324 



THE SPINAL NERVES. 



into the great sciatic nerve : the lower band, which has a more plexiform arrange- 

 ment, results mainly from the junction of the smaller part of the third sacral nerve 

 with the portion of the fourth nerve belonging to the plexus, and is prolonged into 



Fig. 210. THE SACRAL PLEXUS OF 



THE RIGHT SIDE, FROM BEHIND. 



(G. D. T.) 



The peroneal and popliteal divisions 

 of the great sciatic nerve are separated 

 up to their origin. The dorsal offsets 

 of the plexus are represented light, and 

 the ventral offsets dark. 



J-SACRAL CORD 



S'JP. 

 GLUT. 



the pudic nerve. The lower 

 band is, however, joined by a 

 small fasciculus from the second 

 sacral nerve, and according to 

 Eisler it receives fibres also from 

 the first sacral nerve. To the 

 place of union the nerves pro- 

 ceed in different directions, that 

 of the upper ones being obliquely 

 downwards, while that of the 

 lower is nearly horizontal ; and, 

 as a consequence of this difference, 

 they diminish in length from the 

 first to the last. The sacral plexus 

 rests on the anterior surface of 

 the pyriformis muscle, opposite 

 the side of the sacrum, and 

 escaping through the great sacro- 

 sciatic foramen, ends at the lower 

 border of the pyriformis in the 

 great sciatic and pudic nerves. 



BRANCHES. In addition to the terminal offsets, viz., the great sciatic and pudic 

 nerves, the sacral plexus gives origin to a number of collateral branches of smaller 

 size. These are the superior gluteal, inferior gluteal, small sciatic, and perforating 

 cutaneous nerves, and branches to the pyriformis, obturator intern us, gemelli, and 

 quadratus femoris muscles. 



In the description of the sacral plexus a division is sometimes made into two subordinate 

 plexuses. The larger upper part, which ends in the great sciatic nerve and gives off the other 

 branches to the limb, is distinguished as the sciatic plexus {plexus iscMadiciis\ while the 

 smaller lower part, including the pudic nerve together with the visceral and muscular branches 

 of the third and fourth sacral nerves, is designated the pudic plexus (plexus pudendiut}. 



In the usual condition the greater part of the sacral plexus is prolonged, as already stated, 

 into the great sciatic trunk, which divides at a variable level in the thigh into the internal 

 popliteal and external popliteal (or peroneal) nerves. But in many cases (1 in 5 or 6) these 

 nerves spring independently from the plexus, and the external popliteal division passes back- 

 wards through a cleft in the pyriformis muscle, the lower part of which is interposed between 

 the two nerves. In the normal arrangement also the two nerves can be readily separated up 

 to the pelvis by dissecting off the sheath of the great sciatic trunk, and it is then found that 

 the plexus and the nerves entering it are divided into ventral and dorsal portions, the internal 

 popliteal nerve being formed by the union of the ventral divisions of the lumbo-sacral cord 

 and the first three sacral nerves, while the external popliteal receives the dorsal divisions of 

 the lumbo-sacral cord and the first two sacral nerves. The collateral branches of the plexus 

 may also be divided in a corresponding manner according as they arise from the anterior or 



