1008 



THE NER VO US S YSTEM 



lateral side of the internal pudic artery and on the medial side of the sciatic nerve. 

 It gives a branch to the gemellus superior, and turns forward through the small 

 sciatic foramen into the perineum, where it terminates in the inner surface of the 

 obturator internus. 



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 

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

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

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

 as far as the upper end of the popliteal space. In about 10 per cent, of the cases 

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



Fig. 769. — A Dissection of the Lumbar and Sacral Plexuses, prom Behind. 

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



Twelfth nb 



Last thoracic nerve 



Psoas major 



Ilio-hypogastric 



Ilio-inguinal 



Iliac branch of ilio 

 hypogastric 



Gluteus medius 



Superior gluteal artery 



Superior gluteal nerve 



Sciatic nerve 

 Inferior gluteal nerve 



Posterior femoral 

 cutaneous 



^Yj^fy 



Dura mater of cord 



Posterior primary 

 division 



Genito-femoral 



Cauda equina 



f Filum terminale 



Lateral cutaneous 



Obturator 



Lumbo-sacral trunk 

 First sacral nerve 



Fifth sacral nerve 



Visceral branches 



Inferior gluteal artery 

 -jp Sacro-spinous 

 j\ ligament 



Pudic nerve 



Nerve to obturator 

 internus 



Inferior medial clunial 

 of second and third 

 sacral nerves 



part usually pierces the piriformis. The peroneal portion of the nerve consists of 

 fibres derived from the dorsal branches of the anterior i)rimary divisions of the 

 fourth and fifth lumbar and the first and second sacral nerves, while the tibial 

 part is formed by the fil^res from the anterior branches of the fourth and fifth 

 lumbar, and tlie first, second, and third sacral nerves (figs. 7G5, 7(39). The com- 

 mon trunk leaves the pelvis Ijy ])assing through tlie great sacro-sciatic foramen, 

 usually below the piriformis, and des(;ends tiirough the buttock, running midway 

 between the tuber ischii and the great trochanter (fig. 770). Passing down the 

 thigh, the trunk terminates at the upper angle of the popliteal space by dividing 



