1 82 VERTEBRATE ZOOLOGY 



will be seen on each side a white cord, the vagus nerve. Lying 

 opposite to these muscles, at the base of the neck, will be seen four 

 very large spinal nerves, which, together with a delicate branch 

 from the next nerve back of them, form a network called the 

 brachial plexus. From this plexus proceed the nerves that supply 

 the wing and the muscles of the breast. 



Thoroughly clean the plexus and follow the nerves that go 

 to the wing. Note the spinal ganglion at the base of each nerve. 

 The brachial plexus is formed of the eleventh to the fifteenth 

 spinal nerves inclusive. 



Exercise 22. Draw a diagrammatic view of the brachial plexus, show- 

 ing accurately the arrangement of the nerves as observed. 



The five spinal nerves following this plexus proceed laterally 

 straight to the muscles of the trunk. The next two, which are 

 the twenty-first and twenty-second, form the lumbar plexus ; from 

 it nerves go to the muscles of the thigh. The following four 

 nerves, which are the twenty-third to the twenty-sixth inclusive, 

 form the sacral plexus, from which the great sciatic nerve proceeds 

 to the leg. These two plexuses are joined by a cross nerve. 

 Thoroughly clean these nerves and follow the great sciatic nerve 

 into the leg ; observe its branching throughout the leg. Posterior 

 to the sciatic plexus is the plexus pudendus, a simple union of the 

 seven spinal nerves which extend obliquely back to the tail. 



Exercise 23. Draw a diagram of these three plexuses and the nerves 

 proceeding from them, so far as observed. 



Study the spinal nerves of the neck in front of the brachial 

 plexus. Ten pairs are present, of which all but the first two pairs 

 may be seen issuing from among the muscles on the ventral side 

 of the neck and passing dorsally. 



The sympathetic nervous system consists of a pair of longi- 

 tudinal nerve cords with paired ganglia, which lie on either side of 

 the spinal column between the head and the tail, in close con- 

 nection with the spinal nerves. The cord is doubled on each side 

 in the trunk, where it sends off numerous branches to the organs 

 of the digestive and circulatory systems. 



