658 THE LUMBAR PLEXUS. 



linea alba, accompanying the small perforating arteries. Their number and position 

 are very uncertain. They are directed outwards towards the lateral cutaneous nerves. 

 A second set is described by Cruveilhier as existing at the outer edge of the rectus 

 muscle. 



LAST DORSAL NERVE. 



The anterior primary division of this nerve is below the last rib, and is 

 contained altogether in the abdominal wall. The nerve has the general 

 course and distribution of the others between the internal oblique and 

 transversalis, but, before taking its place between those muscles, it passes in 

 front of the upper part of the quadratus lumborum, and pierces the posterior 

 aponeurosis of the transverse muscle. This nerve is connected by offsets 

 with the nerve above, and occasionally with the ilio-hypogastric branch of 

 the lumbar plexus. Near the spine it sometimes communicates with the 

 first lumbar nerve by means of a small cord in the substance of the quad- 

 ratus lumborum. 



The lateral cutaneous branch of the last dorsal nerve^ passing through both 

 oblique muscles, is directed downwards over the iliac crest to the integument, 

 covering the fore part of the gluteal region and the upper and outer part of 

 the thigh, some filaments reaching as far as the great trochanter of the 

 femur. 



ANTERIOR PRIMARY DIVISIONS OF THE LUMBAR NERVES. 



The anterior divisions of the lumbar nerves increase in size from the first 

 to the fifth ; and all, except the fifth, which passes down to join the sacral 

 nerves, are connected together by communicating loops, so as to form the 

 lumbar plexus. On leaving the inter vertebral foramina these nerves are 

 connected by filaments with the sympathetic nerve, these filaments being 

 longer than those connected with other spinal nerves, in consequence of the 

 position of the lumbar sympathetic ganglia on the fore part of the bodies of 

 the vertebrse. In the same situation are furnished small twigs to the psoas 

 and quadratus lumborum muscles. 



LUMBAR PLEXUS. 



The lumbar plexus is formed by the communications between the anterior 

 primary divisions of the four upper lumbar nerves. It is placed in the 

 substance of the psoas muscle, in front of the transverse processes of the 

 corresponding vertebrae. Above, the plexus is narrow, and is sometimes 

 connected with the last dorsal nerve by a small offset from that nerve, 

 named dorsi-lumbar ; below it is wider, and is joined to the sacral plexus 

 by means of a branch given by the fourth lumbar nerve to the fifth. 



The arrangement of the plexus may be thus stated. The first nerve 

 gives off the ilio-hypogastric and ilio-inguinal nerves, and sends downwards 

 a communicating branch to the second nerve. The second furnishes the 

 greater part of the geuito-crural and external cutaneous nerves, and gives a 

 connecting branch to the third, from which some of the fibres of the anterior 

 crural and obturator nerves are derived. From the third nerve, besides the 

 descending branch to the fourth, two branches proceed : one of these, the 

 larger, forms part of the anterior crural nerve ; the other, a part of the 

 obturator nerve. The fourth nerve gives two branches, which serve to 

 complete the obturator and anterior crural nerves, and a connecting branch 

 to the fifth nerve. 



