THE ABDOMEN, INTERIOR. 529 



Nerve Supply. Filaments from the last dorsal and upper 

 lumbar nerves. 



Action. A lateral flexor of the spine. By fixing the 

 last rib it gives the diaphragm a firm point for contraction 

 and so aids inspiration. 



DISSECTION. 

 Trace the branches of the lumbar nerves through the psoas muscle, 



The Last Dorsal Nerve. Figs. 105, 106, 107. 



Courses outward and downward just below the lower 

 border of the last rib, crossing the quadratus lumborum, to 

 disappear through the lumbar fascia, and enter between 

 the transversalis and internal oblique muscles, where it 

 divides into two branches. 



The anterior branch continues to the rectus and pyra- 

 midalis muscles, the lateral or iliac pierces the internal and 

 external oblique muscles about three inches above the crest 

 of the ilium and turns downward to supply the integument 

 over the crest of the ilium in front of and as low down as 

 the great trochanter. 



The Lumbar Plexus. Figs. 103 to 107. 



The lumbar plexus is formed by the anterior divisions of 

 the first four lumbar nerves and a branch from the last 

 dorsal. 



From the fourth lumbar nerve a branch descends to join 

 the fifth (anterior) division, and the trunk thus formed 

 passes to the upper margin of the sacral plexus as the 

 lumbosacral cord. 



The lumbosacral cord lies between the inner border of 

 the psoas muscle and the base of the sacrum, and behind 

 the iliac vessels. The lumbar nerves are connected to the 

 sympathetic ganglia by slender filaments. 

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