24° VERTEBRATE ZOOLOGY 



into an anterior and a posterior division. Look for the first pair 

 of lumbar nerves a short distance behind the last rib and beneath 

 the muscle which forms the inner layer of the abdominal wall. 

 The two divisions of the ventral ramus will here be found. Trace 

 them medially and find the dorsal ramus. Find the second and 

 third lumbar nerves. 



The fourth, fifth, sixth, and seventh lumbar nerves and the 

 three pairs of sacral nerves form the lumbosacral plexus; the 

 nerves issuing from it supply the hind legs and the hinder part 

 of the trunk. This plexus is partly hidden by muscles, and may be 

 best studied by finding the nerves which issue from it and follow- 

 ing them back to the plexus. 



The largest of these nerves are the great sciatic, the obturator, 

 and the femoral nerve. The first of these is the largest nerve in 

 the body ; it is a great cord which will be seen passing straight 

 back through the pelvis. It is formed principally by the union 

 of the sixth and seventh lumbar and the first and second sacral 

 nerves. Trace it forward, thoroughly removing the muscle and 

 fat which obscure it. Note its two largest roots, which are the 

 seventh lumbar and first sacral nerves. Follow the great sciatic 

 into the leg. 



Just in front of the great sciatic is the obturator nerve. Trace it 

 forward ; it will be found to be formed by the union of the sixth 

 and seventh spinal nerves. 



The femoral is a large nerve which is formed by the union of 

 branches of the fifth and sixth lumbar nerves ; it is covered up 

 by a muscle, which must be removed. As it emerges from the 

 abdominal cavity it divides into three nerves which supply the 

 muscles of the thigh. One of these, the saphenous, is the long nerve 

 which accompanies the femoral vein and artery. 



The genitofemoral nerve arises principally from the fourth 

 lumbar nerve and passes to the anterior surface of the thigh. 

 It divides into two main branches: one of these passes through 

 the longitudinal muscle that lies alongside the spinal column 

 in the abdominal cavity, and appears on the ventral surface 

 of this muscle; the other passes posteriorly along its medial 

 surface. 



