COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 337 



ii. The medial cutaneous. This is the small nerve which runs in contact 

 with the ulnar nerve. It turns superficially just above the elbow and is dis- 

 tributed to the skin of the forearm. 



Draw, showing the main parts of the brachial plexus. 



d) The thoracic portions of the vagus and the sympathetic: Trace the two vagi 

 toward the heart. They pass dorsal to the roots of the lungs. The left vagus 

 just caudad of the aortic arch gives off the left recurrent laryngeal nerve which 

 turns cephalad, passing on the dorsal side of the aorta, and ascends along the 

 side of the trachea. At the roots of the lungs the vagi give rise to a plexus 

 the pulmonary plexus to the lungs. This plexus also extends to the heart as 

 the cardiac plexus. The cardiac branches of the sympathetic system, noted 

 above, join the cardiac plexus. The cardiac plexus is situated at the bases of 

 the aorta and pulmonary arteries. In the rabbit the cardiac branches of the 

 vagus may be traced into this plexus. 



Caudad of the pulmonary plexus the two vagi in the rabbit continue posteri- 

 orly along the sides of the esophagus, to which they furnish small branches and 

 penetrate the diaphragm. In the cat each vagus divides just posterior to the 

 root of the lungs into dorsal and ventral branches. The ventral branches of the 

 two sides immediately unite into one trunk which proceeds posteriorly, lying on 

 the left ventrolateral surface of the esophagus. The two dorsal branches con- 

 tinue posteriorly, lying along the right and left sides of the esophagus; near the 

 diaphragm on the dorsal side of the esophagus they unite to one trunk. In this 

 manner are formed the dorsal and ventral divisions of the vagi; they pass through 

 the diaphragm. In their course along the esophagus they furnish branches to it. 



Locate again the inferior cervical ganglion. Note the communicating 

 branches from this ganglion to the brachial plexus. In the cat a particularly 

 stout branch extends anteriorly ventral to the bases of the sixth to eighth cervical 

 nerves, giving branches to them. Trace the sympathetic trunk posteriorly from 

 the inferior cervical ganglion. The contents of the pleural cavities may now be 

 cleaned out. The sympathetic trunk is a white cord lying to each side of the 

 vertebral column, passing ventral to the heads of the ribs. At segmental 

 intervals generally in the places between the ribs, it presents a ganglionic 

 enlargement. 



e) The thoracic spinal nerves: The first thoracic nerve contributes to the 

 brachial plexus as already learned. The ventral rami of the remaining thoracic 

 nerves pass laterally as the intercostal nerves, lying along the posterior side of 

 each rib. These nerves are readily exposed by running the point of an instru- 

 ment along the posterior side of each rib, slitting open the fascia of the intercostal 

 muscles. As each nerve emerges from the intervertebral foramen it receives one 

 or two communicating branches (rami communicantes) from the adjacent sym- 

 pathetic ganglion. These branches are rather delicate and the student may not 

 be able to see them. The dorsal rami of the thoracic spinal nerves supply the 



