110 GENERAL BIOLOGY. 



Lay the frog ventral side up and carefully re- 

 move ike digestive, excretory, and reproductive 

 organs. 

 Observe : 

 1. Along the vertebral column on each side and 



close to the dorsal aorta a slender cord. 

 With great care raise it slightly and note : 

 The yellowish enlargements along its length, 



the sympathetic ganglia. 

 Count the ganglia, being careful not to injure 



them. 



C. The Peripheral Nervous System. 

 1. Spinal Nerves : passing out from between the 



vertebrae. Count those of one side. 

 Gently raise the sympathetic cord, and note 

 the branch from each spinal nerve to a 

 sympathetic ganglion. 



Note the grouping of the spinal nerves into 

 plexuses : 



(a) The sciatic plexus; formed from the 7th, 



8th, and 9th spinal nerves. From the 

 plexus the sciatic nerve is given off. 

 Trace this into the leg. 



(b) The brachial plexus : formed from the 2d 



and 3d spinal nerves. It gives rise to 

 the brachial nerve. Trace this nerve 

 into the arm. 



Between these two plexuses the 4th, 5th, and 

 6th spinal nerves pass out separately as 



