606 ABDOMEN 



second sacral nerves are very large, and of about equal size ; 

 the third is much smaller, and the fourth 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. This descends 

 behind the common iliac artery and over the pelvic brim 

 into the pelvic cavity, where it unites with the first sacral 

 nerve, forming a loop through which the superior glutseal 

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

 of similar loops is formed, and the inferior glutaeal artery 

 frequently runs through one or other of these before it leaves 

 the pelvis. Beyond the loops the plexus usually takes 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 and 

 thickening as it descends, and, passing first between the 

 adjacent borders of the piriformis and coccygeus, and then 

 through the great sciatic foramen, it leaves the pelvis and 

 enters the buttock as the sciatic nerve. 



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

 second, third, and fourth sacral nerves. It also passes be- 

 tween the adjacent borders of the piriformis and coccygeus 

 muscles, and it is continued from the pelvis through the 

 lower part of the great sciatic foramen as the pudendal 

 nerve. 



The student who has already dissected the lower extremity 

 will remember that the sciatic nerve breaks up into common 

 peroneal (ext. popliteal) and tibial (int. popliteal) divisions. 

 It occasionally happens that the two divisions arise separ- 

 ately from the sacral plexus. When this 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 branches of the fourth and fifth lumbar and the first 

 and second sacral nerves, and the tibial nerve from the 

 ventral divisions of the anterior branches of the same nerves, 

 and also from the ventral division of the anterior branch of 

 the third sacral nerve. Moreover, when the common peroneal 





