378 The Iliac Veins 



5 to the outer side of the right common iliac artery, to mark the point 

 of origin of the cava. Then, with a blue pencil, let him place the ex- 

 ternal, and internal, and the beginning of the common iliac veins to 

 the inner side of their respective arteries. Having done that, let him 

 draw in the commencement of the cava. To continue the common 

 veins into the cava, he must bring the left vein away from its artery 

 and beneath the right artery, and send the right vein more quickly 

 beneath the right artery and even to its outer side. 



Tributaries. The sacra media vein conveniently falls into the left 

 common iliac vein as it crosses below the division of the aorta ; the 

 ilio-hnnbar vein enters the corresponding common iliac vein. 



THE SACRAL PLEXUS 



The sacral nerves descend from the lowest part of the lumbar en- 

 largement, which is at the level of the last dorsal vertebra, in the cauda 

 equina, and divide into an anterior and a posterior branch. 



The posterior sacral nerves emerge by the posterior sacral fora- 

 mina and give internal branches to the origin of the erector spinae, and 

 external branches which form loops about the great sacro-sciatic liga- 

 ment, from which twigs pass through the origin of the gluteus maximus 

 to supply the skin of the hinder part of the buttock. 



The sacral plexus is formed by the junction of the lumbo-sacral 

 cord with the anterior divisions of the first three sacraF nerves and 

 with part of that of the fourth. The fourth sacral nerve also sends 

 branches to the rectum, bladder, vagina ; to the coccygeus, levator ani, 

 and external sphincter, and to the skin near the side of the coccyx. 



The anterior division of the fifth sacral is an unimportant twig 

 which passes out between the sacrum and coccyx, for the skin near 

 the coccyx, together with a still smaller nerve, the coccygeal. 



Relations of the sacral plexus. The plexus lies upon the sacrum 

 and pyriformis, and is separated from the rectum and bladder, and 

 from the divisions of the internal iliac artery, by the pelvic fascia. 



The upper part of the plexus consists of the lumbo-sacral cord and 

 of the anterior divisions of the first and second and part of the third 

 sacrals, and forms chiefly the great sciatic nerve ; the rest of the third 

 and the part of the fourth forming the internal pudic. But, in addition to 

 these trunks, the plexus gives off the superior gluteal (from the lumbo- 

 sacral cord), the lesser sciatic, and the inferior gluteal, and muscular 

 branches to the pyriformis, obturator internus, gemelli, and quadratus 

 femoris. The nerve to the obturator internus runs round the ischial 

 spine, with the internal pudic nerve ; and the nerve to the quadratus 

 passes beneath the tendon of the obturator internus and the gemelli, 

 and gives a twig through the back of the capsule of the hip-joint. 



The superior gluteal nerve comes from the lumbo-sacral cord, 



