810 THE XERVOrS SYSTEM 



part of the rectum, and on the right the lower end of the ileum passes in front 

 of it. 



It is formed as follows: a pai't of the fourth lumhar nerve, " nervus furcalis, " 

 joins the tifth lumbar nerve to form the lumbo-saeral cord; this cord is then joined 

 b}'' the first, second, and third sacral nerves to form a great flattened band, which is 

 directed outwards and dowuAvards towards the lower margin of the great sacro- 

 sciatic foramen, and, on crossing that margin, is no longer called the sacral plexus, 

 but takes the name of the great sciatic nerve. A part of the third sacral nerve 

 is joined by branches from the second and fourth sacral nerves to form a second or 

 lower band of relatively small size, which ends in the pudic nerve. Several 

 branches, to be presently enumerated, spring from the plexus; l)Ut l)y far the larger 

 part of the plexus is directly continued into the great sciatic and pudic nerves. 

 The plexus is, therefore, more condensed and more simple in its formation than any 

 other plexus in the body. The two constituents of the lumbo-sacral cord unite to 

 form that structure on, or a little below, the brim of the pelvis. The jjlexus has 

 the following important vascular relations: the internal iliac vessels are placed a 

 little in front of its upper part; the gluteal artery passes lietween the luml)o-sacral 

 cord and the first sacral nerve; the sciatic artery passes through the lower part of 

 the plexus (fig. 466). 



It has been shown (Paterson) that the anterior primary divisions of the lower four lumbar 

 and the ujiper two sacral nerves divide into anterior and posterior parts. The branches derived 

 from the anterior parts (hghtly shaded in fig. 465) always cross in front of the posterior parts and 

 unite to form trunks (e.g. obturator, internal popliteal), which are distributed to what is mor- 

 phologically the anterior aspect of the limb. The trunks which are formed from the posterior 

 parts (e.g. external cutaneous, anterior crural, external ])opliteal) are distributed to the (mor- 

 phologically) posterior aspect of the limb ; these nerves are darkly shaded in fig. 465. 



The 1)ranches of the sacral plexus are classified into collateral and terminal. 

 The collateral branches are the superior gluteal, the inferior gluteal, the nerve to 

 the pyriformis, visceral liranches, the nerve to the quadratus, the small sciatic, the 

 nerve to the obturator internus, and the perforating cutaneous nerve. The terminal 

 branches are the great sciatic and the pudic nerves. 



COLLATERAL BRANCHES. — 1. The Superior gluteal nerve arises by two 

 roots, one from the lumbo-sacral cord, and the other from the first sacral nerve. 

 The upper root contains fibres derived from the fourth and fifth lumbar nerves. 

 The nerve accompanies the gluteal vessels through the great sacro-sciatic foramen, 

 passing above the pyriformis. It then divides into a smaller upi)er and a larger 

 lower branch. The upper branch accompanies the upper division of the deep part 

 of the gluteal artery and terminates in the gluteus medius; the lower brancli crosses 

 the gluteus minimus with the lower branch of the deep part of the gluteal artery, 

 sup]ilying filaments to the gluteus medius and mininnis and a ternnnal branch, 

 which passes between their anterior borders, or through the fibres of the minimus, 

 to the tensor fascia femoris. 



2. The inferior gluteal nerve arises from the posterior aspect of the plexus, and 

 contains fibres derived from tlie fifth lumbar and the first, second, and third sacral 

 nerves. It escapes through the great sacro-sciatic foramen, below the pyriformis, 

 and in this situation is often adherent to the small sciatic nerve. It divides into 

 several stout twigs, which enter the deep surface of the gluteus maximus. 



3. The nerve to the pyriformis is given oft' eitlier from the second or from 

 the third sacral nerve before they join the plexus. 



4. The visceral branches arise from the third and fourth sacral nerves. They 

 will be described in connection with the sympatlietic. 



5. The nerve to the quadratus femoris rises from the front of the plexus, 

 obtaining fibres from the fourtli and fifth lumbar and first sacral nerves. It passes 

 through the great sacro-sciatic foramen lielow the pyriformis. and is usually 

 adherent for some distance to the deep surface of the great sciatic in the part of its 

 course where it lies between the latter nerve and the bone. It then passes, under 

 cover of the tendon of the obturator internus and the gemelli, and, having sup])lied 

 the gemtdlus inferior, ends in the deep or anterior surface of the quadratus femoris, 

 and in the posterior part of the capsule of the hip-joint. 



