300 VERTEBRATE ZOOLOGY 



alongside the vagus ganglion. Identify the eight pairs of cervical 

 spinal nerves ; the first two pairs are very small. 



At each end of the trunk the spinal nerves unite on each side 

 to form a complicated network called a nerve plexus, the anterior 

 one being the brachial plexus and the posterior one the lumbo- 

 sacral plexus ; from each of these plexuses on each side the im- 

 portant nerves proceed which innervate the muscles of the leg. 

 The cervical spinal nerves are also connected together on each side 

 to form a very simple cervical plexus. 



The brachial plexus, which has already been seen in the axillary 

 region, is composed of the ventral rami of the fourth to the eighth 

 cervical and the first thoracic spinal nerves; the nerves which 

 arise from this plexus go to the shoulder, the foreleg, and the 

 diaphragm. 



Identify the roots of these six nerves ; carefully free the plexus 

 from the surrounding tissues and follow the main nerves which 

 spring from it. 



Exercise 25. Draw the brachial plexus, numbering the roots of the 

 spinal nerves; draw the roots of the nerves which issue from it, so 

 far as observed. 



Observe the thoracic and lumbar spinal nerves and their rela- 

 tion to the thoracic and abdominal divisions of the sympathetic 

 system. The first thoracic spinal nerve enters the brachial plexus, 

 as we have just seen. Of the other eleven or twelve pairs, as well 

 as the first three lumbar spinal pairs, the ventral rami pass 

 laterally to the muscles of the back and are called the intercostal 

 nerves; their dorsal rami are very small. As each spinal nerve 

 issues from the intervertebral foramen it sends a delicate branch 

 to the sympathetic ganglion. 



Two large nerve plexuses, the cardiac plexus and the solar 

 plexus, and several smaller ones belong to the sympathetic system. 

 The cardiac plexus lies at the anterior end of the heart and around 

 the base of the aorta and pulmonary artery. The solar plexus lies 

 between the fundus of the stomach and the dorsal portion of the 

 diaphragm; it contains the large cceliac ganglion and the much 

 smaller superior mesenteric ganglion, both of which lie near 



