter- 

 fered with the action of the senii-menibranosus, which usually contracts at the 

 same time. 



Nerve-supply. — From the first, second, and third nerves which enter into the 

 formation of the sacral plexus through the great sciatic nerve which sends l^ranches 

 to the deep surface of the muscle al)out the middle of the thigh. 



Action. — To strongly extend the hip and to flex the knee. When the knee is 

 flexed it will also assist in the internal rotation of the leg, but with less mechanical 

 advantage than the semi-tendinosus and gracilis, as its line of action is so near to 

 the axis of movement. 



Like most penniform muscles, the semi-membranosus is very powerful, as is 

 shown by the thickness of its tendons. This strength is necessitated by the fact 

 that its line of action is so much nearer to the axis of movement both in flexion 

 and rotation inwards of the knee. Like the other muscles which arise from the 

 tuber ischii, it will co-operate powerfully in raising the body from the stooping 

 position, this prominence of bone forming the short arm of the lever by which the 

 trunk is raised. This group of muscles affords a good example of the peculiar 

 action obtained by long muscles passing over two joints. If all three muscles were 

 to remain passive, like so many ligaments, it is oljvious that on flexion of the hip- 

 joint l)y means of the ilio-psoas and other muscles, the hamstrings would ensure 

 the simultaneous flexion of the knee; or, again, on extension of the knee by the 

 action of the powerful muscles of the front of the thigh, the hamstrings would 

 produce a corresponding extension of the hip-joint. Seeing, however, that these 

 hamstrings are not passive, but that they contract powerfully at the same time in 

 man}' of these movements, it follows that in flexion of the hip in such movements 

 as those of running, the knee is at the same time flexed with increased raj^idity; 

 and again, when the knees are extended by the powerful contraction of the 

 quadriceps muscle, the simultaneous action of the hamstring muscles will produce a 

 still more rajiid elevation of the trunk. 



Relations of the semi-membranosus. — Behind, the gluteus maximus, biceps, 

 and semi-tendinosus; in front, the adductor magnus, posterior ligament of knee, 

 and the popliteus. Along its outer border lies the great sciatic nerve, and just 

 before its insertion the inner head of the gastrocnemius hooks round this border, 

 being separated from it by a bursa which communicates with the knee-joint. There 

 is also usually a small bursa l^etween its tendon and the back of the inner tuberosity 

 of the tibia. 



Variations. — The semi-membranosus has occasionally been deficient, or only represented by a 

 tliiu musfulo-tendinous band. It has also been found double. 



AXTERIOR MUSCLES OF THE THIGH 



This group consists of the sartorius and the (quadriceps extensor. 



1. SARTORIUS 

 This musclf has already Ijeen described (see page 350). 



2. QUADRICEPS EXTENSOR 



The quadriceps extensor ( tigs. 287 and 289), as its name implies, consists 

 of four heads. Of these, one, the rectus, arises from the innominate bone; and 

 the three others, the vastus externus, vastus internus, and crureus. from the femur; 

 while the common tendon is inserted into the upper border and sides of the patella. 



