THE LUMBAR PLEXUS. 781 



same disproportion between them and the posterior roots as in the cervical nerves. 

 The roots of these nerves have a vertical direction, and are of considerable length \ 

 more especially the lower ones, since the spinal cord does not extend beyond the first 

 lumbar vertebra. The roots become joined in the intervertebral foramina, and the 

 nerves so formed divide at their exit into two divisions, posterior and anterior. 



The Posterior Divisions of the Lumbar Nerves. 



The posterior divisions of the lumbar nerves (Fig. 415) diminish in size from 

 above downward ; they pass backward between the transverse processes, and divide 

 into internal and external branches. 



The internal branches, the smaller, pass inward close to the articular processes 

 of the vertebrae, and supply the Multifidus spinse and Interspinales muscles. 



The external branches supply the Erector spinre and Intertransverse muscles. 

 From the three upper branches cutaneous nerves are derived which pierce the 

 aponeurosis of the Latissimus dorsi muscle and descend over the back part of the 

 crest of the ilium, to be distributed to the integument of the gluteal region, some 

 of the filaments passing as far as the trochanter major. 



The Anterior Divisions of the Lumbar Nerves. 



The anterior divisions of the lumbar nerves increase in size from above, down- 

 ward. At their origin they communicate with the lumbar ganglia of the sympa- 

 thetic by long, slender filaments, which accompany the lumbar arteries round the 

 sides of the bodies of the vertebrae, beneath the Psoas muscle. The nerves pass 

 obliquely outward behind the Psoas magnus or between its fasciculi, distributing 

 filaments to it and the Quadratus lumborum. The anterior divisions of the four 

 upper nerves are connected together in this situation by anastomotic loops, and form 

 the lumbar plexus. The anterior division of the fifth lumbar, joined with a branch 

 from the fourth, descends across the base of the sacrum to join the anterior division 

 of the first sacral nerve and assist in the formation of the sacral plexus. The cord 

 resulting from the union of the fifth lumbar and the branch from the fourth is called 

 the lumbo-sacral cord. 



The Lumbar Plexus (Fig. 416). 



The lumbar plexus is formed by the loops of communication between the anterior 

 divisions of the four upper lumbar nerves. The plexus is narrow above, and often 

 connected with the last dorsal by a slender branch, the dorsi-lumbar nerve; it is 

 broad below, where it is joined to the sacral plexus by the lumbo-sacral cord. It 

 is situated in the substance of the Psoas muscle near its posterior part, in front of 

 the transverse process of the lumbar vertebrae. 



The mode in which the plexus is arranged varies in different subjects. It differs 

 from the brachial plexus in not forming an intricate interlacement, but the several 

 nerves of distribution arise from one or more of the spinal nerves, somewhat in the 

 following manner: The first lumbar nerve receives a branch from the last dorsal, 

 and gives off a larger branch, which subdivides into the ilio-hypogastric and ilio- 

 inguinal ; a communicating branch which passes down to the second lumbar nerve; 

 and a third branch which unites with a branch of the second lumbar to form the 

 genito-crural nerve. The second, third, and fourth lumbar nerves divide into an 

 anterior and a posterior division. The anterior division of the second divides into 

 two branches, one of which joins with the above-mentioned branch of the first nerve 

 to form the genito-crural ; the other unites with the anterior division of the third 

 nerve, and a part of the anterior division of the fourth nerve to form the obturator 

 nerve. The remainder of the anterior division of the fourth nerve passes down to 

 communicate with the fifth lumbar nerve. The posterior divisions of the second 

 and third nerves divide into two branches, a smaller branch from each uniting to 

 form the external cutaneous nerve, and a larger branch from each, which join with 



