THE PELVIS. 279 



7. Trace out the lateral sacrals to the spine and front of the sacrum. 



8. Follow the superior gluteal out of the pelvis above the pyriformis, 

 through the greater sacro-sciatic foramen, with a nerve of the same name. 



9. Follow the obturator vessels below the obturator nerve through the 

 obturator foramen to the adductor muscles. 



10. Observe the internal pudic leave the pelvis by the greater sacro-sciatic 

 foramen below the pyriformis, cross the ischial spine, reenter the pelvis by the 

 lesser sacro-sciatic foramen, and pass through Alcock's canal to supply the 

 external organs of generation in both the male and female. This artery is 

 attended in its course by the internal pudic nerve. 



1 1. The sciatic artery is a large and important vessel. It crosses the pyri- 

 formis muscle and sacral plexus and leaves the pelvis by the greater sacro- 

 sciatic foramen below the pyriformis muscle, in company with the internal pudic 

 and sciatic nerves. The artery lies under the gluteus maximus muscle and on 

 the external rotation of the thigh. It gives off the inferior gluteal artery, which 

 anastomoses with the superior, and also branches to the pelvic floor and pelvic 

 viscera ; branches to the rotator muscles, and the arteria comes nervi ischiadici, 

 a branch that accompanies the great sciatic nerve. This artery completes the 

 crucial anastomosis, an important collateral circulation about the hip-joint, by 

 inosculating below with the first perforating branch of the profunda, with the 

 internal circumflex internally, and with the external circumflex externally. 



The sympathetic distribution in this region will correspond to the visceral 

 branches given off from the internal iliac artery. The bladder will receive its 

 supply by the vesicals ; the uterus by the uterine ; the ovary by the ovarian ; 

 the rectum by the hemorrhoidals superior, middle, and inferior ; the urethra and 

 erectile tissue of penis and clitoris by the internal pudic. Remember the order : 

 (i) nerve; (2) plexus; (3) ganglion. All trace their sympathetic influence to 

 that part of the sympathetic located in the pelvis, by the side of the bladder, rec- 

 tum, vagina, and uterus the pelvic plexus. 



The Obturator Internus. Cut through the levator ani muscle the whole 

 length of the white line, from spine of ischium to pubic bone ; you may now look 

 down into the ischio-rectal fossa, and see the obturator internus muscle covered 

 by the obturator fascia, forming the outer wall of this fossa ; you can now appre- 

 ciate the origin of the levator ani from the white line and the cut segment of the 

 muscle, and study both the inner and outer walls of the ischio-rectal fossa. (Fig. 

 192.) You will also detach the pyriformis, at its origin, from the anterior surface 

 of the sacrum. Observe it passing through the greater sacro-sciatic foramen. 

 You will also see the coccygeus muscle extending from the spine of the ischium to 

 the side of coccyx. Now find on the outer wall of the ischio-rectal fossa, in 

 Alcock's canal, the internal pudic structures ; trace the same forward and see the 

 relation they bear to the tuber ischii and the triangular ligament, where they 

 perforate the posterior layer of this latter, to enter the deep perineal space. 



The ischio-rectal fossa is bounded by the obturator internus muscle exter- 

 nally, by the levator ani internally ; its apex is at the white line ; it extends from 

 the pubic bone in front to the sacrum behind ; the base is formed by the skin and 

 fasciae. When you dissect the pelvic outlet, you approach this fossa from the 

 base. In the present dissection, however, you approach the fossa from above. 

 When you cut through the white line, you opened into the fossa. You then 

 pulled the levator ani muscle to the mid-line ; this levator ani muscle with 

 its investing anal fasciae, above and below, is the inner wall of the fossa, and 

 you are thus permitted to see the outer wall of the fossa the obturator internus 

 musc le covered by the obturator fascia. 



Iliacus Muscle. Detach this muscle from its extensive origin in the iliac 

 fossa, and find under the same the iliac branches of the ilio-lumbar artery a 



