462 ABDOMEN 



still smaller. Each of the anterior rami, before it joins the 

 plexus, receives a branch from the nearest sympathetic 

 ganglion, and the third and fourth sacral nerves give white 

 rami communicantes to the sympathetic pelvic plexuses. By 

 the union of the branch of the fourth lumbar nerve with the 

 fifth lumbar nerve, in the abdomen, a lumbo-sacral trunk is 

 formed. It descends behind the common iliac artery and 

 across the brim, into the pelvis minor; there it unites with 

 the first sacral nerve to form a loop through which the superior 

 gluteal artery passes. By the union of the other sacral roots a 

 series of similar loops is formed, and the inferior gluteal artery 

 frequently runs through one or other of them before it leaves 

 the pelvis. Beyond the loops the plexuses usually take the 

 form of two flattened bands, viz. an upper or sciatic band 

 and a lower or pudendal band. The sciatic band is very large, 

 and consists of the lumbo-sacral trunk with the first sacral 

 nerve and the greater portions of the second and third sacral 

 nerves. It runs downwards and laterally, narrowing but 

 thickening as it descends, and, passing first between the 

 adjacent borders of the piriformis and coccygeus, and then 

 through the lower part of the greater sciatic foramen, it leaves 

 the pelvis and enters the buttock as the sciatic nerve 

 (Vol. I. Fig. 129). 



The pudendal band is small. It consists of fibres of the 

 second, third, and fourth sacral nerves. It passes between 

 the adjacent borders of the piriformis and coccygeus muscles, 

 and it is continued from the pelvis through the lower part 

 of the greater sciatic foramen as the pudendal nerve. 



The student who has already dissected the inferior extremity 

 will remember that the sciatic nerve breaks up into common 

 peroneal (O.T. ext. popliteal) and tibial (O.T. int. popliteal) 

 divisions. It occasionally happens that the two divisions arise 

 separately from the sacral plexus. When that is the case there 

 is no sciatic band, and it becomes evident that the common 

 peroneal nerve is derived from the dorsal divisions of the 

 anterior rami of the fourth and fifth lumbar and the first and 

 second sacral nerves, and the tibial nerve, from the ventral 

 divisions of the anterior rami of the same nerves, and also from 

 the ventral division of the anterior ramus of the third sacral 

 nerve. Moreover, when the common peroneal arises directly 

 from the sacral plexus in the manner indicated, it usually 

 perforates the piriformis muscle on its way out of the pelvis. 



