382 THE NERVE SYSTEM 



artery, but on three sides of the second part of that 

 vessel. 



The branches of the brachial plexus supply the 

 muscles of arm, forearm, fingers, and the muscles 

 of the chest. 



THE LUMBAR PLEXUS. The lumbar plexus is formed 

 in the substance of the psoas muscle, in the following 

 manner: Each of the first four lumbar nerves divides 

 into an upper and a lower branch. Just before dividing 

 the first receives the twelfth thoracic nerve, and the 

 third and fourth send each a branch to the nerve below. 



The upper branch of the first subdivides into the 

 iliohypogastric and ilioinguinal nerves. The lower 

 branch of the first passes downward and subdivides 

 into two branches, one of which unites with the 

 upper branch of the second to form the genitocrural 

 nerve. The other unites with the lower branch of the 

 second to form a cord. This cord passes downward, 

 and gives off the external cutaneous nerve and a branch 

 to the obturator, after which it unites with the upper 

 branches of the third and fourth to form the anterior 

 crural nerve. The lower branches of the third and 

 fourth unite to form the obturator nerve. 



The branches derived from the above plexus inner- 

 vate the skin and muscles over the anterior and 

 internal aspect of the thigh, leg, instep, and external 

 genital organs. 



THE SACRAL PLEXUS. The sacral plexus is formed 

 by the anterior divisions of the first, second, third, 

 and part of the fourth sacral nerves, together with the 

 lumbosacral cord. 



The lumbosacral cord, with the first, second, and 

 part of the third sacral nerve, is continued into the 

 upper great branch of the plexus, and the remainder of 

 the plexus forms the lower or smaller branch. 



The branches from this plexus supply the muscles 

 and skin over the buttocks, back of thigh and leg, 

 and the sole of the foot. 



