THE ABDOMEN 849 



ment of the great sacro-sciatic foramen, crosses the back of the 

 ischial spine, where it lies internal to the internal pudic vessels, 

 passes through the small sacro-sciatic foramen, and then traverses 

 the outer wall of the ischio-rectal fossa. Having given off the 

 inferior hemorrhoidal ner\'-e, it divides into the perineal and 

 dorsalis penis nerves, which with the internal pudic vessels are 

 contained in Alcock's canal. 



The sacral plexus and its branches being situated behind the 

 parietal pelvic fascia, the branches do not pierce that fascia as they 

 leave the pelvis. The branches of the internal iliac artery, on the 

 other hand, being placed on the superficial surface of the parietal 

 pelvic fascia, have to pierce it, with the single exception of the 

 obturator artery. 



Sacro-coccygeal Plexus. — The nerves which f<Mrxn this plexus are 

 the lower branch of the anterior primary division of the fourth 

 sacral, the anterior primary division of the fifth sacral, and the 

 anterior primary division of the cocc3'geal nerve. The upper branch 

 of the fourth sacral enters the pudic band of the sacral plexus, and 

 the fourth sacral is therefore a nervus fiircalis, inasmuch as it gives 

 a branch to the sacral plexus and one to the sacro-coccygeal plexus. 

 Before entering the sacro-coccygeal plexus the lower branch of the 

 fourth sacral nerve gives off visceral and muscular branches. The 

 visceral branches for the most part reinforce the pelvic plexus of 

 the sympathetic as white rami communicantes, but a few of them 

 pass independently to the pelvic viscera. The muscular branches 

 supply the levator ani, coccygeus, and sphincter ani externus. 

 The branch to the latter muscle reaches the perineum either by 

 piercing the coccygeus, or by passing between it and the levator 

 ani, and, besides suppljdng the external sphincter, it gives branches 

 to the integument between the coccyx and the anus. The nerve 

 is known as the hemorrhoidal or perineal branch of the fourth sacral. 

 The lower branch of the fourth sacral ner%''e, having parted with the 

 foregoing offsets, descends upon the coccygeus where it joins the 

 fifth sacral, which has just entered the pelvis through that muscle. 

 The conjoined nerve now descends, and is soon reinforced by the 

 coccygeal nerve, which also enters the pelvis by piercing the 

 coccygeus. In this manner the sacro-coccygeal plexus is formed. 

 It lies at the lower part of the posterior wall of the pelvis upon the 

 coccygeus, and the nervous loop takes a downward course. Subse- 

 quently it divides into several twigs, which leave the pelvis by 

 piercing the coccygeus, the small sacro-sciatic ligament, and the 

 adjacent portion of the gluteus maximus to be distributed to the 

 integument over the coccyx. 



Ureters. — ^The ureter of each side, having crossed the termina- 

 tion of the common iliac, or the commencement of the external 

 iliac, artery, enters the pelvis. It then passes downwards and for- 

 wards in front of the internal iliac artery to the outer wall of 

 the pelvis, lying beneath the peritoneum, and crossing internally 

 the obturator vessels and nerve, and the obliterated hypogastric 



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