2l8 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 301. — The deep layer of the posterior hip muscles and the superficial layer of the flexors of the 

 thigh region. 

 The glutaeus maximus and medius and the obturator internus have been removed. 



Fig. 302. — The deep layer of the posterior hip muscles and the deep layer of the flexors of the thigh 

 region. 

 The glutaeus maximus and medius, the quadratus femoris, the long head of the biceps, and the semitendinosus have 

 been removed. 



of its position and innervation. It is situated upon the outer and lower surface of the pubis 

 and ischium and is completely covered by the pectineus, the adductor longus, and the adductor 

 brevis. Like the obturator internus within the pelvis, it arises from the pubis, from the ischium, 

 and from the obturator membrane. It becomes narrower and thicker, runs over the lesser tro- 

 chanter, and passes over and behind the insertion of the iliopsoas, along the neck of the femur 

 between the gemellus inferior and quadratus femoris, largely covered by the latter muscle, to the 

 trochanteric fossa, where it is inserted beside the obturator internus. The muscle becomes ten- 

 dinous a short distance before its insertion. 



The obturator externus is supplied by the obturator nerve. It acts as an external rotator. 



THE POSTERIOR GROUP, THE FLEXORS. 



This group consists of but three muscles, the biceps femoris, the semitendinosus, and the semi- 

 membranosus, which have a more or less common origin from the ischial tuberosity; as they pass 

 toward the knee they are grouped in such a way that the biceps is external and the semitendi- 

 nosus and semimembranosus internal. The three muscles lie in the posterior femoral region and 

 their origins are concealed by the glutaeus maximus. 



The biceps femoris (Figs. 301, 302) is a large, strong muscle which is composed of a long 

 biarticular and of a short monarticular head and belly. The long head (Fig. 301) is a rather 

 strong tendon which arises in intimate connection with the semitendinosus from the lower aspect 

 of the tuberosity of the ischium. It passes downward and becomes continuous with a broad mus- 

 cular belly, which at first lies behind the adductor magnus and then passes markedly outward to 

 take up a position behind the vastus lateralis. At the lower third of the thigh it receives the 

 second head or short head (Fig. 302), which is short and rhomboid, and arises from the lower 

 half of the outer lip of the linea aspera. 



At the junction of the two heads, or somewhat above it, the posterior surface of the long 

 head possesses a distinct aponeurosis which is directly continuous with the tendon of insertion. 

 The short head is muscular throughout its entire course. 



The two heads unite just above the knee-joint and the muscle is inserted by a tendon into 

 the capitulum of the fibula. The inner margin of the biceps forms one of the boundaries of the 

 popliteal fossa (see page 220). 



The long head of the biceps is supplied by the tibial nerve, and the short head by the peroneal nerve. 



The semitendinosus (Fig. 301) is muscular in its upper two-thirds and tendinous in its 

 lower third. At its origin from the tuberosity of the ischium it is completely adherent to the long 



