THE NERVOUS SYSTEM. 85 



The only important branch of any of the four upper 

 cervical nerves, which in general supply the neck and 

 shoulders, is the phrenic, which is distributed to the 

 pericardium, the pleura, and the under surface of the 

 diaphragm. 



The brachial plexus, as its name implies, supplies 

 the arms and has a number of important branches, as 

 the circumflex to the shoulder, the musculo-cutaneous 

 to the upper arm, the elbow-joint, and the outer sur- 

 face of the forearm, the internal cutaneous to the inner 

 side of the arm, the median to the pronators and 

 flexors and the fingers on the radial side, and the ulnar 

 to the elbow and wrist-joint. The musculo-spiral runs 

 down the spiral groove to the external condyle of the 

 humerus or upper arm bone, where it divides into the 

 radial and the posterior interosseous, the former going 

 to the thumb and two adjacent fingers and the latter to 

 the wrist-joint and the muscles on the back of the fore- 

 arm. Sometimes, in fracture of the humerus the callus 

 thrown out pinches the musculo-spiral and causes pain. 



The dorsal or thoracic nerves supply the back with their 

 posterior divisions and their anterior divisions are the 

 intercostal nerves. 



The lumbar nerves supply the abdomen, pelvis, and 

 thigh, the chief branches being the ilio-hypogastric to 

 the abdomen and gluteal region, the ilio-inguirial to the 

 inguinal region and scrotum, the external cutaneous and 

 genito-crural to the thigh, and the obturator to the thigh 

 and the hip and knee-joints. The anterior crural de- 

 scends beneath Poupart's ligament and divides into an 

 anterior and a posterior division which supply the thigh 

 muscles, its branches going to the pelvis. 



The sacral plexus supplies the organs of the pelvis, 

 the thigh, and the leg. Its chief branches are the great 

 sciatic, the largest nerve in the body, and the small sci- 

 atic, which go to the buttocks and thigh. The great 

 sciatic runs down the back of the thigh and divides at 

 the lower third of the thigh into the internal and external 



