SPINAL NERVES. 377 



thetic nerve is connected With a chain of ganglia 

 and connectives which cross the heads of the 

 ribs. 



5. The phrenic nerve is a branch of the fourth cervical 

 nerve, which arises just beyond the point of emer- 

 gence of this nerve from the neural canal, and runs 

 back alongside the vertebral column. Entering the 

 thorax it continues its course backwards, lying 

 between the heart and the lungs, and ends in the 

 diaphragm, which it supplies with motor fibres. 



In the neck it crosses the roots of the hinder 

 cervical nerves almost at right angles. It receives 

 a small branch from the fifth cervical nerve, and 

 sometimes one from the sixth, as it passes them. 



To expose the phrenic nerve gently press the heart and 

 lung apart. Follow the nerve back to the diaphragm, and for- 

 wards along the neck to its origin. 



F. The Spinal Nerves. 



The spinal nerves arise in pairs from the sides of the 

 spinal cord. There are eight cervical, twelve thoracic, seven 

 lumbar, four sacral, and six coccygeal. Each spinal nerve 

 arises from two roots, and divides into three branches. The 

 two roots are (a) a dorsal or sensory and ganglionated one, 

 and (Jb) a ventral or motor. The nerve so formed passes out 

 of the spinal canal by the intervertebral foramen. Its three 

 branches are : 



a. The sympathetic nerve, which, together with its 



fellows, forms a longitudinal ganglionated trunk 

 lying under the vertebral column and supplying 

 the vessels and viscera. 



b. The dorsal branch, which supplies the muscles and 



skin of the mid-dorsal region. 



c. The ventral branch, which supplies the lateral and 



ventral muscles and most of the skin. 

 The chief portions of the sympathetic nervous system have 



