THE SCIATIC 



947 



The sciatic nerve (n. ischiadicus). The sciatic is not only the largest nerve 

 of the sacral plexus, but it is also the largest nerve in the body. Its terminal 

 brandies are chiefly muscular, though some of its fibres are cutaneous. Although 

 it is referred to as one trunk, it consists in reality of penuical (external) and tibial 

 (internal popliteal) portions, which are bound together by a sheath of fibrous tissue 

 as far as the upper end of the popliteal --pace. In about if) per cent, of the cases the 

 two parts remain separate, and in such cases the peroneal (external popliteal) part 

 usually pierces the pirifonnis. The peroneal portion of the nerve consists of fibres 

 derived from the dorsal branches of the anterior primary divisions of the fourth and 

 fifth lumbar and the first and second sacral nerves, while the tibial part is formed 



Fio. 693. A DISSECTION OP THE LUMBAR AND SACRAL PLEXUSES, FROM BEHIND. 

 (The anterior crural nerve is placed between the external cutaneous and obturator nerves.) 



LAST THORACIC XERVE 



Fsoas 



ILIO- 

 IDroc.ASTRlC 



ILIO-ISQUINAL 



ILIAC BRA.VCH 



HI-' /LIO- 

 IIYPOGASTRIC 



Oluteua medius 

 Superior glutetil ftrtery _ 

 SUPERIOR 



9CIA Til' XKRVE 

 INFERIOR OH' TEAL 



totnt 



POSTERIOR FEMORAL 

 CUTANEOUS 



Dura mater of cord 



roantiot PKUI.ISY 



DIVISION 



GEXITO-FEMORA L 



CAVDA EQUIJVA 



Fllum terminate 



LA TERA L 

 < 'I TAXEOU8 



OBTURA TOR 



THl-XK 



FIRST SACK A L 

 NERVE 



FIFTH SACRAL 



xi-: n i /; 



V1XI-KRAL 



Inferior glitteal artery 



Saero-BpinotiB ligament 



prnic XERVE 



NERVE TO 

 OHTI'RATOR 

 IXTERXrs 



INFERIOR MEDIAL 



HI' 



THIRD SACRAL 



yssras 



by the fibres from the anterior branchas of the fourth and fifth lumbar, and the first, 

 second, and third sacral nerves (figs. 692, 693). The common trunk leaves the 

 pelvis by passing through the great sacro-sciatic foramen, below the piriformis, 

 and descends through the buttock, running midway between the tuber ischii and 

 the great trochanter (fig. 004). Passing down the thigh, it terminates at the upper 

 angle of the popliteal space by dividing into the common peroneal (external popli- 

 teal) and the tibial (internal popliteal) nerves (fig. OO.'i). The trunk of the nerve 

 lies deeply in the thigh, and it is covered posteriorly by the skin and fascia, the 

 glutens maximus and the long head of the biceps femoris. Anteriorly it is in 



