476 THE MUSCULATURE 



(see above). The long head of the biceps may receive accessory fasciculi from the coccyx, 

 sacrum, sacro-tuberous (great sacro-sciatic) ligament, tuber of the ischium, or the deep surface 

 of the gluteus maximus. These fasciculi suggest the iliac and saero-coccygeal origin of the 

 muscle found in lower vertebrates (see above). Inferiorly, a muscle fasciculus may take 

 the place of the fibrous prolongations from the tendon of the biceps into the sural fascia (the 

 tensores fasciae suralis). This may extend to the tendon of Achilles. The long head may 

 have a tendinous inscription similar to that of the semitendinosus. 



The semitendinosus (figs. 408, 413). — Origin. — Partly from a medio-dorsal facet on the 

 distal margin of the tuber of the ischium by direct implantation of the fibre-bundles, and 

 partly from the medial surface of the tendon common to it and the long head of the biceps. 



Structure and insertion. — The fibre-bundles spread out to form a flat, fusiform belly which, 

 about the middle of the thigh, again contracts toward the tendon of insertion. This begins on 

 the medial margin and dorsal surface of the muscle, becomes free from the muscle slightly above 

 the medial condyle of the femur, passes behind this and curves forward to be inserted by a trian- 

 gular expansion into the proximal part of the medial surface of the tibia behind and distal to 

 the insertion of the gracilis. An aponeurotic expansion is continued into the fascia of the leg. 

 About the middle of the muscle a narrow irregular tendinous inscription more or less completely 

 divides the belly into proximal and distal divisions. 



Nerve-supply. — To the muscle two nerves are commonly given. One arises from the sciatic 

 nerve or directly from the plexus, proximal to the tuber of the ischium, sometimes in com- 

 pany with a branch to the long head of the biceps. It enters the middle third of the deep surface 

 of the proximal portion of the muscle. The other branch arises from the sciatic nerve, usually 

 distal to the ischial tuber, sometimes in common with a nerve to the biceps or the semimem- 

 branosus. It enters about the middle of the deep surface of the distal half of the muscle. 

 Either or both branches may be represented by two nerves. The nerve fibres of the first 

 branch arise chiefly from the first and second sacral nerves, those of the second from the fifth 

 lumbar and first sacral nerves. 



Action. — To extend and adduct the thigh and rotate it medialward and to flex the leg, 

 and with knee flexed, to rotate the leg medialward. 



Relations. — It is covered by the gluteus maximus and fascia lata; on the lateral side lies 

 the biceps; and in front, the semimembranosus and adductor magnus. 



Variations. — It may be completely separated from the biceps at its origin. It may be 

 fused with neighbouring muscles. There may be two tendinous inscriptions. It may have a 

 femoral head (a condition characteristic of many birds). A muscle fasciculus may extend from 

 the body of the muscle to the fascia of the back of the leg. 



The semimembranosus (fig. 408). — Origin. — By a long, flat tendon which hes beneath the 

 proximal half of the semitendinosus, and which arises from the more lateral of the two facets 

 on the back of the tuber of the ischium, between the tendons of the biceps and the quadratus 

 femoris. The tendon is at first adherent to the tendon of the adductor magnus in front and 

 to that of the biceps and semitendinosus behind. It descends to the middle of the muscle. 



Structure and insertion. — From both surfaces of the medial side and distal extremity 

 of the tendon of origin fibre-bundles arise which take an oblique course to their insertion on the 

 aponeurosis of the tendon of insertion. This appears on the deep surface and medial margin 

 of the muscle opposite the end of the tendon of origin and descends on the medial side and deep 

 surface of the muscle. Near the back of the medial condyle of the femur the insertion of muscle- 

 fibres ceases and the tendon is inserted directly on the back of the medial condyle of the tibia, 

 and by aponem-otic expansions into the capsule of the joint, into the lateral condyle of the femur, 

 into the tibial collateral ligament, and into the fascia of the popliteus muscle. 



Nerve-supply. — By several branches from the sciatic nerve, which usually arise from a 

 common trunk in company with the branches to the adductor magnus. These branches enter 

 the deep surface of the muscle about midway between the origin and insertion of the constituent 

 fibre-bundles. 



Action. — ^To flex the leg and rotate it medialward and to extend and adduct the thigh and 

 rotate; it medialward. 



Relations. — It is covered by the gluteus maximus, the long head of the biceps, the semi- 

 tendinosus, and the fascia lata. It lies dorsal to the quadratus femoris, the adductor magnus, 

 and the knee-joint. 



Variations. — It may be fused with the semitendinosus or the adductor magnus. It may 

 be doubled. Its tendons may have a more extensive attachment than usual. The extent of 

 the belly of the mu.scle varies considerably. A muscle fasciculus may be sent into the popUteal 

 space. An extra head may arise from tlu; ischial spine. 



BuRSiE 



B. _m. bicipitis femoris superior. — A fair-sized bursa which frequently lies between the 

 tendou'of origin of tlie long head of the biceps and semitendinosus and the tendon of the semi- 

 membranosuK and the iscliial tuber. B. m. bicipitis femoris inferior. — A small bursa which 

 separates the tendon of insertion from the fihnlar collateral ligament of the knee-joint. B. m. 

 bicipitis gastrocnemialis. — A bursa infre(|uently found i)et\veen the tendon of the biceps and 

 tiie tendons (jf origin of the lateral head of the gastrocnemius and the plaiitaris muscles. B. 

 m. semimembranosus. — This is a large double l)ursa constantly present. One part extends 

 between the semimembranosus, the medial head of the gastrocnemius, and the knee-joint. 

 With the cavity of the joint it frequently connnunicates. The other part extends between the 

 tendon of the semimembranosus and the medial condyle of the tibia. 



