112 ANATOMY OF THE RAT 



The sympathetic system of mammals is centralized in a 

 pair of ganglion-bearing, longitudinal sympathetic trunks, 

 one on each side of the vertebral column. This trunk con- 

 nects with the spinal cord through the spinal nerves, and 

 also sends fibers to sympathetic plexuses in various parts of 

 the body. These plexuses in turn send nerves to the viscera. 

 Locate the two longitudinal sympathetic trunks in the rat, 

 one on each side of the vertebral column. Trace one of 

 these anteriorly to the head and posteriorly to the pelvic 

 region. The superior cervical ganglion is a spindle-shaped 

 enlargement near the bifurcation of the common carotid 

 artery. The cervical portion of the trunk is dorsal to the 

 tenth cranial nerve. The inferior cervical ganglion is the 

 swelling of the trunk at the base of the neck. The thoracic 

 section lies ventral to the ribs and presents ganglionic en- 

 largements. The lumbar portion is median to the dorsal 

 muscle mass in the lumbar region. Locate if possible the 

 splanchnic nerves, which leave the thoracic ganglia in the 

 vicinity of the eleventh and twelfth ribs, enter the abdo- 

 men, and go to the solar plexus. Find other rami of the 

 sympathetic trunk. 



THE SPINAL NERVES 



The study of the spinal cord and the spinal nerves lead- 

 ing from it should be postponed until dissections and draw- 

 ings of all other parts of the body, including the sympa- 

 thetic system, have been completed. Expose the spinal 

 nerves by carefully dissecting away the muscles ventral and 

 lateral to the backbone, from the occipital region of the 

 head to the base of the tail. The spinal nerves are desig- 

 nated according to the region in which they are found, 

 cervical, thoracic, lumbar, sacral, and coccygeal. 



The cervical spinal nerves, eight pairs in all, are in the 

 neck. The first pair leaves the spinal canal through a fora- 



