68 THE ANATOMY OF THE HORSE. 



lower. The tipper nerve passes backwards on the sacro-sciatic liga- 

 ment ; and, after giving branches to the posterior division of the 

 superficial gluteus and to the posterior fleshy slip of the middle 

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

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

 former, turning over the tuber ischii on its outer side, becomes cutane- 

 ous at the back of the thigh about four or five inches below 7 the tuber ; 

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

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



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

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

 sacral plexus, and appears at the great sacro-sciatic 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 interims, the gemelli and common tendon of the obturator 

 interims 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 interims, 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-sciatic 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 

 external sap>henous nerve, which will be followed in the dissection of the leg. 



Directions. — The great sciatic nerve should now be cut 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 pulling the point of the hock outwards. 

 This will put these muscles on the stretch. 



