THE LOWER LIMB 437 



They supply the integument of the back of the leg as low as about 

 the centre of the calf, and communicate with branches of the 

 ramus communicans tibialis from the internal pophteal. 



Muscles. — The muscles of the back of the thigh are called the 

 hamstring muscles, and are three in nmnber, namely, biceps 

 femoris, semitendinosus, and semimembranosus. 



Biceps Femoris or Biceps Flexor Cruris — Origin. — The muscle arises 

 by two heads — long or ischial, and short or femoral, (i) Long or 

 Ischial Head. — This arises, in conjunction with the semitendinosus, 

 from the lower and inner impression on the posterior surface of the 

 tuber ischii. (2) Short or Femoral Head. — This arises from (i) the 

 outer hp of the linea aspera, (2) the upper two- thirds of the external 

 supracondylar ridge, and (3) the external intermuscular septum. 



Insertion. — The upper surface of the head of the fibula in front of 

 the styloid process, by means of a rotmd tendon which sends an 

 expansion downwards to the deep fascia covering the peroneus 

 longus. A few fibres are also inserted into the external tuberosity 

 of the tibia. 



The long external lateral hgament of the knee-joint passes 

 through the tendon of insertion, and divides it into two parts, from 

 which the ligament is separated by the bursa wrapped around it. 

 The two divisions of the tendon are anterior and posterior, the 

 anterior, which is the stronger, being inserted into the external 

 tuberosity of the tibia, as well as into the head of the fibula. The 

 expansion to the deep fascia of the leg is derived from the posterior 

 division of the tendon. 



Nerve-supply. — Great sciatic nerve. The long or ischial head 

 receives branches from the internal popliteal part of the nerve, and 

 the short or femoral head from the external popliteal part. 



Action. — Acting from its origin the muscle is an extensor of the 

 ip- joint and a flexor of the leg upon the thigh, and, having flexed 

 le knee-joint, it acts as an external rotator of the leg. The long 

 3r ischial head alone extends the hip- joint. Acting from its iaser- 

 ion the muscle, by means of its long or ischial head, is an extensor 

 )f the pelvis upon the thigh. In virtue of its double insertion into 

 "le head of the fibula and external tuberosity of the tibia, the biceps 

 smoris contributes materially to the strength of the superior tibio- 

 ibular joint by bracing the bones together. 

 In rare cases the biceps femoris has a third head of origin, which 

 lay arise from the tuber ischii, hnea aspera, or internal supra- 

 condylar ridge. 

 Semitendinosus — Origin. — ^The lower and inner impression on the 

 jsterior surface of the tuber ischii, in conjunction with the long or 

 :hial head of the biceps femoris. 



Insertion. — ^The upper part of the internal surface of the shaft of 



le tibia, behind the sartorius and below the gracilis. From the 



mdon of insertion an expansion is given to the deep fascia of the leg. 



Nerve-supply. — Great sciatic nerve. The branches come from the 



iternal popliteal part of the nerve. 



