DISSECTION OP THE POSTERIOE LIMB. 67 



uperficial gluteus, and to the posterior fleshy slip of the middle 

 gluteus, it enters the biceps femoris. The lower nerve, passing down- 

 yards and backwards, divides into an outer and an inner branch ; the 

 brmer, turning over the tuber isohii on its outer side, becomes cutane- 

 )us at the back of the thigh about four or five inches below the tuber ; 

 ;he latter, after giving twigs to the semitendinosus, joins a branch from 

 ;he internal pudic nerve to be distributed to the perineal structures. 



The Great Sciatic Nerve (Plate 16). This is, at its point of 

 jrigin, the largest nerve in the body. It is furnished by the lumbo- 

 sacral plexus, and appears at the great sacro-soiatic opening as a broad 

 riband. In its downward course in the hip it is covered by the middle 

 gluteus, and rests in succession on the sacro-sciatic ligament, the 

 gluteus internus, the gemelli and common tendon of the obturator 

 internus and pyriformis, and the quadratus femoris. In the thigh it is 

 included between the biceps and semitendinosus outwardly, and the 

 semimembranosus and great adductor inwardly. The trunk of the nerve 

 is continued as the internal popliteal nerve between the two heads of 

 the gastrocnemius, where it will be followed in the dissection of the 

 leg. It gives off in succession the following branches : — 1. A nerve for 

 the supply of the obturator internus, pyriformis, gemelli, and quadratus 

 femoris. This slender branch is given off about midway between the 

 great and small sciatic openings, and it descends at the posterior border 

 of the parent trunk, or between that and the ligament. The nerves to 

 the quadratus and gemelli may arise from the sciatic independently, 

 and the branch to the first of these muscles passes under the gemelli 

 and the common tendon of the pyriformis and obturator internus. 

 2. The external popliteal nerve is a large branch that separates from the 

 great sciatic about the level of the small sacro-soiatic opening ; and, 

 descending in front of the parent nerve, it passes between the biceps 

 and the outer head of the gastrocnemius, where it will be followed at a 

 later stage. The peroneal cutaneous branch of this nerve has already 

 been seen perforating the lower part of the biceps, at the level of the 

 stifle. 3. A branch that divides to supply the semimembranosus and 

 lower portions of the biceps and semitendinosus (Plate 14). 4. The ex- 

 ternal saphenous nerve, which will be followed in the dissection of the leg. 



Directions.— Hhe gi-eat sciatic nerve should now be out at the upper 

 border of the gluteus internus, and turned downwards with its branches. 

 The gluteus internus, and the common tendon of the pyriformis and 

 obturator internus, together with the gemelli, should be carefully 

 cleaned and defined; and to facilitate this, the limb should be rotated 

 inwards as far as possible, by puUing the point of the hock outwards. 

 This will put these muscles on the stretch. 



The Deep Gluteus, or gluteus internus (Plate 16, and fig. 2), is placed 

 above the hip-joint, in immediate contact with the capsular ligament. 



