THE NERVOUS SYSTEM 113 



men in the neural arch of the atlas. The other seven pairs 

 escape through the intervertebral foramina. The first, 

 second, third, and part of the fourth pairs of nerves go to 

 the neck muscles. The phrenic nerve leaves the fourth 

 cervical nerve. Trace it caudad to the diaphragm. 



Dissect the brachial plexus, the conspicuous bundle of 

 intercommunicating nerve trunks in the axillary region. 

 It is formed by the fourth, fifth, sixth, seventh, and eighth 

 cervical, and the first dorsal nerves. Note the size of these 

 nerves. The brachial plexus sends the following branches 

 to the arm, (1) The ulnar nerve goes to the posterior 

 region of the forearm. (2) The median nerve runs across 

 the anterior surface of the elbow joint, supplying the mus- 

 cles of the inner side of the arm. (3) The axillary nerve 

 passes around the posterior side of the humerus, crosses 

 the elbow joint, and as the radial nerve proceeds to the 

 outer side of the arm. Locate, also, the branches going 

 from the plexus to the thorax. 



There are thirteen pairs of thoracic spinal nerves. The 

 first enters the brachial plexus. The second to the twelfth 

 inclusive run in the muscle tissue between the ribs. The 

 thoracic nerves leave the spinal canal through the inter- 

 vertebral foramina. Except for the first, they innervate 

 the walls of the thorax and anterior part of the abdomen. 



The six pairs of lumbar nerves are associated with the 

 thick muscle masses in the lumbar and sacral regions. Care- 

 fully remove these muscles without severing the nerves or 

 their connections. The first two pairs of lumbar nerves 

 innervate the abdominal walls, by way of the lumbar plexus. 

 Find the lumbar plexus, which is formed on each side by 

 part of the first, second, third, fourth, and fifth lumbar 

 nerves. The three following nerves arise from it. The 

 femoral nerve proceeds caudally to the inner aspect of the 

 hind limb. The obturator nerve passes through the obtura- 



